Yield, metrology and inspection characteristics of SCALPEL masks

SCALPEL (Scattering with Angular Limitation in Projection Electron Beam Lithography) masks have been fabricated for use in the SCALPEL Proof-of -Concept (SPOC) and the recently built SCALPEL Proof-of -Lithography (SPOL) tool. 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. For thickness ranging from 100-1500 Å, the yield, defined as having all membranes and struts intact and flat (film is tensile), is in excess of 90%. The mask scatterer (W/Cr) film thickness investigated were 500Å/100Å and 250Å/50Å. Mask blanks, coated with 0.32 μm thick ZEP-520 resist, are patterned on a JEOL JBX6000FS e-beam exposure tool operating at 50 KeV. For the 500 Å thick W films, pattern transfer into the scatterer layer is accomplished using a SF₆/O₂ plasma etch process. Metrology on dry etched tungsten features have been obtained using an Hitachi 4160 SEM and indicates that CD linearity can be achieved over a 0.4-4.0 μm range. The SCALPEL tool prints at a 4X reduction, the above ranges corresponds to imaging 0.1-1.0 μm size features on the wafer. Additional mask metrology data has been obtained and a ̃1% difference is measured between equivalent size features present in an isolated and dense array and oriented in the horizontal and vertical direction. W line-edge roughness using the plasma etch process is ̃15 nm. Minimum contact hole geometries of 0.32 μm (0.08 μm at the wafer) and minimum isolated trenches of 0.24 μm (0.06 μm at the wafer) have been fabricated. SCALPEL masks have been inspected using a KLA SEMSpec in a die-to-die mode on features as small as 0.24 μm on the mask. For a patterned mask structure consisting of 500 Å W/100 Å Cr on 1500 Å SiN_x, all defect types could be identified and no modifications to either the mask structure or SEMSpec operating conditions were required.