Substrate/Wafers Substrate/wafer Material Silicon as substrate Miller Indices mechanical properties of silicon Silicon compounds

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MEMS &
MicrosystemsMICRO-ELECTRO-MECHANICAL SYSTEMS
”Machine on a chip” / “Micro-machine”
Or
”Anything designed and produced using Microelectronics toolset ”

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MEMS &
Microsystems
MICRO-ELECTRO-MECHANICAL SYSTEMS

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Contents:
• Substrate or Wafers
• Substrate Materials
• Silicon as a Substrate Material
• Miller Indices
• Mechanical properties of Silicon
• Silicon Compounds

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Wafers
• A wafer, also called a slice, is a thin slice of
semiconductor material, such as a crystalline silicon,
used in electronics for the fabrication of integrated
circuits and in photovoltaics for conventional,
wafer-based solar cells.
• The wafer serves as the layer for microelectronic
devices built in and over the wafer and undergoes
many microfabrication process steps such as doping
of ion implantation, etching, deposition of various
materials, and photolithographic patterning.
• Finally the individual microcircuits are separated
(dicing) and packaged.

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Wafers
• Wafer is a solid substance onto which a layer of another
substance is applied, and to which that second
substance adheres.
• In solid-state electronics, this term refers to a thin slice
of material such as silicon, silicon dioxide, aluminum
oxide, sapphire, germanium, gallium arsenide (GaAs), an
alloy of silicon and germanium, or indium phosphide
(InP).
• These serve as the foundation upon which electronic
devices such as transistors, diodes, and especially
integrated circuits (ICs) are deposited.

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Wafer
• Wafers are formed of highly pure (99.9999999%
purity), nearly defect-free single crystalline
material.
• The wafer is a slice cut from a larger piece of a
single crystal substrate. Wafers can be of silicon
or other single crystalline material such as quartz
or gallium arsenide.
• A single crystal is a material in which
the crystal lattice of the entire sample
is continuous and unbroken.

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Wafer & Substrate
• In microelectronics, the wafer means a
flat macroscopic object on which
micro-fabrication processes take place.
• In microsystems, a substrate serves an
additional purpose: it acts as signal
transducer besides supporting other
transducers that convert mechanical
actions to electrical outputs or vice
versa.

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Substrates / Wafers Materials
The common substrate materials used in MEMS are silicon (Si), germanium
(Ge), and gallium arsenide (GaAs) all fall in the category of semiconductors.
One major reason for using these materials as principal substrate materials
in both microelectronics and microsystems is that these materials are at the
borderline between conductors and insulators, so they can be made either a
conductor or an insulator as needs arise.
The doping techniques can be used to convert the most commonly used
semiconducting material to an electrically conducting material by doping it
with a foreign material for conducting electricity.

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Substrate Material
substrate materials
for microsystems
with typical
electrical resistivity
of conductors,
semiconductors and
insulators are
shown below:

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Substrate / wafer Material
There are two types of substrate
materials used in microsystems:
1- Passive substrate materials
2- Active substrate materials
The passive material is one that does not play an essential role in the
sensing mechanism.
Passive materials are only used to provide either mechanical structure
or electrical connection.

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Active Substrate Materials
• Active substrate materials are
primarily used for sensors and
actuators in a microsystems and in
other MEMS components.
• Typical active substrate materials
for microsystems include silicon,
gallium arsenide, germanium, and
quartz.
Silicon

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Active Substrate Materials
• These substrate materials have
basically a cubic crystal lattice
with a tetrahedral atomic bond.
• These materials are selected as
active substrates primarily for
their dimensional stability, which
is relatively insensitive to
environmental conditions.
Silicon

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Active Substrate Materials
• Dimensional stability is a critical
requirement for sensors and
actuators with high precision
Silicon

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Silicon as a Substrate Material
Silicon is the most widely used substrate material for MEMS and microsystems.
The popularity of silicon for such application is primarily for the following
reasons:
1. It is mechanically stable and it can be integrated into electronics on the same
substrate.
2. Electronics for signal transduction, such as a p- or n-type piezoresistor, can be
readily integrated with the Si substrate.
3. Silicon is almost an ideal structural material. It has about the same Young’s
modulus as steel (about 2 x 105 MPa), but is as light as aluminum, with a mass
density of about 2.3 g/cm3
.
Materials with a high Young’s modulus can better maintain a linear relationship
between applied load and the induced deformations.

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Silicon as a Substrate Material
4. It has a melting point at 1400°C, which is about twice as high as that of
aluminum. This high melting point makes silicon dimensionally stable even at
elevated temperature.
5. Its thermal expansion coefficient is about 8 times smaller than that of steel,
and is more than 10 times smaller than that of aluminum.
It is thus an ideal candidate material for sensors and actuators.
The silicon wafers are extremely flat and accept coatings and additional thin-
film layers for building micro-structural geometry or conducting electricity.

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Miller Indices
Miller indices form a notation system in crystallography for planes in crystal lattices.
The family of lattice planes is determined by three integers h, k, and ℓ, the Miller
indices.
Crystallography is the experimental science of determining the arrangement of
atoms in the crystalline solids.

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Miller Indices
It is often necessary to be able to specify certain directions
and planes in crystals.
Many material properties and machining processes vary
with direction in the crystal.
The strength of material also vary with direction in the
crystal.
The wafers that are shipped by suppliers normally indicate
in which directions, the cuts have been made to make flat
surface.
Directions and planes are described using three integers
Miller Indices. Miller index is denoted by (hkl).

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Miller Indices
STEP-2 Specify the intercepts in fractional co-ordinates
Fractional Intercepts : a/a , ∞/a, ∞/a i.e. 1 , ∞ , ∞
STEP-1 Identify the intercepts on the x- , y- and z- axes.
It intercepts on the x- axes at point a.
The surface is parallel to the y- and z-axes.
Therefore, intercepts on both axes is ∞.
The point is (x,y,z)=(a, ∞, ∞)
STEP-3 Take the reciprocals of the fractional intercepts
Reciprocals : 1/1 , ∞/1, ∞/1 i.e. 1 , 0 , 0
Therefore: Miller Indices : (100)

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Miller Index

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Miller Indices

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Miller Indices
On <111> plane, the lattice distances between adjacent atoms are shortest.
These short lattice distances between atoms make the attractive forces between atoms
stronger on this plane than those on the other two planes.
Also, this plane contains three of the four atoms that are situated at the center of the faces
of the unit cell.
Thus, the growth of crystal in this plane is the slowest and the fabrication processes, e.g.,
etching, as we will learn will proceed slowest.

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Miller Indices

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TASK # 1
Search publication on following topic.
MICRO MANUFACTURING
Read Abstract and Conclusion
Write a summary of minimum two pages and reply
the following queries.
WHAT, WHY, WHERE, WHO, WHOM, and HOW

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Mechanical Properties of Silicon
• It is a prime material in MEMS microsystems for sensors and actuators.
• Its three dimensional geometry withstand often severe mechanical and
thermal loads, and accommodating electrical instruments such as
piezoresistive integrated into it.
• Silicon is an elastic material with no plasticity or creep below 800°C.
• It shows virtually no fatigue failure under all conceivable circumstances.
• These unique characteristics make it an ideal material for sensing and
actuating in MEMS microsystems.

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Mechanical Properties of Silicon
• It is a brittle material. Therefore, undesirable brittle fracture behavior with
weak resistance to impact loads needs to be considered in the design of
such microsystems.
• It is anisotropic, this makes accurate stress analysis of silicon structures
tedious, since directional mechanical property must be included. .

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Mechanical Properties of Silicon
• For most cases in microsystem design, the bulk material properties of
silicon, silicon compounds, and other active substrate materials
presented in table are used.

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Mechanical Properties of Silicon
• It can be observed from table that copper has the highest thermal
diffusivity, whereas, silicon and aluminum have about the same value.
• The silicon oxide conducts heat more than 150 times slower than silicon
and aluminum.
It observed that copper films are the best material for fast heat
transmission in microsystems, whereas silicon dioxide can be used as an
effective thermal barrier.

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Silicon Compounds
Following three silicon compounds are often used in microsystems:
SiO2 Silicon dioxide
SiC Silicon Carbide
Si3N4 Silicon Nitride

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Silicon Compounds
There are three principal uses
of silicon oxide in
microsystems:
(1) as a thermal and electric
insulator.
(2) as a mask in the etching of
silicon substrates. and
(3) as a sacrificial layer in
surface micromachining.
Silicon oxide has much
stronger resistance to most
etchants than silicon.
Silicon Oxide (SiO2)

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Silicon Compounds
Silicon dioxide can be produced by heating silicon in an oxidant such as oxygen with
or without steam.
Chemical reactions for such processes are:
Dry Oxidation (without steam):
Wet Oxidation (with steam):
The process can be accelerated to much faster rates by the presence of steam; the
highly activated H2O molecules enhance the process.
Silicon Oxide (SiO2)

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Silicon Compounds
Silicon Oxide (SiO2)
The team created silicon dioxide (SiO2)
nanotube anodes for lithium-ion
batteries and found they had over
three times as much energy storage
capacity as the carbon-based anodes
currently being used.
This has significant implications for
industries including electronics and
electric vehicles, which are always
trying to squeeze longer discharges
out of batteries.

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Silicon Compounds
The principal applications of silicon carbide (SiC) in microsystems are:
(1) its dimensional and chemical stability at high temperatures.
(2) It has very strong resistance to oxidation even at very high temperatures.
(3) Thin films of silicon carbide are often deposited over MEMS components to
protect them from extreme temperature.
(4) Using SiC in MEMS is that dry etching with aluminum masks can easily pattern
the thin SiC film. The patterned SiC film can further be used as a protective layer in
micromachining for the underlying silicon substrate, as SiC can resist common
etchants
Silicon exists in the raw materials of carbon (coal, coke, wood chips, etc.), the
intense heating of these materials in the electric arc furnace results in SiC sinking to
the bottom of the crucible.
Silicon Carbide (SiC)

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Silicon Compounds
(1) It provides an excellent barrier to diffusion of water and ions such as sodium.
(2) Its ultra strong resistance to oxidation and many etchants makes it suitable for
masks for deep etching.
(3) Applications of silicon nitride include optical waveguides, encapsulation to
prevent diffusion of water and other toxic fluids into the substrate.
(4) It is also used as high-strength electric insulators and ion implantation masks.
Silicon nitride can be produced from silicon-containing gases and NH3 in the
following reaction:
Silicon Nitride(Si3N4)

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Silicon Compounds
Silicon in polycrystalline form can be deposited onto silicon substrates by chemical
vapor deposition (CVD). It is a principal material in surface micromachining.
The low pressure chemical vapor deposition (LPCVD) process is frequently used for
depositing polycrystalline silicon onto silicon substrates. The temperature involved in
this process is about 600 to 650°C. Polycrystalline silicon is widely used in the IC
industry for resistors, gates for transistors, thin-film transistors.
Polycrystalline Silicon

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Essay Questions
• What is substrate / wafer?
• Explain substrate’s materials?
• Clarify the importance of silicon as substrate material?
• Define Miller Index and its use in Crystallography with diagrams?
• List the thermal diffusivities of silicon, silicon dioxide, aluminum, and
copper, and make an observation on the results?
• Explain the role of silicon dioxide (SiO2); silicon carbide (SiC); and silicon
nitride (Si3N4) in MEMS microsystems.
Fill in the blanks / MCQ Questions