Wet Bulk
Micromachining –
a STIMESI II tutorial
23.02.2011
Per Ohlckers, Vestfold University College
www.hive.no
Per.Ohlckers@hive.no
Daniel Lapadatu, SensoNor Technologies
www.multimems.com
daniel.lapadatu@sensonor.no
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 2
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 3
Manufacturing Processes
 Serial (e.g. Focused Ion Milling - FIB) vs. batch (e.g. bulk Si
micromachining) vs. continuous (e.g. doctor’s blade).
 Additive (e.g. evaporation) or subtractive (e.g. dry etching).
 Projection (almost all lithography techniques) vs. truly 3D.
 Mold vs. final product.
There are many different techniques
that are being used.
However, in general, batch processing
is the most powerful technique and
used mostly.
FIB
Slide 4
Micromachining
 Bulk Micromachining is a process that produces structures
inside the substrate by selective etching.
 Surface Micromachining is a process that creates structures
on top of the substrate by film deposition and selective etching.
Bulk Micromachining
Surface Micromachining
Slide 5
Classification of Bulk Silicon Etching
 Bulk Micromachining is a process that produces structures
inside the substrate by selective etching.
Wet Etching
Dry Etching
Crystal orientation
dependent
Process
dependent
Isotropic
Anisotropic
Isotropic
Anisotropic
Acidic
Etchants
Alkaline
Etchants
BrF3
XeF2
F-based
plasmas
Slide 6
Etching Features
 Etch rate:
► Rate of removal of material/film.
► Varies with concentration, agitation and temperature of etchant,
porosity and density of etched film.
 Etch selectivity:
U
► Relative etch rate of mask,
film and substrate.
 Etch geometry:
► Etching depth (R);
R
S
B
► Mask undercut (U);
► Slope of lateral walls (S);
► Bow of floor (B);
► Anisotropy.
Slide 7
Bulk Silicon Etching: Examples
Deep cavity by wet,
anisotropic etching
Release etch
by RIE
Recess etch
by RIE
Slide 8
Wet Silicon Etching: Examples
Isotropic etching with HNA
(HF : Nitric Acid : Acetic Acid)
Anisotropic etching with KOH
(110)
(100)
Slide 9
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 10
Structure of Single Crystal Silicon
 Face-centred cubic (fcc) structure
(diamond structure) with two atoms
associated with each lattice point of
the unit cell.
 One atom is located in position with
xyz coordinates (0, 0, 0), the other
in position (a/4, a/4, a/4), a being
the basic unit cell length.
 Lattice constant a = 5.43 Å.
The arrangement of the silicon atoms in a unit
cell, with the numbers indicating the height of
the atom above the base of the cube as a
fraction of the cell dimension.
Slide 11
Miller Indices
 Miller indices are a notation system in crystallography for
planes and directions in crystal lattices.
 A lattice plane is determined by three integers h, k and l, the
Miller indices, written (hkl). The indices are reduced to the
smallest possible integers with the same ratio.
Determining the Miller indices for
planes by using the intercepts
with the axes of the basic cell.
Slide 12
Determining Miller Indices
 Example:
► Take the intercepts of the plane
along the crystallographic axes, e.g.
2, 1 and 3.
z
4
► The reciprocal of the three integers
are taken:
1/2, 1/1 and 1/3.
3
► Multiply by the smallest common
denominator (in this case 6):
3, 6 and 2.
2
► The Miller indices of the plane are:
(362).
y
1
1
x
O
1
Slide 13
2
3
Crystallographic Planes and Directions
 (abc) denotes a plane.
 {abc} denotes a family of
equivalent planes.
 [abc] denotes the direction
perpendicular on (abc) plane.
 <abc> denotes a family of
equivalent directions.
 {100}, {110} and {111}
are the most important
families of crystal planes for
the silicon crystal.
Slide 14
Single Crystal Silicon Wafers
 Primary and secondary flats
indicate the dopant type and
surface orientation.
 Wafer diameter in current fab
standards: from 100 to 300 mm.
 Wafer thickness in current fab
standards: from 250 to 600 µm.
 Surface orientation:
► (100) for MOS and MEMS;
► (110) for MEMS;
► (111) for bipolar.
Secondary flat
Primary flat
Slide 15
Standard 100 mm Wafers
 The position of the flat(s) indicates the surface orientation and
the type of doping.
 The primary flat on (100) and (110) wafers is along
the [110] direction.
Orientation of flats for 100 mm wafers
p-(111)
Primary flat
n-(111)
p-(100)
n-(100)
Secondary flat
Slide 16
Wafers Used in MultiMEMS
 P-type, 150 mm Si wafer.
 (100) ± 0.5º Surface.
 [110] ± 0.5º Primary Flat.
z
{100}
Si Wafer
O
x
(100) Surface
<110> Directions
[110]
y
Wafer's Primary Flat
Slide 17
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 18
Isotropic Wet Etching of Silicon
 All crystallographic directions are
etched at the same rate.
 Features:
► Etchants are usually acids;
Mask
with stirring
Si
► Etch temperature: 20... 50 °C;
► Reaction is diffusion-limited;
► Very high etch rate (e.g. up to
50 µm/min);
Si
without stirring
► Significant mask undercutting.
 Masking is very difficult:
► Au/Cr or LPCVD Si3N4 is good.
► SiO2 may also be used for shallow
etching.
Si
Slide 19
Isotropic Etching of Silicon: Etchants
Etchant
(Diluent)
Typical
Composition
Temp.
Etch Rate
[µm/min]
HF
HNO3
CH3COOH + H2O
10 ml
30 ml
80 ml
22 °C
0.7... 3.0
HF
HNO3
CH3COOH + H2O
25 ml
50 ml
25 ml
22 °C
40
HF
HNO3
CH3COOH + H2O
9 ml
75 ml
30 ml
22 °C
7.0
Mechanism
–
hole injection:
oxidation:
oxide removal:
HNO3 + H2O + HNO2 → 2HNO2 + 2OH +
+
2h
4+
–
Si + 4OH → SiO2 + H2
SiO2 + 6HF → H2 SiF6
Slide 20
Silicon Etching with HNA
 HNA: mixture of
► 49,23% HF,
► 69,51% HNO3 and
► acetic acid (CH3COOH) or
water (H2O) as diluent.
(µm/h)
 HNO3 oxidizes the silicon,
HF removes the oxide:
► High HNO3:HF ratio,
- Etch limited by oxide removal.
► Low HNO3:HF ratio
- Etch limited by oxide formation.
 Dilute with water or acetic acid:
► CH3COOH is preferred because it
prevents HNO3 dissociation.
Iso-Etch Curve
(from Robbins et al.)
Slide 21
Isotropic Etching of Glass
 Single- or double-side etching of glass wafers is achieved either
by using HF-water solution or HNA.
 Typical etch rate for borosilicate glass in HNA: 1.9 µm/min.
 Applications:
► Etching cavities and through-holes;
► Etching gas/fluid channels.
Through-hole
Mask
Cavity
Undercut
GLASS
Slide 22
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 23
Anisotropic Wet Etching of Silicon
 Crystallographic directions are
etched at different rates.
<110>
 Features:
► Etchants are usually alkaline;
<100>
► Etch temperature: 85... 115 °C;
54.74°
► Reaction is rate-limited;
► Low etch rate (ca. 1 µm/min);
Mask
<111>
Si
► Small mask undercutting.
 Masking is very difficult:
► LPCVD Si3N4 is good;
► SiO2 may also be used with some
etchants.
Si
Slide 24
Etching Setup
Laboratory setup for wet
chemical etching of silicon.
The principles for industrial
manufacturing equipment
are the same.
Slide 25
Anisotropic Etching of Silicon: Etchants
Etchant
(Diluent)
Typical
Etch Rate Etch Ratio Masking
Temp.
Composition
[µm/min] (100)/(111)
Film
Etylenediamine
Pyrocathecol
Water
750 ml
120 gr
100 ml
35:1
SiO2
Si3N4
metals
Etylenediamine
Pyrocathecol
Water
750 ml
120 gr
240 ml
1.25
35:1
SiO2
Si3N4
metals
115 °C
KOH
Water + Isopropyl
44 gr
100 ml
85 °C
1.4
400:1
SiO2
Si3N4
TMAH
Water + Isopropyl
220 gr
780 ml
90 °C
1.0
100:1
SiO2
Si3N4
115 °C
0.75
Slide 26
Chemistry of Anisotropic Etching
 Etching phases:
► Transport of reactants to the silicon surface;
► Surface reaction;
► Transport of reaction products away from the surface.
 Key etch ingredients:
► Oxidisers;
► Oxide etchants;
► Diluents and transport media.
Mechanism
–
2+
–
Si + 2OH → Si(OH)2 + 4e
–
–
4H2O + 4e → 4OH + 2H2 (gas)
2+
–
2–
Si(OH)2 + 4OH → SiO2(OH)2 +
2H2O
Overall:
–
Si + 2OH + 2H2O →
2–
→ SiO2(OH)2 + 2H2
(gas)
Slide 27
Anisotropic Etching of (100)-Si
 Cavity defined by:
<110> Directions
{111} Planes
► {111} walls
BETCH
– slow-etching planes;
{111} Planes
► {100} floor
– fast-etching plane.
(100) Plane
 Final shape of cavity
depends on:
► Mask geometry;
► Etching time.
Truncated Pyramidal
Cavity
 Shape of cavity:
V-Groove
Cavity
N-LAYERS
► Truncated pyramid;
► V-groove;
► Pyramid.
OXIDE
P-SUBSTRATE
Slide 28
Pyramidal
Cavity
Cavity Geometry for (100)-Si
Anisotropically etched cavity in (100) silicon
with a square masking film opening oriented
parallel to the <110> directions.
Wb = W0 – 2·l·cot(54.7°)
Slide 29
Mask Undercutting
 (a) is a pyramidal pit bounded
by the {111} planes.
 (b) is a type of pit expected
from slow undercutting of
convex corners.
 (c) is a type of pit expected
from fast undercutting of
convex corners.
 In (d), further etching of (c)
produces a cantilever beam
suspended over the pit.
 (e) illustratates the general rule
for undercutting assuming a
sufficiently long etching time.
<100>
<110>
Mask
<111>
(a)
Si
(b)
Si
(c)
Si
(d)
Si
(e)
Slide 30
Anisotropic Etching of (110)-Si
 Cavity defined by:
► {111} walls
Mask
– slow-etching planes;
<111>
► {110} floor
– fastest-etching plane;
{100}
A
<111>
► {100} bottom side walls
– fast-etching planes;
 Final shape of cavity
depends on:
{110}
70.5º
a)
<110>
Mask
<100>
► Mask geometry;
<111>
► Etching time.
 Cavity shape:
► Rhombic prisms;
b)
45º
90º
► Hexahedric prisms.
Slide 31
A'
Cavity Geometry for (110)-Si
Slide 32
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 33
Methods for Selective Etching
 Time etching methods:
► Calculate the needed etching time on the basis of the etching rate.
– Easy, but inaccurate method, as etching rate varies with the chemical condition of
the etchant and geometrical factors limiting the agitation of the etch.
Typical accuracy: ± 20 µm.
► Inspect the depth of the etched cavity in appropriate time intervals
until desired depth is reached.
– Time consuming, but improved accuracy. Uneven etching depth from cavity to
cavity due to chemical and geometrical factors is still a problem.
Typical accuracy: ± 10 µm.
 Chemical selective techniques:
► The etching stops when a chemically resistive layer is reached.
– Typical accuracy: ± 3 µm.
 Electrochemical selective techniques:
► The etching stops on reverse biased pn junctions.
– Typical accuracy: ± 1 µm.
Slide 34
Time-Stopped Etching
 Example of etching stopped at an arbitrary depth, exhibiting a flat
floor.
Slide 35
Etching Stopped by {111} Walls
 Example of etching stopped by the intersecting {111} walls,
exhibiting a pyramidal groove.
Slide 36
Boron Etch-Stop Technique
 Chemical selective etching:
► Etch rate depends on boron
concentration.
► Etching stops if boron
concentration exceeds
5·1018 cm–3.
 Boron stop layer is
manufactured:
► By diffusion deposition,
implantation or both;
► On the opposite surface of the
wafer with respect to the etch
cavity.
Boron-dependent etch rate of
silicon (from Seidel et al.)
Slide 37
Boron Etch-Stop Mechanism
 Interstitial bonds require more energy to be broken.
 The electrons supplied by the etchant recombine with the holes
in the bulk, rather than participating in the chemical reaction.
Interstitial
boron atom
Silicon atom
Substitutional
boron atom
Slide 38
Boron Etch-Stop Shortcomings
 Electronics cannot be integrated in the boron stop layer.
► Solution: depositing an epitaxial layer atop the stop layer, with
appropriate doping as substrate material for integrated devices.
– Controlling the autodoping of the epi-layer is challenging.
n-type epitaxial layer
p+ type boron stop layer
n-type substrate
Slide 39
Electrochemical Etch-Stop (ECES)
 Electrochemical selective etching:
► Etch rate depends on the applied
potential.
+
–
V
oxide
► Etching stops if the applied potential
exceeds a threshold value, called
passivation potential.
► Low-doped material, both p- and
n-type, can be passivated:
– To be used as substrate for integrated
components such as piezoresistors.
p
Pt
n p
etchant
stir bar
 High accuracy, typically ± 1 µm:
► Achieved by using well-controlled
implantation and diffusion techniques.
 KOH and TMAH can be used:
– Both avoid the health dangers of EDP.
Slide 40
Wafer Holder for ECES
A practical way to make a wafer
holder to be used for electrochemical selective etching.
The principles for industrial
manufacturing equipment
are the same.
Slide 41
Electrochemical Etch-Stop Mechanism
 Etch-stop achieved by reverse biasing the pn junctions.
 More in the Bulk Silicon Etching tutorial...
+
P-layers
N-layers
Vp
Etch
Rate
Silicon Etching
Anodic Passivation
Vn
Vpp
V
Slide 42
ECES for MultiMEMS
 Electrochemical etch-stop allows 3 different thicknesses:
► Full-wafer thickness (400 µm)
 For heavy seismic masses;
► Epi-layer thickness (3 µm)
 For thin membrane, springs;
► N-well thickness (23 µm)
 For thick membranes, masses, bosses…
Slide 43
Etch-Stop on Multi-Level Junctions
Very Thin Membrane
Thick Membrane
(Mass/Well)
Thin Membrane
Slide 44
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 45
Undercutting of Convex Corners
 High etch-rate of high-index planes:
► Severe undercutting of convex corners;
► Truncated pyramids or V-grooves as final cavities.
Mask will be undercut until
{111} planes are exposed.
undercut
Mask
{100}
{221}
{111}
{111}
Slide 46
Compensation for Convex Corners
Etching without corner
compensation structure
Etching with corner
compensation structure
Corner compensation of mask is difficult
to establish as a repeatable process;
highly dependant on etching
parameters.
Corner Compensation in Silicon
(from Gupta et al.)
Slide 47
Corner Compensation Structures
Compensation
Structure
Desired Result
A simple approach
to convex corner
compensation
(from Wei Fan et al.)
Slide 48
Designing with Undercut Corners
Fabrication of MEMS - MEMS Technology Seminar
(from Burhanuddin Yeop Majlis)
Slide 49
Thank you
for your attention !
Slide 50