Page 117 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

98 STANDARD MICROELECTRONIC TECHNOLOGIES

G •- -• D

Substrate
effect

Figure 4.33 Intermediate-frequency, small-signal equivalent circuit of an FET showing the prin-
cipal capacitances. The additional capacitors and junction diodes to the substrate (or back-gate) are
given in the dashed box. A full model would include resistances, other parasitic capacitance, and
inductance

depositing thin silicon layers onto insulators. Silicon layers can be deposited by a variety
of methods, such as zone-melt recrystallisation and solid-phase epitaxy (Furukawa 1985).
Insulating layers can be implanted using an oxygen or nitrogen implant to make SiO 2 or
Si3N 4, and (l00)-orientated silicon can be deposited onto glass or sapphire by a process of
heteroepitaxy. Some of the most successful methods to produce SOI presently are based
on wafer bonding. This involves bonding together two silicon wafers at high temperature,
one having a grown oxide on top. Then one of the wafers is thinned down and ends up
with the structure of silicon (substrate)-oxide-thin silicon layer. Once the thin layer of
silicon has been formed, and it is usually less than 1 micron in thickness, standard bipolar
or CMOS can be manufactured on top of this. Figure 4.34 shows the basic structure of
a lateral n-channel and p-channel MOSFET fabricated on top of a sapphire substrate.
Note that the capacitance of the source and drain to the substrate has been considerably
reduced and so SOI CMOS ICs tend to exhibit a high speed at a low power because of
the thermal sinking.
In addition, the substrate under an SOI MOSFET can be removed by a bulk-etching
process to leave a device mounted in a thin membrane. There are a number of potential
uses of SOI technology in microsensor design and Figure 4.35 shows the layout of a
gas-sensitive catalytic-gate MOSFET with an integrated FET heater. In this case, the
temperature of the CMOS-compatible sensing transistor can be raised to 180°C, possibly
using an MOSFET heater transistor, whereas the surrounding standard integrated circuitry