There 3 principal silicon compounds in MEMS & microsystem: Silicon dioxide (SiO2), Silicon Carbide (SiC) & Silicon nitride ($Si_3N_4$) – each has distinct characteristic & unique applications.
Silicon dioxide ($SiO_2$)
- It is least expensive material to offer good thermal & electrical insulation.
- Also used a low – cost material for “mask” in micro fabrication processes such as etching, deposition & diffusion.
- Used as sacrificial material in “surface micromachining”.
- By dry heating of silicon : $Si+O_2→ SiO_2$
- Or by oxide silicon in wet steam $Si+2H_2O→SiO_2+2H_2$
- Silicon Dioxide ($SiO_2$) is used in MEMS due to following reasons,
- as a thermal & electric insulator.
- as a mask & etching of silicon substrate.
- sacrificial layer in surface micromachining
- It offer stronger resistance as compared to silicon in etching process.
![enter image description here](https://i.imgur.com/ZF8cjst.jpg)
Silicon Carbide (SiC)
Its very high melting point and resistance to chemical reactions make it ideal candidate material for being masks in micro fabrication processes.
- Silicon carbide is used in MEMS as it offers very strong resistance to the oxidation at very high temperature.
- SiC is product which is obtained while producing the silicon from the row silicon.
Silicon nitride (Si3N4)
- Produced by chemical reaction $3SiCl_2H_2+4NH_3→Si_3N_4+6HCL+6H_2$
- Used as excellent barrier to diffusion to water and icons.
- Its ultra strong resistance to oxidation & many etchants make it a superior material for masks in deep etching.
- Also used as high strength electric insulators.
- Selected properties SiIts ultra strong resistance to oxidation & many etchants make it a superior material for masks in deep etching.
- Also used as high strength electric insulators.
- Selected properties Si3N4films are as follows:
![enter image description here](https://i.imgur.com/tHu50pL.jpg)
Polycrystalline silicon
- It is usually called “polysilicon”.
- It is an aggregation of pure silicon crystals with randomly orientations deposited on the top of silicon substrates:
- These polysilicon usually are highly doped silicon.
- They are deposited to the substrate surfaces to produce localized “resistors” & “gates for transistors”
- Being randomly oriented, polysilicon is even stronger than single silicon crystals.