Contamination
Damages:
¾ Oxide breakdown, metal
bridging, poly-Si bridging
¾ Low breakdown, VT shift,
low lifetimes, junction
leakage
¾ Change in oxidation
rates, change in
deposition rates
¾ Low breakdown voltages
because of high fields,
low mobility due to
interface scattering
• Particles
• Metallic contaminants
• Organic contaminants
• Surface roughness
Particles
Na
Cu
Photoresist
Fe
Au
N, P
SiO2 or other thin films
Interconnect Metal
Silicon Wafer
Particle Contamination and Yield
in Semiconductors
Particle Removal
• The best approach is to make sure you never get
particles on the wafer
• ≈ 75% of yield loss in modern
VLSI fabs is due to particle
contamination.
– Ultra-clean environments: Class 1 cleanrooms
• <1 particle/ft3 that is >0.12μm diameter
– FOUP box (front opening unified pod)
• Yield models depend on
information about the
distribution of particles.
– Very low particle count solutions (particles / liter of
solution)
Particle Density
0.8
0.6
0.4
0.2
- 0.1 - 0.3 µm
Probability of Particle
Causing Yield Loss
1
Particle Diameter
• Particles on the order of 0.1 - 0.3
µm are the most troublesome:
• larger particles precipitate
easily
• smaller ones coagulate into
larger particles
Cleanroom for micro and nano
fabrication
•
•
•
•
•
NH4OH
H2O2
HF
HCl
H2SO4
≥0.2μm
200
50
1
5
500
≥0.5μm
30
10
0
1
50
Cleanroom
Particle free walls, furniture,
and accessories must be used.
Airflow through 0.3 microns (or
better) filters. Positive pressure
inside clean room ensures
removal of particles.
Cleanroom for mercury program, 1960’s
Semiconductor clean room facility, today
Originally developed by NASA and the aerospace industry for satellite
manufacturing. Cleanrooms now in use for all MEMS and semiconductor
manufacturing.
1
Cleanroom Characterisitcs
Laminar Flow Processing Bench
2
Cleanroom classifications and
applications
Class 10,000 Gowning
Main function of clean rooms is control of particle contamination.
Requires control of air flow, water and chemical filtrations, human
protocol, and operational procedures.
Class 100 Gowning
Class 10,000
Printed circuit boards,
Electronic packaging,
Medical devices
Class 1,000
MEMS,
Electronic packaging,
Hard disk drives
Class 100
MEMS, RF/Photonic ICs
Class 10
Integrated Circuits
Clean Water as a Processing Chemical
Types of Water Filtration
•
Particle Filtration
–
•
• Water (H2O) is the most prevalently used material in
microfabrication processes, and is used mainly for
rinsing and cleaning of wafers.
• Approximately 6000 gallons of de-ionized (DI) water are
required for each 6” CMOS wafer.
• DI water must be manufactured on site to achieve the
quality and purity levels required by modern
microfabrication.
• Each gallon of DI water may require as much as 4-6
gallons of raw city grade water to manufacture.
• DI water must be continuously recirculated in order to
achieve the quality and purity levels.
1-75 μm cartridge filters: cellulose, fiberglass, or polypropylene fibers
Microfiltration
–
•
0.1-1.0 μm cartridge filters: ceramic or polymer membranes
Ultrafiltration
–
•
20-2000 Angstroms: chemically based
Reverse Osmosis (front opening unified )
–
1-200 Angstroms: uses special membrane and high pressure to overcome osmotic pressure
Maximum Contaminant Levels
Contaminant
Ions
Parameter
Resistivity at 25 °C/MΩ•cm
Conductivity at 25 °C/μS•cm
-1
Type I
Type II
Type III
>18.0
>1.0
>0.05
<0.056
<1.0
<20
Organics
Total Organic Content/p.p.b.
<10
<50
<200
Pyrogens
Eu/mL
<0.03
NA
NA
Particulates
size/µm
<0.2
NA
NA
Colloids
Silicia/p.p.b.
<10
<100
<1000
Bacteria
CFU/mL
<1
<100
<1000
Wafer Cleaning
• Cleaning involves removing particles, organics (photoresist) and metals from
wafer surfaces.
• Particles are largely removed by ultrasonic agitation during cleaning.
• Organics like photoresists are removed in an O2 plasma or in H2SO4/H2O2 solutions.
• The “RCA clean” is used to remove metals and any remaining organics.
• Metal cleaning can be understood in terms of the following chemistry.
Si + 2H 2 O ↔ SiO 2 + 4H + + 4e −
M ↔ M z+ + ze −
• If we have a water solution with a Si wafer and metal atoms and ions, the
stronger reaction will dominate.
• Generally (6) is driven to the left and (5) to the right so that SiO2 is formed
and M plates out on the wafer.
• Good cleaning solutions drive (6) to the right since M+ is soluble and will be
desorbed from the wafer surface.
(5)
(6)
Oxidant/
Reductant
Standard
Oxidation
Potential (volts)
1.05
0.84
Mn ↔ Mn2 + + 2e −
Si + 2H2 O ↔ SiO2 + 4H + + 4e −
3+
Cr /Cr
Ni 2 + /Ni
Fe 3+ /Fe
H2 SO 4 /H2SO3
0.71
0.25
0.17
-0.20
Cr ↔ Cr + 3e
2+
−
Ni ↔ Ni + 2e
Fe ↔ Fe3+ + 3e −
H 2 O + H 2 SO3 ↔ H 2SO 4 + 2H + + 2e −
Mn2+/Mn
SiO2/Si
Cu 2+ /Cu
O2 /H2O
-0.34
-1.23
Au3 + /Au
H 2 O 2 / H 2O
-1.42
-1.77
O3 /O2
-2.07
Oxidation-Reduction Reaction
3+
−
Cu ↔ Cu 2 + + 2e −
2H2O ↔ O2 + 4H + + 2e −
Au ↔ Au3+ + 3e −
2H2O ↔ H 2 O 2 + 2H+ + 2e −
O2 + H2O ↔ O 3 + 2H+ + 2e −
• The strongest oxidants are at the bottom (H2O2 and O3). These reactions go to
the left grabbing e- and forcing (6) to the right.
• Fundamentally the RCA clean works by using H2O2 as a strong oxidant.
3
RCA Wafer Cleaning
H2 SO4/H2 O2
1:1 to 4:1
120 - 150°C
10 min
HF/H2 O
1:10 to 1:50
Room T
1 min
DI H2O Rinse
Room T
NH4 OH/H2 O2/H2 O
1:1:5 to 0.05:1:5
SC-1
80 - 90°C
10 min
DI H2O Rinse
Room T
Strips organics
especially photoresist
• Photoresist residue is one of the largest sources of
organic contamination. There are two main ways to
remove organic residue.
Strips chemical
oxide
– Ozone/ashing→dry process that uses very strong oxidizing
agents. Essentially you burn the organic residue converting
everything to CO2
– Wet chemical process
Strips organics,
metals and particles
• RCA clean is “standard process”
used to remove organics, heavy
metals and alkali ions.
• Ultrasonic agitation is used to
dislodge particles.
HCl/H2 O2 /H2 O
1:1:6
SC-2
80 - 90°C
10 min
DI H2 O Rinse
Room T
Organic Removal
•
H2SO4:H2O2 (3:1) at 120°C (SPM)
• Both of these processes leave a chemical native oxide
on silicon.
Strips alkali ions
and metals
SC (standard cleaning) 1
• Wafer cleaning technology has been based on RCA
clean-up technology for years.*
• Sequential cleaning process is based on H2O2 solutions at
low and high pH*
• The first solution SC1 (standard cleaning 1) is based on
NH4OH:H2O2:H2O →usually 1:1:5 at 75°C.
– High pH solution removes organic materials, and some particles,
by oxidation.
– H2O2 is a very strong oxidizing agent and NH4OH acts to solvate
the oxidized material.
– The NH4+ also complexes some group I and group II metals like
Cu, Ag, Ni, and Co.
SC2
• The second part of the cleanup is the low
pH solution, SC2
– HCl:H2O2:H2O at 1:1:6 at 70°C.
– Solution is primarily used to remove heavy
metals
– The peroxide oxidizes the metal and the Cl¯
complexes it
* W. Kern, Handbook of Semiconductor Wafer Cleaning Technology (1993)
Megasonic Cleaning
• One of the more effective methods for removing particles
• Uses a transducer operating at 850-900 kHz and up to
300 W.
• Usually done in SC1 solution.
• High pH solutions work best for removing particles.
• Have to be careful to not let the [H2O2] get too low
because NH4OH will etch the silicon surface and
increase roughness.
• Other cleaning methods are HF:H2O2:H2O
and HF:H2O (≈1%HF)
– HF based cleaning solutions are particularly
good at removing Ca.
– Ca has been shown to be a bad actor in gate
oxide degradation and breakdown
– HF – last -> process leaves a H-terminated
surface which can be stable for hours, as
compared to a non-passivated Si surface
which oxidizes immediately
4
Surface Roughness
• As gate oxide thickness is reduce to 10Ǻ (65 nm
devices), micro-roughness needs to be controlled at the
atomic level.
• SC1 is particularly bad for roughening the surface
because the NH4OH etches silicon (especially if [H2O2] is
too low).
• You can reduce the effect by reducing the amount of
NH4OH, reducing the temperature, and reducing the
time→all of which reduce the effectiveness of cleaning.
• Micro-roughness reduces mobility (interface scattering)
and gate oxide reliability
Contamination Reduction:
Gettering
• Heavy metal gettering relies on:
Gettering consists of
• Metals diffusing very rapidly in silicon.
1. Making metal atoms mobile.
• Metals segregating to “trap” sites.
2. Migration of these atoms to
trapping sites.
3. Trapping of atoms.
• “Trap” sites can be created by SiO precipitates
2
(intrinsic gettering), or by backside damage
(extrinsic gettering).
• In intrinsic gettering, CZ silicon is used and SiO2
precipitates are formed in the wafer bulk through
temperature cycling at the start of the process.
SiO2 precipitates (white dots) in bulk
of wafer.
Modeling Gettering
PSG Layer
Devices in near
surface region
Aus → Au i
Denuded Zone
or Epi Layer
10 - 20 µm
• Step 1 generally happens by kicking
out the substitutional atom into an
interstitial site. One possible reaction is:
1
2
Diffusion
Intrinsic
Gettering
Region
Backside
Gettering
Region
Au S + I ↔ Au i
3
Trapping
*
3
Trapping
*
500+ µm
• 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 or to N+ regions or pair with
effective getters like P (AuP pairs).
(See text.)
• In intrinsic gettering, the metal atoms
segregate to dislocations around SiO2
precipitates.
5
6