A method to characterize pattern density effects: chemical flare and develop loading

Many recent publications have highlighted pattern density effects as a problem in both electron-beam and optical lithography. These effects are manifested as a systematic variation in critical dimension as a function of position on the wafer. It is becoming an increasing problem as the pattern density and diminishing critical dimensions are needed for production nodes 32nm and beyond. One potential source of pattern density effects is acid volatility, where acid is presumed to redeposit during exposure or bake; here we refer to this effect as chemical flare. Another source of density effects is develop loading which refers to the impact of local depletion of developer in highly exposed regions. Both develop loading and chemical flare can cause deviations in feature size that may be difficult to correct for by adjustment of the exposure process. Here we describe a method that allows the detrimental effects of chemical flare and develop loading to be separately characterized. The method makes use of arrays of 248 nm exposure sites and a controlled develop process within a custom liquid flowcell; this combination enables a systematic study of these effects.