The mesa region was defined on the glass substrate using a standard P505-15 chemical structure photolithography technique. The ZnO target (purity = 99.99%, radio-frequency (RF) power = 100 W) and the Al target

(purity = 99.99%, RF power = 15 W) were used as the material source for sputtering the 50-nm-thick Al-doped ZnO (ZnO:Al) film on glass substrates GF120918 as the n-ZnO channel layer of ZnO MOSFETs. The n-ZnO channel layer was deposited using a radio-frequency magnetron co-sputter system under a working pressure of 30 mTorr and an Ar flow rate of 30 sccm. Using the Hall measurement at room temperature, the associated electron concentration and electron mobility of the n-ZnO channel layer were 3.5 × 1017 cm−3 and 9.7 cm2/V s, respectively. The mesa region was then formed using a lift-off process. After the source and drain regions were patterned using a standard photolithography technique, a 20-nm-thick n+-ZnO ohmic enhancement layer was deposited using ZnO target (purity = 99.99%, GDC-0449 datasheet RF power = 100 W) and Al target (purity = 99.99%, RF power = 30 W) in the RF magnetron co-sputter system under a working pressure of 30 mTorr and an Ar flow rate of 30 sccm. The associated electron concentration and the electron mobility of the n+-ZnO ohmic enhancement layer were 4.1 × 1019 cm−3 and 3.6 cm2/V s, respectively.

Ti/Al (20/100 nm) ohmic metals were then evaporated on the n+-ZnO region using an electron beam evaporator. Except for the source and drain regions, the excess n+-ZnO region and Ti/Al metal layers were removed using a lift-off process. To form ohmic contact, the sample was annealed in an N2 ambient at 200°C for 3 min. Figure 2 illustrates the fabrication process of the multiple-gate structure in this work. To avoid the source and drain regions being covered by the consecutively deposited

SiO2 gate insulator, a positive photoresist (AZ6112) Ibrutinib cost layer was patterned on the source and drain regions using a self-aligned technique. In the self-aligned technique, the sample was exposed from the backside illumination by using the mask of the source and drain metal electrodes. After a development process, only the photoresist layer residing on the source and drain electrodes was remained as shown in Figure 2b. A 50-nm-thick SiO2 gate insulator layer was then deposited using the RF magnetron sputter system under a working pressure of 10 mTorr and an Ar flow rate of 30 sccm as shown in Figure 2c. To prevent the source and drain electrodes from contacting with the subsequently deposited Al metal strips, before the process of the laser interference photolithography and the deposition of Al metal strips, the photoresist layer and the deposited SiO2 insulator layer residing on the source and drain electrodes were not removed instantly. After the deposition of the 50-nm-thick SiO2 insulator layer, the periodic strips of the multiple-gate structure were patterned using the laser interference photolithography technique.