Improved SRAM Failure Diagnosis for Process Monitoring via Current Signature Analysis

摘要

SRAM's are frequently used as moinitor circuits for defect related yield, due to the ease of testing and the good correlation to the yield to the yield characteristics of circuitry. For the identification of the failure/fault type and the nature of hte defect causing the failure, measured failure, measured faibitmaps are mapped onto a failbtmap catalog obtained from defectfault simulatio. Often this mapping is not unique. A given failbitmap can be caused by several faults or defects.In this contribution, the application of current signature analysis is demonstrated for a stand-alone 16kx1 SRAM monitor cicuit. It is found that the resolution of the failbitmap-fauolt-defect catalog can be improved considerably by additonal current signature measurements. The interpretation of current measurements is based on simulaiton of the possible faults contained in the failbitmap catalog under the operating conditions in the current test. These was good agreement between the simulated and measured current values. With the aid of current measurements, more yield learning information is obtained from the process monitoring vehicle. In some cases, the shorted nodes inside a SRAM cell can be determined exactly. This eases the localization of the failure and is of practial importance for the sample preparation in physical failure analysis.