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, <[email protected]> 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.
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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, <[email protected]>
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