| Infrared focal plane array detector can be widely used in aerospace,infrared remote sensing,missile guidance,communication,meteorology and other fields.Among the existing infrared focal plane array detector products,InSb infrared focal plane array detector working in 3-5 μm band occupy a prominent position in the field of infrared focal plane array detector due to their good uniformity,low dark current and high quantum efficiency.After mass production of infrared focal plane array detector is completed,in order to suppress background noise and improve signal-tonoise ratio,infrared focal plane detectors usually package under room temperature(300K),but work at liquid nitrogen temperature(77K).When the detector temperature drops rapidly from 300 K to 77 K,the difference of linear expansion coefficient between adjacent materials will induce thermal mismatch and produce thermal stress,excessive thermal mismatch stress will cause InSb IRFPAs chip fragmentation or local delamination between adjacent materials,which restricts the improvement of detector yield.Therefore,it is important to clarify the local delamination mechanism of InSb IRFPAs and the corresponding optimization of thermal stress structure to improve the detector yield.When the prepared InSb infrared focal plane array detectors are subjected to low period liquid nitrogen shock,thermal mismatch will be induced by the difference of linear expansion coefficient between adjacent materials,which often leads to local delamination phenomenon of InSb Infrared focal plane array detector.In order to determine the inducement of local delamination in InSb IRFPAs,we complete cover the interface between the InSb chip and the underlying underfill with cohesion units,and optimize the specified parameters in cohesion model,finally establish the twodimensional local delamination analysis model of InSb IRFPAs.Simulation results are verified by the measured local delamination photographs,that is,1)Most local delamination appear in the surrounding edges of InSb IRFPAs,and occupy a certain width;2)Once the InSb chip is separated from the underlying underfill in the normal direction,the local delamination will expand gradually toward its both sides of the plane.In order to clarify the inducement of the local delamination,we systematically analyze the evolution rule of the local delamination with different mixed-mode ratios under the jointed action of both the opening mode and sliding mode.We find that the simulation results are almost the same as the measured results when the mixed-mode ratio between the opening mode and the sliding mode was set to 4:6.Up to now,we believe that the local delamination of the InSb IRFPAs are ascribed to the jointed action of both the interfacial normal stress and the in-plane shear stress,is the typical mixed-mode failure pattern,furthermore,the sliding local delamination mode is dominant.From the above analysis,we know that thermal mismatch stress is the main reason affecting the local delamination of InSb infrared focal plane array detector,so analysis of thermal stress distribution is an effective means to reduce the probability of local delamination.In liquid nitrogen shock experiment,the thermal mismatching stress will be generated due to the difference of linear expansion coefficient between layers of InSb infrared focal plane array detector,and excessive thermal mismatch stress will lead to the local delamination failure of the detector.In order to reduce the influence of thermal mismatch stress on local delamination,basing on the calculation theory of the thermal stress suitable for the elastic multilayer system,employing the design method of the balanced composite structure,we optimize the thermal strain on the upper surface of the balanced composite structure,that is,making the thermal strain on the upper surface of the silicon readout circuit in the balanced composite structure be closer to the thermal strain on the lower surface of the InSb chip as possible as it can,it is used to reduce the thermal mismatch between the two due to the impact of liquid nitrogen,so as to greatly reduce the thermal stress that causes the local delamination of the detector.Considering the maturity of the device processing technology,Using thermal stress calculation theory of elastic multilayer structure and the design method of the balanced composite structure,after a series of calculations show that: the thickness of the silicon readout circuit is set at 25μm,the thermal strain on the upper surface of the silicon readout circuit is the closest to the thermal strain on the lower surface of the InSb chip in balanced composite structure,at this moment,the in-plane normal stress and shear stress on the bottom surface of the InSb chip are the minimum.The significant reduction of shear stress on the bottom surface of the InSb chip will reduce the probability of local delamination failure and improve the local delamination phenomenon of the detector,it will provide reliable structural design scheme and realization approach for reducing the local delamination probability in liquid nitrogen impact,which has far-reaching significance. |
