Abstract:
A differential pumping system includes a first chamber for storing a light source that emits light, a second chamber that receives light from the first chamber, and a vacuum pump, provided between the first and second chambers, which includes a hollow shaft through which the light passes, and exhausts the hollow shaft.

Description:
[0001]    This application claims a benefit of foreign priority based on Japanese Patent Application No. 2002-261796, filed on Sep. 6, 2002, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a light source and an illumination system for use with an extreme ultraviolet (“EUV”) exposure apparatus that transfers a fine pattern in semiconductor manufacturing.  
           [0003]    In manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology, a reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern.  
           [0004]    The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (such as 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).  
           [0005]    However, the photolithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical apparatus using the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light (referred to as “EUV exposure apparatus”) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.  
           [0006]    The EUV light source uses, for example, a laser plasma light source. It irradiates a highly intensified pulse laser beam to a target material put in a vacuum chamber to generate high-temperature plasma for use as the EUV light with a wavelength of about 13 nm emitted from this. The target material may use Xe gas, droplets, and clusters, and a metallic thin film, such as copper, tin, aluminum, etc., and is supplied to the vacuum chamber by gas jetting means and other means.  
           [0007]    The laser plasma as one mode of the EUV light source irradiates the high-strength pulse laser light onto the target material and generates not only the EUV light from the target material, but also flying particles called debris, which causes pollution, damages and lowered reflectance of an optical element. Accordingly, a method have been conventionally proposed which mitigates influence of debris by providing a foil trap made of a porous material around the target material and circulating inert gas, such as He gas, as buffer gas.  
           [0008]    Since He gas as well as Xe gas as the target material is essential to a light emitting section of the target material, the pressure in a vacuum chamber becomes about 10 Pa although a vacuum pump exhausts the chamber. The atmosphere of a stage subsequent to the light emitting section should be maintained as clean as possible, preferably with the degree of vacuum of about 10 −7  Pa, for intended performance such as reflectance of the optical element, since the EUV light has low transmittance to the air and contaminates an optical element when reacted with a residual gas component (such as high molecule organic gas).  
           [0009]    Differential pumping system have already been proposed which use a thin film window provided between a light emitting section and an optical element in a stage subsequent to the light emitting section (as seen in Japanese Patent Applications Publications Nos. 5-82417, and 2-156200). Several proposals of exposure dose control over a pulsed light source may be seen in U.S. Pat. No. 5,305,364.  
           [0010]    It is difficult to manufacture and handle a self-supported filter material that has high transmittance and is applicable to a wavelength range of the EUV light. A differential pumping method is conceivable, as shown in FIG. 8, which uses a channel or orifice  3900  for differential pumping at a connection between a light source chamber  3110  that accommodates a light emitting section and an illumination system chamber  3120  that stores an optical element  3500 . Here, FIG. 8 is a schematic structure of an EUV light source  3000  that uses a laser plasma light source.  
           [0011]    The differential pumping using the orifice  15  generates a pressure difference of about 10 −2  Pa between the light source chamber  3110  and an illumination system chamber  3120 . When it is considered that the light source chamber  3110  has the pressure of about 10 Pa as discussed, the pressure in the illumination system  3120  becomes about 10 −1  Pa, which is insufficient to maintain the performance such as the reflectance of the optical element  3500 .  
           [0012]    In order to obtain a desired pressure difference between the light source chamber  3110  and the illumination system  3120 , it is conceivable to elongate the channel  3900  that connects the light source chamber  3110  and the illumination system chamber  3200 . On the other hand, for enhanced use efficiency of the EUV light  3400 , a spheroid condenser mirror  3600  should capture the EUV light generated from the target material as much as possible, for example, at about n steradian. However, as the capture angle becomes large, it becomes difficult to elongate the channel  3900  and to obtain a desired pressure difference.  
           [0013]    A demand to maintain the pressure in the illumination system chamber to be the degree of vacuum of about 10 −7  Pa is common to a discharge method that generates the EUV light by circulating Xe gas, etc. in an electrode for discharging and generating plasma, as well as the laser plasma method.  
           [0014]    Thus, it is a very difficult issue to increase the use efficiency of the EUV light while achieving the intended pressure difference in a differential pumping.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    Accordingly, it is an exemplary object of the present invention to provide a differential pumping system that has high differential pumping capacity without harming use efficiency of the EUV, and maintain performance of an optical element, such as reflectance.  
           [0016]    A differential pumping system of one aspect according to the present invention includes a first chamber for storing a light source that emits light, a second chamber that receives light from the first chamber, and a vacuum pump, provided between the first and second chambers, which includes a hollow shaft through which the light passes, and exhausts the hollow shaft. The vacuum pump may include a vane that rotates around the hollow shaft. A wall surface of the hollow shaft may have an aperture, which has a vane section and exhausts gas molecules outside the shaft.  
           [0017]    The differential pumping system may further include a first exhaust unit for exhausting the first chamber, and a second exhaust unit for exhausting the second chamber, wherein pressure of the second chamber is maintained lower than that of the first chamber. The light may be collimated and the hollow shaft may have a cylindrical shape. The light may be condensed, and the hollow shaft may be so tapered that a side of the first chamber is narrower than that of the second chamber. The differential pumping system may further include another vacuum pump for exhausting an atmosphere to the outside which has been exhausted by the vacuum pump provided between the first and second chambers. The light is, for example, EUV light.  
           [0018]    An exposure apparatus of another aspect according to the present invention includes the above differential pumping system, an illumination optical system that introduces the light to a mask that forms a circuit pattern to be transferred onto an object, and a projection optical system that introduces the light from the mask onto the object, wherein the illumination optical system and projection optical system are installed in the second chamber.  
           [0019]    A measurement system of still another aspect according to the present invention includes the above differential pumping system, a light intensity measuring apparatus for measuring light intensity from an object to be measured, an illumination optical system that introduces the light to the object, and a measurement optical system that introduces the light from the object to the light intensity measuring apparatus, wherein the light intensity measuring apparatus, illumination optical system and measurement optical system are installed in the second chamber.  
           [0020]    A device fabrication method of another aspect of this invention includes the steps of exposing a plate by using the above exposure apparatus, and performing a predetermined process for the 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.  
           [0021]    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  
       [0022]    [0022]FIG. 1 is a schematic structure of a differential pumping system as one aspect according to the present invention.  
         [0023]    [0023]FIG. 2 is a schematic structure of one example of a rotational shaft of a turbo molecular pump shown in FIG. 1.  
         [0024]    [0024]FIG. 3 is another schematic structure of one example of a rotational shaft of a turbo molecular pump shown in FIG. 1.  
         [0025]    [0025]FIG. 4 is a schematic structure of a differential pumping system as a variation according to the present invention.  
         [0026]    [0026]FIG. 5 is a schematic structure of an exposure apparatus of one embodiment according to the present invention.  
         [0027]    [0027]FIG. 6 is a flowchart for explaining how to fabricate devices (such as semiconductor chips such as ICs and LCDs, CCDs, and the like).  
         [0028]    [0028]FIG. 7 is a detail flowchart of a wafer process as Step  4  shown in FIG. 6.  
         [0029]    [0029]FIG. 8 is a schematic structure of an EUV light source that uses a laser plasma light source.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    With reference to accompanying drawings, a description will now be given of a differential pumping system of one embodiment according to the present invention. In each figure, the same element is designated by the same reference numeral, and a description thereof will be omitted. Here, FIG. 1 is a schematic structure of the differential pumping system of one embodiment according to the present invention.  
         [0031]    Referring to FIG. 1, the differential pumping system  1  enables the pressure of an illumination system chamber  200  that is connectible to a light source chamber  100 , to be smaller than that of the light source chamber  100  that accommodates plasma  106  as a light source for emitting the EUV light  107 . The differential pumping system  1  is provided with an exhaust part  300  along an optical axis of the EUV light  107 , which allows the EUV light  107  to pass to introduce the EUV light  107  to the second chamber  200 , and exhausts gas molecules from the light source chamber  100  to the illumination system chamber  200  by exhausting the passage of the EUV light.  
         [0032]    Pulsed laser  101  exited from a laser generator (not shown) is condensed on a target  105 , such as Xe gas, supplied from a nozzle  104  via a condenser lens  102  and a transmission window  103 , generating the plasma  106 . The plasma  106  irradiates the EUV light  107 , and a spheroid condenser mirror  108  condenses the EUV light  107  for improved use efficiency, and introduces it into the mirror chamber  150  that accommodates a mirror  151 . The mirror converts the EUV light  107  into collimated light and introduces the collimated EUV light into a hollow part  312   a  of a rotational shaft  312 , which will be described later.  
         [0033]    As discussed, the plasma  106  generates not only the EUV light  107  but also flying particles called debris  109 , which splashes and causes pollution, damages and lowered reflectance of the neighboring condenser mirror  108  and the mirror  151 . Accordingly, a buffer gas supply unit  110  introduces He gas  111  into the light source chamber  100  to reduce splash of the debris  109  using flows of the He gas  111 .  
         [0034]    In order to reduce attenuation of the EUV light  107  and pollution and damages of the condenser mirror  108 , the Xe gas as a target  105  and the He gas  111  as buffer gas are always supplied to the light source chamber  100  while a vacuum pump  113  exhausts the chamber  100 . Therefore, the pressure of the light source chamber  100  becomes about 10 Pa. In order to prevent deterioration of the mirror  151 , the pressure of the mirror chamber  150  should preferably low. Accordingly, an orifice is provided at a connection part with the light source chamber  100  and a pump  152 .  
         [0035]    An exhaust part  300  is implemented as a turbo molecular pump  310  having a rotational shaft  312  having a hollow part  312   a  through which the EUV light  107  passes so that an optical axis of the EUV light  107  is not shielded. A stator  314  is fixed onto the turbo molecular pump  310 . The rotary shaft  312  has vanes  312   b  around the shaft, which has a blade section and exhausts gas molecules outside the rotational shaft  312 . A roughing pump  320  exhausts gas compressed by the turbo molecular pump.  
         [0036]    In this configuration, it is preferable that the EUV light  107  should be thin in order to further enhance the differential pumping performance of the turbo molecular pump  310 , and it is preferable to maintain the low pressure environment of the illumination system chamber  200  and arrange the illumination system chamber  200  close to the light source chamber  100  to prevent pollution and damages of the optical elements  201  and  202  housed in the illumination system chamber  200 .  
         [0037]    On the other hand, it is preferable not to replace the optical elements  201  and  202  accommodated in the illumination system chamber  200 , and the illumination system chamber  200  is always exhausted by a vacuum pump  203 .  
         [0038]    A description will now be given of the inventive differential pumping system  1 . Gas molecules in the light source chamber  100  pass through the orifice  113  and enter the mirror chamber  150 . Part of the gas molecules is exhausted by the pump  152 , but other gas molecules enter the turbo molecular pump  310 .  
         [0039]    [0039]FIG. 2 is a schematic structure of one example of the rotational shaft  312  of the turbo molecular pump  310 . An aperture for exhausting gas molecules to outside is provided in a wall surface of the rotational shaft  312  that includes the hollow part  312   a  and the vanes  312   b  are formed on a wall surface of the aperture. The rotational shaft  312  and the vanes  312   b  rotate at a high speed in an arrow direction shown in FIG. 2. The gas molecules AM that has entered the hollow part  312   a  in the rotational shaft  312  collide with the vanes  312   b  and scattered in a radial direction as shown by arrows in FIG. 2. The compressed gas molecules AM are exhausted to the outside by the roughing pump  320  as exhaust means. The scattered gas molecules AM passes through a section of the rotational shaft  312  and are exhausted to the outside of the rotational shaft  312 . In this portion, the stators  314  and rotors  316  that may rotate around the rotational shaft  312  compress the gas molecules AM in the roughing pump  320 . The compressed gas molecules AM are exhausted to the outside by the roughing pump  320  as exhaust means.  
         [0040]    The rotational shaft  312  may include vanes  312   c  in the hollow part  312   a , which exhaust the gas molecules AM to the outside of the rotational shaft  312 . Here, FIG. 3 is a schematic structure of one example of the rotational shaft  312  of the turbo molecular pump  310 . The vanes  312   c  shield the EUV light  107  but maintains its sectional area as small as possible so as not to reduce the use efficiency of the EUV light  107 . The rotational shaft  312 , and vanes  312   b  and  312   c  rotate at a high speed in an arrow direction shown in FIG. 3. Therefore, the gas molecules AM that move approximately parallel to the optical axis of the EUV light  107  collide with the vanes  312   c  and are scattered in a radial direction as shown by arrows in FIG. 3. The scattered gas molecules AM are further scattered by the vanes  312   b , pass the section of the rotational shaft  312 , and are exhausted outside the rotational shaft  312 , providing the higher differential pumping performance.  
         [0041]    The differential pumping performance will now be calculated when the exhaust part  300  (or turbo molecular pump  310 ) is applied. For simplicity purposes, it is assumed that there is no roughing pump  320  provided in the exhaust part  300 .  
         [0042]    The following equations are met where p 1  (Pa) is the pressure of the mirror chamber  150 , p 2  (Pa) is the pressure of the illumination system chamber  200 , S 1  (m 3 /s) is a pumping speed at which the vacuum pump  152  exhausts the mirror chamber  150 , S 2  (m 3 /s) is a pumping speed at which the vacuum pump  203  exhausts the illumination system chamber  200 , S 12  (m 3 /s) is a pumping speed at which the turbo molecular pump  310  that connects the mirror chamber  150  and the illumination system chamber  200 , Q 1  (Pa·m 3 /s) is degas amount generated from the mirror chamber  150 , Q 2  (Pa·m 3 /s) is degas amount generated from the illumination system chamber  200 , Q 10  (Pa·m 3 /s) and Q 20  (Pa·m 3 /s) are flow rate exhausted by respective vacuum pumps  152  and  203 , and Q 12  (Pa·m 3 /s) is the flow rate that flows through the turbo molecular pump  310 : 
         Q 1 +Q 2 =Q 10 +Q 20   (1) 
         Q 10 =Q 1 +Q 12    (2) 
         Q 10 =S 1 ·p 1   (3) 
         Q 20 =S 2 ·p 2   (4) 
         Q 12 =S 12 ·p 2   (5) 
         [0043]    Equations 6 and 7 are obtained as follows from Equations 1 to 5 by deleting Q 10 , Q 20  and Q 12  and simplifying equations with respect to p 1  and p 2  by setting Q 1 &gt;&gt;Q 2 : 
         p 1 =(S 2 ·Q 1 +S 12 ·Q 1 +S 12 ·Q 2 )/(S 1 ·S 2 +S 1 ·S 12 )≠Q 1 /S 1   (6) 
         p 2 =Q 2 /(S 2 +S 12 )  (7) 
         [0044]    For the pressure p 1 =10 (Pa), the pumping speed S 1 =S 2 =1 (m 3 s) (1000·1/s), the pumping speed S 12 =0.3 (m 3 /s) (300·1/s), the degas amounts Q 1 =10 (Pa·m 3 /s) and Q 2 =10 −5  (Pa·m 3 /s), then p 2 =10 −5 (1+0.3)=7.7·10 −6  (Pa) and p 1 /p 2 &gt;10 6 :  
         [0045]    Thus, as discussed, since the turbo molecular pump  310  uses the rotational shaft that has the hollow part  312   a  so as not to shield the optical axis of the EUV light  107  as illumination light, the differential pumping performance may improve and prevent pollution and deterioration of the optical elements  201  and  202  housed in the illumination system chamber  200 .  
         [0046]    The differential pumping system  1  is useful for applications using a point source X ray source, such as a reflectometer, a wave front measurement device, a microscope, a shape measurement device, medical equipment, a chemical composition analyzer, and a structural analyzer.  
         [0047]    A description will now be given of a differential pumping system  1 A as a variation of the differential pumping system  1 , with reference to FIG. 4. FIG. 4 is a schematic structure of the differential pumping system  1 A of one embodiment according to the present invention. The differential pumping system  1 A is similar to the differential pumping system  1 , but different from it in an exhaust part  400 .  
         [0048]    The differential pumping system  1 A provides the exhaust part  400  between the light source chamber  100  and the illumination system chamber  200 . The EUV light  107  is incident upon the illumination system chamber  200  once condensed by the condenser mirror  108 .  
         [0049]    The exhaust part  400  is implemented as a turbo molecular pump  310  having a rotational axis  412  having a hollow part  312   a  that opens like a taper corresponding to a collection angle of the EUV light  107  so as not to prevent the EUV light  107  from passing. The pressure of the light source chamber  100  corresponds to a molecular flow region, and thus it is effective that the hollow part  412   a  has the smallest opening at a side of the light source chamber  100 . In other words, a position of a condensed point  107   a  of the EUV light  107  corresponds to the opening of the hollow part  412   a  closest to the light source chamber  100 .  
         [0050]    The turbo molecular pump  410  fixes stators  414 . The rotational shaft  412  has vanes  412   b  around it, which have a blade section and exhaust gas molecules to the outside of the rotational shaft  412 . The roughing pump  420  exhausts the gas compressed by the turbo molecular pump  410 .  
         [0051]    Such a configuration may provide the differential pressure between the light source chamber  100  and the illumination system chamber  200 , and reduces the pressure of the entire illumination system chamber  200 . Therefore, it is possible to prevent pollution and deterioration of all the optical elements housed in the illumination system mirror  200 .  
         [0052]    A description will be given of an exemplary inventive exposure apparatus  800  that uses the inventive differential pumping system with reference to FIG. 5. Here, FIG. 5 is a schematic structure of the inventive exposure apparatus  800  of one embodiment.  
         [0053]    The inventive exposure apparatus  800  is a projection exposure apparatus that uses EUV light with a wavelength of 13.4 nm as exposure light for step-and-scan or step-and-repeat exposure of a circuit pattern formed on the mask  820  onto an object  840  to be exposed. This exposure apparatus is suitable for a lithography process less than submicron or quarter micron, and the present embodiment uses the step-and-scan exposure apparatus (also referred to as a “scanner”) as an example. The “step-and-scan manner”, as used herein, is an exposure method that exposes a mask pattern 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 that moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer.  
         [0054]    Referring to FIG. 5, the exposure apparatus  800  includes an illumination apparatus  810 , a mask  820 , a mask stage  825  that mounts the mask  820 , a projection optical system  830 , an object to be exposed  840 , a wafer stage  845  that mounts the object  840 , an alignment detecting mechanism  850 , and a focus position detecting mechanism  860 .  
         [0055]    The illumination apparatus  810  uses arc-shaped EUV light, for example, with a wavelength of 13.4 corresponding to an arc-shaped field of the projection optical system  830  to illuminate the mask  820 , and includes an EUV light source  812  and illumination optical system  814 . The inventive differential pumping system  1  or  1 A is applicable to a connection between the EUV light source  812  and the illumination optical system  814  of the subsequent stage in the illumination apparatus  810 , and the differential pumping system  1  or  1 A may maintain performance, such as reflectance of an optical element of the illumination optical system  814  in a low pressure atmosphere for the illumination optical system  814  without damaging use efficiency of the EUV light. The EUV light source  812  may use any of the above structures, and a detailed description will be omitted.  
         [0056]    The illumination optical system  814  includes a condenser mirror  814   a , an optical integrator  814   b , etc. The condenser mirror  814   a  serves to collect the EUV light that is isotropically irradiated from the laser plasma. The optical integrator  814   b  serves to uniformly illuminate the mask  820  with a predetermined NA. An aperture to limit the illumination area to an arc shape is also provided.  
         [0057]    The mask  820  is a reflection-type mask that forms a circuit pattern or image to be transferred, and supported and driven by the mask stage  825 . The diffracted light from the mask  820  is reflected by the projection optical system  830  and projected onto the object  840 . The mask  820  and the object  840  are arranged in an optically conjugate relationship. The exposure apparatus  800  is a step-and-scan exposure apparatus, and projects a reduced size of the pattern on the mask  820  on the object  840  by scanning the mask  820  and the object  840 .  
         [0058]    The mask stage  825  supports the mask  820  and is connected to a moving mechanism (not shown). The mask stage  825  may use any structure known in the art. A moving mechanism (not shown) may include a linear motor etc., and drives the mask stage  825  at least in a direction X and moves the mask  820 . The exposure apparatus  800  synchronously scans the mask  820  and the object  840 . The exposure apparatus  800  assigns the direction X to scan the mask  820  or the object  840 , a direction Y perpendicular to the direction X, and a direction Z perpendicular to the mask  820  or the object  840 .  
         [0059]    The projection optical system  830  uses plural multilayer mirrors  830   a  to project a reduced size of a pattern formed on the mask  820  onto the object  840 . The number of mirrors is about four to six. For wide exposure area with the small number of mirrors, the mask  820  and object  840  are simultaneously scanned to transfer a wide area that is an arc-shaped area or ring field apart from the optical axis by a predetermined distance. The projection optical system  830  has a NA of about 0.1 to 0.3.  
         [0060]    The instant embodiment uses a wafer as the object to be exposed  840 , but it may include a spherical semiconductor and liquid crystal plate and a wide range of other objects to be exposed. Photoresist is applied onto the object  840 . A photoresist application step includes a pretreatment, an adhesion accelerator application treatment, a photoresist 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 photoresist 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.  
         [0061]    An object to be exposed  840  is held onto the wafer stage  845  by a wafer chuck. The wafer stage  845  moves the object  840 , for example, using a linear motor in XYZ directions. The mask  820  and the object  840  are synchronously scanned. The positions of the mask stage  825  and wafer stage  845  are monitored, for example, by a laser interferometer, and driven at a constant speed ratio.  
         [0062]    The alignment detection mechanism  850  measures a positional relationship between the position of the mask  820  and the optical axis of the projection optical system  830 , and a positional relationship between the position of the object  840  and the optical axis of the projection optical system  830 , and sets positions and angles of the mask stage  825  and the wafer stage  845  so that a projected image of the mask  820  may be positioned in place on the object  840 .  
         [0063]    A focus detection optical system  860  measures a focus position in the direction Z on the object  840  surface, and control over a position and angle of the wafer stage  845  may always maintain the object  840  surface at an imaging position of the projection optical system  830  during exposure.  
         [0064]    In exposure, the EUV light emitted from the illumination apparatus  810  illuminates the mask  820 , and the projection optical system  830  images a pattern formed on the mask  820  onto the object  840  surface. The instant embodiment uses an arc or ring shaped image plane, scans the mask  820  and object  840  at a speed ratio corresponding to a reduction ratio to expose the entire surface of the mask  820 . The exposure apparatus  800  may improve the use efficiency of the EUV light, and reduces the pressure of the subsequent stage to the EUV light source  812  down to the high vacuum state. Thus, the exposure apparatus  800  may maintain the optical performance of the optical element, and provide exposure with good imaging performance and throughput.  
         [0065]    Referring now to FIGS. 6 and 7, a description will be given of an embodiment of a device fabricating method using the above exposure apparatus  800 . FIG. 6 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 ).  
         [0066]    [0066]FIG. 7 is a detailed flowchart of the wafer process in Step  4  in FIG. 6. 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  800  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 a higher quality device than the conventional method. The device fabrication method using the exposure apparatus  800  and devices as the resultant products would constitute one aspect of the present invention.  
         [0067]    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.  
         [0068]    For example, the above differential pumping system  1  or  1 A is applicable to a measurement system. A reflectometer as one example of this measurement system includes, in addition to the above differential pumping system, a light intensity measuring apparatus for measuring light intensity from a multilayer mirror for EUV light as an object to be measured, an illumination optical system that introduces the light to the multilayer mirror, and a measurement optical system that introduces the light from the multilayer mirror to the light intensity measuring apparatus, wherein the light intensity measuring apparatus, illumination optical system and measurement optical system are installed in the above chamber  200 .  
         [0069]    The inventive differential pumping system may thus provide high differential pumping performance without harming the use efficiency of the EUV light and maintain performance of an optical element, such as reflectance.