Anisotropic etching - Rose

Bulk micromachining
 Explain the differences between isotropic and anisotropic  List and explain the most common etch stop techniques
 List and describe the most common dry etching techniques
 Explain the differences between wet and dry etching
 Perform basic calculations for wet etching processes
 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
Bulk micromachining
Silicon etched
Isotropic etch
Silicon wafer
Silicon etched
Anisotropic etch
Silicon wafer
Etching: Chemical reaction resulting in the removal of material
Wet etching: etchants in liquid form
Dry etching: etchants contained is gas or plasma
ionized gas
Etch rate: material removed per time (μm/min)
Selectivity and undercutting
etch rate of one material compared to another
etch rate of one crystalline direction compared to another
(100) Si
SEM image of a SiO2 cantilever formed by undercutting (S. Mohana
Sundaram and A. Ghosh, Department of Physics, Indian Institute of
Science, Bangalore)
Application and properties of different wet etchants
High HF tends to etch SiO2
Acidic etchants tend to etch Si
Basic etchants tend to etch Si
Depend on concentration and
Rate versus diffusion limited etching
Rate limited
limited reaction
Rate limited
reactions are
preferred  easier
to control and
more repeatable
Isotropic etching
Estimate of etch depth
depth ≈ (D-d)/2
• Etch rate is the same in all
• Typically acidic
• Room temperature
• Isotropy is due to the fast chemical
 Reaction or diffusion limited?
• X μm/min to XX μm/min
Isotropic etching
• Used in isotropic etching of silicon
• Also called poly etch
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)  SiO2 (s) + HNO2 (aq)+ H2 (g)
In the second step, the newly formed silicon dioxide is etched by the hydrofluoric acid.
SiO2 (s) + 6HF (aq)  H2SiF6 (aq) + 2 H2O (l)
Isotropic etching
BOE (Buffered Oxide Etch):
• Used in isotropic etching of silicon dioxide and glass
• Basically proceeds from the second step of etching Si:
SiO2 (s) + 6HF (aq)  H2SiF6 (aq) + 2 H2O (l)
Anisotropic etching
• Etch rate is different for different crystal
• Etch depths depend on
plane directions
• Typically basic etchants
• Undercutting also
• Elevated temperatures (70-120°C)
depends on geometry
• Different theories propose for anisotropy
• Slower etch rates, ~ 1 μm/min  Reaction or diffusion limited?
Properties of different anisotropic etchants of Si
Theories for anisotropic etching
Siedel et al.
1 dangling bond
2 dangling bonds
Silicon lattice
The lower reaction rate for the {111} planes is caused by the larger activation energy required to
break bonds behind the etch plane. This is due to the larger bond density of silicon atoms behind
the {111} plane.
Theories for anisotropic etching
Siedel et al. (Continued)
• Reduction of water 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
Elwenspoek et al.
• Suggests surface roughness is reason
• {111} plane is atomically flat, no nucleation sites
(oxidation step)
(reduction step)
Self-limiting etch and undercutting
[111] [111]
• Intersection of {111} planes can cause self-limiting etch.
• Only works with concave corners
Convex corner
exposes other
Resulting undercutting can be used to create
suspended structures
Anisotropic etching of (110) silicon
Mask with large
aspect ratio
Mask with small
aspect ratio
Vertical sidewalls and 90° angles!
{110} planes etch about
twice as fast as {100} planes
in KOH
Top view
Long narrow mask openings can be
used to create long narrow channels
with vertical sidewalls
Anisotropic etching of (111) silicon
How fast does the (111) plane etch?
 usually used as base (Big green Lego®)
for surface micromachining
Sin embargo, todavía es posbile usar lo en “bulk micromachining”
pre-etched pit
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Sketch the cross-sections resulting from anisotropically etching the silicon wafers shown
with the given masks.
Etch stop
Etch stop: Technique to actively stop the etching process
Insulator etch stop
Self-limiting etch
Timed etch
Etch stop via doping
Etch stop via doping
Boron etch stop
Si + 2OH-  SiOH2++ + 4 eSiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2
(oxidation step)
(reduction step)
n type wafer heavily
doped with B
(called a p+ wafer)
n region
p region
p-n junction
p region Si deficient in e-
High level of p-type doping is not
compatible with CMOS standards for
integrated circuit fabrication
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)
e- ep type wafer doped
n-type dopant
p region
n region
“Reverse bias” voltage
applied to p-n junction
keeps current from
p-n junction
Very light doping compared to boron etch stop. OK
with CMOS standards for integrated circuit fabrication.
Dry etching
Etching: Chemical reaction resulting in the removal of material
Wet etching: etchants in liquid form
Dry etching: etchants contained is gas or plasma
- - - - - - - - -
Accelerated to target via the electric field
+ + + + + + + +
Plasma etching: mostly chemical etching
Reactive ion etching (RIE):
In addition to the chemical etching,
accelerated ions also physically etch the
Chemically reactive gas formed by collision of
• molecules of reactive gas with
• energetic electrons
• Excited/ignited be RF (radio frequency) 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
• Can be very directional—can create tall, skinny
If there is no chemical reaction at all, the
technique is called ion milling.
(Intellisense Corporation)
Common dry etchant/material combinations
Reactive gas
Silicon (Crystalline or Chlorine-base: Cl2, CCl2, F2
Fluorine-base: XeF2, CF4, SF6, NF3
Fluorine-base: CF4, SF6, NF3
Chlorine-base: Cl2, CCl4, SiCl4, BCl3
Fluorine-base: CF4, SF6, NF3
O2 (Ashing)
Deep reactive ion etching (DRIE)
Bosch process
• 1st, reactive ion etching step takes place
• 2nd, fluorocarbon polymer deposited to protect
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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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?
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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