Properties of aluminum-doped zinc oxide films deposited by high rate mid-frequency reactive magnetron sputtering

Aluminum-doped zinc oxide films are promising candidates for economic trans parent conducting oxide (TCO) applications. To reach high deposition rates (about 7 nm/s at 4.5 W/cm(2)) in combination with optimum TCO properties by using the reactive mid-frequency sputtering technique, the process window must be precisely controlled, especially for unheated substrates. To overco me the typical hysteresis problem, the process was stabilized by plasma imp edance control for ease of use: this enabled the stabilization of the depos ition process at any setpoint on the S-curve of the corresponding hysteresi s loop. Hence in contrast to former work [B. Szyszka, Thin Solid Films 351, 164 (1999)], wherein a flow control at fixed power restricted the stabiliz ation either to the metallic mode or to the oxide mode, respectively, thr s amples were prepared at various deposition parameters (setpoint, pressure, and temperature) also within the transition made. It turns out that for cer tain process parameters optimum TCO properties were achieved only within th is transition mode, which was not accessible without control. Electrical me asurements indicated that the optimum resistivity of ZnO:Al films deposited on unheated substrates was about 2.5 times higher than at a substrate temp erature of 200 degreesC. In the latter case, an improved value of 290 mu Om ega cm at 1.2 mTorr total pressure and of 250 mu Omega cm at 13.2 mTorr, re spectively, could be reached. Due to band-gap widening, which follows the B urstein-Moss theory, the optimum films showed a neutral color. For those sa mples, the ellipsometric spectra could be well modeled without using interf ace layers, indicating the dense structure of the films. The process state on the stabilized S-curves was characterized by partial pressure measuremen ts and optical emission spectroscopy (OES). These investigations show that it is also possible to stabilize the process at fixed flow with a modified OES control using the process power as the control variable, and using the ratio of an oxygen and a metal emission Line as the controlled variable. (C ) 2001 American Vacuum Society.