Photomask CD metrology at the 100 nm node

At the 100nm technology node, mask level CDs are typically 400nm with assist features and OPC serifs below 160nm. These design rules represent a severe challenge to conventional optical metrology. The use of any method of measurement, which is not representative of the way the pattern information is transmitted from the mask to the wafer, can lead to measurements that do not correlate directly with those at the wafer (excluding MEEF, magnification factors, lens distortion etc.) The most representative tool for Photomask CD metrology would perhaps be an actinic transmitted light tool. This ideal tool however, may not yet be available. When using alternative non-transmitted measurement, higher resolution is only part of the solution. Matching, calibration and sample interaction must also be considered. This paper discusses the relative merits of optical and non-optical metrology. Multiple, feature specific, gauge R & R studies are used to demonstrate the capability indices, for the Leica LWM250DUV (248nm), at each technology node. Furthermore, the specific application of the optical tool in the measurement of 'Assisted' chrome lines, at the 100nm technology node, is demonstrated. The methodology employed includes optical/CDSEM calibration and correlation. Tool specific considerations necessary to achieve a stable and reliable match are detailed.