The document discusses wafer cleaning techniques which are the most important step in the wafer fabrication process. It describes various sources of contamination on wafer surfaces like particles, metals, organics and how they can affect device performance. Standard cleaning procedures using wet chemical solutions like SC1, SC2, sulfuric acid mixtures and advanced techniques like megasonic cleaning are explained. The key steps of cleaning, rinsing with deionized water and drying the wafers are outlined. Priming the wafer surface before photoresist coating is also summarized.

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

3. Device/Wafer
Fabrication
• Fabrication Processes
– Step 1: Wafer Surface Preparation and Cleaning
– Step 2: Photoresist Deposition
– Step 3: Photoresist Soft Baking
– Step 4: Lithography: Alignment and Photoresist Exposure
(More)
– Step 5: Photoresist Development– Step 5: Photoresist Development
– Step 6: Photoresist Hard baking
– Step 7: Development Inspection
– Step 8: Etching/Deposition (More)
– Step 9: Photoresist Removal / Stripping
– Step 10: Final Inspection / Device Testing
recap
Cleaning
Photolithograp
hy
Etching/Depos
ition
Device
Testing

4. Wafer Cleaning
• Wafer Cleaning simply means
– “Get rid of particles and
contamination”
This is the most
important step

5. What to remove? How?
Contamination Possiblesources Effects
Particles Equipment,ambient,gas,
deionised(DI)water,chemical
Lowoxidebreakdown,Poly-Si
andmetalbridging-inducedlow
yield
Metal Equipment,chemical,reactive
ionetching(RIE),implantation
Lowbreakdownfield,Junction
leakage,Reducedlifetimeionetching(RIE),implantation
ashing
leakage,Reducedlifetime
Organic Vapour,residueofphotoresist,
storagecontainers,chemical
Changeinoxidationrate
Micro-roughness Initialwafermaterial,chemical Lowoxidebreakdownfield
Lowmobilityofcarrier
Nativeoxide Ambientmoisture,DIwater
rinse
Degradedgateoxide
Lowqualityofepilayer
Highcontactresistance

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

23. Cleaning Comparison
IMEC +RCA(spray)+HF(bath) 80%
Cleaning methods Yields of
the gate oxide
IPA: isopropyl alcohol
IMEC +RCA(spray)+HF(bath) 80%
IMEC +RCA(spray) 65%
RCA (bath) 60%
IMEC(bath) 83%