This document discusses materials used for MEMS and microsystems, including substrates, active materials, and packaging materials. Common substrate materials include silicon, quartz, and various polymers. Silicon is discussed in detail due to its ideal properties as a substrate. Other materials covered include silicon compounds, piezoelectric crystals, and conductive polymers. The document concludes with a brief overview of packaging materials and methods.
2. Outline
3.1 Introduction
3.2 Substrates and Wafers
3.3 Active Substrate Materials
3.4 Silicon as a Substrate Material
3.5 Silicon Compounds
3.6 Quartz
3.7 Piezoelectric Crystals
3.8 Polymers
3.9 Packaging Materials
3.10 Fabrication of Pressure Sensor.
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3. 3.1 Introduction
• The current technologies used in producing MEMS and microsystems are inseparable from
those of microelectronics.
• Many microsystems use microelectronics materials such as silicon, and gallium arsenide
(GaAs) for the sensing or actuating elements.
• These materials are chosen mainly because they are dimensionally stable and their
microfabrication and packaging techniques are well established in microelectronics.
• Other materials used for MEMS and microsystems products:
• quartz and Pyrex, polymers and plastics, and ceramics―that are not commonly used in
microelectronics.
• Plastics and polymers — used extensively in the case of microsystems produced by the
LIGA processes.
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4. 3.2 Substrates and Wafers
• Substrate:
• Micro fabricated devices are not generally freestanding (separate) devices but
are usually formed over or in a thicker support substrate.
• For electronic applications, semiconducting substrates such as silicon wafers
can be used. For optical devices or flat panel displays, transparent substrates
such as glass or quartz are common.
• The substrate enables easy handling of the micro device through the many
fabrication steps. Often many individual devices are made together on one
substrate and then singulated into separated devices toward the end of
fabrication.
Examples:
• Pressure sensors that convert the applied pressure to the deflection of a thin
diaphragm that is an integral part of a silicon die cut from a silicon substrate.
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8. 3.4 Silicon as a Substrate Material
• The Ideal Substrate for MEMS
• Single-Crystal Silicon and Wafers
• Crystal Structure
• Mechanical Properties of Silicon
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9. The Ideal Substrate for MEMS
• It is mechanically stable and it can be integrated into electronics on the same substrate.
• Silicon is almost an ideal structural material. It has about the same Young’s modulus as steel
(about 2 × 105
MPa), but is as light as aluminum.
• It has a melting point at 1400 , which is about twice as high as that of aluminum. This high℃
melting point makes silicon dimensionally stable even at elevated temperature.
• Its thermal expansion coefficient is about 8 times smaller than that of steel, and is more than
10 times smaller than that of aluminum.
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Single-Crystal Silicon and Wafers
• The puller is slowly pulled up along
with a continuous deposition of
silicon melt onto the seed crystal.
As the puller is pulled up, the
deposited silicon melt condenses
and a large bologna-shaped boule
of single-crystal silicon several feet
long is formed.
• The diameter of the boules ranges
from 100 mm to 300 mm.
pure silicon crystal producing:
15. Crystal Structure
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Silicon has basically a FCC unit
cell.
In a typical FCC (face centered
cubic) crystal, atoms are situated at
the eight corners of the cubic lattice
structure, as well as at the center of
each of the six faces.
16. 3.4.5 Mechanical Properties of Silicon
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• Silicon is an elastic material with no plasticity or creep below 800 .℃
• It shows virtually no fatigue under all conceivable circumstance. These unique
characteristics make it an ideal material for sensing and actuating in microsystems.
• However, 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.
• Another disadvantage of silicon substrates is that they are anisotropic. This makes
accurate stress analysis of silicon structures tedious, since directional mechanical
property must be included.
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3.5.1 Silicon Dioxide
• Principal uses:
• as a thermal and electric insulator,
• as a mask in the etching of silicon substrates,
• as a sacrificial layer in surface micromachining.
• Has much stronger resistance to most etchants
than silicon.
• Production:
• heating silicon in an oxidant such as oxygen
with or without steam. Chemical reactions for
such processes
• “dry” oxidation Si + O2 → SiO2
• “wet” oxidation Si + 2H2O → SiO2 + 2H2
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3.5.2 Silicon carbide
• Principal applications:
• Its dimensional and chemical stability at high temperatures.
• very strong resistance to oxidation even at very high temperatures.
• Thin films of silicon carbide are often deposited over MEMS components to
protect them from extreme temperature.
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3.6 Quartz
• Composition ： Quartz is a compound of SiO2.
• Characteristics and application ：
• an ideal material for sensors because of its near
absolute thermal dimensional stability.
• used in many piezoelectric devices.
• wristwatches, electronic filters, resonators.
• Inexpensive.
• Transparent to ultraviolet light, which is often used
to detect the various species in the fluid.
• Machine ：
• diamond cutting
• ultrasonic cutting
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3.7 Piezoelectric Crystals
• Piezoelectric crystals are the solids of ceramic compounds that can
produce a voltage when a mechanical force is applied between
their faces.
• The reverse situation, that is the application of voltage to the crystal,
can also change its shape.
• This unique material behavior is called the piezoelectric effect.
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3.8 Polymers
• Polymers, which include such diverse
materials as plastics, adhesives,
Plexiglas, and Lucite, have become
increasingly popular materials for MEMS
and microsystems.
• Example:
plastic cards approximately 150 mm
wide containing over 1000 microchannel
have been adopted in microfluidic
electrophoretic systems by the
biomedical industry.
• Structure ： This type of material is made up of
long chains of organic (mainly hydrocarbon)
molecules. The combined molecules, i.e.,
polymer molecules, can be a few hundred
nanometers long.
• Properties ： Low mechanical strength, low
melting point, and poor electrical conductivity
characterize polymers.
• Thermoplastics-easily formed to the desired
shape for the specific product
• Thermosets-have better mechanical strength
and temperature resistance up to 350 .℃
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Usage and Advantages
• Usage ：
• Traditionally--used as insulators, sheathing, capacitor films
in electric devices, and die pads in integrated circuits.
• A special form, the plastics--widely used for machine and
device components.
• Advantages ：
• Light weight
• Ease in processing
• Low cost of raw materials and processes for producing
polymers
• High corrosion resistance
• High electrical resistance
• High flexibility in structures
• High dimensional stability
26. Polymers for MEMS and Microsystems
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• Photoresist polymers are used to produce masks for creating desired patterns on
substrates by photolithography.
• The same photoresist polymers are used to produce the prime mold with the desired
geometry of MEMS components in the LIGA process for manufacturing micro device
components.
• These prime molds are plated with metals such as nickel for subsequent injection molding
for mass production of microcomponents.
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Conductive Polymers
• For polymers to be used in certain applications in
microelectronics, MEMS, and microsystems, they
have to be made electrically conductive with
superior dimensional stability.
• Polymers have been used extensively in the
packaging of MEMS, but they have also been used
as substrates for some MEMS components in recent
years with the successful development of
techniques for controlling the electric conductivity
of these materials.
• Pyrolysis:
• A pyro polymer can be made electrically
conductive by adding an amine heated
above 600 .℃
• conductivity-- 2.7 ×104
S/m>carbon.
• Doping
• Insertion of Conductive Fibers
28. 3.9 Packaging Materials
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• Differences:
• IC—protect the IC die and the interconnects from the often hostile operating
environment.
• microsystem—not only are the sensing or actuating elements to be protected, but they are
also required to be in contact with the media that are the sources of actions. Many of
these media are hostile to these elements.
• Materials: wires made of noble metals, metal layers for lead wires, solders for die/constraint
base attachments, etc., metals and plastics
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3.9 Packaging Materials
• Applications:
• aluminum or gold metal films—ohmic
contacts, lead wires
• plastic or stainless steel—casing
• Glass—constraint bases.
• tin-lead solder alloys or epoxy resins or RTV
—adhesive
• Copper and aluminum—metal
layers(sputtered)
• silicone gel or silicone oil—shield silicon
diaphragm etc.
