Wafer level CD metrology on photomasks using Aerial Imaging Technology

Recently more and more mask designs for critical layers involve strong OPC which increases the complexity for standard CID SEM mask measurements and conclusive interpretation of results. For wafer printing the wafer level CID is the crucial measure if the mask can be successfully used in production. Recent developments in the AIMS (TM) software have enabled the user to use the tool for wafer level CD metrology under scanner conditions. The advantage of this methodology is that AIMS (TM) does see the CD with scanner eyes. All lithographic relevant effects like OPC imaging which can not be measured by other tools like mask CID SEM will be captured optically by the AIMS (TM) principle. Therefore, measuring the CD uniformity of the mask by using AIMS (TM) Will lead to added value in mask metrology. With decreasing feature sizes the requirements for CD metrology do increase. In this feasibility study a new prototype algorithm for measuring the lithographically relevant AIMS (TM) CD with sub pixel accuracy has been tested. It will be demonstrated that by using this algorithm line edge and line width roughness can be measured accurately by an AIMS (TM) image. Furthermore, CID repeatability and tool matching results will be shown.