Frequency Sensitive Soft Fails in SRAM Arrays

Abstract

Abstract Root cause analysis of frequency sensitive “soft” failures in SRAM arrays pose unusual challenges to the failure analyst. Conventional atomic force probe (AFP) DC measurements cannot reliably identify the failure source. The employment of tester based schmoo screening have been shown to correlate with AFP AC quantitative capacitance measurements for the first time. The technique of Nanoprobe Capacitance-Voltage Spectroscopy (NCVS) at contact level (CA) for localization has been previously described [1,2,3]. By exploiting the dC/dV component of the NCVS signal shown in Figure 1 and integrating this output, a quantitative capacitance versus voltage measurement can be demonstrated. This quantitative capacitance measurement identified a frequency sensitve horizontal pair failure (HPF) in the SRAM array. Subsequent process vintage analysis identified the source and eliminated these frequency sensitive HPF characterisics. Given the sensitive nature of these fails, conventional physical analysis methods of TEM EELS, and cross section scanning capacitance analysis were not successful in finding the root cause. This underlies a paradigm shift in failure analysis. Electrical measurements may be the only means to identify a process problem and follow-up process vintage analysis is required to solution the root cause.