Abstract:
An exposure apparatus includes a light source for emitting exposure light, an illumination optical system illuminating an original on which a pattern is formed by the exposure light emitted from the light source, a projection optical system projecting the pattern to a photosensitive object, a first photodetector, disposed in a portion for receiving light from an optical path between the light source and a portion where the original is placed, for monitoring an emission light amount from the light source, and a processing system. The processing system obtains information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system, estimates a change in transmittance of the optical element on the basis of the information obtained and corrects a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of the estimated change of transmittance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates mainly to an exposure apparatus used for a photolithographic operation in a process for manufacturing ICs, LSIs, or other types of semiconductor devices.  
           [0003]    2. Description of the Related Art  
           [0004]    To manufacture semiconductor devices, such as ICs and LSIs, by using photolithography, an exposure apparatus is employed in which a reticle (mask) pattern as an original is directly projected, or reduced at a predetermined proportion and projected, onto a photosensitive material applied to a semiconductor wafer or a photosensitive substrate. Most photosensitive materials applied to wafers have established proper light exposures. In a conventional exposure apparatus, a half mirror is disposed in an illumination optical system, and the amount of exposure light branched by the half mirror is monitored by a photodetecting element (a first photodetecting means). Based on a result of the monitoring, the light exposure is controlled to obtain an appropriate light exposure.  
           [0005]    With the recent trend toward microminiaturization of semiconductor devices, excimer lasers that emit light in a far ultraviolet ray range are being increasingly employed as light sources of exposure apparatuses. It has been found, however, that repeated use of excimer laser beams gradually changes the optical characteristics of an illumination optical system, optical components, such as a half mirror, and coating films. This is considered to be caused by changes primarily in transmittance or refractive index of the vitreous materials of optical components and coating films, the changes being attributed to their exposure to the excimer laser beams. Therefore, the ratio of the light amount of an excimer laser beam branched by the half mirror to the light amount of an excimer laser beam that reaches a wafer changes accordingly. If the light exposure control is conducted on the assumption that the aforesaid ratio stays constant, then the difference between an actual light exposure and an appropriate light exposure may exceed a predetermined permissible value.  
           [0006]    Normally, the amount of light emitted from an excimer laser, which is a light source, is controlled by regulating a voltage corresponding to energy for each pulse from an exposure apparatus, thereby conducting the light exposure control. As the transmittance of an illumination optical system changes, a relationship between the transmittance and a voltage value of an excimer laser determined based on the amount of a laser beam monitored will change accordingly. As the transmittance lowers, a light exposure that has been reduced due to the lowered transmittance will be obtained in relation to a command value. In order to perform accurate light exposure control, if the amount of light of a preceding pulse that reaches a wafer is smaller than a set value, then it is necessary to increase the succeeding pulse energy. This requires a higher voltage to be applied accordingly. If the voltage deviates from a permissible voltage range, then a desired amount of light cannot be obtained, preventing precise light exposure control from being carried out.  
           [0007]    As a solution to the problem described above, there has been known the following method. A photodetecting element (a second photodetecting means) and a light transmitting portion through which exposure light passes to a portion other than a transfer pattern, with a mask resting thereon, are disposed in the vicinity of a wafer. A ratio is obtained of an output of the first photodetecting means that monitors the light amount in the aforesaid illumination optical system relative to an output obtained when exposure light is passed through the light transmitting portion and incident upon the second photodetecting means, while the mask is away from an exposure range, and irradiation to the wafer is OFF. By using the ratio, the sensitivity of the first photodetecting means under exposure is corrected, thereby to perform exposure with an appropriate amount of light. At this time, a relationship between a command voltage applied to a light source that is dependent upon the amount of light emitted from the light source and an output of the first photodetecting means can also be corrected.  
           [0008]    However, in a vacuum ultraviolet range of a wavelength of 200 nm or less, in particular, the transmittance of a vitreous material changes with the irradiation time. The amount of change ranges from 0.1 to 0.3% per one cm, and gradually eases after completion of the irradiation. The time constant is extremely long, e.g., a few tens of seconds. The change amount of the transmittance depends on the pulse energy or oscillation frequency of a laser serving as a light source, an oscillation duty (ratio of a burst oscillation ON versus an oscillation OFF time), exposure duration, the transmittance of a mask, and the amount of light incident upon an optical component in a unit time in an illumination range. A marked change in transmittance is observed, especially immediately after exposure is begun. For this reason, a change in the transmittance of a vitreous constituent located between the aforesaid first photodetecting means and a photosensitive substrate surface presents a significant problem from a standpoint of accuracy in light exposure control. Recently, the diameters of the wafers are being increased, resulting in longer replacement intervals of the wafers. Hence, it is difficult to maintain an appropriate light exposure control accuracy when the relationship between the first photodetecting means, the second photodetecting means, and the voltage of a light source is corrected each time the wafer is replaced. In addition, making frequent corrections inevitably leads to a lower throughput.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, it is a first object of the present invention to provide an exposure apparatus and an exposure method that each enables a minimized drop in throughput to be achieved and, also a proper light exposure control accuracy to be maintained.  
           [0010]    In one aspect, the present invention provides an exposure apparatus that includes a light source, an illumination optical system illuminating an original on which a pattern is formed by the exposure light emitted from the light source, a projection optical system projecting the pattern to a photosensitive object, a first photodetector, disposed in a portion for receiving light from an optical path between the light source and a portion where the original is placed, the first photodetector being used for monitoring an emission light amount from the light source, and a processing system. The processing system obtains information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system, estimates a change in transmittance of the optical element on the basis of the information obtained and corrects a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of the estimated change of transmittance.  
           [0011]    In another aspect, the present invention provides a method for producing devices by use of an exposure apparatus. The method includes steps of illuminating, with an illumination optical system, an original on which a pattern is formed by exposure light from a light source, projecting, with a projection optical system, the pattern to a photosensitive object, receiving light by a first photodetector from an optical path between the light source and a portion where the original is placed, monitoring, by the photodetector, an emission light amount from the light source, obtaining information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system, estimating a change in transmittance of at least the optical element on the basis of the information obtained, correcting a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of the estimated change of transmittance and developing the photosensitive object with a projected pattern, a circuit device being produced from the developed object. The illuminating step is performed on the basis of the corrected proportional coefficient for the first photodetector.  
           [0012]    In yet another aspect, the present invention provides a method for exposing an original and for projecting a pattern formed on the original onto a photosensitive object. The method includes steps of illuminating, with an illumination optical system, the original by exposure light from a light source, projecting, with a projection optical system, the pattern of the original onto the photosensitive object, receiving light by a first photodetector from an optical path between the light source and a portion where the original is placed, monitoring, by the photodetector, an emission light amount from the light source, obtaining information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system and correcting a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of a change in transmittance. The illumination step is performed by using the corrected proportional coefficient for the first photodetector.  
           [0013]    Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a block diagram of an exposure apparatus according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a block diagram illustrating a positional relationship when a sensitivity correction is made in the exposure apparatus according to the embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a diagram showing a relationship among a light receiving range of an irradiated light amount monitor, a light transmitting extent of a light transmitting portion, and an exposable extent;  
         [0017]    [0017]FIG. 4 is a flowchart illustrating a procedure for measuring a relationship between parameters and changes in transmittance by the exposure apparatus shown in FIG. 1;  
         [0018]    [0018]FIG. 5 is a chart showing measurement results of changes in an output ratio obtained by implementing the procedure illustrated by the flowchart of FIG. 4;  
         [0019]    [0019]FIG. 6 is a flowchart showing a process of manufacturing a semiconductor device; and  
         [0020]    [0020]FIG. 7 is a flowchart illustrating the details of a wafer process in the manufacturing process shown in FIG. 6.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings by taking, as an example, a scanning projection exposure apparatus having a reticle as an original. FIG. 1 is a schematic diagram showing the scanning projection exposure apparatus according to an embodiment of the present invention. The scanning projection exposure apparatus is used to manufacture semiconductor devices, such as ICs and LSIs, liquid crystal devices, imaging devices, such as CCDs, magnetic heads, etc., and is equipped primarily with a beam shaping optical system  2 , an optical integrator  3 , a condenser lens  4 , a half mirror  5 , a masking blade  6 , an image forming lens  7 , a mirror  8 , and a light amount detector  12 , which is a first photodetecting means. These components make up an illumination optical system  100  for illuminating a reticle R, which is an original, with exposure light emitted from a light source  1 . The exposure apparatus further includes a reticle stage  9  serving as an original stage, a projection optical system  10 , a wafer stage  11 , and an irradiated light amount monitor  13 , which is a second photodetecting means.  
         [0022]    Referring to FIG. 1, a luminous flux from the light source  1 , which is composed of an excimer laser or the like, is transmitted through an ND filter  20  for adjusting transmittance of the luminous flux, which has a predetermined transmittance, and then is shaped into a beam having a desired shape by the beam shaping optical system  2 . The shaped beam is directed to a light incident surface of the optical integrator  3 , which is composed of a fly-eye lens assembly or the like. The fly-eye lens assembly is composed of a plurality of minute lenses, and has a plurality of secondary light sources in the vicinity of a surface thereof where light exits. An aperture diaphragm  21  determines the magnitude and shape of a secondary light source. The aperture diaphragm  21  is replaced to change a representative value (a value) indicating the magnitude of an illumination extent or to perform oblique incident illumination.  
         [0023]    The condenser lens  4  performs Koehler illumination on the masking blade  6  with a luminous flux from a secondary light source of the optical integrator  3 . The masking blade  6  and the reticle R, which is the original, are disposed to establish a conjugate relationship by the image forming lens  7  and the mirror  8 . The configuration and dimensions of the illumination extent in the reticle R are defined by determining the configuration of the opening of the masking blade  6 . The reticle R, on which a transfer pattern has been formed, is lifted by suction by the reticle stage  9 . Then, the reticle stage  9  and the reticle R are scanned in the directions indicated by an arrow  16  shown in FIG. 1. The reticle stage  9  is provided with a light transmitting portion  14  for allowing exposure light to pass therethrough. Normally, the illumination extent in the reticle R is shaped like a rectangular slit having its short sides oriented in the scan direction, as indicated by the arrow  16 .  
         [0024]    The projection optical system  10  reduces and projects the transfer pattern drawn on the reticle R onto a wafer W, which is a photosensitive substrate on which a photosensitive material has been applied. The wafer stage  11  on which the wafer W has been placed is scanned in the directions indicated by an arrow  17  shown in FIG. 1, and moved in a direction perpendicular to the paper surface so as to form an image of the transfer pattern of the reticle R onto each exposure area of the wafer W. A scanning control system  101  carries out control so that the reticle stage  9  and the wafer stage  11  are accurately scanned at a constant speed at the same ratio as the projection magnification of the projection optical system  10  by a driving device (not shown).  
         [0025]    The light amount detector  12  monitors the amount of light; it divides a portion of an illumination luminous flux from the condenser lens  4  by the half mirror  5 , and monitors the divided luminous flux, thereby indirectly monitoring the light exposure supplied to the wafer W. In the vicinity of the wafer W on the wafer stage  11 , there is disposed an irradiated light amount monitor  13 , which has its light receiving surface adjusted so as to be substantially flush with the wafer W in order to detect the amount of light on a plane corresponding to a surface of the wafer W.  
         [0026]    The light amount detector  12  is disposed to establish a conjugate relationship with the masking blade  6  by the condenser lens  4  and the half mirror  5 , and also to establish a conjugate relationship with the exposure surface of the wafer W, that is, the light receiving surface of the irradiated light amount monitor  13 . A light amount computing unit  102  processes signals from the light amount detector  12  to determine the output energy of the light source  1  so that the output energy provides a proper amount of light. A light source control system  103  decides a voltage to be applied to the light source  1  in response to a command from the light amount computing unit  102 , and controls the output energy of the light source  1 .  
         [0027]    Furthermore, the exposure apparatus according to this embodiment is provided with a main control system  104  constituting a transmittance change estimating means. The main control system  104  estimates a transmittance change of the optical system in the illumination optical system  100  and a transmittance change of the projection optical system  10  based on a voltage, pulse energy, oscillation frequency, and oscillation duty (ratio of a burst oscillation ON time versus an oscillation OFF time), which are laser oscillating conditions, received from the light source control system  103 , and also based on parameters, including the transmittance of the ND filter  20 , the transmittance of the reticle R, and the illumination extent of the masking blade  6 .  
         [0028]    Based on the estimation result, the main control system  104  further estimates a change in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  and a change in the relationship between the output of the light amount detector  12  and the voltage to be applied to the light source  1 , which is decided by the light source control system  103 . Based on the estimation result, the main control system  104  constantly corrects the sensitivities of the light amount detector  12  and the irradiated light amount monitor  13 , and the relationship between the voltage to be applied to the light source  1  and the output of the light amount detector  12  while an exposure operation is being performed.  
         [0029]    [0029]FIG. 2 is a general view of an apparatus demonstrating an example of a positional relationship among the reticle stage  9 , the light transmitting portion  14 , and the irradiated light amount monitor  13  when the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  is measured. When the main control system  104  issues an instruction for correcting the sensitivities of the light amount detector  12  and the irradiated light amount monitor  13 , the reticle stage  9  is driven in the directions indicated by an arrow  18  shown in FIG. 2 by a driving device (not shown) so that the light transmitting portion  14  is positioned on an optical axis of the projection optical system  10 . The wafer stage  11  is also driven by a driving device (not shown) so that the irradiated light amount monitor  13  is positioned on the optical axis.  
         [0030]    [0030]FIG. 3 illustrates a relationship between the light receiving surface of the irradiated light amount monitor  13  and a projection image of the transmitting portion  14  of the reticle stage  9 , which is formed by the projection optical system  10  when the reticle stage  9  is moved to the position shown in FIG. 2 to perform the measurement of the output ratio of the light amount detector  12  to the irradiated light amount monitor  13 . In FIG. 3, an extent defined by the solid line is an exposable extent (illumination extent)  25  illuminated by the illumination optical system  100  and the projection optical system  10 . An extent defined by the dashed line indicates a light transmitting extent  26  of the light transmitting portion  14 , and a hatched area indicates a light receiving area  27  of the irradiated light amount monitor  13 . In the scanning exposure apparatus, the exposable extent  25  is usually a rectangular or arc slit having its short sides oriented in the direction indicated by an arrow  19  shown in FIG. 3.  
         [0031]    Preferably, the light receiving extent  27  of the light receiving surface of the irradiated light amount monitor  13  is longer than the exposable extent  25  in a scan direction, while it is sufficiently shorter than the exposable extent  25  in the direction orthogonal to the scan direction. To maintain a uniform exposure amount in the surface of the wafer W during scanning exposure, the light amount integrated in the scanning direction should remain constant in the lengthwise direction orthogonal to the scan direction. Therefore, by measuring the amount of light at each position while moving the irradiated light amount monitor  13  to a plurality of positions in the lengthwise direction, the integrated illuminance (mw/cm) in the scan direction per unit length in the lengthwise direction of the slit can be determined. This makes it possible to measure an illuminance distribution in the lengthwise direction.  
         [0032]    Under this condition, the light source  1  shown in FIG. 1 is energized at a predetermined applied voltage V 0  by issuing a command from the light source control system  103 , and the amounts of light entering the light amount detector  12  and the irradiated light amount monitor  13  are measured. In this example, a signal obtained by the light amount detector  12  is denoted as S 0 , and a signal obtained by the irradiated light amount monitor  13  is denoted by S 1 . The signals S 0  and S 1  have a value that is equivalent to a light amount in a unit time or per pulse on the plane corresponding to a surface of the wafer W or equivalent to an integrated light amount for each predetermined number of pulses when the light source  1  is energized at the applied voltage V 0 . At initialization or starting of the apparatus, or during periodic maintenance, an absolute illuminometer is mounted on the wafer stage  11  to measure the amount of light and to adjust the sensitivity or gain so as to obtain (gS 0 ) ini =(fS 1 ) ini =E, where E denotes the amount of light on the wafer W measured by the absolute illuminometer, and g and f respectively denote gains for converting the output signals S 0  and S 1  into light amounts.  
         [0033]    At the initialization, the relationship between a voltage V applied to the light source  1  and a signal S measured by the light amount detector  12  is adopted to decide an initial value of a coefficient h of a voltage value relative to a command of the light amount computing unit  102 , the coefficient h being expressed by h=V/S.  
         [0034]    If there is no change in transmittance of the optical system in the illumination optical system  100  and the projection optical system  10 , then the product gS 0  equals the actual amount of light on the wafer surface. If the transmittance changes, then the product gS 0  no longer agrees with the actual amount of light on the wafer surface. To cope with this, the main control system  104  estimates a transmittance change of the optical system in the illumination optical system  100  and the projection optical system  10  from an irradiation amount or optical energy of exposure light entering the optical members during a unit time. The main control system  104  then calculates a correction coefficient a, which will be a value representing a transmittance change, and corrects the gain so that g=g ini ×α thereby to correct a light exposure setting error, where g ini  denotes the gain of the light amount detector  12  in an initialized state wherein no exposure light has been applied. Making the correction enables the amount of light on the wafer W to be accurately estimated based on the output of the light amount detector  12 , permitting exposure with an appropriate light exposure to be achieved.  
         [0035]    Furthermore, in order to perform proper exposure, it is necessary for the optical light control system  103  to instruct a proper applied voltage to the light source  1  to set pulse energy. If no change takes place in the transmittance in the illumination optical system  100 , then the light source control system  103  performs computation of the voltage V=h×S to obtain an output S that provides a target of the light amount detector  12 , and supplies the computation result to the light source  1 . If the transmittance of the illumination optical system  100  changes, then a coefficient h′ based on the transmittance change from the light source  1  to the light amount detector  12  is determined, and the voltage to be informed to the light source  1  is set by V=h′×S, where h′ is calculated by h′=h×β. Reference character p will be a value representing the transmittance change.  
         [0036]    The following will describe the procedures for determining the correction coefficients α and β for each parameter. FIG. 4 is a flowchart illustrating the procedure for the main control system  104  to measure the relationship between parameters related to the amount of irradiation and changes in transmittance in order to estimate transmittance changes in the optical system in the illumination optical system  100  and the projection optical system  10 . In step  1 - 1 , the main control system  104  informs the light source control system  103  of the oscillation conditions, such as pulse energy, oscillation frequency, and oscillation duty, of the light source  1 , selects a desired ND filter  20 , and disposes the reticle R having a desired transmittance on the reticle stage  9 . Obviously, measurement related to parameters other than the transmittance of the reticle R can be performed without employing the reticle R. In this example, the reticle stage  9  is moved to the position shown in FIG. 2 (step  1 - 2 ), the light source  1  is energized at a predetermined applied voltage V 1  according to an instruction given by the light source control system  103 , and an output ratio of the light amount detector  12  to the irradiated light amount monitor  13  in the initial stage is measured (step  1 - 3 ). Subsequently, in step  1 - 4 , the reticle stage  9  is moved to the position shown in FIG. 1, and irradiation is performed in step  1 - 5  under set oscillation conditions for a desired unit time. After performing the irradiation for a preset time, the following steps will be repeated until a predetermined number of irradiations is completed (step  1 - 6 ). The reticle stage  9  is quickly moved to the position shown in FIG. 2 to return to step  1 - 2 , the light source  1  is energized at the predetermined applied voltage V 1  in step  1 - 3  to measure the output ratio of the light amount detector  12  to the irradiated light amount monitor  13 , then the reticle stage  9  is moved to the position shown in FIG. 1 again in step  1 - 4 . In step  1 - 5 , irradiation is carried out under the set oscillation conditions for the desired unit time. This series of steps is repeated for a predetermined number of times, and the changes in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  with respect to exposure time are stored.  
         [0037]    After completing the predetermined number of irradiations (step  1 - 6 ), the light source  1  is periodically energized using the applied voltage V 1 , with the reticle stage  9  moved to the position shown in FIG. 2 (step  1 - 7 ), to measure changes in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  (step  1 - 8 ), then they are left standing in step  1 - 9 . When a predetermined time has elapsed in step  1 - 10 , the transmittance change of the optical system in the illumination optical system  100  and the transmittance change in the projection optical system  10  after being left standing are measured in step  1 - 11 . Repeating a predetermined number of measurements (step  1 - 12 ) completes the measurement process.  
         [0038]    Subsequently, the amount of light incident upon the optical system in the illumination optical system  100  and the projection optical system  10  during the unit time is changed by changing the pulse energy or the oscillation frequency of the light source  1 , or by changing the transmittance of the ND filter  20  in the illumination optical system  100 , or by replacing the reticle R with one having a different transmittance. Then, under the new condition, the changes in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  are measured using the method shown in FIG. 4.  
         [0039]    [0039]FIG. 5 shows the results of the measurement repeated as described above. The exemplary data shown in FIG. 5 indicates the results of measurement that has been performed according to the procedure illustrated in FIG. 4, the measurement being performed at three different transmittances of the ND filter  20 , namely, T 1 , T 2 , and T 3 , as a parameter. The axis of the ordinate indicates the output ratio of the irradiated light amount monitor  13  to the light amount detector  12 , and the axis of the abscissa indicates elapsed time. On the axis of the abscissa, a range defined by A indicates the duration of irradiation, and a range defined by B indicates a duration wherein the units are left standing. Based on the results, the output S 0  of the light amount detector  12  relative to the transmittance of each ND filter  20  is determined, the transmittance of the ND filter  20  being a parameter. Furthermore, a coefficient k 1  of the change in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13 , and a time constant τ 1  are also calculated based on the above results. From the calculated coefficient and the time constant, the output S 0  of the light amount detector  12  within a unit time, and time t, the correction coefficient a of the gain of the light amount detector  12  is computed by α=f (k 1 , S 0 , τ 1 , t, α′), where τ denotes the time elapsed from the moment the preceding correction coefficient was calculated, f denotes a function for computing the correction coefficient α that is obtained by the measurement procedure shown in FIG. 4 and determined from the measurement results shown in FIG. 5, and α′ is a value of the correction coefficient at the time of calculating a new correction coefficient α. In one example, the function f is expressed by α=k 1 ×S 0 +(α′−k 1 ×S 0 )×exp (−t/τ 1 ) during irradiation, and α=α′×exp (−t/τ 1 ) during the left-standing period. The correction coefficient p for correcting the relationship between the applied voltage of the light source  1  and the light amount detector  12  can also be expressed by the similar function system as that of the correction coefficient a by measuring the changes in the output ratio of the applied voltage of the light source  1  to the light amount detector  12  as set forth above. The measurement described above is not required to be performed frequently; it may be performed at, for example, a startup or maintenance of the apparatus.  
         [0040]    In the example illustrated in FIG. 4 and FIG. 5, the method in which the coefficient k 1  for the transmittance of the ND filter  20  in the illumination optical system  100  is calculated has been shown. However, the transmittance change of the optical system in the illumination optical system  100  and the energy applied to the projection optical system  10  during a unit time can be monitored by the light amount detector  12  also when the pulse energy or the oscillation frequency of the light source  1 , or oscillation duty (the ratio of burst oscillation ON time to oscillation OFF time) is used as a parameter. Hence, the coefficient k 1  and the time constant T 1  determined with respect to the output S 0  of the light amount detector  12  can be used. It is obviously possible also to measure the changes in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  and to calculate the coefficient and the time constant individually, while changing the respective parameters independently.  
         [0041]    In an actual exposure operation, the illumination extent of the reticle R and the transmittance of the reticle R change according to the type, layer, etc. of a device. This information regarding such a change cannot be monitored by the light amount detector  12 . It is necessary, therefore, to separately calculate coefficients on the items affecting the amount of exposure light incident upon the optical members during a unit time by using the method illustrated in FIGS. 4 and 5. The calculation of the coefficients must be performed on the wafer W side rather than depending on the light amount detector  12 . This is applied to the illumination extent of the reticle R defined by the masking blade  6  and the transmittance of the reticle R in the example of the exposure apparatus shown in FIG. 1. Hence, regarding the illumination extent A, the coefficient of the change in the output ratio of the light amount detector  12  to the irradiated light amount monitor  13  is denoted as k 2 , and the time constant is denoted as τ 2  in order to individually determine this coefficient and time constant. Similarly, the coefficient of the change in the output ratio with respect to a transmittance RT of the reticle R is denoted as k 3  and the time constant is denoted as τ 3  to individually determine this coefficient and time constant. Then, the correction coefficient α of the gain of the light amount detector  12  is calculated by α=f (So, A, RT, k 1 , k 2 , k 3 , τ 1 , τ 2 , τ 3 , t, α′). The correction coefficient β is calculated in the same manner; however, the illumination extent of the reticle R defined by the masking blade  6  and the transmittance of the reticle R are not involved, so that these two parameters are excluded.  
         [0042]    Referring back to FIGS. 1 and 2, in calculating the correction coefficients α and β, the light transmitting portion  14  on the reticle stage  9  is moved on the optical axis to measure the output ratio of the light amount detector  12  to the irradiated light amount monitor  13 . The light transmitting portion  14 , however, is dispensable. In an exposure apparatus without the light transmitting portion  14 , if the reticle R is not retained, then a light amount can be measured by the irradiated light amount monitor  13  by utilizing a portion where the reticle R of the reticle stage  9  is rested. Furthermore, to measure a change in the transmittance of the projection optical system  10 , using the transmittance of the reticle R as the parameter, if the reticle R itself has a light transmitting portion that can be measured by the irradiated light amount monitor  13 , then the measurement can be performed by utilizing the light transmitting portion.  
         [0043]    The descriptions will now be given of light amount setting and sensitivity correction in the scanning projection exposure apparatus shown in FIGS. 1 and 2.  
         [0044]    First, a job for implementing exposure is loaded. At this time, conditions, such as an NA of the projection optical system  10 , an illumination condition (a value or modified illumination), and the extent of the masking blade  6 , are also loaded. Based on the loaded illumination conditions, the size and shape of the aperture diaphragm  21  of the illumination optical system  100  are selected, and the masking blade  6  is driven so as to light-shield a predetermined area. At this time, based on the values of the correction coefficients α and β at the end of an immediately preceding exposure and the time elapsed from the end of the exposure, the main control system  104  calculates the correction coefficient a of the gain of the light amount detector  12  at that point and the correction coefficient β for correcting the relationship between the applied voltage of the light source  1  and the light amount detector  12 . If a set light exposure is denoted as Ew(mJ/cm 2 ), the scan speed of the stage is denoted as V(cm/sec), and the slit width on the wafer W is denoted as d(cm), then a target output S 0  per unit time of the light amount detector  12  is expressed as follows:  
           S 0=( Ew×d/V )/ E×S 0 ini /α  (1)  
         [0045]    The light amount computing unit  102  determines the oscillation conditions, such as the pulse energy and the oscillation frequency, of the light source  1  to obtain the target output S 0  so as to determine the transmittance of the ND filter  20  in the illumination optical system  100 . Based on a command from the light amount computing unit  102 , the light amount control system  103  sets the applied voltage V of the light source  1  as shown below to obtain a desired pulse energy:  
           V=h×β×S 0   (2)  
         [0046]    Thus, the light source  1  is energized at a predetermined oscillation frequency.  
         [0047]    In actual exposure, the pulse energy of the light source  1  has predetermined energy variations. Hence, the target value SO is slightly changed for each pulse in order to achieve a proper light exposure, and a command voltage applied to the light source  1  is changed according to expression ( 2 ).  
         [0048]    Under the condition described above, the pattern of the reticle R is sequentially transferred onto the surface of the wafer W. Based primarily on the measurement results of the light amount detector  12 , the main control system  104  constantly calculates a most up-to-date correction coefficient a of the gain according to the expression α=f (S 0 , A, RT, k 1 , k 2 , k 3 , τl, τ 2 , τ 3 , t, α′) at each preset timing. In the same manner, the correction coefficient β is also calculated. At this time, the oscillation condition of the light source  1  and the transmittance of the ND filter  20  are set using the most up-to-date correction coefficients a and β calculated by the main control system  104  such that the condition determined according to expression ( 1 ) is satisfied, then the exposure is performed. If the transmittance of the illumination optical system  100  or the projection optical system  10  changes, the applied voltage of the light source  1  that has been calculated to obtain a predetermined pulse energy may exceed a voltage permissible range that takes into account a variation allowance for controlling light exposure. In such a case, the voltage for the light source  1  is determined within a range wherein an appropriate voltage can be set, and based on an estimated value of pulse energy under the condition, the oscillation frequency of the light source  1  and the transmittance of the ND filter  20  are re-determined.  
         [0049]    Thus, the above embodiment of the present invention has the reticle stage  9  that retains the reticle R and is movable in a direction orthogonal to an optical axis, a light transmitting portion  14  that allows exposure light to transmit at a place other than a transfer pattern, with the reticle R rested, and the irradiated light amount monitor  13  that is located in the vicinity of the wafer W and receives exposure light that has been transmitted through the transmitting portion  14 . With this arrangement, the sensitivity of the light amount detector  12  in relation to the illuminance on the surface of the wafer W can be corrected while the reticle R is being retained, thus making it possible to restrain a drop in throughput. Moreover, the main control system  104  estimates a change in the transmittance from the information regarding an output of the light amount detector  12 , the illumination extent, the transmittance of the reticle R, etc. Therefore, based on a change in the transmittance of the optical system, the proportional coefficient of an output of the light amount detector  12  and an emitted light amount of the light source  1  is corrected, and the sensitivity of the light amount detector  12  in relation to the illuminance on the surface of the wafer W is properly corrected. This arrangement provides an advantage in that a drop in throughput is restrained and proper light exposure control accuracy is securely maintained.  
         [0050]    Obviously, the present invention is not limited to the embodiment set forth above, and can be implemented in a variety of modifications. For instance, within a range that would not cause a drop in throughput, based on the measurements of the output ratio of the irradiated light amount monitor  13  to the light amount detector  12 , the method for correcting the sensitivity of the light amount detector  12  and the method for estimating sensitivity corrections shown in the embodiment may be used in combination. In this case, the correction coefficients α and β may be calculated based on the ratio of the outputs of the light amount detector  12  to the irradiated light amount monitor  13  that have passed through the transmitting portion  14  and have been measured, so that the main control system  104  may use the updated correction coefficients α and β, which have been obtained by the measurement, to perform the subsequent computation for estimation.  
         [0051]    In the above embodiment of the present invention, the scanning exposure apparatus has been adopted as an example and described. The same advantages, however, can be expected also when the present invention is applied to a step-and-repeat type projection exposure apparatus (a “stepper”), or a contact or proximity exposure apparatus. In addition, the light source  1  may be any of a KrF excimer laser, an ArF excimer laser, an F 2  laser, or the like.  
         [0052]    The descriptions will now be given of an embodiment of a manufacturing method for a semiconductor device by employing the projection exposure apparatus shown in FIG. 1.  
         [0053]    [0053]FIG. 6 is a flowchart showing a process for manufacturing semiconductor devices, including semiconductor chips, such as ICs or LSIs, liquid crystal panels, or CCDs. In step  1  for designing circuitry, the circuitry of a semiconductor device is designed. In step  2  for fabricating a mask, a mask or the reticle R on which a designed circuit pattern has been formed is produced. In step  3  for manufacturing wafers, wafers (wafers W) are manufactured using a material, such as silicon. Step  4 , which is a wafer process, is known as a pre-process wherein an actual circuit is formed on the wafer by lithography, employing the prepared mask and wafer. The next step, step  5 , which is an assembly step, is known as a post-process wherein a chip is made from the wafer created in step  4 . Step  5  mainly includes an assembly step (dicing and bonding) and a packaging step (sealing chips). In step  6 , which is an inspection step, the semiconductor devices created in step  5  are subjected to inspections that include an operation test, a durability test, etc. The semiconductor devices thus completed are then shipped in step  7 .  
         [0054]    [0054]FIG. 7 shows a detailed flowchart of the aforesaid wafer process. In step  11  for oxidization, the surface of the wafer (wafer W) is oxidized. In step  12  for chemical vapor deposition (CVD), an insulating film is formed on the surface of the wafer. In step  13  for forming electrodes, electrodes are formed on the wafer by deposition. In step  14  for ion implantation, ions are implanted in the wafer. In step  15  for resist treatment, a resist or a sensitive material is applied to the wafer. In step  16  for exposure, the wafer is exposed using the image of the circuit pattern of the mask or the reticle R by the exposure apparatus. In step  17  for development, the exposed wafer is developed. In step  18  for etching, the portion excluding the developed resist area is removed. In step  19  for removing a resist, the resist portion that has become unnecessary after completion of etching is removed. By repeating the above series of steps, circuit patterns are formed on the wafer.  
         [0055]    Employing the manufacturing method according to the embodiment permits easier manufacture of highly integrated semiconductor devices that used to be difficult to manufacture in the past.  
         [0056]    As described above, based on an estimation result provided by a transmittance change estimating means for estimating a transmittance change of an optical system, the proportional coefficient of the output of a first photodetecting means and the amount of light emitted from a light source is corrected, or the sensitivity of the first photodetecting means in relation to the illuminance on a surface of a photosensitive substrate is corrected in addition to correcting the foregoing proportional coefficient. This allows the sensitivity of a light amount monitor to be accurately corrected, so that exposure can be performed with a proper light exposure.  
         [0057]    Except as otherwise disclosed herein, the various components shown in outline or in block form in the figures are individually well known and their internal construction and operation are not critical either to the making or using of this invention or to a description of the best mode of the invention.  
         [0058]    While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.