Give two examples of combination of structural, sacrificial layers and enchants used in MEMS fabrication along with their applications

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

1 Answer

Structural Layer –

  • A layer having the mechanical and electrical properties needed for the component being constructed. (doped polycrystalline silicon, silicon nitride, some metals such as chrome, gold and aluminum-copper)
  • A layer of thin film material that comprises a mechanical device. This layer is deposited on the sacrificial layer, and then released by etching it away.

Sacrificial Layer –

  • A layer deposited between structural layers for mechanical separation and isolation. This layer is removed during a "release etch" to free the structural layers and to allow mechanical devices to move. (Silicon dioxide, photoresist, polycrystalline silicon)
  • A layer of material that is deposited between the structural layer and the substrate to provide mechanical separation and isolation between them.
  • This is removed after the mechanical components on the structural layer are fully formed, by release etch. This approach facilitates the free movement of the structural layer with respect to the substrate.
  • But of course, these materials should be chemically distinct. So that suitable etchant can be selected to remove the sacrificial layer without removing the structural layer and the substrate etc.
  • A sacrificial layer is used to build complicated components, such as movable parts.
  • For example, a suspended cantilever can be built by depositing and structuring a sacrificial layer, which is then selectively removed at the locations where the future beams must be attached to the substrate (i.e. the anchor points). A structural layer is then deposited on top of the polymer and structured to define the beams. Finally, the sacrificial layer is removed to release the beams, using a selective etch process that does not damage the structural layer.
  • Many combinations of structural and sacrificial layers are possible. The combination chosen depends on the process. For example, it is important for the structural layer not to be damaged by the process used to remove the sacrificial layer.

Material Pairs:

a) Poly/SiO2

  • LPCVD deposited poly as structural layer;
  • Thermal or LPCVD oxide as sacrificial layer
  • Oxide dissolves in HF, and not poly.
  • Both materials are used in IC fabrication.

b) Silicon Nitride/Poly-Silicon

  • LPCVD nitride is used as structural layer;
  • Poly Si as sacrificial layer
  • EDP or KOH to dissolve poly.
  • Both materials are used in SOI accelerometer.

c) Tungsten/SiO2

  • CVD tungsten as the structural layer;
  • Oxide as sacrificial layer
  • HF for etchant

d) Polyimide/Aluminum

  • Polyimide as structural layer, aluminum as sacrificial layer
  • Acid based etchants to etch aluminum
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