High Temperature Difference Double-ended Symmetrical MEMS Thermopile Infrared Detector

In recent years,MEMS infrared thermopile detectors have been widely used and come into the public eye because of the COVID-19.The advantages of thermopile detector include low cost,simple operation and a wider spectral response range,mainly detection of constant radiation levels,detection of static objects with no chopping of the output signal;no bias voltage;and simple test amplification circuitry.Which make it suitable for a wide range of civil and military applications such as smart home appliances,automotive control,medical systems and industrial control.In this work,we study the progress of thermopile research,understand the key infrared technologies,briefly explain the advantages and disadvantages of different infrared detectors,choose the infrared thermopile sensor as the research object,in view of the current status of thermopile research,around the high duty cycle,low cost,high performance and some key technologies of the thermopile to carry out specific research work,mainly including high duty cycle low cost high performance thermopile The theoretical heat transfer model of a doubleended symmetrical thermopile with adiabatic grooves;the process development and device fabrication of a double-ended symmetrical thermopile with adiabatic grooves;the refinement of the infrared performance test system and the completion of comparative detector testing and analysis.The content and results of the research in this thesis can be summarised in the following areas:1.In the design of the thermopile structure,in response to the existing research progress in optimizing the absorber of high-performance thermopiles with complex process,difficult preparation and high cost to meet the problems of batch production,combining the characteristics of closed film and cantilever beam,a thermocouple strip is proposed with a double-end symmetrical arrangement,eliminating the independent absorption zone,with the hot ends of the thermopiles on both sides elongated towards the centre to the very centre;silicon nitride is deposited on the entire device surface as a passivated absorption layer;a larger ohmic contact zone at the cold end is designed to improve the reflection at the cold end and reduce the absorption effect at the cold end;an adiabatic slot is opened on both edges perpendicular to the thermocouple strip.The adiabatic grooves are parallel to the thermocouple strip,allowing temperature transfer only in the direction of the thermocouple strip,ensuring a higher temperature difference and temperature utilisation;a theoretical model for heat transfer in a double-ended symmetrical thermopile with adiabatic grooves has been developed in conjunction with the conventional four-ended heat transfer model.2.In the processing and manufacturing of the device,the first developed ultra-thin suspended film structure release method,to avoid direct contact between the frontal protection and the device causing film damage,to ensure the low-cost,rapid batch manufacturing needs of the thermopile structure.Developed a high selectivity ratio etching process for polycrystalline silicon and silicon nitride;developed an extremely narrow aluminium thermocouple strip stripping process by comparing the effects of metal etching and other processes;combined with Slivaco TCAD process simulation,completed a multi-point electrical connection in the thermopile through ohmic contact stability experiments;developed a process for adjusting composite support film matching,deep slot spray glue lithography and the preparation of key structural adiabatic slots.Finally,a mask version of the thermopile flow sheet was designed and a double-ended symmetrical thermopile with adiabatic grooves was fabricated in the Micro and Nano Fabrication Centre,North University of China,and the designed thermopile was encapsulated in a TO-46 package.3.In terms of device testing,in response to the presence of a chopper during traditional infrared thermopile testing,which causes the response to be lower than the actual value,the test system functions were refined and different test systems were used for detailed testing and analysis of different parameter testing requirements.In terms of responsivity testing,a thermopile response test substrate was designed to read the voltage response through the 3cm confined space of the thermopile,avoiding the effects of chopper caused by reading the electrical signal and low response rate;in response time testing,the radiation period of the detector surface was changed through the chopper,thus reading the response time of the device.A field of view test system was developed to read the electrical signal and normalise the field of view of the device by adjusting the angle of the rotary table.Test results show that the addition of adiabatic grooves and a larger ohmic contact zone at the cold end can effectively improve the performance of micromechanical infrared thermopile detector,with the expanded ohmic contact at the cold end increasing the voltage output by 2.8%,the introduction of adiabatic grooves increasing the voltage output by 21.7% and the expanded ohmic contact and adiabatic grooves at the cold end increasing the output by 25.1%.The response time of the optimised thermopile detector is somewhat delayed compared to the detector without the adiabatic grooves due to the increased thermal resistance;the expanded ohmic contact area allows the field of view to be optimised to a certain extent.The double-ended symmetrical thermopile with adiabatic grooves designed in this paper is of great practical importance for the further development of infrared detectors,and the proposed method of releasing ultra-thin suspended films is of some reference value for MEMS processes and device fabrication.