Investigation on an Active Thermoelectric Vacuum Sensor with Low Frequency Modulation

Abstract

In this research, we propose and investigate a thermoelectric-type vacuum sensor with an active heater in the center of sensor under low frequency modulation. Thermal type sensor for vacuum measurement with active-heating is proposed for the suspended thermoelectric element to reach a better performance. Using the TSMC 0.35 μm CMOS-MEMS process, the proposed thermoelectric sensor with active heater is designed and fabricated with central-symmetrical thermocouples positioned. There are 64 pairs of thermocouples placed around the center of membrane and each thermocouple is designed with two kinds of material, n+ Poly and metal. The material of Metal 1 ∼ Metal 4 layers is aluminum, and titanium is used as contact layers to connect these metal layers. The total resistance of the structure with four units is measured to be about 14.8 kΩ and the resistance of each unit is about 3.7 kΩ. Four quarters of sensing units of thermopile were connected and the sensing area at the center will give a temperature difference. According to the Seebeck effect, serial connected thermocouples will produce a weak voltage difference between the hot and cold junctions after heat exchange under air convection related to vacuum pressure. The heating of thermopile by active heater is proceeded under different frequencies and the output signal is acquired with a Phase-Lock-Loop (PLL) amplifier. When the periodic heat exchange will take away the heat to cause a temperature drop of sensing area gives a corresponding periodic weak voltage between the cold and hot end of the thermocouples. According to a careful investigation of the PLL measurement with a wide range of 10 m∼100 Torr, our proposed sensing scheme based on a thermoelectric type sensor is proved for practical vacuum detection. Most of all it is proved as a new approach to use a commercial thermopile with heater, which is easier to include than a special custom design.