Explain Surface Micromachining

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

1 Answer
  • 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:

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

Etching rates for sacrificial layers

Thin Oxide Films Lateral Etch rate (µm/min)
CVD SiO2 (densifiedat 10500C for 30 min) 0.6170
Ion-implanted SiO2 (at 8 x 10115/cm2, 50 KeV) 0.8330
Phosphosilicate (PSG) 1.1330
5%-5% Boromphosphosilicate (BPSG) 4.1670

Mechanical problems

1) Quality of adhesion of layers:

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

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2) Interfacial stresses due to mismatch of CTE:

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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.
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