GaAsMask™

GaAsMask uses a dark-field, "strong" phase-shift mask design to produce quality sub-wavelength images to make discrete GaAs gates.

This patent-pending method uses phase-shifted, subresolution assist features to shape the aerial image of the discrete diffraction pattern to

• eliminate zero order light, improving depth of focus, and
  
  
• place two non-zero orders of light at the edge of the lens pupil plane, reconstructing the desired feature using two-beam interference.

Since there is no electric field at the center of the pupil, the image is called "strong"” shifted. Previous “weak” shifter technologies do not eliminate the zero order energy. 

The figure below compares simulated exposure latitude dependence on depth of focus for a 240 nm isolated space imaged using a binary mask, a weak phase shift mask, and new GaAsMask technology.* With no assist features, the binary mask's depth of focus is 0.00 and 0.85 µm, with exposure latitude limits of 10% and 5%, respectively. At the same exposure latitude, the weak shifted mask had focus latitudes of 0.88 and 1.15 µm. The GaAsMask, however, achieved 1.18 and 1.38 µm focus latitudes.


* Simulations made with PROLITH's lumped parameter model (FINLE Technology). Parameters were: 0.5 NA, 365 nm wavelength, 0.3 sigma, contrast of 23, film thickness of 700 nm, absorbance of 0.2 per micron, and diffusion length of 10 nm

GaAsMask™ is a collaborative project with

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