The manufacture of semiconductor devices, liquid crystal display devices, thin film magnetic heads, or the like, based on a lithographic process uses a projection exposure apparatus for projecting an image of a pattern of a photomask or reticle (hereinafter, simply “reticle”) onto a photosensitive substrate through a projection optical system (e.g., a projection lens). In such a projection exposure apparatus, a decrease of pattern resolving power resulting from aberration of the projection optical system remaining due to the design thereof or the manufacture thereof becomes a large problem.
In consideration of this, technology for very precisely measuring the optical characteristic of a projection optical system, such as aberration, has been desired.
Aberrations of a projection lens such as spherical aberration, image plane (curvature of field), astigmatism (astigmatic aberration), coma (coma aberration), wavefront aberration, etc., are measured, and it is used for practical evaluation and inspection. Among these aberrations, the wavefront aberration is the significant aberration. By approximating this wavefront aberration on the basis of a generally used Zernike polynomial or the like, aberrations such as coma, astigmatism, image plane, and spherical aberration, which are factors of the polynomial, can also be calculated. Also, from the standpoint of prediction of process margin for a wide variety of device patterns based on simulations, the measurement of wavefront aberration is regarded as being important.
Wavefront aberration measuring methods are proposed in U.S. Pat. Nos. 5,828,455 and 5,978,085, for example. In the measuring methods proposed there, a grid-like pattern is provided on a reticle pattern surface and a pinhole is provided right below the center of the grid-like pattern with a small spacing kept from it. Further, on the reticle upper surface, there is a special reticle having a convex lens placed just above the center of the grid-like pattern. When this reticle is illuminated by an illumination system of an exposure apparatus, due to the aforementioned convex lens, the illumination angle (NA) of the illumination light emitted from the illumination system is made into an angle not less than σ1, and this light illuminates the grid pattern below it. Light passed through the grid pattern goes through the pinhole below it. At this time, the light that can pass through the pinhole is limited only to such light having an angle connecting the position of each point on the grid pattern and the aforementioned pinhole. Therefore, light beams emitted from respective points on the grid pattern advance in the form of plural light beams having mutually different angles.
These light beams having mutually different angles arrive at mutually different positions upon a pupil plane of the projection lens and, while being influenced by the wavefront aberration of the projection lens, they reach a wafer surface to image each point of the grid pattern thereon. Here, the images of each point of the imaged grid pattern have been influenced by the wavefront aberration (phase) differently. Namely, since a light ray advances in a direction of a normal to the wavefront, the imaging position of an image of each point on the grid pattern shifts from an idealistic position by an amount corresponding to the tilt at a corresponding point on the wavefront. In consideration of this, by measuring the deviations, from an idealistic grid, of images of each point of the grid pattern, tilts of the wavefront at each point on the pupil plane are obtained and, by using various mathematical techniques, the wavefront is calculated.
The wavefront measuring methods proposed in the aforementioned U.S. Pat. Nos. 5,828,455 and 5,978,085 are a method close to the well-known Hartman's method. In Hartman's method, a pinhole is disposed on a pupil plane of a projection lens to restrict the position of the wavefront and, from a positional deviation of a pattern image, which is formed by light passed therethrough and imaged, the tilt of the wavefront is detected.
In Hartman's method, by placing a pinhole on the pupil plane, the object spectrum bears, according to equation (1) below and by the pinhole filter, only information of a certain small wavefront region.E(x)=F−1[G(f)·p(f)·w(f)]  (1)
F−1: Fourier inverse transform
E(x): optical amplitude function of image
G(f): object spectrum
w(f): pupil (wavefront) function
p(f): pinhole function
Although it is desirable to control the shape of the object spectrum (pupil filtering), accurately, by placing a pinhole on a pupil plane as in Hartman's method, in practical exposure apparatuses, it is difficult because of the space of a barrel, a purge structure for contamination prevention, or the like, and also with respect to the cost.
In the methods proposed in the aforementioned U.S. Pat. Nos. 5,828,455 and 5,978,085, the pinhole filter is placed just below the reticle. Therefore, the object spectrum on the pupil plane is, unlike the equation (1) above, a Fourier transform-including a phase term.
An object of the present invention is to provide an optical characteristic measuring method suitable for high-precision measurement of an optical characteristic of an optical system such as wavefront aberration, for example, and a reticle to be used in such a measuring method.
Another object of the present invention is to provide an optical characteristic measuring method which enables measurement of an optical characteristic such as wavefront aberration, for example, in a completely different way from the methods disclosed by the aforementioned two U.S. patents, and a reticle to used in such a measuring method.
A further object of the present invention is to provide a projection exposure apparatus into which an optical characteristic measuring method or a reticle, described above, can be incorporated.