1. Lecture 4 Fundamentals of Multiscale Fabrication
Multiscale fabrication III:
Silicon surface micromachining, lithography
Kahp-Yang Suh
Associate Professor
SNU MAE

2. Surface Micromachining
Conventional Silicon Technology
Material Deposition
Etching
Lithography

3. Bulk vs. Surface Micromachining
Typical surface micromachining
Bulk + surface

4. Surface Micromachining

5. Advantages and limitations

6. Thin Film Deposition (1)
The transformation of vapors into solids, frequently used to grow solid thin film and powder materials
Physical Vapor Deposition (PVD)
Direct impingement of particles on the hot substrate surface
Electron-beam Evaporation, Sputtering
Chemical Vapor Deposition (CVD)
Convective heat and mass transfer as well as diffusion with chemical reactions at the substrate surfaces
More complex process than PVD
More effective in terms of the rate of growth and the quality of deposition
LP/AP CVD, Thermal/PE/Ph/La CVD

7. Thin Film Deposition (2)
Type
Materials
Physics
Considerations
Thickness
Time
Uniformity
Cost
Damage
Contamination
Adhesion
Material Choice
Spin-on
Organic
Cathode
Sputtering
Metals
Plasma
Anode
Vapor
Chemical Vapor Deposition
Silicon Compounds
Heated Plate
Doping
Conductors

8. Step Coverage Profile (1)
A: Rapid surface migration process (before reaction), yielding uniform coverage since reactants adsorb and move, then react
B: Long mean free path process and no surface migration, with reactant molecule arrival angle determined location on features (local “field of view” effects are important)
C: Short mean free path process with no surface migration, yielding nonconformal coating
It is favorable to performed in high temperature, low pressure but cost problem should be considered.

9. Step Coverage Profile (2)
Key Parameters
Mean Free Path
Surface Migration Energy ( E ∝ Temperature)
Arrival angle
For conformal step coverage
α < l (mean free path)
where α = arctan (w/z)
High Surface Mobility
Process tendency
A: Low Pressure CVD
B: Plasma Enhanced CVD
Evaporated & Sputtered Metal

10. Step Coverage Profile (3)
Step coverage profile example
Good
Bad

11. Step Coverage Profile (4)
Mean free path
Electron’s & ion Mean Free Path
( Considering mean speed of mutual Approach in Ion Mean Free Path)
( Unless T is extremely high , p is the main determinant of l )
Knudsen number :

12. Photolithography

13. Overview

14. General photoresist
Positive resist is suppressing-melt type: Lighted parts melt
Negative resist is linked-structure type: Lighted parts left

15. Procedure

16. Positive PR (1)
Poly(methyl methacrylate) or PMMA

17. Positive PR (2)

18. Negative PR (1)

19. Special photoresist

21. Lithography process (1)

22. Lithography process (2)

23. Lithography process (3)

24. Resolution To get higher Resolution (small R) l Resolution(R) = K1 NA
1. increase NA
2. shorten wavelength 
3. decrease K1
To get higher Depth of Focus
1. decrease NA
2. lengthen 
3. increase K2 l
Resolution(R) = K1 NA l
DOF = K2
NA2
Here,  is wavelangeth of light, NA is numerical aperture of lens,
K1 and K2 are proportional constant of resist process
NA = D/2f (D: diameter of the lens, f: focal length)

25. Projection: Key Parameters

26. Photomask

27. Mask to wafer alignment

28. Exposure technique

29. Exposure source

30. Problems of photolithography (1)

31. Problems of photolithography (2)

32. Problems of photolithography (3)

33. Lift-off process