Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

• Etching process creates 3-D microstructures by removing material from substrates.
• Removed substrate materials are wasted.
• Surface micromachining creates 3-D microstructures by adding material to the substrate.
• Added materialsmay not be same as the substrate material-flexibility.
• Added material layers can be 2-5µm thick each, or as high as 5-20 µm thick each –much more than most etching process can achieve.
• There is little waste of substrate materials.
• Deposition processes are commonly used method-expensive.
• Requires multiple masks-expensive and time consuming.
• Requires sacrificial layers to create cavities – wasteful with technical problems.

General description of process

Illustration of micromachining process – creation of a polysilliconcanfillever beam on silicon substrate base:

Etching Process : 1:1 HF:H2O+1:1HCl:H2O. Rising with deionized water and dried under infrared lamp.

Etching rates for sacrificial layers

Mechanical problems

1) Quality of adhesion of layers:

• The interfaces of layers are the vulnerable areas for structural failures.
• Two possible failures:

2) Interfacial stresses due to mismatch of CTE:

3) Stiction:

• It is the most serious technical problem in surface micromachining.
• It occurs in structures separated by narrow gap that is supported by sacrificial - Stiction phenomenon is the collapsing of the layers supported by the sacrificial layers once they are removed by etching.
• Stiction may occur in the example of the cantilever beam fabricated by surface micromachining.
• Stiction occurs due to Van der Waais and chemical forces between surfaces with narrow gaps.