Repairing x-ray masks with Ta absorbers using focused ion beams

Abstract

A focused ion beam system was used to repair x-ray masks with Ta absorbers. To repair opaque defects, excess Ta is removed by ion milling. Since the wall of the milled pattern is tapered compared to the absorber patterns ofthe mask, milling parameters such as the ion dose are justified by printing the repaired pattern on the resist with the SR exposure system. Clear repairs are made with Ta deposited using a organometallic material. Since the Ta content ofthe deposit was about 30%, a Ta deposition layer thicker than I .2-u m is necessary to keep the contrast of the x-rays high. The repaired Ta absorber patterns have high chemical durability and are not damaged by wet cleaning with strong acid. We printed on resists with repaired masks and confirmed that the defects were completely repaired. INTRODUCTIONDefect-free masks are vital to x-ray lithography. We have developed a Ta absorber x-ray mask fabrication process that virtually eliminates maskThe defect density was reduced to less than 5/cm2, making mask repair an effective method for eliminating defects. For 1 -to-I x-ray lithography, masks require absorber patterns smaller than 0.2-ii m and thicker than 0.5-ji m to block the x-rays. Therefore, the repair must be extremely precise for absorber patterns smaller than 0.2-p m. Furthermore, x-ray mask patterns must be repaired by controlling the sidewall angle of the repaired absorber for high aspect ratios of patterns.To repair x-ray mask defects, we used a focused ion beam (FIB) repair system developed for x-ray mask repair.2 The system uses a 30 kV Ga ion beam focused to less than 25 nm and contains a gas delivery system for Ta film deposition. We examined the printability of repaired patterns by printing on the resist using an SR exposure system with a repaired mask. X-RAY MASK CONSTRUCTIONWe have developed a Ta absorber x-ray mask that consists of a 2-p m-thick SiN membrane and a sputtered Ta absorber.3 Ta films with small stress were deposited on the SiN substrate by RF sputtering.4The density of the Ta films was about 16.5 glcm3, which is the same as the density of bulk Ta. The thickness of the Ta absorber varied from 0.3-ji m to 0.7-i m depending on the absorber pattern. Ta absorber patterns were replicated by ECR plasma etching.Usually, to attain high alignment accuracy exposure with the x-ray stepper, the surface of the mask is covered with an anti-reflection coating (ARC) of an ordinary 0.1-i m thick Si02 film.5 Images of the Ta absorber pattern coated with and without ARC films are shown in Fig. 1. The Ga ion image from the pattern surface expands with ARC films. To obtain real images of the Ta absorber patterns for high-172 /SPIE Vol. 2512 O-8194-1870-6/95/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx