Wafer cleaning


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Wafer cleaning

  1. 1. ECE614: Device Modelling and Circuit SimulationSimulation Unit 1 Wafer Cleaning By Dr. Ghanshyam Singh Sharda University
  2. 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. 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. 4. Wafer Cleaning • Wafer Cleaning simply means – “Get rid of particles and contamination” This is the most important step
  5. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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%