ETTC'2003 - SEE

4.2. Sensitivity according to the heater power
Figure 6 shows that the average temperature of the
heater is close to be proportional to the power
consumption. The slight deviation in high power range is
due to the increase of the air thermal conductivity with
the temperature. Then, the sensor sensitivity was studied
for the distance of 500 µm and it shows a quasi linear
dependence to the heater power (figure 7).
∆T ( °C)
300
200
100
0
0 20 40 60 80 100
P (mW)
Figure 6: Average temperature of the heater ∆T vs. heater power.
∆T det (°C)
4
3
2
1
0
0 20 40 60 80 100
P (mW)
Figure 7: Sensor sensitivity ∆Tdet. vs. heater power.
4.3. Sensitivity according to room temperature
Figure 8 shows the electrical sensitivity of a sensor
versus ambient temperature for a constant heater current
of 41mA. The temperature dependence of the sensitivity
is not linear and the temperature coefficient is about -
1%.K -1 for a low temperature range. In comparison, a
thermal accelerometer with a seismic mass [1-3] presents
a linear temperature dependence and a temperature
coefficient of 0.12 %.K -1 while the thermal
accelerometers without seismic mass, commercialized by
the society MEMSIC, would use a gain adjustment of 0,9
%.K -1 to keep the sensitivity within 5% of its room
temperature value [17]. For an ideal gas, the den y and
e coe nal to
/T and the viscosity is proportional to T 1/2 sit
th fficient of expansion are linearly proportio
1
. Therefore,
the model based on Grashof number can explain
the
sensitivity
decrease when the room temperature
increases.
F. Mailly et Al. ETTC 2003
S (a.u.)
70
60
50
40
30
20
10
0
20 30 40 50 60 70 80 90 100 110 120
T (°C)
Figure 8: Sensor sensitivity. vs. room temperature
for a constant heater current of 41 mA.
4.4. Sensitivity according to the cavity volume
Several packages have been used and have permitted
to obtain different cavity volumes. Figure 9 presents the
sensitivity measurements in arbitrary unit according to
the volume for a fixed heater temperature rise of 250°C.
For a volume higher than 100 mm 3 , the sensitivity is
optimum : ∆T det is about of 4°C when the volume tends to
infinite values and it is about of 0,13°C for a volume of
1,6 mm 3 . This phenomena has been already reported by
Billat et al. [6] but their package volumes were higher
than 300 mm 3 and their sensitivity for slight volumes was
only twice lower than the one for high volume.
S (a.u.)
1000
100
10
1
1 10 100 1000 10000 100000 1000000
V (mm 3 )
Figure 9: Sensor sensitivity ∆Tdet. vs. cavity volume.
4.5. Sensitivity according to the gas and its pressure
For the study of the influence of the gas type, the
sensor is placed in an hermetic chamber and a hole is
made in the TO16. Figure 10 presents in a log-log scale
the sensitivity measurements for different gases - helium,
air and carbon dioxide - according to their thermal
diffusivity. The heater temperature rise was fixed at
180°C by applying different currents according to the gas
type. For the 3 distances heater/detectors, the curve slope
n is close to -2 so, the sensitivity is inversely proportional
to the square of the gas thermal diffusivity as it was
predicted by the formula (3).
3