A thermal flow sensor based on high-resolution thin-film thermistors p
laced on SiN(X) microbridges is presented. A linear array of those bri
dges crossing a silicon micromachined flow channel forms the base of t
he investigated flow sensor. Each thermistor device consists of a vacu
um-evaporated amorphous Ge (a-Ge) resistor passivated by silicon nitri
de layers that were prepared in a low temperature plasma-assisted CVD
process. The top and bottom SiN(X) films serve as the mechanical suppo
rt of the thermistor and as the mask for the micromachining etch proce
ss. The a-Ge thermistor can serve for high resolution temperature sens
ing or as a controlled temperature heater. The SiN(X) bridges are loca
ted in the centre plane of the flow channel yielding maximum thermal c
oupling between thermistor and fluid. A temperature difference between
run-in fluid and heating thermistor of typically 5 K is sufficient fo
r reliable operation. The corresponding heat flux causes an increase o
f fluid temperature of typically less than 1 K. The power per unit are
a required for convenient measurements amounts to 1 mW/mm2 for gaseous
fluids and 4 mW/mm2 for liquids. Measuring ranges from 1 to 4000 ml/h
for liquids and from 10 to 3000 sccm for gases are verified as useful
. With a flow channel cross section of about 0.3 mm2 this corresponds
to average fluid velocities at the upper limit of the flow ranges of a
bout 4 m/s for liquid fluids and 180 m/s for gases, ignoring their com
pressibility. The upper limits of investigated flow rate are determine
d only by the available fluid supply equipment.