Analysis of structural effect on mechanical stress at backside deep trench isolation using finite element method
Section snippets
Introductions
Demand of high-resolution CMOS image sensors keeps increasing in various industries and applications such as mobile phones, digital cameras, and car safety systems. To get a better sensor performance in continuously shrinking of pixel sizes, a backside illumination (BSI) has been preferred to a front side illumination (FSI) [1], [2], [3]. Besides, deep trench isolation (DTI) is known to reduce effectively a pixel crosstalk [4], [5], [6]. Particularly a backside deep trench isolation (BDTI) is
Simulation approach
The ‘Sentaurus interconnect’ of ‘Synopsys’ is used for TCAD simulation based on the finite element method (FEM). Simulation cells of rectangular parallelepiped (1 μm × 1 μm × 2 μm) including a BDTI are illustrated in Fig. 1(d). The BDTI structure before forming a Si-lens and color filters is prepared in this work without any front end of line (FEOL) process. To consider stress situation corresponding to real warpage in our simulation, the displacement of 0.5 nm is applied to outside of cell boundary
Results & discussions
Our BDTIs experimentally formed on silicon wafers were found to be a source of crack under some situations. Even though a main stress resulted in the crack obviously came externally, it is significant to know residual stress (σresidual) distribution at and near the BDTIs [10]. The σresidual may be also obtained from an intrinsic stress (σintrinsic) which exists in spite of no temperature change (△T = 0) during deposition and a thermal stress (σthermal) at △T ≠ 0 by a mismatch of thermal expansion
Conclusions
We simulated a mechanical stress of various BDTI structures under a displacement using the FEM method. The geometry of BDTI depends on an opening length, a Si-etching angle, and a film thickness. Among shape factors, the opening length and thickness of 1st HfOx film are dominant factors to control a maximum stress concentration which occurs at 1st HfOx below an oxide bottom. Comparing only final shapes of various BDTIs, it is consequently very effective to decrease an interfacial curvature
Acknowledgments
C.-W. L. acknowledges support from C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D3A1A01064928).
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