Explain silicon crystal structure. Why silicon is used as substrate material in MEMS ?

Silicon has an uneven lattice geometry for its atoms, but it has basically a face centered cubic [FCC] unit cell.

• A unit cell consists of atoms situated at fixed Locations defined by imaginary lines called a lattice. The dimension b of the lattice is called the lattice constant in the figure.

• In a typical FCC crystal atoms are situated at the right corners of the cubic lattice structure as well as at the center of each of the six faces.

• Foy silicon crystals, the lattice constant b: 0:53mm. In an FCC lattice, each atom is bonded to 12 nearest neighbor atoms.

Silicon is an ideal substrate material for MEMS because of the following reasons:-

• It is mechanically stable and it is feasible to be integrated into electronics on the same substrate.

• Electronics for signal transduction such as the p or n-type piezoresistive can be readily integrated with the si-substrate ideal for transistors.

• Silicon is almost an ideal structure material. It has about the same Young's modulus as steel [nearest to $2*10^5$ mpa] but it is as light as aluminium with a density of about $2.3 g/cm^3$.

• It has a melting point at 1400°c, which is about twice higher than that of aluminium. 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 aluminium.

• Silicon shows virtually no mechanical hysteresis. It is thus an ideal candidate malerial for sensor and actuaters.

• Silicon waters are extremely flat for coatings and additional thin film layers for either being integral structural parts or performing precise electro mechanical functions.

• There is a greater flexibility in design. and manufacture with silicon than with other substrate materials. Treatments and fabrication processes for silicon substrates are well established and documented.