In situ reflectance interferometry (RI) at 400 nm wavelength was used to investigate the effect of the substrate negative bias on the microstructure of alumimium nitride (AIN) films deposited at room temperature on Si substrates by magnetron sputtering. Their surface reflectance recorded during film deposition promptly yields real-time information on the microstructures developed under oxygen contamination and bias change. Specifically, the refractive index n and the extinction coefficient k are deduced from reflectance using appropriate multilayer optical models and validated by spectroscopic ellipsometry. These optical constants correlate appreciably with the microstructure that evolves between columnar-crystallized and purely amorphous phases including in-between amorphous states containing dispersed nano-AIN grains. These microstructures were identified using ex situ energy dispersive X-ray spectroscopy, transmission electron microscopy and diffraction, X-ray diffraction and Auger electron spectroscopy. The simple and cost-effective in situ RI thus appears a powerful tool in controlling the microstructures of thin AIN films for desired applications. (c) 2007 Elsevier B.V. All rights reserved.