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 σװ & σ┴,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= πװ σװ + 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 quartz-based

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