This document summarizes a lecture on micromachining. It discusses clean room fundamentals and maintaining a clean environment for microfabrication. It also covers wafer preparation from silicon ingots, chemical etching processes including isotropic and anisotropic etching, and how crystal geometry affects etching shapes. Bulk and surface micromachining techniques are introduced along with lift-off processing and chemical vapor deposition.

1. 1
Lecture 5:
Micromachining
Prasanna S. Gandhi
Assistant Professor,
Department of Mechanical Engineering,
Indian Institute of Technology, Bombay,
MEMS: Fabrication

2. 2
Recap: Last Class
ƒ E-beam lithography
ƒ X-ray lithography
ƒ Ion beam lithography
ƒ Oxidation

3. 3
Today’s Class
ƒ Clean room fundamentals
ƒ Si wafer preparation
ƒ Chemical etching process
ƒ Anisotropic Etching
ƒ Silicon micromachining
ƒ Surface micromachining
ƒ Bulk micromachining
ƒ How to produce devices

4. 4
Clean Room
Fundamentals
ƒ Need
ƒ Class of a clean environment
ƒ Class X clean room Î not more than X
particles (of size 0.5µm or larger) per
cubic foot of air
ƒ How this cleanliness is produced and
maintained??

5. 5
Clean Room
Fundamentals
ƒ Air conditioning plant
ƒ HEPA filters air recirculation through
these filters

6. 6
Si-wafer preparation
ƒ Czochralski process
ƒ Cutting, CMP, cleaning

7. 7
Secondary flat
Primary
flat
Secondary flat
Primary
flat
{110}
direction
Primary
flat
Primary
flat
Secondary flat
Primary and Secondary wafer flats are used to
identify orientation and type.
(100) n-type
(100) p-type
(111) n-type
(111) p-type

8. 8
Chemical Etching
ƒ Isotropic etching
„ Etchant: HNA mixture.
„ HNA can dissolve 550µm
thick silicon wafer in
about 20 min.
„ HNA mixture removes
silicon equally in all
directions.
ƒ SiO2 etch: 10-30nm/min
Without agitation (5)
With agitation (20)

9. 9
Chemical Etching
ƒ Isotropic etching
ƒ Undercut
ƒ Etch bias
ƒ Materials & etchants*
Without agitation (5)

10. 10
Etch Stop Mechanisms
ƒ Time etch stop
ƒ Dopant B+ (heavy dope) as etch stop
ƒ Pg 45 Spoek
ƒ Thin films
ƒ Electrochemical etch stop
ƒ Anisotropic Etching planes

11. 11
Chemical Etching
Choice of etchant:
ƒ Etch rate
ƒ Topology of the surface to be etched
ƒ Etch selectivity of mask material and
other materials
ƒ Toxicity
ƒ Ease of handling

12. 12
Chemical Etching
ƒ Anisotropic bulk etching
„ Etchant: KOH, EDP
(ethylen diamine pyrocatechol),
TMAH (Tetra methyl
ammonium hydroxide)
„ <111> direction has lower
etching rates than <100>
ƒ Can produce grooves,
slanted/vertical walls
<110> surface wafer
<100>
<010>
<001>
<111>
<100> surface wafer

13. 13
Chemical Etching
ƒ Silicon crystal
geometry*
ƒ Examples of use of the
crystal geometry in
etching
ƒ Fundes regarding etch
shapes under different
conditions*
<100>
<010>
<001>
<111>

14. 14
Anisotropic Etching
KOH, EDP and TMAH
ƒ EDP etches oxide 100 times slower than KOH,
ƒ KOH, TMAH dangerous to eye
ƒ KOH less dangerous than EDP & TMAH
ƒ Etch curves*: 5hrs to etch 300µm thick wafer
ƒ H2 bubbles during KOH etching of Si
ƒ EDP ages quickly in contact with oxygen
producing red brown color, vapor is harmful
ƒ HF dip is necessary for EDP: native oxide problem
<100>
<010>
<001>
<111>

15. 15
A’
A
Top
View
[110]
54.7o
Cross Section A-A’
A square <110>-oriented mask feature results
in a pyramidal pit.

16. 16
A
A’
A
A’
A
A’
Convex corners
are rapidly
undercut
54.7o
[100] Silicon
[110]
[100]
Masking Layer
Cross Section A-A’
[100]
Cross Section A-A’
Convex corners where {111} planes meet are not stable. They are
rapidly undercut. This permits creation of suspended structures.

17. 17
A’
A
Boundary of
rectangular pit
[110]
Undercut
regions
[100] Masking Layer
Cross Section A-A’
Any mask-layer feature, if etched long enough, will result in a
rectangular V-groove pit beneath a rectangular that is tangent
to the mask features, with edges oriented along <110>
directions.

18. 18
The effect of misalignment is to enlarge the
etched region. This figure shows the effect of a
5o misalignment for a rectangular feature.
5o misalignment
Boundary of rectangular pit
[110]

19. 19
Re-entrant resist profile
Wafer
with
photoresist
Directional
evaporation
(e-beam)
Strip resist
and
lift off metal
Illustrating the lift-off method for patterning evaporated metals

20. 20
Wafer with
double-layer
undercut
masking
Directional
evaporation
gradually
closes opening
At closure,
a sharp tip
is formed
The use of a modified lift-off process to create sharp tips.
After
lift-off

21. 21
Conclusions
ƒ Chemical etching
ƒ Isotropic
ƒ Anisotropic
ƒ Bulk and Surface micromachining
ƒ Etch stop mechanisms
ƒ Liftoff process

22. 22
Next class
ƒ Plasma based processes
ƒ Plasma etching
ƒ RIE
ƒ Sputtering
ƒ PE CVD

23. 23
Wafer Cleaning
Process
ƒ Need
ƒ CMP- Chemical mechanical polishing
ƒ RCA cleaning

24. 24
Chemical Vapor
Deposition (CVD)
ƒ Chemical reaction in vacuum
chamber
ƒ High temperatures (>300oC)
ƒ Polysilicon, SiO2, Si3N4,
tungston, titanium, copper
etc. can be deposited
ƒ Low pressure CVD (LPCVD)
ƒ Plasma Enhanced CVD: low
temperatures
ƒ Pressure, temp, gas flow
Wafer
Gases
Temperature > 300oC

25. 25
)
( τ
+
= t
B
x
)
( τ
+
= t
A
B
x
For small time t, and
0
0
2
0
2
2
DN
k
Dd
d
s







 +
=
τ
for large time t, where χ is the thickness of the oxide
layer in the silicon substrate in micrometers at time t,
in hours. A and B are constants, and the parameter τ
can be obtained by:

26. 26
where
„ D=diffusivity of oxide in silicon, e.g.,
„ D=4.4 x 10-16cm2/s at 900oC
„ do=initial oxide layer (~200 in dry oxidation,=0 for wet
„ oxidation)
„ ks=surface reaction rate constant
„ No=concentration of oxygen molecules in the carrier
„ gas
„ =5.2 x 1016molecules/cm3 in dry o2 at 1000oC and
„ 1atm
„ =3000 x 1016 molecules/cm3 in water vapor at the
„ same temperature and pressure
„ N1=number of oxidizing species in the oxide
„ =2.2 x 1022 SiO2 molecules/cm3 in dry O2
„ =4.4 x 1022 SiO2 molecules/cm3 in water vapor
A