The multi-step rapid thermal annealing process of Ti/Al/Ni/Au can make good ohmic contacts with both low contact resistance and smooth surface morphology for AlGaN/GaN HEMTs. In this work, the mechanism of the multi-step annealing process is analyzed in detail by specific experimental methods. The experimental results show that annealing temperature and time are very important parameters when optimizing the Ti/Al layer for lower resistance and the Ni/Au layer for smooth surface morphology. It is very important for good ohmic contacts to balance the rate of various reactions by adjusting the annealing temperature and time. We obtained a minimum specific contact resistance of 3.22 10 7 cm 2 on the un-doped AlGaN/GaN structure with an optimized multistep annealing process.

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A two-step exposure method to effectively reduce the proximity effect in fabricating nanometer-spaced nanopillars is presented. In this method, nanopillar patterns on poly-methylmethacrylate (PMMA) were partly cross-linked in the first-step exposure. After development, PMMA between nanopillar patterns was removed, and hence the proximity effect would not take place there in the subsequent exposure. In the second-step exposure, PMMA masks were completely cross-linked to achieve good resistance in inductively coupled plasma etching. Accurate pattern transfer of rows of nanopillars with spacing down to 40 nm was realized on a silicon-on-insulator substrate.

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