Al2O3/Cr2O3 superlattices with dense structures are successfully deposited on UV grade fused silica substrates and Si wafers by r.f. reactive unbalanced magnetron sputtering in a mixture of argon and oxygen gases at 150 degreesC. The dependence of the optical constant, plasma induced spectral intensity and deposition rate on the deposition parameters such as oxygen flow rate and r.f. sputtering power are examined. It was found that when O-2 gas flow rate increases, the deposition rate and absorption of Al2O3 and Cr2O3 thin films decrease. The O/Cr ratio of the CrOx thin films identified by X-ray photoelectron spectroscopy (XPS) ranges from 1.5 to 3.0 when the sputtering power of the Cr target is increased from 20 to 80 W. The optical constants of Al2O3/Cr2O3 superlattices and Al2O3 and Cr2O3 thin films are determined from the measured transmittance and reflectance by employing the reflection-transmittance (R-T) method. As for an attenuated phase-shifting mask (APSM) blank, it is found that when the thickness percentages of Al2O3 in Al2O3/Cr2O3 superlattices are set between 60% and 70% the optical constants of superlattices can be tuned within the required R-T domain. Chemical and adhesion requirements of Al2O3/Cr2O3 superlattices for APSM applications are also met.