Calibre MDP

The photomask is a critical element in the lithographic image transfer process from the drawn layout to the final structures on the wafer. The non-linearity of the imaging process and the related MEEF impose a tight control requirement on the photomask critical dimensions.Critical dimensions can be measured in aerial images with hardware emulation. This is a more recent complement to the standard scanning electron microscope measurement of wafers and photomasks. Aerial image measurement includes non-linear, 3-dimensional, and materials effects on imaging that cannot be observed directly by SEM measurement of the mask. Aerial image measurement excludes the processing effects of printing and etching on the wafer. This presents a unique contribution to the difficult process control and modeling tasks in mask making.In the past, aerial image measurements have been used mainly to characterize the printability of mask repair sites. Development of photomask CD characterization with the AIMS? tool was motivated by the benefit of MEEF sensitivity and the shorter feedback loop compared to wafer exposures.This paper describes a new application that includes: an improved interface for the selection of meaningful locations using the photomask and design layout data with the Calibre? Metrology Interface, an automated recipe generation process, an automated measurement process, and automated analysis and result reporting on a Carl Zeiss AIMS system.

Optical proximity correction (OPC) is widely used in wafer lithography to produce a printed image that best matches the design intent while optimizing CD control. OPC software applies corrections to the mask pattern data, but in general it does not directly compensate for the mask writer and mask process characteristics. The Sigma7500 deep-ultraviolet (DUV) mask writer projects the image of a programmable spatial light modulator (SLM) onto the mask using partially coherent optics similar to wafer steppers, and the residual optical proximity effects of the mask writer are in principle correctable with established OPC methods.To enhance mask patterning, an embedded OPC function called LinearityEqualizer™ has been developed for the Sigma7500 that is transparent to the user and which does not degrade mask throughput. It employs the Mentor Graphics Calibre OPC engine, selected for the computational speed necessary for mask run-time execution. A multi-node cluster computer applies optimized table-based CD corrections to polygonized pattern data, which is then refractured into a standard writer format for subsequent data processing. This short-range proximity correction works in conjunction with ProcessEqualizer™, a previously developed print-time function that reduces long-range process-related CD errors. OPC flattens the linearity behavior for all linewidths and pitches, which should improve the total CD uniformity on production photomasks. Along with better resolution of assist features, this further extends the application space of DUV mask writing. Testing shows up to a 4x reduction in the range of systematic CD deviations for a broad array of feature sizes and pitches, and dark assist features are reliably printed down to 120 nm at mask scale.