Aluminum nitride (AlN) films have potential applications in high frequency surface acoustic wave (SAW) devices, because of their high SAW velocity and substantial electromechanical coupling coefficient. For the application of AlN film to SAW devices it is essential of the film to be highly oriented preferentially in c-axis in order to obtain a high piezoelectric activity. The effects of deposition conditions and substrates on the degree of c-axis orientation, surface roughness and SAW characteristics of the AlN films deposited by ECR PECVD were investigated.
The effects of deposition conditions on the degree of c-axis orientation of the AlN films deposited on HF-treated Si(111) substrate using ECR PECVD were investigated. The crystallinity for the film deposited with a longer plasma path is greatly superior to the film deposited with a shorter plasma path due to an enhancement of the dissociation efficiency of the precursors. The degree of c-axis orientation of the films deposited using $H_2$ as a scavenging gas is superior to the film deposited with Ar, due to the reduced impurity contents in the film and removal of surface oxide layer. The degree of c-axis orientation of the AlN films improve as the substrate temperature increases: σ of the (0002) peak decreases from 7.8℃ to 4.3℃ as the temperature increases from 300℃ to 500℃. With increasing microwave power, the (0002) peak intensity becomes greater and the degree of c-axis orientation improves: σ decreases from 5.0° to 4.3° as the microwave power increases from 250W to 450W. OES analysis shows that the nitrogen and hydrogen radicals are more highly activated at the higher microwave power.
The selection of the substrate is very important for the epitaxial growth of AlN film, because the matching in the lattice parameter and the crystal structure between the film and the substrate significantly affects the growth habit of the film. The degree of c-axis orientation of the AlN films deposited at 500℃ on various substrates were investigated. The degree of c-axis orientation depends sensitively on the substrate: σ of (0002) peaks are 4.9°, 4.8°, 4.3°, 4.0° and 1.5° on $SiO_2$(100nm)/Si, $Si_3N_4$(200nm)/Si, Si(100), Si(111) and $α-Al_2O_3$(0001) substrates, respectively. The pole figure taken from the AlN film deposited on the $α-Al_2O_3$(0001) substrate exhibites six pole contours spaced by 60°, indicating that the film is grown epitaxially. High-resolution TEM analysis shows that the amorphous interfacial silicon oxide layer on Si substrate has an important effect on the degree of c-axis orientation and in-plane alignment of the AlN film. Therefore, minimization of the interfacial silicon oxide growth or complete removal of the oxide layer prior to AlN film deposition without damaging the Si surface is required for the growth of AlN epitaxial growth. The values of root- mean-squared roughness were 1.9nm, 1.8nm, 1.5nm, 1.4nm and 1.1nm for the films grown on $SiO_2$/Si, $Si_3N_4$/Si, Si(100), Si(111) and $α-Al_2O_3$(0001) substrates, respectively. The AlN films having better epitaxial properties show smoother surface morphology.
SAW delay line is fabricated to investigate the SAW characteristics of the AlN films deposited on $SiO_2$/Si, $Si_3N_4$/Si, Si(100) and Si(111) substrates. Input and output IDT have 20 electrode pairs. The wavelength of IDT is 12㎛ and the aperture size of IDT is 1320㎛. The phase velocity, coupling factor and propagation loss were 3120m/s, 0.15-0.49% and 0.31-0.56dB/λ. The measured phase velocity (3120m/s) is much lower than the simulated phase velocity (4800m/s). A lower stiffness factor resulting from a less rigid atomic Al-N bonding due to the presence of oxygen impurities may contribute to the low phase velocity. The lower coupling factors may result from the lower c-axis orientation.
