Analysis of Structural Stress in InSb Array Detector without Underfill

Hybrid infrared focal plane arrays (IRFPAs) are more and more used in both infrared medium and long wavebands for many different applications. In order to offer more spatial resolution, larger format (2048×2048) start to be available, yet its final yield is so low that its cost is very high, and is not affordable. In this paper, basing on viscoplastic Anand’s model, the structural stress of indium antimonide (InSb) infrared focal plane arrays detector without underfill dependent on both indium bump sizes and array formats is systematically researched by finite element method. For shortening simulation time, three-step method is employed to research Von Mises stress and its distribution in InSb infrared focal plane arrays. First, the structural stress of 8×8 InSb infrared focal plane arrays detector is systematically analyzed by finite element method, and the impacts of design structural parameters including indium bump diameters, heights and InSb chip thicknesses on both Von Mises stress and its distribution are discussed in this manuscript. Simulation results show that as the diameters of indium bump decreases from 36μm to 24μm in step of 2μm, the maximum stress existing in InSb chip firstly reduces, then increases linearly with reduced indium bump diameters, and reaches minimum with indium bump diameter 30μm, the stress distribution at the contacts areas is uniform and concentrated. Furthermore, it seems that the varied tendency has nothing to do with indium bump standoff height. With indium bump diameter 30 μm, as the thickness of InSb chip reduces from 21μm to 9μm in step of 3μm, the varying tendency of the maximum stress value in InSb chip is just like the letter U, as the indium bump thickness decreases also from 21μm to 6μm in step of 3μm, the maximum stress in 8×8 InSb IRPFAs decreases from 260MPa to 102MPa, which is the smallest Von Mises stress value obtained with the indium diameter 30μm, thickness 9μm and InSb thickness 12μm. Basing on the above simulated results, a typical InSb infrared focal plane arrays structure with indium bump diameter 30μm, standoff height 20μm and InSb thickness 10μm is selected, then, InSb IRFPAs array format is doubled once again from 8×8 to 64×64 to learn the effect from array size, thus, the Von Mises stress and its distribution of 64×64 InSb IRFPA is obtained in a short time. Simulation results show that Von Mises stress maximum in InSb chip and Si readout integrated circuit almost do not increases with array scale, and the largest Von Mises stress is located in InSb chips. Besides, stress distribution on the bottom surface of InSb chip is radiating, and decreases from core to four corners, and stress value at contacting area is smaller than those on its surrounding areas, contrary to stress distribution on top surface of InSb chip. Finally, employing the relative displacement theory produced by thermal cycles between InSb detector and silicon readout integrated circuit, an equivalent method is proposed to simulate the relationship between the structure stress of large format InSb infrared focal plane arrays versus array format. The simulated results show that as array format is enlarged from 32×32 to 256×256, the maximal Von Mises appearing in InSb chip fluctuates at 280MPa, simultaneously, the site at which the maximal Von Mises appearing in InSb chip is fixed at the diagonal line of indium bump array with the distance of 5 indium bumps. When the array format is over than 384×384, the maximal Von Mises increases linearly from 326MPa to 1090MPa, which is the maximal Von Mises appearing in InSb chip with format 1024×1024, here the site at which the maximal Von Mises appearing in InSb chip is also fixed at the diagonal line of indium bump array with the distance of