Nanotech 2010

Mechanical Robustness of FPA in a-Si Microbolometer with Fine Pitch

H.Y. Kim, K.M. Kim, B.I. Kim, W.S. Jang, T.H. Kim, T.Y. Kang
National Nanofab Center, KR

Keywords: microbolometer, amorphous silicon, IR sensor, MEMS


Uncooled microbolometer has the various applications in many commercial and military applications. Among many sensor materials, amorphous Si have been widely used as sensor materials for their good TCR (Temperature Coefficient of Resistance), mechanical strength, low thermal mass and high manufacturing yield. One of the key parameters is the low cost driving the pitch reduction approach. Microbolometer FPA (Focal Plane Array) always have the curvature due to the intrinsic residual stresses in the dissimilar layers, leading to undesirable IR absorption useless. In this paper, we report an engineering approach to address this issue: the method we used includes a combination of test structure to analyze the simple beam geometry and FEA analysis of real pixel. In addition, the effects of misalign during fabrication process on mechanical robustness of FPA are investigated. Microbolometer is composed of a thermometer integrated on a micro-bridge which is supported by two legs anchored over the ROIC wafer by metal studs. The micro-bridge and FPA have 0.3 μm thickness of doped amorphous silicon and 0.01 μm thickness of Ti layer and the distance between the mirror and the FPA is 2.0 μm. In simple beam geometry, the effect of beam width on bending was minimized and the effect of contact pad on bending was more significant as the beam length decreased. Residual stresses of all layers consisting of FPA are measured by laser curvature method. The curvature of FPA is mainly controlled by thicker amorphous Si layer. Tilting of FPA is normally observed during process and its relationship with the contact pad positions are analyzed by FEA. Even though the misalign of contact pad position affect small in bending, asymmetric misalign of 1um distance can create the 0.026um tilting of FPA. In conclusion, mechanical robustness of FPA related to the IR absorbance can be implemented by optimization of residual stress and misalign accuracy of each layer consisting FPA.
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