Page 87 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

68 STANDARD MICROELECTRONIC TECHNOLOGIES

4.2.3 Epitaxial Deposition

Epitaxial deposition is a process in which a thin, single-crystal layer can be deposited onto
the surface of a substrate wafer. The substrate wafer acts as a seeding layer and usually
has the same crystal structure as that of the epitaxial layer. Epitaxial growth is now
increasingly used in the production of both discrete devices and ICs. It is most commonly
used to isolate junctions from the substrate, that is, in junction isolation (JI) technology and
to allow formation of buried layers. JI technology permits the fabrication of circuits with
better characteristics, such as lower parasitic capacitances and faster switching speeds.
The most common types of epitaxial deposition are vapour-phase epitaxy (VPE) and
molecular beam epitaxy (MBE).
There are many different designs of epitaxial reactors, and Figure 4.6 shows three
common types: a horizontal epitaxial reactor, a vertical epitaxial reactor, and a barrel
epitaxial reactor. In all cases, there is a reaction chamber in which the chemicals react to
form the epitaxial layer, a heat source (e.g. RF coils) to stimulate the chemical reaction,
and a gas supply and venting system.
In the horizontal reactor, the wafers are placed on the susceptor at a slight angle in
order to compensate for the depletion of the reactive gases as they pass along the surface.
In contrast, the susceptor is rotated in the vertical reactor and the gases can circulate
above the wafers to keep the gas partial pressures constant. Finally, in the barrel reactor,
the wafers are mounted vertically and the susceptor is again rotated to ensure an even
supply of gases to the wafers.

OCX) Induction coil

Gas in Exhaust

(a)

Gas in
Susceptor

Silicon [
wafer
Silicon
Susceptor [ Rotation ] wafer
Induction
coil
O O
Exhaust Exhaust
Gas in Gas in
(b) (c)

Figure 4.6 Some common types of epitaxial reactor: (a) horizontal; (b) vertical; and (c) barrel