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Materials Science in MEMS
GSA: Brooks A. Gross
06.29.2006
Lecture Outline
• Silicon-Compatible Material System
• Other Materials and Substrates
• Important Material Properties & Physical
Effects
Silicon-Compatible Material System
• Silicon (Chemical symbol: Si)
– Economically manufactured in single-crystal
substrates
– Crystalline nature provides electrical &
mechanical advantages
• Electrical conductivity modulated by impurity
doping (key to electronic semiconductor devices)
• Mechanically, it is elastic and robust
• A suitable material platform for integrating
electronic, mechanical, thermal, optical, and
microfluidic functions
Properties of Some MEMS
Materials
Low Cost of Si
• $10 for 100-mm-diameter wafer
• $15 for 150-mm-diameter wafer
Structural Types of Si
•
•
•
•
Crystalline
Polycrystalline (aka - polysilicon or poly-Si)
Amorphous
The latter 2 are usually deposited as thin
films typically under 5μm thick.
Si Wafers
• Commercially available as circular wafers
– Sizes: 100, 150, 200, & 300mm diameter
– Over 0.5mm thick (double-sided polished
wafers usually 100 microns thinner)
– Anything above 150mm is not economical for
MEMS at this time.
• Why?
– Fabrication facility costs for new machines are prohibitive
when the machines are the newest on the market for the
IC industry.
– It’s all about production volume.
Crystal structure of Si
• Diamond-cubic
– Can be discussed as simple cubic
• Primitive unit (smallest repeating block) of Si
• 3 major axes called principle axes
• Reference axes using a notation called Miller
indices
Miller indices
• Directions specified by brackets [xyz] for the
axes (x,y,z)
– No commas between numbers
– Negative #’s have a line over them instead of a minus
sign
• Groups of directions specified with carets (e.g.
<100>: [100] = +x,[010] = +y,[001] = +z, & their
negative counterparts)
• (xyz) specify a plane perpendicular to a vector
• {xyz} specify all equivalent planes.
• What the heck?
Angles Between Planes
• {100} & {110} planes have 45O or 90O
angles between them
• {100} & {111} planes have 54.7O or 125.3O
angles between them
• {111} & {110} planes have 35.3O, 90O or
144.7O angles between them
Why are angles of intersection
important?
• Direction-specific etchants (Ch. 3)
– Takes advantage of the crystal lattice to for
different structures of the MEMS
– Important to start with the best wafer type for
a given process to yield the MEMS with the
least amount of steps
• Saves time and $!!!
– How do you know which type of wafer you
have?
Illustration of Wafer Cuts
Crystalline Si Characteristics
• Hard & brittle
• Tensile yield strength = 7GPa
• Young’s modulus = 169GPa in <110>,
130GPa in <100> (similar to steel)
• Good thermal conductivity
• Not optically active (so no lasers)
• Consistent across wafer lots, making bulk
processing reliable
Poly-Si
• Used to:
– make micromechanical structures
– integrate electrical interconnects,
thermocouples, p-n junction diodes, etc.
• Mechanical properties
– Vary with deposition conditions, but similar to
crystalline Si (except for temperature: Si
stable up to 700O C, poly-Si up to 250O C)
– Important to control conditions so that
mechanical structures like beams do not curl
Silicon Oxide
• Si oxidizes on the surface when exposed to
oxygen.
– At room T, self-limited to a few nm
– Inert, acting as a protective layer against chemicals
• Great electrical & thermal insulators
• Can be used as a sacrificial layer (Ch. 3)
• Can be formed on the Si using various
techniques (Ch. 3)
• Drawback is large intrinsic stress, which can be
hard to control in the manufacturing process
Silicon Nitrides (SiXNY)
• Insulating film
• Barrier to ion diffusion (e.g. sodium or
potassium ions in biological systems)
• Young’s modulus higher than Si
• Intrinsic stress can be controlled
• Can be used as a masking material
Thin Metal Films
• Deposited by sputtering, evaporation,
CVD, and some by elecroplating
• Metal chosen by considering end-use.
• Some metals are used as an adhesion
layer (e.g. chromium)
Polymers
• Used as a photoresist or as structures of
the MEMS
• Thicknesses range between 1 and 100
microns
• Can be used as chemical gas sensors and
humidity sensors due to their unique
adsorption and absorption properties
Other Materials
• Glass
– Can be electrostatically bonded to Si
– Used in making pressure sensors
– Has a different coefficient of thermal
expansion than Si, resulting in interfacial
stresses
• Crystalline quartz
– Piezoelectric
Other Materials
• Si-Carbide & diamond
– Very hard
– High stiffness (high Young’s modulus)
– resistant to harsh chemicals
– Wide bandgap
– Very high thermal conductivity
– More in next Tuesday’s lecture…
Other Materials
• Group III-V compound semiconductors
– Being explored as an alternative to Si for
different mechanical structures
• Different orientation-dependent etching
– Practical way to integrate RF switches,
antennas, and other high-frequency
components for wireless devices
Polymers
• Long chains of carbon atoms or Si atoms
(silicones)
• Can be used to make microfluidic
channels
• Low cost
• Many are flexible
• Can act as barriers to flow of water or
vapor
Other Materials
• Shape-memory alloys
– Return to a predetermined shape when
heated above a transition temperature
(material-dependent)
– Ti-Ni most widely used
– Can generate very large forces
• Good for actuation purposes (unlike piezoelectric
and electrostatic actuators, but they can transition
much more quickly)
Piezoresistivity
• Derived from Greek word piezein meaning to apply
pressure
• Discovered by Lord Kelvin in 1856
• Phenomenon by which an electrical resistance changes
in response to a mechanical stress
• First application was a metal strain gauge to measure
strain, inferring force weight and pressure
• Most resistance change in metals due to dimensional
changes
• C.S. Smith discovered in 1954 that the effect is greater
in Si & germanium than in metals.
• Majority of current commercial pressure sensors use Si
piezoresistors
Physics of Piezoresistivity
• It arises from the deformation of the
energy bands as the result of an applied
stress.
• The deformed bands affect the effective
mass and the mobility of electrons and
holes, therefore modifying resistivity.
Figure 2.4
Piezoresistivity for the Engineer
• The fractional change in resistivity, Δρ/ρ, is to a
1st order linearly dependent on σ‫ & װ‬σ┴,the 2
stress components parallel & orthogonal to the
direction of the resistor, respectively.
• Direction of resistance defined as that of the
current flow:
Δρ/ρ= π‫ װ‬σ‫ װ‬+ π┴ σ┴
– are called the parallel & perpendicular piezoresistive
coefficients
Table 2.4
Piezoresistivity of Poly &
Amorphous Si
• Coefficients lose their sensitivity to
direction
• Use a gauge factor, K, instead
– From -30 to +40—about 1/3 of single-crystal
Si
– K decreases quickly as doping increases
above 1019cm-3
• Main advantage is a reduced TCR (i.e.
much lower dependence on temperature)
Piezoelectricity
• Some crystals produce an electric field when
subjected to an external force.
• Also, they can expand or contract in response to
an externally applied voltage.
• Discovered in quartz by Curie brothers in 1880
• 1st practical application in 1920s as quartzbased sonar.
• Why piezoelectric MEMS?
– They can act as both sensors and actuators.
– They can be deposited as thin layers on Si.
Piezoelectricity
• At the atomic level
– Charge asymmetry within the unit cell
– This forms a net electric dipole.
– Summation of dipoles over entire crystal gives a net
polarization & an effective electric field.
– If the crystal has a center of symmetry, there is no
piezoelectric effect.
• Curie temperature is a critical temp. specific to
the material at which it loses its piezoelectric
properties.
Piezoelectric Material
Non-Piezoelectric Material
Device-level Function
Piezoelectric Properties
Thermoelectricity
• In the absence of an electric field, there
are 3 distinct thermoelectric effects:
– Seebeck – used in thermocouples to measure
temp. differences
– Peltier – used to make thermoelectric coolers
& refrigerators
– Thomson – uncommon
Peltier Effect
• Current flow across a junction of 2
dissimilar materials causes a heat flux,
cooling one side and heating the other
– Large scale appliances, like the mobile wet
bar of the 1950s, have poor energy
conversion efficiency.
– Today, n-type & p-type bismuth telluride
elements are used to cool microprocessors,
laser diodes, & IR sensors.
– Difficult to make thin film versions
Seebeck Effect
• Temperature gradient across an element
gives rise to a measurable E field that
tends to oppose the charge flow resulting
from the T imbalance.
*Board with next slide
Thermocouple