Thermopile Temperature Sensors

Xiang Zheng Tu

POSIFA’s thermopile sensors are based on the technologies of silicon micromachining and CMOS manufacturing. In the thermal sensor fabrication a part of silicon substrate is removed by a porous silicon etching process, leaving on top only a thin sandwich layer membrane of PECVD SiO₂/Si₃N₄, which has low thermal conductivity. On the membrane a resistor and two thermopiles are formed there which are used as the main device elements of the sensor. The resistor is located along the central line of the membrane and made of a deposited thin polysilicon film. Two thermopiles are located on the two opposite sides of the resistor respectively. The thermopiles have alternate hot junctions disposed near the resistor and alternate cold junctions expending out of the membrane and ended on the bulk part of the silicon substrate. The junctions are formed by both a deposited thin polysilicon film and a deposited thin aluminum film.

The thermopile sensors can be used as a lot of different thermal sensors with a little modification which include fluid flow sensors, gas thermal conductivity sensors, vacuum sensors and temperature sensors. In working of the temperature sensors the polysilicon resistor is used as a heater and one thermopile is used for temperature different sensing.

Since the membrane is heated by the heater the temperature different generates between the membrane and the bulk part of the silicon substrate. It is need to know that the low thermal conductivity of the membrane is beneficial to the temperature different maintain.

The power dissipated in the polysilicon resistor causes its temperature to rise above a room temperature by:

T_PR-T_room=(V²/R_PR)θ                                       (1)

Where

T_PR=the raised temperature of polysilicon resistor due to internal power dissipation

T_room  =the reference temperature of polysilicon resistor

V=the driving voltage of polysilicon resistor in V

R_PR=the value of the polysilicon resistor in ohms at T_PR; and

θ=the self-heating polysilicon resistor ting effect in °C/mW.

For an ideal thermocouple, the open-circuit voltage obtained is proportional to the temperature difference between the hot junctions and cold junctions which are constructed of polysilicon film and aluminum film,

  

∆V=S(T_PR-T_room)                                                (2)

where S is the relative Seebeck coefficient, expressed in µV/K. The relative Seebeck coefficient of a junction can be calculated as the absolute value of the each Seebeck coefficient of polysilicon and aluminum; that is,  

S=S_Poly−S_Al                                                                      (3)

Because a voltage is produced when a temperature difference exists between the two junctions of the thermocouple junction pair, the thermocouple can be used as a detector of incident radiation. In open-circuit operation the output voltage produced is usually low, on the order of a tenth of a microvolt per degree celsius of temperature difference for a single junction pair. In order to increase the output voltage, several junction pairs may be connected in series. The responsivity is then increased by n if n thermocouple junction pairs are placed in series; that is,

∆V=n(S_Poly−S_Al) (T_PR-T_room)                                  (4)

Such a device is called a thermopile.

Combining equation (1) and (5) results the equation as:

∆V=n(S_Poly−S_Al ) (V²/R_PR)θ                                     (5)

The resistance of a polysilicon resistor is specified at a room temperature, any other resistance at another temperature is determined by:

R_PR=R_room[1+α(T_PR-T_room)]                                     (6)

Where
R_room=the resistance at a room temperature in ohm
α=the temperature coefficient of the resistance

After replacing equation (6) into equation (5) an equation is obtained as

∆V=n(S_Poly−S_Al ) V²θ / R_room[1+α(T_PR-T_room)]        (7)

From the equation (7) it can be seen that the output voltage ∆V of the thermopile is inversely proportional to the resistance R_room at a room temperature.

An example of a resistance and temperature coefficient of polysilicon resistors with temperature are shown in the following figure.

 

These polysilicon resistors were made of a 700 nm thick polysilicon films that were grown at 580^◦C in furnace by low-pressure chemical vapor deposition (LPCVD). The polysilicon films were partially implanted by boron (p-type) and phosphorus (n-type) from 4×10¹⁵ to 10×10¹⁵ at cm⁻² doses. The implant energy for each group doses was 70-80kev. Afterward, the doped polysilicon films were annealed in furnace at 1000^◦C for 30 min to activate dopants and obtain a uniform doping profile through the whole thickness and repair the defects in the crystalline structure.

As shown in the above figure the resistance of the polysilicon resistor varies with temperature. If the room temperature T_room rises from 15⁰C to 30⁰C the resistance will change about 200Ω for a polysilicon resistor with 15800Ω at 20⁰C. Since the output voltage ΔV of the thermopile is inversely proportional to the resistance of the polysilicon resistor (as a heater) a specified room temperature can be determined from the measured output voltage of the thermopile.

Off cause polysilicon resistors without thermopiles can also be used as temperature sensors. But common types of resistor temperature sensors are made from platinum instead of polysilicon. Platinum has temperature coefficient α = 0.003925 Ω/(Ω·°C) and polysilicon with sheet resistance ranging from 25 to 150Ω/□ has temperature coefficient α =1x10^-3 Ω/(Ω·°C). So the sensitivity of the platinum resistor temperature sensors is much higher than the polysilicon resistor temperature sensors.

Thanks to CMOS manufacturing technology it allows POSIFA to integrate up to 40 pair of thermocouples in each thermopile. That means the responsivity of the thermopile temperature sensors is increased by 40 which is comparable with platinum resistor temperature sensors.

There are three additional advantages that justify the use of MEMS and CMOS manufacturing technologies.

1. CMOS offer the thermopile structure materials with higher Seebeck coefficient.

2. CMOS beneficial to tune the main characteristics of the thermopiles by doping.

3. MEMS allow the thermal capacity of the thermopile to be reduced effectively by miniaturization.

Thus POSIFA’s thermopile temperature sensors have the best potential to satisfy the demands on miniaturization and mass production.