Page 116 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 95

provides a fast thermal time constant and consequently a rapid response time. One
approach to inferring the temperature of the heating element is to drive it with a
constant current and measure its resistance, then calculate the temperature using the
TCR. Alternatively, direct temperature measurement using a p-n diode or a thermo-
couple is equally adequate.
Honeywell, Inc., of Minneapolis, Minnesota, manufactures the AWM series of
bidirectional mass airflow sensors using two adjacent thin membranes, presumably
made of silicon nitride, each containing a heating element and a temperature-
sensitive resistor [16]. The two membranes are small in size, each measuring less
2
than 500 × 500 µm . Gas flow across the membranes cools the upstream heater and
heats the downstream element. The two heaters are part of a first Wheatstone
bridge, and the temperature-sensing resistors form two legs in a second Wheatstone
bridge, whose differential output is directly proportional to the rate of flow (see
Figure 4.13). The direction of flow is reflected in the polarity of the differential
bridge output—a characteristic of the dual sense element configuration. In essence,
this polarity determines which of the two heaters is upstream or downstream.
Laser-trimmed thick- or thin-film resistors provide calibration as well as nulling of
any offsets due to resistance mismatch in the Wheatstone bridges. The Honeywell
AWM series of devices is capable of measuring gas flow rates in the range of 0
to 1,000 sccm. The upper limit is due to pronounced nonlinear effects in the
heat-transfer mechanism. The full-scale output is approximately 75 mV, and the
response time is less than 3 ms. The device consumes less than 30 mW.
While the processing details of the Honeywell series of airflow sensors
are not publicly disclosed, one can readily design a process for fabricating a
demonstration-type device. An example process would begin with the deposition of
a thin layer of silicon nitride, approximately 0.5 µm in thickness, over a {100} sili-
con wafer. Silicon nitride is usually an excellent choice for making thin membranes

Heating
{111} plane Heaters
resistors Downstream Upstream
resistor resistor

Flow Silicon nitride

Silicon substrate

Etched pit Upstream resistor
Silicon nitride
membrane
Downstream resistor <110> <100>

Figure 4.13 Illustration of a micromachined mass flow sensor. Gas flow cools the upstream
heater and heats the downstream heater. Temperature-sensitive resistors are used to measure the
temperature of each heater and consequently infer the flow rate. The etched pit underneath the
heater provides exceptional thermal isolation to the silicon support frame. (After: technical sheets
on the AWM series of mass airflow sensors, Honeywell, Inc., of Minneapolis, Minnesota, and [16].)