What do you understand by high aspect ratio MEMS? Explain fabrication process flow for HARMEMS.
1 Answer

High Aspect ratio micro-structure are manufactured by the LIGA process.Aspect ratio is the ratio of dimension of the height to those of the surface.Most silicon based MEMS and micro-systems use wafers of standard size and thickness as substrate on which etching or thin film deposition takes place to form the desired 3D geometry. Several limitations of depth dimension is thus unavoidable.

LIGA process produces thick micro-structure that have extremely flat and parallel surfaces such as micro-gear trains,motors and generators and micro-turbine made of metals and plastic.

Fabrication steps in LIGA process:

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Process can be explained for micro thin wall metal tube of square cross section.

  1. The process begins with deposition of thick film of PMMA photo-resist on substance as shown in figure (a)

  2. Masks are used in x-ray lithography. To block x-ray transmission thick film of gold is applied to make opaque region. $Si_3 N_4$ is used of 1 – 1.5 $\mu m$

  3. The exposed region will dissolve during development as shown in figure (b). The PMMA photo resist will have outline of the product.

  4. The electroplating is done with desired metals usually nickel to produce the tubular product as shown in figure (c)

  5. The desired tubular product is produced after the removal of PMMA by oxygen plasma or solvents as shown in figure.

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• The short $\lambda$ of x-ray allows line width $0.2 \mu m$ and aspect ratio > 100 : 1

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