Page 124 - MEMS Mechanical Sensors

CHAPTER 6
Pressure Sensors

6.1 Introduction

The application of MEMS to the measurement of pressure is a mature application of
micromachined silicon mechanical sensors, and devices have been around for more
than 30 years. It is without doubt one of the most successful application areas,
accounting for a large portion of the MEMS market. Pressure sensors have been
developed that use a wide range of sensing techniques, from the most common pie-
zoresistive type to high-performance resonant pressure sensors.
The suitability of MEMS to mass-produced miniature high-performance sen-
sors at low cost has opened up a wide range of applications. Examples include auto-
motive manifold air and tire pressure, industrial process control, hydraulic systems,
microphones, and intravenous blood pressure measurement. Normally the pressur-
ized medium is a fluid, and pressure can also be used to indirectly determine a range
of other measurands such as flow in a pipe, volume of liquid inside a tank, altitude,
and air speed. Many of these applications will be highlighted in this chapter, demon-
strating MEMS solutions to a diverse range of requirements.
This chapter will first introduce the basic physics of pressure sensing and discuss
the influence of factors such as static and dynamic effects as well as media com-
pressibility. Following that is a section on the specifications of pressure sensors,
which serves to introduce the terms used and the characteristics desired in a pressure
sensor. Before describing the many MEMS developments that have occurred in the
field of pressure sensing, there is brief discussion on traditional pressure sensors and
diaphragm design. The MEMS technology pressure sensor section then looks at sili-
con diaphragm fabrication and characterization, applied sensing technologies, and
example applications.
Pressure is defined as a force per unit area, and the standard SI unit of pressure is
2
N/m or Pascal (Pa). Other familiar units of pressure are shown in Table 6.1 along

Table 6.1 Units of Pressure and Conversion Factor to Pa (to Two Decimal Places)
Unit Symbol No. of Pascals
Bar bar 1 × 10 5
Atmosphere atm 1.01325 × 10 5
Millibar/hectopascal Mbar/hPa 100
Millimeter of mercury mmHg/torr 133.32
Inch of mercury inHg 3,386.39
2
Pound-force per square inch lbf/in (psi) 6,894.76
Inch of water inH O 284.8
2

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