An infinitely selective repair buffer for EUVL reticles

The three-layer absorber stack for EUVL reticles currently consists of an absorber, repair buffer and etch stop layers. The repair buffer should exhibit high etch selectivity during the absorber etch processes (i.e. pattern transfer and focused ion beam (FIB) repair), be thermally and electrically conductive, optimally thin and have high etch selectivity to the silicon-capping layer over the Mo/Si multi-layer mirror. The absorber materials that have been studied in the past are TaSiN and Cr with SiON as the repair buffer on top of a Cr etch stop layer. The SiON repair buffer is insulating, exhibiting low thermal and electrical conductivity. Also, the required thickness for FEB repair is greater than 750 Angstrom using a standard 30-keV Ga+ FIB tool,(1) while the etch selectivity to the silicon capping layer during pattern transfer is less than five to one necessitating a Cr etch stop. A sputtered carbon repair buffer exhibiting the required qualities has been studied. The carbon film is thermally and electrically conductive and exhibits extremely high reactive ion etch selectivity to the silicon-capping layer. Carbon also has the lowest sputter yield out of all the elements opening a larger FEB repair process window without using gas-assisted etching. A conductive repair buffer also prevents the possibility of static charge buildup on the mask that could damage patterns during an electrostatic discharge.