Quantitative measurement of voltage contrast in SEM images for in-line resistance inspection of incomplete contact

Abstract

An in-line inspection method for estimating defect resistances from the grayscale of voltage contrast in SEM images of manufactured patterns was developed. This method applies a circuit simulator to calculate the intensity of the secondary electrons according to an equivalent-circuit model considering the charge-up voltage on the patterns. To accurately estimate the resistance of defects formed in a device, first, the simulator was improved by taking the variation of defect resistance into account, which strongly depends on the differential voltage between the plug surfaces and the backside wafer. The defect resistances were obtained from the measured I-V characteristics of the deliberately formed defect on the standard calibration wafers, in which some incomplete-contact defects were systematically formed. Next, to consider the effect of the electronic characteristics of the pattern under the normal plugs on the grayscale, the I-V characteristics of the normal plugs were measured. The equivalent circuit of the simulator was improved by taking into account the measured I-V characteristics. The calibration curve for the inspected patterns was calculated from the improved circuit simulator. Finally, the inspection method was applied to estimate the resistance of defects formed on an SRAM pattern. The calculated calibration curve was used to accurately estimate the defect resistance (with an accuracy of about an order of magnitude) from the voltage contrast formed on the defects in the inspected SRAM patterns.