SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 3: WET CLEAN
SEMICONDUCTOR PROCESS TECHNOLOGY
LAB MODULE 3: WET CLEAN
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 3: WET CLEAN
THEORY
Introduction
The Need for Wafer Cleaning
The need for scrupulously clean wafers in the fabrication of microelectronic devices has been
recognized since the dawn of solid-state device technology. Clean substrate surfaces are critical in
VLSI/ULSI fabrication for obtaining maximum device performance, long term reliability, and
high yields. Cleaning techiques are used to remove particulate and chemical impurities so that
pristine surfaces can be obtained. Such techniques must be able to do this without damaging the
wafer surface. Cleaning procedures should also be safe, simple, economical, and produce a
minimum of enviromentally hazardous waste products.
The three general categories of wafer contaminants are: 1) particulates; 2) films; and 3) trace
quantities of contaminant substances in molecular or atomic form. Particulates are any bits of
material present on a wafer surface that have readily defineable boundaries. As feature sizes
shrink, the sizes of particulates that can cause defects also decrease.
Layers of foreign material on wafer surfaces are sources of film contamination. Portion of films
may, however, break loose and become particles, as often happens with photoresist residues.
Metallic and ionic contaminants are often deposited during immersion of wafers in etchant or
resist stripper baths, both of which may contain metal ions and traces of free metal in solution.
Wafer Cleaning Process
Since several classes of contamination exist, there are separate cleaning procedures required to
remove them. Some cleaning procedures are effective in removing more than one class of
contaminant. Both chemical cleaning procedures and particulate cleaning techniques must be
employed to produce a completely clean surface. When one technique follows another, the latter
steps must not re-contaminate the surface nor degrade the effectiveness of former cleaning
procedures.
1. Film Contaminants Removal
Wet chemical cleaning has been the standard technique used to to remove the chemically bonded
films and contaminant molecules and atoms from wafer surface. The goals of chemical cleaning
are defined clearly in the SIA National Technology Roadmap, which separately defines wafer
surface preparation requirements for front-end-of-line (FEOL) and back-end-of-line (BEOL)
processes. The FEOL is defined as the steps that begin with a starting wafer up to first metal
contact etch, and BEOL is defined as all process steps from that point on.
1.1 FEOL Wet Cleaning - RCA
It is assumed that wafer processed up to the end of FEOL contain only silicon, silicon dioxide,
and silicon nitride on their surface prior to high-temperature processes. Consequently, up to this
point, the wafer cleaning operations can utilize highly reactive chemicals that do not attack these
corrosion-resistance materials.
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When bare silicon, or silicon wafer with only thermally grown oxide is chemically cleaned prior
to the furnace process, a two-step cleaning procedure formulated by Kern and Puotinen at RCA is
widely used (and hence is often termed the RCA Clean). The first step in the RCA is called
Standard Clean-1 (SC-1). Its function is to remove organic film contamination, some metals (Au,
Ag, Cu, Ni, Cd and Cr), and particles.
The second step is called Standard Clean-2 (SC-2). Its function is to remove inorganic ions, alkali
ions, and heavy metals.
1.2 BEOL Wet Cleaning
Once metal is deposited on a wafer, the aggressive acids and alkali solutions used in the RCA
clean can no longer be used (since they would rapidly attack the metal). Instead, cleaning must be
done with less reactive solvents. The most commonly used solvent in BEOL cleaning is nmethyl- pyrrolidone (NMP). Such solvents are relatively expensive and their disposal costs are
also quite high.
2. Photoresist Removal
Photoresist must be removed following a veriety of processing steps, including: 1) etching (wet or
dry); 2) ion implantation; 3) photolithography rework. The main objective in resist stripping is to
insure that all the photoresist is removed as quickly as possible without attacking any underlying
surface materials. In fact, no single resist stripping technique is suitable for all applications.
Resist stripping techniques are thus divided into three classes: 1) inorganic strippers; 2) organic
strippers; 3) dry stripping.
Inorganic strippers are solutions of H2SO4 and an oxidant, heated to around 125 ºC (e.g. Piranha
solution). The oxidant typially used is H2O2. It oxidizes the carbon in the resist to form Co2,
which exits from the bath as a gas.
Organic strippers perform resist removal by breaking down the structure of the resist layer. Dry
photoresist strippers (also called plasma resist strip) is done in plasma resist-stripping tools, with
oxygen being the process gas. This offers several advantages over wet resist stripping including
safer operating conditions, no metal contamination, and no attack of most underlying substrate
materials.
3. Particle Removal
Particles are deposited on wafers as aerosols from the air or as particles present in the liquids in
which wafers are immersed. The basis of their removal rests on an understanding of how such
deposited particles adhere to wafer surfaces. If a solid particle wanders close enough to the
vicinity of a solid surface, then van der Waals force can attract and capture it. The particle is then
physisorbed on the surface. Other forces that physisorb particles include static charge on the
particles, electrostatic double-layer repulsion (EDR), and capillary adhesion, but the van der
Waals (VDW) force is usually the strongest.
There are three methods used to remove physisorbed particles: 1) chemically assisted removal
(particle dissolution, particle oxidation, and lift-off by slight etching of the surface; 2) electrical
repulsion of the particle and the surface; 3) mechanical dislodgement.
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The chemically assisted particle removal techniques are implemented in some wet cleaning
procedures. For example, in the SC-1 step of RCA clean, several chemically-assisted particle
removal mechanisms are believed to work in tandem. That is, the SC-1 solution removes organic
particles by dissolution. It removes other type of particles by slightly simultaneously oxidizing
and etching the silicon surface. The SC-2 process dissolves certain types of metal particles.
However, as it does not attack the oxide, it does not remove particles as well as SC-1.
The mechanical removal of particles generally proceeds by momentum transfer. That is, shear
force impact the particle and dislodge it by either sliding it off the surface, or by rotating it until it
detaches. Mechanical particle removal techniques have traditionally involved: 1) vibrational
scrubbing (ultrasonic or megasonic); and 2) a technique that combines high-pressure liquid
spraying and mechanical scrubbing.
4. Rinsing and Drying
After the cleaning chemicals perform their function, they must be removed from the wafer
surface. This is typically done by rinsing the wafers with DI water and then drying them. Rinsing
and drying also represent critical steps in the cleaning sequence since they are performed so
frequently. That is, rinse tanks and dryers may become major sources of particulate
contamination unless they are monitored and properly maintained. In addition, complete removal
of the cleaning chemicals cannot be done by just dunking the wafers in a tank of water. Thorough
rinsing requires a continuous supply of clean water to the wafer surface. The resistivity of the
effluent rinse water is normally monitored, and rinsing is continued until it exhibit the 18 MΩ-cm
reistivity of pure DI water.
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SEMICONDUCTOR PROCESS TECHNOLOGY
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Experiment: Wet Cleaning Process
Objective:
In this experiment, student will carry out three different type of cleaning. At the end of this
experiment, student shall be able:
1. To describe the purpose of wet cleaning process.
2. To observe any defects/contamination on wafer surface.
Equipment / Chemicals
i.
Wet Bench/ Fume Hood
ii.
Hot Plate
iii.
Thermometer
iv.
Chemical Solution, RCA 1
v.
Chemical Solution, RCA 2
vi.
Buffered Oxide Etch, BOE
vii.
DI Water
viii.
Tweezer
Characterization/Testing
1. Particle/defect/color inspection
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SEMICONDUCTOR PROCESS TECHNOLOGY
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Wet Cleaning Process Flow Run Card
Group:
Name:
Lot Number:
Exp No:
Orientation:
Size:
Resistivity:
Lot Start Date:
LP#
Equipment
Wafer Preparation
1
Thickness:
Planner:
Process/Recipe
Time
Out
1. Take on wafer.
Wet Cleaning RCA 1
2
1. Prepare RCA 1 solution
HP
Chemical: NH4OH:H2O2: DIW
Ratio: 1:1:5
2. Heat RCA 1 solution
T:75C
3. Immerse the wafer into RCA 1
t: 10min
4. Prepare next solution.
Wet Cleaning RCA 1
2
1. Prepare RCA 2 solution
HP
Chemical: HCl: H2O2: DIW
Ratio: 1:1:6
2. Heat RCA 2 solution
T:80C
3. Immerse the wafer into RCA 2
t: 10min
4. Rinse in DI water.
t: 15s
5. Spin dry.
t: 15s
Wet Cleaning BOE
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INSEP
Date:
Substrate Type:
Start Wafer Quantity:
Authorized by:
Data
out
Remarks
SEMICONDUCTOR PROCESS TECHNOLOGY
3
WB
1.
Immerse
the
MODULE 3: WET CLEAN
wafer
into
Buffered Oxide Etch (BOE).
2. Rinse in DI water.
t: 15s
3. Spin dry.
t: 15s
Visual Observation
4
HOM
1. Observe the wafer.
2. Record any particle/scratches.
Results and Discussion
1. Explain the principle of wet etching.
2. What is the purpose of RCA 1 cleaning?
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SEMICONDUCTOR PROCESS TECHNOLOGY
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3. What is the purpose of RCA 2 cleaning?
4. Explain the need of a new wafer need to be clean using Hydrofluoric acid (HF) or Buffered
Oxide Etch (BOE).
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