Bulk micromachining
 Explain the differences between isotropic and anisotropic 
etching

 Explain the differences between wet and dry etching
techniques

 Identify several common wet etchants and explain what they 
are commonly used for
 Explain the difference between rate limited and diffusion
limited reactions
 Explain in general terms the different theories behind the
differences in etch rate for different crystal directions in the
anisotropic etching of silicon
 Discern the resulting shapes of trenches (pits) resulting
from the anisotropic etching of Si for different mask and
wafer combinations
List and explain the most common etch stop techniques
Explain the shape of resist profiles and calculate the slopes
of resist layer
List and describe the most common dry etching techniques
Perform basic calculations for wet etching processes
Bulk micromachining
Silicon wafer
Silicon wafer
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Etching
Etching:
___ etching: etchants in ______ ____
___ etching: etchants contained is ___ or ______
_______ ___
Etch rate:
(μm/min)
Selectivity and undercutting
Selectivity:
[ ] [ ]
SEM image of a SiO2 cantilever formed by undercutting (S. Mohana
Sundaram and A. Ghosh, Department of Physics, Indian Institute of
Science, Bangalore)
2
Application and properties of different wet etchants
High __ tends to etch ____
____ etchants tend to etch Si
_____________
_____ etchants tend to etch
Si _______________
Depend on __________
and ___________
Rate versus diffusion limited etching
Etchant
Etchant
Products
Products
_____ _______
reactions are
preferred  easier
to control and
more repeatable
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Isotropic etching
Estimate of etch depth
depth ≈
•
•
•
•
•
Etch rate is the _________________
Typically _______
Room temperature
Isotropy is due to the _________________  Reaction or diffusion limited?
X μm/min to XX μm/min
Isotropic etching
HNA:
HF/HNO3/HC2H3O2
• Used in isotropic etching of silicon
• Also called ______________
HNO3 (aq) + Si(s) + 6HF (aq)  H2SiF6 (aq) + HNO2 (aq) + H2O (l) + H2 (g)
The etching process actually occurs in several steps.
First step, nitric acid oxidizes the silicon
HNO3 (aq) + H2O (l) + Si (s) 
In the second step, the newly formed silicon dioxide is etched by the hydrofluoric acid.
SiO2 (s) + 6HF (aq) 
4
Isotropic etching
BOE (Buffered Oxide Etch):
HF/NH4F/H2O
• Used in isotropic etching of __________ and ________
• Basically proceeds from the second step of etching Si:
SiO2 (s) + 6HF (aq)  H2SiF6 (aq) + 2 H2O (l)
Anisotropic etching
d
[111]
54.7°
[100]
• Etch depths depend on
____________
• Undercutting also
depends on _________
• Etch rate is different for _______________
_________________
• Typically _______ etchants
• Elevated temperatures (___-____°C)
• Different theories propose for anisotropy
• Slower etch rates, ~ 1 μm/min
 Reaction or diffusion limited?
5
Properties of different anisotropic etchants of Si
Theories for anisotropic etching
Siedel et al.
( )
( )
Silicon lattice
The lower reaction rate for the {111} planes is caused by the larger ______________ required to
break bonds behind the etch plane. This is due to the _____________ of silicon atoms behind the
{111} plane.
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Theories for anisotropic etching
Siedel et al. (Continued)
• _________________ believed to be the rate determining step
• OH- believed to be provided by H2O near Si surface
Si + 2OH-  SiOH2++ + 4 eSiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2
(oxidation step)
(reduction step)
Elwenspoek et al.
• Suggests _______________ is reason
• {111} plane is atomically flat, no _________________.
Self-limiting etch and undercutting
[111] [111]
D
D
Resulting ______________ can be used to create
suspended structures
• Intersection of {111} planes can cause __________ etch.
• Only works with ________________
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Anisotropic etching of (110) silicon
Mask with large
aspect ratio
{111}
{111}
{110}
{111}
Mask with small
aspect ratio
{111}
Top view
{110} planes etch about
__________ as {100} planes
in KOH
Anisotropic etching of (111) silicon
How fast does the (111) plane etch?
 usually used as base (Big green Lego®)
for _______________________
Sin embargo, todavía es posbile usar lo en “bulk micromachining”
protected sidewalls
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Te toca a ti
Sketch the cross-sections resulting from anisotropically etching the silicon wafers shown
with the given masks.
Etch stop
Etch stop:
________ etch stop
Etch stop via doping
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Etch stop via doping
Boron etch stop
Si + 2OH-  SiOH2++ + 4 e-
(oxidation step)
SiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2
(reduction step)
n type wafer heavily doped with B
(called a p+ wafer)
p region
Etch stop via doping
Electrochemical etch stop (ECE)
Si + 2OH-  SiOH2++ + 4 eSiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2
(oxidation step)
(reduction step)
p type wafer doped n‐type dopant
V
+
“___________” voltage
applied to p-n junction
keeps _____________
____________
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Dry etching
Etching: Chemical reaction resulting in the removal of material
electrodes
Wet etching: etchants in liquid form
- - - - - - - - -
Dry etching: etchants contained is gas or plasma
+ + + + + + + +
wafer
Plasma etching:
Reactive ion etching (RIE):
Chemically reactive gas formed by collision of
• ________________________
• ____________________
• Excited/ignited be __ (_____ ____) electric field
~ 10-15 MHz
Reactive ion etching
Plasma hits surface with large energy
• In addition to the chemical reaction, there is
physical etching (Parece tirar piedras en la
arena)
• Can be very ________—can create tall, skinny
channels
If there is no chemical reaction at all, the
technique is called ______ __________.
(Intellisense Corporation)
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Common dry etchant/material combinations
Material
Reactive gas
Silicon (Crystalline or Chlorine-base: Cl2, CCl2, F2
polysilicon)
Fluorine-base: XeF2, CF4, SF6, NF3
SiO2
Fluorine-base: CF4, SF6, NF3
Al
Chlorine-base: Cl2, CCl4, SiCl4, BCl3
Si3N4
Fluorine-base: CF4, SF6, NF3
Photoresist
O2 (Ashing)
Deep reactive ion etching (DRIE)
Bosch process
• 1st, reactive ion etching step takes place
• 2nd, _________________________________
_____________________________
Kane Miller, Mingxiao Li, Kevin M Walsh and Xiao-An Fu,
The effects of DRIE operational parameters on vertically aligned
micropillar arrays, Journal of Micromechanics and Microengineering, 23 (3)
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Te toa a ti
Wet etching problems
1. A pattern is etched into a <100> Si wafer as described below. Answer the questions that follow.
A 300 nm thick layer of oxide is grown on the surface of the Si wafer. Photoresist is applied to the oxide surface,
and patterned using standard photolithographic techniques. The pattern is etched into the oxide. The exposed Si is
etched anisotropically to achieve the desired feature.
a. Should the photoresist be removed before the Si etching step? Justify your answer.
b. What etchant will you use for the oxide?
c. What etchant will you use for the Si?
2.
You are asked to make a V-shaped grooves 60 μm deep in an oxidized <100> silicon wafer
a. How wide must the opening in the oxide mask be in order to achieve this result?
b. Will the degree of undercutting, due to etching into the <111> plane, be appreciable compared to the
dimensions of the desired feature? Justify your answer.
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