Copper-Airbridged Low-Noise GaAs PHEMT With Diffusion Barrier for High-Frequency Applications

A GaAs pseudomorphic HEMT (PHEMT) with Cu-metallized interconnects was successfully developed. Sputtered WN_x was used as the diffusion barrier and Ti was used as the adhesion layer to improve the adhesion between WN_x/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/WN_x/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/WN _x/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=1.5V. These results show that the Ti/WN_x/Ti multilayer can serve as a good diffusion barrier for Cu metallization process of airbridge interconnects on GaAs lownoise PHEMTs