EMACS: Efficient MBIST architecture for test and characterization of STT-MRAM arrays

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) is an emerging memory technology which exhibits non-volatility, high density, high endurance and nanosecond read and write times. These attributes of STT-MRAM make it suitable for last level embedded caches. However, defect models, faults and test architectures for emerging memory technologies are relatively unexplored. This is further aggravated by the fact that STT-MRAM, like other post-CMOS technologies rely on novel physics of operation, which can result in unexplored read, write and retention fault models. In particular, the stochastic retention failure of STT-MRAM has a large impact on the test time. Conventional test schemes for retention of STT-MRAM need to be redesigned and optimized for testing large STT-MRAM arrays. This paper presents a comprehensive analysis of read, write and retention tests in STT-MRAM arrays. Resistive and capacitive defects and the corresponding faults are studied. A novel MBIST architecture and associated circuits are presented for measuring thermal stability (and hence retention times) in STT-MRAM bits for characterization and manufacturing tests, amidst variations and magnetic coupling. Trade-offs between fault localization, area overhead and test-times are presented.