Characterization of Oxide Thin Films Deposited by an Atmospheric Pressure Plasma Jet

Deposition zinc oxide and silica-like thin films by the atmospheric pressure plasma jet were studied. To understand the plasma system property and application fields, we first investigate into the plasma optical emission spectra and gas temperature by changing the manipulate variable such as applied voltage, gas type and flowrate. Our research has demonstrated that higher applied voltage and lower gas flowrate will decrease the plasma gas temperature because of the different energy density. But the higher flowrate cause the plasma gas have more excited molecules. By the simulation of the plasma downstream intensity distribution, temperature trend and NO concentration, we found that the turbulence type matchs the downstream flowfield well and it illustrates the non-negligible of outside ambient gas. Futher, our study shows the plasma gas temperature and substrate temperature by the method of simulation molecules emission and thermalcouple measurement. The downstream gas temperature are between 950 K to 1200 K, which makes the plasma system belonging to high temperature plasma type.
To deposit zinc oxide thin films, we use the 1.7 MHz nebulizer and nitrogen carrier gas to spray the precursor solution into the plasma system. The different precursor effect, applied voltage and gas flowrate effect were studied in this research. First, we found the only one precursor, zinc chloride, can deposit the less roughness, smooth and high crystalline thin films. Otherwise the particle-like structure appears. Next, to find the optimized condition of zinc chloride as precursor deposits zinc oxide thin films, the gas flowrate and applied voltage were setting as manipulate variable. In our study, we found that under the applied voltage 275 V and gas flow rate 30 slm, the deposition has best film property with resistivity of 1.4 ohm-cm and transimmion near 90 %. Otherwise the deposition will appear the sheet-like structure and has poor optical and electric property. By the XRD identified, we found the sheet-like structure is the ZHC (Zn5(OH)8Cl2‧H2O), which is the intermedia of the reaction. The appearance of ZHC can be explained by the non-enough power and shorter residence times in those condition.
In next research topic, the silica-like thin films deposited by the APPJ were studied. In this system, we use the bubbler method with nitrogen as the carrier gas to send the saturated vapor pressure to the plasma system. Three different precursors were tested, which are tetraethylorthosilicate (TEOS), Hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDSN). Each precursor has their optimized deposited condition and all deposition show the inorganic-like silica structure. In the TEOS system, we found the deposition rate was much lower than the others but the film has the advantage of smoothness and well-coverage. In the HMDSN system, the deposition rate were rapid but the films have worst leakage current density. It is in regard to its complicated bonding and structure. On the contrary, the HMDSO system has advantage of higher deposition rate, less-roughness and lower leakage current. In this research, we use films deposited from HMDSO as the dielectric layer in the thin films transistor (TFT) and it can work quite well. This TFT result shows the potential and capable of this atmospheric pressure deposited films as the device’s dielectric layer.