SRAM Single Bit Cell Soft Failure and Nano-probing Methods

Abstract

SRAM is often chosen to be the process qualification vehicle during technology development or yield learning vehicle during product manufacturing, and consequently failure analysis of SRAM is the main feedback for process improvement and yield learning. The most common SRAM failure is single bit cell failure. Although its location can be precisely localized by functional test and the defect causing the failure is within the failing bit cell, its failure analysis becomes more and more challenging in advanced technology nodes. As semiconductor technology continuously scales down, SRAM bit cell size and power supply voltage decrease, resulting in increased transistor strength variation and mismatch. SRAM single bit cell soft failures have become more and more common. For such a failure, its defect is usually subtle or even there is not physical defect at most cases. The soft failure is just due to transistor parameter variation. To evaluate the single bit cell soft failure and identify its root cause, electrical nano-probing is an indispensable measure. In this paper, we will first describe the operation of a 6-Transistor (6-T) SRAM single bit cell and three different types of single bit cell soft failures, then discuss the two electrical nano-probing methods for the SRAM single bit cell soft failure.