5.6 Mb/mm $^{2}$ 1R1W 8T SRAM Arrays Operating Down to 560 mV Utilizing Small-Signal Sensing With Charge Shared Bitline and Asymmetric Sense Amplifier in 14 nm FinFET CMOS Technology

Abstract

Multiported high-performance on-die memories occupy significantly more die area than a comparable single-port memory. Among various multiport memory topologies, the 1-read (R), 1-write (W) 8-transistor (T) Static Random Access Memory (SRAM) with a decoupled read port allows separate optimization of the read and write ports when organized without interleaved logical columns. This enables a lower minimum operating voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> ) compared with other dual-port SRAMs that require ports optimized for read stability and write operations. However, the 1R1W 8T SRAM often employs large signal, hierarchical bitline sensing to achieve high performance due to the nondifferential read bitline. This large-signal read architecture necessitates frequently placed local bitline sensing circuits, degrading the array bit density. In this paper, we present two sense amplifier (SA) techniques for small-signal pseudodifferential sensing to facilitate 256 bits per bitline achieving an 8T SRAM array density of 5.6 Mb/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in 14 nm FinFET CMOS. The first design employs a charge sharing SA scheme to generate a reference voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">REF</sub> ) by leveraging the capacitance of otherwise unused metal tracks over the bitcell column. The second design utilizes an asymmetric SA in which the read bitline precharged to V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CC</sub> in the unselected sector acts as a reference voltage and the active bitline side is intentionally upsized to skew the SA. High volume measurement results demonstrate 560 mV V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> at 400 MHz/-10 °C and reaches 2.21 GHz at 1 V supply.