handbook of modern sensors

374 11 Flow Sensors
Fig. 11.14. Structure of a gas microflow sensor utilizing capacitive pressure sensor. (Adapted
from Ref. [9].)
is lower than P 1 , thus creating a pressure differential across the membrane. Therefore,
the flow rate can be calculated from Eq. (11.10).
The pressure differential is measured by a capacitive pressure sensor, which is
composed of a thin, stress-compensated, p ++ boron-doped silicon membrane suspended
above a metal plate. The pressure differential changes the capacitance C x
between the metal plate and the silicon structure with a resolution of 1 mTorr/fF, with
a full pressure of about 4 torr. The overall resolution of the sensor is near 14–15 bits
and the accuracy of pressure measurement about 9–10 bits. At approximately twice
the full-scale pressure differential, the membrane touches the metal plate; hence, a
dielectric layer is required to prevent an electric short, and the substrate glass plate protects
the membrane from rupturing. A capacitance measurement circuit (see Fig. 5.32
of Chapter 5) is integrated with the silicon plate using standard CMOS technology.
11.7 Breeze Sensor
In some applications, it is desirable just to merely detect a change in air (or any other
gas for that matter) movement, rather than to measure its flow rate quantitatively.
This task can be accomplished by a breeze sensor, which produces an output transient
whenever the velocity of the gas flow happens to change. One example of such a
device is a piezoelectric breeze sensor produced by Nippon Ceramic of Japan. The
sensor contains a pair of the piezoelectric (or pyroelectric) elements, 3 where one is
exposed to ambient air and the other is protected by the encapsulating resin coating.
Two sensors are required for a differential compensation of variations in ambient
3 In this sensor, the crystalline element which is poled during the manufacturing process is
the same as used in piezoelectric or pyroelectric sensors. However, the operating principle
of the breeze sensor is neither related to mechanical stress nor heat flow. Nevertheless, for
simplicity of the description, we will use the term piezoelectric.