Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin Film Transistors

This research utilizes plasma treatments as a method to decrease the contact resistance on zinc oxide ZnO thin film transistor TFT. In recent years, researchers have achieved gigahertz RF switch cutoff frequency with ZnO TFTs. To further increase the cutoff frequency, the total resistance of this device must be minimized. This research developed a method to integrate plasma treatments into the fabrication of ZnO TFT to decrease the contact resistance, thus increasing the cutoff frequency of the device. Three plasma treatment methods were used, including Ar plasma generated in an inductively coupled reactive-ion etch ICP-RIE chamber, Ar plasma applied in-situ in a metal-sputter chamber before sputtering tungsten W contacts, and by remotely generated hydrogen plasma. The plasma treatments were implemented after the deposition of ZnO and prior to the deposition of sputtered tungsten W contacts. The plasma treatments increased the conductivity of the ZnO, which allowed for the formation of ohmic contacts on the ZnO TFT. The contact resistance of the untreated ZnO TFT sample was 1.9 ohm-mm, while the ICP-RIE Ar plasma treatment, in-situ Ar plasma treatment, and H plasma treatment demonstrated minimum contact resistances of 1.2, 1.0, and 1.5 ohm-mm respectively. The in-situ Ar plasma treatment demonstrated the best results, with a decrease in contact resistance of 47 percent.