1. Field of the Invention
The present invention relates to an exposure apparatus.
2. Description of the Related Art
Lithography is a well-known technique for forming a predetermined circuit pattern on a semiconductor. Lithography includes a step of forming a latent image pattern by irradiating a substrate coated with a photosensitive agent (photoresist) with light via a reticle to expose the photosensitive agent, and, thereafter, developing the photosensitive agent. Development forms a mask pattern for use in, e.g., etching.
As indicated by equation (1), the resolution of the exposure apparatus is known to be proportional to a wavelength λ of a light source and inversely proportional to a numerical aperture NA of a projection lens.Resolution=k1·(λ/NA)  (1)where k1 is a proportionality constant.
To improve the resolution of the exposure apparatus, it is only necessary to shorten the wavelength of the light source or to increase the numerical aperture of the projection lens.
One of the characteristic features of an optical system of the exposure apparatus is the depth of focus (DOF). The DOF is the allowable blurring range of a projected image, which is expressed by the distance from the focus point. The DOF is given by:DOF=k2·(λ/NA2)  (2)where k2 is a proportionality constant.
As the wavelength of the light source shortens or the numerical aperture of the projection lens increases, to improve the resolution of the exposure apparatus, the DOF noticeably decreases. This shortens a distance, in the optical axis direction, in which accurate processing is possible.
Especially, next-generation devices aiming at an increase in the degree of integration by forming a finer, stereoscopic circuit pattern are facing a serious problem of such a decrease in the DOF. This is because a wide range of sharp focus is necessary, since the formation of a stereoscopic circuit pattern elongates the processing dimension in the optical axis direction and a given DOF is always necessary irrespective of the degree of micropatterning of the circuit.
To solve the above problem, an attempt to increase the DOF is made by projecting the pattern of the reticle onto the substrate using exposure light, including a plurality of wavelengths, to form images at different positions on the same optical axis.
For example, Japanese Patent No. 02-619473 proposes an exposure method of using, as an exposure light beam, the light obtained by synthesizing the light beams from both a light source, which emits the first wavelength light and a light source, which emits the second wavelength light.
Japanese Patent Laid-Open No. 11-162824 proposes a method of performing exposure with exposure light, including a plurality of wavelengths, by inserting, on the light path between the light source and the wafer, a filter, which selectively transmits light in a plurality of wavelength ranges.
The current exposure apparatus must stabilize the light quality of the light source at the time of exposure and its calibration.
FIG. 1 shows the spectral shape of light (single-wavelength light) emitted by the light source. The abscissa represents a light wavelength λ, and the ordinate represents the light intensity as a function of the wavelength λ.
As shown in FIG. 1, the current exposure apparatus employs a central wavelength λ0, FWHM (Full Width Half Maximum), and an E95 value as evaluation indices of the light quality. The FWHM indicates a bandwidth measured at an intensity at half of the peak intensity. The E95 value indicates a bandwidth in which 95% of the entire spectral energy is concentrated.
The exposure apparatus monitors the central wavelength λ0, FWHM, and E95 value immediately before exposure or during exposure to calculate a variation (3σ) in measurement values and an error value (an error with respect to a command value), thereby confirming the stability of the light quality.
Even exposure using exposure light including a plurality of wavelengths requires stabilization of the light quality at the time of exposure and calibration of the exposure apparatus.
However, light including a plurality of wavelengths exhibits a spectrum, such as that shown in FIG. 2. Even when the central wavelength λ0, FWHM, and E95 value as conventional indices are calculated for the entire spectrum of light including a plurality of wavelengths, it is impossible to sufficiently confirm the light quality based on these indices.
For example, when the pattern of the reticle is projected onto the wafer with light including two peaks, as shown in FIG. 2, an adverse effect, such as image blurring, occurs. To prevent this problem, a difference Ediff between the peak intensities should also be controlled/monitored during exposure.