MTr/mm² (mega-transistor per squared millimeter) is a transistor density unit that attempts to quantify a process node. The metric makes use of a weighted system consisting of two typical standard cells found in most libraries - a very small NAND2 cell and a very large SFF cell.

Overview

The metric was proposed by Mark Bohr, Intel's director of process architecture and integration, in 2017. Bohr stated the metric is actually a resurrection of a similar metric that had been abandoned in the past. The metric attempts to take into account second order design rules that the simpler density metrics such as CPPxMMP and CPPxMMPxTracks cannot capture because they do not represent a typical standard cell. It should be noted that the metric explicitly SRAM cell sizes because of the large variance in SRAM-to-logic ratio between popular chips. Therefore, SRAM cell sizes should be reported separately alongside the MTr/mm² unit.

Two cells that are found in every standard library are used: a very small 2-input NAND cell consisting of just four transistors and a very large scan flip-flop cell. The NAND2 is weighted as 0.6 while the SFF is weighted as 0.4.

Example

Intel's 10 nm process consists of a minimum metal pitch of 36nm with 7.56 tracks for a cell height of 272 nanometers. Additionally, the process has a poly pitch of 54 nm. For 0.6 NAND2 + 0.4 SFF, Intel's 10nm has a density of 102.9 MTr/mm² along with a high-density 6T SRAM measuring 0.0312 µm². Note that Intel itself reported their 10nm at 100.8 MTr/mm².