Abstract:
A projection optical apparatus for projection an image of an object onto a workpiece, which is suitable for use, for example, as an exposure apparatus used in the manufacture of integrated circuits. The projection optical apparatus includes an image forming optical system whose optical characteristic is changed by the light energy supplied from the illuminated object. A change of the optical characteristic of the optical system is determined by the use of a predetermined parameter whereby when the distribution of the light energy on the pupil of the optical system is changed, the parameter is correct in such a manner that the change of the optical characteristic determined by the parameter is changed in correspondence to the change in the distribution of the light energy.

Description:
This application is a continuation of application Ser. No. 999,455, filed Dec. 12 1993, now abandoned, which is a continuation of application Ser. No. 859,238, filed Mar. 26, 1992, now abandoned, which is a continuation of application Ser. No. 740,871, filed Jul. 31, 1991, now abandoned, which is a continuation of application Ser. No. 464,621, filed Jan. 3, 1990, now abandoned, which is a continuation of application Ser. No. 326,391, filed Mar. 17, 1989, and now abandoned, which is a continuation of application Ser. No. 945,648, filed Dec. 23, 1986, and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to projection optical apparatus for forming desired images, such as, exposure apparatus used in the fabrication of integrated circuits, and more particularly to improvements in the correction control of changes in the imaging characteristic of the apparatus. 
     2. Description of the Prior Art 
     One of the important optical characteristics of the projection optical apparatus, e.g., reduction projection exposure apparatus is the matching precision and a magnification error of the projection optical system is an important one of various factors which affect the matching precision. 
     In recent years, the scale of integration for integrated circuits has been improved with the resulting tendency toward the realization of finer patterns and consequently there has been an increasing demand for improvement in the matching precision. Thus, there has been a very high degree of necessity for maintaining the projection magnification at a desired value. 
     In this connection, the magnification of the projection optical apparatus varies in the vicinity of the desired magnification due to such causes as small temperature changes of the apparatus, small barometric changes and temperature changes of the atmosphere in the clean room in which the apparatus is installed or the projection of an energy beam to the projection optical system. 
     As a result, reduction projection exposure apparatus of the type employing a magnification correcting mechanism for effecting a fine adjustment of the magnification of the projection optical system and attaining the required magnification have recently been proposed. Such correction mechanisms have been so designed that as, for example, the spacing between a reticle and the projection lens is varied, the lens spacing in the projection lens is varied or the pressure in the suitable air chamber of the projection lens is adjusted as disclosed in U.S. patent application Ser. No. 656,777 filed on Oct. 1, 1984. 
     On the other hand, the focus of the exposure apparatus is also changed due to the similar reasons as the previously mentioned variation factors relating to the magnification. Thus, exposure apparatus employing focus correcting mechanisms have also been proposed. 
     Then, of these imaging characteristic changing factors the accumulation of heat due to the projection of an energy beam to the projection optical system is a heat diffusion phenomenon having a specific time constant. In the conventional exposure apparatus, the numerical apertures of their illuminating systems are generally constant in many cases. Thus, the manner in which the energy beam is incident to the projection optical system is fixed and the time constant of the heat diffusion is constant. As a result, the variation characteristics of the imaging characteristics, e.g., the magnification and focus, are constant and one and the same control method for their adjustment can be used. 
     In recent years, however, an exposure apparatus has been proposed in which the numerical aperture of the illuminating system is varied so as to obtain an improved resolving power for a specific pattern. The results of the experiments conducted on this exposure apparatus have shown that a change of the numerical aperture results in a change in the distribution of the illuminating light beam on the pupil of the projection optical system and therefore the time constant of the heat diffusion is also changed. Thus, there is a disadvantage that even if the previously mentioned control method relating to the constant time constant is applied, the imaging characteristic cannot be adjusted satisfactorily and it is impossible to deal with such change of the time constant. 
     Also, where the total amount of the incident energy beam to the projection optical system or the illuminance is measured on the image plane side of the projection optical system and the imaging characteristic is corrected by using the measured value as a parameter, the change is considered as a simple change in the incident energy quantity from the correction control point of view. Generally, while a change of the numerical aperture of the illuminating system results in the corresponding change in the illuminance at the image plane, the distribution of the light beam on the pupil, i.e., the energy density in the vicinity of the pupil plane, is also changed as mentioned previously. Thus, it is expected that not only the previously mentioned time constant but also the coefficient term of a model formula or the like for specifying the variation of the optical characteristic are changed. Therefor, there is a disadvantage that if such changes of the coefficient are not taken into consideration, the correction control is effected inaccurately. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present to provide a projection optical apparatus so designed that the variation of an imaging characteristic can be adjusted satisfactorily against changes in the distribution of an energy beam incident to a projection optical system. 
     Where a mask is illuminated by the light energy from a light source to form an image of the mask in the desired imaging condition on a workpiece through the projection optical system, the distribution of the light beam on the pupil of the projection optical system is changed as occasion demands. 
     In response to such change in the distribution of the light beam, a parameter (e.g., a time constant or coefficient) for determining a change of an optical characteristic of the projection optical system is corrected. 
     The desired change of the optical characteristic of the projection optical system is determined on the basis of the corrected parameter so that the imaging condition on the workpiece is corrected in accordance with the change of the optical characteristic. By virtue of this correction, the imaging condition is satisfactorily maintained in a desired condition even if the distribution of the light beam is changed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a first embodiment of the present invention. 
     FIG. 2A and 2B are diagrams for explaining the distributions of light in two projection lenses which are set to different numerical apertures. 
     FIG. 3 is a diagram for explaining the numerical aperture. 
     FIG. 4 is a graph showing how the imaging characteristic changes with changes of the numerical aperture. 
     FIG. 5 and 6 are diagrams for explaining the optical systems required respectively for increasing and decreasing the numerical aperture. 
     FIG. 7 is a graph showing how the time constant changes with changes of the numerical aperture. 
     FIG. 8 is a graph showing how the imaging characteristic changes with changes of the numerical aperture. 
     FIG. 9 is a graph showing how the coefficient changes with changes of the numerical aperture(σ value). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The overall construction of a first embodiment of the present invention will now be described with reference to FIG.  1 . In FIG..  1 , the light emitted from a light source  10  for exposure illumination purposes falls on a lens  14  through a shutter  12  so that the light is collimated and then falls on an optical integrator of the type disclosed in U.S. Pat. No. 4,477,015 or fly-eye lens  16 . 
     The light beam passed through the fly-eye lens  16  passes through an aperture stop  18  whose numerical aperture is automatically or manually adjustable and then the light beam falls on a dichroic mirror  20 . Thus, the optical path of the light beam is bent downwardly so that the light beam falls on a main condenser lens  22  and illuminates uniformly a reticle R containing the required pattern. After passing through the reticle R, the light beam further passes through a projection lens  24  and reaches a wafer W on a stage  26 , thereby projecting the pattern of the reticle R onto the wafer W. Note that the aperture stop  18  is arranged conjugate to the entrance pupil of the projection lens  24 . 
     The projection lens  24  includes typical lens elements  24 A,  24 B,  24 C and  24 D and sealed lens chambers  24 E,  24 F and  24 G are respectively defined by these lens elements. With these sealed lens chambers, the internal pressure of the sealed lens chamber  24 F is controlled by means which will be described later, thereby controlling an imaging characteristic of the projection lens  24  for adjustment. 
     Next, its control system will be described. The imaging characteristic is controlled mainly by a main controller  28 . The main controller  28  is connected to a shutter actuator  30 . The actuator  30  controls the opening and closing of the shutter  12 . 
     The sealed lens chamber  24 F of the projection lens  24  is connected to a barometric controller  32  which in turn is connected to a compression system  34  for supplying compressed air and an exhaust system  36  for producing a vacuum, thereby controlling the internal pressure of the sealed lens chamber  24 F through a suitable solenoid value. The sealed lens chamber  24 F is also connected to a pressure sensor  38 , and the pressure sensor  38  and the barometric controller  32  are connected to the main controller  28 . Thus, the internal pressure of the sealed lens chamber  24 F is detected and also the barometric controller  32  is operated by the main controller  28  so as to maintain the internal pressure at a given value. 
     The aperture stop  18  is connected to a selector  40  which in turn is connected to the main controller  28  through a memory  42 . The selector  40  selects from the memory  42  a time constant corresponding to the g value of the illumination aperture stop  18  which will be described later. The memory  42  stores different time constants of change of the imaging characteristic which have preliminarily been determined by experiments in correspondence to different σ values or numerical apertures. The σ value corresponds to the degree of opening and is given as a ratio of the numerical aperture of the illuminating optical system to the object (reticle R) side numerical aperture of the projection optical system. 
     Next, the manner in which the light distribution in the projection optical system changes with changes of the numerical aperture will be described. FIGS. 2A and 2B show schematically the section of the illuminating optical system and the projecting optical system in the apparatus of FIG.  1 . 
     The solid lines LA and LB show the paths of light beams. Symbol PA designates the position of the entrance pupil, PB the principal plane, and PC the image plane. The broken line LC shows the object side or image side aperture of the projecting optical system. 
     The numerical aperture will now be explained with reference to FIG.  3 . If θ 1  represents the angle of the incident light L to the reticle R, θ 2  the angle of the light transmitted through the reticle R, θ 3  the angle of incidence of the light to the image plane PC and n the refraction index of air with respect to vacuum, the numerical aperture of the illuminating optical system or the numerical aperture on the incident side to the reticle R is given by n-sinθ 1  and similarly the numerical apertures of the projection optical system or the projection lens  24  on the reticle R side and the image plane PC side are respectively given by n-sinθ 2  and n-sinθ 3 . Since the refractive index n is usually 1, practically these numerical apertures are given by sinθ 1 , sinθ 2  and sinθ 3 , respectively. 
     FIG. 2A shows the case where the aperture stop  18  has a relatively wide opening and the numerical aperture is also large. As shown in the FIG. 2A, of the illuminating light beam concentrating at three points on the reticle R, the zero-order beam travels straight without diffraction and its path LA up to the image formation spreads throughout the inside of the projection lens  24 . 
     On the other hand, FIG. 2B shows the case where the aperture stop  18  is closed relatively and the numerical aperture is small. In this case, as will be seen from the path LB of the zero-order beam, the light beam concentrates around the optical path inside the projection lens  24 . In this case, while the light beam spreads to the outside of the illustrated extent due to the presence of the diffracted light at the surface of the reticle R, even if this point is considered, the light beam tends to concentrate around the optical axis as compared with the case where the numerical aperture is large. 
     From the foregoing it will be seen that the light beam distribution or the light image area at the pupil position PA (or the principal plane position PB) in the projection lens  24  changes with changes of the numerical aperture or the σ value of the illuminating optical system. Thus, when such change occurs in the light beam distribution, it causes a change in the time constant of change of the optical characteristic due to a temperature rise caused by the abortion of a portion of the light beam by the projection lens  24 . 
     The selector  40  and the memory  42  will now be described. As mentioned previously, the time constant of change of the optical characteristic is varied with changes of the opening or the σ value of the illuminating aperture stop  18 . FIG. 4 shows the variation with time of the magnification (or focus) changes of the projection lens  24  obtained by varying the σ value as a parameter. Note that the changes are shown in normalized form with the saturation point as 100%. In FIG. 4, four exemplary cases for different σ values are respectively shown as characteristics α 1 , α 2 , α 3 ,and α 4 . The shutter  12  is opened from a time t o  to a time t c  and it is closed from the time t c  onward. With the characteristic α 1  for the maximum σ value, the saturation point is attained at the time t 1 , whereas the saturation point is reached at the times t 2 , t 3  and t 4  for the characteristics α 2 , α 3  and α 4 , respectively. From the time t c  on, the four cases differ in the rate of decrease from one another as shown by characteristics β 1  to β 4 . In this way, the time constant of change of the imaging characteristic is varied with changes of the σ value corresponding to the numerical aperture. Thus, as shown in Table 1, preliminarily the corresponding imaging characteristic change data to different values of the σ value or the numerical aperture of the aperture stop  18  are obtained and the corresponding time constants Ti are determined. These time constants Ri are stored in the memory  42  and they are selectively supplied to the main controller  28  by the selector  40  as occasion demands. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Numerical 
                 Time 
               
               
                 aperture 
                 constant 
               
               
                   
               
             
             
               
                 0.1 
                 T 1   
               
               
                 0.2 
                 T 2   
               
               
                 0.3 
                 T 3   
               
               
                 0.4 
                 T 4   
               
               
                 0.5 
                 T 5   
               
               
                 0.6 
                 T 6   
               
               
                 0.7 
                 T 7   
               
               
                 0.8 
                 T 8   
               
               
                 0.9 
                 T 9   
               
               
                 1.0 
                 T 10   
               
               
                   
               
             
          
         
       
     
     The overall operation of the embodiment will now be described. Firstly, the illuminating lift emitted from the light source  10  falls on the lens  14  in response to the opening and closing of the shutter  12  effected under the control of the actuator  30  so that the collimated light beam falls on the fly-eye lens  16  and it is transmitted through the aperture stop  18 . At this time, the spread of the light beam is suitably adjusted in dependence on the magnitude of the σ value or the numerical aperture of the aperture stop  18 . The illuminating light beam of the adjusted spread is projected onto the reticle R through the dichroic mirror  20  and the main condenser  22  and it is then transmitted through the projection lens  24  to reach the wafer W, thereby forming an image of the pattern of the reticle R on the wafer W. 
     On the other hand, the selector  40  selects from the memory  42  the time constant corresponding to the preset numerical aperture of the aperture stop  18  in accordance with the Table 1. For instance, the time constant T 6  is selected when the numerical aperture is 0.6. The selected time constant is supplied to the main controller  28  so that the desired magnification change is determined in accordance with this time constant as shown in FIG.  4 . In accordance with this magnification change and the internal pressure of the sealed lens chamber  24 F detected by the barometric sensor  38 , the main controller  28  applies a control signal to the barometric controller  32 . Thus the internal pressure of the sealed lens chamber  24 F is controlled by means of the compression system  34  and the exhaust system  36 . As a result of this control, the magnification of the projection lens  24  is controlled at a desired value. Any of the known methods of using the time constant for the variation characteristic of the projection lens  24  to control its internal pressure may be used, and more specifically the method disclosed in the previously mentioned U.S. patent application may be used. It is to be noted that the selector  40  may be manually operated at the discretion of the operator to receive an input. 
     The aperture stop  18  may be replaced with a group of lenses forming a Galileian system such as disclosed in U.S. Pat. No. 4,498,742. FIG. 5 shows an example in which the numerical aperture is increased. More specifically, a Galileian system including a concave lens  50  and a convex lens  52  is arranged on the incident side of the fly-eye lens  16  so as to expand the illuminating light beam collimated by the lens  14 . Where no Galileian system is arranged, the illuminating light beam becomes as shown by the broken lines in FIG.  5  and the size of a second-order light source image is IPo. This image is formed on an image plane  16 A positioned conjugate to the pupil of the projection lens  24 . Where the Galileian system is arranged, the illuminating light beam becomes as shown by the solid lines in FIG.  5  and the size of a second-order light source image becomes IP 1 , thereby increasing the numerical aperture. 
     On the other hand, where the concave lens  50  and the convex lens  52  are interchanged with each other as shown in FIG. 6, the illuminating light beam becomes as shown by the solid lines in FIG.  6  and the size of a second-order light source image becomes IP 2 , thereby producing the same condition as if the numerical aperture were decreased. 
     By using these arrangements, it is possible to vary the σ value of the illuminating optical system without using the aperture stop  18  and this has the effect of preventing any vignetting and improving the illumination efficiency. If it is desired to vary the o value continuously, this can be accomplished by arranging a zoom lens system in front of the fly-eye lens  16 . 
     Instead of storing the relation between the numerical aperture and the time constant in the form of Table 1 in the memory  42 , it is possible to approximate the relation between the numerical aperture and the time constant with a suitable function, e.g., n-order function as shown in FIG. 7, and store the function in the memory  42 . Then, the corresponding time constant will be determined by calculation from the preset numerical aperture. In this case, the selector  40  functions as a means for inputting a change of the numerical aperture. 
     Thus, it is possible to deal with cases where the numerical aperture is changed stepwise as well as cases where the numerical aperture is changed continuously. 
     While, in the above-described embodiment, the single time constant is used to represent the change of the imaging characteristic, there are cases where two or more parameters are used to represent a variable characteristic (e.g., attenuation characteristic). In this case, the required two or more parameters are related to the respective numerical apertures. For instance, in the case shown in Table 1, the required numbers of parameters (time constants) are stored in the memory  42  in the form of T 1 , T 2 , T 3 , - - -. In the case of FIG. 7, the functions of the required number of parameters are stored in the memory  42 . Also, there is the correspondence between the numerical aperture and the σ value and either of them may be used, 
     Assume that ΔP represents a change of the magnification or focus, i.e., a variable characteristic, with respect to the instantaneous energy beam projection, T 1 , T 2 , T 3 , (T 1 &gt;T 2 &gt;T 3 ) time constants and a 1 , a 2 , a 3  coefficients, thus establishing the following equation          Δ                 P     =       ∑     i   =   1     3                     ai   ·            -   μ                   Ti                                  
     In this case, if it is also necessary to make a correction for the coefficient ai, it is only necessary to similarly change the coefficient ai in correspondence to the numerical aperture (or the σ value). 
     While the above-described embodiment is mainly concerned with changes of the time constant, there are cases where practically no change occurs in the time constant even if the σ value is changed, 
     FIG. 8 shows four different variation characteristics of the magnification (or the focus) in a case where the value of the aperture stop  18  is varied as a parameter while maintaining constant the total incident quantity of the illuminating light to the projection lens  24  (the illuminance at the image plane). In FIG. 8, the abscissa represents the time t. The shutter  12  is opened from a time t o  to a time t c  and it is closed from the time t c  on. In FIG. 8, the ordinate represents the variation of the magnification (or the focus). 
     These differences in characteristic are caused by different projection lens structures, lens glass materials, etc., and it is considered that a change of the o value does not practically cause the corresponding change of the time constant and it causes a change in the coefficient term serving as a parameter. In such a case, the imaging characteristic change data corresponding to a plurality of σ values (or numerical apertures) of the aperture stop  18  are preliminarily obtained as shown in Table 2, thereby determining and storing the coefficients Ci (corresponding to ai in the previously mentioned equation) corresponding to the σ values in the memory  42  shown in FIG.  1 . Then, the selector  40  of FIG. 1 selects and sends a desired one of the coefficients Ci to the main controller  28 . 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Numerical 
                   
               
               
                 aperture 
                 Coefficient 
               
               
                   
               
             
             
               
                 0.1 
                 C 1   
               
               
                 0.2 
                 C 2   
               
               
                 0.3 
                 C 3   
               
               
                 0.4 
                 C 4   
               
               
                 0.5 
                 C 5   
               
               
                 0.6 
                 C 6   
               
               
                 0.7 
                 C 7   
               
               
                 0.8 
                 C 8   
               
               
                 0.9 
                 C 9   
               
               
                 1.0 
                 C 10   
               
               
                   
               
             
          
         
       
     
     Also, as in the case of FIG. 7, the relation between the σ the coefficient Ci may be approximated by a suitable function, e.g., n-order function or a hyperbola, so as to store its functional formula in the memory  42  and determine the coefficient corresponding to the preset σ value of the illuminating optical system by calculation. Of course, the control may be effected by storing both of the time constants and the coefficients in the memory  42  and suitably reading either one or both of them in response to changes of the σ value. 
     Further, while, in the above-described embodiments, the method of correcting an optical characteristic of the projection lens  24  itself is used as a means of correcting the imaging condition, any other method may be used. For instance, if the reticle side of the projection lens  24  is nontelecentric, the magnification on the wafer W can be changed by automatically shifting the reticle R toward the optical axis. Therefore, by causing the amount of shift to change to follow up in accordance with the calculated variation characteristic, it is possible to always maintain the magnification at a constant value. Further, where changes in the focus of the projection lens  24  present difficulties, it is necessary to arrange so that the offset corresponding to the variation characteristic is applied to the automatic focusing mechanism which maintains the gap between the projection lens  24  and the wafer W constant and the presumed in-focus position of the wafer W is varied in response to a shift of the image plane of the projection lens  24  toward the optical axis. In other words, in accordance with the invention any method may be used provided that the imaging condition of the projected image on the wafer W is corrected. 
     Further, it is desired that when the shape of a light source image (e.g., an of the aperture stop) on the pupil of the projection lens changes from the circular shape to any other shape, the parameter such as the time constant or the coefficient is changed similarly. 
     Still further, where it is desired to change the numerical aperture (the size of the pupil) of the projection optical system itself by the stop or the Like, the same effect can be obtained by changing the parameter in the like manner as in the case where the numerical aperture of the illuminating optical system is changed. In this case, the selector  40  functions as a means of inputting a change of the numerical aperture (the σ value).