Ma. Mccord et al., EFFECT OF MASK ABSORBER THICKNESS ON X-RAY-EXPOSURE LATITUDE AND RESOLUTION, Journal of vacuum science & technology. B, Microelectronics and nanometer structures processing, measurement and phenomena, 11(6), 1993, pp. 2881-2887
Reducing the absorber thickness on an x-ray mask has significant advan
tages in mask fabrication. Masks become easier to pattern and repair,
and pattern placement errors caused by absorber stress are reduced. Ho
wever, thinner absorbers can also affect the x-ray wafer printing proc
ess. As the absorber thickness is reduced, more radiation leaks throug
h nominally opaque areas on the mask. This radiation undergoes a phase
shift as it passes through the absorber which can result in improved
resolution due to partial cancellation of diffraction effects on the w
afer. The principle is similar to the attenuated phase-shift masks bei
ng studied for optical lithography. A mask was fabricated with a range
of absorber thickness from 0.34 to 0.59 mum. The varying thicknesses
were placed in close proximity on the mask to minimize any exposure or
process variations that might occur across the mask and wafer. Prints
were made on silicon wafers using a synchrotron radiation source at t
he IBM advanced lithography facility with a Suss stepper and a commerc
ially available chemically amplified positive resist. Preliminary resu
lts were also obtained with a negative resist. The patterns were inspe
cted for resolution, exposure latitude, and pattern fidelity. Results
show a small improvement in printing performance as the absorber becom
es thinner.