Abstract:
Blind devices and related methods for lithography systems are described. An exemplary system has a vacuum chamber with first and second chamber portions. In a member between the chambers is defined an exposure aperture, relative to which a reticle stage in the first chamber portion moves a reticle. A gas enters the first chamber portion and establishes a thermophoretic condition relative to the reticle or portion thereof. A fixed-blind-aperture assembly, movable relative to the exposure aperture and the reticle to exposure and non-exposure positions, defines an illumination aperture through which light from the second chamber portion and gas from the first chamber portion pass when the fixed-blind-aperture assembly is in the exposure position. A gas-passage aperture in the member conducts the gas, passing through the illumination aperture, from the first chamber portion to the second chamber portion when the fixed-blind-aperture assembly is in the non-exposure position.

Description:
FIELD 
   This disclosure pertains 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 blind devices and methods for preventing particulate contamination of the reticle. 
   BACKGROUND 
   Extreme-ultraviolet lithography (EUVL) is currently regarded as a candidate “next 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. 
   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. 
   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 partial vacuum conditions 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. 
   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. 
   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 suspended 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. 
   General principles of thermophoresis as applied in an EUVL system are described with reference to  FIG. 9 , 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 shield that protects the reticle or a differential pumping barrier used in a vacuum chamber housing the reticle  222 . 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 loses effectiveness in high vacuum, but at a pressure of 50 mTorr thermophoresis is still significant for effectively keeping particles  228  away from the reticle  222 . 
   A conventional thermophoretic scheme as disclosed in the references cited above is generally shown in  FIG. 10 , 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  112  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  112 . 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., ≦5 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. 
   In the configuration shown in  FIG. 10 , to urge particles away from the reticle  112  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  112  and thus prevents the particles from contacting the reticle. 
   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  112 , which would increase the risk of contamination despite the generally enhanced protection afforded by thermophoresis to other regions of the reticle. 
   One manner of solving this problem is described in U.S. patent application Ser. No. 10/898,475, incorporated herein by reference, filed on Jul. 23, 2004, by the current Applicant. Briefly, a space is defined between the reticle and a nearby surface, such as a barrier wall or reticle shield. Gas nozzles are 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 nozzles into the space. The discharged gas, flowing substantially parallel to the reticle and surface, establishes local respective temperature gradients adjacent the reticle and surface. The temperature gradients engender respective thermophoretic forces tending to keep particles entrained in the gas and away from the reticle and surface. 
   A particular configuration of the apparatus  300  described in the &#39;475 application is shown in  FIG. 11 , 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 a “fixed-blind 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 downstream projection optics (not shown). The fixed-blind 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 thus is a “higher-pressure” region), and the projection-optics chamber  310  is typically at less than 1 mTorr (and thus is a “lower-pressure” region). During exposure, to illuminate successive regions of the reticle  302 , the reticle stage  306  moves in a scanning manner relative to the fixed-blind aperture  314 . Flanking the fixed-blind aperture  314  and extending upward (in the figure) toward the reticle  302  are nozzle manifolds  320   a ,  320   b  that define 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 ) away from the nozzle openings  322   a ,  322   b  past the reticle  302  is substantially laminar. The nozzle openings  322   a ,  322   b  may be covered by filters (not shown) that can prevent the admission of particles into the space  324  and can also limit the velocity of gas flow. 
   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 . 
   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. 10 ) that evacuates the reticle chamber  308 . 
   Referring further to  FIG. 11 , the nozzle manifolds  320   a ,  320   b  extend upward (in the figure) and form respective narrow gaps G between the “tops” of the nozzle manifolds 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 fixed-blind aperture  314  to the projection-optics chamber  310 . The gas flow through the gaps G is limited to maintain the desired vacuum level in the projection-optics chamber  310  for minimal attenuation of the EUV illumination and patterned beams. 
   Because of the small distance between the reticle  302  and the top of the nozzle manifolds, maintaining a temperature gradient, and hence thermophoretic protection, within the gaps G can be problematic. Therefore, protection of the reticle is somewhat weaker within the gaps G. However, the flow of gas through the gaps G, from the higher-pressure region  308  to the lower-pressure region  310 , will provide some viscous drag force to convey particles into the lower-pressure region  310  and away from the reticle  302 . Also, during normal reticle scanning, a given area of the reticle  302  spends only a fraction of the time within the gaps G. Much of the time the reticle lies within the space  324  in which thermophoretic protection and gas drag are available. 
   In a conventional EUVL system, illumination of the reticle  302  is non-telecentric. Consequently, movement or displacement of the reticle  302  in the axial direction (vertical direction in the figure) causes corresponding image movement at the wafer, which is problematic. Consequently, the “height” of the reticle  302  must be controlled very accurately and precisely to avoid image distortion at the wafer. An example specification for reticle-height control is 50 nm peak-to-valley over the surface of the reticle  302 . Achievement of such height control requires corresponding measurements of reticle height, which is performed using a very accurate and precise autofocus (AF) system at the reticle  302 . 
   Accurate measurements of reticle height performed using an AF system require that the AF system be calibrated periodically such as during use of the reticle  302  and whenever a new reticle is mounted to the reticle chuck  304 . The AF-system calibration involves scanning the patterned regions of the surface of the reticle  302  with an array of multiple light beams (e.g., 50-70 individual laser beams, at near-grazing incidence on the reticle surface). The beams are reflected from the reticle surface, which is accompanied by some diffraction and scattering of the beams. The reflected beams propagate to respective sensors. At each sensor the respective position of the reflected beam is a function of the reticle “height” at the particular incidence locus of the beam on the reticle. The sensor outputs are averaged to obtain data concerning the mean height of the area actually being measured. The calibration covers an area of the reticle  302  that is larger than the area illuminated at any instant by the EUV illumination beam (i.e., larger than the opening of the fixed-blind aperture  314 ). Consequently, the fixed-blind aperture  314  (with nozzle manifolds  320   a ,  320   b ) is moved out the way (retracted) for the AF-system calibration. 
   For reasons discussed more thoroughly later below, retraction of the nozzle manifolds  320   a ,  320   b  and of the fixed-blind aperture  314  disrupts the gas flow  330  used for establishing differential pressures in the chambers  308 ,  310  and for providing protection of the reticle  302  in the region of the reticle adjacent the gaps G and fixed-blind aperture. (Thermophoretic protection of other portions of the reticle, namely in the space  324 , is maintained.) This situation is shown in the gas-flow image in  FIG. 4 , which shows a gas flow of approximately 50 m/sec in the space  324  but no gas flow in the gap G. As a result, reticle protection from particulate contamination is compromised and the pressure in the projection-optics chamber  310  is undesirably increased. 
   SUMMARY 
   The deficiencies of conventional systems, as summarized above, are addressed by devices and methods as disclosed herein. 
   According to a first aspect, lithography systems are disclosed. An embodiment of such a system comprises a vacuum chamber that includes a first chamber portion and a second chamber portion. A member, situated between the first and second chambers, defines an exposure aperture. A reticle stage is situated in the first chamber portion and is configured to hold a reticle movably relative to the exposure aperture. A gas-discharge port is situated and configured to deliver a gas with a temperature gradient into the first chamber portion so as to establish a thermophoretic condition with respect to at least a portion of the reticle. A fixed-blind-aperture assembly, that is movable relative to the exposure aperture and the reticle to an exposure position and to a non-exposure position, defines an illumination aperture through which light from the second chamber portion and gas from the first chamber portion can pass through the exposure aperture when the fixed-blind-aperture is in the exposure position. A gas-passage aperture is displaced from the exposure aperture so as to conduct the gas, passing through the illumination aperture, from the first chamber portion to the second chamber portion when the fixed-blind-aperture assembly is in the non-exposure position. The gas-passage aperture is defined in the member. 
   In an embodiment the fixed-blind-aperture assembly is situated in the first chamber portion between the member and the reticle and separated from the reticle by a gap. In this configuration the gas flows through the gap from first chamber portion through the exposure aperture to the second chamber portion when the fixed-blind-aperture assembly is in the exposure position, and flows through the gap from the first chamber portion through the gas-passage aperture to the second chamber portion when the fixed-blind-aperture assembly is in the non-exposure position. In this embodiment the fixed-blind-aperture assembly can comprise the gas-discharge port. 
   In another embodiment the system further comprises at least one moving blind situated between the fixed-blind-aperture assembly and the member. The moving blind in this embodiment is configured to move so as to cover the exposure aperture at selected times. The moving blind can be movable to cover the exposure aperture whenever the fixed-blind-aperture assembly is in the non-exposure position. The moving blind can be configured to define an aperture that is situated so as to conduct, when the moving blind is covering the exposure aperture and the fixed-blind-aperture is in the non-exposure position, gas that has passed from the first chamber portion through the illumination aperture to the gas-passage aperture. The gas-passage aperture and the aperture in the moving blind can be aligned with each other when the moving blind is covering the exposure aperture and the fixed-blind-aperture assembly is in the non-exposure position. 
   In many embodiments the reticle extends and the reticle stage is configured to move the reticle in an X-direction and in a Y-direction. In such a configuration the moving blind can comprise a moving X-blind and a moving Y-blind, wherein the aperture in the moving blind is defined in at least one of the X-blind and Y-blind. 
   In embodiments in which the gas-passage aperture is defined in the member, the member further can comprise a collar extending around the gas-passage aperture and toward the moving blind so as to form at least a partial seal for passage of gas through the respective apertures in the moving blind and member when the fixed-blind-aperture assembly is in the non-exposure position. 
   In embodiments in which the moving blind comprises first and second blind portions that are movable relative to each other, at least one of the first and second blind portions can be movable to cover the exposure aperture when the fixed-blind-aperture assembly is in the non-exposure position. In such an embodiment the first and second blind portions can be situated, when the fixed-blind-aperture assembly is in the non-exposure position and the at least one blind portion is covering the exposure aperture, relative to each other to form a gas-passage gap between them. Furthermore, the gas-passage gap can be situated so as to conduct, when the at least one moving blind is covering the exposure aperture and the fixed-blind-aperture is in the non-exposure position, the gas passing through the illumination aperture from the first chamber portion to the second chamber portion. 
   In another embodiment the fixed-blind-aperture assembly can be configured such that, whenever it is in the non-exposure position, a space is provided between the exposure aperture and the reticle. In the space a measurement can be performed of reticle position using at least one laser beam directed to and incident on the reticle at an oblique angle. The measurement can pertain to a reticle-autofocus measurement performed using an array of multiple laser beams directed to and incident on the reticle. 
   Certain embodiments of the lithographic system further comprise at least one of an illumination-optical system, a projection-optical system, and a wafer stage contained in the second chamber portion. 
   Also, in certain embodiments of the lithographic system, the light passing from the second chamber portion through the exposure aperture and the fixed-blind aperture comprises a beam of extreme UV light. 
   Another embodiment of a lithography system comprises chamber means, dividing means, reticle-stage means, gas-introduction means, and fixed-blind-aperture means. The dividing means is for dividing the chamber means into a first chamber portion and a second chamber portion and for defining an exposure aperture by which light passes from the second chamber portion to the first chamber portion and from the first chamber portion to the second chamber portion. The reticle-stage means is for holding a reticle, in the first chamber portion, movably relative to the exposure aperture so as to allow the reticle to receive light from the second chamber portion and to reflect the light to the second chamber portion. The gas-introduction means is for introducing a gas with a temperature gradient into the first chamber portion relative to the reticle. The fixed-blind-aperture means is for defining a fixed-blind illumination aperture and for moving the illumination aperture, relative to the exposure aperture and the reticle, to an exposure position and to a non-exposure position. Thus, light from the second chamber portion and the gas from the first chamber portion pass through the exposure aperture whenever the fixed-blind-aperture means is in the exposure position. The dividing means further can define gas-passage means for conducting the gas, passing through the illumination aperture, from the first chamber portion to the second chamber portion when the fixed-blind-aperture means is in the non-exposure position. 
   In certain embodiments the gas-passage means does not pass significant amounts of the gas when the fixed-blind-aperture means is in the exposure position. 
   Certain other embodiments also comprise moving-blind means for substantially blocking the exposure aperture to passage of light and gas whenever the moving-blind means is in a closed condition, and allowing passage of light through the exposure aperture whenever the moving-blind means is in an open condition. The moving-blind means can comprise at least one moving blind defining an aperture that, when the moving blind is in an open condition, allows passage therethrough of gas passing through the illumination aperture to the gas-passage means. In certain embodiments the moving blind defines collar means extending around the aperture and toward the illumination aperture to provide seal means for gas passing from the illumination aperture to the gas-passage means. 
   In certain embodiments the light passing from the second chamber portion to the first chamber portion comprises a beam of extreme UV light. 
   Yet another embodiment of a lithography system comprises a vacuum chamber comprising a first chamber portion and a second chamber portion separated from the first chamber portion by a barrier plate defining an exposure aperture. A reticle stage is situated in the first chamber portion and is configured to hold a reticle movably relative to the exposure aperture. A fixed-blind-aperture assembly is situated in the first chamber portion between the reticle and the barrier plate and is separated from the reticle by a gap. The fixed-blind-aperture assembly is movable relative to the exposure aperture and the reticle to an exposure position and a non-exposure position, the fixed-blind-aperture assembly comprises a nozzle manifold that is configured to flow a gas with a temperature gradient into the first chamber portion relative to the reticle sufficiently to establish a thermophoretic condition with respect to at least a portion of the reticle. The fixed-blind-aperture assembly defines an illumination aperture through which illumination light from the second chamber portion, patterned light from the first chamber portion, and gas passing through the gap from the first chamber portion can pass through the exposure aperture when the fixed-blind-aperture is in the exposure position. The barrier member defines a gas-passage aperture that is separate from the exposure aperture. The gas-passage aperture is situated so as to conduct the gas passing through the illumination aperture from the first chamber portion to the second chamber portion when the fixed-blind-aperture assembly is in the non-exposure position. 
   Certain embodiments of the system summarized above can further comprise a moving X-blind and a moving Y-blind situated between the fixed-blind-aperture assembly and the barrier plate. The X-blind and Y-blind are cooperatively movable to allow, at selected times when the fixed-blind-aperture assembly is in the exposure position, passage of light and gas through the exposure aperture and to block, at selected times when the fixed-blind-aperture assembly is in the non-exposure position, passage of significant amounts of light and gas through the exposure aperture. 
   The non-exposure position can define a space, between the exposure aperture and the reticle, that is suitable for performing a measurement of reticle position using at least one laser beam directed to and incident on the reticle at a grazing angle of incidence. The measurement can pertain to a reticle-autofocus measurement performed using an array of multiple laser beams directed to and incident on the reticle. 
   Another aspect is set forth in the context of a lithography system that comprises a vacuum chamber including a member separating the vacuum chamber into first and second chamber portions and defining an exposure aperture by which the chamber portions communicate with each other. A stage is mounted in the first chamber portion and is configured to hold a reticle movably relative to the exposure aperture. A fixed-blind-aperture assembly provides a gas flow with a temperature gradient. The fixed-blind-aperture assembly is movable relative to the reticle to an exposure position and to a non-exposure position, and is separated from the reticle by a gap for passage of gas from the vacuum chamber past the reticle. The fixed-blind-aperture assembly defines an illumination aperture through which illumination light and gas can pass through the exposure aperture whenever the fixed-blind-aperture assembly is in the exposure position. In the context of such a system, the aspect is directed to an improvement in which a gas aperture is defined in the barrier member at a location allowing passage of the gas from the first chamber portion through the gap and through the illumination aperture whenever the fixed-blind-aperture assembly is in the non-exposure position. 
   Certain embodiments of such a system can further comprise at least one movable blind that is configured to cover the exposure aperture at selected times including when the fixed-blind-aperture assembly is in the non-exposure position. The movable blind can define a gas aperture situated at a location allowing passage of the gas from the first chamber portion through the gap, illumination aperture, and gas aperture in the barrier member whenever the fixed-blind-aperture assembly is in the non-exposure position. 
   Yet another aspect is set forth in the context of a lithographic method in which a pattern-defining reticle is irradiated by an illumination beam that reflects from the reticle to form a patterned beam. The reticle is mounted inside a first chamber in which a surface of the reticle is irradiated by the illumination beam propagating from a second chamber through an exposure aperture and fixed-blind aperture to the reticle surface to produce a patterned beam that reflects back through the exposure aperture and fixed-blind aperture to the second chamber. In this context, a method is provided for reducing particulate contamination of the reticle surface. In an embodiment of the method, a gas is flowed with a temperature gradient into the first chamber such that the gas contacts the reticle surface and establishes a thermophoretic condition with respect to the reticle surface. For actual irradiation of a region of the reticle surface, the fixed-blind aperture is moved to an exposure position at which the illumination beam can pass through the exposure aperture and the fixed-blind aperture to the region while allowing a flow of a portion of the gas through the fixed-blind aperture and exposure aperture to the second chamber. Thus, the gas flow establishes a protection condition with respect to the irradiated region of the reticle surface in addition to other regions of the reticle surface, and the protection condition serves to reduce particulate contamination of the reticle surface. During a time when the reticle is not being irradiated, the fixed-blind aperture is moved to a non-exposure position while maintaining the flow of the portion of gas through the fixed-blind aperture, at the non-exposure position, to the second chamber. 
   In certain embodiments the flow of the portion of gas through the fixed-blind aperture at the non-exposure position is maintained by passing the gas flow, after passing through the fixed-blind aperture, through an aperture separate from the exposure aperture. 
   Certain embodiments can include the step, during the time when the reticle is not being irradiated, of blocking the exposure aperture. The exposure aperture can be blocked using a movable blind. The flow of the portion of gas through the fixed-blind aperture at the non-exposure position can be maintained further by passing the gas flow, after passing through the fixed-blind aperture, through a gas-passage aperture defined in the movable blind, then through the aperture that is separate from the exposure aperture. 
   Certain embodiments further can comprise the step, during the time when the fixed-blind aperture is at the non-exposure position, of measuring an autofocus position of the reticle. Measuring an autofocus position of the reticle can comprise directing an array of multiple laser beams to be incident at respective locations on the reticle and detecting corresponding beams reflected from the reticle. 
   The foregoing and additional features and advantages of the subject systems and methods will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The Patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       FIG. 1(A)  is a schematic plan view of a conventional blind assembly for an EUV lithography reticle, including a general layout of actuators for the various blinds. 
       FIG. 1(B)  is an elevational view of certain details of the conventional blind assembly of  FIG. 1(A) , showing flow of gas with a temperature gradient relative to a fixed-blind assembly during an exposure situation. 
       FIG. 1(C)  is an elevational view of certain details of the conventional blind assembly of  FIG. 1(A) , showing flow of gas with a temperature gradient relative to the fixed-blind assembly during a calibration situation in which the fixed-blind assembly is retracted to provide space for propagation of at least one calibration beam (such as of a reticle-autofocus system, not shown) to and from the reticle. 
       FIG. 1(D)  is an elevational view of the assembly shown in  FIGS. 1(B) and 1(C)  as viewed along the lines indicated in  FIG. 1(C) , particularly showing the propagation of three exemplary calibration beams to and from the reticle. 
       FIG. 2(A)  is an elevational view of certain details of a representative embodiment of a blind assembly, showing flow of gas with a temperature gradient relative to a fixed-blind assembly during an exposure situation. 
       FIG. 2(B)  is an elevational view of certain details of the blind assembly of  FIG. 2(A) , showing flow of gas with a temperature gradient relative to the fixed-blind assembly during a calibration situation in which the fixed-blind assembly is retracted to provide space for propagation of at least one calibration beam (such as of a reticle-autofocus system, not shown) to and from the reticle. 
       FIG. 3(A)  is a color image produced by a computer program simulating the behavior of the embodiment of  FIGS. 2(A)-2(B) , showing regions of substantial flow of gas with a temperature gradient during the calibration situation. 
       FIG. 3(B)  is a color image produced by a computer program simulating the behavior of the embodiment of  FIGS. 2(A)-2(B) , showing regions of different scalar pressure during the calibration situation. 
       FIG. 4  is a color image produced by a computer program simulating the behavior of the conventional blind assembly of  FIGS. 1(A)-1(D) , showing regions of different gas velocity during 5.4 seconds of a calibration situation. 
       FIG. 5(A)  is a color image produced by a computer program simulating the behavior of the embodiment of  FIGS. 2(A)-2(D) , showing regions of substantial flow of gas with a temperature gradient during an exposure situation. 
       FIG. 5(B)  is a color image produced by a computer program simulating the behavior of the embodiment of  FIGS. 2(A)-2(D) , showing regions of different scalar pressure during the exposure situation. 
       FIG. 6  is a schematic elevational view of an EUV lithography system including a blind apparatus as disclosed herein. 
       FIG. 7  is a process-flow diagram illustrating exemplary steps associated with a process for fabricating semiconductor devices. 
       FIG. 8  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. 7 . 
       FIG. 9  is a schematic diagram showing general principles of thermophoresis. 
       FIG. 10  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. 
       FIG. 11  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 
   This disclosure is set forth in the context of a representative embodiment, which is not intended to be limiting in any way. 
   The subject apparatus is best understood in the context of a conventional blind assembly  10 , which is illustrated in  FIGS. 1(A)-1(D) . Referring first to  FIG. 1(A) , a plan view of the blind assembly  10  is shown, which comprises a fixed-blind-aperture assembly  12 , a Y-blind assembly  14 , and an X-blind assembly  16 . The fixed-blind-aperture assembly  12  comprises a fixed-blind aperture  18  that includes an aperture plate  20  defining an illumination aperture  22  that, in this embodiment, has a fixed arcuate profile. The fixed-blind aperture  18  also includes a nozzle manifold  24  that is similar in configuration and operation to the nozzle manifold summarized above. The aperture plate  20  is mounted to the “top” surface (facing the reticle, which is not shown in the figure but is located above and parallel to the plane of the page) of the nozzle manifold  24 . The fixed-blind aperture  18  is operably coupled to one or more actuators  26  configured to move the fixed-blind aperture  18  in the Y-direction from an exposure zone  28  to a calibration zone  30  and from the calibration zone back to the exposure zone. The Y-blind assembly  14  comprises a first Y-blind  32   a  (left in the figure) and a second Y-blind  32   b  (right in the figure). The first Y-blind  32   a  is operably coupled to a respective actuator  34  that is configured to move the first Y-blind in the Y-direction. Similarly, the second Y-blind  32   b  is operably coupled to a respective actuator  36  that is configured to move the second Y-blind in the Y-direction cooperatively with movement of the first Y-blind  32   a . The X-blind assembly  16  comprises a first X-blind  38   a  (upper in the figure) and a second X-blind  38   b  (lower in the figure). The first X-blind  38   a  is operably coupled to a respective actuator  40  that is configured to move the first X-blind substantially in the X-direction. Similarly, the second X-blind  38   b  is operably coupled to a respective actuator  42  that is configured to move the second X-blind substantially in the X-direction cooperatively with movement of the first X-blind  38   a  and cooperatively with certain movements of the first and second Y-blinds  32   a ,  32   b.    
     FIG. 1(B)  depicts an exposure situation in which the fixed-blind-aperture assembly  12  is in an exposure position (i.e., in a position in which the fixed-blind aperture  18  is situated adjacent an exposure zone  28  of the reticle  44 ). In this embodiment the fixed-blind-aperture assembly  12 , as well as the Y-blind assembly  14  and X-blind assembly  16 , are located upstream of a barrier plate  46  or analogous member. 
   In one embodiment the barrier plate  46  is configured as an actual dividing wall separating, for example, an upstream chamber  66  (e.g., a first vacuum chamber) from a downstream chamber  68  (e.g., a second vacuum chamber), wherein the upstream and downstream chambers are maintained at respective vacuum levels. For example, the upstream chamber  66  can be maintained at 50 mTorr, and the downstream chamber  68  can be maintained at less than 5 mTorr. In another embodiment, the barrier plate is configured as an extension of a housing (not shown) containing some or all the various actuators  26 ,  34 ,  36 ,  40 ,  42 . In yet another embodiment, the barrier plate serves both as a dividing wall and as a housing for some or all the actuators. The barrier plate  46  defines a fixed exposure aperture  48  that transmits illumination light  50  to the exposure zone  28  and transmits patterned light  52  produced by reflection of the illumination light from the exposure zone. 
   The reticle  44  is shown mounted, patterned side facing downward, to a reticle chuck  54 . The reticle chuck  54  is mounted to a reticle stage  56  that is configured to perform desired movements and positioning motions of the reticle chuck, and thus of the reticle  44  itself, as required for making exposures, for performing autofocus (AF) measurements using an AF system (not shown), and for calibrating the AF system. Just downstream of the reticle  44  is the fixed-blind-aperture assembly  12  that includes the aperture plate  20  and nozzle manifold  24 . A narrow gap  58  is defined between the aperture plate  20  and the surface of the reticle  44 . As discussed above, the gap  58  allows passage of a portion of the gas (arrows  59 ), discharged from the nozzle manifold  24 , through the exposure aperture  48  (note arrows  60 ). The nozzle manifold  24  is mounted to a support member  62  (e.g., a plate) that, in turn, is operably coupled to actuators (not shown, but see item  26  in  FIG. 1(A) ). The support member  62  defines an aperture  61  that allows passage of illumination light  50  through the illumination aperture  22  to the reticle  44 , passage of patterned light  52  from the reticle  44 , and passage of the gas  60  from the gaps  58 . In  FIG. 1(B)  the support member  62  is positioned sufficiently to the right (in the figure) so as to abut a reticle shield  63 . 
   In  FIG. 1(B)  the first Y-blind  32   a  and second Y-blind  32   b  are in respective “open” positions that allow transmission of the illumination light  50 , passing through the exposure aperture  48 , to the reticle  44  for exposure. Similarly, the first X-blind  38   a  and second X-blind  38   b  are in respective “open” positions. Placing the X-blinds  38   a ,  38   b  in their respective fully open positions allows use of the full X-dimension width of the exposure aperture  48 , which is as wide in the X-direction as the largest planar dimension (in the X-direction) of the patterned area of the reticle  44 . For example, if the largest planar dimension is 100 mm, then the X-dimension width of the exposure aperture  48  is approximately 100 mm. The width of the exposure aperture  48  in the Y-direction is as required for scanning illumination of the successive exposure regions of the patterned area of the reticle  44 . By way of example, the Y-dimension width of the exposure aperture  48  is approximately 30 mm, which is sufficiently wide to transmit illumination light  50  through the arc-shaped illumination aperture  22  (which has, by way of example, a radial width of 8 mm). 
   The patterned area on the reticle  44  is bounded by a narrow border (not shown but having a width of 1 mm, for example) of non-reflective material that absorbs incident radiation. During normal exposure the X-blinds  38   a ,  38   b  are open sufficiently to provide illumination of the full X-dimension width of the patterned area of the reticle  44  (illumination actually extends into the border) while preventing illumination of the reticle outside the border. During exposure the fixed-blind aperture  18  remains stationary in the position shown in  FIG. 1(B) . Meanwhile, the reticle  44  is moved (by the reticle stage  56 ) in a continuous Y-direction motion past the illumination aperture  22  to illuminate the patterned area extending in the Y-direction. During the Y-direction scan, the Y-blinds  32   a ,  32   b  are opened sufficiently and moved in a coordinated manner to track the exposure. I.e., the Y-blinds  32   a ,  32   b  are open sufficiently (e.g., 30 mm) to follow the leading and trailing edges of the region of the reticle that is actually being illuminated during a particular instant by illumination light  50  passing through the illumination aperture  22 . 
   Turning now to  FIGS. 1(C) and 1(D) , for calibration of the AF system, the X-blinds  38   a ,  38   b  are fully closed and the Y-blinds  32   a ,  32   b  are at their maximally closed positions. Also, the fixed-blind-aperture assembly  12  has been moved (leftward in  FIG. 1(C) ) in the Y-direction parallel to the surface of the reticle  44  by the support member  62  so as to retract the fixed-blind-aperture assembly laterally away from the reticle shield  63 . These motions of the fixed-blind-aperture assembly  12  and blinds  32   a ,  32   b ,  38   a ,  38   b  clear a space  64  in which the array of calibration beams  80  (typically 50-70 beams, but only three are shown in  FIG. 1(D) ) can, without obstruction, propagate to, impinge on, and reflect from respective locations on the surface of the reticle  44 . The area of the reticle surface illuminated by the calibration beams  80  typically is larger than the area that is illuminated at any instant by the illumination light  50 . By way of example, the calibration beams  80 , propagating from a beam source  82 , impinge obliquely on the surface of the reticle  44  at angles of incidence of approximately 84° (i.e., approximately 6° from the surface of the reticle), with respective cone angles of approximately 3° for each beam. The calibration beams  80  propagate substantially in the X-direction from respective sources  82  to respective sensors  84 . 
   Comparing  FIG. 1(B)  to  FIG. 1(C) , it can be seen that, in  FIG. 1(B) , a portion of the gas stream (arrows  59 ) discharged from the nozzle manifold  24  passes through the gaps  58  and travels downward (arrows  60 ) through the illumination aperture  22 , past the open blinds, and through the exposure aperture  48 . This gas stream  60  is sufficient to provide thermophoretic protection in the region of the reticle  44  adjacent the gaps  58  and illumination aperture  22 . In  FIG. 1(C) , in contrast, the first Y-blind  32   a  (or, in some embodiments, both Y-blinds) blocks passage of the gas stream  60  and thus obstructs flow of gas through the gaps  58 . This obstructed flow of gas through the gaps  58  interrupts thermophoretic protection in the regions of the reticle  44  adjacent the gaps  58  and illumination aperture  22 , which creates a condition in which these regions of the reticle  44  are vulnerable to particulate contamination. The flow of gas  59  elsewhere relative to the reticle  44  meanwhile maintains thermophoretic protection in those regions of the reticle. 
   The condition described above is avoided by the embodiment shown in  FIGS. 2(A) and 2(B) , in which are shown a fixed-blind-aperture assembly  412 , a Y-blind assembly  414 , an X-blind assembly  416 , a fixed-blind aperture  418 , an aperture plate  420 , an illumination aperture  422 , a nozzle manifold  424 , a first Y-blind  432   a , a second Y-blind  432   b , a first X-blind  438   a , a second X-blind  438   b , a reticle  444 , a barrier plate  446 , an exposure aperture  448 , a reticle chuck  454 , a reticle stage  456 , a gap  458 , a support member  462  (defining an aperture  461 ), a reticle shield  463 , and a space  464 . These components are similar to corresponding components shown in  FIGS. 1(A)-1(D) . 
     FIG. 2(A)  depicts an exposure situation in which the fixed-blind-aperture assembly  412  is in an exposure position (i.e., in a position in which the fixed-blind aperture  418  is situated adjacent an exposure zone  428  of the reticle  444 ). The barrier plate  446  defines a fixed exposure aperture  448  that transmits illumination light  450  to the exposure zone  428  and transmits patterned light  452  produced by reflection of the illumination light from the exposure zone. In this situation this embodiment functions substantially identically to the configuration shown in  FIG. 1(B) . 
     FIG. 2(B)  depicts a situation in which the components are arranged for calibration of the AF system. In this arrangement the X-blinds  438   a ,  438   b  are fully closed and the Y-blinds  432   a ,  432   b  are at their maximally closed positions. (Regarding the Y-blinds  432   a ,  432   b , in some embodiments they fully come together in their maximally closed positions. In other embodiments, they remain separated from each other, such as shown in  FIG. 2(B) , in their maximally closed positions. Exemplary separations in the fully closed position are 25 mm or 80 mm.) Also, the fixed-blind-aperture assembly  412  has been moved (leftward in the figure) in the Y-direction by the support member  462  so as to retract the fixed-blind-aperture assembly laterally away from the reticle shield  463 . This motion of the fixed-blind-aperture assembly  412  clears a space  464  in which the AF-system-calibration beams (not shown) can, without obstruction, propagate to, impinge on, and reflect from respective locations on the surface of the reticle  444 . The key differences in the embodiment of  FIG. 2(B)  relative to the configuration shown in  FIG. 1(C)  are as follows: In the embodiment of  FIG. 2(B)  the barrier plate  446  defines an aperture  470  that is in communication with the aperture  461  in the support member  462  and with the illumination aperture  422  whenever the support member  462  has retracted the fixed-blind-aperture assembly  412  to the left (in the figure) for AF-system calibration or other purpose. The barrier plate  446  in this embodiment also includes a collar  472  or analogous structure extending around the aperture  470  and upwards in the figure toward the first Y-blind  432   a , but the collar  472  does not actually contact the first Y-blind  432   a . In addition, the first Y-blind  432   a  in this embodiment defines an aperture  474  that is in communication with the aperture  470  and with the aperture  461  whenever the support member  462  has retracted the fixed-blind-aperture assembly  412  to the left (in the figure) for AF-system calibration. 
   As a result of mutual communication established among the apertures  422 ,  461 ,  470 ,  474 , the portion  460  of the gas  459  discharged from the nozzle manifold  424  still can pass through the gaps  458  and out through the apertures  422 ,  461 ,  470 ,  474 . Thus, thermophoretic protection of the reticle  444  is maintained opposite the gaps  458  and illumination aperture  422  whenever the fixed-blind-aperture assembly  412  is retracted. 
   “In communication with” does not require that the aperture  470  be completely (e.g., axially) aligned with the apertures  461  and/or  474 ; but, these apertures  470 ,  461 ,  474  can be so aligned if desired. Not having these apertures  470 ,  461 ,  474  be completely aligned with each other may serve a useful purpose such as creating a baffle effect to gas passing through them, and this effect can be usefully applied for establishing desired differential pressures in the chambers  466 ,  468 . 
   As indicated in  FIGS. 2(A) and 2(B) , the respective vertical distances between the reticle shield  463  and the second Y-blind  432   b , between the support member  462  and the first Y-blind  432   a , between the Y-blinds and the X-blinds  438   a ,  438   b , between the “top” of the collar  472  and the first Y-blind  432   a , and between the X-blinds and the barrier plate  446  are such that actual contact of vertically adjacent components is avoided while providing desired minimal clearances between them. These minimal clearances facilitate differential pumping of the chambers  466 ,  468  as desired. By way of example, because the absolute pressures in the chambers  466 ,  468  are low during normal use, the vertical clearance between these vertically adjacent components can be as large as approximately 1 mm, which avoids having to use sliding seals. 
   The advantageous flow  460  of gas through the gaps  458  and through the apertures  422 ,  461 ,  474 ,  470  during a condition in which the fixed-blind-aperture assembly  412  is retracted (as shown in  FIG. 2(B) ) is shown in  FIGS. 3(A)-3(B)  showing the results of computer simulations. In  FIG. 3(A)  the gas flow  459  is evident by the green color against the blue background. Also evident by green color is gas flow  460  through the gaps  458  and through the apertures  422 ,  461 ,  474 , and  470 , as well as through the lumen of the collar  472 . Establishment of a desired differential pressure is depicted in  FIG. 3(B) , showing clearly the higher scalar pressure in the chamber  466  and lower scalar pressure in the chamber  468 . 
   For comparison purposes, gas-flow velocity and differential-pressure images for the situation shown in  FIG. 2(A) , in which the fixed-blind-aperture assembly  412  is not retracted but rather is in an exposure position, are provided in  FIGS. 5(A) and 5(B) , respectively.  FIG. 5(A)  clearly shows good gas flow  459  past the surface of the reticle  444  as well as good gas flow  460  through the gaps  458  and through the illumination aperture  422 . Note that the collar  472  and first Y-blind  432   a  inhibit substantial gas flow through the aperture  470 .  FIG. 5(B)  shows good maintenance of respective differential pressures in each of the chambers  466  and  468 . 
   In another embodiment, it is possible to eliminate the aperture  474  in the right Y-blind  432   a  by configuring the Y-blinds  432   a ,  432   b  to move to the left (in  FIG. 2(B) ) sufficiently to place the gap between them below the aperture  461 . In other words, in this alternative embodiment, the Y-blinds  432   a ,  432   b  when fully closed still have a gap between them, as shown, but the gap is positioned farther to the left (beneath the aperture  461 ) than shown in  FIG. 2(B) , thereby providing a passage for the gas  460  from the aperture  461  through the aperture  470  into the downstream chamber  468 . 
   Referring now to  FIG. 6 , 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. 
   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. Between the reticle  916  and the barrier wall  920  is a blind apparatus such as that shown in  FIGS. 2(A)-2(B) . 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 . 
   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 . 
   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. 
   Semiconductor devices can be fabricated by processes including microlithography steps performed using a microlithography system as described above. Referring to  FIG. 7 , 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. 
   Representative details of a wafer-processing process including a microlithography step are shown in  FIG. 8 . 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. 
   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. 
   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. 
   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.