Page 92 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

MONOLITHIC PROCESSING 73

conductivity. In addition, SOI technology offers extremely low unwanted parasitic effects
and excellent isolation between devices (see Section 4.3.5).

4.3.1 Bipolar Processing

The bipolar process has evolved over many years, as has its so-called standard process.
Clearly, this is an important issue and the integration of a microsensor, or microactuator,
will depend on the exact details of the process that is employed. As stated earlier, the
possible approaches to microsensors and MEMS integration and the problems associated
with compliance to a standard process are both discussed in some detail in later chapters.
This section presents what may be regarded as the standard bipolar process, which
employs an epi-layer to make the two most important types of bipolar components; that
is, vertical and lateral transistors. Bipolar n-p-n transistors are the most commonly used
components in circuit design as both amplifiers and switches because of their superior
characteristics compared with p-n-p transistors. Let us now consider in detail the process
steps required to make a vertical n-p-n and lateral p-n-p transistors. A similar process
can be defined to make vertical p-n-p transistors or the simpler substrate p-n-p transistors
with slightly different device characteristics.

Worked Example E4.1: Vertical and Lateral Bipolar Transistors
The standard bipolar process begins by taking a p-type substrate (i.e. single-crystal
2
silicon wafer) with the topside polished . A buried n-layer is formed within the p-type
substrate by first growing an oxide layer. The oxide is usually grown in an oxidation
furnace using either oxygen gas (dry oxidation) or water vapour (wet oxidation) at a
temperature in the range of 900 to 1300 °C. The chemical reactions for these oxidation
processes are as follows:

Si(s) + 2H 2O(g) SiO 2(s) + 2H 2(g) (4.6)

Other ways of forming an oxide layer, such as CVD, are discussed in Chapter 5.
The thermal oxide layer is then patterned using a process called lithography. A basic
description of these processes is given in this chapter and a description about more
advanced lithographic techniques is given in Chapter 5. Lithography is the name used
to describe the process of imprinting a geometric pattern from a mask onto a thin layer
of material, a resist, which is a radiation-sensitive polymer. The resist is usually laid
down onto the substrate using a spin-casting technique (see Figure 4.10).

In spin-casting technique, a small volume of the resist is dropped onto the centre of
the flat substrate, which is accelerated and spun at a constant low spin speed of about
2000 rpm to spread the resist uniformly. The spin speed is then rapidly increased to its
final spin speed of about 5000 rpm, and this stage determines its final thickness of 1 to
2 um. The thickness of the spun-on resist, d R, is determined by the viscosity 77 of the
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Double-sided wafers are used if a back-etch is required to define a microstructure.