Paper submitted within the scope of the Master’s Thesis Master of Industrial Sciences

GROUP T – Leuven Engineering College – 2009-2010

Process Risk Assessment of Semiconductor Wet Chemical Cleaning Techniques

Danhua Yao*, Alain Pardon

†, Nausikaä Van Hoornick

†, Patrick Lievens

‡

*Master student <Biochemical Engineering, focus Medical Bio-engineering>, GROUP T – Leuven Engineering College,

Vesaliusstraat 13, 3000 Leuven

†<IMEC>, <Kapeldreef 75 B-3001 Leuven Belgium>

‡Unit <matter>, GROUP T – Leuven Engineering College, Vesaliusstraat 13, 3000 Leuven, <patrick.lievens@groept.be>

Abstract

In semiconductor manufacturing industry, a wide range of hazardous chemicals are involved, which have the potential to

cause harm to persons, property, or environment. Therefore it is necessary and important to do the process risk assessment

of semiconductor manufacturing techniques. This paper is focusing on one of the most important steps in semiconductor

industry-Wet Chemical Cleaning, which is happening throughout the whole fabrication process. For the process risk

assessment, hazards should be identified and the corresponding actions should be taken to avoid reoccurrence and to

achieve the common goal of having healthy and safe working conditions in an environmentally friendly semiconductor

industry.

Keywords:

Risk Assessment, Wet Chemical Cleaning Techniques, Hazardous Chemicals, Health Effects

Introduction

Because of the producing requirement of semiconductor

manufacturing, various small amounts of chemicals are

involved, among which some are special gases (some are

flammable and others toxic) others are liquid chemicals

such as strong acids or alkalines and organic solvents. If

these gases or liquids are not treated properly after the

producing process, damages or even disasters can be

produced. Audits of the occupational hazard in

semiconductor manufacturing industry in recent years

show that above 90% of the hazards are composed of

unsafe behaviors. Hence, not only hardware stability but

also software aspects (safety management) should be

maintained and improved.

A complete enterprising safety management system is

composed of several key factors, none of which can be

lost to operate as a management chain. In this chain, risk

assessment is the vital component. In order to achieve

enterprising safety production and maximize return, an

adequate effective risk assessment method should be

selected. From the variety of methods, Quantitative Risk

Assessment is the most scientific and modernized one at

present and widely accepted and applied all around the

world. [1]

For one of the most important steps in the semiconductor

manufacturing industry-The Wet Chemical Cleaning,

various hazardous liquid solutions and gases are

commonly involved as important roles in daily operation,

such as hydrochloric acid, sulfuric acid, ammonia

hydroxide and ozonated water. These aqueous solutions

can emit corresponding gases that are harmful to the

workers and the environment. Therefore it is necessary

and important to do the quantitative risk assessment of the

Wet Chemical Cleaning techniques.

2

This paper consists of four parts. The first part covers the

principle of the risk assessment, the second part covers

the general shape of the semiconductor manufacturing

process, the third part covers the in-depth study of the Wet

Chemical cleaning techniques, and the last part covers the

actual quantitative risk assessment.

1. Principle of Risk Assessment

What is risk assessment?

Hazards: A hazard is a situation that poses a level of threat

to life, health, property, or environment. [2] And it has the

intrinsic potential to do harm to human beings.

Risk: A risk is the chance or probability that a person will

be harmed or experience an adverse health effect if

exposed to a hazard. It may also apply to situations with

property or equipment loss. [3]

Risk assessment:

Risk assessment is the process where you:

Identify hazards;

Analyze or evaluate the risk associated with that

hazard;

Determine appropriate ways to eliminate or

control the hazard. [3]

2. Semiconductor Processing

In order to make a comprehensive quantitative risk

assessment of semiconductor Wet Chemical Cleaning

techniques, it is important to have a clarified overview of

the whole IC-manufacturing process.

IC-manufacturing process is a multiple-step sequence of

photographic and chemical processing steps during which

electronic circuits are gradually created on a wafer made

of pure semiconducting material. [4] Figure 1 illustrates

one cycle of the main steps and their sequence. [5] The

final products will come out after repeating several times

this basic cycle. And wafer cleaning is happening

throughout the whole fabrication process.

Figure 1: IC-devices manufacturing process [5]

The main steps of the whole semiconductor

manufacturing will be briefly discussed in this chapter,

except the wafer cleaning techniques which is the main

part of the rest of this paper, and will be specifically

discussed in chapter 3.

2.1. Layering

Layering techniques are used to grow thin layers of film

on the surface of a silicon wafer [6].

In general, there are two primary techniques for layer

deposition: chemical vapor deposition (CVD) and

3

physical vapor deposition (PVD). These are schematically

represented in Figure 2. [7]

Figure 2: Schematic representation of PVD and CVD [8]

Chemical vapor deposition (CVD) is the process of

forming a thin film on a substrate by the reaction of vapor

phase chemicals which contain the required constituents.

Heat, plasma, ultraviolet light, or another energy source is

used singly or in combination to activate the reactant

gases on and/or above the temperature-controlled surface

to form the thin film. [9]

Physical vapor deposition (PVD) is the process to deposit

thin-film by the condensation of a vaporized form of the

material on to various surfaces, without chemical

reactions. For instance, the pure physical processes such

as high temperature evaporation, sputtering, or plasma are

belonging to PVD. [9]

2.2. Photolithography

Photolithography is a process used in microfabrication to

selectively remove parts of a thin film or the bulk of a

substrate. It uses light to transfer a geometric pattern from

a photo mask to a light-sensitive chemical (photoresist) on

the substrate. A series of chemical treatments then

engraves the exposure pattern into the material

underneath the photo resist. The main process is

illustrated in figure 3. [10]

Figure 3: The main process in Photolithography [11]

Fine lithographic patterns defining the integrated circuits

can be produced when light interacts with photoresists,

and have been transferred to the photoresist free parts; it

either can be etched away or implanted by dopants.

2.3. Etching

In semiconductor industry, etching is used to remove the

deposited films or substrates which are not protected by

the photoresist. The aim is to form trenches and holes for

other devices or isolation structures to be filled in later.

[12]

Etching processes consist of two main categories: wet

etching and dry etching. Wet etching refers to the removal

of materials (usually in specific patterns defined by

photoresist masks on the wafer) from the wafer by using

liquid chemicals or etchants. Dry etching refers to the

removal of material by exposing the material to a

bombardment of ions (usually plasma of reactive gases

such as fluorocarbons, oxygen, chlorine, boron trichloride)

that dislodge portions of the material from the exposed

surface. [13]

2.4. Doping

Doping refers to the process of introducing impurity

atoms into a semiconductor region in a controllable

manner in order to define the electrical properties of this

region. [14] Two methods are involved: Thermo diffusion

and ion implantation. Ion implantation is however the

primary technology used to introduce doping atoms into a

semiconductor wafer to form devices and integrated

circuits. [14]

2.4.1. Doping by diffusion

Diffusion is the first technique used to dope the

semiconductor. The technique is based on two

mechanisms: in the vacancy model, the dopant atoms

move by filling empty crystal positions; in the interstitial

model, the dopant atoms move through the spaces

between the crystal sites. The wafer is first pre-cleaned

and etched by HF to remove any oxide layer on the

surface. Then it is deposited by the dopant sources in the

tube furnace in the temperature range from 1000°C to

1250°C. [7]

4

Figure 4: Diffusion models. (a)Vacancy model and (b)

interstitial model. [18]

2.4.2. Ion implantation

Ion implantation bring the dopants into the substrate

material mainly due to its ability to accurately control the

number of implanted dopants and to place them at the

desired depth, which works by ionizing the required

atoms, accelerating them in an electric field, select only

the species of interest by an analyzing magnet and direct

this beam towards the substrate. [15]

Figure 5: Schematic representation of ion implantation

2.5. Resist removal

After the etching or ion implantation step, the photoresist

needs to be stripped away. Resist stripping is a critical

process because the removal of the photoresist should not

damage the underlying functional layers.

The newest resist removal technique makes use of ozone

(O3) in a wet environment. In IMEC, three stripping

methods are resulting from ozone. The first method uses

ozonated water defined by ozone together with de-ionized

water (DIW). This technique has less consumption of

chemicals, less waste generation and is more friendly to

the environment. But due to the limitation of O3’s

solubility in the DIW, the maximum O3 concentration that

can be reached in the DIW is only 10ppm, and this is not

strong enough to remove the implanted photoresist layer.

In order to reach a higher concentration of O3, the O3

-boundary layer technique is developed. Till now, the best

technique associated with O3 for photoresist stripping is

using O3 together with sulfuric acid (H2SO4) at 90 °C.

This third technique has a very good performance on

stripping away the doped photoresist layer [12].

3. Wafer cleaning

The need for cleaning wafers has been recognized since

the dawn of semiconductor manufacturing technology.

Clean substrate surfaces are critical for obtaining

maximum device-performance, long-term reliability, and

high yields. Cleaning techniques are used to remove

particulates and chemical impurities so contaminant-free

surfaces can be obtained. The different kinds of

contaminants are demonstrated in Figure 6. However,

such cleaning-methods must also be able to do this

without damaging the surface. Cleaning procedures

should also be safe, simple, economical, and produce a

minimum of hazardous waste-products. [16]

Figure 6: Typical Contaminants on Si Wafer [17]

In order to remove these contaminants, two big groups of

cleaning are used: Wet Chemical Cleaning and Dry

Cleaning.

Wet-chemical cleaning uses a combination of solvents,

acids and water to spray, scrub, etch and dissolve

contaminants from wafer surface.

Dry cleaning uses gas phase chemistry, such as plasma

and ozonated chemistries and relies on chemical reactions

for wafer cleaning, as well as other techniques. [18]

The typical wet chemical cleaning methods are discussed

below.

3.1. RCA Cleaning

The RCA cleaning has been used in the semiconductor

industry since a very long time already and is the most

widely spread technique used.

The purpose of the RCA clean is to remove organic

contaminants (such as dust particles, grease or silica gel)

from the wafer surface; then remove any oxide layer that

may have built up; and finally remove any ionic or heavy

metal contaminants.[18].

During RCA Cleaning four cleaning solutions are used:

SPM, HPM, APM and HF (DHF).

5

SPM (sulfuric peroxide mixture): H2SO4/H2O2- four

volumes of sulfuric acid (H2SO4) 98%, 1 volume H2O2

30%. Sulfuric acid can cause organic dehydration and

carbonization and quickly becomes saturated with carbon,

while hydrogen peroxide can oxidize the carbonized

product into CO or CO2.

APM (ammonia peroxide mixture): NH4OH/H2O2/H2O,

also called the standard clean 1(SC-1), which is usually

composed of 5 volumes of H2O, 1 volume hydrogen

peroxide (H2O2) 30% and 1 volume ammonia hydroxide

(NH4OH) 30%. It contributes to the particle removal, the

reason is H2O2 promotes the formation of a native oxide,

and NH4OH slowly etches the oxide away which contains

the particle contaminants.

HPM (hydrochloric peroxide mixture): HCl/H2O2/H2O,

the standard clean 2(SC-2), which usually consists of 5

volumes of H2O, 1 volume hydrogen peroxide (H2O2)

30%, and 1 volume hydrochloric acid (HCl) 37%. This

solution is mainly used to remove the alkaline and trace

metals. HCl can react with the metal before hydrogen in

Metal activity series.HPM has strong oxidizing and

complexing properties that can oxidize ions and induce

them to react with Cl- to generate soluble complex which

can be rushed away by the rinsing water.

HF (DHF) (dilute HF): usual concentration of 2 % HF.

Sacrificial oxide and native oxide removal. Particles are

removed by directly etching an underlying silicon dioxide

layer to “lift” the particles away from the surface, and

followed by rapid removal of the freed particle from the

vicinity of the surface.

The sequence of RCA Cleaning is illustrated in Table1.

RCA Solution

(Step by step)

Chemicals and Conditions Contaminant Removal

SPM H2SO4:H2O2 = 4:1

at 90°C

Organics

APM NH4OH:H2O2:H2O = 1:1:5

at 35°C

Particles and some metals

HPM HCl:H2O2:H2O = 1:1:5

at 80 °C

Metals: alkaline and trace metals

HF(DHF) HF OR DHF

at 20°C

Sacrificial and native oxide removal

“ Lift-off ” particles

Table 1: The sequence of RCA Cleaning

Note: Rinsing water is used throughout the whole process between each step

3.2. IMEC Cleaning

A very active group of researchers at IMEC in Leuven,

Belgium, has been performing outstanding pioneering

work in developing cleaning technologies for metal and

particle removal preliminary to preparing critical oxide

layers. [16]

The IMEC-Clean concept is based on a two main steps +

one optional step cleaning approach, and is illustrated in

Figure 7.The purpose for the process is indicated below

each step.

6

Figure 7: Schematic illustration of the IMEC-clean concept [16]

The fundamental chemistry of ozone-based cleaning is

due to both direct reactions of the contaminants with

molecular O3 (especially certain organics) and indirect

reactions within oxygen radicals. [19]

Ozone cleaning requires careful optimization of

conditions, because the solubility and decomposition of

ozone is closely related to temperature and pH. The

relationship between ozone decomposition and

temperature or pH value are shown in Figures 8-11.

DI water/ozone solutions (DIO3) provide an effective

replacement for SPM and RCA SC-1 and SC-2 cleans.

Science utilizing ozonated water allows the wafers to

proceed directly from organic removal to the HF-HCl

bath, it could also reduce the large amount of rinsing

water as it is used in RCA Cleaning.

Figure 8: Decomposition of ozone in solution under different pH values at 20°C [38]

Figure 9: Influences on half lives which decomposition of ozone in aqueous solution of pH

values and temperatures [38]

Figure 10: Decomposition of ozone in gaseity under different Temperatures [38]

Figure 11: Half lives of ozone in gaseity under different temperatures [38]

7

The IMEC cleaning sequence was developed in an effort

to provide a lower cost, less environmentally deleterious,

and in some ways more effective cleaning sequence. [18]

3.3. Supercritical CO2 Cleaning

Supercritical fluids are moieties that exist above their

critical temperature and critical pressure. This has been

considered a fourth state of matter where the material is

neither a liquid nor a gas but has the beneficial properties

of both. In the supercritical state the fluid has the

diffusivity of a gas with the solvency of a liquid and zero

surface tension. These become very desirable properties

for cleaning.[18] The region of supercritical fluids are

illustrated for carbon dioxide in Figure12.

Figure 12: Phase diagram of carbon dioxide

3.1.1. Photo resist removal

The mechanisms for removing photoresist using scCO2

processing differ from a typical wet chemistry approach.

Instead of dissolving the photoresist, the scCO2 can be

used to swell the polymer and then debond it from the

surface, and is lifted off in sheets, as shown in Figure 13.

[18]

Figure 13: Photo resist removal by scCO2 Cleaning

3.1.2. Particle removal

Similar approaches to swell and debond could be used for

organic particle removal. However, for inorganic particles

a different method has been demonstrated. Korzenski et

al. have described the use of chemical additives that

include a combination of etchant, surface passivator and

fluorinated surfactant in scCO2 in order to remove Si3N4

particles from Si or SiO2 surfaces. Particles are removed

by an undercut mechanism whereby the particle/surface

adhesion layer is attacked by the etchant. In order to

minimize the etching of the surface, a passivator is added

[18]. Figure 14 shows particle removal results.

Figure 14: Particle removal results by scCO2 Cleaning

3.4. Wafer rinsing and drying

The last steps in wet-chemical wafer cleaning are rinsing

and drying; both are extremely critical because clean

wafers can become recontaminated very easily. Rinsing

should be done with flowing high-purity and ultra filtered

high-resistivity DI H2O at room temperature. It removes

chemicals between each cleaning step to prevent cross

contamination and at the end of the cleaning sequence to

prevent chemical residues from being left on the wafer.

Wafer drying after rinsing must be done by physical

removal of the H2O, rather than by allowing it to

evaporate, which would leave residues. [18]

3.5. Cleaning tools

In Wet Chemical Cleaning, a bunch of cleaning tools is

used, like single-wafer cleaning tool and wet bench

cleaning tool. This thesis is concentrated in wet bench

Nr.32014-00.

The construction and data of the wet bench which we

need for quantitative risk assessment are demonstrated in

the following Figures.

8

Figure 15: Wet bench 32014-00 in IMEC.

Note: In this wet bench, the interior volume is V = 1.9 × 2 × 0.75 = 2.85m3 [20]

Figure 16: Hazardous gas exhausted laminar flow in the wet bench 32014-00 in IMEC

Note: Here the laminar flow rate is 0.668m3/s.[21]

Indicator light

Fire alarm

Rinsing

water Cleaning tank

19

0 cm

200 cm

75 cm

Laminar Flow

9

Figure 17: Expression of the emission surface area

Note: The emission surface area is F = 0.2 × 0.2 = 0.04m2 [20]

The further sketch will be made in the section of Modal based simulation.

Figure 18: Exhaust pipes on wet bench 32014-00 in IMEC

3.6. Potential hazards of the wet chemical cleaning

While cleaning, there are two main hazards existing in

the work place: physical and chemical hazards.

Physical hazards are the most common and will be

present in most workplaces at one time or another. They

include unsafe conditions that can cause injury, illness

and death.

Examples of physical hazards include:

Electrical hazards

Unguarded machinery and moving machinery parts:

guards removed or moving parts that a worker can

accidentally touch

Constant loud noise

High exposure to sunlight/ultraviolet rays, heat or

cold

Chemical hazards are present when a worker is exposed

to any chemical preparation in the workplace in any form

(solid, liquid or gas). Some are safer than others, but to

some workers who are more sensitive to chemicals, even

common solutions can cause illness, skin irritation or

20 cm

20

cm

10

breathing problems.

Beware of:

Liquids like cleaning products, paints, acids,

solvents especially chemicals in an unlabelled

container (warning sign!)

Flammable materials like gasoline, solvents and

explosive chemicals.

Vapours and fumes, for instance those that come

from welding or exposure to solvents [22]

The Primary Route(s) of Exposure are:

Skin contact: liquid, mist

Inhalation: vapor, mist

Eye contact: liquid [23]

This paper will concentrate on the acid/alkaline mist and

harmful gases that emit from the solution during the RCA

Cleaning and IMEC Cleaning in the Wet Chemical

Cleaning field. In the next section an exact quantitative

risk assessment is done.

4. Modal based simulation

In order to estimate the exact amount of acidic or alkaline

mist formed above the liquid or the amount of dangerous

gas remaining in the operating environment of the wafer

cleaning bath, especially when there is sufficient air flow

ventilation inside the cleaning tool, a mathematical

simulation model for this dynamic balance problem was

established. In the following paragraphs, the principle of

the model is illustrated for a few typical gases and the

consequent influence of the ambient environment

conditions onto the residue amount is explained.

4.1. Approach

In wet chemical cleaning processes, a variety of acids or

alkalines are often involved for removing the

contaminants on the semiconductor chips. Among those

acids, hydrochloric acid (HCl), hydrofluoric acid (HF),

sulfuric acid (H2SO4) and ammonia solution (NH4OH) are

most commonly used. Except that some physical and

chemical property factors are different for those different

sorts of acid, we could apply similar simulation models in

our calculation for them. Hence, we choose the ammonia

solution as our analysis object and introduced the

establishment of the model according to its properties.

The very first step in establishing the mathematical

simulation model was to get acquainted with the

simulation environment. According to the picture taken

from the wafer cleaning laboratory, a simplified sketch

can be drawn to explain the operating process and the

theoretical approach we can make:

Figure 19: Simplified sketch of the wet bench

As far as we involve the concept of sampling time, this

continuous model has been converted into a discrete

model. Therefore the continuous emission and exhausting

procedure is divided into millions of infinite small

discrete steps. Regarding the initial stage and end stage of

each sub-step, we have their transient concentration as

Xinitial and Xend.

11

Figure 20: Control volume

Considering the self iterative phenomenon in the density

and residue weight computation, we could regard the

control volume as a balloon. Therefore the fresh air inflow

ventilating process can be seen as the expansion discrete

step of the control volume while the same volume of gases

with different density (after emission) will be emitted

from the control volume like the contraction discrete step

of a balloon. Concerning the transient change, the residue

weight should be the discrepancy between the generated

mass of ammonia mist and the ammonia contained in the

emitted mixed gas.

4.2. Model Calculation

The typical important step for computing the exhausting

model of acid mist is to recognize the way of its emission.

Referring to the “General Analysis and Estimation

Method for Fugitive Emission Resources”, we know the

formula of calculating acid mist emission as:

Gz = M · 0.00352 + 0.000786 ·U ·P ·F [24]

with Gz – Acid Mist Emission (kg/h); M – Molecular

Weight of Acid; U – Interior Wind Velocity (m/s); F –

Emission Surface Area (m2); P – Saturation Partial Vapor

Pressure with respect to solution condition (mmHg), this

value can be taken place by the saturation vapor pressure

of water in corresponding condition when the wt% of acid

solution is smaller than 10%;

Hence we can figure out the related variables are almost

all free from disturbance except the partial vapor pressure

P. Since the ambient conditions are relatively steady,

analyzing the function between P and solution condition

turns to be the key step for the calculation. Unfortunately,

regarding the deionization involved in the dissolution

process of NH4OH, Henry’s Law is no longer available

for calculation here. With the aid of “Chemical Industry

Physical Property Handbook” we note that the function is

a nonlinear function. [25] This means that we cannot

simply apply specific function formula but need to

analyze the experiment data instead. According to the

given solution condition, T=35℃. The experiment data

sheet at this temperature is attached below:

y = 3E+06x6 - 2E+06x5 + 39888x4 + 14217x3 -4378.x2 + 1559.x - 4.369

0

200

400

600

800

1000

1200

1% 6% 11% 16% 21% 26%

Par

tial

Va

po

r P

ress

ure

NH

3

Concentration (wt%)

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2,

Laminar Flowrate = 0.668m3/s, Conc. NH4OH = 4%, T = 35°C in Wet Bench Nr 32014-00.

Tool Nr: 32014-00, T= 35℃

Conc. NH4OH: 4%

Emission Area: 0.04m2

12

By ploting the data sheet, we have the graphical relation simulation by the internal Bezier Curve fitting of MS Excel.

Hence the function of this relation can be expressed according to the sextic trendline approximation function equation of

the Bezier Curve:

y = 3E+06x6 - 2E+06x5 + 39888x4 + 14217x3 – 4378.x2 + 1559.x – 4.369

We continue by programming this equation as an internal function model of MS Excel VBA, defining the function as

“VPamm(Conc)” for future calling.

As the saturation partial vapor pressure P is expressed as

the function of solution concentration--VPamm(Conc) in

the same temperature and atmospheric pressure condition,

we can derivate the transfer function from the

concentration of acid solution to the emitted mass of acid

mist as following:

Gz ·dt = M ·(0.00352 + 0.000786 ·U) ·F ·dt

·VPamm(Xinitial )

Xend =msolution ∙ Xinitial − Gzdt

msolution − Gzdt∙ 100%

Xinitial ,n = Xend ,n−1 , Xinitial ,0 = 4%

Here dt stands for the sampling time of discrete steps

which would be taken as a relative small value, for

instance dt=0.1s. Hereby we complete the calculation

concerning the liquid solution, and go way to the

computation of residue acid mist concentration in the

control volume. According to the reference data we take

the air density of 1.2046kg/m3 at 20℃. Then the transient

internal gas density in the control volume after the

expansion discrete step can be computed as:

ρinternal

= V + Qflow ∙ dt ∙ ρ

air+ Gz ∙ dt + mresidue

V + Qflow ∙ dt + (Gz ∙ dt + mresidue ) ρAmmonia

Here Qflow stands for the in and out flow rate of ventilating

air inside the cleaning tool. With this calculated internal

gas density, we can easily calculate the mass of gas at the

transient moment right after the contraction discrete step

(density of gas doesn’t change in emission process).

mair = ρinternal

∙ V

Then we give way to the computation of the ammonia

residue in the current control volume. The value can be

calculated with the aid of the weight percentage of

generated acid mist in control volume gas as:

mresidue ,n = Gz ∙ dt + mresidue ,n−1 − Qflow ∙ dt +

(Gz∙dt+mresidue,n−1)ρAmmonia∙ρinternal∙(Gz∙dt+m

residue,n−1)mair

4.3. Graphical Result Analysis

Till now, the entire mathematical simulation model is

established with input factors and sampling time dt as

variable. Hereby we take the in and out flow rate of

0.668m3/s, sampling time of 0.1 second and inspection

time of 1600dt to see the concentration response of the

residue ammonia as an example.

13

From the graph we can figure out the saturation effect of

the residue concentration. Regarding the theoretical

analysis, this saturation concentration level can be

understood as the dynamic balance level of the in and out

flow. Hence the conclusion should be that the emission

model reaches the dynamic balance stage at

approximately 20s after initial stage and the saturation

residue concentration is around 7.72 ppm.

Reviewing the previous computation steps, we notice that

there is direct link of the saturation residue concentration

with the in and out flow rate inside the cleaning tool. By

changing the variable of flow rate in the mathematical

model, we conclude the relationship as:

0

1

2

3

4

5

6

7

8

9

0 200 400 600 800 1000 1200 1400 1600 1800

Re

sid

ue

Co

nce

ntr

ati

on

(pp

m)

Time (10-1s)

0

10

20

30

40

50

60

0.1 0.3 0.5 0.7 0.9

Co

nce

ntr

atio

n (p

pm

)

Qflow (m3/s)

0.668@IMEC

Tool Nr: 32014-00, T= 35℃

Conc. NH4OH: 4%

Emission Area: 0.04m2

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2, Laminar

Flowrate = 0.668m3/s, Conc. NH4OH = 4%, T = 35°C in Wet Bench Nr 32014-00.

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2, Laminar

Flowrate = 0.668m3/s, Conc. NH4OH = 4%, T = 35°C in Wet Bench Nr 32014-00.

Tool Nr: 32014-00, T= 35℃

Conc. NH4OH: 4%

Emission Area: 0.04m2

14

Hence as long as the flow rate reaches 0.6m3/s, the

ammonia residue concentration in the interior gas turns to

be smaller than 10ppm. Moreover, each saturation residue

ppm concentration can be looked up from this curve with

corresponding flow rate.

Within the wet bench under investigation a laminar flow

rate of 0.668m3/s has been set. From the curve, the residue

ammonia concentration can be derived to be 7.72 ppm.

The results computed for other acid solutions including

HCl and HF applying similar methodology are contained

in the appendix. Here the saturation residue

concentration for each specific solution is listed in the

table below:

Solution Saturation residue concentration

NH4OH 7.72 ppm

HPM 12.699 ppm

DHCl 3.28*10-4 ppm

HF 6.54*10-3 ppm

4.4. Apply Method for Harmful Gases

Similar to the acid/alkaline mist exhausting model, we

could also establish a discrete mathematical model for the

computation of the emission and residue amount of

dangerous gases generated during the wafer cleaning

processes. Nevertheless, there would of course be some

different formulas involved. Hereby we take the ozonated

water solution as our inspecting object. Notice, an

important upgrade of the simulation system should be

made because of the decomposition phenomenon of

ozone in both air and water environment.

Still according to the “General Analysis and Estimation

Method for Fugitive Emission Resources”, we found the

formula for the harmful gases emission as:

Gs = (5.38 + 4.1 ·U) ·P ·F ·M0.5 [24]

with Gs – Harmful Gas Emission (g/h); M – Molecular

Weight of Acid/Alkaline; U – Interior Wind Velocity

(m/s); F – Evaporation Area (m2); P – Saturation Partial

Vapor Pressure with respect to solution condition (mmHg).

This value would be different from the one of the

ammonia solution model, since the solution is not an

electrolyte solution anymore and the relation of saturation

partial vapor pressure with the ppm concentration of

solution does obey Henry’s Law.

Because of the theoretical linear relationship from

Henry’s Law, we looked up the saturation partial vapor

pressure and ppm concentration of ozonated water at 20℃.

Since the Henry’s Law constant is only relative to the

concentration of ozone in the cleaning solution we

finally use (no matter what % of ozone is generated from

ozone generator), we can gain the following result:

P = 750.2467mmHg; Conc = 368ppm

Therefore the Henry’s Law constant of ozone at T=20℃

can be determined:

KHoz = PConc = 2.038714mmHg/ppm

Regarding the involvement of this constant, the transfer

function from the concentration of the solution to the

emitted mass of dangerous gas can be updated as

following:

Gs ·dt = (5.38 + 4.1 ·U) ·F ·M0.5 ·dt ·KHoz ·Xinitial′

Xend =msolution ∙ Xinitial

′ − Gsdt

msolution − Gsdt∙ 100%

Xinitial ,n′ = Xend ,n−1

′

Another key difference involved here is the difference

between Xinitial′ and Xinitial that we previously used for

the computation of acid mist emission. Considering the

self decomposition half-life of ozone in water solution

with concentration less than 1% and pH value of 7 is

approximately 20 minutes. The sextic trendline

approximation method onto the Bezier Curve can be

applied once again here for the function of decomposition.

The formula below is concluded for the relation

mentioned above:

15

From the simulation, the corresponding internal function of decomposition in water environment should be defined as

OzDW(t) in VBA programming:

This then leads to an easy relation of:

Xinitial′ = Xinitial ∙ OzDW dt

Xinitia ,0′ = 10ppm ∙ OzDW 0.1second

As we continue our computation for the residue ppm

concentration of dangerous gas in the cleaning tool, apart

from the iterative calculation of ρinternal

,

mair and mresidue , the decomposition of ozone in air

should be defined by simulation as well, notice half-life

here is 86 minutes:

y = - 2E-11x3 + 1E-07x2 - 0.000x + 0.998

0%

20%

40%

60%

80%

100%

120%

0 5000 10000 15000

Co

nce

ntr

atio

n (w

t%)

Time (s)

y = - 2E-13x3 + 7E-09x2 - 0.000x + 0.998

0%

20%

40%

60%

80%

100%

120%

0 10000 20000 30000 40000 50000 60000 70000

Co

nce

ntr

atio

n (w

t%)

Time (s)

Tool Nr: 32014-00, T= 20℃

Conc. Ozone: 10ppm

Emission Area: 0.04m2

Tool Nr: 32014-00, T= 20℃

Conc. Ozone: 10ppm

Emission Area: 0.04m2

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2,

Laminar Flowrate = 0.668m3/s, Conc. ozone = 10 ppm, T = 35°C in Wet Bench Nr 32014-00.

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2, Laminar

Flowrate = 0.668m3/s, Conc. ozone = 10 ppm, T = 35°C in Wet Bench Nr 32014-00.

16

The corresponding internal function of decomposition in water environment should be defined as OzDA(t) in VBA

programming:

The detailed computation procedure is listed below:

ρinternal

= V + Qflow ∙ dt ∙ ρ

air+ Gs ∙ dt + mresidue

V + Qflow ∙ dt + (Gs ∙ dt + mresidue ) ρozone

mair = ρinternal

∙ V

mresidue ,n = Gs ∙ dt + mresidue ,n−1 − Qflow ∙ dt +

(Gs ∙ dt + mresidue ,n−1) ρozone

∙ ρinternal

∙

(Gs ∙ dt+mresidue ,n−1)mair

∙ OzDA dt

Take the in and out flow rate of 0.668m3/s, sampling time

of 0.1 second again and the inspecting time of 1600dt, we

gain the following result graph:

Different from the acid/alkaline mist model, with the

involvement of self decomposition effect of ozone, the

curve shows a fast saturation of 9.92ppm at around T=8s

and then drops down with a decreasing velocity till the

lower limit of 1.87 ppm at around T=150s. Hence as soon

as we stop the emission of ozone from the cleaning

process the concentration drops because of the laminar

outflow.

Similar to the acid/alkaline mist model, we also give the

relationship between the saturation limits and the flow

rate as appendix in convenience for operator to check and

select the proper safe laminar flow rate for the cleaning

process.

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200 1400 1600 1800

Ozo

ne

Res

idu

e C

on

cen

tra

tio

n (p

pm

)

Time (10-1s)

Tool Nr: 32014-00, T= 20℃

Conc. Ozone: 10ppm

Emission Area: 0.04m2

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2, Laminar

Flowrate = 0.668m3/s, Conc. ozone = 10 ppm, T = 35°C in Wet Bench Nr 32014-00.

17

In the wet bench under investigation the laminar flow rate

is 0.668m3/s. From the graph we can see that the

maximum residual ozone concentration is reached after 8s

(9.92 ppm) and the final residue ozone concentration is

1.87 ppm after 150s.

5. Health Effects and Relative Limits

5.1. Definition of Common Terms

ACGIH = American Conference of Governmental

Industrial Hygienists

ATSDR = Agency for Toxic Substances and Disease

Registry

NAAQS = National Ambient Air Quality Standards

EPA = Environmental Protection Agency

TLV = Threshold Limit Value (guidelines to assist in the

control of occupational health hazards)

TWA = Time-Weighted Average (8 hours)

STEL = Short-Term Exposure Limit (15 minutes)

5.2. Ammonia

Ammonia, in both its gaseous and liquid form, can be

irritating to the eyes, respiratory tract and skin due to its

alkaline nature. The biological effects of ammonia in

humans after acute exposures are dose-related – they

depend on the ambient concentration, the amount taken in

by the body and the duration of exposure.

ACGIH recommends a threshold limit value (TLV) of 35

ppm as a short-term exposure limit (STEL) and 25 ppm on

a time weighted average (TWA) to avoid irritation of the

eyes, nose and throat. The other effects due to the

different concentration of ammonia have been illustrated

in table 2. [26]

A study conducted on workers exposed to ammonia for 10

to 15 years at levels up to and exceeding 24 ppm did not

find adverse effects. In additional occupational studies, no

relationship was found between exposure to ammonia and

cancer or carcinogenicity. [26]

According to the ATSDR studies, chronic exposure to

ammonia (under 25 ppm) did not result in harm to genetic

material or damage the reproductive system or a

developing fetus.

05

10152025303540

0.1 0.3 0.5 0.7 0.9

Co

nce

ntr

atio

n (p

pm

)Qflow (m

3/s)

Cmin

Cmax

0.668@IMEC

Tool Nr: 32014-00, T= 20℃

Conc. Ozone: 10ppm

Emission Area: 0.04m2

Note: These data of the vapor pressure are collected from the book of “Chemical Industry Physical

Property Handbook”. The solution is under the condition: Emission Surface Area = 0.04m2,

Laminar Flowrate = 0.668m3/s, Conc. ozone = 10 ppm, T = 35°C in Wet Bench Nr 32014-00.

18

Concentration / Time Health Effect

10,000 ppm Promptly lethal

5,000 – 10,000 ppm Rapidly fatal

700 – 1700 ppm Incapacitation from tearing of the eyes and coughing

500 ppm for 30 minutes Upper respiratory tract irritation, tearing of the eyes

134 ppm for 5 minutes Tearing of the eyes, eye irritation, nasal irritation, throat irritation, chest

irritation

140 ppm for 2 hours Severe irritation, need to leave exposure area

100 ppm for 2 hours Nuisance eye and throat irritation

50 – 80 ppm for 2 hours Perceptible eye and throat irritation

20 – 50 ppm (TLV-TWA=25

ppm)

Prolonged repeated exposure produces no injury

Table 2: Concentration/Duration/Effect of NH3 vapor (without protective clothing) [26]

5.3. Hydrochloric acid

Hydrochloric acid is corrosive to the eyes, skin, and

mucous membranes. Acute inhalation exposure may

cause coughing, hoarseness, inflammation and ulceration

of the respiratory tract, chest pain, and pulmonary edema

in humans. [27-29]

Pulmonary irritation, lesions of the upper respiratory tract,

and laryngeal and pulmonary edema have been reported

in rodents acutely exposed by inhalation. [27-29]

Brief exposures (up to a few minutes) to concentrations in

the range of 1,300 to 2,000 ppm are lethal to humans [30].

In workers, exposure to 50 to 100 ppm for 1 hour was

barely tolerable; short exposure to 35 ppm caused

irritation of the throat. In one study, workers chronically

exposed to hydrogen chloride did not exhibit the

pulmonary function changes observed in nine subjects

exposed to similar concentrations, which suggests that

workers become acclimatized to hydrogen chloride [31].

Dental discoloration and erosion of exposed incisors may

occur on prolonged exposure to low concentrations [32].

More effects according to different concentration of

hydrochloric acid are indicated in Table 3:

Exposure Health Effects

5 ppm (TLV-TWA) No health effects are expected.

10 ppm Does not interfere with work

10 ppm mice 90 days Mild rhinitis

> 10 ppm Acute local irritation among acclimatized workers

30 ppm, 1 hr no clinical manifestation

30 ppm upper limit of safety

30 ppm mice 90 days eosinophilic globules in nasal turbinate epithelial tissue

35 ppm irritation of throat for short exposure

50 - 100 ppm barely tolerable

Table 3: Relative limits of HCl vapor [33]

5.4. Hydrogen fluoride

In humans, inhalation of hydrogen fluoride gas may cause

immediate or delayed-onset pulmonary edema after a

1-hour exposure [31]. Repeated exposure to excessive

concentrations of fluoride over a period of years results in

increased density of bone and eventually may cause

crippling fluorosis (osteosclerosis caused by the

deposition of fluoride in bone) [31]. Ingestion of an

estimated 1.5 grams of hydrofluoric acid produces sudden

death; however, repeated ingestion of small amounts of

hydrogen fluoride may cause fluoride osteosclerosis. [34]

The relative limits are indicated in Table 4:

19

Exposure Health Effects

3 ppm (TLV-TWA) Has a strong irritation odor, does not interfere with work over an 8-hour work day

10 to 15 ppm irritate the eyes, skin, and respiratory tract

30 ppm Respiratory symptoms worsen. Can be tolerated for several minutes

50 ppm May be fetal if inhaled for 30-60 minutes

120 ppm Maximum concentration in air that can be tolerated for 1 minute. Smarting of the skin,

conjunctivitis and irritation of the respiratory tract occur

Table 4: Relative limits of HF vapor [35]

5.5. Ozone

The human health effects of ozone have been studied for

over 30 years. The respiratory system is the primary target

of this oxidant pollutant. The degree of adverse

respiratory effects produced by ozone depends on several

factors, including concentration and duration of exposure,

climate characteristics, individual sensitivity, preexistent

respiratory disease, and socioeconomic status [36].

The United States Environmental Protection Agency

(EPA) has classified ozone as a criteria pollutant. EPA

has established National Ambient Air Quality Standards

(NAAQS) of 0.12 parts per million (ppm) averaged over 1

hour (not to be exceeded more than three times in a

3-years period), and 0.08 ppm averaged over 8 hour.

However, recent epidemiological studies have shown that

1-hour ozone levels lower than 0.12 ppm and 8-hour

levels lower than 0.08 ppm produce adverse health effects

in the general population. [37] The relative limits are

indicated in Table 5.

8-hour average ozone

Concentration (ppm)

Air Quality Descriptor Health Effects

0.0 to 0.064 Good No health effects are expected.

0.065 to 0.084 Moderate Usually sensitive individuals may experience

respiratory effects from prolonged outdoor exertion

if you are unusually sensitive to ozone.

0.085 to 0.104 Unhealthy for Sensitive

Groups

Member of sensitive group may experience

respiratory symptoms (coughing, pains when taking

a deep breath).

0.105 to 0.124 Unhealthy Member of sensitive group have higher chance of

experiencing respiratory symptoms (aggravated

cough or pain), and reduces lung function.

0.125 (8-hr) to 0.404 (1-hr) Very Unhealthy Members of sensitive groups experience

increasingly severe respiratory symptoms and

impaired breathing.

0.25-0.75 ppm Very Unhealthy Cause cough, shortness of breath, tightness of the

chest, a feeling of an inability to breathe (dyspnea),

dry throat, wheezing, headache and nausea.

Table 5: Relative limits of ozone [37]

20

Conclusion

Regarding the standard laminar flow rate of 0.668m3/s

used in the wafer cleaning process, we can analyze our

calculation result based on economic consideration. For

the two types of hydrochloric acid we often used: HPM

(1:1:5, 80°C), DHCl (1:1000, 20°C), the saturation

residue concentration are correspondingly 12.699 ppm

and 3.28*10-4 ppm. And for Hydrogen chloride, the

safety limit is 5 ppm. Hence we can figure out for HPM,

the residue is higher than the safety limit (5 ppm), but

after the emission of acid fog halts (put the lid on), the

residue concentration would drop to the safety level

within 4 second. For DHCl the residue is 3.28*10-4 ppm,

we got already relative low ppm concentration, it is even

too low that our flow rate is set extra too high for

controlling DHCl emission.

For hydrofluoric acid, we got similar result as DHCl. The

saturation residue concentration turns to be as low as

6.54*10-3 ppm, and is far lower than the safety limit 3

ppm, which still indicates the waste of high laminar flow

rate.

For ammonia solution, we notice the saturation residue

concentration is 7.72 ppm, which is much lower than the

safety requirement 25 ppm. Therefore considering

economic reasons, we can even reduce the laminar flow

rate to 0.25 m3/s for this specific application to gain a safe

result (20.84 ppm at 0.25m3/s).

For the application in ozonated water, the maximum

saturation residue is 9.92ppm at around T=8s and then

drops down with a decreasing velocity till the lower limit

of 1.87 ppm at around T=150s, and again above the safety

limit (0.06 ppm). So we can still check the curve after we

stop the emission of dangerous gas (put the lid on) and

find out that within 13 seconds the concentration would

become lower than 0.06 ppm.

0

2

4

6

8

10

12

14

0 500 1000 1500 2000

HC

l Res

idu

e C

on

cen

tra

tio

n (p

pm

)

Time (10-1s)

1600

1640(10-1

s)

21

Acknowledgements

The author expresses his gratitude to co-promoters Alain

Pardon and Nausikaä Van Hoornick, IMEC, Belgium and

her promoter Patrick Lievens, GroepT Leuven Belgium

for comprehensive instruction.

Special thank should also be given to Kathleen

Verbraeken, Rita Vos, Jan Coenen, Miel Sledsens and Els

Vanlee from IMEC for their effort and help during the

research stage of this thesis project.

Reference

[1] http://www.mkaq.org/anquanpj/pingjiaal/200905/anquanpj_11236.html

[2] http://en.wikipedia.org/wiki/Hazard

[3] http://www.ccohs.ca/oshanswers/hsprograms/hazard_risk.html

[4] http://en.wikipedia.org/wiki/Semiconductor_device_fabrication

[5] Delfi, IMEC learning site.

[6] Luc Arkens, Judith Staginus, “Process risk assessment of semiconductor layering techniques”, master thesis, Groep T,

Leuven, 2007

[7] Peter Van Zant, “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, 5th ed, Printed and bound

by R. R. Donnelley, 2004, ISBN 0-07-143241-8.

[8] http://python.rice.edu/~arb/Courses/Images/360_02_handout9.jpg

[9] http://141.223.168.19/process.html

[10] http://en.wikipedia.org/wiki/Photolithography

[11] http://www.photochembgsu.com/applications/photoresists.html

[12] Prof, Tayo Akinwande, Lecture 13: “Wet Etching, Etching and Pattern Transfer (1)”, Fall Term 2003

[13] Shuo Wang “Process risk assessment of semiconductor Doping techniques”, master thesis, Groep T, Leuven, 2008.

[14] http://www.iue.tuwien.ac.at/phd/wittmann/node7.html

[15] http://www.gs68.de/tutorials/implant.pdf

[16] Wolf, Stanley, “Microchip Manufacturing” 2004, ISBN 0-9616721-8-8

[17] http://www.glsciences.com/products/application-specificInstruments/swa256_series.html

[18] Karen A. Reinhardt, Werner Kern, “Hand book of silicon wafer cleaning technology”, 2nd ed, USA, 2007, ISBN

978-0-8155-1554-8 (978-0-8155)

0

2

4

6

8

10

12

0 500 1000 1500 2000

Re

sid

ue

Co

nce

ntr

ati

on

(pp

m)

Time (10-1s)

1600 1732

22

[19] http://www.mksinst.com/docs/UR/40330-375.pdf, Clean Rooms, Jan 2005

[20] By measurement in the lab

[21] Personal communication with IMEC employer.

[22] http://www.worksmartontario.gov.on.ca/scripts/default.asp?contentID=2-6-1&mcategory=health

[23] James Unmack, MS, CIH and Michael Kleinman, PhD, “Darft Hydrogen Chloride Health-Based Assessment and

Recommendation for HEAC”, January 29, 2008

[24] Yajun Li, “General Analysis and Estimation Method for Fugitive Emission Resources”, October, 2005

[25] Guangqi Liu, Lianxiang Ma, and Zhiyou Xing, “Chemical Industry Physical Property Handbook”, China, 2002,

ISBN 7-5025-3593-4.

[26] http://www.tfi.org/publications/HealthAmmoniaFINAL.pdf

[27] U.S. Department of Health and Human Services. Hazardous Substances Data Bank (HSDB, online database).

National Toxicology Information Program, National Library of Medicine, Bethesda, MD. 1993.

[28] M. Sittig. “Handbook of Toxic and Hazardous Chemicals and Carcinogens”. 2nd ed. Noyes Publications, Park Ridge,

NJ. 1985.

[29] “The Merck Index. An Encyclopedia of Chemicals, Drugs, and Biologicals”. 11th ed. Ed. S. Budavari. Merck and Co.

Inc., Rahway, NJ. 1989.

[30] Braker W, Mossman AL. “Matheson gas data book”. 6th ed. Secaucus, NJ: Matheson Gas Products, Inc, 1980.

[31] Hathaway GJ, “Chemical hazards of the workplace”. 3rd ed, Proctor NH, Hughes JP, and Fischman ML, New York,

NY: Van Nostrand Reinhold, 1991.

[32] Sittig M. “Handbook of toxic and hazardous chemicals”. 3rd ed. Park Ridge, NJ: Noyes Publications, 1991.

[33] http://www.osha.gov/SLTC/healthguidelines/hydrogenchloride/recognition.html

[34] Gosselin RE, Smith RP, Hodge HC, “Clinical toxicology of commercial products”. 5th ed. Baltimore, MD: Williams

& Wilkins, 1984.

[35] http://www.osha.gov/SLTC/healthguidelines/hydrogenfluoride/recognition.html

[36] White, M.C.. “Exacerbations of childhood asthma and ozone pollution in Atlanta. Environmental Research”. 65:

56-58, 1994.

[37] Marian Fierro, “Ozone Health Effects”, First revision (JB) 12/23/99.

[38] Tan Guixia，Chen Yepu，Xu Xiaoping, “Ozone Decomposition Rules in Gaseity and Aqueous Solution”, School of

Sciences，Shanghai University, China, 2004.