6. Wet Bulk Micromachining
Fundamentals of Microfabrication
Fall 2013
Prof. Marc Madou
MSTB 120
Table of Content
 Single
crystal growth
 Si lattice structure
 Miller Indices
 Wafer flats
 Isotropic and anisotropic etching
 Example
Bulk Micromachining
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Semiconductor grade devices cannot be
fabricated directly from Poly-Si, first we
need to produce single crystal ingots,
also the mechanical properties of single
crystal Si are superior
Major methods are: Czochralski and
Float Zone method
Si crystal growthCzochralsky method
http://www.egg.or.jp/MSIL/english/msilhist0-e.html from
Mitsubishi Materials Silicon Corporation
Bulk Micromachining
Si crystal growth: float-zone
crystal growth
http://www.egg.or.jp/MSIL/english/msilhist0-e.html from Mitsubishi Materials Silicon Corporation
Bulk Micromachining
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The Si diamond lattice is composed of two interpenetrating fcc lattices, one displaced 1/4 of
a lattice constant from the other. Each site is tetrahedrally coordinated with four other sites in
the other sublattice. When the two sublattices are of different atoms, then the diamond lattice
becomes the zincblende or sphalerite lattice. Examples of materials with the diamond crystal
structure are diamond, silicon and germanium.
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Si crystal orientation
Each site is tetrahedrally coordinated
with four other sites in the other sublattice
More atoms per cm 2
(oxidizes faster than 100) but
etches much slower
Equivalent planes i.e. families {}
http://www.novagate.com/~ahines/rocks/vir_cris.htm
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Miller indices
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Miller Indices are a symbolic vector representation for the orientation of an
atomic plane in a crystal lattice and are defined as the reciprocals of the
fractional intercepts which the plane makes with the crystallographic axes
To determine Miller indices of a plane take the following steps: 1. Determine
the intercepts of the plane along each of the three crystallographic directions 2.
Take the reciprocals of the intercepts 3. If fractions result, multiply each by the
denominator of the smallest fraction
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Miller indices
The first thing that must be ascertained is the
fractional intercepts that the plane/face makes
with the crystallographic axes, in other words,
how far along the unit cell lengths does the
plane intersect the axis? in the figure, the plane
intercepts each axis at exact one unit length (1)
Step two involves taking the reciprocal of the
fractional intercept of each unit length for each
axis, in the figure above, the values are all 1/1.
(2)
Finally the fractions are cleared (i.e., make 1 as
the common denominator) (3)
These integer numbers are then parenthetically
enclosed and designate that specific
crystallographic plane within the lattice. Since
the unit cell repeats in space, the notation
actually represents a family of planes, all with
the same orientation. In the figure above, the
Miller indices for the plane are (111)
Miller Indices http://www.gly.uga.edu/schroeder/
gly630/millerindices.html
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This figure shows a 4 inch 100
plane crystal Silicon wafer,
typically between 250-600
microns thick
The current fab standards are up
to 12 inch wafers
For CMOS work (100) and
(111) (for bipolar) wafers are
most important but in MEMS
other orientations are used as
well (especially (110)
Wafer flats indicate orientation
(primary) and conductivity type
(secondary)
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The primary flat on (100) and (111)
wafers marks the <110> direction
(111)
Purpose and Function
(100)
Orientation for automatic equipment
Indicate type and orientation of crystal
Primary Flat = The flat of longest length located in the circumferen
Secondary Flat = Indicates the crystal orientation and doping of th
P type <111> No secondary Flat
P type <100> 90°±5° Clockwise from Primary Flat
N type <111> 45°±5° Clockwise from Primary Flat
N type <100> 180°±5° Clockwise from Primary Flat
m Prima
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Chemical milling: using a maskant
and a scribe followed by acid to
etch the scribed area
–
–
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Chemical milling (15 th century
decorating armor)
Chemical milling by the 1960’s
especially used by the aerospace
industry
Photosenstive masks instead of
scribing by hand (Niepce in 1822)
Printed circuit board (WW II)
Isotropic etching of Si (mid 1950’s)
IC’s (1961)
First Si micromechanical element
(1961-1962)
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Anisotropic etching of Si (mid
1960’s)
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Anisotropic etching: [100] Si
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Proper alignment leads to {111}
sidewalls, (100) bottom, <110>
directed edges and <211> directed
ribs
Consider the unit cube and the offnormal angle  of the intersection
of a (111) sidewall and a (110)
cross-secting plane
Flat [110]
tan  =
L = a* 2
2
L
a
L = a* 2 = 35.26°or
 =arctan
2
54.74° for the complementary angle
(111)
L
(110)
a

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Anisotropic etching: [100] Si
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The width of the square bottom cavity wo
is determined by the etch depth z, the
mask opening and the angle  we just
calculated
W0 = Wm - 2 cotan (54.74°) z
W0 = Wm -

2
z
To create a dense array of vias the Si
wafer must be thinned
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Anisotropic etching: [100] Si
(100) planes

Flat [110]
There are {100} planes perpendicular to
the wafer surface (at a 45° angle with
the wafer flat i.e.the {110} direction)
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Isotropic etching (HF:Nitric Acid: Acetic Acid)
Anisotropic etching (KOH)
(110)
(100)
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Isotropic etchants etch in all
crystallographic directions at the
same rate:
–
–
–
–
–
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Stirring
Usually acidic (HNA i.e. HF,
HNO3 and CH3COOH)
Room temperature or slightly
above (< 50 °C)
Diffusion limited
Etching is very fast (e.g. up to 50
µm min-1)
Undercuts mask
No stirring
Masking very difficult e.g Au/Cr or
LPCVD Si3N4 is good, but SiO2 is
used because it is so simple
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Anisotropic etchants etch at different
rates depending on the orientation of the
exposed crystal plane:
–
–
–
–
–
–
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Usually alkaline (pH> 12 e.g. KOH)
Higher temperatures (> 50 °C e.g. 85 to
115 °C)
Reaction rate limited
Slower e.g 1 µm/min (for <100>
direction)
Does not undercut the mask
Not very agitation sensitive
Masking very difficult e.g. LPCVD
Si3N4
Bulk Micromachining
Example: electrochemical sensor array
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A typical bulk micromachining
example: to make an array of
electrochemical sensors in a catheter
(e.g. to measure pH, O2 and CO2 in
blood)
The etch stop in this case is a sacrificial
oxide layer
Yet smaller structurs could be used to
experiment in picoliter microvials (e.g.
to investigate a single biological cell)go visit
http://pubs.acs.org/hotartcl/chemtech/98
/feb/exper.html
Bulk Micromachining
Example: electrochemical sensor array
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As in most cases the packaging is
the more difficult and more
expensive part of the sensor
fabrication
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The sensor array is mounted in a
catheter (750 µm diameter)
Biocompatible materials is still a
very big issue
CAD of the sensor array
Example: electrochemical sensor array