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

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 V
^BL_RD
.