Post on 10-Dec-2014
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ECE614: Device Modelling and Circuit
SimulationSimulation
Unit 1 Wafer Cleaning
By Dr. Ghanshyam Singh
Sharda University
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
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
Photolithography
Etching/Deposition
Device Testing
Wafer Cleaning
• Wafer Cleaning simply means
– “Get rid of particles and
contamination”
This is the most important step
What to remove? How?
Contamination Possible sources Effects
Particles Equipment, ambient, gas,deionised (DI) water, chemical
Low oxide breakdown, Poly-Siand metal bridging-induced lowyield
Metal Equipment, chemical, reactiveion etching (RIE), implantation
Low breakdown field, Junctionleakage, Reduced lifetimeion etching (RIE), implantation
ashingleakage, Reduced lifetime
Organic Vapour, residue of photoresist,storage containers, chemical
Change in oxidation rate
Micro-roughness Initial wafer material, chemical Low oxide breakdown fieldLow mobility of carrier
Native oxide Ambient moisture, DI waterrinse
Degraded gate oxideLow quality of epilayerHigh contact resistance
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 particlescontamination 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
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
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
wettingTransdu
cer
SC1
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
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
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.
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 flat requirement 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
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 Section
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)
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
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
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
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 Hydrophilicsurface
– 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
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
Wafer Priming
• Types of priming
– Immersion priming (simplest way)• lack of control and not free from contamination
• Expensive
– Spin priming
– vapour primingPR
– 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
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.
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
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%