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9.2 Thermal Flow Sensors 227
Other micromachined flow sensors developed for the car industry, but not lim-
ited to this application are the sensors by HL Planartechnik GmbH, Germany [69],
and by the Fraunhofer Institute for Silicon Technology, Germany [75]. The sensor
by HL Planartechnik is a bidirectional mass airflow sensor. The sensor membrane is
1 µm thick with a nickel heater/sensors. No data about the minimum or maximum
flow rates is available. The sensor from the Fraunhofer Institute can be manufac-
tured at extremely low cost, as the processing is CMOS compatible. Also, the small
chip dimensions enable several hundred sensors to be fabricated on a single wafer.
The sensor can measure bidirectional air mass flow velocities. Photographs of the
sensor chip can be seen in Figures 9.16 and 9.17. Both sensors, HL Planartechnik
and Fraunhofer Institute, are available as unpackaged original equipment manufac-
turer (OEM) solutions excluding the measurement electronics. (An OEM is one that
produces complex equipment from components usually bought from other manu-
facturers.)
The Hahn-Schickard-Gesellschaft–Institut für Mikro- und Informationstechnik
(HSG-IMIT), Germany [61], produces a thermal flow sensor consisting of a heating
element and temperature sensing elements made of doped polysilicon sitting on a
membrane (Figure 9.18). The measurement elements are covered by silicon oxide
Heater/sensor
Reference
resistor
Membrane
Figure 9.16 Photograph of the thermal air flow sensor by Fraunhofer Institute for Silicon
2
Technology, Germany. The chip size is 2.6 × 7.7 mm . (Courtesy Fraunhofer Institute for Silicon
Technology.)
Figure 9.17 Photograph of the thermal air flow sensor by Fraunhofer Institute for Silicon Tech-
nology, Germany. The pictures show a magnified view of the reference resistor and heater/sensors.
The membrane consists of a stack of nitride/oxide/nitride. The resistors are made of titanium and
are covered by a nitride passivation layer. (Courtesy Fraunhofer Institute for Silicon Technology.)
