Phase-shift reticles for the fabrication of subhalf-micron gates in GaAs integrated circuits using optical stepper lithography

Phase shifting mask techniques using an i-line wafer stepper (ASML PAS 2500/40) were applied to our recessed gate process for the fabrication of GaAs/AlGaAs high electron mobility transistors (HEMT). Several methods for making reticles with contrast enhanced shifters, such as an extra patterned resist layer on top of the chromium reticle are presented. This paper gives a short review of the different phase shift processes and layer geometries and the advantages and disadvantages depending on application are discussed.