Exposure apparatus
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor printing and exposing system for maintaining a stable exposure performance in a projection optical system and a method for controlling the same. 
     2. Description of the Prior Art 
     Recently, the patterns of semiconductors such as so-called IC, LSI, VLSI and others are increasingly miniaturized and integrated. The width of line in these patterns is now being decreased to the range of one to two μm. Such miniaturization and integration require an exposure apparatus which has an exposure performance capable of printing finer patterns with the width of line in the range of one to two μm and an alignment performance capable of accurately aligning the patterns with one another through a plurality of steps and which can provide wafers having no defects. In order to satisfy these requirements, various types of projection and exposure systems are energetically being developed. 
     In such projection and exposure systems, a projection optical system has its depth of focus usually in the order of ±1-2 μm depending on the relationship between the effective F-number and the wavelength used therein. For this reason, the projection and exposure system should have a focusing mechanism for exactly imaging the pattern of a photo-mask on the surface of a wafer. In addition, the projection optical system must inherently have its error of magnification and its distortion, but these have to be maintained less than the accuracy of alignment which is required to be in the order of ±0.3 μm for aligning the finer patterns with one another. When the wafer is subjected to exposure operation, the projection optical system is increased in temperature by absorbing part of the heat energy from the light of exposure. As a result, the projection optical system changes in its optical performances to vary or displace its optimum image position. Thus, the magnification and distortion of the projection optical system will unavoidably be affected adversely by the variations of the optical performance. 
     FIG. 1 illustrates the time chart showing illumination system in the prior art system and also displacements Δx of the imaging position in the projection optical system thereof. As shown in FIG. 1, the exposure process consisting of a series of exposure steps is initiated at Time T1 and then terminated at Time T3. During a period from T3 to T4, the exposed wafer is brought out and then a new wafer is brought in. Supposing that the optimum image position of the projection optical system is the saturated position in the displacement Δx, the imaging position is gradually increased from Time T1 at which the exposure process is initiated and reaches the optimum imaging position at Time T2. During the period between T3 and T4 for which no exposure is effected, however, the displacement Δx returns to its original state. In the prior art, thus, the projection optical system thereof is changed in its performances during the entire process, leading to the fault of its focusing function and the decrease of its alignment accuracy. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the present invention to eliminate the above disadvantages in the prior art and to provide an exposure apparatus, more particularly, a semiconductor alignment and exposure apparatus which can maintain a projection optical system at a constant state in the optimum imaging position to provide its stable focusing function, alignment performance and exposure performance at least during the exposure process. 
     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 embodiment of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphic diagram showing the timing chart of illumination and displacements Δx of the image position in the prior art projection optical system; 
     FIG. 2 is a schematic diagram of an exposure apparatus according to the present invention; 
     FIGS. 3 and 4 are graphic diagrams showing the timing charts of illumination and displacements Δx of the image position in the projection optical system according to the present invention; and 
     FIG. 5 is a plan view of a modification of a shutter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, there is shown a die-by-die (shot by shot) alignment and exposure apparatus according to the present invention in which the pattern on a photo-mask M is projected onto a wafer W in the die-by-die exposure manner. The apparatus comprises a base table 1 having a column 2 supported thereon, an illumination system 4 located on the top of the column 2 for emitting an illumination light and containing a shutter 3, a source of light 4a and a condenser lens 4b, a projection optical system 6 consisting of reduction lenses, the system being connected with the column 2 through a focusing mechanism 5. Below the projection optical system 6, an X-Y stage 7 is placed on the base table 1 and adapted to move the wafer W which is placed on the X-Y stage 7. On the top of the projection optical system 6, there is mounted a photo-mask stage 8 on which the photo-mask M is set. The projection optical system 6 is provided with a bottom opening near which a sensor 9 for measuring the spacing between the projection optical system 6 and the wafer W is mounted. The apparatus also comprises sensors 10 and 11 for detecting the positions of the X-Y stage 7 in the X-direction and Y-direction, respectively, and a control 12 for controlling the shutter 3, the focusing mechanism 5 and the X-Y stage 7. 
     The projection and exposure apparatus shown in FIG. 2 is substantially similar to that of the prior art except for the control 12. In the illustrated embodiment, the control 12 controls the illumination system 4 to emit the illumination light therefrom other than in the exposure process, as seen from FIG. 3. For example, the illumination system 4 is repeatedly energized as in the exposure process to provide the heat energy of the illumination light to the projection optical system 6 such that the amount of heat energy per unit time will substantially be equal to that throughout the exposure process. The non-exposure process mainly provides time required to bring wafers in and out of the stage. During this non-exposure process, any wafer W should not be exposed to the illumination light. 
     With experiments, it has been found that the projection optical system 6 can be maintained at the constant optical performances if the ratio of te/tp is constant, where the exposure time is te and the interval or period of exposure cycles is tp. In the present embodiment, time Ts required for the projection optical system 6 to reach its saturated or optimum state is preliminarily stored in the control 12. Prior to the energization of the apparatus, the first exposure time te and the interval tp of the die-by-die exposure are given to the input of the control 12. When the apparatus is started, the control 12 verifies the wafer W located out of the exposure position, based on signals from the sensor 10 and 11 and then supplies a drive signal to the shutter 3. The shutter 3 is controlled to have the first open time to and closed time tc represented by the following relation- ship: 
     
         to/(to+tc)=te/tp . . .                                     (1) 
    
     During the time Ts, the shutter 3 continues to repeatedly open and close in accordance with the command from the control 12 so that the projection optical system 6 will be placed and maintained at its saturated and therefore stable state. After the time Ts, the wafer W is moved to the exposure position by means of the X-Y stage 7. The stepping exposure process is repeated. The stepping exposure process is carried out under the pre-set time schedule such that the projection optical system 6 will be subjected to such a heat energy that the displacement Δx is maintained at its saturated optical performances, when a wafer W has completely been exposed, it is removed from the stage and then a new wafer is placed on the stage. During this operation, the control 12 continues or verifies that no wafer is located on the exposure position based on signals from the sensors 10 and 11 and then supplies a drive signal to the shutter 3 which signal satisfies the formula (1). As a result, the projection optical system 6 will have its displacement Δx which is maintained constant. Even in the non-exposure process, the shutter 3 is continuously opened and closed under the relationship represented by the formula (1) so that the projection optical system 6 will be maintained at its saturated or optimum optical performance. 
     FIG. 4 shows another embodiment of the present invention in which, in the non-exposure process, the projection optical system 6 is subjected temporarily to heat energy under the action of the control 12 to facilitate the removal and charge of wafer W. The displacement Δx of the image position of the projection optical system 6 is over-increased to a predetermined value, and then the illumination is stopped. Thereafter, heat is exhausted from the projection optical system 6 while a new wafer W is being placed in the apparatus. When the displacement Δx in the projection optical system 6 restores its optimum state for exposure, this new wafer W is initiated to expose to the illumination light. 
     FIG. 5 shows a filter disc 3 of the apparatus shown in FIG. 2 located near the shutter 3. The filter disc 13 includes a filter 13a for permitting a light in a wavelength range suitable for the exposure process to be transmitted therethrough, and a filter 13b for absorbing or reflecting the exposure light and for permitting a light in a wavelength range to which the wafer W is not sensitive to be transmitted therethrough. In accordance with signals from the control 12, the filter disc 13 is swung as shown by arrows. In the exposure process, the exposure light can be transmitted through the filter 13a to illuminate the wafer. In the non-exposure process, the non-exposure light is transmitted through the filter 13b. 
     If the performances of these filters 13a and 13b are so selected that heat energy absorbed by the projection optical system 6 per a unit time when such an exposure light as shown in FIG. 3 is irradiated may be equal to that absorbed by the projection optical system 6 when a non-exposure light is irradiated, the projection optical system always absorbs a constant heat energy so that the saturated state can be maintained. This is accomplished by changing these filters 13a and 13b in transmittance, for example. Where the filters 13a and 13b are used, the illumination may continuously be carried out rather than the discontinuous illumination in the non-exposure process. In any event, the wafer W will not be exposed to any exposure light. 
     The irradiation of the non-exposure light can be clearly distinguished from the irradiation in the alignment operation. For the reduction projection lens, it is extremely difficult to compensate chromatic aberration both for the exposure and non-exposure lights. Therefore, the non-exposure light is not used in the alignment operation. 
     As will be understood from the above description, the semiconductor exposure apparatus according to the present invention can maintain the projection optical system at its constant optical performances throughout the exposure process by illuminating the projection optical system in the non-exposure process. Therefore, the projection optical system can be stabilized in focusing function, alignment performances and exposure performances to effect the exposure process with higher accuracy. 
     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.