Chapter Extra-2 Micro-fabrication process• Si wafer fabrication • IC fabrication

– Deposition• Spin coating• PVD – physical vapor deposition• CVD – chemical vapor deposition

– Lithography (Pattern transfer)– Removal (mostly etching process)

• Wet/Dry etching• Plasma etching

• 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:

Spin casting/Spin coatingPVD – physical vapor depositionCVD – chemical vapor deposition

Barrier layer formation• Materials

- 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

Issues: Light source, Barrier layer, Mask, PR, Etching

Light source

UV, EUV (Extreme UV), X-ray, e-beam, etc.

- 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

anisotropic

mask

isotropic undercut <111><100>

Silicon Substrate

54.7

Anisotropic etching of Si

Isotropic vs. Anisotropic etching

Bulk micro-machined cavities

• Anisotropic KOH etch (upper left)• Isotropic plasma etch (upper right)• Isotropic BrF3 etch with compressive oxide (lower right)

Reactive Ion Etching (RIE)

Principle : Plasma is struck in the gas mixture and ions accelerated toward the substrate

Reaction occurs on the surface (chemical)Impact of ion is similar to sputter etching (physical)

Controlling balance between chemical and physicalPhysical : AnisotropicChemical : Isotropic

Deep RIE (DRIE) : altering two gas compositions High aspect ratio of 50:1, High etching rate

MEMS-specific fabrication processes

MEMS utilizes IC fabrication process for electric circuit, MEMS-specific process for mechanical structure.

• Bulk micro-machining• Surface micro-machining• Deep reactive ion etching (DRIE)• LIGA• Other materials/processes

• Terminology MEMS ( 미국 ) Micro-machining ( 일본 ) Micro system (EU)

Bulk & Surface micro-machining, DRIE & LIGA

• Bulk micro-machining involves removing material from silicon wafer itself.

– Typically wet etched– Traditional MEMS industry– Artistic design, inexpensive equipment

• Surface micro-machining leaves the wafer untouched, but adds/removes additional layers above the wafer surface. First widely used in 1990s.

– Typically plasma etched– IC-like design philosophy, relatively expensive equipment

• Deep Reactive Ion Etching (DRIE) removes substrate but looks like surface micromachining.

Bulk vs. Surface micro-machining

Bulk micro-machining

Surface micro-machining

bonding

Effects of residual stress

Example of Bad Luck !!

DRIE and LIGA

DRIE

LIGA

DRIE and LIGA: for high aspect ratio products but different process

LIGA process

• LIthographie Galvanoformung Adformung (Lithography/Electroplating/Plastic molding)

• 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)

Think of applying your major to micro-scale !