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Proceedings Paper

Applications of holographic gratings in x-ray mask metrology
Author(s): Matthew E. Hansen
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Paper Abstract

X-ray lithography is a proximity printing process. After a resist-coated wafer is positioned 10-40 m behind the x-ray mask, the mask and wafer are aligned and secured for the 'x-ray exposure. Securing the mask may create distortions in the membrane. The lx nature of proximity printing demands that these distortions be tightly controlled. At O.25,tm critical dimension, the distortion budget allocated to the mask is 25 nm. This corresponds to 0.7 ppm over a field of 1" . It is thus necessary to develop a metrology tool capable of accurately and rapidly measuring distortions induced in the mask. Interferometry, because of its non-contact nature and high sensitivity, is ideally suited to the task. The construction of an x-ray mask begins with the deposition of a 1-2 m film onto a silicon wafer substrate.' After carrier deposition, the mask wafer is bonded to a glass or silicon mounting ring. The mounting ring gives the mask rigidity and provides a mechanism for mounting the mask during x-ray exposure. Mask distortions may occur in the bonding process due to non-fiat wafers or bonding ring irregularities. The central region of mask blank wafer is then back-etched with KOH to create an x-ray transparent membrane. The redistribution of stress in the carrier during the membrane construction is a second source of distortion in the mask-making process. The mask blank is subsequently patterned and metallized. Localized stresses occurring at the absorber-membrane boundaries can lead to additional mask distortions. Several interferometric techniques can be employed to characterize the sources of the above-mentioned x-ray mask distortions. Out-of-plane distortions (OPD) of x-ray masks have been measured with the Michelson interferometer. In-plane distortions (IPD) in the mask have been characterized using Moire interferometry. In Moire, diffraction gratings printed on the mask pre-exposure are examined post-exposure in a virtual grating. Both OPD and IPD have been observed in-situ using shear interferometry.

Paper Details

Date Published: 1 March 1991
PDF: 2 pages
Proc. SPIE 1396, Applications of Optical Engineering: Proceedings of OE/Midwest '90, (1 March 1991); doi: 10.1117/12.47750
Show Author Affiliations
Matthew E. Hansen, Univ. of Wisconsin/Madison (United States)


Published in SPIE Proceedings Vol. 1396:
Applications of Optical Engineering: Proceedings of OE/Midwest '90

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