1. ECE614: Device
Modelling and Circuit
SimulationSimulation
Unit 1 Wafer Cleaning
By Dr. Ghanshyam Singh
Sharda University
2. Outline
• Section 1:
– Sources of Contaminations
– Problems
– How to rectify? How to clean?– How to rectify? How to clean?
• Section 2:
– Wet Chemical Solutions
– Cleaning Techniques
– Wafer Priming
6. How?
Since 1960s…….
SC1(RCA): NH4OH-H2O2-H2O (1:1:5 to1:2:7) @
70-80 ºC
29% 30%
At a high pH, SC1 attacks organic and particlesAt a high pH, SC1 attacks organic and particles
contamination by oxidation
SPM : H2SO4(98%)-H2O2(30%) (4:1)
SC2: HC1- H2O2-H2O (1:1:6 to1:2:8) @ 70-80 ºC
37% 30%
At a low pH, SC2 can remove metal contamination by
forming a soluble complex
*SPM: Sulphuric peroxide mixture
7. Particles
Oxidising
In an alkaline
OH- provide
SC 1 /
SC 2
The most effective commercial method is the
Megasonic cleaning process+SC1---> remove
organic and inorganic particles at temperature
of 40 ºC
In an alkaline
solution
electric
repulsion
H2O2 oxidise the
surface
OH provide
the negative
charge
8. Megasonic
• 700-1200 kHz
(ultrasonic<400kHz)
• Generated using a ceramic,
piezoelectric crystal, which is
excited by a high-frequency
AC voltage.
• SMALLER BUBLES THAN
ULTRASONIC
• Significantly reduces the risk
of surface damageof surface damage
• Removing 0.15 µm particle
• Not for large particles
• NO CONTACT ,
BRUSHLESS
wetting
Transdu
cer
SC
1
9. Particle Cleaning
• Use of high pressure Nitrogen
gas from a handheld gun
(Blow-off)(Blow-off)
• Mechanical wafer surface
scrubbers
– Expensive
• High-pressure water cleaning
10. Metal Contamination
• Sources : RIE etching, chemical solution,
ion implantation
• Problems:
– Induce leakage current of p-n junctions
– Reduce minority carrier lifetime– Reduce minority carrier lifetime
– fault built up during regrow
• Wet Cleaning: dilute HF(0.5%)-
H2O2(10%) and SC1 SC2
11. Organic Contamination
• Sources : Vapour, photoresist, containers,
chemical, fingerprints (oil),
carbon(compound)
• Problems:
– Incomplete cleaning of surface, leaving– Incomplete cleaning of surface, leaving
contaminations such as native oxide or metal
impurities
– micromasking-->RIE process
– photoresist is the main contamination source in
IC processes
• Cleaning process:
– Ozone-injected ultrapure water(strong oxidising
agent O3)
– Acetone and alcohol (simple way)
Native Oxide: own oxide of the solid; e.g. SiO2 in the case of silicon and
Al2O3 in the case of aluminum.
12. Surface Microroughness
• Surface microroughness is an important
factor in the manufacture of high
performance and quality devices
• For growing 100Å thin film, the surface
requirement should be atomically flatrequirement should be atomically flat
• Sources : SC1 cleaning, NH4OH (etchant) ,
H2O2(oxidant)
• Problems: low performance, low yield
• Problem solving:
– reduce the proportion of NH4OH, temperature,
time
– use SPM and SC 2
13. Native Oxide
• Sources : Oxidation, exposure time,
organic contamination, metallic impurities
• 7 days of exposure to cleanroom air for
silicon----> 6.7 Å
• Problems:• Problems:
– Uncontrollable ultrathin oxide growth, high
contact resistance, hard for epitaxial growths
(MBE and MOCVD)
– A problem for high performance devices.
• Problem solving:
– shorten processing time
– HF(0.5%)-H2O2(10%)
– Etching
own oxide of the solid; e.g. SiO2 in the case of silicon and Al2O3 in the case of aluminum.
End
of
Secti
on
14. Wet-chemical cleaning
Techniques
• Sulfuric acid:
– The most common chemical cleaning solution is
hot (90-125ºC) sulfuric acid, removing most
inorganic residues and particles.
• Sulfuric acid+H2O2 : (Very effective)
– Oxidants are added to remove organic residues.– Oxidants are added to remove organic residues.
C+O2--->CO2 (gas)
– Photoresist stripper
• Sulfuric acid + (NH4)2SO4:
– A drawback of adding H2O2 (strong oxidiser).
Precaution needed.
– H2O2 decays rapidly
• Standard procedures:
– Chemical cleaning-->DI water rinsing--
>Drying (Nitrogen blow-off + baking)
15. Cleaning Techniques
• Immersion Cleaning: common, easy
– Expensive (lot of chemical solutions)
– Present potential recontamination
– Cannot reaches smaller and deeper pattern/structure
• Spray Cleaning:
– Chemical costs are decreased (Spray, less chemical)– Chemical costs are decreased (Spray, less chemical)
– Free from recontamination
– Cleaning efficiency improved due to high pressure of
the spray assists in cleaning small patterns and holes
– Immediate spray rinsing in one station (save time)
• Dry Cleaning: (to rectify some problems in wet tech. I.e.
Particle generation/drying)
– Ultraviolet-ozone clean, vapor clean, plasma,
thermal
16. Water Rinsing
• Wet cleaning chemicals can also be contaminants if
left on the surface.
• Therefore, chemical cleanings follow with DI water
rinsing
• DI water rinsing also serves after etching
• Overflow rinsers
• Continuous supply of DI water
• Enhanced by a stream of nitrogen bubbles
• Minimum 5 min
• flow rate =5 times the volume of the rinser /min
• DI water 18 ohm
• 15-18 ohm on exit side (Cleaned)
• Rinse time is determined by measuring the resistivity
of the water as it exists the rinser.
N
2
DI
water
17. Water Rinsing
• Cascaded rinser
– Two or three overflow rinsers connected to each
other
– Water enter only the end rinser and cascades through
the downstream rinsers
– Very efficient when several boats of wafers are being
rinsed simultaneouslyrinsed simultaneously
• Sonic assisted cleaning/rinsing
– Adding ultrasonic/megasonic
– Cavitation effect
– Speed up wetting process
N
2
DI
water
18. Drying
• Nitrogen blow off: Remove residue water droplets
– Remaining water may interfere with any subsequent
operation
• Spin-rinse dryers (SRDs)
– Complete drying is accomplished in a centrifuge like
equipment.
Hydrophobic Hydrophilic
surface
– Start with rinsing of DI water(slow rpm), then
Heated N2 injection (high rpm)
• Vacuum Dehydration baking
– Before applying photoresist(PR)
– Hydrophilic : When exposed to moistures
– PR adheres well on a hydrophobic surface
– PR cannot adhere on a hydrophilic surface
– Temperature ~200ºC for 30min
– Can be used if delay in substrate preparation
(growing, sputtering)
Water and heated N2
Wafer cassette
19. Wafer Priming
• A process where wafers are exposed to a vapour of
HMDS to prime the wafer surface prior to
photoresist coating. (Silicon)
• Primers form bonds with surface and produce a
polar (electrostatic) surface
• Resist adhesion factors• Resist adhesion factors
• Moisture content on surface
• wetting characteristics of resist
• type of primer
• delay in exposure and softbake
• surface smoothness
• stress from coating process
• surface contamination
Ideally want no
H2O on wafer
surface
15 min 80-90 ºC
in convention
oven
20. Wafer Priming
• Types of priming
– Immersion priming (simplest way)
• lack of control and not free from contamination
• Expensive
– Spin priming
– vapour priming
PR
– vapour priming
• Free from contamination
• Cheap
• For GaAs wafer:
– No necessarily, because GaAs already has a polar
surface
vacuum
PR
Primer
wafer
chuck
End of section
21. Standard Cleaning
Procedures
1. A hot H2SO4: H2O2 (2:1 to 3:1) at
120 ºC mixture is used to remove the
greasy contamination, which may be
from the cassette or residues from the
photoresist layers.photoresist layers.
2. SC1
3. After the SC1 process, a 15 s
immersion in 1% HF-H2O solution
may be beneficial for removing any
trace impurity.
22. Advanced Cleaning
Procedures
1. H2O +O3 : organic contamination
2. NH4OH+H2O2 (0.05:1:5): Particle, organic
and metallic impurities
3. HF+H2O2 (0.5%: 10%): Native oxide,
metallic impurities
4. Ultrapure water: Rinsing
IMEC
1. H2SO4+ H2O2 (4:1) @ 90ºC for 10 min
2. HF(0.5%)/IPA(0.1%) @ room temperature
for 2 min
4. Ultrapure water: Rinsing
IPA: isopropyl alcohol
