Techniques to detect and analyze photomask CD uniformity errors

Abstract

With increasing mask error enhancement factors (MEEF), detecting and analyzing photomask critical dimension (CD) uniformity errors is critical for understanding how photomasks can be manufactured to afford high wafer yields. Using UV pattern inspection tools, recent improvements in automated inspection algorithms can now provide CD error detection below 50 nm. This level of sensitivity is necessary in order to provide a clear picture of the reticles' contribution to the final wafer image and possibly function. However, dispositioning CD errors less than 100 nm is very challenging at defect review which has previously resulted in misclassification of true CD errors. Classifying very small errors require the high precision and resolution of a metrology tool. A process has been developed for detecting very small CD errors with a UV inspection tool and the coordinates of areas of concern transferred to a CD SEM for sizing, review and disposition. In this study, we have characterized the sensitivity and false defect performance of a new algorithm with production masks and a new programmed defect test mask. The inspection results were transferred to a CD SEM for analysis. SEM measurements were taken to validate the sensitivity of the algorithm and to quantify the calibration accuracy of the review tools of the inspection system.