Patent Number: 056065868
Section: summary

FIELD OF THE INVENTION AND RELATED ART The present invention relates to an exposure method and an X-ray exposure apparatus and a device manufacturing method in which a substrate such as a wafer is exposed to X-ray radiation (light). Recently, an X-ray exposure apparatus is under development in which rays from a charged particle accumulating ring, which will hereinafter be called "SR-X-rays" are used as illumination energy to expose the wafer or another substrate (which will hereinafter be called a "substrate"). Generally, a divergence angle of SR-X-rays is small in a direction perpendicular to an orbit plane of the charged particle accumulation ring, which will hereinafter be called a "y-axis direction", and therefore, a convex mirror is disposed in a path of the rays (optical path) from the point of radiation of the charged particle accumulation ring to increase the divergence in the y-axis direction. The X-ray intensity of the SR-X-rays which has been enlarged in the y-axis direction by the mirror is uniform in the direction perpendicular to the y-axis direction, which will hereinafter be called an "x-axis direction", but it has a curved distribution indicated by a solid line in FIG. 2 in the y-axis direction. For this reason, a shutter is disposed between the mirror and the substrate to control the exposure period so as to provide a uniform exposure amount of the surface of the substrate. Referring to FIG. 1, there is shown an example of an X-ray exposure apparatus using the convex mirror. The SR-X-rays L in the form of a sheet generated from the point of radiation 101 of the charged particle accumulation ring are expanded in the y-axis direction by a mirror 102 having a convex reflecting surface. The X-rays are introduced into an unshown reduced pressure chamber through an X-ray transmitting film 103, and are incident on a substrate 105 supported on a substrate stage 104, through an opening of the shutter 107. Adjacent the surface of the substrate 105, a mask having a pattern opening (not shown) is disposed. At an upper end of the shown substrate stage 104, an X-ray detector 106 is disposed. Before the start of the exposure of the substrate 105, the substrate stage 104 is moved downwardly in the Figure, and the X-ray detector 106 detects the X-ray intensity distribution in the exposure area. The exposure period at each position in the y-axis direction is controlled by the shutter 107 so as to be reversely proportional to the X-ray intensity distribution determined by the X-ray detector 106. SUMMARY OF THE INVENTION The radiation from the charged particle accumulation ring has a continuous spectrum from X-rays of a wavelength of several tens of pm to infrared rays. In the X-ray exposure apparatus, the radiated rays are reflected by at least one mirror, and are transmitted through a Be window functioning as a vacuum isolator. They are then reflected by or transmitted through a mask, and then are absorbed by a resist. The component having a wavelength longer than several nm is reflected or absorbed by the Be window. The short wavelength component of approximately 0.5 nm or shorter is not reflected by the mirror, and therefore, is absorbed by the resist. The component to which the resist is exposed has approximately 1 nm wavelength. Strictly speaking, however, the spectrum distribution of the radiation is different depending on the angle formed with the SR orbit plane, and the reflectance is different depending on the incident angle to the mirror and the wavelength. Therefore, the spectral distribution of the rays reaching to each point in the exposure region is different depending on the type of rays. The energy effect to expose the resist is not the energy per unit area incident on the resist, but the energy per unit area absorbed by the resist. In the case of soft X-rays (hereinafter soft X-rays are included in the class of X-rays) of 1 nm approx., the ratio of the X-ray energy per unit area incident on the resist to the X-ray energy per unit area absorbed by the resist changes significantly, even if the wavelength change is small. For this reason, the measurement of the energy distribution of the X-rays given to the resist in the exposure area does not necessarily mean the measurement of the distribution of the exposure intensity of the resist. In addition, a detector for detecting X-ray intensity has a different sensitivity depending on the difference of the wavelength (this is said to be that a spectral sensitivity is different). The X-ray intensity distribution detected by an X-ray detector is generally different from the intensity distribution of the X-rays per unit area incident on the resist. The exposure intensity distribution is a distribution of the exposure amount in the exposed area, and is reversely proportional to the time period required for the optimum exposure. The exposure intensity is considered as being proportional to the energy per unit area absorbed by the resist. When the material of the resist is different, exposure intensity distribution may change. In addition, even if the material of the resist is the same, exposure intensity distribution may change if the thickness is changed. As for the method of determining the exposure intensity distribution, there is a method in which an exposure area is exposed for a predetermined period under fixed process conditions, and the remaining resist film ratio is measured at each point in the exposure area, a method in which an accuracy of a line width of the resist is detected relative to the line width of the mask, a method in which a resist profile is measured, or the like. The amount of exposure is defined as the intensity of exposure radiation multiplied by the time period of exposure at the point. It is called an optimum exposure amount when the time period of the exposure is the same as the time period required for the optimum exposure. Exposure amount non-uniformity means deviation from an optimum exposure amount, or it means an amount of deviation. It is caused by a variation of the exposure intensity and the exposure period. In the X-ray exposure apparatus, the tolerable exposure amount nonuniformity is approx. 2%. Of the tolerable nonuniformity, the error permitted to the measurement of the exposure intensity is approx. 0.5%. According to the method in which the exposure intensity distribution is detected, it is obtained only after the exposure operation is actually carried out. The problem thereof is that the exposure intensity distribution changes by the change in the relative positional relationship among the point of radiation, at least one mirror, the mask and the wafer. The change of the accumulation current in the SR generating apparatus results in a change in .sigma.y, .sigma.y' of the electron beam of the SR generating apparatus, and therefore, the change in the exposure intensity distribution. It is cumbersome to detect the exposure intensity distribution for each of such changes, and it is difficult to specify the change among various changes. Thus, the problem of the reduction of yield attributable to the exposure amount nonuniformity arises. For this reason, a method of measuring the exposure intensity distribution in a short period of time and with the accuracy corresponding to the tolerable exposure non-uniformity is necessary. Accordingly, it is a principal object of the present invention to provide an X-ray exposure apparatus, an X-ray exposure method and a device manufacturing method using the same in which the exposure intensity distribution can be quickly detected with the accuracy corresponding to the tolerable exposure non-uniformity. The recent investigations have revealed that there is a shape instability of the SR light stemming from the SR radiation itself, as well as the position instability of the electron beam from the point of radiation of the radiation source and the position instability of the exposure position attributable to an angle instability of the X-rays from the point of radiation. The shape of the electron beam and the angular component of the velocity are both in the form of a Gaussian distribution or close thereto, and are expressed by: ##EQU1## where y is a position in a direction perpendicular to the SR orbit plane of the electrons in the electron beam, and y' is an angular component perpendicular to the SR orbit plane of the velocity of the electrons in the electron beam. By the positional instability of the SR radiation, that is, by the changes of Y.sub.0 and y'.sub.0 in equation (A), the angle of incidence to the mirror changes (the change in the incident position on the mirror causes the change in the incident angle to the mirror), and the reflectance of the SR radiation changes due to the change of the angle of incidence significantly. As a result, the shape of the reflected radiation from the mirror changes. This has been known. In consideration, as disclosed in Japanese Laid-Open Patent Application No. 129188/1993, an X-ray detector is disposed before the mirror, and the output thereof is fed back, so that the position of incidence of the SR radiation on the mirror is stabilized. The shape instability of the SR radiation attributable to the SR source itself can occur due to the changes of the .sigma.y and .sigma.y', and is the instability by nature even if no mirror is used. It is a cause of exposure non-uniformity. The shape instability of the SR radiation is dealt with in D. Laundy and S Cummings, "Electron Beam 63, Measurements on the Daresbury SRS, "Rev. Sci Instrum 63, January 1992, p. 554-556, together with the results of experiments. It is another object of the present invention to provide an X-ray exposure method and apparatus and a device manufacturing apparatus in which the exposure non-uniformity attributable to the shape instability of SR radiation stemming from the variation in the accumulation current can be reduced. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.