Aluminum nitride (ALN) thin films have been deposited on silicon and glass substrates by reactive RF magnetron sputtering at various deposition conditions. In order to investigate the growth mechanism of ALN thin films, the preferred orientation and texture morphologies have been characterized by changing deposition parameters such as sputtering pressure, nitrogen concentration, negative bias voltage, and hydrogen concentration.
The effects of sputtering pressure on the ALN films are summerized as follows. As the sputtering pressure decreases, the films show higher (0002) preferred orientation, smoother surface, and smaller grain size. The above results are attributed to the the enhancement of adatom mobility on the film surface due to the increased kinetic energy transfer from plasma, as a result of the increased mean free path with less gas scattering. The increase of compressive stress with decreasing sputtering pressure is regarded as the evidence of the increase of kinetic energy transfer from plasma to the film surface. Dense ALN film (low void fraction) with well developed columnar structure have been directly verified by the transmission electron microscopy analysis.
Nitrogen concentration is one of the most important deposition parameters for ALN film fabrication. As the nitrogen concentration increases, the c-axis orientation of ALN films changes from a parallel to a normal orientation with respect to substrate surface. At high nitrogen concentration, ALN films have shown smaller grain size, smaller column diameter and higher compressive stress. According to the plasma analysis, $N_2$ and $N_2^{+}$ ions play the important role with increasing the nitrogen concentration. Since the flux of bombarding particles ($N_2^{+}$ and $N_2^{+}$ ions) increases with the nitrogen concentration, the adatom mobility on the film surface is augmented.
When the negative bias voltage has been applied to the substrate holder, the c-axis orientation of the film shows the maximum behavior with respect to the magnitude of negative bias voltage. As the negative bias voltage increases, the grain size increases due to the increase of column diameter. These results can be understood considering that the kinetic energy and the flux of $N_2^{+}$ ions from the plasma to the film surface increases with the magnitude of negative bias voltage. According to the composition analysis of the films, the nitrogen concentration increases and the oxygen content decreases with the negative bias voltage. Above results reveals that negative bias voltage affects the kinetic energy of $N_2^{+}$ ions in the plasma.
Though a little hydrogen gas has been added to the sputtering gas mixture, the c-axis orientation changes dramatically from a normal to a parallel orientation with respect to film surface. In addition, the deposition rate decreases remarkably and the grain morphology is altered from round to needle shape. It has been found, with the microstructural study, that the film growth of c-axis orientation is proceeded through ledge mechanism and the secondary nucleation occurs at film surface of primary grain. From the supplementary experiments, it can be concluded that the hydrogen gas disturbs the ALN synthesis and etches the nitrogen atom selectively on the film surface.
A simple SAW delay line has been fabricated with 3.5㎛-thick ALN film on $SiO_2/Si$(100) wafer. The IDT electrodes of aluminum film have been formed on the ALN film, using chemical etching. From the measurement of frequency response with spectrum and network analyzer, the center frequency was about 32 MHz but the quality of signal response is not so good due to the relative thin ALN film.
