Chapter 3:

Clean room, wafer cleaning and gettering

1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.

1
 Introduction
Yield improvement is the biggest challenge in integrated circuit fabrication. This has
become even more important now, since device dimensions are currently in the nm
range.

Cause of Contamination

Figure 1: Defects between (a) metal lines and (b) on

the surface of a wafer. Surface defects can affect the
growth of new layers while defects between
metal lines can cause electrical shorts. Adapted from
Microchip fabrication- Peter van Zant.
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Introduction

3
 Type of contaminants
Contaminants can be divided into 4 main classes:
1. Particles
2. Metal Ions
3. Chemicals
4. Airborne Molecular Contaminants

4
Particle contaminants
Particle sources: air, people, equipment and
chemicals.
A typical person emits 5-10 million particles per
minute.
Particle density (number/ml)
for ULSI grade chemicals

>0.2m >0.5m
NH4OH 130-240 15-30

H2O2 20-100 5-20

HF 0-1 0

HCl 2-7 1-2

H2SO4 180-1150 10-80

ULSI: ultra-large-scale integration

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 Metal Ion contamination
Dopant concentrations in semiconductors are very small, of the order of 1015 to 1017 ions
per cm3 (typically ppm or ppb). Presence of electrically active impurities or contaminants
can alter device performance. These impurities are called mobile ionic contaminants
(MICs). These are ions that have high mobility in the semiconductor. They can cause failure
even after packaging (they might not be detected during sort). Sodium is the most common
MIC, which is commonly found in chemical sources.

Sources:
chemicals, ion implantation,
reactive ion etching, resist
removal, oxidation.

Effects: defects at interface

degrade device; leads to leak
current of p-n junction, reduces
minority carrier life time.

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 Chemicals
- Another source of contaminants are unwanted chemicals that contaminate
process chemicals and deionized water that are used in various steps in the
fabrication process. They can affect the regular processing e.g. contamination in
the etchant can cause non uniform etching or change the etching rate.
- Chlorine is a common contaminant that is found in these chemicals.
Bacteria is another common contaminant that can grow on unwashed surfaces.
These can act as particulate contaminants and also as a source of
metallic ions.

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 Airborne molecular contaminants
Airborne molecular contaminants (AMCs) are contaminants from process
tools or chemical delivery systems. They enter the fabrication area and
cause defects on the wafers. AMCs can be gases, dopants, process
chemicals, moisture, and/or organics. A common source of AMCs is during
wafer transfer in the in the fab. Wafer transfer and storage usually happens
through FOUPs (front opening universal pods). The FOUP is a plastic
container with grooves for holding wafers and outgassing of the FOUP can
contaminate the wafers. Thus, wafers stored in the fab for long time can
pick up dust just by sitting in these FOUPs. One solution is to use nitrogen
purged FOUPs to minimize dust particles.

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Airborne molecular contaminants

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Chapter 4 Clean room, wafer cleaning and gettering

1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.

10
 Contamination Problems and sources
The presence of contaminants can cause three major effects:
1. Device yield: this is the most obvious effect and can be easily be
detected. Contaminants can cause the die to fail electrical tests and
thus reduce yield.
2. Device performance: contamination can cause a lowering of device
performance with time. This is a more serious problem because it causes
lowering of device life.
3. Device reliability: this is the hardest to detect because this can lead to
failure in service. Sometimes, it might not even be detected during
electrical testing.

The general sources of contamination are

1. Air
2. Fabrication facility
3. Cleanroom personnel
4. Process water, chemicals, and gas
5. Static charge.
6. Process equipment. 11
 Clean factory is the first approach against contamination
Modern IC factories employ a three tiered approach to
controlling unwanted impurities:
1. clean factories
2. wafer cleaning
3. gettering

Clean factory Wafer cleaning Gettering

12
 Clean room

Factory environment is cleaned HEPA: High Efficiency Particulate Air

by: • HEPA filters composed of thin porous sheets of
• HEPA filters and recirculation ultrafine glass fibers (<1m diameter).
for the air. • It is 99.97% efficient at removing particles from air.
• “Bunny suits” for workers. • Large particles trapped, small ones stick to the
• Filtration of chemicals and fibers due to electrostatic forces.
gases. • The exit air is typically better than class 1.
• Manufacturing protocols.

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Clean room

14
 Class of a clean room
• Air quality is measured by the “class” of the facility.
• Class 1-100,000 mean number of particles, greater than 0.5m, in a cubit foot of air.
• A typical office building is about class 100,000.
• The particle size that is of most concern is 10nm – 10m. Particles <10nm tend to
coagulate into large ones; those >10m are heavy and precipitate quickly.
• Particles deposit on surfaces by Brownian motion (most important for those <0.5m)
and gravitational sedimentation (for larger ones).
FED-STD-209E (federal standard)
Particle diameter (m)
Class 0.1 0.3 0.5 5.0
1 35 3 1
10 350 30 10
100 300 100
1000 1000 7
10000 10000 70
100000 100000 700
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Class of a clean room

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 Particle contamination and yield
• Generally, particles on the order of the technology minimum features size or
larger will cause defect.
• 75 yield loss in modern VLSI fabrication facilities is due to particle
contamination.
• Particles on the order of 0.1-0.3m are the most troublesome: larger particles
precipitate easily; smaller one coagulate into larger particles.

Precipitate: lắng xuống

Coagulate: kết lại
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Chapter 4 Clean room, wafer cleaning and gettering

1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.

18
 Modern wafer cleaning
• Cleaning involves removing particles, organics and metals from wafer surfaces.
• Particles are largely removed by ultrasonic agitation during cleaning.
• Organics (photoresist) are removed in O2 plasma or in H2SO4/H2O2 (Piranha) solutions.
• The “RCA clean” is used to remove metals and any remaining organics.

A cassette of wafers

Typical person emit 5-10

million particle per
minute.
Most modern IC plants
use robots for wafer
handling.
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 Standard RCA H2SO4/H2O2
1:1 to 4:1
120 - 150ÞC
10 min
Strips organics
especially photoresist
cleaning procedure
HF/H2O Room T Strips chemical
RCA clean is “standard 1:10 to 1:50 1 min oxide
process” used to remove and all contaminants on
top of it, but induces H
organics, heavy metals and passivated surface (bad)
alkali ions. DI H2O Rinse Room T

Ultrasonic agitation is used

to dislodge particles. NH4OH/H2O2/H2O 80 - 90ÞC Strips organics,
SC: Standard Cleaning 1:1:5 to 0.05:1:5 10 min metals and particles
SC-1 Less NH4OH will reduce
RCA: Radio Corporation of surface roughness
America, now makes TV,
stereos… DI H2O Rinse Room T

HCl/H2O2/H2O 80 - 90ÞC Strips alkali ions

1:1:6 10 min and metals
SC-2 not removed by SC-1

DI water: de-ionized water HF dip added to remove oxide

DI H2O Rinse Room T 20
 Standard cleaning (SC)
SC-1:
NH4OH(28%):H2O2(30%):H2O=1:1:5 - 1:2:7; 70-80C, 10min, high pH.
• Oxidize organic contamination (form CO2, H2O…)
• Form complex such as Cu(NH3)4+2 with metals (IB, IIB, Au, Ag, Cu, Ni, Zn, Cd, Co, Cr).
• Slowly dissolve native oxide and grow back new oxide, which removes particles on
oxide.
• But NH4OH etches Si and make the surface rough, thus less NH4OH is used today.

SC-2:
HCl(73%):H2O2(30%):H2O=1:1:6 - 1:2:8; 70 - 80C; 10min, low pH.
• Remove alkali ions and cations like Al+3, Fe+3 and Mg+2 that form NH4OH insoluble
hydroxides in basic solutions like SC-1.
• These metals precipitate onto wafer surface in the SC-1 solution, while they form
soluble complexes in SC-2 solution.
• SC-2 also complete the removal of metallic contaminates such as Au that may not
have been completely removed by SC-1 step.
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 Deionized water system
Water is used extensively in fabrication e.g. in any cleaning process. Typically,
wafers are rinsed in water to remove excess chemicals, after a process step, and
then dried in nitrogen. This procedure is repeated in many stages during the
fabrication process. Normal water contains dissolved minerals, particulates,
organics and dissolved gases that make it unsuitable for use in the fab. Dissolved
minerals can contain electrically active ions, simple common salt contains Na+ ions,
an MIC, that can destroy the wafer functionality, while particulates and organics can
increase contamination. These chemicals have to be removed before use in the fab
and the purified water used is called deionized water (DI water). The purity of DI
water is given by its resistivity.

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Chapter 4 Clean room, wafer cleaning and gettering

1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.

23
 Gettering
Gettering is defined as the process of removing device-degrading impurities from
the active circuit regions of the wafer. Gettering, which can be performed during
crystal growth or in subsequent wafer fabrication steps, is an important ingredient
for enhancing the yield of VLSI manufacturing

The general mechanism by which

PSG layer
gettering removes impurities from
device regions may be described Devices in
by the following steps: near
1) the impurities to be gettered surface
are released into solid solution region
from whatever precipitate they're Denuded
in; zone or

10-20m
500+m
2) they undergo diffusion through epitaxy
the silicon; layer
3) they are trapped by defects
such as dislocations or
precipitates in an area away from
device regions.
Intrinsic
gettering
region 24
 Gettering
• For the alkali ions, gettering generally uses dielectric layers on the topside; PSG for
trapping, or Si3N4 layer for blocking them from getting into the device region.
• For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk.
Here gettering works because the metals (Au…) do not “fit” in the silicon lattice easily
because of their very different atomic size, thus they prefer to stay at defect sites.
• Therefore, the idea of gettering is to create such defect sites outside of active device
region.
• Backside = external gettering: roughing/damaging the backside of the wafer, or depositing
a poly-silicon layer, to provide a low energy “sink” for impurities.
• Bulk = intrinsic (or internal) gettering: using internal defects to trap impurities, thus moving
them away from the active region of the wafer.

PSG: phosphosilicate glass, is a P2O5/SiO2 glass that is normally deposited by CVD,

usually contains 5% by weight phosphorus.
PSG traps alkali ions (Na+, K+) and form stable compounds.
At higher than room temperature, alkali ions can diffuse into PSG from device region
and trapped there.
Problems with PSG: it affects electric fields since dipoles exist in PSG, and it absorb
water, leading to Al corrosion. 25
 Fast diffusion of various impurities
They can diffuse from front-
side to backside of the wafer
(>0.5mm distance)
Diffusivity (cm2/sec)

Those metal diffuses fast

because they do so as
interstitials.
Whereas dopants are
substitutional and diffuse by
interacting with point defects.

I: interstitial
S: substitutional

Heavy metal gettering relies on metal’s very high diffusivity (when in

interstitial sites) in silicon, and its preference to segregate to “trap” sites.
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