Growth of "haze" defects on photomasks exposed in ArF lithography is recognized as a serious problem. Haze defects
that have grown to detectable sizes can be analysed in situ by techniques such as EDX or Raman, but to analyze at the
photomask manufacturing stage requires extraction of residues by solution in DI water. The effect of extraction
conditions, including surface area and material, water volume, time, and temperature, has been studied. A standard
method to compare residual ion levels is proposed.
Various methods for reducing residual ion levels from the photomask cleaning process have been published. These
include SPM reduction, oxygen plasma, SC1 dilution, Megasonic agitation, hot rinse, UV exposure, thermal bake, ozone
water, ozone gas, and hydrogenated water. Critical parameters for the cleaning process, besides residual ion levels and
contamination removal efficiency, include CD shift, AR/chrome damage, scatter bar damage, and on phase shift masks,
the change in phase and transmission.
An optimized process combining conventional and novel techniques is described. Data is presented to show the
importance of controlling all resist strip and clean processes, not just the final clean. It has achieved sulphate levels of
0.2ng/cm2 (well below the critical level for haze growth), as well as improved results for the other critical parameters.
This process has been demonstrated to allow ArF exposure of large numbers of wafers without the appearance of haze