The most important factor in extreme ultra violet (EUV) mask process is thickness variations which
caused by resist dark loss, absorber etching and capping layer durability of cleaning chemical at each
layer. For example if multilayer (M/L) is damaged due to 2.5nm capping layer loss after cleaning, it means it is impossible to get sufficient reflectance to make proper EUV mask.
Process control of line width and etch depth on the photomask production is more important as the industry moves
toward 50nm node and beyond. In this paper, we report the ellipsometer-based scatterometry based metrology system
that provides line width and resist thickness measurements on sub 50 nm node test masks for a mask process monitoring.
Measurements were made with spectroscopic rotating compensator ellipsometer system. For analysis we made up
modeling libraries with a 200 nm half pitch and checked and applied them to ADI and ACI measurements of binary and
phase shift mask (PSM). We characterized the CD uniformity, linearity, thickness uniformity. Results show that linearity
measured from fixed-pitch, varying line/space ratio targets show good correlation to top-down CD-SEM with R2 of more
than 0.99. Resist thickness results show that depth bias is about 2nm between AFM and OCD in ADI step. The data
show that optical CD measurements provide a nondestructive way to monitor mask processes with relatively little time
loss from measurement step.
Spectrophotometry has been applied to the characterization of pattered mask line-width. Variations in the line-width by few nanometers can be distinguished by comparing spectrum profiles of reflectance or transmittance in spectrophotometry. It can be theoretically explained that the variations in the spectrum profiles are caused by CD bias of the patterned film. Experimental results also show that the positions of the spectrums along wavelength axis are related to the CD bias measured under CD-SEM. As a result, both spectra could be used to estimate quickly the line-width of patterned mask without in-depth analysis.