EFFECT OF MASK ABSORBER THICKNESS ON X-RAY-EXPOSURE LATITUDE AND RESOLUTION

Reducing the absorber thickness on an x-ray mask has significant advantages in mask fabrication. Masks become easier to pattern and repair, and pattern placement errors caused by absorber stress are reduced. However, thinner absorbers can also affect the x-ray wafer printing process. As the absorber thickness is reduced, more radiation leaks through 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 wafer. The principle is similar to the attenuated phase-shift masks being 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 the IBM advanced lithography facility with a Suss stepper and a commercially available chemically amplified positive resist. Preliminary results were also obtained with a negative resist. The patterns were inspected for resolution, exposure latitude, and pattern fidelity. Results show a small improvement in printing performance as the absorber becomes thinner.