Chapter 4 Clean room, wafer cleaning and gettering
1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.
5. Measurement methods.
NE 343: Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
1
Effect of defect and contamination
on semiconductor industry
LLS: localized light
scatters (use laser to
detect and count
particles)
GOI: gate oxide
integrity, by electrical
measurement
Requirement different
for DRAM and logic
chip, due to greater
gate insulator area on
DRAM chip.
109/cm2  0.0001%
monolayer
Importance of unwanted impurities increases with shrinking geometries of devices.
75% of the yield loss is due to defects caused by particles (1/2 of the min feature size).
2
Type of contaminants
Contaminants may consist of particles, organic films,
photoresist, heavy metals or alkali ions.
Particles
Fe
Na
Cu
Photoresist
Au
N, P
SiO2 or other thin films
Interconnect Metal
Silicon Wafer
Modern IC factories employ a three tiered approach
to controlling unwanted impurities:
1. clean factories
2. wafer cleaning
3. gettering
3
Effects on MOSFET: two examples
MOSFET threshold voltage is given by:
If tox=10nm, then a 0.1V Vth shift can be caused by Na+ or K+ of
QM=2.151011 ions /cm2 (<0.1% monolayer or 10ppm in the oxide).
0=8.8510-12F/m, ox=3.9
For MOS DRAM, refresh time of several msec
requires a generation lifetime of
This requires trap density Nt1012/cm3, or 0.02ppb
(1012/(51022)=0.02ppb).
( is trap capture cross-section, vth is minority carrier
thermal velocity; Vth107cm/sec, 10-15cm-2)
Deep-level traps (Cu, Fe, Au etc.) pile
DRAM: Dynamic Random Access Memory
up at the surface where the devices are
located. This causes leak current. Need
refresh/recharge the MOS capacitor.4
Effects of cleaning on thermal oxidation
Residual contaminants affect kinetics of
processes, here oxidation.
5
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
6
Metal contamination
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.
Fe, Cu, Ni, Cr,
W, Ti…
Na, K, Li…
Ion
implantation
Dry etching
Photoresist
removal
Fe Ni Cu
Wet oxidation
9
10
11
12
Log (concentration/cm2)
13
7
Chapter 4 Clean room, wafer cleaning and gettering
1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.
5. Measurement methods.
NE 343 Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
8
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
9
Wafer cleaning
Gettering
Clean room
Factory environment is cleaned
by:
• HEPA filters and recirculation
for the air.
• “Bunny suits” for workers.
• Filtration of chemicals and
gases.
• Manufacturing protocols.
HEPA: High Efficiency Particulate Air
• HEPA filters composed of thin porous sheets of
ultrafine glass fibers (<1m diameter).
• It is 99.97% efficient at removing particles from air.
• Room air forced through the filter at 50cm/sec.
• Large particles trapped, small ones stick to the fibers
due to electrostatic forces.
• The exit air is typically better than class 1.
10
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).
Particle diameter (m)
Class
0.1
0.3
0.5
1
35
3
1
10
350
30
10
300
100
100
5.0
1000
1000
7
10000
10000
70
100000
100000
700
by definition
11
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.
• Yield models depend on information about the description of particles.
• Particles on the order of 0.1-0.3m are the most troublesome: larger particles
precipitate easily; smaller one coagulate into larger particles.
12
Chapter 4 Clean room, wafer cleaning and gettering
1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.
5. Measurement methods.
NE 343 Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
13
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.
14
Standard RCA
cleaning procedure
RCA clean is “standard
process” used to remove
organics, heavy metals and
alkali ions.
Ultrasonic agitation is used
to dislodge particles.
SC: Standard Cleaning
RCA: Radio Corporation of
America, now makes TV,
stereos…
DI water: de-ionized water
H2SO4/H2O2
1:1 to 4:1
120 - 150ÞC
10 min
HF/H2O
1:10 to 1:50
Room T
1 min
DI H2O Rinse
Room T
NH4OH/H 2O2/H2O
1:1:5 to 0.05:1:5
SC-1
80 - 90ÞC
10 min
DI H2O Rinse
Room T
HCl/H2O2/H2O
1:1:6
SC-2
80 - 90ÞC
10 min
DI H2O Rinse
Room T
Strips organics
especially photoresist
Strips chemical
oxide
and all contaminants on
top of it, but induces H
passivated surface (bad)
Strips organics,
metals and particles
Less NH4OH will reduce
surface roughness
Strips alkali ions
and metals
not removed by SC-1
HF dip added to remove oxide
15
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.
16
Principles of metal cleaning
If we have a water solution with a Si wafer and metal atoms and ions, two reactions take
place.
Si  2H 2 O  SiO 2  4H   4e 
M  Mz  ze 
(1)
(2)

 directions, one providing electrons, which will
The two reactions will proceed in opposite
then be consumed by the other (forming an oxidation/reduction couple). In this couple, the
stronger reaction will dominate.
Generally, (2) is driven to the left and (1) to the right so that SiO2 is formed and M plates
out on the wafer.
Good cleaning solutions drive (2) to the right since M+ is soluble and will be desorbed from
the wafer surface.
17
Principles of metal cleaning
The strongest oxidants are at the bottom (H2O2 and O3). These reactions go to the left,
grabbing electrons and forcing (2) in previous slide to the right.
Fundamentally the RCA clean works by using H2O2 as a strong oxidant.
Reaction goes to the left
18
Ultrasonic cleaning and DI water
RCA cleaning with ultrasonic agitation is more effective in removing particles.
Ultrasonic cleaning:
• Highly effective for removing surface contaminants
• Mechanical agitation of cleaning fluid by high-frequency vibrations (between 20 and 45
kHz) to cause cavitation - formation of low pressure vapor bubbles that scrub the surface.
• Higher frequencies (>45kHz) form smaller bubbles, thus less effective.
• However, megasonic (1MHz) cleaning is also found effective in particle removal.
DI (de-ionized) water is used for wafer cleaning.
One monitors DI water by measuring its resistivity, which should be >18Mcm.
H2O  H++OHDiffusivity of:
of :

[H+]=[OH-] = 6x10-13cm-3
H+ ≈ 9.3x10-5cm2s-1  µH+=qD/kT=3.59cm2V-1s-1
OH- ≈ 5.3x10-5cm2s-1  µOH-=qD/kT=2.04cm2V-1s-1
q([ H  ] H 
1
 18.5Mcm
 [OH  ]OH  )
Einstein relation: µ=qD/kT, http://en.wikipedia.org/wiki/Einstein_relation_%28kinetic_theory%29
19
Other cleaning methods
Ohmi cleaning: room temperature, fewer chemicals
Dry (vapor phase) cleaning:
Energy may come from plasma, ion
beam, short-wavelength (UV)
radiation or heating.
• HF/H2O vapor cleaning
• UV-ozone cleaning (UVOC)
• H2/Ar plasma cleaning
• Thermal cleaning
20
Chapter 4 Clean room, wafer cleaning and gettering
1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.
5. Measurement methods.
NE 343 Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
21
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.
22
Gettering
3
4
5
6
7
H
1.008
3
Li
6.941
11
Na
22.99
19
K
39.10
37
Rb
85.47
55
Cs
132.9
87
Fr
223
Alkali Ions
IIA
A
III IV
4
Be
9.012
12
Mg
24.31
20
Ca
40.08
38
Sr
87.62
56
Ba
137.3
88
Ra
226
5
B
B
21
Sc
44.96
39
Y
88.91
57
La
138.9
89
Ac
227.0
22
Ti
47.88
40
Zr
91.22
72
Hf
178.5
104
Unq
261
III IV
B
Deep Level Impurites in Silicon
10.81
13
VIII
Al
B
B
B
B
B
V VI VII
I
II 26.98
23
24
25
26
28
29
30
31
27
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
50.94 51.99 54.94 55.85 58.93 58.69 63.55 65.39 69.72
41
42
43
44
45
46
47
48
49
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
92.91 95.94 98 101.1 102.9 106.4 107.9 112.4 114.8
73
74
75
76
77
78
79
80
81
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
180.8 183.9 186.2 190.2 192.2 195.1 197.0 200.6 204.4
105 106 107
Unp Unh Uns
262 263 262
Deep level impurities in silicon:
large diffusivity, easily trapped by
mechanical defects or chemical traps.
A
6
C
12.01
14
Si
28.09
32
Ge
72.59
50
Sn
118.7
82
Pb
207.2
V
A
7
N
14.01
15
P
30.97
33
As
74.92
51
Sb
121.8
83
Bi
209.0
A
VI VII
8
O
16.00
16
S
32.06
34
Se
78.96
52
Te
127.6
84
Po
209
9
A
F
19.00
17
Cl
35.45
35
Br
79.90
53
I
126.9
85
At
210
2
He
4.003
10
Ne
20.18
18
Ar
39.95
36
Kr
83.80
54
Xe
131.3
86
Rn
222
Shallow Donors
2
I
Noble
Gas es
Elemental
Semiconductors
1
1
A
Shallow Acceptors
Period
Figure 4-6 Periodic table indicating the elements that are of most concern in gettering.
23
Fast diffusion of various impurities
Diffusivity (cm2/sec)
They can diffuse from frontside to backside of the wafer
(>0.5mm distance)
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.
24
Gettering mechanism
Denuded zone
or epitaxy layer
Intrinsic
gettering region
500+m
Devices in near
surface region
10-20m
PSG layer
Backside
gettering region
25
Gettering mechanism
Gettering consists of:
1. Making metal atoms mobile.
2. Migration of these atoms to trapping sites.
3. Trapping of atoms.
Step 1 generally happens by kicking out the substitutional (s) atom into an interstitial (i)
site. One possible reaction is: (I = interstitial Si) Au S  I  Au i
Step 2 usually happens easily once the metal is interstitial since most metals diffuse rapidly
in this form.
Step 3 happens because heavy metals segregate
preferentially to damaged regions

(dislocation or stacking fault) or to N+ regions, or pair with effective getters like P (AuP).
Step 1 can be facilitated by introducing large amount of Si interstitials, by such as high
density phosphorus diffusion, ion implantation damage or SiO2 precipitation.
26
Intrinsic gettering
Out-diffusion of O
Temperature oC
1100
900
700
500
denuded zone =
oxygen free;
thickness several
tens of µm
Precipitates (size) grow @ high T
Density of nucleation sites grow @ low T
Precipitation
(growth of SiO2) Therefore, low T to increase density, and
high T to grow its size.
Slow ramp
Oxygen diffusivity:
50-100nm in size
  2.53  2
1
D0  0.13 exp 
cm sec
 kT 
D0 >> Ddopants but D0<< Dmetals
Nucleation of SiO2
1-3 nm min size of nuclei,
concentrations ≈ 1011cm-3
Time
In intrinsic gettering, the metal atoms segregate to dislocations (formed because of volume
mismatch of SiO2 and host Si lattice) around SiO2 precipitates.
15 to 20 ppm oxygen wafers are required:
<10 ppm - precipitate density is too sparse to be an effective getterer.
>20 ppm - wafers tend to warp during the high temperature process.
Note: devices that use the entire wafer as the active region (solar cells, thyristors, power
diodes, etc...) can not use this technique, but can use extrinsic gettering.
Today, most wafer manufactures perform this intrinsic gettering task that is better controlled.
27
Intrinsic Gettering: SiO2 precipitates
No SiO2 on top surface
(denuded zone)
no SiO2
SiO2
precipitates
(50-100nm)
SiO2 precipitates (white dots) in bulk Si
Figure 4-13
28
Chapter 4 Clean room, wafer cleaning and gettering
1. Introduction.
2. Clean room.
3. Wafer cleaning.
4. Gettering.
5. Measurement methods.
NE 343 Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
29
Particle contamination detection
Un-patterned wafers (blank)
• Count particles in microscope
• Laser scanning systems that give maps of particles down to ≈ 0.2 µm
Patterned wafers
• Optical system compares a die with a “known good reference” die
(adjacent die, chip design - its appearance)
• Image processing identifies defects
• Test structure (not in high volume manufacturing)
Test structures design
to detect defects
30
Monitoring the wafer cleaning efficiency
Concentrations of impurities determined by surface analysis.
Excite  Identify (unique atomic signature)  Count concentrations
works with SEM
emitted
Primary beam electron
Detected beam electron
X-ray
ions (SIMS)
ions (RBS)
He+ 1-3 MeV
O+ or Cs+
sputtering
and mass
analyses
good lateral resolution (e can be focused, but not x-ray)
good depth resolution and surface sensitivity
poor depth resolution and poor surface sensitivity
excellent
good depth resolution, reasonable sensitivity (0.1 atomic%)
31