Abstract:
An exposure apparatus includes a light supplying device for supplying exposure light, an optical system for directing the exposure light to a substrate to be exposed, a cover for covering a predetermined portion of the optical system, a gas supply device for supplying an inert gas to the inside of the cover, a detector for detecting a quantity of the inert gas inside the cover and a controller for controlling the light supplying device on the basis of an output of the detector.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     This invention relates to an exposure apparatus and, more particularly, to a projection exposure apparatus using an excimer laser, for example, and being suitable for the manufacture of various devices such as semiconductor devices, for example. 
     In order to meet the miniaturization of microdevices such as semiconductor devices (ICs, LSIs) or liquid crystal panels, those light sources (exposure light sources) which provide light of a shorter wavelength have been used in exposure apparatuses. More specifically, in exposure apparatuses for semiconductor device manufacture, for example, g-line light of a super high pressure Hg lamp has been replaced by i-line light, for reduced wavelength. Currently, the wavelength of KrF excimer laser light (248 nm) or ArF excimer laser light (193 nm), which is deep ultraviolet light of a shorter wavelength, is used. 
     Since light of a KrF excimer laser or an ArF excimer laser in the deep ultraviolet region has a high photon energy, there is a possibility that, due to the action of the energy, minute dust along the path of the light reacts with oxygen in air along the path, with a result of adhesion of impurities to the optical system (lens surface or mirror surface), or that the lens coating material of the optical system reacts with oxygen in the air, with a result of contamination of the optical system. In order to avoid adhesion of impurities to the optical system or contamination of the optical system, conventionally, an inert or inactive gas is charged to flow along the path of deep ultraviolet light to thereby prevent reaction of dust or the lens coating material with oxygen. To this end, a serviceman or operator is required to do the following work in relation to the portion of the optical system which is filled with an inert gas: that is, in order to avoid suffocation with the inert gas, first, the serviceman or operator closes the main tap of the inert gas, which is usually located at the back of the exposure apparatus, and then checks stoppage of the flow of inert gas into the portion of the apparatus, filled with the inert gas. After an elapse of a certain time period, the serviceman or operator opens the cover of the portion filled with the inert gas. 
     When the apparatus is to be re-started after completion of the above-described work, the serviceman or operator closes the cover of the portion to be filled with inert gas, and opens the main tap of the inert gas at the back of the apparatus. The serviceman or operator has to wait there for an elapse of a certain time period, until the portion to be filled with inert gas is filled with the inert gas. 
     This procedure needs many and time-consuming steps: to close the main tap of the inert gas at the back of the apparatus; to check stoppage of flow of inert gas into the portion filled with inert gas; to leave the apparatus for a certain time period; and to open the cover of the portion filled with inert gas. If the operator makes an error and opens the cover of the portion filled with inert gas, without shutting off the main tap, for example, there is a danger to the life of the operator since the operator may be exposed to the inert gas. Even if the main tap is closed, if the cover is opened before the density of inert gas is decreased sufficiently, the operator may similarly be exposed to the inert gas, which is very dangerous to the life of the operator. 
     Also, in relation to a re-start of the apparatus after completion of the above-described work, the procedure needs many and time-consuming steps: thereafter, to close the cover of the portion to be filled with inert gas; to open the main tap of inert gas at the back of the apparatus; and thereafter, to leave the apparatus for a certain time period until that portion is filled with the inert gas. If the operator makes an error to miss opening the main tap after closing the cover of the portion to be filled with inert gas, there is a possibility that the excimer laser is oscillated before inert gas charging. Laser emission in the state in which the inert gas charging is not started or completed causes adhesion of impurities to the optical system or contamination of the lens coating material of the optical system, as described hereinbefore, and it results in decreased optical performance. If such impurity adhesion of or contamination occurs, the optical element (or elements) damaged has to be demounted from the optical system. Thus, an enormous volume of work is necessary to recover adequate optical performance of the optical system. This directly decreases the operation efficiency of the apparatus. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved exposure apparatus which assures a smaller decrease in optical performance and enables enhanced safety. 
     In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: an optical system for directing exposure light to a substrate to be exposed; a cover for a predetermined portion of said optical system; gas supplying means for supplying an inert or inactive gas to the inside of said cover; detecting means for detecting a quantity of inert gas inside said cover; and notifying means for notifying information related to the quantity of inert gas, on the basis of an output of said detecting means. 
     Said detecting means may detect a density of the inert gas. 
     Said detecting means may detect a flow rate of the inert gas. 
     Said cover may have one of a door and a lid, and said apparatus may further comprise fixing means for fixing said door to said cover as said door or said lid is being closed. 
     Said apparatus may further comprise a sensor for detecting closure/opening of said door or said lid. 
     Said apparatus may further comprise an excimer laser for providing the exposure light. 
     Said notifying means may include display means for displaying the information. 
     Said optical system may include an illumination optical system for illuminating a mask, and the predetermined portion to be covered by said cover may be a portion of or the whole of said illumination optical system. 
     Said optical system may further include a projection optical system for projecting a pattern of the mask, illuminated by said illumination optical system, onto the substrate to be exposed. 
     Said apparatus may further comprise an excimer laser for providing the exposure light. 
     Said detecting means may detect a density of the inert gas. 
     Said detecting means may detect a flow rate of the inert gas. 
     Said cover may have one of a door and a lid, and said apparatus may further comprise fixing means for fixing said door to said cover as said door or said lid is being closed. 
     Said notifying means may include first display means for displaying the information. 
     Said cover may have one of a door and a lid, and said apparatus may further comprise a sensor for detecting closure/opening of said door or said lid. 
     Said supplying means may include an electromagnetically openable/closable valve for adjusting flow of the inert gas. 
     Said electromagnetically openable/closable valve may be controlled on the basis of an output of said sensor. 
     Said notifying means may further include second display means for displaying closure/opening of said electromagnetically openable/closable valve. 
     Said apparatus may further comprise control means wherein, when an output of said sensor represents a change of said door or said lid from a closed state to an open state, said control means is serviceable to close said electromagnetically openable/closable valve, is serviceable to cause said second display means to display closure of said electromagnetically openable/closable valve, and is serviceable to cause said first display means to display whether a detected value, detected by said detecting means, exceeds a predetermined value. 
     When an output of said sensor represents a change of said door or said lid from the closed state to the open state, said control means may be serviceable to produce a signal to stop light emission of said excimer laser. 
     Said apparatus may further comprise control means wherein, when an output of said sensor represents a change of said door or said lid from an open state to a closed state, said control means is serviceable to open said electromagnetically openable/closable valve, is serviceable to cause said second display means to display opening of said electromagnetically openable/closable valve, and is serviceable to cause said first display means to display whether a detected value, detected by said detecting means, exceeds a predetermined value. 
     When an output of said sensor represents that the detected value, detected by said detecting means, does not exceed the predetermined value, said control means may be serviceable to continue stoppage of light emission of said excimer laser, and wherein, when an output of said sensor represents that the detected value exceeds the predetermined value, said control means may allow light emission of said excimer laser. 
     Said apparatus may further comprise third display means for displaying a state of light emission of said excimer laser. 
     Said apparatus may comprise different covers for different portions of said illumination optical system, and said gas supplying means may supply inert gases to insides of said covers, respectively. 
     In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: light supplying means for supplying exposure light; an optical system for directing the exposure light to a substrate to be exposed; a cover for a predetermined portion of said optical system: gas supplying means for supplying an inert gas to the inside of said cover; detecting means for detecting a quantity of inert gas inside said cover; and discriminating means for discriminating whether a detected value, detected by said detecting means, exceeds a predetermined value, wherein, when said discriminating means discriminates an excess of the detected value beyond the predetermined value, said discriminating means produces a signal for allowing supply of exposure light by said light supplying means. 
     Said supplying means may include an excimer laser. 
     In accordance with a further aspect of the present invention, there is provided a device manufacturing method including a step of transferring a device pattern onto a photosensitive layer of a substrate by use of an exposure apparatus as recited, and a step of developing the substrate after the device pattern transfer step. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an illumination system of an exposure apparatus according to a first embodiment of the present invention. 
     FIG. 2 illustrates a cover member of the first embodiment as it is opened. 
     FIG. 3 is a flow chart for illustrating a procedure of stoppage and supply of inert gas, in accordance with the present invention. 
     FIG. 4 is a flow chart for illustrating another example of a procedure of stoppage and supply of inert gas, in accordance with the present invention. 
     FIG. 5 is a perspective view of an exposure apparatus according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 of the drawings illustrate the structure of a semiconductor device manufacturing exposure apparatus according to a first embodiment of the present invention. The exposure apparatus includes a light source which comprises a KrF excimer laser (laser head)  1 . Laser light  12  is emitted from an emission port  9  of the excimer laser  1  into a housing  101 . The laser light  12  goes along a light path  12 A and  12 B inside a laser light directing illumination optical system  5 , and it is projected onto a reticle  11 . The optical system  5  comprises a vertical portion  5 A and a horizontal portion  5 B. The optical system  5  includes mirrors M 1 , M 2  and M 3  illustrated as well as lenses, a fly&#39;s eye lens, a rotatable prism, and a beam splitter, for example. These elements may have a structure as disclosed in U.S. Pat. Nos. 5,153,773 or 5,363,170, for example. 
     The reticle  11  has a device pattern such as a semiconductor circuit device pattern, for example, which is formed thereon. Projection lens  6  serves to project and image the pattern of the reticle onto a wafer  10  in a reduced scale, by which the pattern is printed on the wafer. The wafer  10  is placed on an X-Y stage  7  which is movable in X and Y directions. Through simultaneous whole-surface exposure or scan exposure followed by stepwise motion, circuit device patterns are printed on different zones of the wafer. 
     The vertical portion  5 A and the horizontal portion  5 B of the optical system  5  are provided with separate covers (barrels). Each cover is provided with a door or lid mounted thereon, for maintenance or any other operation. When the door (lid)  5 C of the vertical portion  5 A is opened, a door opening/closing detecting sensor  4 A provided in the vertical portion  5 A detects that the door  5 C is opened. When the door (lid)  5 D of the horizontal portion  5 B is opened, another door opening/closing detecting sensor  4 B detects that the door  5 D is opened. About the door opening/closing sensors  4 A and  4 B inside the covers of the vertical and horizontal portions  5 A and  5 B, there are gas density sensors  14 A and  14 B for detecting the degree of inert gas charging. Inert gas flows into the vertical and horizontal portions  5 A and  5 B, through inlet ports formed in the covers of the portions  5 A and  5 B, respectively. Also, the gas flows out of the vertical and horizontal portions  5 A and  5 B, through outlet ports formed in the covers of them, respectively. 
     In FIG. 1, there is an inert gas supply tank  2  for inert gas charging, which is communicated with the optical system  5  through piping. The piping is provided with an electromagnetically openable/closable valve  3  for controlling discharge of inert gas. In this embodiment, nitrogen (N 2 ) gas is contained in the tank  2 . The door opening/closing detecting sensors  4 A and  4 B, the inert gas sensors  14 A and  14 B, the electromagnetically openable/closable valve  3  and the excimer laser  1  described above are all connected to a controller  8 . The controller  8  serves to control the valve  3  and light emission of the excimer laser  1 , on the basis of output signals from the door opening/closing detecting sensors  4 A and  4 B and the gas density sensors  14 A and  14 B. 
     Next, the operation of the apparatus controlled by the controller  8 , will be explained. FIG. 2 illustrates the state in which the door of the optical system  5  is being opened by a serviceman or operator, for maintenance of the optical system  5 . Here, a case for maintenance of the vertical portion will be described. 
     For maintenance of the vertical portion  5 A of the optical system, first the light emission of the excimer laser  1  is stopped. Then, the relevant door  5 C is opened. The door opening/closure detecting sensor  4 A detects that the door  5 C is in the open state, and it produces an electrical signal that represents the open state and applies the same to the controller  8 . The procedure is illustrated in the flow of FIG. 3, and checking opening/closure corresponds to step  1  of FIG.  3 . 
     On the basis of the signal from the sensor  4 A, the controller  8  discriminates that the door  5 C has changed from its closed state to the open state. The controller  8  then operates to close the electromagnetically openable/closable valve  3  mounted on the piping of the inert gas. Also, it applies a laser light emission stopping signal to the excimer laser  1  to shut the beam. If the laser is provided with a beam shutter, the beam shutter may be closed. This operation corresponds to step  2  of the flow chart of FIG.  3 . Table  1  below lists four states of the apparatus based on the outputs of the door opening/closure detecting sensor and of the inert gas density sensor. The change in state of the apparatus described just above corresponds to a transition from State 1 to State 2 in Table 1. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 DOOR 
                   GAS DENSITY 
                  GAS 
                   LASER 
               
               
                   
                 SENSOR 
                 SENSOR 
                 CHARGE 
                 EMISSION 
               
               
                   
               
             
             
               
                     STATE 1 
                 CLOSED 
                 ABOVE THRES. 
                   ON 
                   EMITTABLE 
               
               
                 STATE 2 
                 OPEN 
                 ABOVE THRES. 
                 OFF 
                 UNEMITTABLE 
               
               
                 STATE 3 
                 OPEN 
                 BELOW THRES. 
                 OFF 
                 UNEMITTABLE 
               
               
                 STATE 4 
                 CLOSED 
                 BELOW THRES. 
                 ON 
                 UNEMITTABLE 
               
               
                   
               
             
          
         
       
     
     In accordance with the above-described sequential operation, the controller  8  causes a monitor thereof to display the information that the supply of inert gas, having been discharged into the vertical portion  5 A of the optical system  5 , is stopped. This corresponds to step  3  of the flow of FIG.  3 . 
     Subsequently, the controller  8  operates to check the degree of inert gas charge within the vertical portion  5 A, by using the inert gas density sensor  14 A. If the sensor  14 A detects that the degree of inert gas charge has decreased to a predetermined level or lower, the controller  8  causes its monitor to display that the degree of inert gas charge (density) in the vertical portion has reached a safe level. This operation corresponds to step  4  of the flow chart of FIG. 3, and it means that the state of the apparatus has changed from State 2 to State 3 in Table 1. 
     As the safeness is confirmed through the above-described procedure and by means of the controller  8 , the operator or serviceman can start the maintenance operation for the vertical portion  5 A of the optical system, safely. 
     A similar procedure applies to performing maintenance for the horizontal portion  5 B of the optical system  5 . That is, for the maintenance of the inside components of the horizontal portion  5 B of the optical system  5 , first the light emission of the excimer laser is stopped. Then, the door  5 D of the horizontal portion  5 B of the housing is opened. The door opening/closure detecting sensor  4 B detects that the door  5 D has changed into an open state, and it produces an electrical signal, representing the open state, and applies the same to the controller  8 . This corresponds to step  1 . 
     On the basis of the output signal from the sensor  4 B, the controller  8  discriminates that the door  5 D has changed from its closed state to the open state. The controller  8  then operates to close the electromagnetically openable/closable valve  3 , mounted on the piping of the inert gas, and also it supplies a laser light emission stopping signal to the excimer laser  1  to stop the beam. If the excimer laser  1  is provided with a beam shutter, the beam shutter may be closed. This operation corresponds to step  2 . If the opening operation for the door  5 C of the vertical portion  5 A is being performed in parallel and if the closing operation of the electromagnetic valve  3  and supply of a light emission stopping signal to the excimer laser head  1  are already done, a further supply of an emission stopping signal from the controller  8  may of course be omitted. 
     In accordance with the sequential operations described above, the controller  8  caused its monitor to display that the supply of inert gas, having been discharged into the horizontal portion  5 B of the optical system  5 , is stopped (step  3 ). 
     The controller  8  continues measurement of the degree of inert gas charge within the horizontal portion  5 B through the inert gas density sensor  14 B. If the sensor  14 B detects that the degree of inert gas charge has decreased to a predetermined level or lower, the controller  8  causes its monitor to display that the degree of inert gas charge (density) in the horizontal portion  5 B has reached a safe level. When the safeness is confirmed through the above-described procedure and by means of the controller  8 , the operator or serviceman can start the maintenance operation for the optical system  5  safely. 
     Next, a description will be made of how the apparatus should be restarted after a completion of maintenance by a serviceman or an operator. As the maintenance operation by the serviceman or operator is finished, the serviceman or operator closes the door of the portion of the optical system  5  to which the maintenance operation has been done (e.g., door  5 C is closed if the maintenance operation has been done to the vertical portion  5 A). As the door  5 C of the vertical portion  5 A is closed, for example, the sensor  4 A detects that the door  5 C is closed, and it applies an electrical signal, representing the closed state, to the controller  8 . In response to reception of the signal from the sensor  4 A, the controller  8  discriminates the door closure. Also, the controller  8  checks the state of the other door sensor or sensors of the other portion of the optical system. 
     If all the other sensors are in the closed state, the controller  8  operates to open the electromagnetic valve  3  of the inert gas piping. This operation corresponds to step  5  of the flow chart of FIG. 4, and it means a transition of the state of the apparatus from State 3 to State 4 in Table 1. 
     Under this situation, the controller  8  causes its monitor to display that inert gas charging to the charging path (light path) has started. This corresponds to step  6  of FIG.  4 . Simultaneously therewith, the controller  8  checks the result of measurement through the inert gas density sensor  14 A, mounted on the vertical portion  5 A of the optical system  5  to which the operation has been done. The sensor  14 A continuously measures the inert gas density along the path of charging, and if the degree of gas charge (density) in that portion is not greater than a predetermined level, it means that the degree of inert gas charge to the optical path is insufficient. Thus, the sensor  14 A applies a corresponding electrical signal to the controller  8 . As the measurement continues and when the degree of inert gas charge reaches a predetermined level or higher, the gas density sensor  14 A discriminates that the inert gas density has become sufficient and it applies a corresponding electrical signal to the controller  8 . The controller  8  also monitors output signals from other inert gas density sensors mounted in the other portion or portions of the optical system, and it concludes a total inert gas charging state of the apparatus as a whole. If at least one of those inert gas density sensors indicates a low degree of inert gas charge, lower than the predetermined level, stoppage of laser light emission of the excimer laser  1  is held. 
     If all the inert gas density sensors disposed along the inert gas charge path show that the degree of inert gas charge in the portions of the gas charge path has reached a predetermined level or higher, the controller  8  applies a laser light emission allowance to the excimer laser  1 . This operation corresponds to step  7  of FIG. 4, and the apparatus changes from State 4 to State 1 of Table 1. 
     Simultaneously therewith, the controller  8  causes its monitor to display that the inert gas charge level has become high and thus, emission of the excimer laser is enabled. This operation correspond to step  8  of FIG.  4 . 
     Through the operations described above, the inert gas charge level has been confirmed and the apparatus has recovered to its operation mode. A start of operation after the recover may be done automatically or it may be done manually with an input by the serviceman or operator. 
     FIG. 5 illustrates a main portion of a second embodiment of the present invention. This embodiment differs from the first embodiment in that there are fixing mechanisms  13 A and  13 B mounted to fix the doors  5 C and  5 D of the optical system  5 . These fixing mechanisms  13 A and  13 B are connected to the controller  8 , and they operate to control fixation/release of the doors  5 C and  5 D in response to a signal from the controller  8 . 
     When maintenance for the vertical portion  5 A of the optical system  5  is to be done by an operator or serviceman, first, a maintenance start command is inputted from a keyboard of the controller  8 . As the command is inputted, the controller  8  applies a laser light emission stopping signal to the excimer laser  1 . Simultaneously therewith, the controller  8  operates to close the electromagnetic valve  3  mounted on the inert gas piping. 
     After the sequential operations described, the controller  8  causes its monitor to display that the supply of inert gas, having been discharged into the vertical portion  5 A of the optical system  5 , is stopped. Further, the controller  8  checks the degree of inert gas charge in the charge path  5 A, through the gas density sensor  14 A. As the inert gas charge level becomes lower than a predetermined value, the controller  8  causes its monitor to display that the inert gas charge level (density) has reached a safe level. Simultaneously therewith, it applies a door opening signal to the fixing mechanism  13 A. 
     As the fixing mechanism  13 A releases the door  5 C in response to the door opening signal, the controller  8  causes its monitor to display that the door  5 C is rendered into an openable state. On the basis of this display, the operator or serviceman may open the door  5 C to start the maintenance operation. 
     Similar operations in relation to the vertical portion  5 A apply to the maintenance operation for the horizontal portion  5 B of the optical system  5 . That is, first, a maintenance start command is inputted from the keyboard of the controller  8 . As the command is inputted, the controller  8  applies a laser light emission stopping signal to the excimer laser  1 . Simultaneously therewith, the controller  8  operates to close the electromagnetic valve  3  mounted on the inert gas piping. 
     After the sequential operations described, the controller  8  causes its monitor to display that the supply of inert gas, having been discharged into the vertical portion  5 A of the optical system  5 , is stopped. Further, the controller  8  checks the degree of inert gas charge in the charge path  5 B, through the gas density sensor  14 B. As the inert gas charge level becomes lower than a predetermined value, the controller  8  causes its monitor to display that the inert gas charge level (density) has reached a safe level. Simultaneously therewith, it applies a door opening signal to the fixing mechanism  13 B. 
     As the fixing mechanism  13 B releases the door  5 D in response to the door opening signal, the controller  8  causes its monitor to display that the door  5 D is rendered into an openable state. On the basis of this display, the operator or serviceman may open the door  5 D to start the maintenance operation. 
     The procedure for restoring the apparatus after the maintenance by the operator or serviceman is completed, is essentially the same as that of the first embodiment. 
     Although in the first and second embodiments, the inert gas density is measured by using gas density sensor means, the sensor means may be replaced by an inert gas flow meter for measuring the flow rate. 
     While the invention has been described with reference to examples of semiconductor device manufacturing exposure apparatus, the invention is similarly applicable to other types of semiconductor device manufacturing apparatuses using inert gas. 
     In accordance with the embodiments as described above, there is provided a system such as an exposure apparatus which uses an excimer laser and which needs filling with an inert gas, wherein sensor means for detecting an inert gas charge level is disposed in the portion filled with or to be filled with the inert gas, and wherein the system is controlled on the basis of an output of the sensor means. This avoids the risk of suffocation of an operator or serviceman by the inert gas or exposure to the inert gas, due to an error by the operator or serviceman. Thus, danger to the life of the operator or serviceman is reduced. 
     Further, the sufficiency of the degree of inert gas charge is detected automatically and a light emission signal is outputted to an excimer laser. This prevents emission of light in the state in which the inert gas is not charged for an error of the operator or serviceman. Thus, adhesion of impurities to the optical system or contamination of lenses of the optical system is avoided. This effectively reduces the volume and time of the maintenance operation of the apparatus, and it increases the operation efficiency of the apparatus and enhances the lifetime of the apparatus. 
     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.