Explain fabrication of any one of the MEMS devices using bulk micromachining technique
1 Answer
  • Micromachining is defined as the process of shaping Si or other material to realize 3 dimensional mechanical structure in miniature form.
  • The oldest micromachining technology is bulk micromachining.
  • This technique involves the selective removal of the substrate material in order to realize miniaturized mechanical components.
  • Bulk micromachining can be accomplished using chemical or physical means.
  • A widely used bulk micromachining technique is chemical wet etching, which involves the immersion of a substrate into a solution of reactive chemical that will etch exposed regions of the substrate at measurable rates.
  • Chemical wet can provide a very high etch rate and selectivity.
  • There are two general types of chemical wet etching in bulk micromachining: isotropic wet etching and anisotropic wet etching.

Isotropic wet etching:

  • The etch rate is not dependent on the crystallographic orientation of the substrate
  • The etching proceeds in all directions at equal rates.
  • Common masking materials for isotropic wet silicon etching include silicon dioxide and silicon nitride.

Anisotropic wet etching:

  • The etch rate depends upon the crystallographic orientation of the substrate.
  • Anisotropic etching etches more slowly along the <111> planes than all the other.
  • It provides high resolution etch capability with reasonably tight dimensions control.
  • Anisotropic etching techniques are commonly used in manufacturing of Si pressure sensors as well as bulk micro-machined accelerometers.

Unit processes required in Bulk Micromachining

  • Deposition of thin films - Physical or Chemical vapor deposition (for metals, polySi, SiO2, Si3N4). Spin coating (for resists, SU-8, other polymeric materials)
  • Doping- To change conductivity of PolySi and to change chemical (etch rate) characteristics
  • Pattern transfer & Etching- Lithography, Wet etching of Silicon, Dry Etching of Silicon

Bulk micromachining for a cantilever

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