Aluminum nitride (AlN) thin films were deposited by a helicon plasma sputtering system with a radical cell. We investigated the effects of eight sputtering control factors on the crystal orientation of the films by design of experiments and the analysis of variance (ANOVA) in order to prepare highly oriented AlN thin films on silica glass substrates. Consequently, it was proved statistically that the distance between a target and a substrate, the sputtering pressure and the substrate temperature are significant control factors for the crystal orientation of the films. Especially, the distance is the most important factor of the eight central factors, which has not been reported so far. On the other hand, the effects of the cathode r.f. coil power, the radical cell power, the nitrogen concentration, the sputtering time and the cathode power are not statistically significant. Moreover, a detailed investigation of the dependence of the orientation on the three important control factors was carried out to optimize the sputtering conditions. The full width at half-maximum (FWHM) of the X-ray rocking curve of the film deposited under the optimized sputtering conditions is 2.4 degrees (sigma = 1.3 degrees). This orientation is the highest in the ALN thin films deposited on amorphous substrates reported to our knowledge.