Copper-airbridged low-noise GaAs PHEMT with Ti/WNx/Ti diffusion barrier for high-frequency applications

Abstract

A GaAs pseudomorphic HEMT (PHEMT) with Cu-metallized interconnects was successfully developed. Sputtered WNx was used as the diffusion barrier and Ti was used as the adhesion layer to improve the adhesion between WNx/Cu interface in the thin-metal structure. After copper metallization, the PHEMTs were passivated with silicon nitride to avoid copper oxidation. The Cu-airbridged PHEMT showed the saturation I-DS was 250 mA/mm and the g(m) was 456 mS/mm. The Ti adhesion layer plays a significant role on the g(m) and V-p uniformity of the Cu-metallized PHEMTs. The GaAs PHEMTs with Ti/WNx/Ti/Cu multilayer have better noise figure and associated gain than those of the devices without the Ti adhesion layer. The fabricated Cu-metallized GaAs PHEMT with Ti/WNx/Ti/Cu multilayer has a noise figure of 0.76 dB and an associated gain of 8.8 dB at 16 GHz. The cutoff frequency (f(T)) is 70 GHz when biased at V-DS = 1.5 V. These results show that the Ti/WNx/Ti multilayer can serve as a good diffusion barrier for Cu metallization process of airbridge interconnects on GaAs low-noise PHEMTs.