Patent Number: 053655613
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, there is shown a major portion of an X-ray exposure apparatus to which the present invention is applied. In FIG. 1, a wafer 101 has a surface to which a radiation-sensitive material (resist) is applied. Mask 102 has formed thereon a pattern for manufacture of semiconductor devices. Mask stage 112 is adapted to hold the mask 102 parallel to the wafer 101 with a clearance (mask-to-wafer gap) of about 40 microns in a Z-axis direction. Wafer moving stage 103 is adapted to hold the wafer 101 and to move the same in each of the X-axis and Y-axis directions. The exposure apparatus of the present embodiment is arranged such that, by means of the stage 103, different shot areas (exposure regions) of a wafer 101 are placed, in a predetermined order, to be opposed to an irradiation field 120 (see FIG. 2), whereby the whole surface of the wafer 101 is exposed with radiation. At any one time, a pattern bearing portion of the mask 102 and one exposure region of the wafer 101 are placed at the irradiation field 120. In other words, the exposure apparatus of this embodiment is structured as a step-and-repeat type exposure apparatus, called a "stepper". Sensor 104 is provided on the wafer moving stage 103 and is movable in the X and Y directions with the movement of the moving stage 103 so as to measure the illuminance at each of desired points in the irradiation field 120. Denoted generally at 105 is a main shutter mechanism having a movable aperture member. In this embodiment, the movable aperture member is provided by an endless belt having openings (apertures). The main shutter mechanism cooperates with an auxiliary shutter mechanism 106, to provide an exposure shutter device. The operation of each of the main shutter mechanism 105 and the auxiliary shutter mechanism 106 is controlled by a controller 113. More specifically, this controller 113 operates to control the main shutter mechanism 105, in a manner which will be described later, on the basis of the illuminance at each point within the irradiation field 120 as measured by use of the sensor 104. Also, the controller 113 operates to control the auxiliary shutter 106 so as to open the same before initiation of the exposure control by the main shutter mechanism 105 and to close the auxiliary shutter 106 after completion of the exposure control. Denoted at 107 is a beryllium thin film (partition film). The mask 102 side of this beryllium film 107 is maintained, for example, in a reduced-pressure He gas ambience, while a high-vacuum ambience is maintained on a radiation-source side of the film 107. Denoted at 108 is a radiation emitting point of a synchrotron orbitration radiation (SOR) ring which functions as an X-ray source. The synchrotron radiation (X-rays) from the emission point 108 has a uniform intensity distribution in a direction (X-direction in this embodiment) parallel to the orbital plane of an electron beam 110 (see FIG. 2), but has a substantially symmetric intensity distribution with respect to a direction (Y direction) perpendicular thereto. As best seen in FIG. 2, a convex-surface cylindrical mirror 109 functions to expand the synchrotron radiation from the emission point 108 in the direction (Y direction) in which the radiation has a substantially symmetric intensity distribution. The cylindrical mirror 109 is used to provide an irradiation field 120 of a required dimension, at the position at which the mask 102 and one exposure region of the wafer 101 are placed. Usually, when a cylindrical mirror whose reflection surface has a single curvature is used, there occurs in the irradiation field 120 a one-dimensional (Y direction) illuminance distribution (non-uniformness) having one peak. An example of such an illuminance distribution is illustrated in FIG. 6. The curve in FIG. 6 has been drawn by plotting the results of the calculation of the illuminance distribution as weighted in consideration of the dependency, upon wavelength, of the sensitivity of a resist material applied to a wafer 101. The axis of the abscissa denotes the position, while the axis of the ordinate denotes a specific illuminance in which the illuminance is standardized by using the illuminance I.sub.L or I.sub.U (=I.sub.L) at the position Y.sub.1 or Y.sub.N as a reference (=1). Reference character I.sub.P denotes a peak illuminance, and reference character Y.sub.P denotes the position in the Y direction corresponding to the peak Y.sub.P. FIG. 3 exemplifies a specific structure of a main shutter mechanism usable in the FIG. 1 embodiment. Belt 105 having an aperture 204 comprises a thin metal belt made of stainless steel, for example. This thin metal belt has a thickness sufficient for intercepting X-rays from the source. The belt 105 is wound around rollers 201 and 202. By rotating one of these rollers by a driving motor 203, the aperture 204 of the belt 105 can operate as a movable aperture. Arrow 210 corresponds to the Y direction and denotes in FIG. 3 the direction of movement of the aperture 204, which direction is coincident with the direction in which the synchrotron radiation has non-uniformness in intensity. The belt 105 is provided with two apertures, one of which is denoted at 204, and another of which is denoted at 208. The aperture 204 functions as a movable aperture which is operable to locally control, along the Y direction, the exposure time for the exposure region. The aperture 204 has a leading edge 206 and a trailing edge 207. When the leading edge and the trailing edge of the aperture 204 moves through the exposure region in the Y direction, their moving speeds are controlled independently by the controller 113. The other aperture 208 is defined by an opening of a size and shape necessary for avoiding interception of the radiation from the source in a time period in which the movable aperture member provided by the opening 204 is in an open state. Photosensor 209 is operable to detect the position of the aperture 204 and is used to determine the timing of passage of the aperture 204 through the exposure region. FIG. 4 exemplifies the manner of movement of the leading edge 206 and the trailing edge 207 of the aperture 204 (movable aperture). In FIG. 4, the axis of the abscissa denotes the time and the axis of the ordinate denotes the position of the movable aperture in its moving direction, namely, the position coordinate in the direction of non-uniformness in intensity of the radiation being projected. The zone as depicted at L.sub.O (from Y.sub.1 to Y.sub.N) corresponds to the exposure region (irradiation field) in FIG. 4, while reference character L.sub.E denotes the interval between the leading edge 206 and the trailing edge 207 of the aperture 204 in the Y direction. It is an important feature that, within the exposure region L.sub.O, the moving speeds of the leading edge 206 and the trailing edge 207 change independently of each other. In an initial state, the aperture 204 is located at a position under the exposure region (exposure field), in FIG. 4. When exposure start is instructed, the controller 113 operates to actuate the motor 203 to move the belt 105 with the cooperation of the rollers 202 and 203, so that the positions of the leading edge 206 and the trailing edge 207 of the aperture 204 are changed with the lapse of time along the curves shown in FIG. 4. When the leading edge 206 reaches the position Y.sub.1, the exposure starts. The leading edge 206 displaces between the positions Y.sub.1 and Y.sub.N in such manner that the position of the leading edge 206 changes relatively slowly in the neighborhood of the position Y.sub.1 whereas it changes relatively quickly in the neighborhood of the position Y.sub.N. At this time, the trailing edge 207 has not reached the exposure region, because there is a relationship "L.sub.O &lt;L.sub.E ", After this and when a middle portion of the aperture 204 reaches the exposure region, the controller 113 stops the drive of the motor 203 and, thereafter, restarts the drive of the motor 203 at a suitable timing. When the trailing edge 207 reaches the position Y.sub.N, the exposure is completed, and, thereafter, the drive of the motor 203 is stopped. The trailing edge 207 displaces such that the position of trailing edge 207 changes relatively quickly in the neighborhood of the position Y.sub.1, whereas it changes relatively slowly in the neighborhood of the position Y.sub.N. The reason for controlling the moving speed (displacement) of each of the leading edge 206 and the trailing edge 207 will be explained later. FIG. 7 shows changes in the position of each of the leading edge 206 and the trailing edge 207, moving between the positions Y.sub.1 and Y.sub.N in FIG. 4. The axis of the abscissa denotes the position coordinate in the Y direction, while the axis of the ordinate denotes time. FIG. 8 shows the exposure time (T.sub.E -T.sub.F) at each point Y.sub.i between the positions Y.sub.1 and Y.sub.N. In these Figures, reference character T.sub.F denotes the time at which the leading edge 206 reaches the point Y.sub.i, and reference character T.sub.E denotes the time at which the trailing edge 207 passes the point Y.sub.i. It is seen from FIG. 8 that, in the FIG. 4 example, the exposure time is shorter at the central portion of the exposure region (zone between Y.sub.1 and Y.sub.N) than that at marginal portions. In FIG. 9, the illustrated curve shows the illuminance at each point Y.sub.i (FIG. 6) as multiplied by the exposure time at each point Y.sub.i (FIG. 8). Namely, the graph of FIG. 9 shows the amount of exposure at each point Y.sub.i within the exposure region (actually, this corresponds to the amount of radiation absorbed by a resist material). It is seen from FIG. 9 that, by the provision of a speed-controlled movable aperture of the present embodiment, the amount of exposure can be made substantially uniform throughout the exposure region irrespective of that the radiation contains non-uniformness as shown in FIG. 6. In the above-described example, the movable aperture moves through the exposure region by a constant acceleration motion. To be exact, accordingly, the amount of exposure in the exposure region is not completely uniform. However, depending on the manner of control of the movable aperture, it is in principle possible to make the amount of exposure completely uniform, over every point Y.sub.i. An example of this will be described below in detail. Assume that, within the exposure region, a coordinate is set in the direction in which the illuminance changes. Points in the exposure region of a number N are selected and reference characters Y.sub.1, Y.sub.2, . . . and Y.sub.n are assigned to these points. Actual illumination intensity at a position Y.sub.i, as calculated on the basis of the data obtained by the measurement by the sensor 104 and having been weighted in accordance with the sensitivity-to-wavelength characteristics of a resist material on a wafer, is denoted by I.sub.i. Assuming that the required amount of exposure is E, then a correct exposure time at the position Y.sub.i in the exposure region can be expressed as follows: EQU T.sub.i =E/I.sub.i (1) On the other hand, the time at which the leading edge 206 of the movable aperture passes the point Y.sub.i is denoted by T.sub.Fi (P.sub.1, P.sub.2, . . . ). These reference characters P.sub.1, P.sub.2, . . . are parameters for controlling the operation of the movable aperture. Similarly, the time at which the trailing edge 207 of the movable aperture passes the point Y.sub.i is denoted by T.sub.Ei (P.sub.1, P.sub.2, . . . ). For each parameter P.sub.k, such a one by which the value E, determined by the following equation, becomes minimum is determined by the following calculation: ##EQU1## The motion of the movable aperture is controlled accordingly. By doing so, it is possible to minimize the error caused in relation to the control of the movable aperture. If control parameters of a number not less than N are set, it is possible to establish the following relationship with respect to each point Y.sub.i on the exposure region: EQU T.sub.Ei -T.sub.Fi -T.sub.i .+-.0 (3) In this case, it is possible to provide the same amount of exposure at every point Y.sub.i in the exposure region. Therefore, it is possible to assure, at high precision, uniform exposure over the whole exposure region. Taking this case as an example, a moving method by which the exposure can be completed in the shortest time will be explained below. It is assumed now that Y.sub.1 and Y.sub.N denote the ends of the exposure region; the movable aperture is moved in the direction from the point Y.sub.1 to the point Y.sub.N ; an illuminance distribution such as shown in FIG. 6 is provided; I.sub.p denotes the peak of the illuminance distribution in the exposure region; Y.sub.p denotes the position which provides the peak; I.sub.L =I.sub.1 ; and I.sub.U =I.sub.N. In an example when, as shown in FIG. 4, the movable aperture is provided by one endless belt having an aperture 204, and if the length L.sub.E of the aperture satisfies the following relation: EQU L.sub.E .gtoreq.max(.vertline.Y.sub.P -Y.sub.1 .vertline.,.vertline.Y.sub.N -Y.sub.P .vertline.), then, the movable aperture may be moved such a manner that the time T.sub.Fi and the time T.sub.Ei at which each point Y.sub.i is passed by the leading edge 206 and the trailing edge 207 of the aperture 204, with a maximum speed Vmax of the movable aperture, can be expressed by the following equations: ##EQU2## wherein T.sub.F0 is the time at which the leading edge 206 passes the point Y.sub.1 (T.sub.F1 =T.sub.F0). The manner of motion of the leading edge 206 and the trailing edge 207 as determined by equations (4), (5), (6) and (7) is shown in FIG. 10. Also, in this example, like the FIG. 7 example, the leading edge 206 displaces relatively slowly in the neighborhood of the point Y.sub.1. The moving speed gradually increases and, in the neighborhood of the position Y.sub.N, the leading edge moves relatively quickly. On the other hand, the trailing edge 207 moves relatively quickly in the neighborhood of the position Y.sub.1 and the moving speed gradually decreases, such that the trailing edge moves relatively slowly in the neighborhood of the position Y.sub.N. Also, the exposure time (T.sub.Ei -T.sub.Fi) in the exposure region is shortest at the position Y.sub.P. In this case, the time period .DELTA.T from the entrance of the leading edge 206 of the aperture 204 into the exposure region to the exit of the trailing edge 207 of the aperture 204 from the exposure region is given by the following equation: ##EQU3## Among various types of the motion of the movable aperture, satisfying equation (3), the above-described is the one that provides the minimum of the time (.DELTA.T). Particularly, when L.sub.O .ltoreq.L.sub.E, the leading edge 206 and the trailing edge 207 do not exist in the exposure region at the same time. Therefore, the maximum speed Vmax can be determined as desired within a range of E/I.sub.L &gt;L.sub.O /Vmax. The motion according to equations (4) to (7) provides a minimum time .DELTA.T. However, it should be noted that the motion represented by these equations is not the sole solution. Any motion may be adopted, provided that equations (5) and (7) are satisfied. Further, according to another important feature of the present invention, as long as the illuminance distribution is continuous, uniform exposure is attainable with regard to any illuminance distribution. Equations (4) to (7) show one solution for such a moving method. Namely, the leading edge 206 may be moved in accordance with equation (4) in such a region wherein T.sub.i decreases in the moving direction, whereas the leading edge may be moved in accordance with equation (6) in such a region wherein T.sub.i increases. Referring now to FIG. 5, another embodiment of a shutter device will be described. Denoted in FIG. 5 by reference numerals 211 and 212 are two separate movable aperture members each being in the form of a belt member, in this embodiment. The movable aperture member 211 is wound around driving rollers 216 and 219, while the other aperture member 212 is wound around driving rollers 217 and 218. Arrow 215 denotes the moving direction of these aperture members for the exposure control. In this embodiment, the origin sensor and the driving actuator of the FIG. 3 embodiment are omitted. Reference numeral 214 denotes an edge of an opening of the aperture member 212, and this edge has a function substantially the same as that of the leading edge 206 of the FIG. 3 embodiment. Reference numeral 213 denotes an edge of an opening of the aperture member 211, and this edge has a function substantially the same as that of the trailing edge 207 of the FIG. 3 embodiment. When the leading edge and the trailing edge of a shutter device is provided by two separately movable members, as in the present embodiment, an auxiliary shutter member such as at 106 in FIG. 1 may be omitted. Further, in the present embodiment, the length L.sub.E of the opening in the FIG. 4 example can be made apparently variable. Therefore, a higher degree of freedom is provided with respect to the control. In the extension of the method of the present invention, which assures uniform exposure by use of movable aperture means, equation (8) provides an optimization method for the design of the illumination system shown in FIG. 2. This will be described below in greater detail. FIG. 11 shows the illuminance distribution in the exposure region which varies with the change in the curvature 1/R of the convex-surface cylindrical mirror 109 of the illumination system shown in FIG. 2. In the particular example shown in FIG. 11, calculations were made under the conditions that: the critical wavelength .lambda..sub.L of the synchrotron radiation was 10.2 .ANG.; the intrinsic diversive angle .sigma. at the critical wavelength .lambda..sub.L was 0.44 mrad; the distance between the emission point 108 of the synchrotron radiation and the reflection center of the mirror 109 was about 5 m; the distance between the reflection center of the mirror 109 and the movable aperture 105 was about 7 m; and the size L.sub.O of the exposure region in the Y direction was about 30 mm (the Y direction is substantially vertical, although the inclination is emphasized in FIG. 1). Under these conditions, and while taking into account the angle of incidence of the radiation upon the mirror 109, the calculations were made. FIG. 12 is a graph wherein the axis of the abscissa denotes 1/R while the axis of the ordinate denotes .eta. which, on an occasion when the movable aperture means is moved in accordance with equations (4) to (7), can be expressed by the following equation (9): EQU .eta.=1/I.sub.U +1/I.sub.L -1/I.sub.P (9) EQU .DELTA.T=.eta.E+L.sub.0 /Vmax (10) As seen in this Figure, there exists such a radius of curvature (Rc) of the convex-surface cylindrical mirror 109 that provides the minimum ordinate value. While the value Rc is variable depending on the profile of the synchrotron radiation source and the size of the exposure field as well as the disposition of the radiation source, the mirror and the mask, for example, it is possible to assure an exposure system having a shortest opening time for the shutter and having a good efficiency with respect to time, by selecting, as the radius of curvature of the convex-surface cylindrical mirror, such an Rc that provides the minimum of .eta. as represented by equation (9). However, upon actual determination of the radius of curvature, the selection of Rc does not always provide a best result. In the method of the present invention making the exposure uniform, error factors which affect the same include an error in the measurement of illuminance, an error in the control of the moving speed of the movable aperture, and so on. The tolerances for these errors are determined with respect to the required precision for the amount of exposure. Each error can be reduced more easily, with smaller non-uniformness in the intensity of radiation. This is in the tendency to reduce the radius of curvature of the cylindrical mirror and, therefore, increases the exposure time. To enlarge the radius of curvature of the cylindrical mirror beyond Rc causes larger non-uniformness in the radiation and, additionally, the prolongation of the exposure time. Therefore, to do so provides no merit. As will be understood from the above, it is desirable to take the value Rc as an upper limit for the radius of curvature of the cylindrical mirror and to select a smaller radius of curvature within a range that does not increase the exposure time beyond a tolerable level. When the radius R is small, on the other hand, the non-uniformness in the intensity of radiation becomes small and the effectiveness of the present invention, making uniform the exposure by the action of the movable aperture means, may be reduced. But, the illuminance itself becomes too low. In consideration of this, when the radius of curvature of the cylindrical mirror that provides the minimum .eta. min is denoted by Rc, the radius R may desirably be within the limit of about 3.times..eta. min. In the particular example of FIG. 12, the radius R=30 m defines an intensity distribution which provides "3.6.times..eta. min". In the foregoing embodiments, the motion of the movable aperture has been described with reference to an example wherein the illuminance distribution has one peak. However, as long as the illuminance distribution is one dimensional and the illuminance changes continuously, the present invention is applicable to any profile of distribution and assures uniform exposure. Thus, the present invention is applicable not only to an illumination system having a cylindrical mirror, but also to any other illumination system using any other optical component such as a non-cylindrical mirror, for example. With regard to the illuminance distribution I, it may be measured after execution of actual exposure and development. Alternatively, it may be detected by calculations based on the ray tracing the synchrotron radiation. In accordance with the present invention, as described hereinbefore, a speed-controllable movable aperture means is provided, as an exposure shutter, in an exposure apparatus having an illumination system which provides radiation containing non-uniformness in the illuminance within an exposure field, and the aperture means is controlled to locally control the exposure time. By this, the non-uniformness in the amount of exposure (the amount of radiation absorbed by the resist material) resulting from the non-uniformness in the illuminance can be reduced remarkably, with the result that the resolution of the exposure apparatus can be improved significantly. Further, since the precision itself for the uniformness in the exposure as required from the point of resolution in the exposure process can be satisfied by the local exposure time control of the present invention using the movable aperture means, the tolerance for the non-uniformness in illuminance as can be set for the illumination system side is determined by the performance of this local exposure time control, and a significantly wider tolerance is allowed as compared with that for the non-uniformness as allowed conventionally. Usually, the magnitude of the illuminance and the uniformness thereof are somewhat incompatible factors with respect to the design of an illumination system. Therefore, if the tolerance for the non-uniformness becomes wider, as in the present invention, the illuminance can be made higher correspondingly. This makes it possible to reduce the exposure time and to reduce the exposure error due to the variation with time during the exposure. Additionally, this makes it possible to increase the processing speed of the apparatus, called "throughput". In the foregoing, it is shown that an exposure apparatus according to the present invention having a movable aperture means can accomplish more precise and correct exposure even when it is used with an illumination system that provides a much higher intensity of illumination than that of prior art illumination systems. By using the exposure time as an evaluation function, an index is given in the optimum designing of an illumination system. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.