3. Once the design engineer selected a specific manufacturing
technique,whether it is bulk micromanufacturing or surface
micromanufacturing or the LIGA process, a search for
appropriate micromanufacturing process begins.
So,here we discussing about some of the microfabrication
process that involved in micromanufacturing.
4. PROCESS DESIGN
Microfabrication process is classified mainly into three
i. Photolithography
ii. Thin-Film Fabrication
iii. Geometry Shaping
5. PROCESS DESIGN
Photolithography
Photolithography is used to produce micro patterns that show
the 3D structural geometry of microsystem.
Also used in the process for producing masks for all
micromanufacturing techniques that include,
i. Masks for etching in bulk manufacturing
ii. Thin-film deposition and for etching in surface micromachining
iii. Micromolds in the LIGA process
6. PROCESS DESIGN
Photolithography
Photolithography process involves the use of an optical image
and a photosensitive film to produce desired patterns on a
substrate.
The “optical image” is originally in macro scale, but is
photographically reduced to the micro-scale to be printed on
the silicon substrates.
The desired patterns are first printed on light-transparent
mask, usually made of quartz.
7. PROCESS DESIGN
Photolithography
The mask is then placed above the top-face of a silicon
substrate coated with thin film of photoresistive materials.
The mask can be in contact with the photoresistave material,
or placed with a gap, or inclined to the substrate surface:
9. PROCESS DESIGN
Photolithography
Thee tasks that need to be carried out in systems design
process are
i. Design of patterns for substrates
ii. Design of masks for lithography
iii. Fabrication process for mask sets
10. PROCESS DESIGN
Photolithography
For example, take the silicon die used in micropressure sensor
Pressure of medium is applied at back side of silicon die.
The front side of the die has four piezoresistors diffused
beneath the surface.
Two masks are required for the silicon die,
i. etching of the cavity
ii. Diffusion of the piezoresistors and the deposition of the thin film
13. PROCESS DESIGN
Photolithography
Both SiO2 and Si3N4 can be used for masks.
But Si3N4 is a more suitable masking materials, because deep
etching is required for the production of many cavities in
silicon dies.
Patterns for both masks are quite different.
The yellow lines for the SiO2 masks are for the electrical
connection of the resistors in a WheatStone bridge.
14. PROCESS DESIGN
Thin Film Fabrication
There are several ways to produce thin film over the substrate
surface.
Some methods include CVD, Oxidation, Ion Implantation,
Diffusion, Sputtering, Electroplating.
Design engineer may use any of methods for production of
thin films according to needs.
Most of these methods are carried out in high temperature
environmentwhich result in residual stress and strain.
15. PROCESS DESIGN
Thin Film Fabrication
So effort must be taken to reduse these residual effects.
In electroplating, rate of deposition is given by,
m=(1/z)iAt(1/F)M
Where
A =electrode surface i=current density
t =electroplating time
z =electrons involved in reaction Ni2+ + 2e -→ Ni
M =atomic weight of Ni
F =Faraday constant(96487 Å/s-mol)
16. PROCESS DESIGN
Geometry Shaping
The complex geometry of Si based microdevice components
can be produced .
It is either by depositing thin film of various materials over the
substrate or by removing portions of materials from the bulk
substares.
An effective prcocess for removing material from substrate is
etching.
Either dry or wet etching can be used.
