Page 260 - MEMS Mechanical Sensors

9.11 Conclusion 249

Out-of-plane bent
silicon beam structure Bond pads
Hot-wires

Polysilicon
hot-wire
Cured
polyimide
Metal

Silicon beam

Figure 9.38 Schematic drawing of a triple-hot-wire anemometer. The polysilicon hot-wires are
3
500 × 5 × 2 µm .(After: [64].)

only in the research field but also from industry, which has already commercialized
millions of MEMS flow sensors. Examining these in detail, it is noted that, to date,
gas flow sensing is more popular than liquid flow sensing. Devices are used for car
air intake modules or air-conditioning systems. The BioMEMS field is a promising
candidate for further commercializing of microfluidic devices and systems, includ-
ing flow sensors working with liquids.
CMOS fabrication compatibility is an enormous advantage for a
micromachined sensor. The fabrication of MEMS devices can run in parallel with
other processes in microelectronics fabrication cleanrooms, thereby reducing the
costs enormously. This means that metals like platinum or gold, or KOH etching to
form thermal isolation structures cannot be used. Specialized MEMS-only clean-
rooms would not be economical, as the selling numbers of MEMS sensor chips are
still far below microelectronic devices (apart from ink jet printer nozzles and hard
disk drive heads). An overview of micromachined thermally based CMOS sensors
was presented by Baltes et al. [126].
Packaging of a flow sensor is not an easy task. This has a great influence on the
sensor’s performance, as described in Chapter 4. For example, the diameter of the
channel in which a sensor sits has an impact on the minimum or maximum flow
rate. The packaging can protect the sensor from damage by particles, as seen in the
aerodynamic bypass developed by Bosch GmbH. Sensors for wall shear stress
measurement need to be mounted flush to the wall. Up to now, each sensor needs to
have an individual packaging solution, depending on the measurement principle
adopted, the required flow range, and the measurement environment.
Ultrasonic macroflow measurement systems are commonly used [127]. They
are based on drift, Doppler, and attenuation or diffraction effects. Ultrasound is
normally generated by piezoelectric transducers. A miniaturized ultrasonic wave
velocity and attenuation sensor for liquids was developed in 1993 by Hashimoto et
al. [128]. The device is made of silicon and glass with sputtered ZnO to create the