Chapter 5: Lithography

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Chapter 5: Lithography


Introduction

The mechanism to print 2-D patterns to a thin film layer on the wafer surface.

Masks are glass plates (soda lime or quartz glass) that contain the patterns.

The patterns are first transferred from the mask to photoresist (PR), a light-sensitive polymer.

After opening windows in the PR, the pattern is transferred to the thin film using etching techniques.

Complexity of a fabrication process is often measured by the number of photolithographic masks used in the process.


Introduction

The concept is simple– Spin on a thin layer of light-sensitive photoresist– Selectively expose it to UV light

Causing chemical bonds to either form or break

– Develop to selectively remove the lighter weight PR The resist may be used as a mask for either etching or

for ion implantation Because of constraints of resolution, exposure field,

accuracy, throughput, and defect density, the implementation is not so simple– Very expensive– Very complex


Introduction

• Steps in the mask fabrication process:

Designing 2D layout using CAD

tools

Transfer data to pattern generator

(mask maker)

Pattern generation on the mask plate coated with Cr&PR

Etching PR and then Cr

Inspection

Stripping PR

Glass plate with Cr

Only Glass


Introduction

Mask Maker


Introduction

• Steps in the photolithography

Clean wafer

deposit film (oxide, nitride, metal, …)

Coat with PR

Soft bake

Align masks

Expose Pattern

Develop PR

Hard Bake

Etch the deposited film

Remove PR

Typical for 1800 Series PR:Soft Bake: 110°C for 1min on a hotplateHard Bake: 110°C for 3min on a hotplate

PR1813 1.3µm @ 4krpm & 30secPR1827 2.7µm @ 4krpm & 30sec


Introduction

Spinner

Hotplate


Introduction

Mask Aligner


Introduction

1

2

3,4

5,6

6

UV


Introduction

7,8

9

10

Be aware that there are two different types of PR:

Positive PR: exposed areas will be developed

Negative PR: exposed areas will not be developed

Some common PRs:

1800 series (for thin) will be developed in MF 319

9200 series (for thick) will be developed in AZ 400


Alignment Markers

Once a photolith process is done, the pattern developed is used to perform some additional process selectively on the wafer

– Etching trenches in Si or SiO2

– Making metalization runs– Implantation of dopants

Then the wafer will come back for another photolith step

Alignment markers are registration patterns that mate from one mask to another so that the multiple pattern sets match one another.


Introduction

Positive resists provide better controllability for small features.

Positive resists are easier to work with and use less corrosive developers and chemicals.

Positive resists are the dominant type of photoresists today.


Clear Field and Dark Field Masks

Most photolith engineers prefer clear field masks when possible– Easier to detect pattern on the wafer itself

as there is more clear glass in the mask


Introduction

Demands placed on this process for– Resolution: smaller device structures– Exposure field: ever-increasing chip sizes– Placement accuracy: aligning with existing

layers– Throughput: manufacturing cost– Defects: yield and cost


NTRS Lithography Requirements


Introduction

The National Technology Roadmap for Semiconductors defines the future needs

Note especially– The driving force is the reduction of feature size– For every factor of two in reduction of area, there is

a reduction of 0.7 in the linear dimensions– The reduction is required every three years– The most commonly quoted feature size is not as

small as isolated MOS gate lines– Critical dimension (CD) control must improve (about

10% of minimum feature size)– Alignment accuracy must be about 1/3 of minimum

feature size– The printing area increases with time since we must

print one full die at a time


Introduction

About 1/3 of the cost of a wafer cost (about $1000 for an 8-inch wafer) is associated with lithography; we have only a few hundred dollars per wafer to spend– Optical lithography is used down to

0.13 m (130nm) generations– For smaller dimensions, X-ray, direct e-

beam, or extreme UV (EUV) processes are used.


Basic Concepts

We generally separate lithography into three parts– The energy source (photons or electrons)– The exposure system– The resist

The exposure tool, which includes the light source and the exposure system, creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations)– Optimization of the photoresist with the settings on

the exposure tool transfers the aerial image from the mask to the best thin film replica of the aerial image


Light Source

Historically, light sources have been arc lamps containing Hg vapor

A typical emission spectra from a Hg-Xe lamp

Low in DUV (200-300nm) but strong in the UV region (300-450nm)


Light Source

A much smaller set of wavelengths used to expose the resist– to minimize optical distortion associated

with the lens optics.– to match the properties of the resist

Pick the wavelength that is heavily absorbed and causes changes in resist chemical properties

Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm.


UV Light Sources

To expose < 250nm wide lines, we need to use shorter wavelength light– Two excimer lasers (KrF at 248 nm and ArF at 193

nm)– These lasers contain atoms that do not normally

bond, but if they are excited the compounds will form; when the excited molecule returns to the ground state, it emits UV light

– These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation; can get several mJ of energy out


Excimer Lasers

Low reliability due to etching of the electrodes and the optical windows by the energitic F ions


E-beam Source

http://cmi.epfl.ch/metrology/img/LEO1550/LEOColumn.gif

Field Emission Gun (3), which provides the source of Field Emission Gun (3), which provides the source of the electron beam, is a W or LaFthe electron beam, is a W or LaF66 filament. filament.

Condenser Lens (7) are pairs of electromagnets that Condenser Lens (7) are pairs of electromagnets that are used to collimate the beam of electrons.are used to collimate the beam of electrons.

Beam Booster, composed of Anode (5), Vacuum Beam Booster, composed of Anode (5), Vacuum Tube (6), Apertures (8), Alignment Coils (9a, b, c), Tube (6), Apertures (8), Alignment Coils (9a, b, c), Stigmator (13), and Isolating Valve (15) is used to Stigmator (13), and Isolating Valve (15) is used to determine the energy of the electrons and to remove determine the energy of the electrons and to remove the electrons moving off-axis.the electrons moving off-axis.

Objective Lens (10,11) is another set of Objective Lens (10,11) is another set of electromagnets that focuses the electron beam onto electromagnets that focuses the electron beam onto the specimen (12), also containing the Deflecting the specimen (12), also containing the Deflecting System (14), which is another set of electromagnetics System (14), which is another set of electromagnetics that sweep the electrons across the field of view and that sweep the electrons across the field of view and off of the sample .off of the sample .


X-Ray Source

High energy electrons collide with a metal. The transfer of energy results in the release of x-rays (short wavelength photons).


Exposure System

There are three classes of exposure systems– Contact– Proximity– Projection


Exposure System

Contact printing is the oldest and simplest The mask is put with the absorbing layer face

down in contact with the wafer This method

– Can give good resolution– Machines are inexpensive– Cannot be used for high-volume due to

damage caused by the contact– Still used in research and prototyping

situations


Wafer Exposure Systems

Proximity printing solves the defect problem associated with contact printing– The mask and the wafer are kept about

5 – 25 m apart – This separation degrades the resolution– Cannot print with features below a few

microns– The resolution improves as wavelength

decrease. This is a good system for X-ray lithography b/c of the very short exposure wavelength (1-2 nm).


Projection/Step and Repeat

For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer.– Masks are now called reticules

Projection printing is the dominant method today– They provide high resolution without the defect

problem– The mask is separated from the wafer and an optical

system is used to image the mask on the wafer.– The resolution is limited by diffraction effects– The optical system reduces the mask image by 4X to

5X– Only a small portion of the wafer is printed during

each exposure– Steppers are capable of < 0.25 m– Their throughput is about 25 – 50 wafers/hour


Optics Basics

We need a very brief review of optics If the dimensions of objects are large

compared to the wavelength of light, we can treat light as particles traveling in straight lines and we can model by ray tracing

When light passes through the mask, the dimensions of objects are of the order of the dimensions of the mask

We must treat light as a wave


Snell’s Law and Reflectivity

n1 sin(1) = n2 sin(2)

1 = T+R+A, where T is transmissionR is reflectionA is absorption

If 1 = /2, 2 = sin-1(n1/n2)

R = [(n1-n2)/(n1+n2)]2

http://scienceworld.wolfram.com/physics/SnellsLaw.html


Refractive index of SiO2

http://www.ioffe.ru/SVA/NSM/nk/Oxides/Gif/sio2.gif

R = 3.5 in air

= 365nm

Transmission through two air-glass surfaces is

less than 93.1%.

http://www.mellesgriot.com/products/optics/images/fig5_12.gif


Snell’s Law/Antireflective Coatings

when the layer thickness,t, is

t = (m+1)/4; m = 0,1,2…

R = 0 when n = (n1n2)1/2

tt

nn11 nn22

nn

http://en.wikipedia.org/wiki/File:Optical-coating-1.png

2

21

221

nnn

nnnR


Young’s Single Slit Experiment

sin = /d

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html


Amplitude of largest secondary lobe at point Q, Q, is given by:

Q = (A/r)f()d

where A is the amplitude of the incident wave, r is the distance between d and Q, and f() is a function of , an inclination factor introduced by Fresnel.

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html


Young’s Double Slit Experiment

http://micro.magnet.fsu.edu/optics/lightandcolor/interference.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/interference.html


Diffraction of Light



The Huygens-Fresnel principle states that every unobstructed point of a wavefront at a given time acts as a point source of a secondary spherical wavelet at the same frequency – The amplitude of the optical field is the sum

of the magnitudes and phasesFor unobstructed waves, we

propagate a plane waveFor light in the pin-hole, the ends

propagate a spherical wave.


Basic Optics


Basic Optics

Information about the shape of the pin hole is contained in all of the light; we must collect all of the light to fully reconstruct the pattern– If only part of the diffraction pattern is

collected and focused on the substrate, the image created is not identical to the one on the mask.

The light diffracted at higher angles contains information about the finer details of the structure and are lost


Basic Optics

The image produced by this system is


Basic Optics

The diameter of the central maximum is given by

Note that you get a point source only if d

light ofh wavelengtλ

length focal f

diameter lens focusing

22.1 maximum central ofDiameter

dd

f


Basic Optics

There are two types of diffraction– Fresnel, or near field diffraction– Fraunhofer, or far field diffraction

In Fresnel diffraction, the image plane is near the aperture and light travels directly from the aperture to the image plane.

In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane.

Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems


Fraunhofer Diffraction

We define the performance of the system in terms of– Resolution– Depth of focus– Field of view– Modulation Transfer Function (MTF)– Alignment accuracy– throughput



Imagine two sources close together that we are trying to image (two features on a mask)– How close can these be together and we

can still resolve the two points? The two points will each produce an Airy disk.

– Lord Rayleigh suggested that the minimum resolution be defined by placing the maximum from the second point source at the minimum of the first point source.



With this definition, the resolution becomes

For air, n=1 is defined by the size of the lens, or by an

aperture and is a measure of the ability of the lens to gather light

light diffracted theof angle half maximum

lens andobject ebetween th material theof refraction ofindex

sin

61.0

sin2

22.122.1

n

nfn

f

d

fR



This is usually defined as the numerical aperture, or NA

Defined only for point sources as the point source Airy function was used to develop the equation

A more generalized equation replaces 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems.

NAk

NAR

nNA

1

61.0

sin



From this result, we see that we get better resolution (smaller R) with shorter wavelengths of light and lenses of higher numerical aperture

We now consider the depth of focus over which focus is maintained.

We define as the on-axis path length difference from that of a ray at the limit of the aperture. These two lengths must not exceed /4 to meet the Rayleigh criterion


Depth of Focus


Depth of Focus

From this criterion, we have

For small

cos4/

22114/

22

2222

22sin

NAk

NADOF

NAf

d



From this we note that the depth of focus decreases sharply with both decreasing wavelength and increasing NA.

The Modulation Transfer Function (MTF) is another important concept

This applies only to strictly coherent light, and is thus not really applicable to modern steppers, but the idea is useful



Because of the finite aperture, diffraction effects and other non-idealities of the optical system, the image at the image plane does not have sharp boundaries, as desired

If the two features in the image are widely separated, we can have sharp patterns as shown

If the features are close together, we will get images that are smeared out.


Modulation Transfer Function



The measure of the quality of the aerial image is given by

The MTF is really a measure of the contrast in the aerial image

The optical system needs to produce MTFs of 0.5 or more for a resist to properly resolve the features

The MTF depends on the feature size in the image; for large features MTF=1

As the feature size decreases, diffractions effects casue MTF to degrade

MINMAX

MINMAX

II

IIMTF


Change in MTF versus Wavelength


Contact and Proximity Systems

These systems operate in the Fresnel regime– If the mask and the resist are separated by some

small distance “g” and a plane wave is incident on the mask, light is diffracted at the aperture edges.

– As shown in next slide, there is 1. Small maximum at the edge from

constructive interference

2. Ringing caused by constructive and destructive interference

To minimize effects, multiple wavelengths of light may be used to expose PR


Fresnel Diffraction


Fresnel Diffraction

As g increases, the quality of the image decreases– The aerial image can be computed accurately when

where W is the feature size

– Within this regime, the minimum resolvable feature size is:

– Proximity aligner with a 10 m gap and an i-line source can resolve ~ 2 m features.

2Wg

gW min


Resolution

A more exact solution for the theoretical resolution for proximity or contact aligners is given by:

Where is the wavelength of light used to exposure the pattern, g is the distance between the bottom of the mask and the top of the photoresist, z is the thickness of the photoresist (typically 0.8-1.2m).

22

3 zgR


Fresnel Number

Fresnel diffraction when F ≥ 1 Fraunhofer diffraction when F << 1

g

WF

2


Depth of Focus

http://www.research.ibm.com/journal/rd/411/holm1.gif


Summary of the Three Systems


Photoresists

Parameters that determine the usefulness of the resist include:– Sensitivity: a measure of how much light is

required to expose the resist - typically 100mJ/cm2

– Resolution where the effects of exposure, baking, developing should not degrade the quality of the image

– Chemical and physical properties: it must withstand chemical etching, mild temperature excursions, ion implantation


Photoresists

Photoresists usually contain three components– Inactive resin (usually a hydrocarbon which

forms the base material)– Photoactive compound (PAC) – Solvent which is used to adjust the viscosity

The most common g- and i-line resists use– Diazonaphthoquinones (DNQ) as the PAC– Novolac as the resin– Propylene glycol monomethyl ether acetate

(PGMEA) as the solvent (this has replaced Cellosolve acetate, which is a toxic hazard)


Basic Structure of Novolac

Novolac is a polymer containing hydrocarbon rings with 2 methyl groups and 1 OH group

The basic ring structure is repeated to form a long chain polymer

Novolac readily dissolves in developer at about 15 nm/s


Diazoquinone

The photoactive part of the molecule is the part above the SO2


Diazoquinone

The function of the PAC is to inhibit the dissolution of the resin in the developer– DNQ is essentially insoluble in developer

prior to exposure to light– When dissolved in the resin, DNQ reduce

the resist dissolution rate from ~ 15nm/s to 1-2 nm/s

When the resist is exposed to light, the diazoquinone molecule changes chemically and increases the dissolution rate to ~100nm/s.


Properties and Characteristics of Resists

Two parameters are used to define the properties of photoresists– Contrast– Critical modulation transfer function (CMTF)


Contrast

The ability of the photoresist to distinguish between various levels of light intensities.– It is experimentally determined by exposing

the resist to differing amounts of light, developed for a fixed time and measuring the thickness of resist remaining after developing.


Photoresist Contrast



For positive resists, material exposed to low light will not be attacked by the developer; material exposed to large doses will be completely removed

Intermediate doses will result in partial removal The contrast is the slope of this curve and is given by

Typical g- and i-line resists will achieve a contrast of = 2-3 and Qf values of 100 mJ/cm2

O

f

Q

Q10log

1



The contrast is not a constant, but depends on process variables such as – development chemistry, – bake times, – temperatures before and after exposure,– wavelength of light, and – underlying structure

It is desirable to have as high a contrast as possible in order to produce the sharpest edges in the developed pattern


Modulation Transfer Function (MFT)

Defined in two points of the lithographic system.– MTF: Measure of the dark versus light intensities in

the aerial image produced by the projection system– CMTF: Measure of the exposed versus unexposed

regions in the high contract image focused on the PR

The CMTF is the minimum optical transfer function necessary to resolve a pattern in the resist– For g- and i-line resists, CMTF 0.4

110

110/1

/1

0

0resist

QQ

QQCMTF

f

f


Effect of Resist Thickness

Resists usually do not have uniform thickness on the wafer– Edge bead: The build-up of resist along the

circumference of the wafer- There are edge bead removal systems

– Step coverage

Centrifugal ForceCentrifugal Force


Effect of Resist Thickness

The resist can be underexposed where it is thicker and overexposed where it is thinner– This can lead to linewidth variations

Light intensity varies with depth below the surface due to absorption

where is the optical absorption coefficient– Thus, the resist near the surface is exposed first

A process called bleaching in which the exposed material becomes almost transparent (i.e., decreases after exposure)– Therefore, more light goes to deeper layers after

bleaching the near surface layer of PR

)exp()( 0 xIxI


Photoresist Absorption

If the photoresist becomes transparent and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to.– This leads to the possibility of standing

waves (due to interference), with resultant waviness of the developed resist

We can solve this by putting an antireflective coating on the surface of the substrate before spinning the photoresist increases process complexity


Standing Waves Due to Reflections


Standing Waves Due to Reflections

http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html


(a) (b) (c)

Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves.

Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.

http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html

Removal of Standing Wave Pattern


Mask Engineering

There are two ways to improve the quality of the image transferred to the photoresist– Optical Proximity Correction (OPC)– Phase Shift Masks (PSM)

We note that the lenses in projections systems are both finite and circular but most features on the mask are square.– The high frequency components of the pattern are

lost and the “squareness” of the corners of the pattern disappear.

– Can be taken into account by adjusting feature dimensions and shapes in the masks


Mask Engineering


Phase Shift Masks

In a projection system the amplitudes at the wafer add so that closely spaced lines interact; the intensity at the wafer is smeared– If we put a material of proper index of refraction on

part of the mask, we can retard some of the light and change its phase by 180 degree and the two portions of light interfere and cancel out.

The thickness of the PS layer is

n is the index of refraction of the phase shift material

12

nd


Phase Shift Masks (PSM)

Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.


Scanning Projection Aligners

Projection aligners have been industry standard for about 20 years– It is easier to correct for aberrations in small

regions than in large Scan a small slit across the mask while the

wafer is simultaneously scanned– Scanning projection aligners must use 1:1

masksPattern on the mask is the same size as the one imaged on the wafer.


Scanning Projection Printer


Scanning Projection Systems

Cost effective and has high throughput– Linewidth control for smaller devices is

difficult– As chips became larger, it is more difficult

to produce good full wafer masks– With ULVI and WSI, this system could not

scale and was replaced by systems that exposed only a single die at a time


Step-and-Repeat Projection Aligners

Exposed a limited portion of the wafer at a time– The image on the wafer is 4-5 times smaller

than the image on the mask or reticule.– Masks thus are much larger, and thus

repairable to some extent Steppers also allow better alignment because

they align on the exposure field rather than for the entire wafer– Wafer can be moved vertically to keep

image plane at some location as the PR


Off-Axis Illumination

By changing the angle of incidence of the light on the mask, change the angle of the diffracted light– Although some of the diffracted light is lost

in this scheme, much of the higher order diffraction is captured

– As the resolution is decreased, it is harder to make these optics work


Off-Axis Illumination


Step and Scan

A hybrid has been developed called a “step-and-scan”, but is very complex and very expensive.

https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm


DNQ/Novolac Resist Process

The details of the process are more complex that described earlier



We first must consider adhesion– There can be one or more operations

depending on what is under the resist The wafer must be clean before resist is

applied It may need to be heated to a few hundred

degrees to drive off water Adhesion to Si is not as good as to metals and

silicon dioxide– Adhesion promoter, Hexamethyldisilane

(HMDS), may be needed



Dispensing the resist can be done either with a stationary or a slowly spinning wafer

The solvent evaporates rapidly after dispensing the resist and during the spin– Generally more uniform resist thicknesses

are obtained the faster the wafer is accelerated.

– The faster the final speed, the thinner the resist.



Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources– Exposure time is increase by increasing the bake

temperature (due to decomposition of the PAC and thus decreased sensitivity)

Post-exposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern

Post-development bake is done to remove standing wave pattern by flowing resist (90-100oC) or increase chemical/mechanical strength of resist (120-150oC) Long UV exposure can also be used to cross-link the

polymer chains in the remaining photoresist


http://www.research.ibm.com/journal/rd/411/holm4.gif


Measurement Methods

Measurement of– Mask Features and Defects– Resist Patterns– Etched Features– Alignment

Measure resist pattern after development– The aerial image is not generally

measurable Because of the complexity of the masks, the

inspection must be fully automated—manual observation under a microscope is not possible


Mask Inspection System


Measurement of Mask Features and Defects

Here, light is passed through the mask and collected by an image recognition system

Solid state detectors are used to collect the light The information is compared against the database of

the mask design or with an identical mask The inspection process is more difficult if the mask

contains OPC or is a PSM Often, defects found in this process can be corrected

– Lasers can burn off excess Cr or Fe oxide.– Adding absorber to clear areas is harder


SEM Measurement


State-of-the-Art

Capable of exposing down to ~ 10nm – E-beam lithography– X-ray lithography– Extreme UV lithography

E-beam and EUV are performed under vacuum– Throughput is very slow

New resist families are required– Most are very difficult to remove after use

Research needed on mask material for x-ray and EUV– Glass absorbs– Thickness of metal needed to block x-rays is very

thick (20-50m)

Chapter 5: Lithography

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