Abstract:
Apparatus and methods are disclosed for reducing particle contamination of a surface of an object such as a reticle used in an EUV lithography system. An exemplary apparatus includes a thermophoresis device and an electrophoresis device. The thermophoresis device is situated relative to and spaced from the surface, and is configured to produce a thermophoretic force, in a gas flowing past and contacting the surface, sufficient to inhibit particles in the gas from contacting the surface. The electrophoresis device is situated relative to a region of the surface contacted by the gas and is configured to deflect particles, having an electrostatic charge, in the gas away from the region as the gas flows past the region. In an example, the thermophoresis device produces the thermophoretic force by establishing a temperature gradient in the gas relative to the surface, and the electrophoresis device includes an electrode situated and configured to produce a voltage gradient relative to the region of the surface sufficient to attract charged particles away from the region of the surface.

Description:
FIELD  
       [0001]     This disclosure pertains, inter alia, to microlithography, which is a key imaging and pattern-transfer technology employed in the fabrication of semiconductor devices such as integrated circuits, displays, and the like. More specifically, the disclosure pertains to microlithography in which extreme ultraviolet (EUV) light is used for transferring a pattern from a pattern-defining reticle to an exposure-sensitive substrate such as a semiconductor wafer. Even more specifically, the disclosure pertains to reducing particulate contamination of the reticle.  
       BACKGROUND  
       [0002]     Extreme-ultraviolet lithography (EUVL) is currently regarded as a candidate “generation lithography” (NGL) that offers prospects of substantially finer pattern resolution than currently obtainable using conventional “optical” lithography (i.e., lithography performed using deep-ultraviolet wavelengths of light). These expectations of increased resolution from EUVL stem largely from the fact that, whereas current optical lithography is performed using a wavelength in the range of 150-250 nm, EUVL is performed using a wavelength in the range of 11-15 nm, which is at least ten times shorter than the conventional “optical” wavelengths. Generally, the shorter the wavelength of light used for pattern imaging in microlithography, the finer the obtainable resolution.  
         [0003]     In view of the extremely small pattern elements (currently less than 100 nm) that can be resolved using microlithography, including EUVL, the accuracy and precision with which pattern transfer is performed lithographically must be extremely high to ensure proper placement and registration of multiple pattern layers on a substrate and to ensure that the pattern elements are transferred to the substrate with high fidelity. To obtain such high accuracy and precision, extreme measures are taken to control and remove extraneous causes of performance degradation. For example, with current expectations being demanded of microlithography systems to produce pattern features of less than 100 nm, eliminating significant particulate contamination has become paramount.  
         [0004]     EUV light is highly attenuated by the atmosphere, and no currently known materials are adequately transmissive and refractive to EUV light for use as EUV lenses. Consequently, EUVL must be performed under high vacuum using reflective optics (mirrors) for illumination of the reticle and for projection of the illuminated pattern from the reticle to the substrate. Even the reticle is reflective rather than being a transmissive reticle as used in conventional optical microlithography.  
         [0005]     In optical microlithography the reticle during use typically is protected by a pellicle from particulate contamination. (The pellicle is a transmissive thin film on a frame that covers the patterned surface of the reticle to prevent deposition of particles on the reticle surface.) A pellicle cannot be used with a reticle for EUVL because, in view of the lack of EUV-transmissive materials, the pellicle would absorb and thus block the EUV beam incident to the reticle, leaving substantially no EUV light for projecting the pattern image to the substrate. Thus, the EUVL reticle must be used naked, which leaves the reticle vulnerable to particulate contamination during use. In optical lithography in which the reticle is protected by a pellicle, a particle deposited on the pellicle is sufficiently displaced from the plane of the reticle (i.e., outside the depth of focus) to be unresolved (or at most poorly resolved) on the wafer. A particle on a naked EUVL reticle, on the other hand, is in the plane of the reticle pattern and hence is resolved on the wafer where it likely will flaw the projected pattern image. Hence, for EUVL reticles, particle protection as effective as a pellicle is required.  
         [0006]     In EUVL systems currently under development, the reticle is used facing downward, which is helpful in preventing deposition of particles on the reticle surface. However, merely facing the reticle downward is insufficient for keeping the reticle completely clean, and various techniques currently are being developed that are aimed at preventing deposition of any particulate contamination on the reticle without having to use a pellicle. One technique that exhibited remarkable success in preventing particulate deposition on the reticle (by preventing particles from hitting the reticle) is termed “thermophoresis,” discussed in Rader et al., “Verification Studies of Thermophoretic Protection for EUV Masks,”  Proceedings SPIE  4688:182-193, 2002. See also U.S. Pat. Nos. 6,153,044 and 6,253,464. Thermophoresis refers to a force exerted on particles in a gas where a temperature gradient is present, wherein the particles are driven by a thermophoretic “force” (imparted by the gas) from a warmer region to a cooler region. Thus, a surface can be protected from particle deposition by maintaining the surface at a warmer temperature than its surroundings.  
         [0007]     General principles of thermophoresis as applied in an EUVL system are described with reference to  FIG. 8 , which depicts a reticle  222  and a nearby surface  226  that is maintained at a cooler temperature than the reticle  222 . The cooler surface  226  may be, for example, a differential pumping barrier used in a vacuum chamber housing the reticle  222  or a shield that protects the reticle. A gas in the vicinity of the reticle  222  and the surface  226  exhibits a temperature gradient in which the gas is warmer near the reticle  222  and cooler near the surface  226 . The thermophoretic “force” associated with the gradient urges particles  228  away from the warmer reticle  222  toward the cooler surface  226 . Some particles  228  may actually become attached to the surface  226 . Thermophoretic forces are greatest in the presence of a sufficient gas pressure in which the mean free path of the gas molecules is a small fraction of the distance from the reticle  222  and the surface  226 . As pressure is decreased (i.e., as vacuum is increased), thermophoretic forces decrease correspondingly. In other words, thermophoresis is poorly effective in high vacuum, but at a pressure of 50 mTorr thermophoresis is still significant for effectively urging particles  228  away from the reticle  222 .  
         [0008]     A conventional thermophoretic scheme as disclosed in the references cited above is generally shown in  FIG. 9 , which depicts a portion of an EUVL system  100  in the vicinity of the reticle. The depicted system  100  comprises a vacuum chamber  104  including a first region  108  and a second region  110 . The first region  108  contains a reticle stage  114  that supports a reticle chuck  118  configured to hold a reticle  122  face-down. The second region  110  contains projection optics  124  and a wafer stage (not shown). The first and second regions  108 ,  110  are substantially separated from each other by a barrier wall  126  through which an opening  130  is defined. The barrier wall  126  and opening  130  collectively form a differential pumping barrier. The opening  130  is sufficiently large to pass EUV light incident to and reflected from the reticle  122 . Gas at a pressure of approximately 50 mTorr is supplied to the first region  108  via a gas-supply port  132  in the vacuum chamber  104 . To minimize EUV-absorption losses to ambient gas, the second region  110  is maintained at a lower pressure (i.e., higher vacuum; e.g., ≦1 mTorr) than the first region  108 . Maintaining these two respective pressures in the regions  108 ,  110  is achieved by differential evacuation of the regions, performed using respective vacuum pumps  134 ,  136  and facilitated by the differential pumping barrier.  
         [0009]     In the configuration shown in  FIG. 9 , to remove particles away from the reticle  122  by thermophoresis, the reticle is maintained at a higher temperature than the barrier wall  126 . This temperature differential, as discussed above, results in attraction of the particles to the barrier wall  126 , which causes some particles (entrained in gas passing through the opening  130 ) to enter the second region  110  via the opening  130 . The flow of gas from the region  108  to the region  110  also helps convey particles away from the reticle  122  and thus prevents the particles from contacting the reticle.  
         [0010]     While placing a cooler surface proximal to a warmer reticle helps reduce particulate contamination of the reticle, maintaining surfaces of different temperatures within the EUVL system can be problematic. For example, maintaining surfaces at different temperatures can complicate temperature control of critical subsystems and can generate issues relating to thermal expansion and distortion of critical components. For example, thermal expansion or distortion of the reticle can compromise the performance of the overall EUVL lithography process and hence of the semiconductor-device-fabrication process. Also, flowing gas from the region  108  to the region  110  may sweep particles originating in the region  108  toward the reticle  122 , which would increase the risk of contamination despite the general protection afforded by thermophoresis.  
         [0011]     Other manners of solving this problem are described elsewhere by the current Applicant, namely U.S. patent application Ser. No. 10/898,475, entitled “Extreme Ultraviolet Reticle Protection Using Gas Flow Thermophoresis,” filed on Jul. 23, 2004, and a corresponding PCT CIP Application, entitled “Extreme Ultraviolet Reticle Protection,” filed on Jul. 23, 2005, both of which being incorporated herein by reference. Briefly, a space is defined between the reticle and a nearby surface, such as a barrier wall or reticle shield. At least one gas nozzle is situated in the space. A gas, cooled to below the temperature of the reticle and surface (the reticle and surface normally have substantially the same temperature), is discharged from the nozzle(s) into the space. The discharged gas, flowing substantially parallel to the reticle, establishes local temperature gradients adjacent the reticle and surface, respectively. The temperature gradients engender respective thermophoretic forces tending to urge particles away from the reticle and surface so that the particles become or remain entrained in the gas.  
         [0012]     A particular configuration of the apparatus  300  described in the &#39; 475  application is shown in  FIG. 10 , which depicts a reticle  302  supported by a reticle chuck  304  mounted face down on a reticle stage  306 . The reticle stage  306 , reticle chuck  304 , and reticle  302  are contained in a reticle chamber  308  that is separated from a projection-optics chamber  310  by a barrier wall  312  (e.g., a differential pumping barrier or reticle shield). The barrier wall  312  defines an aperture  314  that is sized and configured to allow illumination EUV light  316  to impinge on the desired region of the reticle  302  and to pass patterned EUV light  318  reflected from the reticle to the projection optics (not shown). The aperture  314  also helps establish and maintain the differential pressures in the two chambers  308 ,  310  (the reticle chamber  308  is typically at approximately 50 mTorr, and the projection-optics chamber  310  is typically at less than 1 mTorr). During exposure, to illuminate successive regions of the reticle  302 , the reticle stage  306  moves in a scanning manner relative to the aperture  314 . Flanking the aperture  314  and extending upward (in the figure) toward the reticle  302  is a nozzle manifold  320   a,    320   b  that defines nozzle openings  322   a,    322   b  for discharging the gas. The nozzle openings  322   a,    322   b  are oriented so as to discharge the gas into the space  324  (between the reticle  302  and the barrier wall  312 ) in a direction substantially parallel to the reticle. The flow of gas (note arrows  326 ) past the reticle  302  is approximately laminar.  
         [0013]     As noted above, the gas can be cooled before discharging the gas into the space  324  between the reticle  302  and barrier wall  312 . Alternatively, the nozzle openings  322   a,    322   b  are sized and configured to establish a substantially higher gas pressure at the nozzle openings than in the space  324 . Thus, discharge of the gas is accompanied by adiabatic cooling of the gas. I.e., as the gas is discharged into the space  324 , it expands rapidly out of the nozzle openings  322   a,    322   b  and cools significantly in the process. With such a configuration, the discharged gas is colder than the reticle  302  and barrier wall  312  and establishes the desired temperature gradient without having to pre-cool the gas. In addition, the relatively high gas pressure at the nozzle openings  322   a,    322   b  produces a high gas-flow velocity through the space  324 . This high-velocity flow establishes a substantial viscous-drag force on particles and tends to convey the particles out of the space  324  and thus away from the reticle  302 .  
         [0014]     As indicated by the multiple arrows  326 , most of the discharged gas (and entrained particles) flows laterally as shown, substantially parallel to the reticle  302 , through the space  324  and is exhausted via the vacuum pump (not shown but see item  134  in  FIG. 9 ) that evacuates the reticle chamber  308 .  
         [0015]     Referring further to  FIG. 10 , the nozzle manifold  320   a,    320   b  extends upward (in the figure) and forms respective narrow gaps G between the “tops” of the nozzle manifold and the surface of the reticle  302 . These gaps G, each approximately 1 mm or less, allow limited movement of the reticle  302  (in the vertical, or “z,” direction) as required for focus control and reticle-wafer alignment movements. The narrow gaps G also allow a limited flow of gas (note single arrows  330  compared to multiple arrows  326 ) from the space  324  through the aperture  314  to the projection-optics chamber  310 . The gas flow through the gaps G is limited so as to maintain the desired vacuum level in the projection-optics chamber  310  for minimal attenuation of the EUV illumination and patterned beams.  
         [0016]     In the scheme summarized above the pressure in the gaps G is substantially lower than, for example, the pressure in the space  324  between the reticle  302  and the barrier wall  312 . This reduced pressure produces a correspondingly reduced thermophoretic “force” in the gaps G compared to elsewhere on the reticle  302 . Consequently, particles urged into these gaps by the higher-velocity gas flow near the nozzle openings  322   a,    322   b  have opportunities to contact the reticle  302  and become attached to it.  
         [0017]     Therefore, a need exists for methods and devices for reducing this source of particulate contamination of the reticle.  
       SUMMARY  
       [0018]     The foregoing need is met by apparatus and methods as disclosed herein.  
         [0019]     According to a first aspect, apparatus are provided for reducing particle contamination of a surface of an object. An embodiment of such an apparatus comprises a thermophoresis device and an electrophoresis device. The thermophoresis device is situated relative to and spaced from the surface and is configured to produce a thermophoretic force, in a gas flowing past and contacting the surface, sufficient to inhibit particles in the gas from contacting the surface. The electrophoresis device is situated relative to a region of the surface contacted by the gas and is configured to deflect particles, having an electrostatic charge, in the gas away from the region as the gas flows past the region. The thermophoresis device desirably produces the thermophoretic force by establishing a temperature gradient in the gas relative to the surface. In this configuration the electrophoresis device desirably comprises an electrode that is situated and configured to produce a voltage gradient relative to the region of the surface sufficient to attract charged particles away from the region of the surface.  
         [0020]     The object can be a lithographic reticle having a patterned surface including the region, wherein the region is irradiated by a lithographic energy beam propagating to the reticle along an irradiation path. In this configuration the thermophoresis device desirably is configured to deliver a flow of the gas in a first direction relative to the surface, wherein the gas desirably is at a temperature appropriate for producing the thermophoretic force. The thermophoresis device desirably directs a portion of the gas flow, in the vicinity of the region, in a second direction that is opposite the first direction and then away from the region along a trajectory including at least a portion of the irradiation path so as to allow the lithographic energy beam to impart charges to the particles in the directed portion of the gas flow in the second direction. The electrophoresis device desirably is configured to deflect the charged particles, in the directed portion of the gas flow, away from the region. If the lithographic energy beam is a beam of extreme ultraviolet light, the reticle typically is a reflective reticle.  
         [0021]     Another embodiment of an apparatus for reducing particle contamination of a surface of an object comprises a thermophoresis device and an electrophoresis device. The thermophoresis device is configured to flow a gas stream in a space substantially parallel to the surface of the object, wherein the gas stream has a temperature lower than a temperature of the surface so as to establish a thermophoretic temperature gradient between the gas and the surface. The temperature gradient creates a thermophoretic force that inhibits particles from contacting the surface. The electrophoresis device is situated relative to the surface and the thermophoresis device and comprises at least one electrode situated in the gas stream. The at least one electrode is configured, when energized, to establish a voltage gradient in the gas stream urging migration of charged particles in the gas away from the surface. The thermophoresis device desirably comprises a gas-discharge nozzle that produces the gas stream by discharging the gas stream in a manner by which the gas is adiabatic cooled so as to establish the thermophoretic temperature gradient. The thermophoresis device can be further configured to bleed off a portion of the gas stream, in a space adjacent a region of the surface, to the electrophoresis device. The at least one electrode of the electrophoresis device desirably is connected to a DC-voltage source.  
         [0022]     The object can be a lithographic reticle having a patterned surface that is configured to be irradiated by a lithographic energy beam under a vacuum condition. In this configuration the lithographic reticle can be contained in a first vacuum chamber that is separated by a barrier wall from a second vacuum chamber, wherein the barrier wall defines an opening allowing the first and second vacuum chambers to communicate with each other. The opening can be situated so as to allow the lithographic energy beam to pass from the second vacuum chamber to the first vacuum chamber in which the lithographic energy beam illuminates a selected region on the lithographic reticle and reflects, as a patterned beam from the selected region, back into the second vacuum chamber.  
         [0023]     The reticle in the first vacuum chamber can be situated substantially parallel to the barrier wall, thereby defining a substantially uniform-width space between the reticle and the barrier wall. In this configuration the thermophoresis device can be configured to flow the stream of gas substantially parallel to the reticle and barrier wall through the space, and to bleed off a portion of the stream through the opening from the space into the second chamber away from the reticle. The at least one electrode of the electrophoresis device attracts the charged particles, in the bled-off portion of the stream exiting the first chamber through the opening, to move away from the reticle.  
         [0024]     As noted above, the lithographic energy beam can be a beam of extreme ultraviolet light. In this configuration the lithographic energy beam can have sufficient energy to impart charges to particles entrained in the bled-off portion of the gas stream, so as to render the particles attractive to the at least one electrode.  
         [0025]     Yet another embodiment of an apparatus for reducing particle contamination of a surface of an object comprises a member having a surface that is proximal to the object surface but separated a first distance from the object surface so as to define a first space between the surfaces. The member defines an opening adjacent a region of the object. The apparatus includes a nozzle manifold that is mounted to the member in the first space and is configured to supply a gas into the first space such that a first portion of the supplied gas flows substantially parallel to the object surface and in a manner that establishes a thermophoretic force in the first space. The thermophoretic force inhibits particles from contacting the object surface. A second portion of the supplied gas flows substantially parallel to the region and then away from the object through the opening, wherein the flow of the second portion is constrained relative to the flow of the first portion of the supplied gas. The apparatus also includes at least one electrophoresis electrode that is mounted to the nozzle manifold and is configured to produce an electric field sufficient to attract charged particles, in the second portion of the supplied gas, away from the object surface as the second portion flows through the opening. The electrophoresis electrode can be configured to establish, when energized, the electric field between the object and the electrophoresis electrode by a voltage applied to the electrode relative to the object. The first and second portions of the supplied gas can flow substantially parallel to the object but in different respective directions.  
         [0026]     The nozzle manifold can be situated adjacent the opening. In this configuration the nozzle manifold can be oriented and configured to supply the gas such that the first portion of the supplied gas flows approximately laminarly through the first space away from the opening, and the second portion of the supplied gas flows approximately laminarly toward the opening.  
         [0027]     The nozzle manifold can extend from the member surface toward the object surface and can define a surface that is separated from the object surface a second distance that is less than the first distance so as to define a second space that is narrower than the first space, thereby providing a lesser flow of the supplied gas through the second space compared to through the first space. The surface of the nozzle manifold can comprise the electrophoresis electrode. The nozzle manifold can be configured to discharge the gas while cooling the gas adiabatically as the nozzle manifold supplies the gas into the first space. The gas supplied by the nozzle manifold can have a lower temperature than either the member surface or the object surface.  
         [0028]     The opening defined by the member can be configured as a fixed aperture through which a radiation beam is directed to the region of the object surface. In this configuration the radiation beam can be a beam of extreme ultraviolet light having sufficient energy to impart a charge to at least some of the particles as the second portion of the gas, including particles, enters the opening.  
         [0029]     The object can be situated in a first chamber that is maintained at a first pressure, wherein the member separates the first chamber from a second chamber that is maintained at a second pressure that is lower than the first pressure. The resulting pressure difference causes the second portion of the supplied gas to flow from the first chamber through the opening to the second chamber. By way of example, the opening can be configured as a differential pumping aperture. The opening also can be configured as an illumination aperture for transmitting a radiation beam to the object.  
         [0030]     Yet another embodiment of an apparatus for reducing particle contamination of a surface of an object comprises thermophoresis means for producing a thermophoretic force, in a gas flowing past and contacting the surface, sufficient to inhibit particles in the gas from contacting the surface. The apparatus also comprises electrophoresis means for deflecting particles, having an electrostatic charge, in the gas away from a region of the object surface contacted by the gas as the gas flows past the region.  
         [0031]     Yet another embodiment of an apparatus for reducing particle contamination of a surface of an object comprises thermophoresis means for flowing a gas stream, having a temperature lower than a temperature of the surface, in a space substantially parallel to the surface so as to establish a thermophoretic temperature gradient between the gas and the surface. The thermophoresis means is also for creating, from the temperature gradient, a thermophoretic force that inhibits particles from contacting the surface. The apparatus also comprises electrophoresis means for establishing a voltage gradient that urges migration of charged particles in the gas stream away from the surface.  
         [0032]     Yet another embodiment of an apparatus for reducing particle contamination of a surface of an object comprises member means for providing a surface that is separated a first distance from the object surface, for defining a first space between the surfaces, and for defining an opening adjacent a region of the object. The apparatus also comprises gas-discharge means for supplying a gas into the first space such that a first portion of the discharged gas flows substantially parallel to the object surface and a second portion of the discharged gas flows substantially parallel to the region and then away from the object through the opening. The gas has a lower temperature than the object surface, which establishes a thermophoretic force in the first space that inhibits particles in the gas from contacting the object surface. The apparatus includes electrophoresis means for producing an electric field sufficient to attract charged particles, in the second portion of the supplied gas, away from the object surface as the second portion flows through the opening.  
         [0033]     Another aspect is directed, in the context of an EUV lithography system, to apparatus for reducing particulate contamination of a surface of a reticle that is contained in a reticle chamber evacuated to a vacuum level during use of the reticle. An embodiment of such an apparatus comprises a reticle-shield member having a surface that is proximal to and substantially parallel to the surface of the reticle but separated a first distance from the reticle surface so as to define a first space between the surfaces. The shield member defines an opening that is configured to transmit a beam of EUV radiation to a selected region of the reticle surface for illuminating the region. The apparatus includes a gas nozzle manifold that is situated and configured to supply a gas into the first space such that a first portion of the supplied gas flows in the first space. The gas is at an appropriate temperature relative to the temperature of the reticle-shield member, the gas flow is at an appropriate pressure and flow rate, and the gas flow is substantially parallel to the reticle surface to produce a thermophoretic force in the first space sufficient for inhibiting particles in the gas from contacting the reticle surface. A second portion of the supplied gas flows relative to the region and then away from the reticle through the opening. The apparatus includes at least one electrophoresis electrode situated relative to the opening and the region. The electrode is configured to produce an electric field sufficient to attract charged particles, in the second portion of the supplied gas, away from the reticle surface in the region as the second portion flows through the opening. The opening desirably has a shape corresponding to a shape of an illumination region on the reticle surface, and can be configured as a fixed aperture. The nozzle manifold can be situated adjacent the opening and can extend toward the reticle surface so as to define a second space, between a top surface of the gas port and the reticle surface, that is thinner than the first space. The second space establishes a bleed rate, relative to gas flow through the first space, of gas flow through the second space and out through the opening.  
         [0034]     The at least one electrode can be situated on the top surface of the gas port. In this configuration the apparatus further can comprise a dielectric layer situated between the at least one electrode and the top surface of the gas port.  
         [0035]     The reticle-shield member can separate the reticle chamber from a downstream optics chamber. In this configuration the nozzle manifold can be situated adjacent the opening and can extend toward the reticle surface so as to define a second space, between a top surface of the nozzle manifold and the reticle surface, that is thinner than the first space. Thus, a bleed rate (relative to gas flow through the first space) of gas flow is established through the second space and out through the opening. The at least one electrode can be situated on the top surface of the nozzle manifold.  
         [0036]     According to another aspect, methods are provided for reducing particulate contamination of a surface of an object. In an embodiment of such a method, one step comprises delivering a gas relative to the object such that the gas contacts the object surface, wherein the gas has a lower temperature than the object surface. In another step a thermophoretic force is produced in the gas that is sufficient to inhibit particles from contacting the surface. In yet another step an electrophoretic force is produced in a region of the surface contacted by the gas. The electrophoretic force is sufficient to deflect particles, having an electrostatic charge, in the gas away from the region.  
         [0037]     In one example the gas is delivered so that the gas flows substantially parallel to the object surface. In another example the gas is delivered such that a first portion of the gas flows substantially parallel to the object surface in a first direction and a second portion of the gas flows in a second direction opposite the first direction and then away from the region. In this example the electrophoretic force can be produced so as to deflect particles in the second portion as the second portion flows past the region.  
         [0038]     The method further comprises the step of irradiating the region, which can be performed using EUV radiation, wherein the object can be a reticle having a patterned surface including the region.  
         [0039]     In another embodiment of a method for reducing particle contamination of a surface of an object, one step comprises producing a gas stream having a temperature lower than a temperature of the surface. The gas stream is substantially parallel to the surface so as to contact the surface and produce a thermophoretic force between the gas and the surface sufficient for inhibiting particles in the gas stream from contacting the surface. In another step an electrophoretic force is produced that is sufficient for urging migration of charged particles in the gas away from the surface.  
         [0040]     In another embodiment of a method for reducing particle contamination of a surface of a lithographic reticle, one step comprises placing the reticle relative to a member having a surface that is separated a first distance from the reticle surface, so as to define a first space between the surfaces. The member defines an opening adjacent a region of the reticle surface. A gas flow is supplied into the first space such that a first portion of the gas flows relative to the reticle surface and a second portion of the gas flows relative to the region and then away from the reticle surface through the opening. A thermophoretic force is established in the first space to inhibit particles in the gas from contacting the reticle surface. An electric field is produced that is sufficient to attract charged particles, in the second portion of the supplied gas, away from the reticle surface as the second portion flows through the opening. This method embodiment further can comprise irradiating the region of the reticle surface while establishing the thermophoretic force and producing the electric field. The region can be irradiated with an electromagnetic radiation sufficient to ionize the particles, thereby producing at least some of the charged particles.  
         [0041]     The gas can be supplied in a manner resulting in adiabatic cooling of the gas as the gas is supplied into the first and second spaces. The second portion of the gas can be made to flow at a bleed rate compared to the first portion.  
         [0042]     The foregoing and additional features and advantages of apparatus and methods as disclosed herein will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]      FIG. 1  is a schematic diagram showing general principles of electrophoresis as applied in various embodiments described herein.  
         [0044]      FIG. 2 ( a ) is a schematic elevational view of a portion of a representative embodiment of a reticle-protection device.  
         [0045]      FIG. 2 ( b ) is a schematic plan view of a portion of the device shown in  FIG. 2 ( a ), depicting an exemplary shape of the fixed aperture and the gas-discharge nozzle structures.  
         [0046]      FIG. 3  is schematic elevational view showing details of the nozzle structures and electrodes.  
         [0047]      FIG. 4  is a schematic elevational view similar to  FIG. 3  but also showing an exemplary manner in which the electrodes can be connected to a voltage source.  
         [0048]      FIG. 5  is a schematic elevational view of an EUV lithography system including a reticle-protection device as disclosed herein.  
         [0049]      FIG. 6  is a process-flow diagram illustrating exemplary steps associated with a process for fabricating semiconductor devices.  
         [0050]      FIG. 7  is a process-flow diagram illustrating exemplary steps associated with processing a substrate (wafer), as would be performed, for example, in step  704  in  FIG. 6 .  
         [0051]      FIG. 8  is a schematic diagram showing general principles of thermophoresis.  
         [0052]      FIG. 9  is a schematic elevational view of a portion of an EUV lithography system that exploits thermophoresis for protecting the reticle according to one manner as known in the art.  
         [0053]      FIG. 10  is a schematic elevational view in the vicinity of a reticle stage of an EUV lithography system that exploits thermophoresis for protecting the reticle according to another manner as known in the art. 
     
    
     DETAILED DESCRIPTION  
       [0054]     This disclosure is set forth in the context of representative embodiments, which are not intended to be limiting in any way.  
         [0055]     The embodiment described below exploits not only principles of thermophoresis but also a phenomenon termed “electrophoresis.” Thermophoresis is defined above. In the current context, electrophoresis generally involves the migration, in space, of charged particles under the influence of an applied voltage gradient in the space. In this regard, reference is made to  FIG. 1 , which depicts a space  10  flanked by a first electrode  12  and a second electrode  14 . The electrodes  12 ,  14  are respectively charged by connection to a voltage source  16 . If a particle  18   a,    18   b  entering the space  10  has a net electrical charge, then the particle  18   a,    18   b  will experience an electrostatic attraction to one or the other of the electrodes  12 ,  14 , depending upon the polarity and magnitude of charge of the particle and the polarity and magnitude of the charge of the respective. electrode. In general, a positively charged particle  18   a  will be attracted to the negatively charged (cathodic) electrode  14 , and a negatively charged particle  18   b  will be attracted to the positively charged (anodic) electrode  12 .  
         [0056]     A representative embodiment of the subject apparatus  20  is shown in FIGS.  2 ( a )- 2 ( b ). The apparatus comprises a first (upper) chamber  22  and a second (lower) chamber  24  separated from each other by a barrier wall  26 . Each of the first and second chambers  22 ,  24  is a respective vacuum chamber that is evacuated by a respective vacuum pump  28 ,  30  to a respective vacuum level. The first chamber  22  contains a reticle stage  32  on which a reticle chuck  34  is mounted, facing downward in a manner characteristic of a reticle chuck intended to hold an EUVL reticle. The reticle chuck  34  is shown holding a reticle  36 .  
         [0057]     The barrier wall  26  defines a “fixed aperture”  38  that serves the following two functions: a differential pumping aperture and a reticle-illumination aperture. As a differential pumping aperture, the fixed aperture  38  is configured (especially due to its nozzle manifold and electrode, described later below) to facilitate attainment, by the vacuum pumps  28 ,  30 , of the desired vacuum levels in each of the first and second chambers  22 ,  24 . For example, the first and second chambers (the first chamber  22  is typically at approximately 50 mTorr (6.6 Pa), and the second chamber  24  is typically at less than 1 mTorr (0.1-0.2 Pa). The higher pressure in the first chamber is advantageous for effective viscophoresis and thermophoresis, and the lower pressure in the second chamber is advantageous for high transmission of EUV radiation in the second chamber. As a reticle-illumination aperture, the fixed aperture  38  is sized and configured (typically with an arc-shape; see  FIG. 2 ( b )) to pass an EUV illumination beam  40  to illuminate a desired region on the reticle  36  and to pass patterned EUV light  42  reflected from the reticle and propagating to projection optics  44  contained in the second chamber  24 .  
         [0058]     The reticle stage  32  and reticle chuck  34  cooperatively hold the reticle  36  such that a space  46  is defined between the lower surface  48  of the reticle and the top surface  50  of the barrier wall  26 . By way of example, the vertical dimension (thickness) of the space  46  is 10-13 mm. During exposure, to illuminate successive regions of the reticle  36  through the fixed aperture  38 , the reticle stage  32  moves in a scanning manner relative to the fixed aperture. The reticle chuck  34  desirably is temperature-controlled to establish a desired temperature of the reticle  36 . Temperature control of the reticle  36  is advantageous for effective thermophoresis, as described later below.  
         [0059]     Flanking the fixed aperture  38  are nozzle manifolds  52   a,    52   b  that extend upward (in the figure) from the upper surface  50  of the barrier wall  26  toward the reticle  36 . The nozzle manifolds  52   a,    52   b  define gas-discharge nozzle openings  54   a,    54   b  that are configured and oriented for discharging gas  56   a,    56   b  (supplied by conduits  58   a,    58   b ) laterally into the space  46 , substantially parallel to the reticle  36 . The flow of gas laterally through the space  46  is approximately laminar, and the number, size, and shape of the nozzle openings  54   a,    54   b  desirably are selected so as to establish and maintain this approximately laminar flow. At the low pressures employed in this invention, laminar flow can be only approximately achieved. As indicated by the multiple arrows  56   a,    56   b,  most of the discharged gas flows laterally, substantially parallel to the reticle  36  and outwardly toward the edges of the reticle, and ultimately is exhausted via the vacuum pump  28 . This approximately laminar flow of gas pulls particles with it by viscous drag (“viscophoresis”); thus, the region between the nozzle openings  54   a,    54   b  and the edge of the reticle  36  is termed the “viscophoresis region” VR in which particles are moved, while entrained in the gas, laterally away from the reticle  36  and are not allowed to move toward the reticle surface  48 . The flow of gas in the viscophoresis region VR remains largely unchanged as the reticle  36  is scanned back and forth by the reticle stage  32  during exposure. The flow velocity of the gas desirably is in the range of approximately 5 m/sec to several hundred m/sec. Reticle-stage velocities are determined by the throughput of the lithography system, but a typical reticle-stage velocity is approximately 1 m/sec. Thus, movement of the reticle stage would have little effect on the gas flow.  
         [0060]     The nozzle manifolds  52   a,    52   b  are topped by an electrode plate (described later below) having respective “top” surfaces  60   a,    60   b  that define respective narrow gaps  62   a,    62   b  (approximately 1 mm or less) relative to the lower surface  48  of the reticle  36 . These gaps  62   a,    62   b  allow limited vertical movement of the reticle  36  as required for focus control and alignment purposes. These gaps  62   a,    62   b  also allow a flow of gas (albeit constrained, note single arrows  64   a,    64   b  indicating a “bleed” flow, compared with the greater flow indicated by the arrows  56   b ) from the space  46  through the fixed aperture  38  to the projection-optics chamber  24 . Note that this constrained flow is in directions that are opposite the respective directions of most of the gas flow from the nozzle openings  52   a,    52   b.    
         [0061]     The gas can be pre-cooled before being discharged into the space  46  between the reticle  36  and barrier wall  26 . Alternatively, and more desirably, the nozzle discharge openings  54   a,    54   b  are sized and configured to establish a substantially higher gas pressure at the nozzle openings than in the space  46  between the reticle  36  and barrier wall  26  or in the space  66  above the reticle, for example. Thus, in this alternative scheme, discharge of the gas is accompanied by adiabatic cooling of the gas. I.e., as the gas is discharged into the space  46 , it expands rapidly out of the nozzle openings  54   a,    54   b  and cools significantly in the process. In such a configuration, the supply temperature of the gas relative to the temperature of the reticle  36  is of little consequence because in any event the discharged gas expanding into the space  46  becomes colder than the reticle  36  and barrier wall  28 , thereby establishing the desired temperature gradient for thermophoresis without having to pre-cool the gas. The relatively high gas pressure at the nozzle openings  54   a,    54   b  produces high-velocity gas flow through the viscophoresis region VR toward the edges of the reticle  36 , which establishes a substantial viscous drag on any particles present in the space  46 . Thus, particles are conveyed out of the space  46  away from the reticle  36 . Most of the gas ultimately is removed by the vacuum pump  28 .  
         [0062]     The number, size, and shape of the nozzle openings  54   a,    54   b  desirably are selected so as to establish the conditions for adiabatic cooling of the gas. Alternatively, the nozzle openings  54   a,    54   b  may be single large openings that are covered by respective particle filters (not shown). The effective pore size of the particle filters is small, thereby providing both protection of the reticle from particles that may enter the space  46  with the gas flow and conditions appropriate for adiabatic cooling of the expanding gas. The gas flow at the entrance to the space  46  desirably is subsonic because supersonic gas flow tends to generate shock waves and heating of the ambient gas rather than the desired cooling. The gas emerges into the space  46  at subsonic velocities if the nozzle openings  54   a,    54   b  or the filter pores are smaller than the molecular mean path of the gas. At the ambient pressures the molecular mean path is typically a fraction of a millimeter.  
         [0063]     The gas discharged from the nozzle openings  54   a,    54   b  may be a light gas such as helium or hydrogen, a heavier gas such as nitrogen, or one of the noble gases, or a mixture of two or more of these gases. Since nitrogen is relatively inexpensive, and is used as a system gas in gas bearings (not shown) such as in the reticle stage  32 , nitrogen is an advantageous candidate gas for discharge by the nozzle openings  54   a,    54   b.  Any of these gases tend to absorb EUV radiation.  
         [0064]     Desirably, the reticle  36  and barrier wall  26  are at substantially the same temperature. (The reticle  36  and barrier wall  26  can have different temperatures, so long as they are warmer than the temperature of the gas discharged into the space  46 .) To maintain the reticle  36  and the barrier wall  26  at their respective desired temperatures as heat is being removed from them by the discharged gas, the reticle and barrier wall can be heated. In addition, to facilitate temperature control of the barrier wall  26 , thermal insulation (not shown) can be used to isolate the barrier wall thermally from nearby structure. The reticle  36  and the barrier wall  26  can be heated by any suitable mechanism. By way of example, the reticle  36  can be heated by the EUV radiation  40  passing through the fixed aperture  38  and impinging on the reticle  36 . This radiative heating alone may be sufficient for heating the reticle  36 . Alternatively or in addition, the reticle  36  can be heated by thermal conduction via a temperature-controlled reticle chuck  34 . The barrier wall  26  can be heated hydraulically using fluid-circulation coils, electrically using a resistance heater, or radiatively using a radiative heater. Removal of heat from the reticle  36  and barrier wall  26  by the discharged gas is typically proportional to the heat capacity of the gas. Because of the low pressure of the discharged gas, its heat capacity is relatively small, and the rate of heat removal from the reticle  36  and barrier wall  26  by the gas is typically small. Consequently, the amount of heat energy to supply to the reticle  36  and barrier wall  26  is typically small.  
         [0065]     Referring now to  FIG. 3 , details of the nozzle manifolds  52   a,    52   b  and nozzle openings  54   a,    54   b  are shown. In the depicted embodiment, each nozzle manifold  52   a,    52   b  includes, extending along its “top” surface  60   a,    60   b,  a respective electrode  68   a,    68   b  (The nozzle manifolds  52   a,    52   b  can be constructed as a single unit (see  FIG. 2 ( b )) having a “left” portion  52   a  and a “right” portion  52   b .) The electrodes  68   a,    68   b  desirably are configured a single unit (with a “left” portion  68   a  and a “right” portion  68   b ), but they may be separate if desired. The electrodes  68   a,    68   b  collectively form a “fixed” blind, usually having an arc-shaped opening (see  FIG. 2 ( b )). It is this fixed blind that gives the fixed aperture  38  its name. If the nozzle manifolds  52   a ,  52   b  are made of metal, the electrodes  68   a,    68   b  desirably are electrically insulated from the respective nozzle manifolds by a region of dielectric material  70   a,    70   b.  Structures that may absorb EUV radiation typically are electrically conductive or are provided with a conductive coating, and are connected to electrical ground. Insulated surfaces exposed to EUV radiation typically charge up to a finite voltage, which creates an electric field that may attract or divert particles to the vicinity of the reticle or other particle-sensitive surfaces. If the electrodes  68   a,    68   b  are electrically insulated, the insulation desirably is protected from exposure by the EUV by means of an electrical shield or by placement in the shadow of a conducting body.  
         [0066]     Turning now to  FIG. 4 , the electrodes  68   a,    68   b  are electrically connected to a voltage source  72 , typically to the negative pole  74  of a DC-power source  72  (producing a DC voltage in the range of, e.g., 1-50 V DC). The reticle  36  in such a scheme is connected to the positive pole  76  of the DC-power source  72  and is typically also connected to electrical ground. When energized, the electrodes  68   a,    68   b  produce an electrical field having field lines  78 , of which some are shown in  FIG. 4 . The electrodes  68   a,    68   b  constitute respective portions of an electrophoresis unit situated at the fixed aperture  38  and relative to the gaps  62   a,    62   b.  The electrophoresis unit deflects particles, in the gas passing through the gaps  62   a,    62   b  toward the fixed aperture  38 , away from the portion of the reticle surface  48  located adjacent the fixed aperture and gaps. More specifically, an initially uncharged particle  80  being carried into a gap  62   a,    62   b  by the higher-pressure flow of gas (arrow  82 ) near the nozzle openings  54   a,    54   b  becomes positively charged by the incident EUV radiation  40  and reflected EUV radiation  42 . The now-charged particle  80  experiences the field produced by the electrodes  68   a,    68   b  and thus is deflected downward toward the fixed aperture  38 . The particle  80  is assumed to be uncharged initially, so its trajectory  84   a  in the gap  62   a  is unaffected by the field lines  78  of the electric field. (The trajectory  84   a  is shown as a straight line. However, forces from the gas flow may in fact alter the trajectory  84   a  from a straight line.) After encountering the EUV radiation the particle becomes charged, and its trajectory  84 b is influenced by the electric field.  
         [0067]     Particles in the gas entering the fixed aperture  38  from the gaps  62   a,    62   b  encounter EUV radiation  40  ( FIG. 2 ( a )) illuminating the reticle  48  through the fixed aperture. The EUV radiation  40  imparts charges to the particles by the photoelectric effect, causing the particles to acquire mostly a net positive charge. The electric field  78  created by the negatively charged electrodes  68   a,    68   b  near the reticle  36  deflects the positively charged particles downward, away from the reticle  36 . The electric field  78  is localized near the reticle  36  to minimize the possibility of positively charged particles below the electrodes  68   a,    68   b  from being attracted toward the reticle.  
         [0068]     Any particles in the space  46  have been assumed to be electrically neutral. Some particles may become positively charged by the photoelectric effect, notably from EUV radiation that has scattered from the surface of the reticle  36  into the space  46 . These particles would be attracted even more efficiently to the electrodes  68   a,    68   b.  Although a mechanism is not currently known, if some particles do acquire a negative charge in the space  46  and are conveyed by the gas flow near the electrodes  68   a,    68   b,  they undesirably will be driven by the electric field toward the surface of the reticle  36 . A means of avoiding this situation is to provide a flow of positive gas ions from the nozzle manifolds  52   a,    52   b  into the space  46 . These ions can neutralize any negatively charged particles in the space  46  and prevent them from being driven toward the surface of the reticle  36  by the electrodes  68   a,    68   b.  These ions may be produced by, e.g., a positive corona discharge, whereby a positively charged set of sharp conductive points removes electrons from adsorbed gas atoms, giving them a positive charge and repelling them from the points.  
         [0069]     Referring now to  FIG. 5 , an embodiment of an EUVL system  900  is shown. The depicted system  900  comprises a vacuum chamber  902  including vacuum pumps  906   a,    906   b  that are arranged to enable desired vacuum levels to be established and maintained within respective chambers  908   a,    908   b  of the vacuum chamber  902 . For example, the vacuum pump  906   a  maintains a vacuum level of approximately 50 mTorr in the upper chamber (reticle chamber)  908   a,  and the vacuum pump  906   b  maintains a vacuum level of less than approximately 1 mTorr in the lower chamber (optical chamber)  908   b.  The two chambers  908   a,    908   b  are separated from each other by a barrier wall  920 . Various components of the EUVL system  900  are not shown, for ease of discussion, although it will be appreciated that the EUVL system  900  can include components such as a reaction frame, a vibration-isolation mechanism, various actuators, and various controllers.  
         [0070]     An EUV reticle  916  is held by a reticle chuck  914  coupled to a reticle stage  910 . The reticle stage  910  holds the reticle  916  and allows the reticle to be moved laterally in a scanning manner, for example, during use of the reticle for making lithographic exposures. An illumination source  924  produces an EUV illumination beam  926  that enters the optical chamber  908   b  and reflects from one or more mirrors  928  and through an illumination-optical system  922  to illuminate a desired location on the reticle  916 . As the illumination beam  926  reflects from the reticle  916 , the beam is “patterned” by the pattern portion actually being illuminated on the reticle. The barrier wall  920  serves as a differential-pressure barrier and can serve as a reticle shield that protects the reticle  916  from particulate contamination during use. The barrier wall  920  defines an aperture  934  through which the illumination beam  926  may illuminate the desired region of the reticle  916 . The incident illumination beam  926  on the reticle  916  becomes patterned by interaction with pattern-defining elements on the reticle, and the resulting patterned beam  930  propagates generally downward through a projection-optical system  938  onto the surface of a wafer  932  held by a wafer chuck  936  on a wafer stage  940  that performs scanning motions of the wafer during exposure. Hence, images of the reticle pattern are projected onto the wafer  932 .  
         [0071]     The wafer stage  940  can include (not detailed) a positioning stage that may be driven by a planar motor or one or more linear motors, for example, and a wafer table that is magnetically coupled to the positioning stage using an EI-core actuator, for example. The wafer chuck  936  is coupled to the wafer table, and may be levitated relative to the wafer table by one or more voice-coil motors, for example. If the positioning stage is driven by a planar motor, the planar motor typically utilizes respective electromagnetic forces generated by magnets and corresponding armature coils arranged in two dimensions. The positioning stage is configured to move in multiple degrees of freedom of motion, e.g., three to six degrees of freedom, to allow the wafer  932  to be positioned at a desired position and orientation relative to the projection-optical system  938  and the reticle  916 .  
         [0072]     An EUVL system including the above-described EUV-source and illumination-optical system can be constructed by assembling various assemblies and subsystems in a manner ensuring that prescribed standards of mechanical accuracy, electrical accuracy, and optical accuracy are met and maintained. To establish these standards before, during, and after assembly, various subsystems (especially the illumination-optical system  922  and projection-optical system  938 ) are assessed and adjusted as required to achieve the specified accuracy standards. Similar assessments and adjustments are performed as required of the mechanical and electrical subsystems and assemblies. Assembly of the various subsystems and assemblies includes the creation of optical and mechanical interfaces, electrical interconnections, and plumbing interconnections as required between assemblies and subsystems. After assembling the EUVL system, further assessments, calibrations, and adjustments are made as required to ensure attainment of specified system accuracy and precision of operation. To maintain certain standards of cleanliness and avoidance of contamination, the EUVL system (as well as certain subsystems and assemblies of the system) are assembled in a clean room or the like in which particulate contamination, temperature, and humidity are controlled.  
         [0073]     Semiconductor devices can be fabricated by processes including microlithography steps performed using a microlithography system as described above. Referring to  FIG. 6 , in step  701  the function and performance characteristics of the semiconductor device are designed. In step  702  a reticle (“mask”) defining the desired pattern is designed and fabricated according to the previous design step. Meanwhile, in step  703 , a substrate (wafer) is fabricated and coated with a suitable resist. In step  704  (“wafer processing”) the reticle pattern designed in step  702  is exposed onto the surface of the substrate using the microlithography system. In step  705  the semiconductor device is assembled (including “dicing” by which individual devices or “chips” are cut from the wafer, “bonding” by which wires are bonded to particular locations on the chips, and “packaging” by which the devices are enclosed in appropriate packages for use). In step  706  the assembled devices are tested and inspected.  
         [0074]     Representative details of a wafer-processing process including a microlithography step are shown in  FIG. 7 . In step  711  (“oxidation”) the wafer surface is oxidized. In step  712  (“CVD”) an insulative layer is formed on the wafer surface by chemical-vapor deposition. In step  713  (electrode formation) electrodes are formed on the wafer surface by vapor deposition, for example. In step  714  (“ion implantation”) ions are implanted in the wafer surface. These steps  711 - 714  constitute representative “pre-processing” steps for wafers, and selections are made at each step according to processing requirements.  
         [0075]     At each stage of wafer processing, when the pre-processing steps have been completed, the following “post-processing” steps are implemented. A first post-process step is step  715  (“photoresist formation”) in which a suitable resist is applied to the surface of the wafer. Next, in step  716  (“exposure”), the microlithography system described above is used for lithographically transferring a pattern from the reticle to the resist layer on the wafer. In step  717  (“developing”) the exposed resist on the wafer is developed to form a usable mask pattern, corresponding to the resist pattern, in the resist on the wafer. In step  718  (“etching”), regions not covered by developed resist (i.e., exposed material surfaces) are etched away to a controlled depth. In step  719  (“photoresist removal”), residual developed resist is removed (“stripped”) from the wafer.  
         [0076]     Formation of multiple interconnected layers of circuit patterns on the wafer is achieved by repeating the pre-processing and post-processing steps as required. Generally, a set of pre-processing and post-processing steps are conducted to form each layer.  
         [0077]     In the embodiment described above, the barrier wall  26  was described as defining the fixed aperture  38 . Alternatively, the fixed aperture can be defined in any suitable structure, such as a reticle shield, which may be stationary or movable. Also, in certain embodiments, the nozzle manifolds  52   a,    52   b  (with attached electrodes  68   a,    68   b ) can constitute an independently movable assembly, relative to the barrier wall  26  or analogous structure. Such movability can facilitate certain calibration operations (e.g., focus position of the reticle) that are more easily performed with the nozzle manifolds and electrodes moved out of the way.  
         [0078]     It will be apparent to persons of ordinary skill in the relevant art that various modifications and variations can be made in the system configurations described above, in materials, and in construction without departing from the spirit and scope of this disclosure.