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Abstract: Amorphous (Al2O3) x –(TiO2)1−x composite films are prepared using r.f. unbalanced magnetron sputtering in an atmosphere of argon and oxygen at room temperature. The optical constants of (Al2O3) x –(TiO2)1−x composite films are linearly dependent on the Al2O3 mole fraction in the Al2O3–TiO2 composite film. The optical constants of these Al2O3–TiO2 composite films can be made to meet the optical requirements for a high transmittance attenuated phase shift mask (HT-APSM) blank by tuning the Al2O3 mole fraction. The Al2O3 mole fraction range that would allow the films to meet the optical requirements of an HT-APSM blank for ArF immersion lithography is calculated to be between 76% and 84%. One π-phase-shifted Al2O3–TiO2 composite thin film to be used as an HT-APSM blank for ArF immersion lithography is fabricated and is shown to satisfy the optical requirements. [Copyright &y& Elsevier]