Download Class 2 updated Sep 30

MEMS
Class 2
Materials for MEMS
Mohammad Kilani
MEMS Materials
Silicon-Compatible Material System
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Silicon
Silicon Oxide and Nitride
Thin Metal Films
Polymers
Other Materials and Substrates
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Glass and Fused Quartz Substrates
Silicon Carbide and Diamond
Gallium Arsenide and Other Group III-V Compound
Semiconductors
Polymers
Shape-Memory Alloys
Silicon
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The second most abundant element in the Earth's crust
coming after oxygen, making up 25.7% of it by weight.
In its crystalline form, it has a dark gray color and a
metallic luster.
Occurs in clay, granite, quartz and sand, mainly in the
form of silicon dioxide (also known as silica) and silicates
(compounds containing silicon, oxygen and metals).
The principal component of glass, cement, ceramics,
most semiconductor devices, and silicones, the latter a
plastic substance often confused with silicon.
Reacts with halogens and dilute alkalis, but most acids
do not affect it (except for a combination of nitric acid and
hydrofluoric acid).
One of very few materials that is economically
manufactured in single crystal substrates.
Tremendous wealth of information accumulated on silicon
and its compounds over the last few decades.
Silicon Oxides
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The surface of silicon oxidizes immediately upon
exposure to the oxygen in air (referred to as native
oxide). The oxide thickness self-limits at a few
nanometers at room temperature. As silicon dioxide is
very inert, it acts as a protective layer that prevents
chemical reactions with the underlying silicon.
Silicon Oxide is stable and is electrically and thermally
insulating.
Can be preferentially etched in hydrofluoric acid (HF) with
high selectivity to silicon.
Silicon dioxide (SiO2) is thermally grown by oxidizing
silicon at temperatures above 800°C,
A drawback of silicon oxides is their relatively large
intrinsic stresses, which are difficult to control. This has
limited their use as materials for large suspended beams
or membranes
Silicon nitride (SixNy) is also a widely used insulating thin
film and is effective as a barrier against mobile ion
diffusion—in particular, sodium and potassium ions found
in biological environments.
Silicon in the Periodic Table
Has the symbol Si and atomic number 14
The silicon atom
The Silicon atom
has 4 electrons in its
outer shell
A silicon unit cell
consists of eight
silicon atoms.
Silicon and
germanium have a
diamond crystal
structure. The silicon
structure belongs to
the class of face
center cubic unit
cells.
Silicon Crystallography
The structure can be seen as two interpenetrating face centered crystal sublattices
with one sublattice displaced from the other by one quarter of the distance along the
body diagonal of the cube.
Miller Convention
As a crystal is periodic, there exist families of equivalent directions and
planes. Notation allows for distinction between a specific direction or plane
and families of such.
Use the [ ] notation to identify a specific direction (ie [1,0,-1]).
Use the < > notation to identify a family of equivalent directions (ie <110>).
Use the ( ) notation to identify a specific plane (ie (113)).
Use the { } notation to identify a family of equivalent planes (ie {311}).
A bar above a index is equivalent to a minus sign.
Vectors in the Silicon Lattice
[100],[100]
[110],[1 10]
[111],[1 1 1]
[010],[0 10]
[101],[10 1]
[111],[1 1 1]
[001],[00 1]
[011],[0 1 1]
[1 11],[11 1]
[110],[1 10]
[11 1],[1 11]
[101],[10 1]
[0 11],[01 1]
3x2=6
3 x 22 = 12
1 x 23 = 8
equivalent <100> vectors
equivalent <110> vectors
equivalent <111> vectors
Crystallographic Planes
6 (100) planes,
corresponding to 6 faces.
Each two opposite faces
results in the same plane
12 (110) planes,
corresponding to 12 edges.
Each two opposite edges
result in the same plane
8 (111) planes,
corresponding to 8 vertices.
Each two opposite vertices
result in the same plane
3 {100} planes
6 {110} planes
4 {111} planes
The Four {111} Planes
Angles between crystallographic planes
a .b  a b cos 
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ax bx  ay b y  az bz
a
.
b
1
1 
  cos 
  cos
2
2
2
2
2
2
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a
b
a

a

a
b

b

b


y
z
x
y
z
 x
Example:
Angle between (001) and (111) planes
 [001].[111]
  cos 
 [001] [111]
1

1  1 
  cos 
  54.7
 3





{111} planes has slow etch rate in KOH solution
perspective view of a {100} wafer and
a KOH-etched pit bounded by {111}
planes
ODE etching with concave corners in the mask openings
Square and rectangular openings in mask with the silicon etched for short or
long time. Anisotropic etching or Orientation Dependent Etching (ODE).
ODE etching with convex corners in the mask openings
Etching at convex corners and the formation of
suspended beams of a material that is not etched
(e.g., silicon nitride, p++ silicon). The {411} planes
are frequently the slowest etching and appear at
convex corners.
Etching through arbitrarily shaped opening in mask.
Flat assignments
Flats at 180 deg for n-type and 90 deg for p-type
Flats at 45 deg for n-type, no secondary for p-type.
Wafer Processes
Silicon Production
Silicon is commercially prepared by the heating
of high-purity silica in an electric arc furnace
using carbon electrodes. At temperatures over
1900 °C, the carbon reduces the silica to silicon
according to the chemical equation
SiO2 + C → Si + CO2
Liquid silicon collects in the bottom of the
furnace, and is then drained and cooled. The
silicon produced via this process is called
metallurgical grade silicon and is at least 99%
pure. Using this method, silicon carbide, SiC,
can form. However, provided the amount of
SiO2 is kept high, silicon carbide may be
eliminated, as explained by this equation:
2SiC + SiO2 → 3Si + 2CO
In 2000, metallurgical grade silicon cost about $
0.56 per pound ($1.23/kg)
Silicon Purificattion
(Zone Melting Method)
The first silicon purification method to be
widely used industrially is the zone
method. Rods of metallurgical grade
silicon are heated to melt at one end.
Then, the heater is slowly moved down the
length of the rod, keeping a small length of
the rod molten as the silicon cools and
resolidifies behind it. Since most impurities
tend to remain in the molten region rather
than resolidify, when the process is
complete, most of the impurities in the rod
will have been moved into the end that
was the last to be melted. This end is then
cut off and discarded, and the process
repeated if a still higher purity was desired
Silicon Purificattion
(Siemens Process)
Today, silicon is purified by first converting it to a liquid silicon compound
that can be purified by successive fractional distillation, and converting the
compound back to Silicon. In the Siemens process, Trichlorosilane
(HSiCl3) is the silicon compound used as an intermediate. Trichlorosilane,
which has a boiling point of 318 °C is formed via a reaction with HCl as
follows:
Si + 3HCl → SiHCl3 + H2 at 600 K
High-purity silicon rods are exposed to trichlorosilane gas at 1150 °C. At
this high temperature, the trichlorosilane gas decomposes and deposits
additional silicon onto the rods, enlarging them according to chemical
reactions like
2 HSiCl3 (g) → Si + 2 HCl + SiCl4 at 1400 K
Silicon produced from this and similar processes is called polycrystalline
silicon. Polycrystalline silicon typically has impurity levels of 1 part per
billion or less.
Silicon Crystallization
Czochralski process
The majority of silicon crystals grown for device production are produced by the Czochralski
process, since it is the cheapest method available. A small seed crystal of the material to be
grown is lowered to the surface of the melt and then drawn upwards, slowly. As the seed crystal is
pulled from the melt, it draws with it a layer of molten material. This material cools gradually,
taking on the same crystalline structure as the seed crystal.
Silicon Crystallization
Czochralski process
Pellets of dopant material are
added to the melt if extrinsic
semiconductor material is
required. The process is
named after Jan Czochralski,
who discovered the method in
1916 while investigating the
crystallization rates of metals.
Silicon Crystallization
Float-Zone process
When silicon is grown by the Czochralski method the melt is contained in a silica
(quartz) crucible. During growth the walls of the crucible dissolve into the melt
and Czochralski silicon therefore contains oxygen impurities with a typical
concentration of 1018cm − 3. For certain electronic devices, particularly those
required for high power applications, silicon grown by the Czochralski method is
not pure enough. For these applications, float-zone silicon (FZ-Si) can be used
instead. Up to 99.99999999999% (11 nines) purity may be obtained
Wafer Preperation
•Size the boule
•X-ray orient and grind
one or more flats
•slice
•lapping
•etching
•polishing and cleaning
Fabrication Processes