1. Lecture 3 Fundamentals of Multiscale Fabrication
Multiscale fabrication II:
Silicon bulk micromachining
Paper reading: “Bulk Micromachining of Silicon”, Proceedings of The IEEE, GREGORY T. A. KOVACS, NADIM I. MALUF, AND KURT E. PETERSEN, Vol 86, No. 8, pp , 1998.
Kahp-Yang Suh
Associate Professor
SNU MAE

2. Further readings (ISI web search)
Title: Fabrication techniques of convex corners in a (100)-silicon wafer using bulk micromachining: a review Author(s): Pal P, Sato K, Chandra S Source: JOURNAL OF MICROMECHANICS AND MICROENGINEERING   Volume: 17   Issue: 10   Pages: R111-R133   Published: OCT 2007
Times Cited: 7
Title: Micromachining for optical and optoelectronic systems Author(s): Wu MC Source: PROCEEDINGS OF THE IEEE   Volume: 85   Issue: 11   Pages:   Published: NOV Times Cited: 181
Title: Silicon microstructuring technology Author(s): Lang W Source: MATERIALS SCIENCE & ENGINEERING R-REPORTS   Volume: 17   Issue: 1   Pages: 1-55   Published: SEP Times Cited: 94
Title: Development of surface micromachining techniques compatible with on-chip electronics Author(s): French PJ Source: JOURNAL OF MICROMECHANICS AND MICROENGINEERING   Volume: 6   Issue: 2   Pages:   Published: JUN Times Cited: 25

3. Micromachining

4. Si Micromachining Methods
SFB: Silicon Fusion Bonding

5. Bulk Micromachining

6. Benefits of Si bulk micromachining

7. Etching Process Students need to notify the process and related terms.
Shapes of cut originated from Si characteristics.

8. Bulk Micromachining The purpose of bulk micromachining
Selectively remove significant amounts of silicon from a substrate
Broadly applied in the fabrication of micromachined sensors, actuators, and structures
Fabrication method: dry/wet etching
Undercut structures that are required to physically move
Form membranes on one side of a wafer
Make a variety of trenches, holes, or other structures
Bulk micromachining is used for mass removal of substrate.
Usually wet etching is used but some are performed by gas.

9. Bulk Micromachining
Isotropic etching performed by acid, and anisotropic etching is performed by alkali on the other hand.
Etched sides are form a trapezoid or a rectangle.
Boron layer stops etching. (used for etch stop layer)
Figure 1. Bulk silicon micromachining: (a) isotropic etching; (b) anisotropic etching;
(c) Anisotropic etching with buried etch-stop layer; (d) dielectric membrane released by
back-side bulk etching; (e) dopant dependent wet etching; (f) anisotropic dry etching

10. Production-Line Automation
Bulk Micromachining
Terminology:
Etch Rate – How fast material is removed
Selectivity – Ratio of etch rates of materials exposed to etching
Anisotropy – Degree of lateral etch to vertical etch
Wet Etching – chemical bath
Dry Etching – chemical / physical material removal using gas / vapor
Dry Etching
Wet Etching
Production-Line Automation
Good
Poor
Cost chemicals
Low
High
Selectivity
Can be very good
Sub-micron features
Applicable
Not Applicable
Etch Rate
Slow (0.1um/min)
Fast (1 um/min)
(Madou)

11. Silicon Crystallography (1)

12. Silicon Crystallography (2)
Refer to open courseware -> readings -> etc
(see MEMS Open Courseware: Readings_Etc)

13. Isotropic Wet Etching Isotropic wet etching
Etching with chemical reaction
 etching in all directions a substrate
Induces etch reaction with silicon through
the opening region
 agitation required: stirring, ultrasonic
Considering opening area to size of reaction bubble
Bubble disturbs exchange of etchant
Slow down etch rate
The most common isotropic wet silicon etchant: HNA
HNA = HF + HNO3 + CH3COOH
Reaction: 18HF + 4HNO3 + 3Si  2H2SiF6 + 4NO(g) + 8H2O

14. Anisotropic Wet Etching
Anisotropic etchants etch much faster
in on direction than in another
 exposing the slowest etching crystal
planes over time
 (111) planes have the slowest etch rate
Several solutions:
Alkalic OH (KOH, NaOH)
Tetramethylammonium hydroxide (THAH, (CH3)4NOH)
Ethylenediamine pyrocatechol (EDP, NH2 (CH2)2 NH2+C6H4(OH)2)
Etching at concave corners on (100), stop at (111) intersections.
Convex corners are under cut

15. Example of Anisotropic Wet Etching

16. Anisotropic Etching Mechanism

17. Dry Etching Xenon Difluoride Etching (XeF2)
Non-plasma, isotropic dry etch process
Very high selectivity for Al, SiO2, Si3N4, PR
Reaction: 2XeF2 + Si  2Xe + SiF4
Etch rate: 1~ 3 m/min
Drawback: Rough surface
reaction with water (HF)
silicon fluoride polymer
Use of BF3 ~ much smooth surface

18. Example of isotropic Dry Etching

19. Dry Etching Plasma/Reactive Ion Etching (RIE)
Anisotropic dry etch process
Process in which chemical etching is accompanied by ion bombardment
Combination of physical and chemical etching

20. RIE principles

21. Mask materials for RIE
Etch mask: PR (Photo Resist), Hard mask (SiO2, Al)
Selectivity
= etch rate of etching material / etch rate of mask
Usually, standard PR (for CMOS) is not adequate for O2
plasma etch, for which hard mask is required
Selectivity of silicon: AZ1512
Cl based etch (physical etch): <2
F based etch (chemical etch): <10
(if O2 gas is inserted into the chamber, the selectivity would be smaller than 10)

22. Reaction in RIE process (1)
Physical reaction is superior for Cl-based process. It has high selectivity!!!
Chemical reaction is superior for F-based process. It has low selectivity!!!

23. Reaction in RIE process (2)

24. Example using BCl3

25. Reaction in RIE process (3)

26. Example using SF6

27. Deep reactive ion etching (DRIE)

28. Wet etching vs. Dry etching
In wet etchants, the etch reactants come from a liquid source
In dry etchants, the etch reactants come from a gas or vapor phase source and are typically ionized
Atom or ions from the gas are the reactive species that etch the exposed film
Selectivity: in general, dry etching has less selectivity than wet etching
Anisotropy: in general, dry etching has higher degree of anisotropy than wet etching
Etch rate: in general, dry etching has lower etch rate than wet etching
Etch control: dry etching is much easier to start and stop than wet etching
Dry etching is easily automated.
If selectivity is high, we could etch deep and long.

29. MEMS: Deposition + Lithography + Sacrificial Etching