Delft University of Technology (TU Delft) and nanoelectronics research center imec, introduced 3D-COSTAR, a new test flow cost modeling tool for 2.5/3D stacked integrated circuits (ICs). 3D-COSTAR aims to optimize the test flow of 3D stacked ICs (SICs), taking into account the yields and costs of design, manufacturing, packaging, test, and logistics.

Due to many high-precision steps, semiconductor manufacturing is defect-prone. Consequently, every IC needs to undergo electrical tests to weed out defective parts and guarantee outgoing product quality to the customer. For TSV-based 2.5D- and 3D-SICs that typically contain complex die designs in advanced technology nodes, testing is even more critical. In addition, there are many possible test moments in their manufacturing flow: pre-bond (before stacking), mid-bond (on a partial stack), post-bond (on a completed stack), and final testing (on a packaged device). Although testing is expensive, filtering out the bad components in an early stage is critical to save costs later on in the production process.

“There is not a ‘one-size-fits-all’ test flow that covers all stacked-die products. The test flow needs to be optimized based on yield and cost parameters of an individual product and that is a complex optimization problem,” stated Dr. Said Hamdioui, Associate Professor at TU Delft. “And different test flows, executed after manufacturing, may require different design-for-test features, which need to be incorporated in the various dies during their early design stages.”

3D-COSTAR uses input parameters that cover the entire 2.5D-/3D-SIC production flow: 1) design; 2) manufacturing; 3) test; 4) packaging; and 5) logistics. It is aware of the stack build-up (2.5D versus 3D, multiple towers; face-to-face or face-to-back) and stacking process (die-to-die, die-to-wafer, or wafer-to-wafer). The tool produces three key analysis parameters: 1) product quality, expressed as defect level (test escape rate) in DPPM (defective parts per million); 2) overall stack cost; and 3) breakdown per cost type.

“3D-COSTAR has proven to be a crucial tool to analyze the many complex trade-offs in 3D test flows, in terms of both cost and DPPM,” said Erik Jan Marinissen, ‎Principal Scientist at imec. “Among others, we have used 3D-COSTAR to determine when pre-bond testing of the interposer in 2.5D-SICs pays off and what its maximum-allowed test cost can be. In some cases, the overall stack cost reduction amounts to 40%, showcasing that upstream testing can help avoid downstream costs. The tool also demonstrated under which circumstances mid-bond testing (on partially-completed stacks) can be avoided without compromising a high stack yield.”

“Together with imec, Cascade Microtech has recently demonstrated the feasibility of direct probing large-array fine-pitch micro-bumps to avoid the usage of dedicated pre-bond pads,” stated Ken Smith, Principal Engineer, Cascade Microtech, Inc. “Analysis with 3D-COSTAR clearly showed up to 50% overall cost benefit of doing microbump probing using an advanced probe cell such as was demonstrated with Pyramid Probe® RBI technology on our CM300 probe station.”

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