Design And Characteristic Measurement Of A Thermopile Infrared Detector

In the infrared system, infrared detectors are one of key elements, which can transfer infrared radiation into electrical output signal based on the infrared detecting theory. Thermopile infrared detectors are one of non-cooled detectors, with the following characteristics:1) they can work under room temperature with low cost of fabrication; 2) they are sensitive to infrared radiation with various wavelengths; 3) when they receive infrared radiation, they generate phonon to make the temperature increase of crystal lattice which will influence the electrical performance of the detectors; 4) they can detect the radiation with stable amount and have electrical response. Due to their advantages, they are broadly applied in the military and commercial fields. In this thesis, thermopile infrared detectors are studied from the point of view of theory and fabrication process, and then the performance of thermopile infrared detectors is measured.First, for the thermopile infrared detector mentioned above, one-dimensional Fourier's stationary heat model is built to analyze the temperature gradient between hot junctions and cold junctions of thermo couples. The influences of absorbing area, thermocouple's size, supporting membrane's size, back-etching window's size and measurement condition on the performance of the thermopile infrared detector are established. Moreover, heat static state finite simulation and transient state finite simulation are conducted. Thus, the figures of temperature distribution, heat gradient, heat flux, temperature path analysis and heat response time are obtained to make direct impression of heat response of the detector, after the detector receives infrared radiation.The whole fabrication process of the thermopile infrared detector is conducted at Institute of microelectronics of Chinese academy of Sciences (IMECAS). In order to finish the whole fabrication process, firstly, a series of single experiments for key fabrication process are conducted, including the fabrication of a single layer of low-stress SiNx membrane, square resistance testing of poly-Si, dry etching of poly-Si and SiNx, fabrication of the passivation layer, fabrication of the reflective coating, and wet etching of Si substrate. Based on these single key experiments, the whole fabrication process of the detector is designed, and the thermopile infrared detected based on a single layer of low-stress SiNx membrane is fabricated successfully.In this thesis, a series of experiments are conducted to quantify the performance of the thermopile infrared detector, such as measurement of infrared transmission spectra of the detector, performance comparison of the detector under different structural size, frequency response of the detector, blackbody temperature response of the detector, performance enhancement experiments of the detector. All these testing results show that the performance of the detector is distinct under different absorbing area, the length of thermo-couples and back-etching window's size; due to heat capacity of the detector itself, the performance of the detector decreases with the adding of the frequency of the chopper; during the blackbody temperature range of 403K-823K, the detector in this thesis shows higher performance in low temperature region because of its equivalent absorbing ratio; and the application of carbon-black in the absorbing area and metal reflective-coating in cold junctions of the detector enhances the performance of the detector distinctly.This thesis brings forward the main innovation spots as follows:a) Breaking through the traditional the fabrication method of the SiO2-Si3N4 sandwich-type supporting membrane, a single layer of low-stress SiNx membrane is used as the supporting membrane of the detector to achieve easy fabrication method of the low-stress supporting membrane with adjustable depth.b) The anti-etch protection of the thermo-couples is easy to be achieved combining APIE wax with CRY wax, and the productivity of the detector is enhanced distinctly.c) In this thesis, infrared transmission spectra of the absorbing layer of the thermopile infrared detector are taken into consideration. The infrared transmission spectra of the absorbing layer with different depth and that of the absorbing layer with carbon-black and without carbon-black are obtained and fitted to quantify the actual amount of infrared radiation by the detector more accurately.d) Carbon-black, applied to the absorbing area of the detector, and Titanium/Gold coating, applied to the cold junctions of the detector, enhance the performance of the detector.