Stoichiometric (ZrO2)(x)-(Al2O3)(1-x) composite thin films are obtained by using rf unbalanced magnetron sputtering in an atmosphere of argon and oxygen. All the deposited composite thin films are amorphous. The ZrO2-Al2O3 composite thin films will be nearly completely oxidized when the O-2/Ar flow rate ratio exceeds 0.5. The optical constants of the thin films are calculated and measured. The measured values agree well with the ones calculated by applying the Hunderi effective medium model. The optical constants can be tuned by controlling the volume fraction of the ZrO2 and Al2O3 films. The optical requirements for a high-transmission attenuated phase-shifting mask (HT-APSM) blank can be satisfied by a suitable choice of volume fraction. An optimized ArF lithography transmittance value of 18%-20%, which conforms to the typical value for an HT-APSM blank, can be realized for a ZrO2 volume fraction in the range of 54.8% to 48.8%. To be used as a HT-APSM blank, one pi-phase-shifted ZrO2-Al2O3 composite thin film with optimized transmittance is fabricated and is shown to satisfy the optical requirements of an HT-APSM blank. (C) 2004 American Vacuum Society.