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Example of difference in dose vs. actual dimensions for HSQ as a function of beam current and pitch.
Process conditions:
60nm drawn CD on 3um pitch, 90nm HSQ thickness
100kV, 100pA, 2nm pixel pitch, 60um aperture
100kV, 10nA, 16nm pixel pitch, 100um aperture
Result:
In this case, the higher current (10 nA) and larger pitch (16 nm) requires a higher dose to print the target dimension of 60 nm. The first reason for this is likely the shot pitch fill. At 2 nm pitch, the 60 nm square is evenly divisible into 30 x 30 shot pitches, and so the square is uniformly filled by beam shots. With 16 nm pitch, the 60 nm square can only be filled by 3 x 3 shot pitches which makes a square of 48 nm, which is 12 nm smaller. Therefore, this "48 nm" square needs to be exposed at a higher dose in order to become 60 nm. The second reason for the difference between 2 nA and 10 nA may be that the 10 nA beam may have a less dense or slightly less Gaussian spot size shape and so the beam spot density is lower.
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