Structure Design And Simulation Of Dewar Cold Head For Low Thermal Stress In Infrared Detector

The cooled infrared focal plane arrays(IRFPA)is widely used in military and civil fields,such as aerospace,forest fire prevention,etc.The IRFPA usually need to work in a cold environment,which is aimed at reducing the dark current of the detector and improving the response rate and ratio of signal to noise of the detector.The coefficients of expansion of the materials of the cold head are different,which will lead to large thermal stress in the chip during the cooling process of detector.The thermal stress will lead to the increase of defective pixels,what is worse,it can make the chip break.In addition,when the separation stress between the materials caused by thermal stress is greater than the adhesion of the adhesive,it will lead to the separation phenomenon between the materials of the cold head.In this paper,three different cold head structures,which are no balance layer structure,ear shape balance layer structure,and cone shape balance layer structure,were studied by Ansys finite element analysis method.On this basis,the research also found the relationship between the thickness of balance layer and the thermal stress of the chip,the thicker the balance layer is,the smaller the thermal stress of the chip is,but growth slowing as thickness of balance layer increases.Finally,two infrared chips were packaged with two different cold head structures respectively.The blind pixels number and Gpol-Vt1 curve are taken as the evaluation criteria of the thermal stress,which proves that the balance layer can reduce the thermal stress of the chip.In addition,the effects of different cold head structures and different materials on the separation stress between the balance layer and the metal plate are studied.The simulation results show that the thicker the balance layer is,the greater the separation stress is.In addition,the separation stress between the balance layer and the metal plate is greater with L605 thin-walled tube than that with TC4 thin-walled tube.The separation stress of thin-walled tube with ear shape is higher than that of straight tube.In order to verify the simulation analysis results,this paper packaged two chips with the straight TC4 cold head structure and the ear L605 cold head structure respectively.Through the liquid nitrogen tests,it is proved that the separation stress of the straight TC4 cold head structure is smaller than that of the ear L605 cold head structure.