Abstract:
An exposure apparatus includes a light source, one or two or more housings each for accommodating therein an optical element disposed along an exposure light path extending from the light source to a substrate, a first substitution system for substituting the interior of the housing with an inert gas ambience, and a second substitution system for substituting the interior of a holding mechanism for holding the optical element accommodated in the housing, with an inert gas ambience. The structure enables reduction in time for substitution of the exposure light path with an inert gas, and assures enlargement of a throughput of the exposure apparatus.

Description:
FIELD OF THE INVENTION AND RELATED ART  
         [0001]    This invention relates generally to an exposure apparatus for projecting and printing a circuit pattern formed on a mask, onto a substrate being coated with a photosensitive material, in a reduced scale. More particularly, the invention is concerned with an exposure apparatus which uses deep ultraviolet light or an excimer laser as an exposure light source.  
           [0002]    Reduction type projection exposure apparatuses are used in a process of manufacturing a semiconductor device which is formed with a very fine pattern such as LSI or VLSI. Miniaturization of a pattern has been required strongly due to increases in the integration density of a semiconductor device, and exposure apparatuses have been modified to meet such miniaturization, as well as improvements in a resist process.  
           [0003]    The resolving power of an exposure apparatus can be improved by two methods, that is, a method in which the exposure wavelength is shortened, and a method in which the numerical aperture (NA) of a projection optical system is enlarged. Generally, the resolution is proportional to the exposure wavelength and it is inversely proportional to the NA. Besides the improvement of resolution, many attempts have been made to keeping the depth of focus of a projection optical system. Generally, the depth of focus is proportional to the exposure wavelength, and it is inversely proportional to the square of NA. Thus, improving the resolution and keeping the depth of focus are contradictory matters. As an attempt to solving such problem, a phase shift method and a FLEX (Focus Latitude Enhancement Exposure) method, for example, have been proposed.  
           [0004]    As regards the exposure wavelength, recently, KrF excimer lasers having an emission wavelength of about 248 nm are prevalently used in place of i-line of 365 nm. Also, ArF excimer lasers having an emission wavelength of about 193 nm are currently being developed, as a next generation exposure light source.  
           [0005]    From the viewpoint of the production cost of a semiconductor device, further improvements in the throughput of an exposure apparatus have been attempted. For example, the power of an exposure light source is enlarged to thereby shorten the exposure time per one shot. Another example is enlarging the exposure area to thereby increase the number of chips per one shot.  
           [0006]    In recent years, in order to meet the requirement of enlargement in chip size of a semiconductor device, the stream is shifting from step-and-repeat type exposure apparatuses (steppers) in which a mask pattern is printed sequentially in association with stepwise motion, to step-and-scan type exposure apparatuses in which a mask and a wafer are scanningly exposed in synchronism with each other, followed by stepwise motion to place a subsequent shot. In such step-and-scan type exposure apparatuses, the exposure field has a slit-like shape and, therefore, the exposure area can be enlarged without enlargement in size of the projection optical system.  
           [0007]    Where ultraviolet light is used as an exposure light source, as described above, there may occur a phenomenon that, due to long-period use, ammonium sulfate (NH 4 ) or silicon dioxide (SiO 2 ) is deposited on the surface of an optical element disposed on the light path, to cause considerable degradation of the optical characteristic. The deposition is produced because of chemical reaction of ammonia (NH 3 ), sulfurous acid (SO 2 ) or silicon compound contained in the surrounding ambience caused in response to irradiation with ultraviolet light. In order to prevent such deterioration of optical elements, conventionally, the whole of the light path is purged by use of a clean dry air or an inert gas such as nitrogen.  
           [0008]    As regards deep ultraviolet light, particularly, ArF excimer lasers having a wavelength of about 193 nm, it is known that there are plural absorbing bands for oxygen (O 2 ) in the bandwidth about that wavelength. Also, ozone (O 3 ) will be produced when oxygen absorbs light, and this ozone acts to increase light absorption, causing considerable decrease of transmission factor. Additionally, various products, as described above, attributable to the ozone will be deposited on the surface of an optical element, thus causing a decrease of the efficiency of the optical system.  
           [0009]    In consideration of it, in an exposure optical system for projection exposure apparatuses having a deep ultraviolet light source such as an ArF excimer laser, for example, purge means using an inert gas such as nitrogen, for example, may be provided to keep the oxygen density along the light path at a low level.  
           [0010]    An example of such inert gas purge means for an illumination optical system in a projection exposure apparatus, will be described with reference to FIG. 8.  
           [0011]    As illustrated in the drawing, there are an excimer laser  201 , and a container or housing  202  for the illumination optical system. Further, there are a reticle  203  and mirrors  204 ,  205  and  206 . Denoted at  207  is a beam shaping optical system, and denoted at  208  is an optical integrator. Also, there are condenser lenses  209 ,  210  and  211 .  
           [0012]    A laser beam emitted by the excimer laser  201  is shaped by the beam shaping optical system  207  into a predetermined beam shape. Thereafter, the light enters the optical integrator  208  and, in response, secondary light sources (not shown) are produced near the light exit surface of the optical integrator  208 . The light rays from the secondary light sources are directed through the condenser lenses  209 ,  210  and  211  to uniformly illuminate the reticle  203 . Thus, the arrangement provides a Koehler illumination optical system.  
           [0013]    In order to provide an inert gas ambience around the optical elements described above and along the light path of them, inert gas supply means (not shown) supplies a nitrogen gas, for example, into the housing  202  through a gas inlet port  202   a . The thus applied inert gas flows through the interior of the illumination optical system. After substitution to remove any residual gas such as atmospheric gas, for example, the inert gas is discharged outwardly through a gas outlet port  202   b , by gas discharging means (not shown).  
           [0014]    The gas supply quantity may be controlled so as to minimize the substitution time by the inert gas, to thereby increase the system throughput, or minimize the consumption quantity of the inert gas after substitution, to thereby decrease the system running cost (Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994).  
           [0015]    On the other hand, a currently prevailing illumination method is a variation illumination method (e.g., Japanese Laid-Open Patent Application, Laid-Open No. 204114/1994) wherein the distribution of secondary light source as described above is changed in various ways. This is to accomplish both of a high resolution and a large depth of focus. In order that the illumination condition is made variable, many optical elements of an illumination optical system should be made interchangeable. With the above-described inert gas substitution method, in that occasion, it is very difficult to forcibly substitute the inside space of a mechanism (barrel) for holding optical elements to be interchanged. Particularly, in a case where an ArF excimer laser having an emission wavelength about 193 nm is used, there is a problem, as described, that the light absorption occurs due to any oxygen remaining along the light path which causes a serious decrease of optical efficiency. Therefore, forcible substitution of the interior of the movable barrel, if desired, needs a complicated structure for the gas flow passageway, and it causes an increase of the system cost as well as prolongation of the time for completion of the substitution which results in a decrease of the system throughput.  
         SUMMARY OF THE INVENTION  
         [0016]    It is accordingly an object of the present invention to accomplish reduction of a substitution time to an inert gas ambience along an exposure light path, still with a minimum cost, and thereby to increase the system throughput.  
           [0017]    In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: a light source; at least one (one or two or more) housing for accommodating therein an optical element disposed along an exposure light path extending from said light source to a substrate; first substitution means for substituting the interior of said housing with an inert gas ambience; and second substitution means for substituting the interior of a holding mechanism for holding the optical element accommodated in said housing, with an inert gas ambience.  
           [0018]    The second substitution means may preferably include control means for controlling an inert gas supply quantity in accordance with the state of substitution of the inert gas ambience inside said holding mechanism and the state of substitution of the inert gas ambience inside said housing.  
           [0019]    Each of the first and second substitution means may include control means for controlling an inert gas supply quantity, each control means being operable independently to set an inert gas supply quantity and a control operation timing.  
           [0020]    The holding mechanism may comprise a barrel for movably holding said optical element in said housing.  
           [0021]    The housing may accommodate therein the whole of or a portion of an illumination optical system for directing light from said light source to a reticle, and the holding mechanism may movably hold an optical element which may serve to variably or interchangeably set an illumination condition of said illumination optical system.  
           [0022]    The housing may comprise a barrel for a projection optical system, and the holding mechanism may movably hold a lens inside said projection optical system, for variably or interchangeably set an optical characteristic of said projection optical system.  
           [0023]    The light source may comprise a light source of one of deep ultraviolet light and excimer laser.  
           [0024]    In accordance with another aspect of the present invention, there is provided a device manufacturing method including a process for producing a device by use of an exposure apparatus as recited above.  
           [0025]    In accordance with a further aspect of the present invention, there is provided an exposure method, comprising the steps of: preparing at least one housing for accommodating therein an optical element disposed along an exposure light path extending from a light source to a substrate; substituting, by use of first substitution means, the interior of the housing with an inert gas ambience; and substituting, by use of second substitution means, the interior of a holding mechanism for holding the optical element accommodated in the housing, with an inert gas ambience, whereby the inside of the housing is substituted with an inert gas ambience.  
           [0026]    In the exposure method described above, the second substitution means may be used to control an inert gas supply quantity in accordance with the state of substitution of the inert gas ambience inside the holding mechanism and the state of substitution of the inert gas ambience inside the housing.  
           [0027]    Alternatively, each of the first and second substitution means may include control means for controlling an inert gas supply quantity, each control means being operable independently to set an inert gas supply quantity and a control operation timing.  
           [0028]    Further, the holding mechanism may comprise a barrel for movably holding the optical element in the housing.  
           [0029]    In the exposure method described above, the housing may accommodate therein the whole of or a portion of an illumination optical system for directing light from the light source to a reticle, and the holding mechanism may movably hold an optical element which may serve to variably or interchangeably set an illumination condition of the illumination optical system.  
           [0030]    In the exposure method described above, the housing may comprise a barrel for a projection optical system, and wherein the holding mechanism movably holds a lens inside the projection optical system, for variably or interchangeably set an optical characteristic of the projection optical system.  
           [0031]    In the exposure method described above, the light source may comprise a light source of one of deep ultraviolet light and excimer laser.  
           [0032]    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  
       [0033]    [0033]FIG. 1 is a schematic view of a projection exposure apparatus according to an embodiment of the present invention.  
         [0034]    [0034]FIG. 2 is a schematic view for explaining scan exposure to be made in the exposure apparatus of FIG. 1.  
         [0035]    [0035]FIG. 3 is a schematic view for explaining details of an input lens portion in the exposure apparatus of FIG. 1.  
         [0036]    [0036]FIG. 4 is a schematic and diagrammatic view of an inert gas supplying and discharging system in the exposure apparatus of FIG. 1.  
         [0037]    [0037]FIGS. 5A, 5B,  5 C,  5 D,  5 E and  5 F are graphs, respectively, for explaining a nitrogen flow rate and a change in the state of nitrogen substitution, in the exposure apparatus of FIG. 1.  
         [0038]    [0038]FIG. 6 is a schematic and diagrammatic view of a modified example of the gas supplying and discharging system of FIG. 4.  
         [0039]    [0039]FIG. 7 is a schematic view of a projection exposure apparatus according to another embodiment of the present invention.  
         [0040]    [0040]FIG. 8 is a schematic view of a conventional projection exposure apparatus.  
         [0041]    [0041]FIG. 9 is a flow chart of microdevice manufacturing processes.  
         [0042]    [0042]FIG. 10 is a flow chart for explaining details of a wafer process in the procedure shown in FIG. 9. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
         [0044]    [First Embodiment] 
         [0045]    [0045]FIG. 1 shows a first embodiment of the present invention.  
         [0046]    Denoted in the drawing at  1  is a deep ultraviolet light source such as an ArF excimer laser, for example, and denoted at  2  is a mirror. There are a beam shaping optical system  3  and an input lens  4 . Further, there are an imaging lens  5 , a mirror  6 , an optical integrator  7 , a stopper  8 , a condenser lens  9 , and a blind  10 . Denoted at  11  is another condenser lens, and denoted at  12  is a mirror. Denoted at  13  is a condenser lens.  
         [0047]    A laser beam emitted by the ArF excimer laser  1  is directed via the mirror  2  to the beam shaping optical system  3 , by which the light is shaped into a predetermined beam shape. Thereafter, the light goes via the input lens  4 , the imaging lens  5  and the mirror  6 , and it enters the optical integrator  7  which comprises small lenses arrayed two-dimensionally. In response, secondary light source images are produced near the light exit surface  7   a  of the optical integrator  7 . There is the stopper  8  disposed adjacent the plane where the secondary light sources are produced. Thus, by changing the stopper  8  by another in association with interchanging the input lens  4 , a desired distribution of secondary light sources can be produced. Denoted at  14  and  15  are actuators for the switching drive of the input lens and the stopper, respectively.  
         [0048]    The light from the secondary light sources is collected by the condenser lens  9 . Adjacent a plane orthogonal to the optical axis and containing the point of light convergence defined by the condenser lens, there is the blind  10  disposed which functions to determine the illumination range for a mask  16 . The light from the light convergence plane goes via the condenser lenses  11  and  13  and the mirror  12 , such that a Koehler illumination optical system for illuminating the mask  16  uniformly is provided.  
         [0049]    The whole illumination optical system described above is accommodated in a container or housing  17 , so that they are isolated against gas communication with the outside atmosphere.  
         [0050]    Denoted at  18  is a mask stage, and denoted at  19  is a mirror. Denoted at  20  is an interferometer, and denoted at  22  is a projection optical system. Denoted at  23  is a wafer, and denoted at  24  is a wafer chuck. Denoted at  25  is a wafer stage, and denoted at  26  is a mirror. Denoted at  27  is an interferometer.  
         [0051]    As shown in FIG. 2, the illumination optical system described above serves to illuminate a portion of a pattern  28  formed on the mask  16 , with slit-like light  29  (i.e., slit illumination). The portion of the pattern  28  is projected by the projection optical system  22  onto the wafer  23 , in a reduced scale.  
         [0052]    Here, the mask  16  and the wafer  23  are scanningly moved relative to the projection optical system  22  and the slit-like illumination  29 , in opposite directions as depicted by arrows in the drawing, at a speed ratio corresponding to the reduction magnification of the projection optical system  22 , while on the other hand multiple-pulse exposure based on the pulse light from the ArF excimer laser is repeated. In this manner, the whole pattern  28  on the mask  16  can be transferred to a single or plural chip regions on the wafer  23 .  
         [0053]    Referring back to FIG. 1, denoted at  18  is a mask stage for holding a mask  16  thereon. It can be scanningly moved in a direction of an arrow C, by means of a driving system (not shown). Denoted at  19  is a mirror fixedly mounted on the mask stage  18 , and denoted at  20  is a laser interferometer for detecting the movement speed of the mask stage  18 . Denoted at  24  is a wafer chuck for holding a wafer  23  thereon, and denoted at  25  is a wafer stage for holding the wafer chuck  24  thereon. It can be scanningly moved in a direction of an arrow D, by means of a driving system (not shown). Denoted at  26  is a mirror fixedly mounted on the wafer stage  25 , and denoted at  27  is a laser interferometer for detecting the movement speed of the wafer stage  25 .  
         [0054]    Denoted at  31  is inert gas supplying means which operates, in this embodiment, to provide a gas supply to the housing  17  of the illumination optical system as well as to two locations in the projection optical system  22 , as illustrated. The gas supply to the housing  17  of the illumination optical system is based on a supply system  32 , connected to a portion near the laser emission end of the ArF excimer laser  1 . Thus, the supplied gas flows along the laser light path while passing the optical elements sequentially, so that the gas inside the housing  17  is discharged outwardly. Finally, the gas is discharged outwardly, from a portion near the condenser lens  13  and through an evacuation system  33 , by means of evacuation means  34 .  
         [0055]    The supply of an inert gas to the projection optical system  22  is made through a supply system  35  connected at an end of the projection optical system  22 . The gas passes inside optical elements (not shown) sequentially, and it is discharged outwardly from the other end and through an evacuation system  36 , by use of the evacuation means  34 .  
         [0056]    Denoted at  37  is another inert gas supplying means which operates, in this embodiment, to provide gas supply to the input lens  4 , separately, with use of a supply system  38 .  
         [0057]    [0057]FIG. 3 illustrates details of the input lens  4  and components around it. Denoted in the drawing at  41   a - 41   b  are optical elements, among the optical elements of the beam shaping optical system  3 , described with reference to FIG. 1. Denoted at  42  is a case, and denoted at  43   a  and  43   b  are spacers. Denoted at  44  is a holding ring, and denoted at  45   a - 45   b  are pipings. Denoted at  46  is a container or housing, and denoted at  47  is a gas deflecting plate.  
         [0058]    Denoted at  48   a  and  48   b  are optical elements, among the optical elements of the imaging lens system  5  having been described with reference to FIG. 1. Denoted at  49  is a case, and denoted at  50  is a spacer. Denoted at  51   a  and  51   b  are pipings.  
         [0059]    Denoted at  14  is an actuator having been described with reference to FIG. 1. Denoted at  52  is a housing, and denoted at  53  is a shaft. Denoted at  54  is a sealing member, and denoted at  55  is a bearing. Denoted at  56  is a rotary plate. Denoted at  57   a  and  57   b  are first input lens elements, and denoted at  58  is a barrel. Denoted at  59   a  and  59   b  are holding rings, and denoted at  60  is a gas outlet port. Denoted at  61   a  and  61   b  are second input lens elements, and denoted at  62  is a barrel. Denoted at  63   a  and  63   b  are holding rings, and denoted at  64  is a gas outlet port.  
         [0060]    The functions and operations of these components will be described with reference to FIG. 3.  
         [0061]    As has been described with reference to FIG. 1, the light beam passing through the beam shaping optical system is shaped into a predetermined beam shape. The components of this optical system at a trailing end thereof are the optical elements  41   a  and  41   b  shown in FIG. 3. These optical elements  41   a  and  41   b  as well as the other optical elements (not shown) which are components of the beam shaping optical system are all accommodated in the case  42 . In this embodiment, these elements are mounted in an order of the spacer  43   a , the optical element  41   a , the spacer  43   b  and the optical element  42   b . Also, these elements are held fixed by means of the holding ring  44  which is thread-engaged with the inside circumferential surface of the case  42 , at an end thereof. The case  42  and the spacers  43   a  and  43   b  are provided with gas communication bores. Thus, the inert gas flowing along the light path, from the light source side, reaches the optical element  43   a  portion as depicted by an arrow A in the drawing. Thereafter, the gas flows through the communication bores and the pipings  45   a  and  45   b , so that it is directed into the housing  46  while performing inert gas substitution of the space for the optical elements  41   a  and  41   b . The deflecting plate  47  is disposed so that the inert gas thus introduced can flow throughout the housing  46  without stagnation. The inert gas passed through the housing  46  is directed by means of a piping  51   a  to the space between the optical elements  48   a  and  48   b  and, after inert gas substitution of that space, it flows through a piping  51   b  toward-a succeeding optical system (not shown).  
         [0062]    These optical elements  48   a  and  48   b  as well as the other optical elements (not shown) which are components of the imaging lens system are all accommodated in the case  49 . In this embodiment, these elements are mounted in an order of the optical element  48   a , the spacer  50  and the optical element  48   b . Also, these elements are held fixed inside the case  49 , by use of fixing means (not shown). The connection between the case  42  and the housing  46  as well as the connection between the housing  46  and the case  42  are gas-tightly closed and isolated against gas communication with the outside atmosphere. Therefore, there occurs no outward leakage of the inert gas.  
         [0063]    The first input lens elements  57   a  and  57   b  are mounted at opposite ends of the barrel  58 , and they are held fixed by means of the holding rings  59   a  and  59   b  each being thread-engaged with the inside circumferential surface of the barrel  58 , at an end thereof. Similarly, the second input lens elements  61   a  and  61   b  are mounted at opposite ends of the barrel  62 , and they are held fixed by means of the holding rings  63   a  and  63   b  each being thread-engaged with the inside circumferential surface of the barrel  62 , at an end thereof. The first input lens and the second input lens elements are provided by different optical elements. Thus, by selectively and interchangeably inserting either the first or second input lens into the laser light path, the intensity distribution of the laser beam impinging on the optical integrator  7  through the imaging lens  5  of FIG. 1, can be controlled.  
         [0064]    The interchanging mechanism for the input lenses will now be described in detail.  
         [0065]    The barrels  58  and  62  are fixedly mounted on the rotary plate  56 . The rotary plate  56  is connected to the actuator  14  through the shaft  53 . Further, the shaft  53  is supported by the housing  52  through the bearing  55 . Also, the actuator  14  is fixedly mounted on a holder, not shown. The positioning of the first and second input lenses is performed by using an angular sensor (not shown) accommodated in the actuator.  
         [0066]    The housing  52  is provided with a gas inlet port  65  to which a gas supply is made separately by the inert gas supplying means  37  shown in FIG. 1. An inert gas is thus supplied as depicted by an arrow B, and it is directed to an inside circumferential groove  66  which is formed in the inside circumferential surface of the housing  52  and which is communicated with the inlet port  65 .  
         [0067]    The shaft  53  is provided with a gas communication bore  67  being communicated with the groove  66 . Thus, even when the shaft  53  rotates, the communication with the groove  66  is kept. There is a sealing member  54  as illustrated, between the shaft  53  and the housing  52 , such that rotational motion can be made without leakage of inert gas in the communication between the groove  66  and the bore  67 .  
         [0068]    The gas communication bore  67  is further communicated with a communication bore  68  which is formed in the rotary plate  56 . Thus, inert gases can be introduced through the communication bores  69  and  70  of the barrels  58  and  62 , respectively, into the barrels  58  and  62 , respectively. The gas having substituted the space between the first input lens elements  57   a  and  57   b  is discharged from a gas outlet port  60 , while the gas having substituted the space between the second input lens elements  61   a  and  61   b  is discharged from a gas outlet port  64 , respectively, both being directed into the housing  46 .  
         [0069]    As compared with the gas flow rate as supplied from the inert gas supply means  31  of FIG. 1 (arrow A), the gas flow rate as supplied from the inert gas supply means  37  of FIG. 1 (arrow B) is very small. Namely, while the gas of arrow A should function to perform the substitution of the inside of the housing  17  for the whole illumination optical system of FIG. 1, the gas of arrow B is used for the substitution of only the inside of the barrels  58  and  62  of the first and second input lenses. Thus, as compared with the inside volume of the housing  17 , the inside volumes of the barrels  58  and  62  are very small. For this reason, there is substantially no possibility that the flow of gas discharged from the outlet ports  60  and  64  applies a large adverse influence to the flow of gas along the arrow A to cause a decrease of substitution efficiency. However, as regards the shape of the discharging ports  60  and  64 , it should preferably be determined so as not to make the gas flow throughout the housing  46 , unstable.  
         [0070]    Details of the inert gas supply means  31  and  37  shown in FIG. 1 will now be described, with reference to FIG. 4.  
         [0071]    [0071]FIG. 4 is a block diagram of a system, from inert gas supply means to units to which gases should be supplied. Denoted at  81  is a gas supply line which is connected to a supply source (not shown) of nitrogen (inert gas). This line is forked into two, i.e., one connected to the inert gas supply means  31  and another connected to the inert gas supply means  37 .  
         [0072]    The inside structure of the inert gas supply means  31  will be described first. The gas supply line  81  further separated into lines  82  and  83 . The line  82  is connected to a first electromagnetic valve  84  and, after passing through a first pressure gauge  85 , it is branched and connected to throttle valves  86  and  87 , respectively. The gas flow rate setting for the throttles  86  and  87  will be described later. The throttle  86  is connected to the supply system  32 , while the throttle  87  is connected to the supply system  35 .  
         [0073]    On the other hand, the line  83  is connected to a second electromagnetic valve  88  and, after passing through a second pressure gauge  89 , it is branched and connected to throttle valves  90  and  91 , respectively. The throttle  90  is connected to the supply system  32 , while the throttle  91  is connected to the supply system  35 .  
         [0074]    Next, the inside structure of the inert gas supply means  37  will be described.  
         [0075]    The line branched from the gas supply line  81  is connected to a third electromagnetic valve  92  and, after passing through a third pressure gauge  93 , it is connected to a throttle valve  94 .  
         [0076]    As has been described with reference to FIG. 1, the supply system  32  is connected to the housing  17  which accommodates the whole illumination optical system therein, and, through the gas discharging system  33 , it is connected to the gas evacuation means  34 . Also, the supply system  35  is connected to the projection optical system  22  and, through the gas discharging system  36 , it is connected to the evacuation means  34 . The throttle valve  94  is connected to the input lens  4  disposed inside the housing  17  for the illumination optical system, and the gas from the input lens  4  is discharged through the housing  17  and through the discharging system  33 , into the evacuation means  34 .  
         [0077]    The evacuation means  34  is, in turn, connected to an evacuation instrument (not shown) through an evacuation line  95 .  
         [0078]    The settings and operations of the components will be described below, in conjunction with FIG. 4.  
         [0079]    The supply of nitrogen starts as the first and third electromagnetic valves  84  and  92  are opened in response to signals from a controller (not shown). Here, the second electromagnetic valve  88  is kept closed. The pressure gauges  85  and  93  are connected to the controller (not shown), and they function to check whether a predetermined pressure is reached as the first and third electromagnetic valves  84  and  92  are opened. If any disorder occurs in the gas supply system and the predetermined pressure is not accomplished, a signal of malfunction is applied to the controller, in response to which an appropriate reaction such as interruption of operation is made.  
         [0080]    The throttles  86  and  87  are set to their optimum flow rate levels so that the gas, such as atmospheric air, inside the projection optical system can be substituted with nitrogen, in a necessary and shortest time.  
         [0081]    As regards the throttle  94 , the flow rate is set to such level that the substitution of the inside of the input lens can be completed before the substitution of the housing  17  is accomplished as described above, thus substantially enabling the light emission of the ArF excimer laser.  
         [0082]    The unshown controller operates to close the first and third electromagnetic valves  84  and  92  after elapse of a predetermined time and, on the other hand, it operates to open the second electromagnetic valve  88 . The pressure gauge  89  detects whether a predetermined gas pressure is reached or not as the second electromagnetic valve  88  is opened. If any disorder occurs in the gas supply system and the predetermined pressure is not accomplished, a signal of malfunction is applied to the controller, in response to which an appropriate reaction such as interruption of operation is made.  
         [0083]    Since at that time the substitution of the interiors of the housing  17  and the projection optical system  22  with a nitrogen gas has been accomplished, nitrogen may thereafter be supplied at a level that maintains this substitution state. Thus, when the set flow rate of the throttle  86  is denoted by Q 86 , and similarly the set flow rates of the throttles  87 ,  90  and  91  are denoted by Q 87 , Q 90  and Q 91 , respectively, then there may be relations of Q 90 &lt;Q 86  and Q 91 &lt;Q 87 .  
         [0084]    FIGS.  5 A- 5 F schematically illustrate the nitrogen flow rate and changes in the state of nitrogen substitution, in accordance with the procedure described above.  
         [0085]    Among these drawings, FIG. 5A shows the quantity of nitrogen supply to the housing  17 , wherein the axis of abscissa denotes time and the axis of ordinate denotes the nitrogen flow rate. At time to, the nitrogen supply starts. The flow rate is at Q 86  and is constant. At time ti, the nitrogen flow rate is changed to Q 90 . FIG. 5B shows changes in oxygen density, during this procedure, as an index of the state of nitrogen substitution inside the housing  17 . The axis of abscissa denotes time, and the axis of ordinate denotes the oxygen density within the housing  17 . The initial oxygen density at the nitrogen supply start time (t 0 ) is depicted as a level d 0 , and the oxygen density with which the exposure process can be started substantially without any inconveniences is depicted as a level d 1 . If the time where the oxygen density reaches the level d 1  is t 2 , the timing for changing the nitrogen flow rate may be set to satisfy a relation that t 2 ≦t 1 .  
         [0086]    [0086]FIG. 5C shows the quantity of nitrogen supply to the input lens  4 , and FIG. 5D shows changes in the oxygen density inside the input lens. In FIG. 5C, the nitrogen supply starts at time to. The flow rate is at a level Q 94 , and it is constant. At time t 3 , the nitrogen supply is interrupted. In FIG. 5D, the time where the oxygen density inside the input lens  4  reaches a level d 1  with which the exposure process can be started substantially without any inconveniences is denoted at t 4 . The timing for changing the nitrogen flow rate may be set to satisfy a relation t 4 ≦t 3 . It is seen from FIG. 5C that, even if the nitrogen supply is interrupted after time t 3 , the ambience surrounding the input lens  4  is the nitrogen ambience because it is placed inside the housing  17 , and that there does not occur undesirable degradation of the nitrogen substitution level.  
         [0087]    [0087]FIG. 5E shows the quantity of nitrogen supply to the projection optical system  22 , and FIG. 5F shows changes in oxygen density inside the projection optical system  22 . In FIG. 5E, the nitrogen supply starts at time t 0 . The flow rate is at a level Q 87 , and it is constant. At time t 1 , the flow rate of nitrogen is changed to a level Q 94 . In FIG. 5F, the time where the oxygen density inside the projection optical system  22  reaches a level d 1  with which the exposure process can be started substantially without any inconveniences is denoted at t 5 . The timing for changing the nitrogen flow rate may be set to satisfy a relation t 5 ≦t 1 .  
         [0088]    As described above, if the time until a predetermined nitrogen substitution level is accomplished is predetected, the switching timing for the electromagnetic valves  86 ,  87 ,  90 ,  91  and  92  shown in FIG. 4 can be set in the controller, as desired. Further, if it is desired to best optimize the timing for changing the nitrogen flow rate to thereby reduce the nitrogen consumption, appropriate substitution level monitoring means such as an oxygen density gauge, for example, may be disposed inside the housing  17 , the input lens  4  and/or the projection optical system  22 , so that the nitrogen flow rate may be changed by the controller, in accordance with an output of the monitor.  
         [0089]    Further, while the description has been made above with reference to an example where the nitrogen supply to the input lens  4  is interrupted or stopped at a predetermined time, if it is desired to continue supply of a very small amount of nitrogen, rather than stopping the nitrogen supply, a structure such as shown in FIG. 6 may be used. In FIG. 6, components corresponding to those of FIG. 4 are denoted by like reference numerals. Description therefor will be omitted here.  
         [0090]    In FIG. 6, denoted at  96  is a fourth electromagnetic valve, and denoted at  97  is a fourth pressure gauge. Denoted at  98  is a throttle valve. In this example, the nitrogen flow rate Q 94  through the throttle  94  and the nitrogen flow rate Q 98  through the throttle  98  may be placed in a relation Q 98 &lt;Q 94 . As the third electromagnetic valve  92  is closed by the controller (not shown), the fourth electromagnetic valve  96  is opened, whereby a predetermined amount of nitrogen is supplied to the input lens  4 .  
         [0091]    [Second Embodiment] 
         [0092]    [0092]FIG. 7 shows an embodiment wherein, in addition to the structure of the embodiment shown in FIG. 1, a system for separate nitrogen supply to a space for a particular lens inside the projection optical system  22  is added. In FIG. 7, components corresponding to those of FIG. 1 are denoted by like reference numerals. Description therefor will be omitted here.  
         [0093]    The projection optical system  22  has such structure to be described below.  
         [0094]    In FIG. 7, denoted at  101   a - 101   g  are lenses, and denoted at  102   a - 102   e  are lens holders. Denoted at  103  is a group holder. Denoted at  105  is a container or housing for the projection optical system, and denoted at  105  is an actuator. Denoted at  106  is a connector.  
         [0095]    The lens  101   a  is held by the lens holder  102   a . Similarly, the lenses  101   d ,  101   e ,  101   f  and  101   g  are held by the lens holders  102   b ,  102   c ,  102   d  and  102   e , respectively. Also, the lenses  101   b  and  101   c  are held by the group holder  103 , and they can be moved along an optical axis direction by means of the actuator  105 , for correction of aberration of the projection optical system  22 . The actuator  105  is connected to the group holder  103 , through the connector  106 .  
         [0096]    In this embodiment, the inert gas supply means  31  operates to supply a gas into the projection optical system housing  104 , at a portion near the top end of housing. Through gas communication bores formed in the lens holders  102   b ,  102   c  and  102   d , nitrogen substitution is performed sequentially to the lens spaces. Finally, the gas is discharged to the evacuation means  34 . Further, the inert gas supply means  37  functions to supply nitrogen into the group holder  103 . After nitrogen substitution of the space between the lenses  101   b  and  101   c  is accomplished, the gas is discharged into the housing  104  through a gas communication bore formed in the group holder  103 . Subsequently, like that described above, the gas passes sequentially through the lens spaces through gas communication bores formed in the lens holders, and finally it is discharged to the evacuation means  34 . As has been described with reference to the embodiment of FIG. 1, also in this embodiment, the flow rate of nitrogen from the inert gas supply means  37  is very small as compared with the nitrogen flow rate from the inert gas supply means  31 . For this reason, there is substantially no possibility that a gas discharged from the group holder  103  disturbs the flow of nitrogen inside the housing  104 .  
         [0097]    As regards the nitrogen supply to the group holder  103 , the supply may be stopped in a predetermined time period, like the first embodiment. Alternatively, the flow rate may be changed to a low level, rather than stopping the same.  
         [0098]    While two embodiments of the present invention have been described above, the object to which an inert gas should be supplied separately from the inert gas supply means  37  is not limited to an optical element which is to be driven, as described. For example, the separate inert gas supply may be made to any location where inert gas substitution is not easy because of the disposition of an optical element or elements or of any structure around it.  
         [0099]    Further, while the foregoing description has been made with reference to examples where nitrogen is used as an inert gas, any other inert gas may of course be used, with a result of similar advantageous effects of the invention.  
         [0100]    [Third Embodiment] 
         [0101]    Next, an embodiment of a semiconductor device manufacturing method which uses an exposure apparatus such as described above, will be explained.  
         [0102]    [0102]FIG. 9 is a flow chart of procedure for manufacture of microdevices such as semiconductor chips (e.g. ICs or LSIs), liquid crystal panels, CCDs, thin film magnetic heads or micro-machines, for example.  
         [0103]    Step  1  is a design process for designing a circuit of a semiconductor device. Step  2  is a process for making a mask on the basis of the circuit pattern design. Step  3  is a process for preparing a wafer by using a material such as silicon. Step  4  is a wafer process (called a pre-process) wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. Step  5  subsequent to this is an assembling step (called a post-process) wherein the wafer having been processed by step  4  is formed into semiconductor chips. This step includes an assembling (dicing and bonding) process and a packaging (chip sealing) process. Step  6  is an inspection step wherein operation check, durability check and so on for the semiconductor devices provided by step  5 , are carried out. With these processes, semiconductor devices are completed and they are shipped (step  7 ).  
         [0104]    [0104]FIG. 10 is a flow chart showing details of the wafer process.  
         [0105]    Step  11  is an oxidation process for oxidizing the surface of a wafer. Step  12  is a CVD process for forming an insulating film on the wafer surface. Step  13  is an electrode forming process for forming electrodes upon the wafer by vapor deposition. Step  14  is an ion implanting process for implanting ions to the wafer. Step  15  is a resist process for applying a resist (photosensitive material) to the wafer. Step  16  is an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above. Step  17  is a developing process for developing the exposed wafer. Step  18  is an etching process for removing portions other than the developed resist image. Step  19  is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer.  
         [0106]    With these processes, high density microdevices can be manufactured at a lower cost.  
         [0107]    In accordance with the embodiments of the present invention as described hereinbefore, inert gas substitution along an exposure light path can be performed satisfactorily. Consequently, the present invention can provide an exposure apparatus of higher efficiency and a higher throughput.  
         [0108]    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.