New concept phase shifting masks based on projection lens design

Abstract

2Absrrac-For the purpose of shortening turn around time (TAT) and costs of Phase-Shifting Mask (PSM) fabrication, we proposed a new concept of PSM composed of an opaque pattern substrate and a phase shifter substrate. It was considered that this PSM has the disadvantage of high wave-front aberration. In order to solve this problem, we have designed a high NA (0.75), very low aberration (0.0017 I ) reduction projection lens system.Based on this design, we found that wave-front aberration of phase shirter substrate (0.09 inches thickness) between the mask and projection lens was reduced from 033141 to O.DO891by projection lens system optimization of mask and imaging positions. We measured the intensity profiles of this prototype PSM using AMIS (Zeiss optical simulation microscope) and checked the intensity profies of this PSM were better than that of the conventional PSM. I IntroductionOpticaI exposure method using ultraviolet light is main way of LSI fabrication. This method has a resolution limitation fixed by the wavelength of exposure light and NA of the projection lens. To improve the above limitation, several types of PSM exposure methods have been proposed [l], [2], [3]. Some of them had been succeeded to practical use.Recently, shortening TAT and lowering the costs of the PSM fabrication for sub-micron LSI became a more significant task. For this purpose we have proposed a two-layer PSM composed of an opaque pattern substrate and a phase shifter substmte [4], [SI. In a previous study, we have reported a quarter micron pattern obtained by using a two-layer PSM and an i-line, low NA (0.43) stepper [6j. Two-layer PSM is able to compensate the printed CD (critical dimension) imbalance between shifter and non-shifter region, because of shielding the passing light at phase shifter edge [7], [8].In fig. 1 light intensity simulation data of conventional and two-layer alternating PSM are presented. Because the opaque pattern and the phase shifler pattern are formed on the separate substrates, this PSM can be easily fabricated. (a) ~mwntarsl alterst rg E M (b) hwleyerdtemiw E M Llgm 0 8 (AU) 02 Int-ty 06 0 4 0 -0 4 02 0 02 0 4 ++ -0 2 0 02 0 4 lmtlm (!Ami P h a l (lid 1-m n 6 3 Md5. DISumli-on& Fig. 1 and two layer PSM Light intensity profiles of conventional PSM In fig. 2 it is shown a comparison of fabrication time chart between conventional PSM and two-layer PSM. Costs of PSM are depend on TAT and fabrication process yield. Two-layer PSM had TAT and costs ments of PSM fabrication, because opaque pattern and phase shifter pattern were formed independently. TAT of two-layer PSM was less than 3 days. On the other hand, average TAT of conventional PSM was 7 days. T A d s f d -W A U -b n mu1 --------.-, it TI I Fig.2 Time chart of conventional and Two layer PSM fabricationHowever, generally it was considered two-layer PSM had high aberrations of shifter glass plate between opaque mask pattern and projection lens. The projection lens system for LSI fabrication has a very high precision and complexity, and is treated as ...