EMBER: Efficient Multiple-Bits-Per-Cell Embedded RRAM Macro for High-Density Digital Storage

Designing compact and energy-efficient resistive RAM (RRAM) macros is challenging due to: 1) large read/write circuits that decrease storage density; 2) low-conductance cells that increase read latency; and 3) the pronounced effects of routing parasitics on high-conductance cell read energy. Multiple-bits-per-cell RRAM can boost storage density but has further challenges resulting from reliability problems due to conductance relaxation and slow write due to narrow conductance levels. This work presents a multiple-bits-per-cell RRAM macro called Efficient Multiple-Bits-per-Cell Embedded RRAM (EMBER), which: 1) demonstrates read/write circuit compaction through constrained optimization of driver and pass gate transistor sizes; 2) introduces a common-mode bleed conductance at the sense amplifier inputs, reducing read settling time by
$11.35\times $
for low-conductance cells, and 3) cuts read path capacitance to further reduce read access time and energy. To address reliability and write speed, EMBER contains a configurable on -chip read/write controller. We present a level allocation scheme that uses array-level characterization data to find sufficiently reliable allocations, while simultaneously maximizing write bandwidth. EMBER is the first embedded RRAM storage macro to achieve fully integrated multiple-bits-per-cell readout and write-verification without any off -chip reference generation or sensing. The macro operates at 100 MHz with 64k
$\times $
48 = 3 M cells in TSMC 40-nm CMOS, achieving 1 b/cell read operation with 1.0 pJ/bit energy at 2.4 Gbps, and 2 b/cell read with 1.1 pJ/bit at 1.6 Gbps. 1 b/cell write-verify operates with 0.40 nJ/bit energy at 12.4 Mbps (BER < 6e-4), and 2 b/cell write-verify operates with 1.2 nJ/bit at 3.8 Mbps (BER < 3e-3). The array-level endurance is found to be 10 K for 1–2 b/cell. Normalizing for process scaling, the macro demonstrates the highest effective RRAM cell density to date of 5.6e-3 b/
$\text{F}^{2}$
for 1 b/cell and 1.3e-2 b/
$\text{F}^{2}$
for 2 b/cell, an improvement of 31% and 204%, respectively, over the best prior work.