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Chapter Extra-2 Micro-fabrication process• Si wafer fabrication • IC fabrication
• Micro-machining processes (MEMS-specific) Si based: Bulk vs. Surface micro-machining DRIE: Deep reactive ion etch Non-Si based: LIGA & Stereo lithography
(*) 최근의 추세 : Without Lithography… Why? LG 전자 : FIPA (?) 공정 : 이승기 박사
Example of MEMS products
Surface micro-machiningproduct
LIGA product
Process flow of IC & MEMS fabrication
Deposition Lithography Etch
Wafers
Chips
• Processes of IC and MEMS are almost same• Process complexity/yield depends on repetition of central loop
Silicon wafer fabrication
Czochralski process: widely-used to make single crystal Si
Silicon wafer fabrication – slicing & polishing
Smart cutting process? CMP is used.. Why?
Deposition processes- Issues of deposition : Compatibility, Conformability- Process:
- SiO2 : most common - Si3N4, polysilicon, metals etc…
• Process - Thermal oxidation, Evaporation, Sputtering, CVD
Spin Casting/Coating
• Viscous liquid is poured on center of wafer• Wafer spins at 1000-5000 RPM for ~30s (thickness control)• Baked on hotplate 80-500oC for 10-1000s (volume reduction by 1/2)• Application of etchants and solvents, rinsing• Deposition of polymers, sol-gel precursors (SOG)
E-beam
substrate
target
heating
Vacuum
(1) Heating target with desired material to evaporate in the vacuum chamber(2) Thin film is formed on the substrate
Disadvantage: high temperature, high vacuum
Physical Vapor Deposition - Evaporation
Physical Vapor Deposition - Sputtering
• Sputtered metals and dielectrics– Argon plasma sputters material (small #s of atoms) off target– Ejected material takes ballistic path to wafers
• Typically line-of-sight from a distributed source• Requires high vacuum depending on material
Mechanism: Physical process by impact of ions (plasma state)(1) impacting target surface with accelerated ions (Ar+)(2) knocking out atoms from the target surface (3) transporting atoms to the substrate for deposition(4) spin the substrate to achieve uniform thickness
Ar+
target
substrateatoms
Plasma
-
+
RF source
Chemical Vapor Deposition - CVD
To exhaust system
Wafers
SusceptorN2 H2SiCl4+ H2
HCl Dopant+ H2
Process(1) Gas phase is injected into the chamber(2) Thermal decomposition and/or reaction of gaseous compounds occur on the substrate surface(3) Desired material is deposited directly from the gas phase to form thin layer
LithographyPattern transfer: transferring a mask pattern onto wafer
Procedure
(1) Deposit barrier layer (SiO2, Si3N4, metal, etc.)(2) Coat with PR(3) Soft baking (curing)(4) Align mask(5) Expose pattern and develop PR(6) Hard baking and Etching(7) Remove PR
- Shorter wavelength for higher resolution (e.g. UV) - UV is difficult to use for nano-scale due to diffraction - X-ray or electron beam or EUV for finer resolution (on-going research topic)
Etching process
- Classification: (Wet vs. Dry), (Isotropic vs. Anisotropic)
- Wet vs. Dry etchingWet etching : liquid etchantDry etching : gas or plasma
Physical vs. Chemical Plasma, Sputter, RIE
- Issues of etching : Anisotropy, Selectivity
• Isotropic etchants etch at the same rate in every direction
• Developed during 1980s at Research Center KarlsruheDeveloped during 1980s at Research Center Karlsruhe
• Possible to produce microstructures with Possible to produce microstructures with very highvery high aspect ratios (up to 100),aspect ratios (up to 100), very small structures (in the submicron range),very small structures (in the submicron range), and with very smooth walls (surface roughness and with very smooth walls (surface roughness < < 50 nm)50 nm)
• To manufacture a tool in a molding step to replicate the microstructure of polymerTo manufacture a tool in a molding step to replicate the microstructure of polymer
• Processes : Basic LIGASLIGA (Sacrificial LIGA)LIGA-like process
Basic LIGA process
Lithography/Electroplating/Plastic molding
LIGA process to produce polymer replica
Summary
Think of SCALE-BRIDGING concept (micro-macro, nano-micro)