Abstract:
A gas supply unit supplies gas to a certain space via a channel, and includes a first switch mechanism located in the channel for selectively changing the channel of the gas.

Description:
[0001]     This application is a divisional application of co-pending U.S. application Ser. No. 10/446,328, filed May 27, 2003, entitled “Gas Supply Unit, Gas Supply Method And Exposure System”. Aforementioned, U.S. application Ser. No. 10/446,328, filed May 27, 2003, is incorporated by reference herein in its entirety.  
         [0002]     This application claims the right of priority under 35 U.S.C. § 119 based on Japanese Patent Application No. 2002-153008, filed on May 27, 2002, which is hereby incorporated by reference herein in its entirety as if fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates generally to gas supply units and methods for supplying inert gas to an exposure apparatus, and exposure systems using the same. In particular, the present invention is suitable for a gas supply unit, as well as to an exposure system, for supplying inert gas to an exposure light path of a projection exposure apparatus that uses far UV light and an excimer laser beam as a light source.  
         [0004]     Along with recent demands on smaller and lower profile electronic devices, fine semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. The conventional printing or photolithography for fabricating semiconductor devices has used a projection exposure apparatus.  
         [0005]     In general, a projection exposure apparatus includes an illumination optical system that uses light emitted from a light source to illuminate a mask, and a projection optical system arranged between the mask and an object to be exposed. For a uniform illumination area, the illumination optical system introduces light from a light source into a light integrator, such as a fly-eye lens composed of multiple rod lenses, and uses a light exit plane of the light integrator as a secondary light source plane to Koehler-illuminate the mask plane through a condenser lens.  
         [0006]     The minimum critical dimension to be transferred by the projection exposure apparatus (resolution) is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture of the projection optical system. The shorter the wavelength is, the better the resolution is.  
         [0007]     Accordingly, the light source in recent years has been in transition from an ultra-high pressure mercury lamp (g-line with a wavelength of approximately 436 nm) and i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). Practical use of F 2  excimer laser (with a wavelength of 157 nm) has been promoted.  
         [0008]     It is known that i-line or other exposure light with a shorter wavelength results in a photochemical reaction between the impurity in the air and oxygen (O 2 ) due to its short wavelength, which generates products to adhere to and opaque an optical element, such as a lens and a mirror in an optical system.  
         [0009]     The products typically include ammonium sulfate ((NH 4 ) 2 SO 4 ), for example, which is produced by sulfuric acid (SO 2 ) that reacts with oxygen in the air or oxidizes when it absorbs light energy and gets excited. Ammonium sulfate is whitish and opaques an optical element, such as a lens and mirror, when it adheres to a surface of the optical element. Ammonium sulfate disperses and absorbs the exposure light, and lowers the transmittance of an optical system, thus greatly reducing an exposure light intensity or transmittance down to an object to be exposed and throughput.  
         [0010]     The far UV light, such as excimer laser with a wavelength of 250 nm or less, particularly, ArF excimer laser having an oscillation wavelength of about 193 nm includes multiple oxygen absorption bands in this wavelength region. For example, as shown in  FIG. 10 , inert gas supplied from a plant facility  1100  is supplied to a tube port  1210  in an exposure apparatus  1200  to purge its optical system and reduce oxygen concentration in the exposure light path to a very low level for exposure light with a less absorbent and purified oscillation wavelength. Here,  FIG. 10  is a schematic block diagram of a conventional exposure apparatus.  
         [0011]     It is also known that the F 2  excimer laser with an oscillation wavelength of about 157 nm includes consecutive oxygen absorption bands in this wavelength region, and does not allow exposure light with a less absorbent wavelength to be selected like the ArF excimer laser. The vacuum UV light with a wavelength of about 157 nm includes continuous steam absorption bands that cannot be observed around 193 nm. The vacuum UV light with 157 nm is easily absorbed by ammonia (NH 3 ), carbon dioxide (CO 2 ), organic gases, etc., and a light absorption in the exposure light path increases substantially, which is not a problem for the vacuum UV light with a wavelength of 160 nm or less.  
         [0012]     A fluctuant concentration of a light absorbent material in the exposure light path during exposure would result in an error or discord of the actual exposure dose relative to the target exposure dose, and deteriorate the above throughput and an exposure-dose control precision.  
         [0013]     Accordingly, the impurity concentration should be monitored in gas constituents in the exposure light path in a projection exposure apparatus that uses the far UV light or excimer laser for controls over optical systems in their product adhesion, efficiency and the exposure dose.  
         [0014]     However, the conventional exposure apparatus shown in  FIG. 10  cannot detect the impurity concentration of the supplied gas, and might cause the projection exposure apparatus to accept the inert gas, etc. with an impermissible impurity concentration due to malfunctions etc. of the plant facility. The inert gas, etc., with an impermissible impurity concentration supplied to the exposure apparatus would cause the following disadvantages:  
         [0015]     (1) The light absorption increases in the exposure light path and considerably lowers the throughput of the apparatus. (2) The fluctuant light absorption in the exposure light path during an exposure operation causes a change or error in the actual exposure dose to the target exposure dose, and deteriorates the exposure-dose control accuracy. (3) Impurities in the inert gas, etc. in the exposure light path photochemically react and cause resultant products to adhere to an optical element, such as a lens and a mirror in an optical system. The products lower performance, such as the optical efficiency, and might require an exchange for an expensive optical element depending on adhesions. (4) The impurities adhere to a pipeline system for guiding the inert gas, etc. to the exposure light path, and might require its cleansing or exchange.  
       BRIEF SUMMARY OF THE INVENTION  
       [0016]     Accordingly, it is an exemplary object of the present invention to provide a gas supply unit and method, and an exposure system having the same, which detect the inert gas with an impurity concentration beyond a permissible value, and prevent the inert gas from entering the exposure apparatus.  
         [0017]     A gas supply unit of one aspect according to the present invention supplies gas to a certain space via a channel, and includes a first switch mechanism located in the channel for selectively changing the channel of the gas.  
         [0018]     The gas supply unit may further include a first detector, provided in the channel, for detecting an impurity concentration in the gas, wherein the first switch mechanism is located downstream in the channel from the first detector in a direction supplying the gas, the first switch mechanism switching the channel of the gas when the first detector detects an impermissible impurity concentration.  
         [0019]     The first switch mechanism switches the channel to a predetermined channel that has a filter for removing the impurity. The first switch mechanism may switch the channel to a predetermined channel connected to a reserve gas container that contains gas with a permissible impurity concentration. The gas supply unit may further include a first delay part, located between the first detector and the first changing mechanism, for delaying a flow of the gas.  
         [0020]     The gas supply unit may further include a second detector for detecting an impurity concentration of the gas that has passed through the filter, and a shut-off valve, provided between the second detector and the certain space, which shuts off the gas, the shut-off valve shutting off the gas when the second detector detects that the impermissible impurity concentration. The gas supply unit may further includes a second switch mechanism, located downstream from the shut-off valve in a direction supplying the gas in the channel, for switching the channel, the switch mechanism selecting another channel that is connected to a unit for supplying the gas with a permissible impurity concentration, the second switch mechanism switching the channel to the different channel via when the shut-off valve shuts off the channel.  
         [0021]     The gas supply unit may further include a controller for controlling operations of the first or second switch mechanism based on a detection result of the first or second detector. The impurity may include one or more of ammonia, carbon oxide, organic substances, inorganic substances, oxygen, and water. The gas supply unit may further include a second delay part, located between the second detector and the shut-off valve, for delaying a flow of the gas. The first or second delay part may be a delay tube or a tank. The gas supply unit may further include an exhaust part, located between the shut-off valve and the certain space, for exhausting the gas. The gas supply unit may further include an alarm that notifies of the impermissible impurity concentration of the gas. The gas supply unit may further include a power supply of uninterruptible power.  
         [0022]     A gas supply method of another aspect of the present invention that detects an impurity concentration of a gas in a certain space, and switches a gas supply channel such that the certain space has a permissible impurity concentration of the gas includes the steps of storing information on a permissible value, comparing the permissible value stored in the storing step with a detected impurity concentration, and switching the supply channel based on a result of the comparing step. The gas supply method may include the step of stopping supplying the gas to the supply channel based on the result of the comparing step.  
         [0023]     An exposure system of still another aspect includes the above gas supply unit, and an exposure apparatus that exposes an object by using ultraviolet light, far infrared light and vacuum ultraviolet light as exposure light, and the channel filled with the gas supplied by the gas supply unit.  
         [0024]     A device fabrication method of still another aspect of the present invention includes the steps of exposing an object using the above exposure system, and performing a predetermined process for the projected and exposed object. Claims for a device fabrication method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.  
         [0025]     Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a schematic block diagram of an exposure system of a first embodiment according to the present invention.  
         [0027]      FIG. 2  is a view showing an inert-gas flow introduced into the gas supply unit shown in  FIG. 1  with a permissible inert-gas impurity concentration.  
         [0028]      FIG. 3  is a view showing an inert-gas flow introduced into the gas supply unit shown in  FIG. 1  with an impermissible inert-gas impurity concentration.  
         [0029]      FIG. 4  is a view showing an inert-gas flow with impermissible impurity concentration that passes through a filter shown in  FIG. 1 .  
         [0030]      FIG. 5  is a view showing cleansing-gas and inert-gas flows when impurities are cleansed from the pipeline in the gas supply unit shown in  FIG. 1 .  
         [0031]      FIG. 6  is a schematic block diagram of a gas supply unit as a variation of the gas supply unit shown in  FIG. 1 .  
         [0032]      FIG. 7  is a schematic block diagram of an exposure system of a second embodiment according to the present invention.  
         [0033]      FIG. 8  is a flowchart for a device fabrication method including an inventive exposure system.  
         [0034]      FIG. 9  is a flowchart for a wafer process shown in  FIG. 8 .  
         [0035]      FIG. 10  is a schematic block diagram of a conventional exposure apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     A description will now be given of an exposure system as one embodiment according to the present invention with reference to accompanying drawings. However, the present invention is not limited to this embodiment, but each element may be replaced with an alternative element within the spirit and scope of the present invention. Here,  FIG. 1  is a schematic block diagram of an exposure system  1  as a first embodiment of the present invention. As shown in  FIG. 1 , the exposure system  1  includes a plant facility  100 , a gas supply unit  200 , a power supply  300 , an exhaust facility  400 , a spare gas supply unit  500 , and an exposure apparatus  700 . The exposure system  1  of the present embodiment is a system that supplies the exposure apparatus  700  with inert gas for exposure with an impurity concentration equal to or less than a permissible value from the plant facility  100  via the gas supply unit  200 .  
         [0037]     Although the instant embodiment describes as if the gas supply unit, the spare gas supply unit, the exhaust facility, etc. are separate members from the exposure apparatus and the exposure system has all of them, the exposure apparatus may include the gas supply unit, the spare gas supply unit, the exhaust facility, etc. However, the factory facility  100  (for supplying inert gas and clean gas) is preferably a separate member from the exposure apparatus.  
         [0038]     The plant facility  100  produces gas supplied to the exposure apparatus  700  via the gas supply unit  200 . The gas produced by the plant facility  100  is inert gas or clean dry air with a permissible concentration of impurities including ammonia (NH 3 ), carbon dioxide (CO 2 ), organic and inorganic matters, oxygen (O 2 ), and water (H 2 O). The instant embodiment separately forms the plant facility  100  that produces the inert gas and the gas supply unit  200 , but they may be integrated into one body so as to serve as the gas supply unit  200 , which will be described later.  
         [0039]     The gas supply unit  200  detects an impurity concentration of gas produced from the plant facility  100  and supplies the exposure system  700  with the inert gas with a permissible impurity concentration. The gas supply unit  200  includes a port  210 , a first detector  220 , a delay tube  230 , a channel  240 , a second detector  250 , a delay tube  260 , a shut-off valve  270 , valves  280  and  282 , a controller  290  and an alarm  292 . Preferably, the delay tubes  230  and  260  at least partially have an S shape.  
         [0040]     The port  210  is connected to the plant facility  100 , and separates the plant facility  100  and the gas supply unit  200  from each other. The port  210  introduces the inert gas produced by the plant facility  100  to the gas supply unit  200 .  
         [0041]     The first detector  220  detects the concentration of one or more of ammonia, carbon dioxide, organic and inorganic substances, oxygen, and water as impurities contained in the inert gas introduced from the port  210 . The first detector  220  feeds a detected impurity-concentration result of the inert gas to the controller  290 . The first detector  220  may use a dry ammonia analyzer, a zirconia oxygen densitometer, a thin-film aluminum oxide moisture meter, etc.  
         [0042]     The delay tube  230  is arranged between the first detector  220  and the channel  240 , which will be described later. The delay tube  230  is a first delay member that delays a flow of the inert gas with an impermissible impurity concentration (hereinafter called “contaminated inert gas”) until a supply channel for the inert gas is switched to the channel  240  so that the contaminated inert gas may not enter the exposure apparatus  700 .  
         [0043]     The channel  240  includes a filter  242 , and valves  244  and  246 , and serves to remove the impurities of the contaminated gas. More specifically, when the first detector  220  detects that the gas supplied to the gas supply unit  200  from the plant facility  100  is the contaminated inert gas, the valve  244  switches the supply channel for the contaminated inert gas to the channel  240  and the filter  242  removes the impurity. The filter  242  may use a chemisorption refiner, and a porous substance, such as activated carbon and zeolite. The inert gas whose impurities are removed by the filter  242  returns to the original supply channel, and enters the second detector  250 . The controller  290 , which will be described later, switches the supply channel for the inert gas.  
         [0044]     The second detector  250  detects a concentration of impurities, e.g., one or more of ammonia, carbon dioxide, organic and inorganic substances, oxygen, and water contained in the inert gas which have been removed by the filter  242  located in the channel  240 . The second detector  250  determines whether the filter  242  has removed the impurity of the contaminated inert gas and whether the impurity concentration becomes permissible. The second detector  250  feeds an impurity concentration result of the inert gas to the controller  290 . The controller  290  includes a microprocessor etc., determines whether the impurity concentration sent from the first and second detectors  220  and  250  is permissible or equal to or less than a predetermined value, and switches the valves if it is impermissible.  
         [0045]     The delay tube  260  is arranged between the second detector  250  and the shut-off valve  270 , which will be described later. The delay tube  260  is a second delay member that delays a flow of the inert gas that has passed through the filter  242  located in the channel  240  and its impurity concentration exceeds the permissible value, or when the filter  242  does not remove impurity sufficiently until the shut-off valve  270  works to prevent the contaminated gas from entering the exposure apparatus  700 .  
         [0046]     The shut-off valve  270  stops supplying the inert gas to the exposure apparatus  700  when the second detector  250  detects that the inert gas having passed through the channel  240  is contaminated.  
         [0047]     The valve  280  switches channels for flowing cleansing gas to the exhaust the facility  400  when the impurity adheres to the pipeline system in the gas supply unit  200 , and the pipeline system needs to be cleaned.  
         [0048]     The valve  282  switches connections so that the inert gas may be supplied to the exposure apparatus  700  from the spare gas supply unit  500  to allow the exposure apparatus  700  to continue actions while the impurity that sticks to the tube in the gas supply unit  200  is cleansed from the tube.  
         [0049]     The controller  290  is connected to and controls the first detector  220 , the second detector  250 , the valves  244 ,  246 ,  280 , and  282 , and the shut-off  270  in the gas supply unit  200 . The controller  290  receives the impurity concentrations of the inert gas detected by the first and second detectors  220  and  250 , compares them with the permissible value, and, controls switching actions of the valves  244 ,  246 ,  280  and  282 , and the shut-off valve  270  based on these comparison results. The controller  290  has stored a permissible impurity-concentration value of the inert gas as a threshold in advance. A detailed description of its operations will be given later.  
         [0050]     The alarm  292  informs via sounds, light, displays, etc. that the first and second detectors  220  and/or  250  have detected an impermissible impurity concentration of the inert gas. The alarm  292  can also inform of an operational status, such as cleaning and pipeline cleansing of the current gas supply unit  200 , and identify a supply channel for the inert gas.  
         [0051]     The power supply  300  supplies all parts of the gas supply unit  200  with power. The power supply  300  is an uninterruptible one for privately generating electric power to prevent the gas supply unit  200  from stopping its actions due to a power failure, etc. during actions of the exposure system  1  and the contaminated inert gas from entering the exposure apparatus  700  by mistake.  
         [0052]     The exhaust facility  400  exhausts cleansing gas via the valve  280  when the impurity adheres to the tube of the gas supply unit  200 , and the tube needs to be cleaned.  
         [0053]     The spare gas supply unit (gas container)  500  supplies the exposure apparatus  700  with inert gas via the valve  282  in order to allow the exposure apparatus  700  to continue its actions even when the impurity has adhered to the tube and tube is being cleansed. The spare gas supply unit  500  may be of the same structure as that of the gas supply unit  200 , or it may be a unit that supplies the inert gas whose purity is assured in advance.  
         [0054]     Referring to  FIGS. 2-5 , a description will be given of the gas supply unit  200 &#39;s operation and a flow of inert gas that fills the exposure light path in the exposure apparatus  700 . Here,  FIG. 2  is a view showing a flow of inert gas with a permissible impurity concentration, which has been introduced from the plant facility  100  to the gas supply unit  200  in a normal state. As illustrated, the flow of the inert gas is shown by an arrow.  
         [0055]     Referring to  FIG. 2 , the inert gas produced by the plant facility  100  is initially introduced to the port  210  of the gas supply unit  200 . The inert gas introduced to the port  210  enters the first detector  220 , which in turn detects its impurity concentration. The concentration detected by the first detector  220  is sent to the controller  290 , and compared with the permissible value. When the controller  290  determines that the inert gas has a permissible impurity concentration, it opens the valves  244  and  246 , the shut-off valve  270 , and the valves  280  and  282 . The delay tube  230  then delays a flow of the inert gas. Then, the inert gas is supplied to the exposure apparatus  700  through the valve  244 , the valve  246 , the second detector  250 , the delay tube  260 , the shut-off valve  270 , the valve  280 , and the valve  282 .  
         [0056]     The second detector  250  in the present embodiment does not work since the inert gas has a permissible impurity concentration, but it may work when the impurity is likely to be mixed into the supply channel from the first detector  220  to the second detector  250 . In this case, the impurity concentration of the inert gas is sent to the controller  290 , which in turn closes the shut-off valve  270  to stop supplying the inert gas to the exposure apparatus  700  when determining that it exceeds the permissible value.  
         [0057]      FIG. 3  is a view showing an inert-gas flow introduced from the plant facility  100  to the gas supply unit  220  when its impurity concentration exceeds the permissible value in an abnormal state. As illustrated, the flow of the inert gas is shown by an arrow.  
         [0058]     Referring to  FIG. 3 , at first, the inert gas produced by the plant facility  100  is introduced to the port  210  of the gas supply unit  200 . The inert gas introduced to the port  210  enters the first detector  220 , which in turn detects its impurity concentration. The concentration detected by the first detector  220  is sent to the controller  290 , and compared with the permissible value. When the controller  290  determines that the impurity concentration of the inert gas exceeds the permissible value, it switches the valves  244  and  246  to supply the contaminated inert gas to the channel  240 , and prevent the contaminated inert gas from flowing downstream. The controller  290  uses the alarm  292  to notify an operator via sounds, light, displays, etc., that the inert gas has an impermissible impurity concentration, and switches the valves  244  and  246 . The delay tube  230  delays a flow of the inert gas during this period, and the contaminated inert gas never enters the second detector  250  through the valves  244  and  246 . The filter  242  removes the impurity from the contaminated inert gas in the channel  240 . The inert gas whose impurity has been removed by the filter  242  enters the second detector  250  via the valve  246 , which in turn detects a concentration of its impurities. The impurity concentration detected by the second detector  250  is sent the controller  290 , and compared with the permissible value. When the controller  290  determines that the inert gas has a permissible impurity concentration, it opens the shut-off valve  270  and the valves  280  and  282 . The delay tube  260  delays a flow of the inert gas during this period. The inert gas is then supplied to the exposure apparatus  700  through the shut-off valve  270  and the valves  280  and  282 .  
         [0059]     When the controller  290  determines that the inert gas that has passed through the filter  242  of the channel  240  has an impermissible impurity concentration, it closes the shut-off valve  270  and stops supplying the inert gas to the exposure apparatus  700 , as shown in  FIG. 4 . The controller  290  uses the alarm  292  to notify an operator via sounds, light, displays, etc. that the inert gas has an impermissible impurity concentration, as well as switching the valves  244  and  246 . The delay tube  260  delays a flow of the inert gas this time, and the contaminated inert gas never enters the exposure apparatus  700  through the valves  280  and  282 . Here,  FIG. 4  is a view showing an inert-gas flow with impermissible impurity concentration in an abnormal state that passes through the filter  250  in the channel  240 . As illustrated, the flow of the inert gas is shown by an arrow.  
         [0060]      FIG. 5  is a view showing cleansing-gas and inert-gas flows when the impurity is cleansed from the pipeline in the gas supply unit  200  when the impurity concentration of the inert gas introduced from the plant facility  100  to the gas supply unit  200  exceeds the permissible value. As illustrated, the flow of the cleansing gas and inert gas is shown by an arrow.  
         [0061]     A cleansing gas supply unit  600  introduces a cleansing gas to the gas supply unit  200  via the port  210 . The cleansing gas removes the impurity adhering to the pipeline system in the gas supply unit  200 . The cleansing gas uses inert gas of nitrogen, etc. with a confirmedly permissible impurity concentration. Referring to  FIG. 5 , the valve  282  is switched such that inert gas is supplied to the exposure apparatus  700  from the spare gas supply unit  500  that has the inert gas with a confirmedly permissible impurity concentration. Thus, the exposure apparatus  700  may act even when the pipeline of the gas supply unit  200  is being cleaned. The port  210  is disconnected between the plant facility  100  and the gas supply unit  200 , and connected to the cleansing gas supply unit  600 . Then, the valve  280  is switched to connect the flow channel for the cleansing gas to the exhaust facility  400 .  
         [0062]     The cleansing gas is introduced from the cleansing gas supply unit  600  to the port  210  in the gas supply unit  200 . The cleansing gas introduced to the port  210  is exhausted to the exhaust facility  400  through the first detector  220 , the delay tube  230 , the valves  244  and  246 , the second detector  250 , the delay tube  260 , the shut-off valve  270  and the valve  280 . The cleansing gas then removes the impurity that clings to the pipeline in the gas supply unit  200 .  
         [0063]     Upon completion of removal of the impurity adhering to the pipeline in the gas supply unit  200 , the port  210  is disconnected from the cleansing gas supply unit  600  and connected to the plant facility  100 , which has the inert gas with a confirmedly permissible impurity concentration. The inert gas is introduced from the plant facility  100  to the gas supply unit  200 , and exhausted from the exhaust facility  400 . The first and second detectors  220  and  250  detect the impurity concentration of the inert gas. The concentrations detected by the first and second detectors  220  and  250  are sent to the controller  290  for comparison with the permissible value. When the inert gas has a confirmedly permissible impurity concentration, the valves  280  and  282  are switched such that the inert gas from the plant facility  100  is supplied to the exposure apparatus  700 . When the impurity concentration of the inert gas exceeds the permissible value, the plant facility  100  and the port  210  are disconnected, and instead, the cleansing gas supply unit  600  is connected to repeat the cleaning of the gas supply unit  200 &#39;s pipeline by using the cleansing gas.  
         [0064]     Referring now to  FIG. 6 , a description will be given of a gas supply unit  200 A as a variation of the gas supply unit  200 . The gas supply unit  200 A differs from the gas supply unit  200  in the delay tubes  230  and  260  as the first and second delay parts. Here,  FIG. 6  is a schematic block diagram of a gas supply unit  200 A as a variation of the gas supply unit  200  shown in  FIG. 1 .  
         [0065]     Similar to the gas supply  200 , the gas supply unit  200 A detects an impurity concentration of the inert gas produced by the plant facility  100  and supplies to the exposure apparatus  700  the inert gas with a permissible impurity concentration.  
         [0066]     A delay tank  230 A is arranged between the first detector  220  and the channel  240 , and serves as a first delay member that delays a flow of the inert gas with an impermissible impurity concentration until the supply channel for the inert gas is switched to the channel  240  so that the contaminated inert gas may not enter the exposure apparatus  700 .  
         [0067]     A delay tank  260 A is arranged between the second detector  250  and the shut-off valve  270 , and serves as a second delay member that delays a flow of the inert gas that has passed through the filter  242  of the channel  240  and includes an impermissible impurity concentration or when the filter  242  does not remove the impurity sufficiently until the shut-off valve  270  works, so that the contaminated inert gas may not enter the exposure apparatus  700 .  
         [0068]     The gas supply unit  200 A&#39;s action and the flow of the inert gas filling the exposure light path in the exposure apparatus  700  are the same as those of the gas supply unit  200 , and a description thereof will be omitted.  
         [0069]     Turning back to  FIG. 1  again, the exposure apparatus  700  includes an illumination apparatus  710  that illuminates a mask or reticle (these terms are used interchangeably in the present application)  720  which forms a pattern, a stage  745  that supports a plate, and a projection optical system  730  that projects diffracted light arising from the illuminated mask pattern to the plate  740 , and a piping unit  750 .  
         [0070]     The exposure apparatus  700  is a projection exposure apparatus that exposes a circuit pattern formed on the mask  720  onto the plate  740 , e.g., in a step-and-repeat or step-and-scan manner. Such an exposure apparatus is suitable for a photolithography process of a sub-micron or a quarter-micron or less. A description will be given below of a step-and-scan exposure apparatus (which is also referred to as a “scanner”) as an example. The “step-and-scan” manner, as used herein, is one mode of exposure method which exposes a pattern on a mask onto a wafer by continuously scanning the wafer relative to the mask, and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The “step-and-repeat” manner is another mode of exposure method which moves a wafer stepwise to an exposure area for the next shot every shot onto the wafer.  
         [0071]     The illumination apparatus  710  illuminates the mask  720  which forms a circuit pattern to be transferred, and includes a light source section  712 , a delivery optics unit  714 , and an illumination optical system  716 .  
         [0072]     The light source section  712  employs, e.g., laser as a light source. The laser may use ArF excimer laser with a wavelength of approximately about 193 nm, KrF excimer laser with a wavelength of about 248 nm, F 2  excimer laser with a wavelength of about 153 nm, etc. However, a kind of laser is not limited to excimer laser. For example, YAG laser can be used, and the number of laser units is not limited. For example, if two units of solid laser that operates independently are used, no coherence between these solid laser units exists, and thus, speckles arising from the coherence will be reduced considerably. In order to reduce speckles, it would be preferable to oscillate an optical system in a straight or rotating manner. When the light source section  712  uses laser, it is desirable to employ a beam shaping optical system that shapes a parallel beam from a laser source to a desired beam shape, and an incoherently turning optical system that turns a coherent laser beam into an incoherent one. A light source applicable to the light source part  712  is not limited to the laser, but may use one or more lamps such as a mercury lamp, xenon lamp, etc.  
         [0073]     The delivery optics unit  714  guides light from the light source section  712  to the illumination optical system  716 . The illumination optical system  716  is an optical system that illuminates the mask  729 , including a lens, a mirror, a light integrator, a stop, and the like, for example, in the order of a condenser lens, a fly-eye lens, an aperture stop, a condenser lens, a slit, and an imaging optical system. The illumination optical system  716  can use any light whether it is axial or non-axial light. The light integrator may include a fly-eye lens or an integrator formed by stacking two sets of cylindrical lens array plates (or lenticular lenses), and be replaced with an optical rod or a diffractive element.  
         [0074]     The mask  720  forms a circuit pattern or an image to be transferred, and is made, for example, of quartz and supported and driven by a mask stage (not shown). Diffracted light through the mask  720  is projected onto the plate  740  through the projection optical system  730 . The plate  740  is an object to be exposed such as a wafer or a liquid crystal plate, onto which resist is applied. The mask  720  and plate  740  are located in a conjugate relationship. When the exposure apparatus  700  is a scanner, it transfers a pattern on the mask  720  onto the plate  740  by scanning the mask  720  and plate  740 . When the exposure apparatus  700  is a stepper (or a “step-and-repeat” exposure apparatus), it exposes while resting the mask  720  and plate  740 .  
         [0075]     The projection optical system  730  may use an optical system including plural lens elements, an optical system including plural lens elements and at least one concave mirror (a catadioptric optical system), an optical system including plural lens elements and at least one diffractive optical element such as a kinoform, a full mirror type optical system, and so on. Any necessary correction of the chromatic aberration may use plural lens units made from glass materials having different dispersion values (Abbe values), or arrange a diffractive optical element such that it disperses in a direction opposite to that of the lens unit.  
         [0076]     Photoresist is applied onto the plate  740 . A photoresist application step includes a pretreatment, an adhesion accelerator application treatment, a photo-resist application treatment, and a pre-bake treatment. The pretreatment includes cleaning, drying, etc. The adhesion accelerator application treatment is a surface reforming process so as to enhance the adhesion between the photo resist and a base (i.e., a process to increase the hydrophobicity by applying a surface active agent), through a coat or vaporous process using an organic film such as HMDS (Hexamethyl-disilazane). The pre-bake treatment is a baking (or burning) step, softer than that after development, which removes the solvent.  
         [0077]     The stage  745  supports the plate  740 . The stage  745  may use any structure known in the art, and thus a detailed description of its structure and operations is omitted. For example, the stage  745  may use a linear motor to move the plate  740  in directions X and Y. The mask  720  and plate  740  are, for example, scanned synchronously, and the positions of the stage  745  and mask stage (not shown) are monitored, for example, by a laser interferometer and the like, so that both are driven at a constant speed ratio. The stage  745  is installed on a stage stool supported on the floor and the like, for example, via a damper, and the mask stage and the projection optical system  730  are installed on a lens barrel stool (not shown) supported, for example, via a damper on a base-frame placed on the floor and the like.  
         [0078]     The piping unit  750  has a pipeline port  752 , and supplies the inert gas into the exposure light path while decompressing its pressure and adjusting its flow rate, the inert gas that is supplied with a permissible impurity concentration from the gas supply unit  200  connected via the pipeline port  752 . The pipeline unit  750  allows the exposure light path to be filled with the inert gas with a permissible impurity concentration. This may prevent extremely lowered throughput due to augmented light absorption in the exposure light path caused by impurity of the inert gas, the degraded exposure-dose control accuracy due to the fluctuant light absorption in the exposure light path during exposure and fluctuant or erroneous exposure dose, and lowered performance such as optical efficiency as a result of adhesions of impurity onto an optical element, such as a lens and a mirror in an optical system and its photochemical reactions in the exposure light path and product generated by the reactions.  
         [0079]     In exposure, light emitted from the light source section  712  uses the illumination optical system  716  to Koehler-illuminate the mask  720 . Light passing the mask  720  and reflecting the mask pattern is imaged onto the plate  740  by the projection optical system  730 . Since the inside of the exposure apparatus  700 &#39;s exposure light path is filled with the inert gas supplied with a permissible impurity concentration by the inventive gas supply unit  200 , UV, far UV light and vacuum UV light are transmitted with high transmittance, and may provide devices (such as semiconductor devices, LCD devices, image pick-up devices (such as CCDs), thin-film magnetic heads, etc. with high throughput and high economical efficiency.  
         [0080]     Referring now to  FIG. 7 , a description will be given of an exposure system  2  of a second embodiment according to the present invention.  FIG. 7  is a schematic block diagram of the exposure system  2  of the second embodiment according to the present invention. The exposure system  2  in  FIG. 7  is similar to the exposure system  1  in  FIG. 1 , but differs in the structure of the gas supply unit  200 . Incidentally, for the same elements as are shown in the exposure system  1  in  FIG. 1 , the same reference numerals are assigned such that a duplicate description of them is avoided.  
         [0081]     As shown in  FIG. 7 , the exposure system  2  includes the plant facility  100 , a gas supply unit  800 , an electric power supply  300 , the exhaust facility  400 , the spare gas supply unit  500 , and the exposure apparatus  700 . The exposure system  2  of this embodiment is a system that uses the gas supply unit  800  to supply the exposure apparatus  700  with the inert gas with a permissible impurity concentration from the plant facility  100  for exposure.  
         [0082]     The gas supply unit  800  supplies the exposure apparatus  700  with the inert gas with a permissible impurity concentration and detects the impurity concentration of the inert gas produced by the plant facility  100 . As illustrated, the gas supply unit  800  has a gas container  812  in place of the filter  242  and the valves  244  and  246 , the shut-off valve  814 , and the valve  816  in the channel  810 .  
         [0083]     The channel  810  includes the gas container  812 , the shut-off valve  814  and valve  816 . The channel  810  is to provide the exposure apparatus  700  with the inert gas from the gas container  812  that is filled with the apparently purified inert gas, when the first detector  220  has detected that the inert gas supplied from the plant facility  100  to the gas supply unit  800  is contaminated inert gas. The shut-off valve  814  opens and closes the gas container  812 . The valve  816  switches a supply channel of the inert gas to the channel  810 . The controller  290  controls switching of the supply channel of the inert gas or the opening/closing of the shut-off valve  814  and the switching of the valve  816 .  
         [0084]     A description will be given of the operations of the gas supply unit  800  and a flow of the inert gas filling the exposure light path in the exposure apparatus  800 .  FIG. 7  indicates the flow of the inert gas by an arrow.  
         [0085]     At first, the inert gas produced by the plant facility  100  is introduced to the port  210  of the gas supply unit  800 . The inert gas introduced to the port  210  enters the first detector  220 , which in turn detects its impurity concentration. The concentration detected by the first detector  220  is sent to the controller  290 , and compared with the permissible value.  
         [0086]     If the controller  290  determines that the inert gas has a permissible impurity concentration, it controls the valve  816  to switch the channel supplying the inert gas to the channel  810 , and prevents the contaminated inert gas from flowing downstream. The controller  290  notifies an operator through sounds, light, displays, etc. of an impermissible impurity concentration of the inert gas via the alarm  292 . The delay tube  230  delays a flow of the inert gas this time, and the contaminated inert gas never enters the valve  816  and the second detector  250 . The controller  290  then opens the shut-off valve  814 , which allows the inert gas to flow from the gas container  812  via the valve  816  into the second detector  250 , which in turn detects the impurity concentration. The impurity concentration detected by the second detector  250  is sent to the controller  290 , and compared with the permissible value. When the controller  290  determines that the inert gas has a permissible impurity concentration, it opens the shut-off valve  270 , and the valves  280  and  282 . The delay tube  260  delays a flow of the inert gas during this time. Then, the inert gas is supplied to the exposure apparatus  700  through the shut-off valve  270 , and the valves  280  and  282 .  
         [0087]     When the controller  290  determines that the inert gas that is supplied from the gas container  812  in the channel  810  and has an impermissible impurity concentration, it closes the shut-off valve  270  and stops supplying the inert gas to the exposure apparatus  700 . The controller  290  closes the shut-off valve  270  and notifies an operator through sounds, light, displays, etc. from the alarm  292  that the inert gas has an impermissible impurity concentration. The delay tube  260  delays a flow of the inert gas during this time, and the contaminated inert gas never enters the valves  280  and  282  and flows into the exposure  700 .  
         [0088]     Referring now to  FIGS. 8 and 9 , a description will be given of an embodiment of a device fabricating method using the above exposure apparatus  1 .  FIG. 8  is a flowchart for explaining a fabrication of devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of a fabrication of a semiconductor chip as an example. Step  1  (circuit design) designs a semiconductor device circuit. Step  2  (mask fabrication) forms a mask having a designed circuit pattern. Step  3  (wafer preparation) manufactures a wafer using materials such as silicon. Step  4  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  5  (assembly), which is also referred to as a posttreatment, forms into a semiconductor chip the wafer formed in Step  4  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  6  (inspection) performs various tests for the semiconductor device made in Step  5 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  7 ).  
         [0089]      FIG. 9  is a detailed flowchart of the wafer process in Step  4 . Step  11  (oxidation) oxidizes the wafer&#39;s surface. Step  12  (CVD) forms an insulating film on the wafer&#39;s surface. Step  13  (electrode formation) forms electrodes on the wafer by vapor disposition and the like. Step  14  (ion implantation) implants ion into the wafer. Step  15  (resist process) applies a photosensitive material onto the wafer. Step  16  (exposure) uses the exposure apparatus  200  to expose a circuit pattern on the mask onto the wafer. Step  17  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  19  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. The device fabrication method of this embodiment may manufacture higher quality devices than the conventional one.  
         [0090]     The above gas supply unit and method, and exposure systems may detect the inert gas with an impermissible impurity concentration, and prevent that inert gas from entering an exposure apparatus. This may prevent extremely lowered throughput due to augmented light absorption in the exposure light path caused by impurity of the inert gas, the degraded exposure-dose control accuracy due to the fluctuant light absorption in the exposure light path during exposure and fluctuant or erroneous exposure dose, and lowered performance such as optical efficiency as a result of adhesions of impurity onto an optical element, such as a lens and a mirror in an optical system and its photochemical reactions in the exposure light path and product generated by the reactions. This may also reduce cost incurred in exchanging optical elements and pipelines to which the impurities have adhered.  
         [0091]     Only when an impurity concentration of supplied inert gas exceeds a permissible value, a filter removes the impurity or a gas container of the inert gas works with the purified inert gas. Thus, a regular exchange of the filter or gas container is unnecessary, restraining the running cost.  
         [0092]     Further, the present invention is not limited to these preferred embodiments, and various variations and modifications may be made without departing from the scope of the present invention.  
         [0093]     The gas supply unit, the supply method, and the exposure system of the instant invention can detect the inert gas with an impermissible impurity concentration, and prevent that inert gas from entering the exposure apparatus.