Fabrication and commercialization of scalpel masks

SCALPEL (SCattering with Angular Limitation in Projection Electron Lithography) masks have been fabricated for use in the Proof-of-Lithography system and to demonstrate the feasibility of having them produced by a commercial blank manufacturer and optical mask shops. Mask blanks are formed from 100 mm diameter(100) silicon wafers. A 100-150 nm thick SiNx layer is LPCVD deposited onto the wafers followed by magnetron sputter deposition of a thin (30-60 nm) Cr/W metal layer which is used as the scatterer layer for the mask. The mask is supported by an underlying network of struts which are arranged to be compatible with the step and scan writing strategy of the exposure tool and to provide robustness to the mask. Crystallographic wet etching of the (100) silicon wafer forms membranes and struts. To date over 300 mask blanks have been formed and yield data as a function of the thickness of the silicon nitride membrane has been quantified. Recent developments in the mask blank formation process include the production of blanks by MCNC who serve as a commercial source of SCALPEL mask blanks. They have successfully delivered 36 blanks that exhibit equivalent properties to those produced at Lucent. Mask patterning has been performed at the commercial optical mask shops of PHOTRONICS and DUPONT. In this investigation a MEBES exposure system has been used to write patterns. The resist used is ZEP-520 and development and pattern transfer processes are performed in the STEAG-Hammatech spray/spin processing tool. Metrology is performed using a KMS 310 RT optical microscope. Pattern placement accuracy is measured on the LMS 2020 system without modification. The masks are inspected for defects using the optical based KLA 300 series inspection system in a die to die mode and in transmission. Results to date suggest feasibility of producing SCALPEL masks by a commercial blank supplier and by merchant optical mask shops.