Cu pattern density impacts on 2.5D TSI warpage using experimental and FEM analysis

Through Silicon Interposer (TSI) needs to fulfill multi-die stacking in one packaging which can bring high integration density, short interconnection length and small size for next generation devices. Die stacking is a key process in the TSI manufacturing flow, and within that process, die warpage is of central concern. This is because the large warpage of the Si-mterposer induces poor joining of u-bump interconnection, lithograph missing and die breakage during the assembly process. According to our experience, the Cu metal density of redistribution layer (RDL) significantly effects on TSI warpage. [2] The work proposes to design four different Cu metal densities, namely code A (10∼25%), B (25∼40%), C (40∼55%) and D (55∼70%) and combine the different metal density to investigate the relationship between Cu metal pattern density and TSI warpage under four different TSI size. Each unit of Cu metal density area is 12 × 12 mm
²
. The TSI size is ranging from unit 1 × 1 (12 × 12 mm
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), 1 × 2 (12 × 24 mm
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), 2 × 2 (24 × 24 mm
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) and 2 × 3 (24 × 36 mm
²
). In this work, the TSI warpage modeling methodology has been developed and carried out to find the correlation between pattern density and warpage behavior using the finite element method (FEM). A simplified pattern-inclusion model has been constructed to obtain its warpage response under a thermal loading. Furthermore, the equivalent model has been established to obtain the effective mechanical properties of the equivalent layer (i.e., the composite layer of Cu traces and IMD) through adapting the warpage results to correctly match the simplified pattern-inclusion model. Therefore, the TSI warpage can be calculated during the reflow process after acquiring the effective mechanical properties of different Cu pattern densities. The simulation warpage variation of 1 × 1 (12 × 12 mm
²
) TSI die size for code А, В, С and D are 8 μm, 19 μm, 32 μm, and 47 μm, respectively. The simulation results indicate that large TSI with high Cu metal density possesses high warpage. After fabricating the different Cu metal density wafer, the wafer was grinded to 100 μm for dicing, then using thermal shadow moiré to measure the TSI warpage after one time reflow process. The experimental TSI warpage variation of 1 × 1 (12 × 12 mm
²
) TSI die size with code A, B, C and D are 6–10 μm, 21–26 μm, 31–36 μm, and 43–70 μm, respectively. The experimental results imply that large TSI with high Cu metal density possesses high warpage. After measuring the warpage of those dies during the thermal curve, we compared the measurement data with our simulation result and explored 3 subjects: different pattern density, die size, and symmetry. We found the TSI warpage between simulation and measurement result are consistent. The finite element simulation results in the work correlate well with the experimental test results. Therefore, we are highly confident that this finite element model can help generate design guidelines for wafer patterns and predict the die warpage variation during the assembly process.