Spin-transfer Torque Random Access Memory (STT-RAM) emerges for on-chip memory in microprocessor architectures. Thanks to the magnetic field based storage STT-RAM cells have immunity to radiation induced soft errors that affect electrical charge based data storage, which is a major challenge in SRAM based caches in current microprocessors. In this study we explore the soft error resilience benefits and design trade offs of 3D-stacked STT-RAM for multi-core architectures. We use 3D stacking as an enabler for modular integration of STT-RAM caches with minimum disruption in the baseline processor design flow, while providing further interconnectivity and capacity advantages. We take an in-depth look at alternative replacement schemes in terms of performance, power, temperature, and reliability trade-offs to capture the multi-variable optimization challenges microprocessor architectures face. We analyze and compare the characteristics of STT-RAM, SRAM, and DRAM alternatives for various levels of the cache hierarchy in terms of reliability. © 2011 IEEE.