Page 13 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

Preface

The miniaturisation of sensors has been made possible by advances in the technolo-
gies originating in the semiconductor industry, and the emergent field of microsensors
has grown rapidly during the past 10 years. The term microsensor is now commonly
used to describe a miniature device that converts a nonelectrical quantity, such as pres-
sure, temperature, or gas concentration, into an electrical signal. This book basically
reports on the recent developments in, firstly, the miniaturisation of a sensor to produce a
microsensor; secondly, the integration of a microsensor and its microelectronic circuitry
to produce a so-called smart sensor; and thirdly, the integration of a microsensor, a
microactuator, and their microelectronic circuitry to produce a microsystem.
Many of the microsystems being fabricated today employ silicon microtechnology and
are called microelectricalmechanical systems or MEMS in short. Consequently, the first
part of this book concentrates on the materials and processes required to make different
kinds of microsensors and MEMS devices. The book aims to make the reader familiar
with these processes and technologies. Of course, most of these technologies have been
derived from those currently employed in the semiconductor industry and so we also
review the standard microelectronics technologies used today to produce silicon wafers,
process them into discrete devices or very large-scale integrated circuits, and package
them. These must be used when the microelectronics is being integrated to form either
a hybrid device, such as a multichip module (MCM), or a fully integrated device, such
as a smart sensor. We then describe the new techniques that have been developed to
make microsensors and microactuators, such as bulk and surface silicon micromachining,
followed by the emerging technology of microstereolithography that can be used to form
true three-dimensional micromechanical structures.
The reader is now fully prepared for our description of the different types of microsen-
sors made today and the way in which they can be integrated with the microelectronics
to make a smart device (e.g. an electronic eye, electronic nose, or microtweezers) or
integrated with a microactuator to make a microsystem. Several of these chapters have
been dedicated to the important topic of IDT microsensors, that is, surface acoustic wave
devices that possess an interdigital transducer and so can be used to sense a wide variety
of signals from mechanical to chemical. This type of microsensor is attractive, not only
because it offers both high sensitivity and compatibility with the microelectronics industry
but also because it can be operated and even powered by a wireless radio frequency link.
The latter overcomes the initial constraints of communicating with small, low energy
budget, and even mobile MEMS - now referred to as micromachines!