This document describes three different designs for multifunction thermopile sensors fabricated using a MEMS process. Each sensor can measure temperature, pressure, humidity, and gas concentration. The three designs are an overlap-type, non-overlap elongated type, and symmetric non-overlap type. Experiments show the sensors respond to changes in these conditions, with different designs more sensitive to particular measurements like the symmetric non-overlap type performing best for humidity and pressure sensing. The sensors could allow multiple measurements on a single chip.

1. Multifunction Thermopile
Sensors Fabricated With
a MEMS-Compatible Process
By
D. Rajeev kumar
13207101
M.Tech (Mechatronics)
K L University

2. ABSTRACT
This work is about micromachined
multifunction thermopile sensors based on the
Seebeck effect.
A single sensor is able to perform
measurement on temperature, pressure,
humidity, and percentage of a gas
constituent.
Three different designs are employed and
compared.

3. INTRODUCTION
Physical sensors convert physical quantities into
electrical signals which can be further processed.
Particularly, microelectro-mechanical systems
(MEMS)-based sensors make easy the
measurement of physical quantities such as force,
angular rate, acceleration, etc., which cannot be
readily done by common electronic circuits.
Most sensors perform a single function.but to
achieve a multi function sensor different sensing
mechanisms have to be integrated within a die.

4. INTRODUCTION
Thermopiles have been used as sensors for
temperature , droplets , flow , acceleration ,
humidity , pressure , and gas concentration,
etc.
In this presentation several micro machined
multi-function thermopile sensors of different
designs, each of which alone is capable of
measuring the temperature , pressure,
humidity, and gas constituent percentage.

5. INTRODUCTION
The devices in this presentation are all
fabricated with the same MetalMUMPs
MEMS process offered by MEMSCAP Inc.
This means that if more than one device for
different purposes is needed, they can be easily
integrated on a single die while bypassing the
compatibility issue and troublesome cross-die
wiring.

6. DESIGN AND FABRICATION
The MetalMUMPs fabrication process offers n-
doped polysilicon and nickel as the conductive
materials, which are used to construct our
thermopile sensors.
The thicknesses of the polysilicon and nickel are
0.7 and 20μm, respectively.
They have designed three different devices,
which we call
overlap-type thermopile sensor
non-overlap-type elongated thermopile sensor and
symmetric non-overlap type thermopile sensor

7. Overlap-type thermopile
sensor
‘Overlap’ indicates the
polysilicon and nickel
structures overlap when
viewing the device from
the top, with a 1.1-μm
air gap sandwiched
between them.

8. Non-overlap-type elongated thermopile
sensor
Within the ‘elongated’
thermopile sensor, the
lengths of polysilicon
and nickel structures are
both 900μm, in contrast
to the shorter 600μm for
other devices.

9. Symmetric
non-overlap-type thermopile sensor
In a ‘symmetric’ device,
the top-view shapes of
the polysilicon and
nickel structures are
identical and are the
mirror image of each
other.

10. 3-D schematic drawing of a MEMS multi-
function thermopile sensor – the non-overlap
type.
 In each device, the thermopile
is folded and consists of 40
thermocouples.
 A heating wire goes above the
device central axis to create
the temperature gradient
leading to a voltage output.
 25-μm deep trenches are
etched in the silicon substrate
underneath the thermopiles,
making the devices more
sensitive to the heat convection
change which is related to the
gas pressure, humidity, and gas
constituents.

11. Experimental setup
In our experiments, we have two choices for the chamber:
one with humidity and temperature control and the other with
pressure and temperature control.
The heating current is set as 1.5 A.

12. Output Voltage Vs Pressure (Degree-of-Vacuum)
 The output voltage of each
device increases with the
degree-of-vacuum.
 As the vacuum degree goes up,
the amount of air within the
chamber decreases, reducing
the heat which convection can
carry away.
 Hence, the hot ends of the
thermocouples are heated to a
higher temperature,
consequently resulting in a
larger temperature difference
between the hot and cold ends,
and therefore, a higher output
voltage.

13. Sensitivity to degree-of-vacuum change Vs ambient
temperature
 We define the sensitivity to
pressure change as the
average slope of the curve.
 The symmetric non-overlap-
type device exhibits the
largest sensitivity.
 The sensitivity of the
overlap-type device is least
susceptible to tempearature
change.

14. Output voltage versus ambient temperature
 The output voltage increases with
the ambient temperature.
The sensitivity of the overlap-type
thermopile sensor is the highest among
the three, and it is also least dependent of
the vacuum degree.

15. Output Voltage versus CO2 Partial Pressure
 Total pressure is fixed at
760 mmHg.
 the output voltage decreases
when the CO2 partial
pressure goes up.
 This is due to the fact that
CO2 has a lower viscosity
than common air and leads
to better heat convection,
lowering the hot-end
temperature, and therefore,
the temperaute difference
across the thermocouple

16. Sensitivity to CO2 partial pressure change versus temperature
 The non-overlap-type
elongated thermopile sensor
exhibits the highest
sensitivity.

17. Output Voltage versus Relative Humidity
 For the overlap-type device. The
curves of 50 and 60°C exhibit
turns in themselves. Converting
the relative humidity into the
H2O vapor pressure can yield

18. Output voltage versus relative humidity
 For the symmetric non-
overlap-type and non-
overlap-type elongated
devices, the output voltage
monotonically decreases
with the relative humidity,
i.e. no turn in the curve is
observed.
 The reason for the different
behaviors of these two
devices and the overlap-type
thermopile sensor is still
under investigation.

19. CONCLUSION
 Demonstrated three different designs of micromachined multi-
function thermopile sensors.
 Each device alone is capable of measuring the temperature,
pressure, humidity, and gas constituent percentage (gas partial
pressure).
 No significant hysteresis has been observed during the testing.
 In terms of sensitivity, the symmetric non-overlap-type device
is the best for humidity and degree-of-vacuum monitoring.
 The overlap-type device prevails in temperature sensing while
the non-overlap-type elongated thermopile sensor is best for
CO2 partial pressure measurement.