Subject: MEMS Technology

Topic: MEMS Fab Processes II

Difficulty: Medium

Substrates in LIGA process must be electrical conductive to facilitate subsequent electroplating over photoresists mould.

Metals such as: steel copper plates, titanium and nickel, orSilicon with thin titanium or silver / chrome top layer, glass with thin metal layers.

Photoresists materials

Basic requirements:

• Must be sensitive to x-ray radiation.

• Must have high resolution and resistance to dry and wet etching.

• Must have thermal stability up to 1400C.

• The unexposed part must be absolutely insoluble during development.

• Good adhesion to substrate during electroplating.

• PMMA appears most popular for LIGA process, but other polymers are available:

Electroplating

The inner surfaces of the photoresists mold produced by X-ray lithography need to be plated with thin metal layers for securing permanent microstructure geometry.

Metals available for the plating are: Ni, Cu, Au, NiFe and N/W.

In case of plating with Ni, the process is:

Nickel ions (Ni2+) are produced from electrolysis of NiCl2 solution.They are attracted to the electrons at the cathode:

Ni2 + 2e  Ni

There could be H+ ions presence at the same cathode in the process.These H+ ions may form H2 bubbles on the cathode, and thus Ni plate.

Proper control of the pH in the solution is important to miligate this effect.