Annealing-free copper source-drain electrodes based on copper–calcium diffusion barrier for amorphous silicon thin film transistor

The copper (Cu) source-drain electrodes based on copper–calcium (CuCa) diffusion barrier were fabricated without annealing process and in one wet etching step in order to develop the applications of Cu in hydrogenated amorphous silicon thin film transistor (α-Si:H TFT). The results show that oxygen flux and substrate temperature in depositing CuCa buffer layer affect greatly the adhesion of source-drain electrodes, and a perfect adhesion was obtained by an increasing oxygen flux to 4 sccm or an increasing substrate temperature to 150 °C, despite no annealing process. The specific resistance of source-drain electrodes has a slight increase with the increasing oxygen flux or substrate temperature or CuCa thickness. Auger electron spectroscopy (AES) show that the CuCa alloy barrier layer has perfect anti-diffusion between Cu film and α-Si:H. A much-desired taper angle of 43.4° and a little critical dimension (CD) bias of 0.91 μm for the Cu/CuCa electrodes were obtained in one wet etching step. The α-Si:H TFT with the Cu/CuCa source-drain electrodes demonstrated the field-effect mobility of 0.73 cm²/Vs, the subthreshold slope of 0.73 V/dec, the threshold voltage of 0.45 V, and the I_on/I_off ratio of 10⁶ due to the superior performances of the source-drain electrodes with the desired adhesion, specific resistance and taper angle despite no annealing process.