INFLUENCE OF SPUTTERING PRESSURE ON THE MICROSTRUCTURE EVOLUTION OF AIN THIN-FILMS PREPARED BY REACTIVE SPUTTERING

Aluminum nitride (AlN) films have been deposited on Si (100) wafers by reactive r.f, magnetron sputtering in a mixed Ar-N-2 discharge. It is important to control the preferred orientation and the texture morpho logy of the films with deposition parameters for the application of th e surface acoustic-wave device. The change of preferred orientation an d microstructure with sputtering pressure has been investigated using X-ray diffraction and transmission electron microscopy (TEM). It is fo und that highly c-axis oriented films are deposited at low pressures a nd the films with mixed (100) and (110) planes are deposited at high p ressure. Through the cross-sectional TEM study, better aligned columns with smaller diameter are observed in the AIN films prepared at lower sputtering pressure. It is confirmed, by plan-view TEM micrographs, t hat an AlN film deposited at low pressure is composed of small and rou nded grains, but the film prepared at high pressure is composed of elo ngated and open porous grains. The better microstructure of the AlN fi lm prepared at low pressure must be attributed to the increased adatom mobility on the growing surface owing to the enhanced kinetic energy transfer and the longer mean free path of the gas molecule in the plas ma. From the measurement of internal stress, it is found that the magn itude of compressive stress increases with the decrease of sputtering pressure mainly due to the enhanced kinetic energy transfer from the p lasma to the film surface.