In exposure apparatuses used in the manufacture of semiconductors, a circuit pattern formed on a mask surface that is used as an object plane is projected and transferred onto a substrate such as a wafer via an image focusing optical system. The substrate is coated with a resist, and the resist is made photosensitive by exposure, so that a resist pattern is obtained.
The resolution w of an exposure apparatus is determined mainly by the exposure wavelength λ and the numerical aperture NA of the image focusing optical system, and is expressed by the following equation:w=kλ/NA k: constant
Accordingly, in order to improve the resolution, it is necessary either to shorten the wavelength or to increase the numerical aperture. Currently, exposure apparatuses used in the manufacture of semiconductors use mainly the i line with a wavelength of 365 nm, so that a resolution of 0.5 μm is obtained at a numerical aperture of approximately 0.5. Since it is difficult from the standpoint of optical design to increase the numerical aperture, it will be necessary to shorten the wavelength of the exposing light in order to further improve the resolution in the future. Excimer lasers may be cited as an example of exposing light with a wavelength shorter than the i line; since the wavelength is 248 nm in the case of a KrF excimer laser and 193 nm in the case of an ArF excimer laser, a resolution of 0.25 μm is obtained in the case of a KrF excimer laser, and a resolution of 0.18 μm is obtained in the case of an ArF excimer laser, in cases where the numerical aperture is set at 0.5. Furthermore, if extreme ultraviolet light (hereafter also referred to as “EUV light”) which has an even shorter wavelength is used as exposing light, a resolution of 0.1 μm or finer is obtained (for example) at a wavelength of 13 nm.
A conventional exposure apparatus is constructed mainly from a light source, an illumination optical system and a projection image focusing optical system. The projection image focusing optical system is constructed from a plurality of lenses, reflective mirrors, or the like, and focuses an image of a pattern on the mask on the wafer.
Meanwhile, if an attempt is made to design a projection optical system for EUV in order to obtain a higher resolution, the exposure field becomes smaller, so that the desired area cannot be exposed at one time. Accordingly, a method is employed in which a semiconductor chip with a size of 20 mm square or greater is exposed using a projection optical system with a small exposure field by scanning the mask and wafer during exposure. By doing this, it is possible to expose the desired exposure area even using an extreme ultraviolet projection exposure apparatus. For example, in cases where exposure is performed using EUV light with a wavelength of 13 nm, a high resolution can be obtained by forming the exposure field of the projection optical system in the form of an ring shape.
FIG. 5 shows a schematic diagram of an extreme ultraviolet projection exposure apparatus. 1 indicates a point that generates EUV light. The light source is not shown in the figure; however, various types of extreme ultraviolet (EUV) light sources such as a laser plasma light source and discharge plasma light source can be used. The EUV light that is emitted from the EUV light generating point 1 is collected by a light collector mirror 3, and is directed to a mask 19 by illumination reflective mirrors 4 through 12. It is desirable to use an optical system that forms a secondary light source as the illumination optical system that includes the illumination reflective mirrors 4 through 12; in the present example, fly-eye reflective mirrors 5 and 6 are disposed as such a secondary light source. The EUV light that is reflected by the mask 19 is directed onto the wafer 20 by projection reflective mirrors 13 through 18, and an image of the mask 19 is projected and focused on the wafer 20. Multi-layer optical films (e.g., films in which Mo and Si are alternately laminated) are formed on the respective reflective mirrors 4 through 18 and mask 19 in order to increase the reflectivity of EUV light. The mask 19 is illuminated with extreme ultraviolet light so as to have a ring shape exposure field.
The mask 19 and wafer 20 are moved in synchronization at a rate corresponding to the demagnification ratio (e.g., ¼) of the projection optical system so that the desired area (e.g., an area equivalent to one semiconductor chip) is exposed.
In such an extreme ultraviolet optical system, beginning with the extreme ultraviolet projection optical system, since a transparent glass material is not obtained, the EUV exposure apparatus is constructed entirely from a reflective optical system (normal incidence by means of multi-layer films and grazing incidence by means of total reflection).
Exposure apparatuses using conventional light mainly utilize a refractive optical system. FIG. 4 shows the construction of an exposure apparatus using conventional light. The light emitted from the light source 101 passes through an illumination system 102, and illuminates a mask 103. The light beam that passes through the mask 103 focuses the pattern of the mask 103 on the wafer 105 by a projection system 104. The illumination system 102 and projection system 104 are respectively formed into units, and a mask stage (not shown in the figure) on which the mask 103 is mounted is disposed between these units. In an optical system using a refractive optical system, since the light always advances in the forward direction, the successive disposition of the necessary units in this manner is easy.
On the other hand, in the case of an EUV exposure apparatus such as that shown in FIG. 5, since [the optical system is] a reflective optical system, the respective mirrors must be disposed so that the light beams that are incident on the mirrors and the light beams that are reflected by the mirrors do not overlap. In particular, in the case of the final mirror 12 of the illumination system (i.e., the mirror that is closest to the mask), the angle of incidence of the light rays on the mask 19 must be close to perpendicular; accordingly, this mirror must be disposed in a position that is in close to the projection system.
The respective reflective mirrors are relatively positioned by being mechanically held in a optical housing (optical housing unit). However, in cases where six projection system mirrors 13 through 18 are held in such a optical housing, there is a possibility of mechanical interference between this projection system optical housing and the illumination system mirror 12; accordingly, disposition is difficult, and even in cases where such disposition is possible, the problem of an extremely restricted disposition remains. Furthermore, in cases where a disposition that is devised to avoid mechanical interference is used, there is a problem in that the optical performance must be sacrificed to some extent.