Influence of sputtering pressure on the microstructure evolution of AlN thin films prepared by reactive sputtering

Abstract

Aluminum nitride (AlN) films have been deposited on Si (100) wafers by reactive r.f. magnetron sputtering in a mixed Ar-N2 discharge. It is important to control the preferred orientation and the texture morphology of the films with deposition parameters for the application of the surface acoustic-wave device. The change of preferred orientation and microstructure with sputtering pressure has been investigated using X-ray diffraction and transmission electron microscopy (TEM). It is found that highly c-axis oriented films are deposited at low pressures and the films with mixed (100) and (110) planes are deposited at high pressure. Through the cross-sectional TEM study, better aligned columns with smaller diameter are observed in the AlN films prepared at lower sputtering pressure. It is confirmed, by plan-view TEM micrographs, that an AlN film deposited at low pressure is composed of small and rounded grains, but the film prepared at high pressure is composed of elongated and open porous grains. The better microstructure of the AlN film 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 plasma. From the measurement of internal stress, it is found that the magnitude of compressive stress increases with the decrease of sputtering pressure mainly due to the enhanced kinetic energy transfer from the plasma to the film surface.