The present invention relates to an exposure apparatus, and more particularly to an exposure apparatus that exposes an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses extreme ultraviolet (“EUV”) light, such as soft X-ray, for an exposure light source.
In manufacturing such fine semiconductor devices as semiconductor memories and logic circuits in photolithography technology, reduction projection exposure apparatus have been conventionally employed which use a projection optical system to project a circuit pattern formed on a mask or a reticle onto a wafer, etc. to transfer the circuit pattern. Proximity exposure apparatuses have also conventionally been employed which directly transfer a circuit pattern on a reticle onto a wafer.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer semiconductor devices have promoted a shorter wavelength of ultraviolet light from an ultra-high pressure mercury lamp (i-line with a wavelength of about 365 nm) to KrF excimer laser (with a wavelength of about 248 nm) and ArF excimer laser (with a wavelength of about 193 nm). The lithography using the ultraviolet light, however, has the limit to satisfy the rapidly progressing fine processing of semiconductor devices. Accordingly, there has been developed a reduction projection optical system using EUV light with a wavelength less than about 100 nm (referred to as an “EUV exposure apparatus” hereinafter) for clear transfers of very fine circuit patterns. The EUV light's wavelength is shorter than that of F2 laser (with a wavelength of about 157 nm) as the UV light.
Absorptive materials, such as oxygen molecules, water molecules and carbon dioxide included in the air greatly absorb the optical energy of light in a wave range between 100 nm and 200 nm in the vacuum UV range, and thus such light has a difficulty in transmitting through the air. Accordingly, an exposure apparatus that uses ArF excimer laser and F2 laser as a light source needs to purge an optical path for the exposure light with inert gas, such as nitrogen and helium. In particular, the EUV exposure apparatus needs to maintain the optical path for the exposure light vacuum, because gas in the optical path absorbs and scatters the exposure light and prevents its transmissions.
Since materials greatly absorb the light of a wave range of the EUV light, a refractive optical system (that utilizes lenses or refractions of light) for use with visual light and UV light is not viable because of the low transmittance of the EUV light to an optical element, such as a lens. Therefore, a catoptric optical system is used which utilizes reflections of light.
An optical element for use with the catoptric optical element includes an oblique incidence total reflection mirror and a multilayer mirror. The wave range of the EUV light has a refractive index's real part of slightly smaller than 1, and is totally reflected by increasing an incident angle to introduce the light almost parallel to the reflective surface. The oblique incidence total reflection mirror utilizes this feature. Usually, the reflectance of 80% or greater is available to an oblique incidence with an angle between several degrees to 10° from the reflective surface (or an incident angle between a little larger than 70 and 90°). However, the oblique incidence total reflection mirror disadvantageously makes an optical system large due to little degree of freedom of the optical design cause by the limited incident angle.
On the other hand, the multilayer mirror alternately layers two different types of thin films having different optical constants or refractive indexes, and can use an incident angle close to perpendicular incidence. Proper selections of materials for these thin films and the number of layers would increase the reflectance up to about 70%.
The EUV exposure apparatus thus uses for a projection optical system a multilayer mirror that has larger degree of freedom than that of an oblique incidence total reflection mirror.
An exposure apparatus needs an alignment between a reticle and a wafer in exposure, and includes plural alignment optical systems. FIG. 15 is a schematic structure of a conventional exposure apparatus 1000 that uses KrF excimer laser and ArF excimer laser etc. for an exposure light source. The alignment optical system can be roughly classified into two types, i.e., an off-axis optical detection system 1400 that detects an alignment mark on a wafer 1300 for use with a wafer alignment, and a Through The Reticle (“TTR”) alignment optical system 1500 that detects a position of an alignment mark on a reticle 1100 relative to the alignment mark on the wafer 1300 through a projection optical system. The TTR alignment optical system is sometimes referred to as a Through The Lens (“TTL”) alignment optical system.
The off-axis alignment optical system 1400 serves to detect a position of the wafer 1300 for alignments of the wafer 1300 at a position different from an exposure position. Therefore, a precise alignment needs to maintain a baseline between the exposure position and the alignment position. Accordingly, the TTR alignment optical system 1500 needs to measure the baseline for alignments with higher precision than the baseline stability.
The TTR alignment optical system 1500 introduces light from an exposure light source (not shown) into an illumination part 1520 through an optical fiber 1510, etc., illuminates an alignment mark on the reticle 1100, and forms an image of the alignment mark on the illuminated reticle 1100 onto an image pickup device 1550 while enlarging the image through an objective lens 1530 and a relay lens 1540. The TTR alignment optical system 1500 preferably uses a light source that has the same wavelength as an exposure wavelength, and usually employs an exposure light source. A light source can use non-exposure light, but this configuration undesirably needs to correct the chromatic aberration in the projection optical system 1200.
The light that has transmitted through the reticle 1100 and the projection optical system 1200 illuminates an alignment mark on a wafer-side reference plate 1352. An image of the illuminated alignment mark is formed on the reticle 1100 via the projection optical system 1200, and an enlarged image is formed on the image pickup device 1550 through the objective lens 1530, the relay lens 1540, etc.
Use of the exposure light would maintain the same imaging relationship between the alignment marks on the reticle 1100 and the wafer-side reference plate 1352, as that for the exposure-time, and enable the same optical system to simultaneously detect these marks. An exposure position of a pattern or a mark on the reticle 1100 can be measured with precision without being affected by the optical system's errors, etc. In addition, the baseline or an arrangement between the exposure position (or reticle mark) and a position of the off-axis alignment optical system 1400 can be assured by driving the wafer stage 1530 and detecting the alignment mark on the wafer-side reference plate 1352 through the off-axis alignment optical system 1400.
Some TTR alignment optical systems illuminate an alignment mark on a wafer-side reference plate from a backside of the wafer-side reference plate (or an opposite side to the projection optical system) to form an image of the alignment mark on a reticle-side reference plate through a projection optical system, and illuminate the alignment mark on the reticle-side reference plate to image the transmitted light on an image pickup device.
An alternative type makes alignment marks on a reticle and a wafer-side reference plate of repetitive patterns of a light-shielding part and a light-transmitting part. These patterns are different in size by a magnification of the projection optical system. This type illuminates the repetitive pattern on the reticle from a backside of the reticle (or backside of the projection optical system) to project the pattern onto the repetitive pattern on the wafer-side reference plate via the projection optical system, and detects the light that has transmitted through the wafer-side reference plate while moving the wafer stage.
The TTR alignment optical system thus can detect a position of the wafer-side alignment mark relative to the reticle-side alignment mark, or a position of the reticle-side alignment mark relative to the wafer-side alignment mark.
When the EUV exposure apparatus applies exposure light to the alignment optical system, the TTR alignment optical system cannot use a conventionally available refractive element, such as a lens, but use only mirrors to form an image of an alignment mark on an image pickup device. Therefore, a conventionally available compact optical system is inapplicable.
In general, the alignment optical system forms an image of the alignment mark of twenty magnifications or greater on the image pickup device, and the enlarged magnification lowers the light intensity. For example, the alignment optical system of twenty optical magnifications lowers the light intensity on the reticle surface down to 1/400 on the image pickup device. Moreover, the multilayer mirror has reflectance of about 70% to the EUV light, posing a problem of efficiency or reflectance of the optical system: If the alignment optical system of twenty optical magnifications uses about ten multilayer mirrors, although the number of mirrors depends upon the permissible size of the alignment optical system, the reflectance becomes about 2.8%. On the other hand, the efficiency (or transmittance) of a lens-using alignment optical system in a conventional exposure apparatus is 90.5% when the alignment optical system uses ten lenses and each lens has reflectance of 0.5%. Therefore, the efficiency is 30 times as large as that for the EUV light. Therefore, the TTR alignment optical system that uses the EUV light cannot configure a detection system as high-magnification and precise as the prior art.
Even without the enlargement optical system, a light-receiving sensor for detecting the EUV light would raises a durability problem. In addition, since the EUV exposure apparatus is stricter in overlay precision than the prior art, frequent calibrations with the TTR alignment optical system are required for baseline corrections, etc. However, an EUV light source requires a high running cost, and frequent calibrations would increase the cost.