1. 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

2. Example of MEMS products
LIGA product
Surface micro-machining
product

3. Process flow of IC & MEMS fabrication
Processes of IC and MEMS are almost same
Process complexity/yield depends on repetition of central loop
Deposition
Lithography
Etch
Wafers
Chips

4. Silicon wafer fabrication
Czochralski process: widely-used to make single crystal Si

5. Silicon wafer fabrication – slicing & polishing
Smart cutting process?
CMP is used.. Why?

6. Deposition processes Barrier layer formation
- Issues of deposition : Compatibility, Conformability
- Process:
Spin casting/Spin coating
PVD – physical vapor deposition
CVD – chemical vapor deposition
Barrier layer formation
Materials
- SiO2 : most common
- Si3N4, polysilicon, metals etc…
Process
- Thermal oxidation, Evaporation, Sputtering, CVD

7. Spin Casting/Coating Viscous liquid is poured on center of wafer
Wafer spins at RPM for ~30s (thickness control)
Baked on hotplate oC for s (volume reduction by 1/2)
Application of etchants and solvents, rinsing
Deposition of polymers, sol-gel precursors (SOG)

8. Physical Vapor Deposition - Evaporation
(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
Vacuum
substrate
E-beam
target
heating

9. 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
substrate
atoms
Plasma - + RF
source

10. Chemical Vapor Deposition - CVD
Process
Gas phase is injected into the chamber
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
To exhaust
system
Wafers
Susceptor N2 H2
SiCl4+ H2
HCl
Dopant+ H2

11. Lithography Pattern transfer: transferring a mask pattern onto wafer
Procedure
Deposit barrier layer (SiO2, Si3N4, metal, etc.)
Coat with PR
Soft baking (curing)
Align mask
Expose pattern and develop PR
Hard baking and Etching
Remove PR
Issues: Light source, Barrier layer, Mask, PR, Etching

12. 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)

13. Etching process
- Classification: (Wet vs. Dry), (Isotropic vs. Anisotropic)
- Wet vs. Dry etching
Wet etching : liquid etchant
Dry etching : gas or plasma
Physical vs. Chemical
Plasma, Sputter, RIE
- Issues of etching : Anisotropy, Selectivity

14. Isotropic vs. Anisotropic etching
Isotropic etchants etch at the same rate in every direction
mask
<111>
<100>
Silicon Substrate
54.7
isotropic
undercut
Anisotropic etching of Si
anisotropic

15. Bulk micro-machined cavities
Anisotropic KOH etch (upper left)
Isotropic plasma etch (upper right)
Isotropic BrF3 etch
with compressive oxide (lower right)

16. 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 physical
Physical : Anisotropic
Chemical : Isotropic
Deep RIE (DRIE) : altering two gas compositions
High aspect ratio of 50:1, High etching rate

17. 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)

18. 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.

19. Bulk vs. Surface micro-machining
Bulk micro-machining
bonding
Surface micro-machining

21. Effects of residual stress
Example of Bad Luck !!

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

24. LIGA process LIthographie Galvanoformung Adformung
(Lithography/Electroplating/Plastic molding)
Developed during 1980s at Research Center Karlsruhe
Possible to produce microstructures with very high aspect ratios (up to 100),
very small structures (in the submicron range),
and with very smooth walls (surface roughness ＜ 50 nm)
To manufacture a tool in a molding step to replicate the microstructure of polymer
Processes : Basic LIGA
SLIGA (Sacrificial LIGA)
LIGA-like process

25. Lithography/Electroplating/Plastic molding
Basic LIGA process
Lithography/Electroplating/Plastic molding

26. LIGA process to produce polymer replica

27. Summary Think of SCALE-BRIDGING concept (micro-macro, nano-micro)
Think of applying your major to micro-scale !