Page 80 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

4

Standard Microelectronic

Technologies

4.1 INTRODUCTION

A large number of different microelectronic technologies exist today, and they are used for
making conventional microelectronic components, such as operational amplifiers, logic
gates, and microprocessors. Many technologies, and in some cases, all of a standard
process are used to fabricate the type of microdevices that are of interest to us here.
For example, a thermal microsensor can be made by using a standard bipolar process in
which, for instance, the forward voltage of a p-n diode (under constant current) or the
base-emitter voltage of an n-p-n transistor is proportional to the absolute temperature
(details may be found in Section 8.2.1). In this case, there is no significant difference
between the processing of a thermodiode and a conventional diode, although there will
be a difference in the device design and, perhaps, package. In most practical situations,
it is highly desirable to employ, wherever possible, the standard technologies and then
'bolt-on' one or more nonstandard pre- or postprocessing steps. The part or full integration
of standard microelectronics into a microsensor or microactuator is also often required
1
to enhance its functionality and, in doing so, make a so-called smart microtransducer .
The degree of integration is a critical design issue; hence, designers should first consider
the hybrid solutions that, once again, draw upon standard microelectronic process and
packaging technologies. Because of the complexity of the end product, the integration of
microelectronics into a microelectromechanical system (MEMS) device is an even more
difficult problem. Consequently, it is covered in some detail in other chapters, wherein
we consider the different possible approaches, such as microelectronics first or MEMS
first. For all these reasons, this chapter seeks to provide a brief account of the standard
microelectronic technologies and thus provide a sound foundation on which to add other
nonstandard materials and processes - such as those described in later chapters on new
silicon-processing technologies (Chapters 5 and 6) and interdigitated transducer surface
acoustic wave (IDT SAW) microsensors (Chapter 12). We shall not attempt to describe all
the standard technologies, or indeed, even the subtle details of employing some of them.
Instead, our aim is to make the reader aware of the common choices currently available
and provide an appreciation of the nature of the technologies involved.

1
We will use the term microtransducer to describe a microsensing (or microactuating) device.