Description about the materials used in MEMS .

1. Chapter 2: MEMS Materials
Materials for MEMS
 Materials for MEMS
• Silicon (majority)
• Silicon compatible materials: SixOy, SixNy
• Glass
• Ceramics
• Polymers: photoresist, polyimide.
• Compound semiconductors
• Metals: Al, Ti, W, Cu, etc.
• Silicon Carbides (SiC).
• Other materials
Silicon Material for MEMS
 Single Crystalline Silicon (SCS): Si, hard, fragile, cleave
along crystal plane.
 Why Silicon?
• Atomic number: 14, atomic weight: 28.
• Density: ρ=2.33g/cm3,
• Young’s modulus: E=170GPa (along <110> direction)
• Abundant on earth: inexpensive
• Compatible with existing VLSI technology (easy integration)
• Excellent electrical properties: conductivity modulated with
impurity doping (n-type/p-type)
• Excellent mechanical properties: elastic and robust
• Good thermal conductivity: 1.56W/cm·K at 300K.
• Types: amorphous, polycrystalline, crystalline

5. Slicing and Dicing of Silicon
Slicing silicon ingot into wafers
Dicing silicon wafer into chips

6. Crystal Structure of Single-crystalline Silicon Miller Indices of Crystalline Plane/Direction
 It is often necessary to be able to specify certain directions
and planes in crystals. Many material properties and
processes vary with direction in the crystal.
 Directions and planes are described using three integers -
Miller Indices.
 x, y, z are the axes (on arbitrarily positioned origin)
 a, b, c are lattice parameters (length of unit cell along a side)
 h, k, l are the Miller indices for planes and directions -
expressed as planes: (hkl) and directions: [hk]
 Conventions for naming
 There are NO COMMAS between numbers
 Negative values are expressed with a bar over the number
Example: -2 is expressed 2.
 Miller plane/direction symbols:
plane: (hkl) (round brackets)
family of planes: {hkl} (curly brackets)
direction: [hkl] (square brackets)
family of directions <hkl> (angle brackets)

7. Miller Indices of a Crystalline Direction
 Miller Indices of a crystalline direction:
1. Draw the vector of the direction, find the coordinates of the
head, h1,k1,l1, and the tail h2,k2,l2. If vector starts from
origin, then h1,k1,l1=0,0,0.
2. Subtract coordinates of tail from coordinates of head: h2-
h1,k2-k1,l2-l1.
3. Multiply the result by a common factor to convert them into
the smallest integer set, enclose it in square bracket, this
is the Miller Indices of direction.
Family of Directions
 All the parallel directions have the same Miller Indices.
 Indices in angle brackets denote a family of directions
which are equivalent due to symmetry operations. For
example, [100], [010], [001] or the negative of any of those
directions are equivalent. They are put in angle brackets
and called <100> family of directions.
Miller Indices of a Plane
 Miller Indices of a Plane:
1. Determine the intercepts (x, y, z) of the plane along each of
the three crystallographic directions. If the plane passes
through the origin, select a parallel equivalent plane not
passing the origin. If the plane is in parallel to an axis, the
intercept is ∞.
2. Take the reciprocals of the intercepts (1/x, 1/y, 1/z).
(Note:1/∞=0)
3. If fractions results, multiply each by the denominator of the
smallest fraction, resulting in smallest integer set (h, k, l),
which are the Miller indices of a plane.
Miller Indices of a Plane
 Miller Indices of a Plane:
Planes and their negatives
are equivalent:
 In the cubic system, a plane and a direction
with the same indices are orthogonal. E.g.
[001] direction is orthogonal to (001) plane.

8. Linear and Planar Density
 Linear and planar density, Why do we care?
- Properties, in general, depend on linear and planar density.
Linear and Planar Density
 Example: Find the linear density of the [110] and the
[100] direction in the FCC cell in terms of the atomic
radius R.
Miller Indices of a Plane
 Miller Indices of a Plane:
Planes and their negatives
are equivalent:
 In the cubic system, a plane and a direction
with the same indices are orthogonal. E.g.
[001] direction is orthogonal to (001) plane.

9. Silicon Wafers
 Identify doping type (n or p) and surface orientation of silicon
wafers by judging the flats:
 The angle between primary and second flats can be 0°, 45°,
90° and 180°.
 For smaller angles (0°, 45°): (111) wafer; for larger angles
(90°, 180°): (100) wafer
 Within each surface orientation, smaller angle (0°and 90°): p-
type; larger angle (45°, 180°): n-type.

10. Silicon Wafer Cleaning
 Chemicals used for wafer cleaning
• H2SO4 (sulfuric acid)
• HCl (hydrocloric acid)
• H2O2 (hydrogen peroxide)
• NH4OH (ammonium hydroxide)
• HF (hydrofluoric acid)
• DI water (deionized water)
Poly-silicon as MEMS Material
 Advantages
- Compatible with VLSI technology
- Good for surface micromachining
 Disadvantages
- Residual stress during deposition
- Thickness limitation: <10µm
 Chemical properties of Si: stable and resistant to many
chemicals, but can be etched with certain etchants with
good controllability.

11. Poly-Si MEMS micromotor ADXL150 poly-Si MEMS accelerometer
Silicon Dioxide as MEMS Material
 Silicon oxides: SiO2, SixOy, silicate glass, etc.
 SiO2: very stable, electrical insulating, also used as
sacrificial layers in surface micromachining, easily
removed with HF.
 How to obtain SiO2
- Thermal oxidation
Si+O2SiO2
- CVD, sputtering, spin-on.
Limitations: large residual stress