Patent Publication Number: US-2007121091-A1

Title: Extreme ultraviolet reticle protection using gas flow thermophoresis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application is a national phase filing of International Application No. PCT/US2005025958 (internationally filed Jul. 21, 2005) which claims priority to U.S. patent application Ser. No. 10/898,475, filed Jul. 23, 2004, now U.S. Pat. No. 7,030,959, which are each incorporated herein by reference in their entireties.  
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates generally to equipment used in semiconductor processing. More particularly, the present invention relates to a mechanism which is arranged to reduce the amount of particle contamination on a reticle used in an extreme ultraviolet lithography system.  
      2. Description of the Related Art  
      In photolithography systems, the accuracy with which patterns on a reticle are projected off of or, in the case of extreme ultraviolet (EUV) lithography, reflected off of the reticle onto a wafer surface is critical. When a pattern is distorted, as for example due to particle contamination on a surface of a reticle, a lithography process which utilizes the reticle may be compromised. Hence, the reduction of particle contamination on the surface of a reticle is crucial.  
      Photolithography systems typically use pellicles to protect reticles from particles. As will be appreciated by those skilled in the art, a pellicle is a thin film on a frame which covers the patterned surface of the reticle to prevent particles from becoming attached to the patterned surface. Pellicles, however, are not used to protect EUV reticles, since thin films generally are not suitable for providing protection in the presence of EUV radiation. Principles of thermophoresis may also be applied to protect reticles from particle contamination by maintaining reticles at a higher temperature than their surroundings, and, therefore, causing the particles to move from the hotter reticle to the cooler surroundings, e.g., cooler surfaces.  
      Since thermophoresis generally may not be used in a high vacuum environment, in order for thermophoresis to be used in an EUV system to protect a reticle mounted in a reticle chuck, gas at a pressure of approximately fifty milliTorr (mTorr) or more may be introduced to substantially flow around the reticle. With the gas at a pressure of approximately fifty mTorr or more flowing around the reticle, particles may be effectively pushed away from the reticle towards a cooler surface. As will be appreciated by those skilled in the art, at pressures close to zero, thermophoretic forces are relatively insignificant. However, at low pressures of approximately fifty mTorr, thermophoretic forces are generally significant enough to convey particles from a hotter area to a cooler area.  
       FIG. 1  is a diagrammatic side view representation of a portion of an EUV lithography or exposure system. An EUV lithography system  100  includes a chamber  104  which includes a first region  108  and a second region  110 . First region  108  is arranged to house a reticle stage  114  which supports a reticle chuck  118  that holds a reticle  122 . Second region  110  is arranged to house projection optics (not shown) and a wafer stage arrangement (not shown). Sections  108 ,  110  are substantially separated by a differential pumping barrier  126  through which an opening  130  is defined.  
      Gas at a pressure of around fifty mTorr or more is supplied to first region  108  through a gas supply opening  132  in chamber  104 . In order for EUV radiation absorption losses in the gas to be minimized, second region  110  is maintained at a lower pressure, e.g., less than approximately one mTorr, than the pressure maintained in first region  108 . Hence, independent differential pumping of first region  108  and second region  110  is maintained by pump  134  and pump  136 , respectively, so that the pressure in second region  110  may be maintained at approximately one mTorr or less while gas of a higher pressure is supplied through opening  130  into first region  108 .  
      In order for particles (not shown) located between reticle  122  and barrier  126  to be conveyed away from reticle  122  by the gas using the principles of thermophoresis, a temperature differential must be maintained between reticle  122  and the surroundings of reticle  122 . Typically, in order for thermophoresis to convey particles away from reticle  122 , reticle  122  is maintained at a higher temperature than barrier  126 . When reticle  122  is maintained at a higher temperature than barrier  126 , particles (not shown) present between reticle  122  and barrier  126  may be attracted towards barrier  126 , as will be discussed below with respect to  FIG. 2 . In come cases, particles (not shown) that are attracted towards barrier  126  may pass into second region  110  through opening  130 . The flow of gas from region  108  to region  110  will also convey particles away from reticle  122 , which helps in keeping particles from coming into contact with reticle  122 .  
      With reference to  FIG. 2 , the use of thermophoresis to substantially repel particles away from the surface of a reticle will be described. A reticle  222 , which is maintained at a first temperature, may be positioned in proximity to a cooler surface  226 . Cooler surface  226  may be a differential pumping barrier in a chamber used in EUV lithography, or may be a shield which is arranged to protect reticle  222 . A variation in gas temperature is generally formed between reticle  222  and cooler surface  226  that goes from being relatively warm near reticle  222  to being relatively cool near cooler surface  226 . This creates a temperature gradient in the gas which is an essential condition for the existence of thermophoresis. Particles  228  are generally repelled from reticle  222  towards cooler surface  226 . That is, thermophoretic forces are such that particles are driven away from the hotter reticle  222  towards cooler surface  226 . Some particles  228  may become substantially attached to cooler surface  226 .  
      While the positioning of a surface in proximity to a reticle that is cooler than the reticle reduces particle contamination of the reticle, maintaining surfaces of different temperatures within an EUV apparatus is often problematic. For example, maintaining surfaces at different temperatures may complicate temperature control of critical systems. In addition, issues relating to thermal expansion and distortion typically arise when a reticle and adjacent components are maintained at different temperatures. When there is thermal expansion or distortion within an EUV apparatus, e.g., with respect to a reticle or a shield, the integrity of an overall lithography process or, more generally, a semiconductor fabrication process may be compromised. Also, the flow of gas from region  108  of chamber  104  to region  110  may sweep particles originating in region  108  into proximity with reticle  122 , thereby increasing the risk of contamination despite the protection afforded by thermophoresis.  
      Therefore, what is desired is a system which allows an EUV reticle to be efficiently and effectively protected from particle contamination substantially without adversely affecting an overall EUV lithography process. That is, what is needed is a system which enables a reticle such as an EUV reticle to be protected from particle contamination without a significant risk of thermal expansion and distortion issues arising.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention relates to using a flow of a relatively cool gas to establish a temperature gradient between a reticle and a reticle shield such that particle contamination on the reticle may be reduced. According to one aspect of the present invention, an apparatus that reduces particle contamination on a surface of an object includes a member having a surface proximate to the object, e.g., a plate, and a gas supply. The plate is arranged to be positioned in proximity to the object such that the plate, which is of a second temperature, and the object, which is of a first temperature, are substantially separated by a space. The gas supply supplies a gas flow to the space. The gas is of a third temperature that is lower than the first temperature and lower than the second temperature. Heat flow between the gas, the plate, and the object create a temperature gradient in the gas and, hence, a thermophoretic force that is suitable for conveying particles in the space away from the object.  
      In one embodiment, the plate includes at least a first opening defined therein that enables the gas flow to pass therethrough and into the space. In such an embodiment, the plate may also include a second opening defined therein. The second opening enables the gas flow to pass therethrough and out of the space to convey the particles in the space away from the object and away from the plate.  
      Allowing a reticle and a nearby surface, e.g., a reticle shield, to remain at substantially the same temperature while allowing for thermophoretic effects to convey particles away from the reticle reduces particle contamination without causing relatively significant thermal distortion effects and performance issues. By maintaining a reticle and a nearby surface at substantially the same temperature while providing a cooled or chilled gas in a space between the reticle and the nearby surface, a temperature gradient may be created between the reticle and the nearby surface. The presence of the temperature gradient allows thermophoretic forces to convey particles away from both the reticle and the nearby surface. The source of the gas is local, and the gas may be locally filtered, so the likelihood of the gas sweeping additional particles into the vicinity of the reticle is quite small.  
      According to another aspect of the present invention, a method for reducing particle contamination on a surface of an object includes providing a shield in proximity to the surface of the object that is positioned such that there is a space defined between the surface of the object and the shield. The shield has a first opening defined therein, and the surface of the object is of a first temperature while the shield is of a second temperature. The method also includes providing a flow of a gas in the space defined between the surface of the object and the shield, the gas being of a third temperature that is lower than both the first temperature and the second temperature. The flow of the gas is provided through the first opening.  
      In one embodiment, the flow of the gas in the space creates a temperature gradient in the space that enables the flow of the gas to convey any particles in the space away from the surface of the object. In another embodiment, providing the flow of the gas in the space includes cooling the gas to the third temperature and controlling the amount of the gas that flows through the first opening.  
      According to still another aspect of the present invention, an apparatus arranged to reduce particle contamination on a surface of an object includes a chamber, a first scanning arrangement, and a gas supply. The chamber has a first region and a second region where the first region has a pressure of at least approximately 50 mTorr while the second region has a pressure that is less than the pressure of the first region. The first scanning arrangement scans the object, and is positioned in the first region. The first scanning arrangement includes a plate that is arranged in proximity to a first surface of the object such that a first surface of the plate and the first surface of the object are substantially separated by a space in the first region. The first surface of the object is of a first temperature and the first surface of the plate is of a second temperature. The gas supply supplies a gas flow to the space. The gas is at a third temperature that is lower than the first temperature and lower than the second temperature, and cooperates with the plate and the object to create a thermophoretic force to convey any particles in the space away from the object.  
      In accordance with yet another aspect of the present invention, an apparatus arranged to reduce contamination on a surface of a first object includes a member having a first surface proximate to the first object and a second surface proximate to the second object. The member is in proximity to the second object such that the member and the second object are substantially separated by a space, and has a nozzle defined therethrough. The nozzle has an associated aperture that is closer to the second object and an opening, which is larger than the aperture, that is closer to the first object. The nozzle also has a gas supply that supplies a gas flow to the space. The apparatus also includes a pumping arrangement that causes the gas flow to be conveyed through the space substantially away from the aperture. In one embodiment, the first object is a mirror associated with an optical arrangement and the second object is a reticle mounted on a reticle stage assembly and enclosed in a vacuum chamber.  
      These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a diagrammatic side view representation of a portion of an extreme ultraviolet lithography or exposure system.  
       FIG. 2  is a diagrammatic representation of a reticle, a nearby surface, and particles which are attracted away from the reticle through the use of thermophoresis.  
       FIG. 3   a  is a diagrammatic representation of the layers of gas flow between a reticle and a reticle shield in accordance with an embodiment of the present invention.  
       FIG. 3   b  is a diagrammatic representation a temperature gradient associated with a gas located between a reticle and a reticle shield in accordance with an embodiment of the present invention.  
       FIG. 4   a  is a diagrammatic cross-sectional side view representation of a portion of an EUV lithography chamber which uses a cooled gas to create thermophoretic forces in accordance with an embodiment of the present invention.  
       FIG. 4   b  is a diagrammatic bottom view of one configuration of openings, i.e., openings  432  of  FIG. 4   a , through which a gas may flow between a reticle and a barrier in accordance with an embodiment of the present invention.  
       FIG. 4   c  is a diagrammatic bottom view of another configuration of openings, i.e., openings  432  of  FIG. 4   a , through which a gas may flow between a reticle and a barrier in accordance with an embodiment of the present invention.  
       FIG. 5   a  is a diagrammatic representation of a reticle in a first position with respect to a differential pumping barrier in accordance with an embodiment of the present invention.  
       FIG. 5   b  is a diagrammatic representation of a reticle in a second position with respect to a differential pumping barrier, i.e., reticle  512  and differential pumping barrier  528  of  FIG. 5   a , in accordance with an embodiment of the present invention.  
       FIG. 5   c  is a diagrammatic representation of a reticle in a third position with respect to a differential pumping barrier, i.e., reticle  512  and differential pumping barrier  528  of  FIG. 5   a , in accordance with an embodiment of the present invention.  
       FIG. 5   d  is a diagrammatic representation of a reticle i.e., reticle  512  of  FIG. 5   a , in two extreme positions, illustrating the application of an embodiment of the present invention.  
       FIG. 5   e  is a diagrammatic side view of a reticle with a second differential pumping barrier in accordance with an embodiment of the present invention.  
       FIG. 5   f  is a diagrammatic side view of yet another embodiment of the present invention.  
       FIG. 6  is a block diagram side-view representation of an EUV lithography system in accordance with an embodiment of the present invention.  
       FIG. 7  is a process flow diagram which illustrates the steps associated with fabricating a semiconductor device in accordance with an embodiment of the present invention.  
       FIG. 8  is a process flow diagram which illustrates the steps associated with processing a wafer, i.e., step  1304  of  FIG. 7 , in accordance with an embodiment of the present invention.  
       FIG. 9  is a diagrammatic cross-sectional side-view representation of a reticle stage assembly which utilizes a reticle shield to protect a reticle in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Particle contamination on critical surfaces of reticles such as reticles used in extreme ultraviolet (EUV) lithography systems may compromise the integrity of semiconductors created using the reticles. Hence, protecting critical surfaces of reticles from airborne contaminants is important to ensure the integrity of lithography processes. Some reticles are protected from airborne particles through the use of pellicles. However, pellicles are not suitable for use in protecting surfaces of EUV reticles. While thermophoresis is also effective in protecting reticle surfaces from particle contamination when at least a slight gas pressure is present, maintaining a surface that is in proximity to a reticle at a lower temperature than that of the reticle to enable thermophoretic forces to act often causes thermal expansion and distortion within an overall EUV lithography system.  
      By introducing a gas that flows between a reticle and a nearby surface, e.g., a reticle shield, that is at a cooler temperature than those of the reticle and the nearby surface, thermophoresis may be used to convey particles away from the reticle while the reticle may be maintained at substantially the same temperature as the nearby surface. The cooler gas will typically establish local temperature gradients adjacent to both the reticle and the nearby surface, thereby establishing thermophoretic forces which will effectively sweep particles away from both the reticle and the nearby surface. Since the reticle and the nearby surface are maintained at substantially the same temperature, particle contamination of the reticle may be reduced, while the potential for thermal expansion and distortion effects is also significantly reduced.  
      The introduction of a gas between a surface of a reticle and a surface of a reticle shield at a temperature that is cooler than the temperatures of the reticle and the reticle shield allows a temperature gradient to be formed in the gas between the reticle and the reticle shield. With reference to  FIGS. 3   a  and  3   b , the formation of a temperature gradient between the reticle and the reticle shield will be described in accordance with an embodiment of the present invention. As shown in  FIG. 3   a , when a cooled gas  312  is substantially introduced between a reticle  304  and a surface  308  near reticle  304 , as for example a reticle shield, a boundary layer  316  is formed near a surface of reticle  304  and a boundary layer  318  is formed near surface  308 . Boundary layers  316 ,  318  are generally warmer than the rest of cooled gas  312 , as will be understood by those skilled in the art, since the gas in boundary layers  316 ,  318  may be partially heated by reticle  304  and surface  308 , respectively.  
      Cooler gas  312  typically establishes local temperature gradients  320 , and cause thermophoretic forces to be established which will generally cause particles to move away from reticle  304  and surface  308 , and effectively be swept into the flow of cooled gas  312 . Hence, particle contamination of reticle  304  as well as particle contamination of surface  308  may be reduced.  
       FIG. 3   b  is a diagrammatic representation of cooled gas between a reticle and a nearby surface, e.g., cooled gas  312  of  FIG. 3   a , and a temperature gradient in accordance with an embodiment of the present invention. A temperature gradient  320  associated with cooled gas  312  may be such that the temperature distribution is approximately gaussian, as indicated by distributions  326 , with the coolest temperature being substantially midway between boundary layer  316  and boundary layer  318 . More generally, the temperature distribution is such that the coolest temperature is approximately halfway between boundary layer  316  and boundary layer  318 , while the warmest temperatures are at boundary layer  316  and boundary layer  318 , as indicated at  322 . It should be appreciated that the actual profile of a temperature distribution may vary widely.  
      A cooled gas such as cooled gas  312  may be introduced into an EUV lithography apparatus using a gas source or supply that is substantially external to the apparatus.  FIG. 4   a  is a diagrammatic cross-sectional side view representation of a portion of an EUV lithography chamber which uses a cooled gas to create thermophoretic forces in accordance with an embodiment of the present invention. An EUV lithography chamber  400  includes a first region  410  and a second region  411  that are substantially separated by a differential pumping barrier  428  or a reticle shield. A pressure of approximately fifty milliTorr (mTorr) or more is maintained in first region  410 , while a pressure of less than approximately 1 mTorr, i.e., a near vacuum, is maintained in second region  411 .  
      A reticle  412 , which is held by a reticle chuck  408  that is coupled to a reticle stage arrangement  404 , and barrier  428  are maintained at approximately the same temperature. A gas which is supplied by gas supplies  416  and is cooled using coolers  424  may be introduced through openings  432  into a space between reticle  412  and barrier  428 . The flow of the gas is approximately laminar, and may be controlled by gas flow controllers  420 . In one embodiment, filters  438  may be used to filter particles out of the gas as the gas passes through openings  432  into the space between reticle  412  and barrier  428 .  
      Openings  432  may generally be slots or orifices of various shapes and sizes. As shown in  FIG. 4   b , openings  432  may be a series of substantially round openings. Alternatively, openings  432 ′ may be slots as shown in  FIG. 4   c . It should be appreciated that the number of openings  432 , as well as the size and the shapes of openings  432 , may vary widely. In general, the shape and the configuration of openings  432  may be chosen to enable an approximately laminar flow of gas to be efficiently established.  
      Gas that flows through openings  432  into the space between reticle  412  and barrier  428  establishes local temperature gradients adjacent to reticle  412  and barrier  428 , and causes thermophoretic forces to convey particles away from reticle  412  and barrier  428 . The particles may be conveyed through an opening, or differential pumping aperture  436 , defined within barrier  428  which is generally arranged for an EUV beam to pass through. It should be appreciated that although gas may escape from between reticle  412  and barrier  428  and into the remainder of first region  410  or into second region  411 , the amount of gas that escapes is typically not excessive enough to significantly alter the pressure in first region  410  or to compromise the vacuum in second region  411 .  
      The gas introduced between reticle  412  and barrier  428  may be a light gas such as helium or hydrogen. In general, the gas is a pure gas that absorbs EUV radiation. In addition to being a light gas such as helium or hydrogen, the gas may be argon or nitrogen. Since nitrogen is relatively inexpensive, and is used in gas bearings (not shown) which are typically a part of reticle stage arrangement  404 , nitrogen may often be used as the gas introduced between reticle  412  and barrier  428 .  
      During lithographic exposure, reticle  412  is scanned back and forth above the opening  436  by means of reticle stage arrangement  404 . As reticle  412  scans, variations in temperature, and therefore thermophoretic force, that are caused by the gas, i.e., the cooled gas, warming up as the gas flows in contact with reticle  412  and barrier  428  may generally be substantially averaged out. Such a warming of the gas may be at least partially compensated for by the thermodynamic cooling of the gas as the gas approaches opening  436 , which often results in a temperature drop of the gas.  
      In order to maintain reticle  412  and barrier  428  at substantially the same constant temperature, as heat is removed by the cold flowing gas, a mechanism (not shown) for effectively heating reticle  412  and barrier  428  may be provided. To facilitate temperature control of barrier  428 , thermal insulation  425  may be used to thermally isolate barrier  428  from the surrounding structures. The mechanism for effectively heating reticle  412  and barrier  428  may generally be any suitable mechanism. By way of example, reticle  412  may be sufficiently heated by EUV radiation that passes through opening  436 , and no other mechanism may need to be used to heat reticle  412 . The removal of heat by the flowing gas is typically proportional to the heat capacity of the gas. Because of the low pressure of the gas, the heat capacity is relatively small, and the amount of heat removed from reticle  412  and barrier  428  is typically not excessive.  
      To reduce the amount of cooled gas that may effectively escape from between a reticle and a barrier and into a surrounding area, part of the flow of cooled gas may be shut down at times depending upon the positioning of the reticle. For example, when a reticle is near an extreme point of travel, gas flow through an opening or openings which are not effectively covered by the reticle may be shut off. As shown in  FIG. 5   a , when a reticle  512  that is supported by a reticle chuck  508  is scanned by a reticle stage arrangement  504  over a barrier  528  or shield, reticle  512  may be positioned such that openings  532   a ,  532   b  are both effectively covered by reticle  512 . However, when reticle  512  is at an extreme point of travel such that opening  532   b  is not effectively covered by reticle  512 , as shown in  FIG. 5   b , a gas flow through opening  532   b  may be shut off. Alternatively, when reticle  512  is at another extreme point of travel such that opening  532   a  is not effectively covered by reticle  512 , as shown in  FIG. 5   c , a gas flow through openings  532   a  may be shut off. By shutting down the flow of gas through one of openings  532   a ,  532   b  as appropriate, gas may be substantially prevented from being directly pumped into a surrounding environment.  
       FIG. 5   d  shows another embodiment which reduces the amount of cooled gas escaping from between a reticle and a barrier. Skirts  540   a  and  540   b , attached to stage arrangement  504 ″, effectively extend the length of reticle  512 , so that normal gas flow patterns are maintained even when reticle  512  is at an extreme position of travel. In one embodiment, a surface of skirts  540   a  and  540   b  which opposes barrier  528  is at substantially the same level as a surface of reticle  512  which opposes barrier  528 . Such skirts  540   a  and  540   b  experience no forces, save for the acceleration and deceleration of the stage arrangement  504 ″ itself, nor does their location need to be highly precise. Thus, skirts  540   a  and  540   b  may be constructed of very light thin materials, so that their addition has no effect on stage performance.  
       FIG. 5   e  shows an embodiment which allows less gas flow from the region between a reticle  512 ′ and a barrier  528 ′ into a region  511 ′ below barrier  528 ′. A nozzle  545  is attached to barrier  528 ′, and a gap  560  between the top surface of nozzle  545  and reticle  512 ′ is reduced to a relatively small value, thereby limiting gas flow into region  511 ′. Gap  560  may be approximately 1 mm or less, for example. Gas inlets  550   a  and  550   b  installed on nozzle  545  provide a flow of gas largely parallel to the surface of reticle  512 ′. This flow is largely undisturbed as reticle  512 ′ is scanned back and forth by a stage arrangement  504 ′. Gas flow into region  510  will typically fluctuate as stage arrangement  504 ′ scans, but the EUV radiation does not pass through region  510 , so the fluctuations will not significantly affect the EUV intensity.  
       FIG. 5   f  shows another embodiment of the invention. Gas is introduced into the region  521  between reticle  512 ′ and barrier  528 ′ through gas inlets  550   a  and  550   b . The gas pressure at the inlets is substantially higher than the ambient gas pressure in region  521  and the ambient pressure in region  510 ′. Thus the gas expands rapidly out of the inlets and cools significantly in the process. The initial temperature of the gas at the inlets may be adjusted to be warmer than, equal to, or cooler than the temperature of reticle  512 ′ or barrier  528 ′, but as it expands into region  521  a substantial fraction of it becomes cooler than reticle  512 ′ and barrier  528 ′. Thus the desired temperature gradient in the gas may be established under these conditions without the need to initially cool the supply of gas with a cooler such as  424 . In addition the high gas pressure at inlets  550   a ,  550   b  causes the gas flow to achieve a high velocity as it flows through region  521  into region  510 ′. This imparts a substantial drag force on any particle present which tends to quickly convey it out of region  521  and away from reticle  512 ′. Thus in this embodiment reticle  512 ′ is protected by both a thermophoretic force arising from the temperature gradient in the gas, and a drag force arising from the high velocity of the gas flow in region  521 .  
      In the embodiment described in  FIG. 5   f , the gas expanding out of gas inlets  550   a  and  550   b  exits the inlets at subsonic velocity. If the gas enters region  521  at supersonic velocity, it will collide with the ambient gas, creating shock waves and heating of the gas rather than the desired cooling. A subsonic flow into region  521  may be substantially insured if gas inlets  550   a  and  550   b  have openings whose dimensions are less than approximately the molecular mean free path of the expanding gas. If gas inlets  550   a  and  550   b  are each relatively large openings, they may be covered by particle filters whose effective pore size is less than approximately the molecular mean free path of the expanding gas.  
      With reference to  FIG. 6 , an EUV lithography system will be described in accordance with an embodiment of the present invention. An EUV lithography system  900  includes a vacuum chamber  902  with pumps  906  which are arranged to enable desired pressure levels to be maintained within vacuum chamber  902 . For example, pump  906   b  may be arranged to maintain a vacuum or a pressure level of less than approximately 1 mTorr within a second region  908   b  of chamber  902 . Various components of EUV lithography system  900  are not shown, for ease of discussion, although it should be appreciated that EUV lithography system  900  may generally include components such as a reaction frame, a vibration isolation mechanism, various actuators, and various controllers.  
      An EUV reticle  916 , which may be held by a reticle chuck  914  coupled to a reticle stage assembly  910  that allows the reticle to scan, is positioned such that when an illumination source  924  provides beams which subsequently reflect off of a mirror  928 , the beams reflect off of a front surface of reticle  916 . A reticle shield assembly  920 , or a differential barrier, is arranged to protect reticle  916  such that contamination of reticle  916  by particles may be reduced. In one embodiment, reticle shield assembly  920  includes openings  950  through which a cooled gas, which is supplied through a gas supply  954  with a temperature controller  958 , may flow.  
      As discussed above, reticle shield assembly  920  includes an opening through which beams, e.g., EUV radiation, may illuminate a portion of reticle  916 . Incident beams on reticle  916  may be reflected onto a surface of a wafer  932  held by a wafer chuck  936  on a wafer stage assembly  940  which allows wafer  932  to scan. Hence, images on reticle  916  may be projected onto wafer  932 .  
      Wafer stage assembly  940  may generally include a positioning stage that may be driven by a planar motor, as well as a wafer table that is magnetically coupled to the positioning stage by utilizing an EI-core actuator. Wafer chuck  936  is typically coupled to the wafer table of wafer stage assembly  940 , which may be levitated by any number of voice coil motors. The planar motor which drives the positioning stage may use an electromagnetic force generated by magnets and corresponding armature coils arranged in two dimensions. The positioning stage is arranged to move in multiple degrees of freedom, e.g., between three to six degrees of freedom to allow wafer  932  to be positioned at a desired position and orientation relative to a projection optical system reticle  916 .  
      Movement of the wafer stage assembly  940  and reticle stage assembly  910  generates reaction forces which may affect performance of an overall EUV lithography system  900 . Reaction forces generated by the wafer (substrate) stage motion may be mechanically released to the floor or ground by use of a frame member as described above, as well as in U.S. Pat. No. 5,528,118 and published Japanese Patent Application Disclosure No. 8-166475. Additionally, reaction forces generated by motion of reticle stage assembly  910  may be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and published Japanese Patent Application Disclosure No. 8-330224, which are each incorporated herein by reference in their entireties.  
      As described above, a reticle may be protected from particles using a reticle shield that covers the reticle except where the reticle is illuminated by EUV, in conjunction with a nozzle which generates a flow of gas substantially parallel to the reticle surface. In one embodiment, the nozzle may be a part of a fixed blind assembly. The gas flow drags particles with it, away from the reticle surface. The dragging of particles away from the reticle surface using gas flow may be referred to here as viscophoresis. Gas also expands and cools from an inlet to provide some thermophoretic protection.  
       FIG. 9  is a diagrammatic cross-sectional side-view representation of a reticle stage assembly which utilizes a reticle shield to protect a reticle in accordance with an embodiment of the present invention. A reticle stage  1200  supports a reticle chuck  1204  which, in turn, supports a reticle  1208 . Reticle  1208  is shielded by a reticle shield  1220 . A fixed blind aperture  1224  is arranged substantially within reticle shield  1220 , and reticle shield  1220  is arranged to define a nozzle  1228 . Nozzle  1128  opens up into a projection optics environment  1216 , while reticle stage  1200 , reticle chuck  1204 , and reticle  1208  are substantially within a reticle stage environment  1212 . It should be appreciated that, in one embodiment, projection optics environment  1216  may be a projection optics chamber and reticle stage environment  1212  may be a reticle stage chamber. In general, projection optics environment  1216  is arranged to include components such as a mirror (not shown) of an optical arrangement.  
      Gas flows between reticle  1208  and reticle shield  1220 , as represented by arrows  1230 . The gas may be supplied by a gas supply associated with or included in the nozzle. Some of the gas is pumped from the reticle stage environment  1212 , in one embodiment, by means of vacuum pumps attached to a reticle stage environment vacuum chamber (not shown). It should be appreciated that a reticle stage environment vacuum chamber may be such that reticle  1208  is substantially enclosed within the vacuum chamber. Some of the gas exits through fixed blind aperture  1224  into projection optics environment  1216 . Projection optics environment  1216  is maintained at a lower pressure than reticle stage environment  1212  and the space between reticle  1208 , and fixed blind aperture  1224  effectively serves as a differential pumping aperture. The higher pressure in reticle stage environment  1212  allows for viscophoresis and thermophoresis, and the lower pressure in projection optic environment  1216  allows for a relatively high transmission of EUV radiation through gas.  
      Projections optics mirror reflectivities are typically sensitive to hydrocarbon and water vapor contamination. Less than a monolayer absorbed on the surfaces of the mirrors may result in a relatively significant reduction in reflectivity and, hence, lithographic throughput. Outgassing of hydrocarbons or water vapor from structures in the reticle stage environment  1212 , such as reticle stage  1200  or reticle chuck  1204  or cables or hoses attached thereto, is substantially contained within reticle stage environment  1212  by the flow of gas represented by arrows  1230 . Therefore, projection optics mirrors within projection optics environment  1216  may be protected from contamination as a result of outgassing. The containment of products and byproducts of outgassing may be achieved in part through the use of differential pumping between projection optics environment  1216  and reticle stage environment  1212 . However, the containment of products and byproducts of outgassing is generally when the gas flows from nozzle  1228  effectively prevent outgassing from parts of the reticle stage environment  1212  from getting to fixed blind aperture  1224  and, hence, projection optics in projection optics environment  1216 .  
      Gas flow enables outgassing of a hydrocarbon such as methane, i.e., CH4, from the side of reticle stage  1200  or reticle chuck  1204  to be substantially confined to the vicinity of reticle stage  1200 . The concentration of CH4 may be reduced by approximately two orders of magnitude or more near the nozzle  1228  by the flow of gas.  
      When a situation arises in which reticle stage  1200  is moved to one extreme end of motion such that the outgassing region is closer to fixed blind aperture  1224  than it is in  FIG. 9 , containment of CH4 outgassing due to gas flow and the pressure differential between projection optics environment  1216  and reticle stage environment  1212  is generally still in effect. This typically assumes that the differential pumping condition between reticle stage environment  1212  and projection optics environment  1216  is maintained, which may involve the inclusion of reticle skirts such as reticle skirts  540  of  FIG. 5   d , in one embodiment. The concentration of CH4 in projection optics environment  1216  may still be reduced, as for example by approximately two orders of magnitude over the concentration if there were no pressure differential or gas flow.  
      An EUV lithography system according to the above-described embodiments, e.g., a lithography apparatus which may include a reticle shield, may be built by assembling various subsystems in such a manner that prescribed mechanical accuracy, electrical accuracy, and optical accuracy are maintained. In order to maintain the various accuracies, prior to and following assembly, substantially every optical system may be adjusted to achieve its optical accuracy. Similarly, substantially every mechanical system and substantially every electrical system may be adjusted to achieve their respective desired mechanical and electrical accuracies. The process of assembling each subsystem into a photolithography system includes, but is not limited to, developing mechanical interfaces, electrical circuit wiring connections, and air pressure plumbing connections between each subsystem. There is also a process where each subsystem is assembled prior to assembling a photolithography system from the various subsystems. Once a photolithography system is assembled using the various subsystems, an overall adjustment is generally performed to ensure that substantially every desired accuracy is maintained within the overall photolithography system. Additionally, it may be desirable to manufacture an exposure system in a clean room where the temperature and humidity are controlled.  
      Further, semiconductor devices may be fabricated using systems described above, as will be discussed with reference to  FIG. 7 . The process begins at step  1301  in which the function and performance characteristics of a semiconductor device are designed or otherwise determined. Next, in step  1302 , a reticle (mask) in which has a pattern is designed based upon the design of the semiconductor device. It should be appreciated that in a parallel step  1303 , a wafer is made from a silicon material. The mask pattern designed in step  1302  is exposed onto the wafer fabricated in step  1303  in step  1304  by a photolithography system. One process of exposing a mask pattern onto a wafer will be described below with respect to  FIG. 8 . In step  1305 , the semiconductor device is assembled. The assembly of the semiconductor device generally includes, but is not limited to, wafer dicing processes, bonding processes, and packaging processes. Finally, the completed device is inspected in step  1306 .  
       FIG. 8  is a process flow diagram which illustrates the steps associated with wafer processing in the case of fabricating semiconductor devices in accordance with an embodiment of the present invention. In step  1311 , the surface of a wafer is oxidized. Then, in step  1312  which is a chemical vapor deposition (CVD) step, an insulation film may be formed on the wafer surface. Once the insulation film is formed, in step  1313 , electrodes are formed on the wafer by vapor deposition. Then, ions may be implanted in the wafer using substantially any suitable method in step  1314 . As will be appreciated by those skilled in the art, steps  1311 - 1314  are generally considered to be preprocessing steps for wafers during wafer processing. Further, it should be understood that selections made in each step, e.g., the concentration of various chemicals to use in forming an insulation film in step  1312 , may be made based upon processing requirements.  
      At each stage of wafer processing, when preprocessing steps have been completed, post-processing steps may be implemented. During post-processing, initially, in step  1315 , photoresist is applied to a wafer. Then, in step  1316 , an exposure device may be used to transfer the circuit pattern of a reticle to a wafer. Transferring the circuit pattern of the reticle of the wafer generally includes scanning a reticle scanning stage which may, in one embodiment, include a force damper to dampen vibrations.  
      After the circuit pattern on a reticle is transferred to a wafer, the exposed wafer is developed in step  1317 . Once the exposed wafer is developed, parts other than residual photoresist, e.g., the exposed material surface, may be removed by etching. Finally, in step  1319 , any unnecessary photoresist that remains after etching may be removed. As will be appreciated by those skilled in the art, multiple circuit patterns may be formed through the repetition of the preprocessing and post-processing steps.  
      Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, while the use of a cooled gas to establish thermophoretic forces between a reticle and a reticle shield has been described, a cooled gas may be used in proximity to a wafer surface to establish thermophoretic forces to keep particles from being attracted to the wafer surface. In addition, the introduction of a cooled gas flow in proximity to a wafer surface may further enable outgassing products of the wafer surface to be conveyed away from the wafer surface.  
      A gas that is to be introduced into a space between a reticle and a reticle shield has generally been described as being cooled by coolers that are in proximity to openings in the reticle shield. That is, a cooled gas has been described as being locally cooled. It should be appreciated, however, that a gas may be cooled by substantially any suitable mechanism in a suitable location. In addition, the gas may be any suitable gas, as for example a light gas such as helium or hydrogen.  
      Substantially any suitable mechanism may be used to maintain the temperature of the reticle and the temperature of a reticle shield at a temperature that is warmer than the temperature of a cooled gas that is provided in the space defined between the reticle and the reticle shield. The configuration of such suitable mechanisms may generally vary widely.  
      A fixed blind aperture, e.g., fixed blind aperture  1224  of  FIG. 9 , has generally been described as being the only channel between a reticle stage environment or chamber and a projection optics environment or chamber. It should be understood, however, that there may be other channels between a reticle stage environment and a projection optics environment. By way of example, openings may exist in a reticle shield to accommodate alignment microscopes and an interferometer fixed mirror. As gas flow is arranged such that contamination or particles may be kept away from the reticle shield, some contamination or particles may be transported through any other openings in the reticle shield. However, as the conductances between a reticle stage environment and a projection optics environment is generally small compared to the conductances which occur through the fixed blind aperture, any contamination transported through other openings in the reticle shield may likely be considered to be relatively negligible.  
      While the use of a gas flow in conjunction with a reticle shield may be suitable for protecting projection optics, the use of a gas flow in conjunction with a reticle shield may be suitable for protecting other components of an overall system which utilizes an EUV reticle. For instance, illumination optics may also be protected using a gas flow and a reticle shield.  
      A reticle and a barrier or a reticle shield have been described as having substantially the same temperature. In one embodiment, the reticle and the barrier may have different temperatures that are warmer than the temperature of a cooled gas introduced into a space between the reticle and the barrier. That is, the reticle and the barrier may have slightly different temperatures as long as the different temperatures are both higher than the temperature of the cooled gas provided between the reticle and the barrier without departing from the spirit or the scope of the present invention. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.