Fabrication and characterization of aluminum oxide/chromium oxide superlattice for attenuated phase-shifting mask working at 193 nm wavelength

Abstract

Al 2O 3/Cr 2O 3 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 °C. 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 Al 2O 3 and Cr 2O 3 thin films decrease. The O/Cr ratio of the CrO x 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 Al 2O 3/Cr 2O 3 superlattices and Al 2O 3 and Cr 2O 3 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 Al 2O 3 in Al 2O 3/Cr 2O 3 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 Al 2O 3/Cr 2O 3 superlattices for APSM applications are also met.