SOT-MRAM Bitcell Scaling with BEOL Read Selectors: A DTCO Study

TL;DR

This paper evaluates the potential of using BEOL read selectors to improve the scaling of SOT-MRAM bitcells at 7 nm technology, addressing area, power, and performance tradeoffs through extensive DTCO analysis.

Contribution

It introduces BEOL read selectors as a solution to the scaling challenges of SOT-MRAM bitcells, demonstrating area reduction and performance considerations at 7 nm.

Findings

BEOL RSs enable 10-40% bitcell area reduction.

BEOL RS-based bitcells can meet writability requirements with proper magnetic layer engineering.

Tradeoffs include increased latency and energy costs with certain selector types.

Abstract

This work explores the cross-node scaling potential of SOT-MRAM for last-level caches (LLCs) under heterogeneous system scaling paradigm. We perform extensive Design-Technology Co-Optimization (DTCO) exercises to evaluate the bitcell footprint for different cell configurations at a representative 7 nm technology and to assess their implications on read and write power-performance. We crucially identify the MTJ routing struggle in conventional two-transistor one-resistor (2T1R) SOT-MRAMs as the primary bitcell area scaling challenge and propose to use BEOL read selectors (BEOL RSs) that enable (10 -- 40) % bitcell area reduction and eventually match sub-N3 SRAM. On writability, we affirm that BEOL RS-based bitcells could meet the required SOT switching current, provided the magnetic free layer properties be engineered in line with LLC-specific, (0.1 -- 100) s retention targets. This is particularly to attribute to their (i) more available Si fins for write transistor and (ii) lower bitline resistance at reduced cell width. We nevertheless underscore the read tradeoff associated with BEOL RSs, with the low-drive IGZO-FET selector sacrificing the latency up to (3 -- 5) ns and the imperfectly rectifying diode selectors suffering (2.5 -- 5)$\times$ energy cost relative to 2T1R. This article thus highlights the realistic prospects and hurdles of BEOL RSs towards holistic power-performance-area scaling of SOT-MRAM.