Sensor - The Electronic Nose Artificial Olfaction Technology - page 64

Doping/Diffusion:
The introduction of certain impurities into a semiconductor can change its elec-
trical, chemical, and even mechanical properties. This process consists of the
introduction of a few tenths to several micrometers of impurities by the solid-state
diffusion of dopants into selected regions of a wafer to form junctions. Most of
these diffusion processes occur in two steps: the pre-deposition and the drive-in
diffusion. In the pre-deposition step, a high concentration of dopant atoms are
introduced at the silicon surface by a vapor that contains the dopant at a tem-
perature of about 1,000 C. Diffusion is an isotropic process, the doped area will
also extend underneath the mask. In micro-fabrication, diffusion is mainly used in
the formation of very highly doped boron regions (p++), which are usually used as
etch steps in bulk micromachining. In recent years Ion Implantation is used.
Ion Implantation:
This is a process of introducing dopants into selected areas of the surface of the
wafer by bombarding the surface with high-energy ions of the particular dopant. In
ion implantation, the impurities are ionized and accelerated toward the semicon-
ductor surface.
After implantation, an annealing process is needed to activate the impurities
and to repair the damage in the crystal structure generated by the ion collisions. A
drive-in process to redistribute the impurities, performed in a standard furnace like
those used for oxidation or diffusion may also be required.
Ion implantation allows more precise control of the dose (total amount of
impurities introduced per area unit) and the impurity profile (concentration versus
depth).

Photolithography:
Photolithography is the standard process to transfer a pattern, which has been
designed with computer-aided-engineering (CAE) software packages, on to a
certain material. Basically, photolithography comes down to applying a layer of a
light-sensitive material (photoresist) on a flat substrate, illuminating it by some
source using some pattern (a mask) thereby making the illuminated parts either
soluble or insoluble and then removing the soluble parts [ 15 ].

Epitaxy:
Epitaxy is the process of the controlled growth of a crystalline doped layer of
silicon on a single crystal substrate. Epitaxy is used to deposit Non N+ silicon,
which is impossible to accomplish by diffusion. It is also used in isolation between
bipolar transistors where in N- is deposited on P.
Selective epitaxial growth is of particular interest for the formation of

microstructures.
Metallization and interconnections:
After all semiconductor fabrication steps of a device or of an IC are completed, it
becomes necessary to provide metallic interconnections for the IC and for external
connections to both the device. Figure 2.21 shows the example of fabrication
process implemented in accelerometer.