An extremely fine pattern of a solid element such as large-scale semiconductor integrated circuit or the like is formed by using chiefly a reduction projection exposure method that is one of optical lithography methods. This method is a method of reducing and transferring a mask pattern formed by a photomask or a reticle (called a mask hereinafter), onto a substrate by use of an imaging optical system.
Improvement of resolution in the reduction projection exposure method is advanced by high numerical aperture in the imaging optical system and short wavelength of exposed light. However, since there are needs of more extreme fineness for least process size of the solid element than the above-mentioned improvement, a deformed illumination exposure method or a phase shift mask exposure method, so-called a super resolution exposure method is developed and applied.
The phase shift mask exposure method includes, for example, a Levenson type phase shift mask, a halftone type phase shift mask, an auxiliary pattern arrangement type phase shift mask, and the like. The Levenson type phase shift mask is a mask generating a phase difference of 180 degrees between light beams that permeate regions between adjacent apertures (light permeating region) on the mask. The Levenson type phase shift mask also has an effect on further improvement of the resolution thereof within regions in which pattern-arranged pitches are extremely fine. For example, if a Levenson type phase shift mask is used, a reduction projection exposure method using KrF excimer laser light can eminently improve resolving characteristics even within a size region less than a least process size whose sufficient resolution is difficult to obtain in the case of use of normal masks. Further, the halftone type phase shift mask is a mask in which a halftone film is formed on a mask substrate instead of a light shield film. The halftone film has functions of making exposed light beams be permeated some per cents and of generating a phase difference of 180 degrees between exposed light beams permeating the halftone film and permeating apertures around which the halftone film is removed.
And, the auxiliary pattern arrangement type phase shift mask is a mask having such a size as not to resolve on a semiconductor wafer around a main aperture and arranging auxiliary patterns for generating a phase difference of 180 degrees between exposed light beams permeating the main aperture. The auxiliary pattern arrangement type phase shift mask can be used when mask patterns are not arranged densely. For example, in a mask pattern for transferring isolated hole patterns, there is a structure of arranging auxiliary patterns which have such a size as not to be transferred on the semiconductor wafer of a plane surface containing upper, lower, right and left side of the main aperture and which generate a shift difference of 180 degrees relative to exposed light beams permeating the main apertures. This results in improvement of a light intensity profile of the main aperture and enhancement the resolving characteristics. This method is described in Japanese Patent Laid-open No. 5-19446, which discloses a technique of disposing auxiliary patterns on an end of dense patterns and around isolated patterns in order to enhance resolution of the dense patterns end and the like. Further, for example, Japanese Patent Laid-open No. 6-123963 discloses a technique of disposing respective auxiliary patterns such that light beams permeating respective adjacent patterns do not interfere with one another, or a technique of disposing one auxiliary pattern relative to the main aperture when the auxiliary patterns are arranged between the adjacent patterns. And, for example, Japanese Patent Laid-open No. 6-289591 discloses a technique of disposing auxiliary patterns in a symmetrically shifted manner in order to enhance flexibility in arrangement of the main apertures. Further, for example, Japanese Patent Laid-open No. 8-297359 discloses a technique of making layouts of mask patterns such that one main aperture and one auxiliary pattern are handled as one unit in order to facilitate the layouts of the mask patterns. And, for example, Japanese Patent Laid-open No. 11-84625 discloses a structure of disposing main apertures, auxiliary patterns, and shifters arranged like zigzag at dense main apertures, and of arranging the auxiliary patterns at each end of memory mats.