Research Directory of the Brandenburg University of Applied Sciences

A typical thermopile used as an IR
sensor is shown in Fig. 3.22. It consists
of 72 thermocouples with one
type of junction (“hot junction”) in
the center of a thin micromachined
Si 3 N 4 membrane and the other type
(“cold junction”) connected to the
outside silicon substrate [8, 9]. The
IR radiation must be absorbed on
the thermopile to be detected, so
the center is coated with a absorption
layer (500 μm diameter). The
thermogram of a homogeneous tempered
thermocouple depicts the
emissivity difference between the absorption layer and
the surrounding sensor area, see Fig. 3.22. The absorption
layer should define the radiation sensitive area. But
the incident radiation is also partially absorbed on the
uncovered membrane area causing a contribution to a
temperature increase of the membrane. Therefore the
sensor exhibits a larger radiation sensitive area as defined
by the size of the absorption layer. This behavior of
an undefined radiation sensitive sensor area can cause
a lot of problems in application of the sensors.
Wissenschaftliche Beiträge – Fachbereich Technik
Scientific Articles – Department of Engineering
Fig. 3.23: Thermopile operated as a Peltier element with an electrical voltage applied to the thermopile
detector such that the thermocouples in the center are heated (left). For comparison an
image without applied voltage is shown (right).
The Peltier effect represents the opposite effect to the
Seebeck effect and a thermopile consequently represents
a Peltier element if it is operated in the opposite
way [10]. If a voltage is applied to the thermopile one
type of junctions is heated up and the other type of junctions
is cooled down. The direction of the applied voltage
determines what type is heated or cooled, respectively.
Fig. 3.23 depicts such operation of the thermopile.
The used polarity of the applied voltage causes a hea-
Fig. 3.24: Thermogram of a thermopile operated as a
Peltier element with heated thermocouples in the
center (top, left). Comparison of the temperature line
profile of the center heated thermopile (right, top a)
with the 3D - sensitivity profile of the thermopile
(right, top b) measured by a laser spot measurement
(top, right). False color representation of the spatial
sensitivity distribution with laser scanning (bottom,
left) and of the temperature distribution due to the
Peltier effect with IR imaging (bottom right).
Forschungsbericht Research Report 2007 – 2010 87