Question

Discuss why bulk micromachining of silicon could generate high-aspect ratio features (e.g. deep valleys) with well-defined side walls instead of undercutting into the material.

Answer 1

Bulk micromachining

1. Allows selective removal of significant amounts of silicon from a substrate to form i)membranes on one side of the wafer ii) A variety of holes iii) Or other structures

2. BM can be divided into two;

i) Wet etching : Liquid etchant (aqueous chemicals)

ii) Dry etching: Vapor and plasma etchant

Some important points:

i)Emerged during 1960s

ii)Used pressure sensors, Si valves

iii)Used to realize structures within bulk of a single crystal Si wafer by selectively removing wafer material

iv) Structures may have thickness range from sub-micron to full wafer thickness, and lateral dimensions as large as few mm

v)Key step is etching

1.Wet isotropic 2. Wet anisotropic 3. Plasma isotropic 4. Reactive ion etching

High Aspect Ratio Microsystems

i)Wafer Bonding techniques

ii)High Aspect Ratio Methods for Silicon

iii)LIGA and other molding techniques

iv)Polymeric microsystems (low cost fabrication)

v)Low Temperature Co-fired Ceramics (LTCC)