Low Stress Tungsten Absorber for X-ray Masks

Abstract

Abstract We have developed a process for consistently depositing, by rf sputtering, low stress ( 7 dynes/cm 2 ) alpha-phase tungsten (μ-W) on the Si and Si 3 N 4 membranes of x-ray masks. Consistency of the μ-phase is ensured by first evaporating a 10nm-thick μ-W seed layer and then sputtering W at a substrate temperature of 200°OC. Low stress is ensured by controlling the Ar pressure to within 10 -4 Torr (0.013Pa). Stress was determined by measuring, in a Linnik interferometer, the membrane deflection at a W step edge. This method can detect stress as low as 5x10 7 dynes/cm 2 . Both the thermal and radiation stability of μ-W patterns on Si membranes were tested. Zero stress (i.e., 7 dynes/cm 2 ) W patterns on Si membranes exhibited no changes after annealing at 410°OC for 24 hours and 200°OC for 50 hours. Also, no change was observed after an x irradiation of 9800 J/cm 2 using a synchrotron. This is equivalent to between 10 5 and 10 6 x-ray lithography exposures. Tungsten etching was studied using various gas combinations and reactive ion etching (RIE) parameters. Using Microposit 1400-25 resist patterns, and a 10:1:5 mixture of CHF 3 /CCl 2 F 2 /O 2 , we obtained reasonable etch rates and vertical sidewalls (slope less than 3 O ). We are developing a system for measuring the resonant frequency of membranes in-situ, during rf sputtering of W. The resonant frequency depends on W stress and hence can be used in a feedback loop to control stress under computer control.