Biomedical Engineering Reference
In-Depth Information
11.2.1
Microfabrication Techniques
Microfabrication techniques can be classified as follows: (1) property modification, (2) patterning, (3)
subtractive processes, (4) additive processes and packaging
[10]
.
11.2.1.1 Property Modification
Property modification techniques include alteration of the substrate by (i)
doping
(the process of
intentionally introducing impurities into an extremely pure semiconductor substrate like silicon in
order to change its electrical properties), (ii)
ion implantation
(a technique by which ions of a mate-
rial are implanted into another solid substrate, thereby changing the physical properties of the solid)
[10]
.
11.2.1.2 Microfabrication by Patterning
Microfabrication by patterning is based on lithography techniques (or microprinting). It involves (i)
photolithography and (ii) soft-lithography. (i)
Photolithography
(also known as optical lithography) is
a process used to selectively remove parts of a thin film (or bulk of a substrate). It uses light to trans-
fer a geometric pattern from a photomask to a light-sensitive chemical (photoresist) on the substrate.
A series of chemical treatments then engraves the exposure pattern into the material underneath the
photoresist. (ii)
Soft-lithography
refers to a group of techniques for fabricating or replicating struc-
tures using elastomeric stamps, moulds, and photomasks
[11]
.
11.2.1.3 Additive Microfabrication
Additive microfabrication techniques involve microfabrication by deposition or growth of specific
layers on a substrate
[10]
. A few examples of this technique are (i)
thermal oxidation
, to produce a
thin layer of oxide (usually silicon dioxide) on the surface of a wafer (for semiconductor applica-
tions), (ii)
chemical vapor deposition (CVD)
, a chemical process employed to produce high-purity,
high-performance solid materials. The process is often used in semiconductor industry to produce
thin films. In a typical CVD process, the wafer (substrate) is exposed to one or more volatile pre-
cursors, which react and/or decompose on the substrate surface to produce the desired deposit.
Frequently, volatile by-products are also produced which are removed by gas flow through the reac-
tion chamber. (iii)
Physical vapor deposition (PVD)
is an additive microfabrication technique used
to describe any of a variety of methods to deposit thin films by condensation of vaporized form of
the materials onto various surfaces (e.g., onto semiconductor wafers). The coating method involves
purely physical processes such as high-temperature vacuum evaporation or plasma sputter bombard-
ment rather than involving a chemical reaction at the surface to be coated as in CVD. (iv)
Epitaxy
is
an additive microfabrication method of depositing a monocrystalline film on a monocrystalline sub-
strate. Epitaxial films may be grown from gaseous or liquid precursors
[10]
.
11.2.1.4 Subtractive Microfabrication
Subtractive microfabrication techniques employ removal or etching of a portion of substrate to fab-
ricate microstructures/patterns. Dry etching and wet etching are two types of subtractive microfab-
rication techniques. (i)
Dry etching
refers to the removal of material, typically a masked pattern
of semiconductor material by exposing the material to a bombardment of ions (usually plasma of
reactive gases such as fluorocarbons, oxygen, chlorine, boron trichloride; sometimes with addition
of nitrogen, argon, helium, and other gases) that dislodge portions of the material from the exposed
