The present invention relates generally to an adjustment method and apparatus for adjusting an exposure apparatus that exposes an object, such as a single crystal substrate for a semiconductor wafer (plate or ball), and a glass plate (wafer) for a liquid crystal display (LCD). The present invention is particularly suitable, for example, for an exposure apparatus that uses ultraviolet light and extreme ultraviolet (“EUV”) light as a light source for exposure.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.
The minimum critical dimension 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. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical system using extreme ultraviolet (“EUV”) light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
As the light absorption in a material remarkably increases in a wavelength range of the EUV light, making impractical a refraction-type optical system for visible light and ultraviolet light, a reflection-type or cataoptric optical system is used for an exposure apparatus that uses the EUV light (“EUV exposure apparatus”). A reflection-type reticle is used which forms a pattern to be transferred, on an absorber on a mirror.
The EUV exposure apparatus uses as a cataoptric element a multilayer mirror that alternately forms or layers two kinds of materials, e.g., molybdenum (Mo)/silicon (Si), having different optical constants on a precisely polished glass plate. For example, a molybdenum layer is about 2 nm thick, and a silicon layer is about 5 nm thick. A sum of thickness of two kinds of materials is generally called a coating cycle, which is 2 nm+5 nm=7 nm in the above example.
The multilayer mirror reflects EUV light with a specific wavelength when receiving EUV light. Efficiently reflected EUV light is one within a narrow bandwidth around λ that satisfies Equation 1 below where λ is a wavelength of the reflected EUV light, θ is an incident angle and d is a coating cycle and the bandwidth is about 0.6 to 1 nm:2·d·cos θ=λ  (1)
The reflectance of the EUV light would be about 0.7 at most. Non-reflected EUV light is absorbed in the multilayer film or plate, and most of the energy is consumed as heat. The number of multilayer mirrors should be maintained minimum for maximum reflectance in the entire optical system.
A typical projection optical system for the EUV light includes about four to six multilayer mirrors, which have a plane, concave or convex spherical or aspheric reflective surface.
A multilayer mirror in the projection optical system requires very high precision for its surface shape. For example, a permissible figure error σ (rms value) is given in Equation 2 below where n is the number of multilayer mirrors in the projection optical system, and λ is a wavelength of the reflected EUV light:
                    σ        =                  λ                      28            ×                          n                                                          (        2        )            
For example, four multilayer mirrors in the projection optical system that uses the EVU light with a wavelength of 13 nm is permitted to have a figure error σ of 0.23 nm. The wave front aberration amount is about 0.4 nm for resolution of 30 nm pattern transfer, which is permitted for the whole projection optical system.
Only polishing has a difficulty in making a figure error fall within the above permissible range. In addition, errors are inevitable due to gravity drawdown and alignment errors in combining plural polished multilayer mirror even when they are sufficiently precisely polished. For example, wave front aberration of about 1 nm remains in the whole projection optical system even after repetitive alignments according to the disclosure in “2nd International Workshop on EUV Lithography Source Oct. 17–19, 2000”, DESCRIPTION: At Wavelength Testing of an EUVL Four Mirror Ring Field System, PRESENTOR/AUTHOR: Goldberg, et al., COMPANY: LLBL, UC Berkeley, LLNL. In other words, since multilayer mirrors or plates in the projection optical system includes figure errors, alignment errors, errors due to their own weights, an object to be exposed, such as a wafer, has wave front offset from an ideal wave front obtained through calculation or so-called wave front aberration. As a result, the projection optical system exhibits insufficient imaging performance, lowered resolution and contrast and cannot transfer a fine pattern transfer satisfactorily.
USPTO Backgrounds
USPTO Backgrounds