Low Distortion X-Ray Mask With W-Ti Absorber

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Abstract
The performance of W-Ti alloy as an x-ray mask absorber material was investigated in order to obtain a low distortion x-ray mask. The W-Ti films were deposited by sputtering the W-Ti (1wt% Ti content) target using Ar + N2 gas with a DC magnetron sputtering system. It was found that the internal stress and the density in the W-Ti films strongly depend on the gas pressure and N2 content of the Ar + N2 sputtering gas. The internal stress of W-Ti films deposited using Ar + N2 of 30% N2 content decreased with increasing gas pressure from a very large compressive stress, and became small in the region of above 2 Pa. The stress in W-Ti films deposited in this pressure region was very small (0.5-2.0 x 107 N/m2). This value is fully satisfactory for an x-ray mask absorber. The density of the low-stress W-Ti film is also satisfactory for the x-ray mask absorber. The etching properties of W-Ti film were evaluated for CF4 + 02 gas plasma. It was found that the etching rate becomes the maximum (1500 Å/min.) at the 02 content of 15-20%. In order to demonstrate the accuracy of the W-Ti x-ray mask, the full exposure mask (BN/Polyimide membrane) and the stepper mask (SiN/Polyimide membrane) were fabricated. As a result, x-ray masks with very small distortion (less than 0.1pm for 15 x 15mm field ) were obtained. It is concluded that the low distortion W-Ti mask is practical for use in submicron VLSI fabrication.

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