| Summary: | 碩士 === 國立交通大學 === 材料科學與工程系所 === 92 === In this work, a new multilayer Ti/WNx/Ti/Cu thin metal system was used for the study of copper-airbridged GaAs devices. The attempt is to find a reliable diffusion barrier structure with good adhesion for copper metallization of GaAs devices in order to reduce the production cost of the GaAs devices and to provide better thermal and electrical performances.
For airbridge fabrication, tungsten nitride was chosen as the diffusion barrier for copper-airbridged pseudomorphic high electron mobility transistor (PHEMT) due to its compatibility with the conventional airbridge process. In Ti/WNx/Ti/Cu structure, sputtered WNx was used as the diffusion barrier and Ti was used as the adhesion layer to avoid the peeling problem between Au/WNx and Cu/WNx. And silicon nitride film was deposited on the copper airbridges to prevent copper surface from oxidation. Then, the samples were annealed at high temperature to investigate the thermal stability of the copper metallized devices. We also developed the optimal selective etching processes for etching the thin metal system of Ti/WNx/Ti/Cu multilayer scheme in the copper airbridge fabrication.
A gallium arsenate (GaAs) PHEMT with copper airbridge using of Ti/WNx/Ti/Cu multilayer as the thin metal system was successfully developed and the electrical performance of these devices were better than those only using WNx/Cu scheme and are comparable to those with the conventional gold airbridges.
When tested at 17 GHz, the fabricated low noise GaAs PHEMT had the lowest noise figure of 0.96 dB and the associated gain was 10.68dB under Vds=1.5 V and Vg = -0.5 V. After thermal annealing at 2000C for 3 hours there’s no atomic interdiffusion between copper and underlying contact metals and the devices showed little change in device performance. The equivalent circuit modeling shows the Ti adhesion layer decreases the source resistance of the devices.
And the most benefit is that the Ti/WNx/Ti/Cu scheme improves the yield of the devices because the Ti layer provides good adhesion and solves the copper airbridge peeling problem in GaAs PHEMTs.
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