Short note: Thin-film deposition techniques

Thin films (nm to a few µm) form the structural, sacrificial, insulating and conducting layers of MEMS. Deposition methods fall into two groups.

Method	Principle	Typical films
Evaporation (PVD)	Heated source vaporises in vacuum and condenses on the substrate	Al, Au contacts
Sputtering (PVD)	Ion bombardment ejects target atoms that coat the substrate	Metals, ZnO, PZT
LPCVD	Gas reaction at low pressure and high temperature; uniform films	Polysilicon, silicon nitride
PECVD	Plasma allows reaction at a lower temperature	Silicon oxide, nitride
Thermal oxidation	Silicon reacts with O₂ or steam at high temperature	SiO₂
Electroplating	Metal ions deposited from solution by current	Thick Ni, Cu, Au (LIGA)
Spin coating	Liquid spread by centrifugal force, then cured	Photoresist, polymers, SU-8

Selection factors

Film uniformity, step coverage, stress, deposition temperature, rate and cost.

Exam tip

Divide into PVD and CVD, then give one-line principles and examples in a table.