Novel method to find the best process point in e-beam mask making

To this day there is no established practical method available to identify the “optimal” base dose in e-Beam lithography for mask making. Methods range from choosing the “zero-bias” exposure dose that yields CDmeasured = CDtarget to real isofocal calibrations that require to print the structures at different blurs/focus settings. While the latter yields an optimal base dose, it requires a significant experimental procedure, whereas the former uses a pretty simple experiment but will likely yield a non-optimal base dose due to process effects (e.g., lateral resist development). Here we present a novel method to find the optimum base dose for e-Beam mask exposures that requires only a simple experiment. Surprisingly, the exposure doses required to print features with different densities at the same CD does depend on the process point, adding an error term to the proximity effect correction in case the correction is not aligned to this. This is due to the fact that for a given stack and acceleration voltage, a PEC method will deliver one fixed dose range, whereas the experimentally required dose range depends on the process point chosen. This observation can be used to calibrate the base dose such that it matches the dose range obtained from PEC. Moreover, it can be used to add a new criterion to proximity effect correction – the iso-focal condition - to not only correct for line width (also called critical dimension, CD) linearity and density dependent effect, but to also add immunity to process variations such as focus and blur variations.