Fabrication of MEMS Based Air Quality Sensors

by

Faysal Ahmed

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering University of Toronto

© Copyright by Faysal Ahmed 2011

ii

Fabrication of MEMS Based Air Quality Sensors

Faysal Ahmed

Master of Applied Science

Mechanical and Industrial Engineering University of Toronto

2011

Abstract

This thesis deals with the fabrication of MEMS air quality sensors for automotive applications.

The goal of this project is to design, fabricate and test an integrated sensor that measures three

important air quality components inside the automotive cabin, which are temperature, relative

humidity and carbon monoxide (CO) concentration. The sensors are fabricated on silicon

substrate covered with thermal oxide and LPCVD nitride. Various deposition and etching

techniques were utilized to fabricate these sensors including E-beam evaporation, thermal oxide

growth, PECVD, LPCVD, RIE, KOH and HF etching. The temperature and humidity sensor use

nickel as the sensitive material while the CO sensor was designed to use SnO2 although it was

not fabricated to completion. A chamber was created where the temperature and humidity are

controlled and the sensors were tested. Curves of sensor resistance vs. temperature and sensor

resistance vs. humidity were created and the two sensor’s sensitivity was calculated.

iii

Acknowledgments

In the Name of Allah (God), the Most Beneficent, the Most Merciful.

This thesis would not be possible without the encouragement and support from the following:

My mother and father, my aunt Samiya and my uncle Abdullahi, my wife Fahima and my

daughter Leila. I love you all.

My supervisors, Professor Cleghorn and Professor Mills for giving me the opportunity to work

with them and do research without boundaries.

AUTO21 for their generosity and financial support.

Dr. Edward for his expertise, guidance and training with the various microfabrication methods

and equipments. He has also guided me in the planning and execution of the sensor fabrication.

My research colleagues, Henry Chu, Dominik Wyss, Issam Bait, Shael Markin, Adam Yi Lee,

Steven Haley, Cong Lu and Masih Mahmoodi. Dominik for his Microsoft Word wizardry and

Henry for helping me when I was new to the lab and throughout my two years of research and

TA.

Professor Cleghorn once again for his continued guidance and support for the past two years and

for his fascinating lectures. He is in my opinion and the majority of my colleagues at the

University of Toronto, the best lecturer. I have never seen a professor that puts that much

enthusiasm and effort into his lectures.

Last but not least, my relatives and friends for their constant support, especially Mohamed Mire.

Much love to you all.

Thank You.

iv

Table of Contents

Abstract ........................................................................................................................................... ii

Acknowledgments.......................................................................................................................... iii

1.0 Introduction to research problem ......................................................................................... 1

2.0 Micro-fabrication overview ................................................................................................. 5

2.1 What is a wafer? ............................................................................................................... 5

2.1.1 Wafer size?................................................................................................................ 6

2.1.2 Wafer Orientation ..................................................................................................... 8

2.1.3 Wafer Doping: .......................................................................................................... 9

2.1.4 Wafer Resistivity .................................................................................................... 10

2.1.5 Wafer Cleaning ....................................................................................................... 10

2.2 Photoresist ...................................................................................................................... 10

2.3 Photomasks..................................................................................................................... 13

2.4 Photolithography ............................................................................................................ 15

2.4.1 Prebaking ................................................................................................................ 15

2.4.2 Exposure ................................................................................................................. 15

2.4.3 Post-Exposure Bake ................................................................................................ 16

2.4.4 Developing .............................................................................................................. 16

2.4.5 Hard Bake ............................................................................................................... 16

2.5 Etching ........................................................................................................................... 16

2.6 Deposition ...................................................................................................................... 20

2.6.1 Chemical Vapor Deposition .................................................................................... 21

2.6.3 Physical Deposition ................................................................................................ 24

3.0 Sensing Theory .................................................................................................................. 29

3.1 Temperature ................................................................................................................ 29

3.2 Humidity ..................................................................................................................... 30

3.3 Carbon Monoxide ....................................................................................................... 30

3.3.1 Selectivity and Performance ................................................................................... 32

3.3.1.1 Temperature Control ........................................................................................... 32

3.3.1.2 Additives ............................................................................................................. 33

3.3.1.3 Sensor Array ........................................................................................................ 33

v

3.3.2 Sensitivity ............................................................................................................... 34

3.3.2.1 Temperature Control ........................................................................................... 34

3.3.2.2 Geometry ............................................................................................................. 34

3.3.2.3 Grain Size Effects................................................................................................ 34

4.0 Sensor Design .................................................................................................................... 37

4.1 Fabrication Steps ............................................................................................................ 37

4.1.1 Carbon Monoxide Sensor Design Overview .............................................................. 37

4.1.2 Fabrication Steps and Methods ................................................................................... 37

Passivation Layer: ..................................................................................................................... 38

4.1.3 Base Structure Design (Experimental) ....................................................................... 39

4.1.4 Metal Deposition (Experimental) ............................................................................... 45

4.2 Mask designs .................................................................................................................. 50

4.2.1 Heater Component .................................................................................................. 50

4.2.2 Temperature Component ........................................................................................ 51

4.2.3 Contact Holes .......................................................................................................... 53

4.2.4 Contact Holes .......................................................................................................... 54

4.2.5 Sensitive Material Layer Component ..................................................................... 55

4.2.6 Bridge Membrane ................................................................................................... 56

4.2.7 Bridge Membrane (Etching Experiments) .............................................................. 57

4.2.8 Metallization ........................................................................................................... 59

4.2.9 Etching (Experimental) ............................................................................................... 60

4.3 Overall CO Fabrication Steps ........................................................................................ 72

Heater: ....................................................................................................................................... 72

Temperature: ............................................................................................................................. 72

4.4 Overall Temperature Sensor Fabrication Steps .............................................................. 73

4.5 Overall Humidity Sensor Fabrication Steps ................................................................... 74

5.0 Final Fabrication ................................................................................................................ 76

6.0 Testing and Results ............................................................................................................ 80

6.1 Temperature ................................................................................................................ 80

6.2 Humidity ..................................................................................................................... 82

6.3 Carbon Monoxide ....................................................................................................... 87

vi

7.0 Conclusions ........................................................................................................................ 88

8.0 Future Work ....................................................................................................................... 91

Bibliography ................................................................................................................................. 93

1

Chapter 1

Introduction

1.0 Introduction to research problem

The invention of the automobile has altered life as we know it. A journey that would take days, if

not months, without an automobile has become simple and easy, allowing widespread travelling

across lands. As the number of automobiles on the roads increased, the safety features increased.

Automotive safety has improved astronomically through the years starting with the seatbelt and

the padded dashboard. Features like the antilock brakes, driver air bags, third brake lights and

head restraints were then introduced and widely spread. Evidently, as the number of automobiles

on the road increased so did the number of accidents involving automobiles. The concern over

automotive safety has resulted in the development of certain safety measures like safety crash

tests and the safety rating, which is awarded to vehicles with the highest safety standards.

As automobiles became more affordable to the average person, people started to move further

away from the cities, commuting to work. This commuting lifestyle eventually leads people to

spend more time on the road [1] and in turn causing an increased level of pollution.

Although the research and development in the automotive sector has improved vehicle emissions

causing them to pollute less, the sheer quantity of automobiles on the road has exponentially

increased. The number of new automobiles in North America every year has been increasing

along with the population and all of this add up to more pollution being emitted into the air.

Similarly, as the roads become more crowded, the number of traffic jams increases, which means

hundreds and thousands of automobiles idling, and polluting without going anywhere and this

decreases the air quality we breathe. High quality of air is an essential for a good quality of life.

On average, an adult breathes in about 20,000 [2] liters of air every day. The characterization of

air quality and detection of harmful gases therefore are crucial for providing the quality of air

needed for our very being.

2

The seat belt or the airbags were not important in the early automobiles simply because the

number of vehicles on the road was limited and their maximum speed was low [3]. As these

factors started to change, so did the safety standards, which made the seat belt, the head rest and

the air bags more of a necessity than a form of luxury.

Similarly, the air quality that drivers, passengers and pedestrians require is becoming more

important in today’s automotive design as the seatbelt and the air bags. The driver, passengers

and pedestrians have to be protected from dangerous gases and air quality conditions that could

cause short and long term health complications. The air quality in vehicles is worse than that

typically found in homes or workplaces, especially when exhaust pipes are only inches away

from the adjacent vehicles [4].

Traffic jams and bumper to bumper traffic conditions worsen the air quality inside the cabin, and

the more time drivers and passengers spend in the automobile, the more they are exposed to

these dangerous gases.

The ratio of the volume of the interior material (seats, carpets, compartments, etc) inside the

cabin and the volume of the cabin are a lot higher than the ratios of a typical room. The air

pollution that is present in the cabin is predominantly escaping from the various interior

materials and components. Independent studies have also shown that vehicle cabins commonly

show concentrations of toxic gases such as carbon monoxide (CO), hydrocarbons (HC), volatile

organic compounds (VOC), and oxides and nitrides (NOx) higher than safety limits set by

occupational Safety and Health Administration (OSHA) and World Health Organization (WHO)

[4].

Amongst one of the most dangerous poisonous gases is CO, which is a byproduct from the

incomplete combustion of fuels such as gasoline, natural gas and wood. CO reduces the ability

of the blood to carry oxygen which causes dizziness, headaches, nausea and eventually kills a

person, especially people with cardiovascular disease or women who are pregnant [5].

Concentrations as small as 400ppm can cause headaches within an 1 hour and can be life

threatening in 3 hours, while 1600 PPM causes headaches, dizziness and nausea within 20

minutes and death within 1 hour [6]. The threshold limit value of CO is a mere 50ppm, so the

3

devices used to measure these small concentrations have to be both highly sensitive and

selective.

The current CO sensors are bulky and neither highly selective nor sensitive. The Gas Research

Institute (Gill) estimates that 80% of emergency calls triggered by CO sensors are false alarms

[7], leaving a dire need for more accurate sensors in the market.

Aside from dangerous gases, the temperature and humidity inside the automobile cabin also

contribute to the total air quality present. The air pollution that is being emitted from the interior

materials increases with the interior temperature of the automobile cabin. Controlling the

interior temperature not only reduces the concentrations of these gases, it also improves the

comfort level of the passengers.

Controlling the humidity levels inside the cabin also improves the overall air quality. Low

humidity causes dry skin/hair, itching and chapping, dry nose and throat, increasing discomfort

and susceptibility to colds and respiratory illnesses [8]. High humidity on the other hand, causes

fogged windows, musty and foul smelling odors, which also contribute to the overall discomfort

of the passengers [8].

The temperature, humidity and CO levels inside the automobile cabin are vital components of

the overall air quality, so they must be measured, monitored and maintained at a safe level at all

times.

The aim of this thesis project is to fabricate a sensor that can monitor the air quality of the

automobile cabin by measuring the interior temperature, humidity and CO levels. All three

sensors will be integrated into one global sensor which will then measure and monitor the overall

air quality which will enhance passenger comfort, health and safety.

Since the CO levels of interest are in the parts per million ranges, the sensors to detect them

needs to have the micro-size sensitivity, which requires them to be made using micro and/or

nanotechnology. The sensor has to be made at the micro level because the sensitivity improves

when the surface area to volume ratio is minimized. Minimum sensor volume also reduced the

amount of power intake and material cost. The temperature and humidity components of the

sensor will also be fabricated at the micro level to create a miniature global sensor.

4

Aside from the required high sensitivity, micro sensors are proven [9] to be rugged, have a low

fabrication cost and low power consumption. Due to their size, they can also be placed in

multiple places in the vehicle, producing accurate data at multiple locations at a time.

Fabricating these micro-electrical-mechanical-systems (MEMS) will be done using micro

fabrication tools and techniques that have been proven to work for the integrated circuits

industry.

5

Chapter 2

Microfabrication

2.0 Micro-fabrication overview

In order to use micro/nanotechnology approach to design an air quality sensor, we shall cover the

basic foundations of this process. The next few pages will discuss the different equipments

needed to create a micro and/or nano sized devices and the different technology, methodology,

processes and stages required when producing a device of this size.

2.1 What is a wafer?

The initial material used when making MEMS or any other micro-device is called a wafer. A

wafer is a thin slice of silicon crystal, which is a semiconductor material. Wafers are formed of

highly pure (99.9999%) nearly defect-free single crystalline material. One of the most popular

and highly used methods of forming crystalline wafers is known as Czochralski growth invented

by the Polish chemist Jan Czochralski [10]. In this process, a cylindrical ingot of high purity

monocrystalline silicon is formed by pulling a seed crystal from a melt. Dopant impurity atoms

such as boron or phosphorus can be added to the molten intrinsic silicon in precise amounts in

order to dope the silicon, thus changing it into n-type or p-type extrinsic silicon [11]. In an n-

type semiconductor, the dopant atoms provide extra conduction electrons to the host material

which creates an excess of negative (hence n-type) electron charge carriers.

The ingot is then sliced with a wafer saw and polished to form wafers. Typical wafers for

electronics applications use wafers with sizes from 100mm - 300mm diameter, and the largest

wafer made has a diameter of 450mm, but is not yet commercially available [12].

The wafer serves as a substrate for microelectronic devices built in and over the wafer and

undergoes many micro-fabrication process steps such as doping, deposition, etching and

photolithography.

6

2.1.1 Wafer size?

Wafers can be made into various sizes as shown in Figure 1, with some limitations of course, but

there are general standard sizes summarized in Table 1.

Table 1: Diameter and Thicknesses of Common Silicon Wafers [12]

Wafers made from materials other than silicon are normally not

available in sizes greater than 100 mm, and have various thicknesses

unlike wafers from silicon which have general sizes.

Wafers can be made into various sizes, but each size has its

advantages and disadvantages. The smaller wafers, like the 50.8 mm

(2 inch) or 76.2 mm (3 inch), have more marginal space on the edges

as a percentage of total space, and this decreases the yield per wafer.

A solution to this problem is to use a bigger wafer, like the 200 mm (8

inch) because with larger wafers, less marginal space is present on the

edges as a percentage of total space which increases the yield.

Increasing yield decreases price, so going from 200 mm to 300 mm

reduces price per die by up to 40% [13].

Diameter Thickness

1 2 inch (50.8 mm) 275 µm.

2 3 inch (76.2 mm) 375 µm.

3 4 inch (100 mm) 525 µm.

4 5 inch (127 mm) or 125 mm (4.9 inch) 625 µm.

5 6 inch (150 mm), (5.9 inch, but usually referred to as "6 inch"). 675 µm.

6 8 inch (200 mm) (7.9 inch, but usually referred to as "8 inch") 725 µm

7 300 mm (11.8 inch) 775 µm.

8 450 mm ("18 inch") 925 µm (future)

Figure 1: Common Wafer Sizes [8]

7

Figure 2: Wafer Area Increase (Percent) [12]

The area along the edges of the wafer is not used because that is where it is held during

processes, so the smaller the wafer is, the greater the ratio of the unused area gets. This is the

reason why the area of a 127 mm (5 inch) wafer has 56% more usable area than a wafer with 100

mm (4 inch) Figure 2.

Figure 3: Dice per Wafer Based on Die Size and Wafer Size [12]

8

Cost per Square cm 25 – 30% less

Usable Portion of Wafer 3 – 14% more

Cost Per Chip 27 – 39% less

Labor About Equal

Tool Capital Cost 20 – 40% more

Material Use About Equal

Emissions About Equal

Process/Probe Yield Slightly Better

Table 2: Comparison between 200 mm and 300 mm Wafer Sizes [12]

Although there are huge advantages to increasing wafer size, there are also downsides to it as

well as can be seen in Figure 3 and Table 2. Increasing the wafer size cause vibrational effects,

gravitational bending (known as sag), and problems with surface flatness. These problems

increase with increased wafer size, especially weight. It is estimated that going from 300 mm to

450 mm wafer translates to the crystal ingot being much heavier. It will also take 2-4 times to

cool, and the process time will double [14].

Another reason why wafers are generally made into a limited number of standard sizes is

because this saves time and money during processing. During processing, the wafer goes

through many steps in various equipments and these highly specialized machines have holders

and other compartments that hold the wafer during the process. If the wafer is made into any

size, the various machines would not support it, so it is more effective from a fabrication point of

view to have a very few wafer sizes, to minimize the number of adjustments in equipment. The

wafers can later be cut into smaller pieces for various uses, but it is more economical to have it

made from a standard sized wafer.

2.1.2 Wafer Orientation

As mentioned earlier, wafers are made or “grown” from a crystal with a regular crystal structure,

having diamond cubic structure with a lattice spacing of 0.543 nm. The wafers are cut, with the

9

surface aligned with one of the several crystal orientations, defined by the Miller index, i.e.

[100], [110], [111] as shown in Figure 4. This alignment is done because silicon’s single crystal

structure and electronic properties are very anisotropic, i.e., change with orientation. Factors like

ion implantation depths, wafer cleavage (dicing), or even etching rates are dependent and work

well in a few well known directions, so the orientation of the wafer is important. Each wafer

orientation has properties that are suitable for certain purposes, so it is not a one type fits all

procedure. One must know what he/she intends to do with the wafer, before using it as a

substrate.

Figure 4: Planes in a Wafer of 100 Orientation [15]

2.1.3 Wafer Doping:

Wafers are made from pure silicon, but for certain purposes, their properties have to be

enhanced, which requires doping. Doping is done by artificially creating an impurity in the melt,

during the wafer making process. Impurities as in the range of 1013

and 1016

per cm3 of various

materials like boron, phosphorus, arsenic are added, which then makes the wafer either a bulk n-

type or p-type [16]. An n-type wafer has boron as the main dopant while an n-type wafer has

phosphorus, antimony or arsenic as the main dopant. This added impurity still leaves the wafer

99.9999% pure, due to silicon’s high single crystal density.

10

2.1.4 Wafer Resistivity

The resistivity of a wafer is very important because it determines the quality of the oxide that can

be grown on the wafer. Thermal oxide can be grown much faster in areas with low resistivity

[17] so the more uniform the resistivity is across the wafer, the more uniform the thermal oxide

you can grow.

2.1.5 Wafer Cleaning

Wafers are always cleaned before they are used to remove organic or inorganic contamination.

The wafer is initially heated to a temperature sufficient to evaporate any moisture that might be

present. Wafers that have been in storage for a while have to be chemically cleaned. There are

various cleaning chemicals and procedures but they all have the same purpose, and it is to

remove those two types of contaminants.

2.2 Photoresist

A photoresist, or resist is a material that has a light sensitive property and it is used in many

industries including photolithography. Resists are classified into two groups (tones); positive or

negative resists. For a positive resist, the area exposed to light becomes soluble to the developer

and washes out, while the area not exposed to light becomes insoluble to the developer and stays

as seen in Figure 5. Negative resist on the other hand, the area exposed to light becomes soluble

to the developer and washes out. Area not exposed to light becomes insoluble to the developer

and stays.

11

Figure 5: Photoresist Types [18]

The two types of resist have other difference than their solubility with the developer when

exposed to light. These characteristic differences are tabulated in Table 3:

Table 3: Characteristics of Photoresist Tone

Characteristic Positive Negative

Adhesion to silicon fair excellent

Relative cost more less

Developer base aqueous organic

Minimum 0.5 μm 2 μm

Step coverage better lower

Wet chemical resistance fair excellent

12

The resist is dispensed on the wafer in the liquid form and then spun rapidly using a resist

spinner (Figure 6), to create a resist layer that is uniform. The spin coating is generally run at

1200 to 4800rpm for 30 seconds to a minute, which leaves a layer of 0.25 to 0.5 μm, with a

uniformity of 5 to 10nm. The high uniformity is because the resist moves faster at top than it

does at the bottom because of viscous forces [19]. The fast moving resist on top gets ejected

from the substrate’s edge as the substrate spins, while the bottom layer moves slowly radially

along the wafer, removing any bump or ridge of resist [19]. This leaves a very clean and flat

layer of resist, even if the substrate itself had micro bumps or ridges. The final thickness is

calculated based on both the spin speed and by the evaporation of liquid solvents from the resist.

Figure 6: Resist Spinner [20]

The thickness of the resist has to be controlled because it plays a role in not only the dimensions

of the patterns it creates, but also the quality. For small dense features of 125nm or less, the

resist has to be 500nm thick. Typically the minimum feature size to resist thickness ratio of 1:4

is used.

Amongst the most important steps in photolithography is the resist deposition because all other

features will depend on the resist quality. If the resist thickness is uneven and/or there are

regions with minimal or no coverage, then future depositions or etches will be incorrect. Areas

that are not properly covered with resist will not have the proper mask that is required during

further fabrication steps, which can create short circuits and other undesirable effects. These

effects can be prevented by using a clean pipette and dropping the resist in the center of the

Wafer

Holder

Spinner

Cover

25 cm

25 cm

13

substrate, then spreading it around until full coverage is achieved. Bubbles are also undesirable

because they are full of air and they create areas with resist thickness that is not sufficient. The

drops should be done in a way so that there are no sharp corners because once the resist spinner

starts to spin, the resist will streak down the wafer and there will be areas of no coverage. The

spin speed should also be decreased/increased if a thinner or thicker resist is to be used.

2.3 Photomasks

A photomask is an opaque plate with transparencies or sometimes holes, which permit light

through these transparencies/holes in a defined pattern mostly used in photolithography. These

masks usually consist of chromium patterns on a transparent quartz glass plate. The pattern on

the mask is first generated with computer CAD software, and then the data file is converted to

series of polygons. A laser writer or an E-beam writer shown in Figure 8 is then used to write

those patterns on a glass plate. The steps used to fabricate photomasks are given in Figure 7. In

a complex design, numerous masks are fed into a photolithography stepper/scanner and then are

individually selected for exposure.

Figure 7: Mask Making Process [21]

(A) (B) (C)

(D) (E) (F)

(G)

14

Figure 8: Mask Laser Beam and E-beam Writing [21]

Although there are different kinds of photomasks, they can be categorized in groups and types.

One of these types is used in contact lithography also known as contact printing. These types of

masks are pressed against the photoresist, and then UV light is shined above the mask,

transferring the design onto the resist. The resist that lies under the transparency/hole pattern is

exposed to the light and washes away after developing (positive resist). This results in a 1:1 ratio

between the design on the mask and the design transferred onto the substrate (sample) using

various micro-fabrication processes.

The biggest advantage of contact lithography is the elimination of the complex projection optics

between the object and image. Contact masks are also cheaper in price, but are not used for

production purposes because they can be damaged due to wear. Defects on the masks can widen

the gap between the mask and the substrate, which causes images based on evanescent waves or

surface plasmon interference to disappear [22].

Complex designs normally use multiple photomasks to create multi-stage designs. The features

on each mask have to be aligned with the previous mask using markers as shown in Figure 9.

Each mask has two marker sets, one to align with the previous mask, and one to create new

15

markers on the substrate, so it could be used by the next mask. These markers should be

designed so that they are easy to find and they are placed in a location that does not interfere

with the rest of the process.

Figure 9: Photomask (with markers)

2.4 Photolithography

2.4.1 Prebaking

When a wafer is coated with a resist, it has to be prebaked to evaporate the excessive photoresist

solvent, typically at 100⁰C for 30 to 60 seconds on a hotplate. This also affects the final

thickness of the photoresist.

2.4.2 Exposure

Once the photomasks are ready, and the resist is deposited on and prebaked, the next step is the

exposure. Exposure is basically the step where the designs on the masks are being transferred

onto the wafer. Ultraviolet light is shined on top of the mask, and the light goes through the

holes/transparent areas, which causes the resist beneath these areas to become exposed. For

positive resist, which is the most common type used, the areas of the resist that are exposed wash

off when it comes in contact with the developer solution.

Wafer

Markers

16

2.4.3 Post-Exposure Bake

The post exposure bake, unlike the pre-baking stage, is used to help reduce standing wave

phenomena caused by the constructive and destructive interference patters of the incident light.

This is a very time critical stage because it is here that the UV light during the exposure stage

reacts with the resist. This stage is also temperature dependant.

2.4.4 Developing

For the stage of the developing, the resist that has been exposed to the UV light is removed by

the developer which is delivered on a spinner, similar to the photoresist. A typical developer is

Tetramethylammonium hydroxide (TMAH), and it does not have any metallic ions because they

are considered extreme contaminants [23]. The developer can also be placed in a container and

the wafer is submerged in it, stirring it frequently if needed.

2.4.5 Hard Bake

For the stage of the hard bake, the wafer is baked for 2 minutes at 100° Celsius to solidify the

remaining photoresist to make it stronger and durable for further fabrication processes.

2.5 Etching

Etching is basically removing material from the surface of a wafer during micro-fabrication as

shown in Figure 10. There are many types of etching techniques, but they all involve removing

material in a pre-designed pattern using masks. In a typical etching process, a part of the

material is protected from the etching chemical using a masking material, which has a much

lower etching rate compared to the material that is intended to be etched. Normally a resist is

layered on top of the material, and then is patterned, before the etching chemical is introduced.

The depth of the etch cavity is controlled by two factors; the etch rate and the etch time. The

etch rate of the material using the etching chemical is normally known, so the only factor that has

to be controlled is the etch time in order to achieve a certain depth. The selectivity of the etchant

has to be known, and it has to be high, so that the masking material is not etched. If the masking

17

material gets etched, the area that it is covering is next, which leads to incorrect etching patterns.

The high selectivity is also needed in case the etching depth goes too far, and material below is

etched as shown in Figure 11. An etchant with a high sensitivity is always desired because it

only etches the exposed areas and it stops when it etches through a material; it does not etch the

material underneath.

Typical Photolithography Etching Process:

Figure 10: Photolithography Steps [24]

Figure 11: Unselective (1) vs. Highly Selective (2) Etchant [25]

18

Not all etchants etch only in the vertical direction. There are etchants that go both ways, creating

what is called bias. Etchants with large bias are called isotropic, simply because they go in all

directions. Isotropic etchants erode the material in all directions equally, which is sometimes

undesired. Anisotropic etches are mostly preferred because they produce sharp edges and

features that are controlled [26].

There are two different kinds of etchants: wet and dry. Wet etchants are liquid, while dry

etchants are gaseous (plasma phase). Wet etchants were originally used because they are simple

and inexpensive. The wafer is immersed in a bath of the etchant solution, agitated, then taken

out. The etchant solution attacks the substrate and the exposed areas get etched. Dry etchants on

the other hand are basically in the plasma form, and it also attacks the exposed areas of the

substrate. Unlike wet etchants which are used in 50⁰C – 100⁰C temperature, dry etchants are

used in a much higher temperature, and they have to be highly energized so they can bombard

the surface of the exposed substrate and knock its particles.

Wet etchants are normally isotropic, and therefore it etches in all directions, which creates large

biases. This bias has to be considered beforehand because it could collapse structures which can

lead to failures. There are also anisotropic wet etchants, and they etch different planes at

different rates. A <100> plane Silicon substrate etches patterns vertically, along the <111>

plane, which is 54.7⁰ [21], Figure 12.

Figure 12: [100] Plane Etching of Silicon [25]

19

Some of the most common anisotropic wet etchants for silicon substrates are EDP, KOH/IPA

and TMAH [27] [28], and their operating conditions, etching rates and mask materials are given

in Table 4. The selectivity is the etching rates of one direction to another. High selectivity is

always preferred because the etching shape can be predicted beforehand without having to rely

on trial and error. A fast etching is also preferred simply because micro-fabrication normally has

multiple steps, and the quicker each step is, the better. The etchant used in this research is KOH,

simply because it has low operating temperature, and highly selectivity, along a with fast etch

rate.

Etchant Operating Temp

(⁰C)

R100

(µm/min)

S =

R100/R111

Mask Materials

Ethylenediamine

Pyrocatechol (EDP)

110

0.47

17

SiO2, Si3N4,

Au, Cr, Ag, Cu

Potassium

Hydroxide/Isopropyl

Alcohol (KOH/IPA)

50

1.0

400

SiO2, Si3N4

Tetramethylammonium

Hydroxide (TMAH)

80

0.6

37

SiO2, Si3N4

Table 4: Anisotropic Etchants for Silicon Substrates [27] [28]

When using wet etchants, stirring affects the etch pattern by increasing the vertical etch depth. If

the etching solution is not stirred, the undercut is increased. The affects of stirring can be seen

in Figure 13 and Figure 14.

Figure 13: Wet Etching with Stirring

Nitride Mask

Substrate

Nitride Mask

20

Figure 14: Wet Etching without Stirring

2.6 Deposition

In micro-fabrication, materials are either added or subtracted when patterns are being made;

subtraction occurs during etching while addition occurs during deposition. There are various

methods of depositing material onto a substrate and the method consists of steps very similar to

the steps taken when etching.

Depositions types are typically categorized into two main groups; chemical deposition and

physical deposition. Each one of these groups has its advantages and disadvantages, so choosing

what method to use depend on what the purpose of the deposition is, the preferred quality and

cost considerations.

The chemical being used can be either in the liquid or the vapor form as can be seen in Figure

15, but the most common method is the vapor phase method due to its high uniformity.

Amongst the liquid chemical deposition methods are plating and chemical solution deposition

(CSD), which are both inexpensive and highly proven.

Substrate

21

Figure 15: Deposition Methods [22]

2.6.1 Chemical Vapor Deposition

In a chemical deposition process, the substrate is covered with a chemical and that chemical

undergoes a chemical change leaving a solid layer. Since the chemical covers the substrate in

all directions, the solid layer of material is deposited on every surface, with little regard to

direction. Chemical Vapor Deposition (CVD) requires high temperature and low/high pressure

environment to work properly. The main advantage of CVD is that it creates highly pure and

highly performing solid materials.

2.6.2 Plasma Enhanced Chemical Vapor Deposition

Another highly used method is the Plasma Enhanced CVD (PECVD) where the chemical is in

the plasma stage. This method uses ionized vapor or plasma as a precursor and relies on

electromagnetic means rather than a chemical reaction to produce plasma. The components of a

typical PECVD machine can be seen in Figure 16.

22

Figure 16: Plasma Enhanced Chemical Vapor Deposition [29]

When depositing a material like nitrite for example, the balanced chemical reaction looks is:

3SiH4 + 4NH3 = Si3N4 +12H2 [30] Equation 1

The problem is that when this chemical reaction occurs on the surface of the substrate, the

chemical reaction is not balanced and looks like:

3SiH4 + 4NH3 = Si2.5N3.1 +12H2 Equation 2

This is because the chemical reaction is not fully stoichiometric so the 3:4:1:12 ratio is not

always satisfied. The PECVD works by taking two gases and then adding High Frequency (HF)

energy to it to convert these gases into plasma. This plasma is then bombarded onto the surface

and the different gases do not always combine fully to produce the desired material. In this

example, the deposited material is not fully a nitrite; it is more of a nitride. The thickness of the

nitride also varies, since the deposition of the saline (SiH4) and ammonia (NH3) deposit on the

23

surface randomly. PECVD requires that the reaction chamber be kept in a very low pressure.

The lower the pressure, the easier the deposition gets.

The thickness of the deposited material using PECVD is calculated using a film measurement

system like the NanoSpec AFT/400 shown in Figure 17. This is a metrology tool and it

measures the thickness of dielectric thin films like photoresists, oxides, nitrides, polysilicon and

similar materials that are deposited on silicon substrates. It measures the reflected light to

determine film thickness based on interference effects. Using measurement algorithms, the

Nanospec compares a bare silicon wafer to the sample being tested to yield thickness information

without causing damage to the sample [31].

Figure 17: Nanospec Film Thickness Measurement System (Nanospec AFT Model 410)

24

2.6.3 Physical Deposition

This method of deposition uses mechanical or thermodynamic means to produce a thin film of

solid. The material that is being deposited is placed in an energetic environment so that its

particles can escape its surface. Across from the material that is to be deposited is the substrate,

which has to be at a much cooler temperature so that the particles from the deposition material

can arrive and form a solid layer. This deposition technique requires the deposition chamber to

be operating at a low pressure so that the material being deposited can move freely. Something

to note about this deposition technique is that the material being deposited moves in a straight

line, and the thickness is more of a normal distribution as shown in Figure 18.

Figure 18: Thickness distribution using physical deposition

Amongst the most popular physical deposition processes are thermal evaporator, electron

evaporator and sputtering, the latter two being the ones discussed in this report.

25

2.6.3.1 Electron Beam Evaporation

This method uses an electron gun and shoots high energy beam onto the material to cause it to

evaporate. The beam gun is focused using a magnet and it hits the target metal which is kept in a

crucible. The beam is focused onto a small area, causing that material to be evaporated. The

evaporated material then travels upwards and deposits on the above substrate as shown in Figure

19. The thickness of the target material follows a normal distribution as well, but this can be

compensated for if the substrate is rotated. The crucible has to be made of a material that does

not react with the metal to be deposited since the electron beam can cause a reaction between the

two materials.

Figure 19: E-beam Components [32]

26

Figure 20: Crucible and Target Metal [32]

When an electron beam evaporator is used to deposit metal, the thickness can be measured using

one of few methods including a profilometer, Figure 21 and scanning electron microscope

(SEM), Figure 22. Any one of these devices can be used to measure the thickness of the thin

film, but the profilometer can damage the sample so a witness coupon needs to be used in order

to measure the thin film thickness.

27

Figure 22: Scanning Electron Microscope [33]

2.6.3.2 Sputtering

This deposition method uses plasma made up of a noble gas to knock material from a target

material as shown in, and unlike the E-beam evaporation, this method does not require the

material to be heated and it is done at relatively low temperatures. The surface of the target

material is hit with energized electrons (plasma), as illustrated in Figure 23, and this causes the

ejection of atoms. The rate of sputtering can be increased by increasing the energy of the

incoming plasma [34]. The typical sputtering chamber is shown in Figure 24.

Figure 21: Profilometer [50]

28

Figure 23: Sputtering of Target Material Using Plasma [34]

Figure 24: Sputtering Chamber [35]

29

Chapter 3

Sensing Theory

3.0 Sensing Theory

In this thesis, the goal is to sense three distinct air quality properties: temperature, humidity and

CO levels. In order to sense these conditions, an appropriate sensing technique has to be utilized

to achieve accurate results in a timely manner for each sensor.

3.1 Temperature

Most metals can be used as a temperature sensor due to the fact that their resistance changes with

respect to temperature. Amongst the common metals available in the market, the most suitable

metal for temperature sensing is platinum because it has a linear resistance to temperature slope

as can be seen in Figure 25.

Figure 25: Relative Resistance Vs Temperature of Metals [36]

30

The temperature sensing component is a thin layer of platinum, which has a linear thermal to

resistance properties as mentioned earlier. A small current is passed through a thin platinum

metal with a known resistance and its voltage is measured. When the temperature

increases/decreases, the resistance of the platinum metal also changes. This causes the output

voltage readings to change. This change in voltage is measured and correlated to calculate the

ambient temperature.

3.2 Humidity

The humidity is measured using a hotplate technique and platinum as the sensitive material. The

platinum is kept at an elevated temperature, a current is passed through it, and the voltage is

measured. This voltage change is then used to calculate the change in resistance of the platinum.

When the temperature is between 100°C and 200°C, the resistance of platinum does not change

with changes in humidity because in that temperature range, the thermal conductivity of dry air

and humid air are similar [37]. When the temperature is elevated above 350°C, the resistance of

the platinum filament changes with humidity. The second method used is to apply a voltage

across the filament and measure the current. The temperature of the filament increases linearly

with the supplied current and when the temperature is about 350°C; the change in resistance is

correlated with the change in humidity. In this method, the heating filament is not required.

3.3 Carbon Monoxide

There are various principles for detecting gases including electrical resistance or conductance

change due to ionization of adsorbed gas, calorification due to burning of the gas, electromotive

force [38], etc.; but most of these methods are either too expensive, too inconvenient, have a

large response time or have low sensitivity. Amongst the various gas detection methods, thin

film SnO2 has become a well documented sensing material for CO [39], [40], [41], [42], [43],

[44]. SnO2 is a sensitive material that is highly popular because of its favorable chemical

behavior. Oxygen adsorbs onto its surface and stays there in a stable form as ionic species which

have acquired electrons from the lattice [45].

Oxygen is adsorbed on to the surface of a thin SnO2 layer at about 200 to 400°C [45], removing

electrons from the conduction band, and hence increasing the surface resistivity. This is a

31

characteristic of n-type materials. When a combustible gas like CO is introduced, it reacts with

the adsorbed oxygen, oxidizing in the process, releasing electrons into the conduction band as

described in Equation 3. This increase of electrons onto the surface increases conductivity which

in turn reduces resistivity, which can be measured. What makes SnO2 an ideal sensitive material

is that it can hold on to this O2-, O

- and O

2- on its surface until they are desorbed at high

temperatures [45]. The overall reaction is the removal of adsorbed oxygen ions in a neutral

form, and hence the electrons trapped on these ions will be donated back to the oxide lattice to

become re-associated with the impurity centers [46].

Equation 3

The downside is that all combustible gases oxidize when they react with the adsorbed oxygen on

the SnO2 surface, and they all release an electron, so a change in the resistance can be caused by

any one of them. Therefore, SnO2 is sensitive to all various gases, and not just CO, as shown in

Table 5.

Table 5: Conduction in SnO2 at 200°C

Gas Concentration Conductance

(v/v) (*10-4

Ω-1

)

Air 2% 1

Hydrogen 2% 27.5

Methane 2% 21.1

Propylene 2% 28.3

Butane 2% 28.3

Carbon Monoxide 100ppm 6.8

Hydrogen Dioxide 5% 1

Hydrogen Sulphide 20ppm 18.5

Sulphur Dioxide 10ppm 22.5

Fortunately, these gases all have different oxidation rates and oxidation rates are dependent on

temperature, so each gas has its own temperature in which it needs to oxidize.

32

3.3.1 Selectivity and Performance

3.3.1.1 Temperature Control

The problem with SnO2 and other metal oxide gas sensing materials is that they have problems in

terms of cross sensitivity which reduces their selectivity against the measured gases. By

controlling the working temperature of the SnO2, it can be made more selective to CO since its

distinct oxidation rate is a function of temperature. Since adsorption and the surface reaction

between the gases and the sensitive material are all temperature activated processes, their rates

increase as the operating temperature increases. Therefore, the response time and recovery time

can be reduced by raising the operating temperature.

Raising the operating temperature can enhance the sensitivity of SnO2 to CO, but it too has its

limits. The operating temperature should be below the melting temperature as well as be within

a sensitivity range. The fractional change in conductivity produced by a gas of a given

concentration increases as the temperature of the SnO2 is increased before passing through a

maximum and decreasing again [46], as seen in Figure 26.

Figure 26: Effects of temperature on conductivity of SnO2 in given concentration of CO and

CH4 [46]

33

The ideal operating temperature of SnO2 for sensing CO is between 175°C and 380°C, while

methane (CH4) is between 325°C and 475°C [46], as seen in Figure 26. In order to avoid sensing

CH4 when trying sense CO, the operating temperature of the SnO2 would have to be kept

between 175°C and 325°C.

3.3.1.2 Additives

A second way of increasing selectivity is by adding additives to the sensitive material to do one

of three things. The additives can speed up the transient behavior leading to a faster response. It

can also lower the overall resistance making for simple electronics. An example of these

additives includes palladium, gold, copper, etc. [45], [47], [48]. These additives cannot be

used as a multi-gas sensing ingredient because each one enhances selectivity of a particular gas

by acting as a catalyst during the reaction between the sensing material and the gas. Palladium

and platinum enhance the selectivity of sensing CO when the sensing material is SnO2 because

the adsorption of CO is increased by incorporating it into the SnO2.

3.3.1.3 Sensor Array

The presence of CO or other gases can be sensed using metal oxides as mentioned earlier. The

problem is that they have drawbacks in terms of their cross sensitivity. Various gases at various

concentrations can cause the same resistance change, making it harder to decipher what gases are

present. One of the latest solutions to improve sensor selectivity has been to use an array of gas

sensors having different sensitive layers fabricated on a single chip [49], [50]. Although the

temperature control and dopants can be used to be more selective, there are still some levels of

cross sensitivity. To overcome this problem for the CO sensor, an array of sensors operating at

various temperatures, having various dopants have to be utilized.

34

3.3.2 Sensitivity

3.3.2.1 Temperature Control

Once the sensor has desirable selectivity, and can narrow down the particular desired gas, the

second most important step is the sensitivity aspect, especially for CO sensors due to the toxic

properties of the gas. Temperature control is important for selectivity, but it can also be used for

sensitivity. Although the operating temperature can be at 250°C or even 300°C for selectivity

purposes, it occurs at a slower rate. Having CO oxidize at a slower rate means the rate of drop in

resistance is slower, which translates to a longer transient behavior. The maximum sensitivity

for sensing CO occurs at about 380°C.

3.3.2.2 Geometry

Other than operating temperature control, the geometry of the sensor can be used for sensitivity

control due to the fact that the sensing of CO is a surface chemistry phenomenon. Increasing the

surface area to volume ratio increases sensitivity [51], because it increases the chemical

interaction between the SnO2 and CO. The limit in the past has been the limited resolution of the

readily available photolithography process. This problem is being tackled by using a state of the

art E-Beam machines that can deposit and pattern in the sub nano scale. To increase the surface

area to volume ratio, the sensor has to be designed similar to heat transfer fins to increase the

sensitive material and gas interaction at the surface.

3.3.2.3 Grain Size Effects

In order to increase the surface area, the SnO2 has to be patterned in thin parallel bars. The

dimensions of these thin SnO2 bars will have to be as small as possible to increase the surface

area to volume ratio which will lead to higher sensitivity. As can be imagined, the dimensions

must have lower limits. These limits are set by the microstructure of the polycrystalline element,

in this case SnO2. Each crystallite of semiconductor oxide included in the element has an

electron-depleted surface layer (space-charge layer) to a depth L in air, where L is determined by

the Debye length and the strength of the oxygen chemisorptions [52]. If the diameter, D, of the

35

crystallite decreases to be comparable to 2L, the whole crystallite is depleted of electrons which

limit the gas sensitivity. For a thin sputtered film of SnO2, L has been reported to be as small as

3nm [53], [52], so the critical crystallite size is approximately 6nm, which becomes the

minimum dimension of the parallel SnO2 bars.

These crystallites of SnO2 are only stable below 400°C, so if sintering or other heat treating

processes are to take place, the critical crystallite size increases according to Figure 27 [52].

Most sensors utilizing SnO2 as the sensitive material have to be heated from between 400°C and

600°C in order to refresh the sensing area by burning out adsorbed dust, oil, and also to reduce

resistance drift or during sintering. Since the SnO2 has to be heated at these high temperatures,

the minimum crystallite size is about 13 nm [52] or above. The smaller the crystallite size, the

greater the resistivity of the SnO2 to reducing gases like CO as can be seen in Figure 28 [52].

Figure 27: Crystallite size of SnO2 as a function of calcinations temperature (calcinations: 1

hour) [52]

Although sensitivity can be increased exponentially by decreasing the crystallite size, there is a

limit of how small it can be made depending on the operating temperature or other heat treating

processes. The lower the operating or sintering temperature, the smaller the crystallite size which

translates to an increase in sensitivity.

36

Figure 28: Correlation between crystallite size and gas sensitivity to 800 PPM CO for pure SnO2

element (sintered at 400°C)

37

Chapter 4

Sensor Design

4.0 Sensor Design

4.1 Fabrication Steps

4.1.1 Carbon Monoxide Sensor Design Overview

The CO sensor is fabricated on a silicon substrate which has to be processed using various

micro-fabrication techniques. The sensing material, which is the SnO2, has to be operated from

between 300°C and 400°C and sintered as high as 600°C and this heat is generally generated

using metal or polysilicon layer beneath the SnO2. The temperature of this heating component is

controlled by adjusting its current supply; increasing the current causes the material to heat up.

Since the operating temperature of the SnO2 is crucial, it has to be monitored. This is achieved

by utilizing a patterned metallic layer under the sensitive material which is used to monitor the

operating temperature.

Although this high operating temperature is necessary in order to sense CO, it has to be

contained within the sensing area. The heat has to be dissipated in order to not overheat the

surrounding components. To avoid this excessive heat, these types of sensors are typically built

on a free-standing membrane with four bridges connecting it to the rest of the wafer.

The various currents are supplied to the sensing area using multiple wires and these wires are

patterned along the four bridge structures.

4.1.2 Fabrication Steps and Methods

Heater:

1. Deposit and Pattern Resist Using Mask 1

2. Deposit 150 nm of Chrome Using E-Beam Evaporation

38

3. Remove Resist

Temperature:

1. Deposit and Pattern Resist Using Mask 2

2. Deposit Titanium and Platinum Using E-Beam Evaporation

3. Remove Resist

Passivation Layer:

1. Deposit 300nm of Silicon Nitride Using PECVD at 100⁰C

Contact Holes:

1. Deposit and Pattern Resist Using Mask 3

2. Dry Etch Holes Through Silicon Nitride Layer Using Resist Ion Etching (RIE)

3. Remove Resist

Contact Metal:

1. Deposit and Pattern Resist Using Mask 5

2. Deposit 400nm of Aluminum Using E-Beam Evaporation

3. Remove Resist

Bridge Structure:

1. Deposit and Pattern Resist Using Mask 4

1. Dry-Etching Through Nitride and Oxide Layers to the Silicon Surface Using (RIE)

2. Remove Resist

3. KOH Silicon Etching (KOH Solution) to Create Bridge Structure

39

Sensitive Material:

1. Deposit and Pattern Resist Using Mask 6

2. Deposit 250 nm of SnO2 Using E-Beam Evaporation

3. Remove Resist

4.1.3 Base Structure Design (Experimental)

These sensors will be built using a silicon substrate of Type N, with an orientation of [100] and a

resistivity of 0-100 ohms. The wafer has a diameter of 100 mm and a thickness of 500µm +/- 20

µm with a polished surface.

The silicon wafer has to be etched underneath the sensitive material to create a hotplate held by

the four bridges, connecting it to the surface of the wafer. In order to create a bridge that can

withstand the thermal and residual stresses, the silicon is first covered with:

1. Silicon oxide

2. Silicon Nitride

3. Silicon Oxide

These layers have been used in literature [49], [54] and are proven to provide the right

mechanical strength if their thicknesses are controlled. In order to prevent membrane bending

due to the residual stress during fabrication, the oxides are deposited on both sides of the nitride

to keep it straight.

The typical thicknesses of these layers in the literature are:

1. SiO2 = 400 nm

2. Si3N4 = 800 nm

3. SiO2 = 400 nm

As can be seen, the oxides on both sides of the nitride have to be equal so that their residual

stresses cancel out and the membrane will not bend in either direction.

A 100mm N-type silicon wafer with 100 orientation has been bought and the next step was to

deposit the oxide, followed by the nitride then the second oxide.

40

There are various methods of depositing oxide and each one has its advantages and

disadvantages. From an economical prospective, the lowest cost and most readily available

method is to deposit the oxide using Plasma Enhanced Chemical Vapor Deposition (PECVD),

Figure 29 located inside the cleanroom shown in Figure 30. The downside to using this method

of deposition is its low adhesion and since this oxide is the first layer to be deposited on the

silicon wafer, it has to have the highest adhesion. Having a bottom layer with low adhesion is

very risky because the whole sensor depends on a strong foundation; if the bottom oxide

deteriorates, everything else on top of it will go with it. To overcome this adhesion problem due

to the PECVD deposition method, the oxide was grown on top of the silicon using a furnace as

shown in Figure 31. A thermal oxide was grown using the steps shown in Figure 32.

Figure 29: PECVD Chamber [56]

Plasma

Chamber

Wafer

Holder

Water

Cooler

Electronics

and Hardware

Vacuum

and Sliding

Mechanism

41

Figure 30: Bahan Microfabrication Cleanroom

Figure 31: Thermal Oxide Furnace [55] Thermal Silicon

Oxide Deposition

Furnace

42

Figure 32: Gas Control Schematic for Thermal Oxide Growth [55]

Although the thermal oxide has better adhesion compared to the oxide layer obtained using a

PECVD method, it has its own limitations. The thickness of the oxide that can be achieved is

dependent on the oven operating temperature, and the highest temperature that can be obtained is

the ultimate constraint.

The maximum thickness that was achieved using the thermal oxide approach was about 200 nm

and even this amount was difficult. The wafers were left in the oven for 8 hours the first day,

then an additional 4 hours the following day. Multiple oxide growth was used to maximize the

thickness of the oxide.

The thickness of the oxide was measured using a thickness measurement device (Nanospec) and

it was noticed that the thickness of the oxide was not increasing anymore after the second day,

and that this thickness was the highest that could be achieved using this equipment.

43

Since the desired thickness of the three base structure layers were SiO2 (400 nm), Si3N4 (800 nm)

and SiO2 (400 nm), and the maximum thermal oxide layer that was achieved was 225 nm, the

other layers have to be adjusted. The new thicknesses of the oxide, nitride and oxide should be

225 nm, 450 nm and 225 nm, respectively, so that the thickness ratios are still kept constant.

Once the thermal oxide was deposited, the next step was the nitride and it was deposited using

PECVD since the nitride adhesion is not a problem here since it is not the bottom layer. The

problem with using PECVD is that it does not deposit silicon nitride (Si3N4) in the pure form.

The silicon nitride that it deposits has coefficients that are unknown. The deposited material has

the coefficients: Si2.5N3.1. The deposited material consists of both silicon nitride and other

particles of various materials.

The PECVD machine combines saline (SiH4) and ammonia (NH3) which produces Si2.5N3.1 and

H2. The balanced equation is:

3SiH4+ 4NH3 = Si3N4 +12H2 Equation 4

The reaction inside the chamber is not stoichiometric so it does not produce the proper ratios

(3:4:1:12). The saline and ammonia are added along with high frequency (HF) energy, causing

the two gases to break down and become plasma. The nitrogen particles and the silicon particles

are being deposited randomly causing the silicon nitride to have varying concentrations. The

energy input is switched from HF to low frequency (LF), one to break down the gases into

plasma and bombard it on the surface until it sticks and the other to bombard the particles on the

surface but not stick, respectively. The LF serves to reduce stresses in the nitride since the

random bombardment causes uneven stresses on the surface.

The deposition rates of the nitride using the PECVD equipment varies with the recipe. The

deposition rates are a function of time but they are average rates which are not linear. In order to

deposit 400nm of silicon nitride, a recipe with a deposition rate of 100nm/minute was used.

Instead of using a single wafer and experimenting with only one silicon nitride thickness, three

different trials were done, each one having a different oxide thickness. The thermal oxide that

44

was deposited as the first layer is about 225nm and the thickness of the two layers that go on top

of it will be experimented with. To do this, three different wafers were used and the desired

experimental oxide/nitride/oxide thickness profiles are:

1. SiO2 (225 nm)/ Si3N4 (200 nm)/ SiO2 (150 nm)

2. SiO2 (225 nm)/ Si3N4 (400nm)/ SiO2 (300nm)

3. SiO2 (225 nm)/ Si3N4 (600nm)/ SiO2 (450nm)

All three wafers have the first oxide layer so the next step is the PECVD nitride which was

deposited using a known recipe with a deposition rate of 100nm/minute. In order to achieve the

desired thicknesses of 200, 400 and 600nm, the deposition times used were 2 minutes, 4 minutes

and 6 minutes, respectively. When the thickness of the nitride was measured on each wafer,

their thicknesses were almost zero with high standard deviations.

This error was because the plasma takes time to form and using a high deposition rate and a low

timing does not work. Using a recipe that has a deposition rate of 10nm/minute and setting the

deposition time for 2 minutes to get 20nm thickness does not work. It was found that each recipe

has a known recipe and that deposition rate is based on an average using a higher time.

Using trial and error, PECVD nitride and oxide recipes with a deposition rate of 10nm/minute

were found. Using these recipes, with the given deposition rate, the desired nitride and oxide

thicknesses were achieved by setting the correct deposition rate. To achieve the 200, 400 and

600nm thick nitride on wafers 1, 2 and 3 respectively, the 10nm/minute rate was used. The

deposition time was adjusted to 20 minutes, 40 minutes and 60 minutes for wafer 1, 2 and 3,

respectively. The fabrication steps to achieve the PECVD nitride and oxide on top of the

thermal oxide layers are shown in Figure 33.

45

Figure 33: Base Structure Deposition Steps (A, B C)

4.1.4 Metal Deposition (Experimental)

Once the wafer was covered with the desired bottom layers, the next step was to experiment with

depositing thin metals. Both the heater and the temperature monitoring components are thin

metals that have to be deposited using an E-beam evaporator [57].

46

Figure 34: Crucible with Aluminum Pellet (Target)

Figure 35: E-Beam Evaporator Interior [57]

In order to obtain an accurate deposition rate of the E-beam evaporator, a few depositions were

done using aluminum as the target material as shown in Figure 34. Photo resist was deposited on

Crucible and

aluminum

Target

Wafer

Thickness

Measurement

Device

Crucible

and Target

Holder

47

a wafer and patterned using photo-lithography, followed by the deposition of aluminum using the

E-beam evaporator.

Aluminum was used because it is a readily available metal that is inexpensive and it would allow

one to find the actual metal deposition rate for future use. A small silicon wafer was utilized as a

witness coupon and placed next to the test wafer in order to measure the aluminum thickness on

the witness coupon instead of the one on the wafer.

There are various methods to measure the thickness of a deposited material and a profilometer

was chosen because most other methods either require the wafer to be broken or they are not

readily available within the facility that the fabrication is being done. The profilometer requires

a step along the wafer surface and since this cannot be achieved due to the uniform deposition, a

witness coupon with a step has to be used instead as shown in Figure 36. This step causes a

thickness change, which the profilometer can record as its needle moves from one corner of the

wafer across the wafer. Running the profilometer needle along the wafer surface can also

damage the wafer so this is another reason why the witness coupon was used.

Figure 36: Wafer and Witness Coupon on the E-beam Evaporator Plate

Wafer

E-Beam

Deposition

Plate

48

Figure 37: E-Beam Evaporator Spinning Top with Wafer

Figure 38: E-Beam Evaporator Deposition Metal

E-Beam

Target

Material

Evaporating

Wafer placed

inside E-Beam

Deposition

Chamber

Metal Plate

49

For uniformity and material cost reduction purposes, the metal plate was spun as the aluminum

was deposited on the wafer. The evaporated metal deposits on the silicon wafer in a normal

distribution which causes a thickness gradient along the wafer. By spinning the metal plate, as

shown in Figure 37, the metal thickness becomes more uniform and more than one wafer can be

deposited at once, as illustrated in Figure 38, decreasing the equipment and material cost.

Layer uniformity is important here because this allows the sensors to have similar components.

If the heating or the temperature metal thicknesses are not constant, then their resistance will also

vary, causing uncertainties when measuring voltages.

The operating temperature of the sensor is critical because it is needed for sensitivity and

selectivity of the gases that are being sensed. This operating temperature has to be set and

monitored by controlling the input current and measuring the voltage change. The voltage

change in all the different metals depends on their resistance, which depends on their

dimensions. Having the thicknesses of the heater and the temperature metals to vary along the

wafer makes all the sensors to behave differently. Correlating the voltage drops to the

temperature of the sensor would not be possible if the resistance of these thin metals is unknown.

The thickness of these metals is very critical to the overall sensor performance so it has to be

kept constant and uniform from one sensor to another.

50

4.2 Mask designs

4.2.1 Heater Component

Figure 39: Heater Mask Designs

Figure 40: Single Heating Filament Design

Heating

Filament Heating

Filament

Pads

51

The heater filament mask shown in Figure 39 and Figure 40 is used to heat the sensor to the

desired operating temperature by passing a current through the filament. The current is passed

through the filament using the contact pads on the left side of the filament. The heater is

designed to enclose the sensor in order to achieve a constant temperature. This filament which is

deposited on the silicon substrate before being covered with a nitride layer transforms the bridge

structure into a hot plate. The nitride layer covers the heating component so that the sensor

layers can be deposited on top of it.

4.2.2 Temperature Component

Figure 41: Temperature Mask Design

52

Figure

Figure 42: Single Temperature Design

The temperature component mask shown in Figure 41 and Figure 42 is designed to measure the

temperature of the hot plate at any given time. The sensor component consists of zigzag design

which is placed on the interior of the heating coil before both components are covered with the

passivation nitride layer. A current is passed through the temperature filament similar to the

heater filament, and the resistance of the wire is measured and correlated to the temperature of

the filament. The current is passed through the temperature component using the contact pads

similar to the heating filament.

Temperature

Filament Pads

Temperature

Filament Zigzags

Width:

30 µm

53

4.2.3 Contact Holes

Figure 43: Contact Holes Mask Design

54

4.2.4 Contact Holes

Figure 44: Single Contact Holes Design

The contact holes mask shown in Figure 43 and Figure 44 is used to create the needed holes to

connect the heater and the temperature component pads to the circuit. The heating and the

temperature filaments are covered with a nitride layer to keep it passivated from the rest of the

sensor above it, so these contact holes are needed. Contact metal is deposited in the holes

created using the contact holes mask which connect the temperature and the heating components

to the circuit.

Contact Holes to

Temperature

Filament

Contact Holes to

Heater Filament

55

4.2.5 Sensitive Material Layer Component

Figure 45: Sensitive Material Mask

Figure 46: Sensitive Material Mask (Zoomed in)

56

4.2.6 Bridge Membrane

Figure 47: Membrane Mask Design

Figure 48: Single Membrane Design

Sensing Area

of Membrane

Etched

Silicon

Cavitie

s

57

The membrane mask, shown in Figure 47 and Figure 48 is used to create the desired bridge

membrane needed to make the CO sensor. The mask has a hole on each side of the hot plate area

so that when etched, a thin membrane is created. The etchant etches down the four holes, at 54.7

degrees, meeting at a point, which is what creates the suspended bridge.

4.2.7 Bridge Membrane (Etching Experiments)

Figure 49: Experimental Membrane Mask Design

58

Figure 50: Experimental Membrane Mask Design (close-up)

The mask shown in Figure 49 and Figure 50 was used to experiment with various etching

patterns until a desired membrane is achieved. The goal was to etch the four holes until they

create a membrane without the etchant eating away parts of the sensor area. Some of the designs

ate away the parts of the four bridges that hold the sensor area or made it too thin to be

structurally acceptable. The various designs were needed to experimentally find a design that

would create the needed membrane while structurally strong.

59

4.2.8 Metallization

Figure 51: Metallization Mask Design

Figure 52: Single Metallization Design

Contact Metal to

Temperature

Filament Contact Metal to

Heater Filament

Contact Metal

to Sensitive

Material

Contact Metal

to Sensitive

Material

60

The metallization mask shown in Figure 51 and Figure 52 is used to deposit the contact metal on

the silicon wafer.

4.2.9 Etching (Experimental)

Although this step is one of the last steps in the sensor fabrication, it was chosen to be

experimented with next because it is a very critical step. A thin membrane will be created,

where the sensor will be constructed, held by four bridges which will act as thermal bridges to

dissipate the heat. Experimenting with the membrane etch at this stage will not only verify if the

right oxide/nitride/oxide layers have been deposited, it will also tell us if the whole design should

be resized. Since this step involves etching under the sensor area, we have to make sure that it

etches the intended areas and directions.

Figure 53: Phantom Etcher (near) [58]

Computer

Etching

Chamber

Electronics

61

There are various methods of creating a free standing membrane and one of the most utilized

methods in carbon monoxide sensors is to use a sacrificial layer, a surface microfabrication

method. This method does not require the etching of the silicon substrate and is quite easy, but it

has stress problems. A sacrificial layer is deposited followed by one or two layers of oxide and

silicon. The sensor is constructed then the sacrificial layer is removed, creating a free standing

membrane.

The membrane fabrication method that was chosen is bulk microfabrication instead of the usual

surface microfabrication. Etching the free standing membrane using this method involves two

types of etches applied consecutively.

The first step will etch the desired pattern on the oxide/nitride/oxide layers until the bare silicon

is exposed. The second etch will etch the silicon in the desired pattern until the desired

membrane is constructed.

This process starts with using photolithography to pattern, expose and develop resist on the

wafer to be used as a mask later on. The wafer is then etched using Resist Ion Etching (RIE)

Phantom Etcher as shown in Figure 53 and the patterned resist acts as a mask. This RIE process

etches all three oxide/nitride/oxide layers in the areas that are not protected by the resist mask.

Since there are three layers of material on the silicon wafer, the etching rate had to be found

experimentally, using two to three etching runs, where the time and depth would be measured

and then averaged out. Initially, the etching runs were unsuccessful so wet etching techniques

like hydrofluoric (HF) acid solution was used to etch the oxide/nitride/oxide.

Once the desired patterns were used to etch the oxide/nitride/oxide down to the bare silicon, the

next step is to etch the silicon itself to form the thin membrane where the sensor will be built

upon.

Once the pattern was etched down to the silicon wafer, the resist was removed from the wafer,

exposing the oxide/nitride/oxide layers in the areas that were not etched. A solution of

potassium hydroxide (KOH) acid was then prepared using 30% KOH and 70% water by mass.

The solution was heated to 85°C and mixed thoroughly at a constant speed to maintain a constant

temperature throughout before the wafer was inserted, as seen in Figure 54.

62

Figure 54: KOH Solution at 85 Degrees Celsius

The etching rate of KOH is a function of temperature and concentration and this recipe has been

tried and tested to give a constant etching rate and a good surface roughness finish [59].

It is known that KOH etches different materials at different rates [60], so it will etch the top

oxide at the same time that it is etching the silicon substrate to create the membrane. Once the

wafer was deposited into the KOH, it started to etch everywhere, including the exposed silicon.

Since the thermal oxide that was deposited initially was on both sides, and the following

nitride/oxide layers were deposited on the front alone, the KOH etched from the back faster.

This leads to the wafer becoming thinner and thinner before being completely eaten away due to

the fact that the KOH was attacking from the back.

To overcome this problem, a wafer with a low pressure chemical vapor deposition (LPCVD)

nitride was used to see if the KOH would penetrate through it. The back of the wafer was also

covered with a Protek PSB polymer and its primer to protect it from being etched from that side.

The wafer was kept and monitored in the KOH solution, the etch depth being measured every

half hour. It was found that the polymer was protecting the back of the wafer exceptionally well

and only the etch areas are etched slightly.

63

Figure 55 to Figure 60 show the fabrication steps taken to produce the free standing membranes,

from top to bottom.

Figure 55: Silicon Wafer with Thermal Oxide, Nitride, Oxide (Base Structure)

Figure 56: Base Structure Covered with Resist

Figure 57: Resist Photo-lithographically patterned

Figure 58: Oxide-Nitride-Oxide Layers Etched (RIE)

Figure 59: Silicon Substrate Etched (KOH)

64

Figure 60: Resist Removed

The etching pattern was experimented with, so the etching depth varies from one design to the

other. After 150 minutes, the majority of the membranes were created and only a few collapsed

as can be seen in Figure 62 to Figure 65

In order to complete the etching patterns, the wafer was kept in the KOH, and in the third hour it

was noticed that the KOH completely etched through the nitride mask. A thin nitride is normally

used as a mask when using KOH etching, but it too was etched, just at a slower rate than other

materials. Continuing the etching process any further would lead to the complete loss of the

wafer once the KOH etches through the nitride. The other issue with this process is that the

PECVD oxide at the front is always lost to the KOH solution which defeats the whole purpose of

depositing it the first place. The PECVD oxide was deposited for stress purposes, but the KOH

will etch it during the membrane fabrication.

65

Figure 61: Silicon Wafer Being Etched in KOH Solution

Figure 62: 30 Minutes into Etching

66

Figure 63: 120 Minutes into Etching

Figure 64:120 Minutes into Etching

67

Figure 65: Bridge Eaten Away by KOH

From these experiments, it was found that in order to create a thin membrane, the silicon wafer

needs a nitride layer that is deposited using LPCVD instead of PECVD. The LPCVD is much

denser, stronger and it has a theoretical etch rate that is slower than the PECVD. The lower the

etch rate, the better, since this material could potentially be used a mask if the KOH eats through

the thermal oxide. But since the KOH will eventually consume the thermal oxide, we need to

protect it as well. One solution would be to cover it with another layer of LPCVD, but then we

would have four layers of materials as a base and the surface stress would be an issue.

Since there were no in-house capabilities to deposit LPCVD nitride, the silicon wafers were sent

to an outside foundry to perform the deposition. The new wafer has the following base

materials:

1. Wet Thermal Silicon Oxide: 250nm

2. Low Stress LPCVD Silicon Nitride: 500nm

68

Figure 66: LPCVD Nitride Covered Wafer in KOH Etchant

The new wafer with the LPCVD nitride and thermal oxide was taken through photolithography

steps using the bridge mask. The pattern was then etched using the phantom etcher to etch to the

silicon surface, followed by KOH etching as can be seen in Figure 66. The wafer was etched and

its etching depth was monitored at 30 minute intervals as shown in Figure 67 to Figure 73. In

Figure 67, the bridges are starting to be etched under. In Figure 68 the bridge arms have been

etched under completely making them thin membranes, while in Figure 69 and Figure 70, the

sensing area predominantly a membrane. Figure 71 is a zoomed in version of Figure 70, where

the microscope reveals how the structure looks like below the surface. The bridge becomes the

desired free standing membrane in Figure 72. Although most of the free standing membranes

were formed, there were a few membranes that broke during the nitrogen drying of the wafer as

can be seen in Figure 73.

69

Figure 67: LPCVD Nitride Etch for 60 Minutes

Figure 68: LPCVD Nitride Etch for 90 Minutes

Bridge

Formed

Bridge

Arm Being

Formed

70

Figure 69: LPCVD Nitride Etch for 120 Minutes

Figure 70: LPCVD Nitride Etch for 135 Minutes

Not Hollow

Under Yet

71

Figure 71: LPCVD Nitride Etch for 135 Minutes (Below Membrane Focus)

Figure 72: LPCVD Nitride Etch for 150+ Minutes

Four Sides

Not Met Yet

Complete

Membrane

72

Figure 73: LPCVD Nitride Etch for 150 Minutes (Broken Bridge)

4.3 Overall CO Fabrication Steps

Heater:

1. Deposit and Pattern Resist Using Mask 1

2. Deposit 100 nm of Chrome Using E-Beam Evaporation (200nm later)

3. Remove Resist

Temperature:

1. Deposit and Pattern Resist Using Mask 2

2. Deposit Titanium and Platinum Using E-Beam Evaporation

3. Remove Resist

Passivation Layer:

Broken

Bridge

73

1. Deposit 300nm of Silicon Nitride Using PECVD @ 100⁰C

Contact Holes:

1. Deposit and Pattern Resist Using Mask 3

2. Dry Etch Holes Through Silicon Nitride Layer Using Resist Ion Etching (RIE)

3. Remove Resist

Contact Metal:

1. Deposit and Pattern Resist Using Mask 5

2. Deposit 400nm of Aluminum Using E-Beam Evaporation

3. Remove Resist

Bridge Structure:

1. Deposit and Pattern Resist Using Mask 4

2. Dry-Etching Through Nitride and Oxide Layers to the Silicon Surface Using (RIE)

3. Remove Resist

4. KOH Silicon Etching (KOH Solution) to Create Bridge Structure

Sensitive Material:

1. Deposit and Pattern Resist Using Mask 6

2. Deposit 250 nm of SnO2 Using E-Beam Evaporation

3. Remove Resist

4.4 Overall Temperature Sensor Fabrication Steps The design and fabrication steps of the temperature sensor are the same as the temperature

component within the carbon monoxide sensor. The same masks, material and fabrication

process are used with the omission of the components i.e. heater that are not required. The same

74

wafer as the one used for the CO sensor with the thermal oxide and LPCVD nitride is used. The

fabrication steps are as follows:

Temperature:

1. Deposit and Pattern Resist Using Mask 2

2. Deposit Titanium and Platinum Using E-Beam Evaporation

3. Remove Resist

Contact Metal:

1. Deposit and Pattern Resist Using Mask 5

2. Deposit 400nm of Aluminum Using E-Beam Evaporation

3. Remove Resist

4.5 Overall Humidity Sensor Fabrication Steps The humidity sensor is also fabricated using similar fabrication procedure as the carbon

monoxide. Since platinum is used as the sensitive material and the operating temperature is also

elevated similar to the CO sensor, the overall fabrication steps are made similar for fabrication

simplicity. The same wafer as the CO sensor is used, along with the same thermal oxide and

LPCVD nitride. The same masks will also be used, except in different order. The fabrication

steps are as follows:

Heater:

1. Deposit and Pattern Resist Using Mask 1

2. Deposit 100 nm of Chrome Using E-Beam Evaporation (200nm later)

3. Remove Resist

Passivation Layer:

1. Deposit 300nm of Silicon Nitride Using PECVD @ 100⁰C

75

Humidity:

1. Deposit and Pattern Resist Using Mask 2

2. Deposit Titanium and Platinum Using E-Beam Evaporation

3. Remove Resist

Contact Metal:

1. Deposit and Pattern Resist Using Mask 5

2. Deposit 400nm of Aluminum Using E-Beam Evaporation

3. Remove Resist

Bridge Structure:

1. Deposit and Pattern Resist Using Mask 4

2. Dry-Etching Through Nitride and Oxide Layers to the Silicon Surface Using (RIE)

3. Remove Resist

4. KOH Silicon Etching (KOH Solution) to Create Bridge Structure

76

Chapter 5

Fabrication

5.0 Final Fabrication

The initial goal was to fabricate all three sensors on the same chip but there were numerous

challenges that could not be overcome. One of the first problems was that the PECVD machine

that was required to deposit the passivation layer was down for a few months. This layer is what

keeps the various layers of the sensors from touching each other and creating a short circuit. A

secondary problem was the deposition rates of the platinum.

Initially, chromium was used during the trial runs of the fabrication as the heating element for

the CO sensor, the sensitive material for the temperature and the humidity sensor. Chromium

was used because it is much cheaper than platinum and it was not feasible to have multiple trial

runs using this precious metal. Using one target metal was also preferred over multiple metals

because it shrinks the number of deposition runs which saves time and money.

When chromium was deposited using the E-Beam evaporation machine, it had a high deposition

rate, about 0.07 nanometers per second (nm/s) using a current of only 20 to 40mA and it does not

cause a huge chamber pressure increase.

When platinum was finally used to fabricate the temperature, humidity and CO sensor, its

deposition rate was low. I was able to deposit 4nm in almost 3 hours using the maximum current

allowed of 250mA. When the current was set to max, the high current caused the platinum target

metal to spark, potentially over exposing the photoresist on the wafer. The machine also started

to heat up and the deposition had to be stopped. The deposition was tried a few times but with

little success because the platinum target pellets would not melt thoroughly as can be seen in

Figure 74.

77

Figure 74: Target Metal and Crucible

In order to overcome this problem the sensor were attempted to be fabricated using alternate

materials. The temperature and the humidity sensors were fabricated on their own using nickel

and chromium instead of platinum. Although nickel was found to also have a very low

deposition rate, I was able to deposit 25nm without exceeding the maximum current and

chamber pressure of 6 E-6 millibar (mb). The deposition process is shown in Table 6.

Current

(mA)

Deposition Rate

(nm/s)

Chamber Pressure (*E-5

mb)

60 0.05 1.00

90 0.06 1.30

100 0.05 1.30

120 0.05 1.10

140 0.06 1.30

150 0.07 1.30

165 0.07 1.00

175 0.06 1.40

Table 6: Current, Deposition Rate and Chamber Pressure of E-Beam

Platinum Pellets

Not Melted

Crucible

78

Figure 75: Heater Filament of CO Sensor

Figure 76: Temperature and Humidity Filament

79

The low deposition rate of the nickel reduces the thickness of the sensor from being 100nm to

25nm. To overcome this problem, the width of the filaments of the sensors were increased from

10-40µm to 1mm so that when their resistance is being measured, they would not be too large.

The filaments are shown in Figure 75 and Figure 76. The limited thickness of the sensors was

compensated by increasing the size of the sensors so that reasonable resistance could be achieved

during the testing phase.

80

Chapter 6

Testing and Results

6.0 Testing and Results

6.1 Temperature Once the temperature sensor was fabricated, its resistance was measured at various temperatures

using a hotplate with an adjustable temperature and a BK Precision LCR/ESR meter to measure

the sensor resistance as seen in Figure 77.

Figure 77: Temperature Sensing Set Up

BK Precision

LCR/ESR meter

Wafer with

Temperature

Sensors

Hotplate

81

The temperature and resistance of the sensor are tabulated in Table 7, Figure 78 and Figure 79.

Temperature

(⁰F) Resistance Trials (KΩ)

Average Resistance

(KΩ)

32 5.367 5.374 5.37 5.374 5.371

77 5.824 5.830 5.815 5.825 5.824

100 6.144 6.13 6.16 6.148 6.146

200 6.175 6.163 6.189 6.228 6.189

300 6.361 6.364 6.359 6.358 6.361

400 6.518 6.515 6.535 6.526 6.524

500 6.619 6.612 6.617 6.618 6.617

600 6.619 6.606 6.593 6.579 6.599

Table 7: Temperature and Corresponding Resistance

Figure 78: Resistance vs. Temperature (Nickel)

Looking at these graphs, we can see that the resistance of the nickel sensor is almost linearly

proportional to the temperature. Various sizes of these sensors were fabricated, and although

their resistance was different, their relative resistance vs. temperature curves was very similar.

The relative resistance is the resistance at various temperatures divided by the resistance at 32⁰F.

This is done because resistance of every sensor is a function of its size, and a change of

resistance would not mean anything. The increase in resistance due to the increase in

temperature has to be normalized using a set resistance so that sensors of varying sizes can be

5.400

5.600

5.800

6.000

6.200

6.400

6.600

6.800

100 200 300 400 500 600

Re

sist

ance

(K

Ω)

Temperature (⁰F)

Resistance vs. Temperature (Nickel)

82

compared for their sensitivity. This sensor has a sensitivity of 0.463⁰F/Ω when range of 32⁰F to

600⁰F is used.

Figure 79: Relative Resistance vs. Temperature (Nickel)

A filament made of platinum was attempted a few times, but the maximum thickness of 4nm was

not enough to get accurate resistance readings. .

6.2 Humidity

It is known that nickel is more sensitive to temperature than platinum which was shown in Figure

25, therefore, humidity sensors were fabricated using various metals including nickel and

chrome. Since nickel is far less expensive than platinum and it has a higher sensitivity than

platinum, I was curious to know if it can also be used as a humidity sensor. This would allow me

to use nickel for both the temperature and humidity, which would save fabrication time and

material cost.

Once the humidity sensor was fabricated, it was tested using the test set up, consisting of a glass

chamber, a hotplate with an adjustable temperature and a BK Precision LCR/ESR meter to

measure the sensor resistance, a humidifier with an adjustable humidity level and a humidity

1.100

1.150

1.200

1.250

100 200 300 400 500 600

Re

lati

ave

Re

sist

ance

(R

/R(3

2⁰F

)

Temperature (⁰F)

Relative Resistance vs. Temperature (Nickel)

Series1

83

measuring probe as shown in Figure 80. The sensor was placed on top of the hotplate at various

temperatures and the humidifier was turned on to a set level. The humidity was allowed to

increase inside the chamber as can be seen in Figure 81. The relative humidity was monitored

using the probe shown in Figure 82.

Figure 80: Temperature and Humidity Test Set Up (No Humidity)

BK Precision

LCR/ESR Meter

Humidity

Chamber

Humidifier

Hotplate

84

Figure 81: Temperature and Humidity Test Set Up (With Humidity)

Figure 82: Humidity Measuring Probe (EL-USB-2-LCD, RH, Temp Data Logger)

Humidifier

Turned On

Humidity

Measuring

Probe

Humidity

Measuring

Probe

85

Figure 83: Humidity and Change in Resistance @ 77⁰F

Humidity

%RH Resistance (KΩ)

Average Resistance

(KΩ)

53 6.200 6.252 6.235 6.233 6.230

68 6.286 6.300 6.400 6.327 6.328

73 6.326 6.469 6.429 6.353 6.394

90 6.459 6.621 6.323 6.423 6.457

Table 8: Humidity and Change in Resistance @ 400⁰F

Figure 84: Humidity and Change in Resistance @ 400⁰F

5.76

5.78

5.8

5.82

5.84

5.86

5.88

5.9

5.92

50 66 74 91

Re

sist

ance

(K

Ω)

Relative Humidity (%RH)

Resistance vs. Humidity (Nickel)

6.100

6.150

6.200

6.250

6.300

6.350

6.400

6.450

6.500

53 68 73 90

Re

sist

ance

(K

Ω)

Relative Humidity (%RH)

Resistance vs. Humidity (Nickel)

86

Figure 85: Humidity and Change in Resistance @ 600⁰F

The resistance of the nickel seems to rise with increase of the relative humidity as can be seen in

Figure 84 and Figure 85. This increase in resistance is mostly likely due to the increase in

temperature since the humidifier causes an increase in chamber temperature. It is known in

literature that platinum’s resistance decreases with an increase in relative humidity at

temperatures above 600⁰F, but this is not the case with nickel. It actually increases as seen in

Figure 84 and Figure 85, but this is probably due to the increase in temperature of the hotplate or

the humidity chamber. The change in resistance due to the increase in humidity is within the

random variations of the data from temperature sensor at those operating temperatures. This

random variation can be seen in Table 7. Since the resistance of the sensor at each specific

humidity level was taken 4 times, the results were averaged out to minimize the random variation

in the measuring and in the sensor readings. The sensitivity of the humidity sensor at 77⁰F is

0.487 Ω/1%RH, at 400⁰F it is 5.94 Ω/1%RH and at 600⁰F it is 0.448 Ω/1%RH. We can see that

the sensitivity of the humidity sensor at the 600⁰F is not in line with the other two so there must

be some sort of discrepancy.

6.71

6.715

6.72

6.725

6.73

6.735

6.74

6.745

50 63 76 89

Re

sist

ance

(K

Ω)

Relative Humidity (%RH)

Resistance vs. Humidity (Nickel)

87

6.3 Carbon Monoxide The carbon monoxide sensor was not fabricated to completion and is left as future work. The

goal was to fabricate it and then fabricate multiple variations with varying parameters so that it

can be made more selective to the desired gas. The sensor was designed so that its different

components could also be used as temperature and humidity sensor. Every component

experimented with and fabricated on its own for testing except for the SnO2 deposition due to

time constraints. The passivation layer that was required to separate the heating/temperature

sensing filaments from the sensitive material layer could not be deposited because of equipment

failure.

88

Chapter 7

Conclusions

7.0 Conclusions

The goal of this project was to design and fabricate a micro sensor that has three components;

temperature, humidity and carbon monoxide sensing abilities to be used by the automotive

industry. All three sensors have been designed, fabricated and proven to work in literature but

the goal of this project was to design it so that they could be fabricated on the same chip using

the least expensive fabrication steps and materials.

The temperature sensor is fairly simple to design and fabricate yet highly sensitive. The

sensitive component can be made from various metals but only platinum has a linear relative

resistance to temperature ratio which makes it suitable for this application. Unfortunately,

platinum would not evaporate in the E-Beam evaporator so metals like chrome and nickel were

used as a substitute. It was proven that nickel has a good sensitivity to temperature.

The humidity sensor is similar to the temperature sensor in terms of design and fabrication, and it

too was designed to use platinum but was later fabricated using nickel. It was proven that nickel

did not have high sensitivity to humidity and that its change in resistance is probably due to the

change in temperature instead. Once platinum is able to be deposited, data of the humidity

sensor at various ambient temperatures and conditions should be collected and used to determine

the relative humidity. The temperature, pressure etc. have to be measured first, and then the

correct resistance vs. relative humidity curves should be used to determine the actual humidity.

This would require generation of numerous humidity curves but it will give correct results.

The CO sensor designed and fabricated by parts, excluding the sensitive material, but it was not

completed as a whole due to the passivation layer not being able to be deposited. This sensor

was designed using components from the temperature sensor and the humidity sensor with the

addition of the sensitive material component. Various sizes and shapes of thermal bridges were

successfully fabricated for the CO sensor with only a few breaking.

89

Amongst the major goals of this project was to fabricate all three sensors using the least

expensive methods available.

Instead of using LPCVD nitride and oxide, PECVD was experimented with because it is a less

expensive substitute since this option was available in the cleanroom. Since PECVD cannot be

deposited on both sides, a PSB polymer was experimented with to protect the backside from

being etched by the KOH. Unfortunately, the PECVD nitride was not strong enough to resist

KOH. The number of masks was also minimized by designing masks that could be used by all

three sensors to reduce mask cost, material cost, fabrication time and cost. Although platinum is

known to be sensitive to humidity, nickel was experimented with to determine if it can be used

instead. Nickel is known to have a higher temperature sensing ability than platinum so it would

be an ideal material for humidity sensing.

Although nickel could be used as a substitute for platinum in temperature sensing, since it is of

lower cost, more sensitive and easier to deposit, it would not reduce the overall cost if a single

chip with all three sensors is to be fabricated using sputtering techniques. Since nickel cannot be

used for humidity, an equal amount of platinum has to be sputtered on the wafer regardless,

whether one sensor or hundreds of sensors are being made. If one sensor has one component that

has an expensive material, it would cost the same as if every sensor needed that material.

To make the sensors more durable, the sensors were fabricated using bulk micromachining

instead of the traditional surface micromachining. Bulk micromachining translates to less stress

on the sensor components, increasing the lifetime of the sensors making it more reliable. The

thermal bridges that were fabricated were strong and almost all of them stayed intact even after

numerous nitrogen drying processes.

Another cost savings factor was the use of chrome as the heating filament for the CO sensor

instead of the traditional polysilicon used commonly. Although polysilicon has a lower

resistance than chrome, making it a better choice, chrome is lower cost and can be fabricated

using readily available methods.

One of the greatest challenges in making these sensors was the numerous fabrication steps that

had to be taken to fabricate it. The fabrication procedure had to be kept in mind when designing

the masks for various components. The different fabrication steps and processes had to be

90

experimented separately to make sure that they can be achieved. Once each fabrication step had

been experimented with on its own, they were performed in their proper sequence as a test run.

Each fabrication step had to be done in a way that allows the following step to be done

effectively.

Multiple sensors with multiple variations were fabricated on each substrate so that features can

be optimized without having to carry out numerous fabrications. This minimized fabrication

time and cost while increasing the number of sensors available for testing. The different

components and processes of the MEMS sensor were designed with the goal of simplifying the

fabrication and testing stage.

In conclusion, MEMS sensors for the automotive industry are very promising. These sensors can

be used to sense and monitor the air quality inside the automotive cabin for safety and comfort.

The temperature, humidity and carbon monoxide levels can be sensed at various places inside the

cabin with high accuracy. These sensors are also inexpensive to fabricate because they utilize

fabrication techniques that are tried and tested in the IC industry and are inexpensive. Hundred

of these sensors can be fabricated from a single 4 inch wafer and they have low power

consumption because of their miniature size. They can also be made rugged and have low

maintenance which makes them suitable for these types of applications.

Overall, MEMS air quality sensors have the potential to be in every automobile in the near

future. They are highly sensitive, have low maintenance and power consumption while being

inexpensive to manufacture. There are few obstacles with the CO sensor and the humidity

sensor in terms of selectivity and ambient air conditions, respectively, that have to be solved to

complete the capabilities of these sensors.

91

Chapter 8

Future Work

8.0 Future Work

There are a number of areas that should be considered as future work, including the full

fabrication of the final sensor and the fabrication of all three sensors on the same chip. There are

also various modifications that should be done to each sensor to enhance performance.

For the CO sensor, there are various doping materials used in literature to increase selectivity and

sensitivity of SnO2 and combinations of them should be investigated.

Increasing the surface area to volume ratio of the SnO2 should also be done by experimenting

with E-beam etching. The SnO2 should be deposited on a silicon substrate followed by a layer of

photoresist. The photoresist should then be patterned using the E-Beam and then etched to create

long and thin filaments. The width of these filaments should be varied from 100µm, 50 µm,

25µm, 10µm, 1 µm and 0.5 µm. The thickness of the SnO2 should also be varied from 200nm,

100nm, 50nm, 25nm and 10nm. Decreasing the width of the SnO2 filaments and its thickness

will increase surface area to volume ratio.

The material properties of the SnO2 should be investigated before and after the photolithography

(E-beam patterning and etching). The material properties of the SnO2 might change when it

reacts with the resist or with the E-beam or with the etchant.

A laser can also be used to create the long thin filaments required to increase the surface to

volume ratio of the sensitive material. The SnO2 can be deposited on a silicon substrate and then

patterned into thin filaments with various width sizes by simply evaporating the areas between

two consecutive filament bars. This should not increase the surface to volume ratio without

changing the material properties of the SnO2.

Finding a way to remove nickel and platinum during lift off should also be found. Nickel takes

an hour or so to remove while platinum takes about 2 to 3 hours, even when its thickness is as

small as 4nm.

92

One of the most cost effective methods to fabricate sensors is to utilize the least number of steps

and materials. A material that could be used as humidity and temperature sensing while still

being less expensive than platinum and cheaper to evaporate should be found if E-Beam

evaporation is to be used.

93

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