Intelligent thermal vacuum sensors based on multipurpose thermopile MEMS chips

Realization of intelligent thermal vacuum sensors based on multipurpose thermopile microelectromechanical (MEMS) chips is presented in this work. These vacuum sensors satisfy the main requirements for contemporary sensors, they are cost-effective in both fabrication and operation, and simple to use. Intelligent devices based on two types of thermal sensors, A-type with Al heater, and P-type with p+Si heater, were developed. Both types have two thermopiles with 30 p+Si/Al thermocouples each. Thermal and electrical isolation is provided by a sandwich membrane (residual n-Si and sputtered oxide). The intelligent functionality is achieved by using a signal processing unit the authors developed earlier for pressure sensing based on their proprietary Si piezoresistive pressure sensing elements. The main issues which were of key importance for realization of intelligent vacuum sensors are addressed in this work: design and fabrication of the intelligent vacuum sensors, adaptation of the software module, linearization of the input signal using digital signal processing and temperature compensation. A three-stage test procedure is presented. The A-type sensor, with a thinner membrane, achieved 19 times better thermopile voltage-pressure sensitivity compared with the P-type sensor, with a thicker membrane. Since the estimated noise level of the measuring system is comparable with the useful signal of the P-type device, the improved voltage-pressure sensitivity provides a better resolution and signal to noise ratio of the intelligent vacuum measuring system.