Stress-Induced Performance Shifts in 3D DRAMs

3D-stacked DRAMs can significantly increase cell density and bandwidth while also lowering power consumption. However, 3D structures experience significant thermomechanical stress due to the differential rate of contraction of the constituent materials, which have different coefficients of thermal expansion. This impacts circuit performance. This article develops a procedure that performs a performance analysis of 3D DRAMs, capturing the impact of both layout-aware stress and layout-independent stress on parameters such as latency, leakage power, refresh power, area, and bus delay. The approach first proposes a semianalytical stress analysis method for the entire 3D DRAM structure, capturing the stress induced by through-silicon-vias (TSVs), micro bumps, package bumps, and warpage. Next, this stress is translated to variations in device mobility and threshold voltage, after which analytical models for latency, leakage power, and refresh power are derived. Finally, a complete analysis of performance variations is performed for various 3D DRAM layout configurations to assess the impact of layout-dependent stress. We explore the use of alternative flexible package substrate options to mitigate the performance impact of stress. Specifically, we explore the use of an alternative bendable package substrate made of polyimide to reduce warpage-induced stress, and show that it reduces stress-induced variations and improves the performance metrics for stacked 3D DRAMs.