A 28nm 32Kb embedded 2T2MTJ STT-MRAM macro with 1.3ns read-access time for fast and reliable read applications

Abstract

Many IoT and wearable devices require an on-chip small-to-mid-capacity nonvolatile memory (NVM) with a fast read-access time (T AC ) and reliable read operations: for applications including data-logging, configurable look-up tables (LUT), eFuse, and physically unclonable functions (PUF). STT-MRAM [1-4] is a good candidate for these applications due to its fast write speed, low-voltage write, and high endurance. However, STT-MRAM suffers from a small read-signal margin (RSM) due to a small tunnel magnetoresistance ratio (TMR: (R AP -R P )/R P ) between the cell resistance of parallel (P, R P ) and anti-parallel (AP, R AP ) states [1-6]. Moreover, the read-disturb behavior of STT-MRAM cells is sensitive to the BL read voltage (V BL_RD ) and the stress/read time. Compact 1T1MTJ arrays are suitable for high-density applications [5-6]; however, they use a power-hungry current-mode read scheme with a slow read speed due to the small RSM. Researchers have proposed 2T2MTJ (Fig. 30.3.1) arrays [1-4] with differential bitlines (BL and BLB) and a voltage-mode read scheme, with an enlarged RSM (V RSM ), for fast, low-power read operations. V RSM refers to the voltage difference between BL (V BL ) and BLB (V BLB ). 2T2MTJ STT-MRAM read operations still face the following challenges: (1) V BL and V BLB both drop from V BL_RD to 0V quite quickly due to the large cell read current (V P and V AP ) or low R-value in both R P and R AP , resulting in small sensing window (T SMW ), which is the period when V RSM >offset; (2) the maximum V RSM (V RSM_MAX ) occurs at different times (t RSM_MAX ) for different cells due to TMR (R AP /R P ) variation; and (3) a degraded V RSM due to the use of a low V BL_RD to avoid read disturbs for high data-reliability applications. (1) and (2) lead to a decrease in V RSM after reaching its peak (t RSM_MAX ), despite an increase in BL development time (t BL ). When using a conventional voltage-mode sense amplifier (CNV-VSA) with a common activated (SAEN=1) timing (f SAEN ) under the effects of (1)-(3), the signal to be amplified (ΔV IN „ RSM_MAX ) is subject to degradation at the VSA's differential inputs, resulting in a sensing failure at a low VBL_RD.