Abstract:
Optical systems are disclosed for use in lithography systems, especially extreme-ultraviolet (EUV) lithography systems. An exemplary optical system includes at least a first optical-element set and a second optical-element set that are collectively configured to perform an optical function in which, for example, the first and second optical-element sets receive an EUV light flux from an EUV source and direct the EUV light flux to a pattern master so as to illuminate a selected region of the pattern master. At least one of the first and second optical-element sets is provided as an ensemble of multiple counterpart optical-element sets that are individually selectable for positioning and use at an operational position for performing the optical function. For each ensemble, a respective exchange mechanism holds the ensemble and moves a selected counterpart optical-element set of the ensemble from an off-line position for placement at the operational position and to move another counterpart optical-element set of the ensemble from the operational position to an off-line position.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/652,437, filed Feb. 10, 2005, which is incorporated herein by reference in its entirety. 
     
    
     FIELD  
       [0002]     This disclosure pertains to, inter alia, microlithography, which is a key imaging technology used in the formation of circuit layers in semiconductor integrated circuits, displays, memory devices, and the like. More specifically, the disclosure pertains to microlithography systems employing, for imaging purposes, a wavelength of electromagnetic radiation that must propagate in a subatmospheric (“vacuum”) environment to avoid significant scattering and attenuation of the electromagnetic radiation. Even more specifically, the disclosure pertains to microlithography systems utilizing extreme ultraviolet (EUV) light (also termed “soft X-ray” light) for imaging purposes.  
       BACKGROUND  
       [0003]     Microlithography involves the “transfer” of a pattern, having extremely small features, from a pattern-defining object to an imprintable object. In “projection-microlithography” the pattern-defining object is usually termed a “reticle” or “mask,” and the imprintable object is termed a “substrate,” which usually is a semiconductor wafer that may or may not already have previously formed circuit layers on its surface. So as to be imprintable with an image, the substrate is coated with a radiation-sensitive composition termed a “resist.” 
         [0004]     Projection-microlithography systems are used extensively, for example, for manufacturing integrated circuits, microprocessors, memory “chips,” and the like. These products characteristically comprise multiple functional layers of microscopic circuit elements, all interconnected together in 3-dimensional space. Typically, microlithography is used for patterning most, if not all, the functional layers. In each microlithographic step, the pattern-defining object (usually a mask or reticle) defines the respective pattern for the particular functional layer to be formed. A beam of exposure radiation, termed an “illumination beam,” is directed by an “illumination-optical system” from a source to the pattern-defining object. Interaction of the illumination beam with the pattern-defining object (i.e., selective transmission of the illumination beam through the pattern-defining object or selective reflection of the illumination beam from the pattern-defining object) results in patterning of the beam (now termed a “patterned beam” or “imaging beam”), which renders the patterned beam capable of forming an aerial image of the illuminated pattern. The patterned beam is projected by a “projection-optical system” onto a desired location on the resist-coated substrate where an actual image of the illuminated pattern is formed. Thus, a projection-microlithography system is a type of camera that projects and forms an image on the resist-coated substrate (analogous to a sheet of photographic paper) corresponding to the pattern defined by the pattern-defining object (analogous to a photographic negative, for example). For simplicity herein, the pattern-defining object is generally termed a “reticle.” 
         [0005]     For exposure, the reticle usually is held on a device called a “reticle stage,” and the substrate usually is held on a device called a “substrate stage.” These stages also are typically equipped to perform highly accurate positional measurements and positioning in response to the measurements. Some microlithography systems have multiple reticle stages and/or multiple substrate stages which allow, for example, pre-exposure or post-exposure manipulations to be performed on other reticles and substrates, respectively, as an exposure is being performed on a particular substrate.  
         [0006]     Before being exposed, and to prepare the substrate for exposure, the substrate is usually primed and then coated with a layer of a suitable resist. Before actual exposure, the resist usually is treated such as by a soft-bake step (“pre-exposure bake”). After exposure, the substrate may be soft-baked again (“post-exposure bake”), followed by development of the resist and a hard-bake step to prepare the resist for downstream process steps such as etching, doping, metallization, oxidation, or other suitable step in which the remaining resist on the substrate surface serves as a process mask. Thus, the respective layer is formed on the substrate. As noted above, multiple layers must be formed on the substrate in order to fabricate actual semiconductor devices, so these or similar process steps usually need to be repeated multiple times during the fabrication of the devices. During formation of each layer, steps must be taken to ensure proper and accurate registration of the new layer with the previously formed layer(s).  
         [0007]     The substrate usually is sufficiently large to allow formation of multiple semiconductor devices at respective locations (“dies”) on the substrate. Exposure of multiple dies on the substrate can be die-to-die in one shot per die (characteristic of a “step-and-repeat” exposure scheme) or by scanning each die (characteristic of a “step-and-scan” exposure scheme). In step-and-scan each die typically is exposed by scanning in a scanning direction, wherein both the reticle and the substrate are moved during scanning. Movement of the reticle and substrate can be in the same direction or in opposite directions. If the projection-optical system has a magnification factor (M) other than unity, then the scanning velocity of the substrate typically is M times the scanning velocity of the reticle.  
         [0008]     After completing the fabrication of all the required layers on the surface of the substrate, the dies are cut one from the other. Individual dies are mounted on a packaging substrate, connected to pins or the like, and encased to form finished semiconductor devices. The finished devices typically undergo rigorous testing before being released for sale.  
         [0009]     Accompanying the acknowledgement of an apparent limit (not yet defined) of the minimum feature size of a pattern that can be transferred at an acceptable resolution by optical microlithography, a substantial ongoing effort currently is being directed to the development of a practical “next-generation lithography” (“NGL”) technology. One promising NGL approach is EUV lithography (“EUVL”) performed generally in the wavelength range of 5-20 nm and more specifically at a wavelength in the range of approximately 11-14 nm.  
         [0010]     One challenge posed by EUVL is the substantial scattering and attenuation of an EUV beam by normal-pressure air. Consequently, the propagation path of an EUV beam in an EUVL system must be maintained at high vacuum. Another challenge posed by EUVL is the lack of any known material that is EUV-transmissive and capable of refracting EUV light. Consequently, all the optical elements in an EUV optical system must be reflective rather than refractive. These reflective optical systems and elements include the illumination-optical system, the projection-optical system, and the reticle itself.  
         [0011]     The respective reflective elements making up the reticle, the illumination-optical system, and the projection-optical system of an EUVL system must be fabricated extremely accurately to obtain the level of optical performance currently being demanded. The elements also must perform their intended functions without exhibiting significant degradation of performance caused, for example, by repeated or prolonged exposure to the EUV radiation and/or by accumulation of dust, other debris, and/or contamination on the reflective surfaces of the elements.  
         [0012]     Yet another challenge posed by EUVL is the source of the EUV light. A particularly suitable source is an EUV beam produced by a synchrotron, undulator, or analogous device. But, synchrotrons and undulators are very large, enormously expensive, and enormously complex devices, and very few facilities have access to a synchrotron. Other sources have been developed, including discharge-plasma and laser-plasma sources. Whereas these other sources are advantageously compact and relatively portable, they unfortunately produce substantial amounts of debris that tends to become deposited on the optical elements especially of the illumination-optical system. Consequently, current EUVL systems must be shut down frequently for optical-maintenance tasks such as mirror cleaning and/or replacement.  
         [0013]     Certain aspects of a conventional EUVL system  800  are shown in  FIG. 20 . The depicted system  800  includes an EUV source  802 , an illumination-optical system  804 , and a projection-optical system  806 . The EUV source  802  produces pulses of EUV light from, for example, a laser-induced plasma or electrical-discharge-induced plasma. Light from the plasma is reflected, within the source, from a concave collector mirror  808 , which collects the light produced by the plasma and directs the collected light  810  to the illumination-optical system  804 . The EUV source  802  typically includes a filter  812  that removes, from the EUV light produced by the plasma, extraneous and unwanted wavelengths of light (including visible light) as the EUV light  810  exits the source  802 . Thus, the light  810  exiting the source  802  consists almost exclusively of the particular wavelength (e.g.,  13 . 4  nm) of EUV light desired for making lithographic exposures.  
         [0014]     The depicted illumination-optical system  804  includes a collimator mirror  814 , a first fly-eye mirror  816 , a second fly-eye mirror  818 , a first condenser mirror  820 , a second condenser mirror  822 , and a grazing-incidence mirror  824 . The mirrors  814 ,  816 ,  818 ,  820 ,  822  are mounted at respective locations on a rigid frame (not shown) so as to place the mirrors in proper respective positions relative to each other. The collimator mirror  814  collimates the EUV light  810  from the source  802  as the EUV light reflects from the collimator mirror. The collimated light  824  propagates to the first fly-eye mirror  816 , from which the light  826  reflects to the second fly-eye mirror  818 . The first fly-eye mirror  816  typically is arc-shaped (corresponding approximately to the illumination field), and the second fly-eye mirror  818  typically has a rectangular profile. The fly-eye mirrors  816 ,  818  make the illumination intensity of the EUV light substantially uniform over the illumination field. From the second fly-eye mirror  818  the EUV light  828  assumes a gradually convergent characteristic as the EUV light propagates to and reflects from the first and second condenser mirrors  820 ,  822 . From the second condenser mirror  822  the EUV light  830  reflects (at grazing incidence) from the grazing-incidence mirror  824  (usually a planar mirror) to the reticle  832  where the illumination field illuminates respective selected portions of the reticle pattern at particular instances in time. During illumination, the reticle  832  is mounted (reflective-side facing downward) on a reticle chuck  834  that is mounted on a movable reticle stage  836 . The reticle stage  836  positions the reticle  832  as required for illumination of the desired portions of the reticle pattern by the illumination field at respective instances in time.  
         [0015]     The particular type of illumination-optical system  804  shown in  FIG. 20  is a 6-mirror system. So as to be reflective to incident EUV light at less than grazing angles of incidence, the collimator mirror  814 , fly-eye mirrors  816 ,  818 , and condenser mirrors  820 ,  822  have surficial multilayer-interference coatings (e.g., multiple superposed and very thin layer pairs of Mo and Si) that render the surfaces of these mirrors reflective to incident EUV light. Due to the manner in which the EUV light reflects from the grazing-incidence mirror  824  (i.e., at grazing angles of incidence), the grazing-incidence mirror need not have a multilayer coating. In the EUV source  802 , the collector mirror  808  also has a multilayer-interference coating.  
         [0016]     The EUV light  838  from the grazing-incidence mirror  824  is incident on the reticle  832  at a small angle of incidence (approximately 5 degrees). So as to be reflective to EUV light at such a small angle of incidence, the reticle  832  also has a multilayer-interference coating as well as EUV-absorbent bodies that define, along with spaces between the bodies, the particular pattern on the reticle that is to be transferred to a substrate. Thus, as the EUV light reflects from the irradiated region of the reticle  832 , the EUV light  840  acquires an aerial image of the pattern on the reticle and thus is rendered capable of imaging the illuminated pattern on the surface of the substrate  842 . The substrate  842  is mounted on a movable substrate stage  846 .  
         [0017]     The illumination-optical system  804  must be under high vacuum during operation. Hence, the illumination-optical system  804  is contained in a respective vacuum chamber  848 . Similarly, during operation the projection-optical system must be under high vacuum, and hence is contained in a respective vacuum chamber  850 .  
         [0018]     To form the image on the surface of the substrate  842 , the “patterned” EUV light  840  reflected from the reticle  832  passes through the projection-optical system  844 , which also contains multiple reflective mirrors. Depending upon its particular configuration, the projection-optical system  844  usually has two, four, or six mirrors (not detailed) each having a respective multilayer-interference coating.  
         [0019]     As mentioned above, plasma-based EUV sources  802  tend to produce debris and other contamination that poses a substantial maintenance problem with respect to optical components located in the EUV source itself and in neighboring systems. Also, the plasma is very hot and the light produced by the plasma is very intense; this combination of elevated temperature and illumination intensity can deteriorate nearby surfaces. For example, in the EUV source  302 , the collector mirror  808  is closest to the plasma and thus tends to experience a rapid rate of debris accumulation from the plasma as well as deterioration caused by high temperature and intense illumination. As a result, the collector mirror  808  tends to require frequent maintenance (e.g., cleaning) and/or replacement, both of which involve a substantial interruption in the operation of the EUVL system. Similarly, the mirrors  814 ,  816 ,  818 ,  820 ,  822  in the illumination-optical system  304  (especially the collimator mirror  814 ) also become contaminated during use, and the closer a mirror is to the plasma, generally the more rapid the rate of contamination. Debris accumulation and other contamination of EUV optical elements are substantial problems because these phenomena cause substantial reductions in reflectivity (and thus performance) of the elements. Unfortunately, whenever the time for a maintenance event arrives, the EUVL system  800  must be shut down, the vacuum must be vented, and the optical systems opened up to remove the element(s) requiring maintenance. After cleaning, repair, or replacement of the element(s), the optical system(s) must be reassembled and aligned, the optical systems closed and evacuated, and the system recalibrated to restore the system to normal operational status. These various maintenance-related tasks consume enormous amounts of time and thus impose substantial detriments to system throughput, which can cause intolerable reductions in profitability of the semiconductor-fabrication facility in which the EUVL equipment is located. At the same time, these maintenance tasks must be performed quickly, without contaminating the system and without harming other parts of the system.  
         [0020]     In addition, it currently is desirable that the EUVL system  800  have sufficient flexibility so as to be useful under various conditions and situations. For example, it currently is desirable that the system provide variable sigma (a) and/or field-size settings. As used herein, sigma is the “partial coherence parameter,” which is the ratio of the numerical aperture of the illumination system to the numerical aperture of the projection system. The numerical aperture of the illumination system conventionally is adjusted by exchanging apertures in the illumination system. (See U.S. Pat. No. 6,771,350.) But, use of these apertures reduces overall light intensity, which correspondingly reduces the overall productivity of the lithography tool. Furthermore, in a conventional EUVL system, imparting these changes typically requires shut-down of the system, as summarized above, to allow exchange of apertures and/or one or more optical elements that establish the respective parameters (e.g., sigma or size and shape of the illumination field).  
       SUMMARY  
       [0021]     The shortcomings of conventional apparatus and methods as summarized above are cured by apparatus and methods as disclosed herein. Certain of the various  20  embodiments disclosed herein provide at least one mirror (e.g., the collimator mirror) or mirror set (e.g., the fly-eye set, the condenser set, the fly-eye/collimator set, or the fly-eye/condenser set) that is selectable from multiple such mirrors or sets that are provided on an exchanger mechanism. The mirrors or sets are provided in respective “frames” or “modules” that can be manipulated, moved, and positioned without human handling of the constituent mirrors. Other sets of mirrors (e.g., fly-eye set, condenser set) that are not exchanged or exchangeable can be housed in respective frames or modules. The exchangers generally are configured to occupy minimal space. The exchanger mechanism desirably includes a non-contacting interface between the frame placed in the operational position and the adjacent frame, which allows frame exchange with minimal disturbance to non-exchanged optical elements and shortens the required time for any alignment performed after exchange. Also, exchanges can be performed with less particle generation, which minimizes contamination of the illumination-optical system and other systems with which the illumination-optical system is used. The actuators used for moving frames into position, as well as sensors used for detecting proper positioning of just-moved frames at their respective operational positions, serve to position and stabilize the frames and mirrors relative to a reference frame such as the projection-optics box (POB). In addition, the exchangers desirably are situated, with respect to any surrounding structure such as a vacuum chamber, so as to allow accessibility to the exchangers for service.  
         [0022]     According to a first aspect of the disclosure, optical systems for lithography systems are provided. An embodiment of such an optical system comprises at least a first optical-element set and a second optical-element set collectively configured to perform an optical function in which the at least first and second optical-element sets receive a light flux from a light source and direct the light flux for lithographic-imaging purposes. At least one of the optical-element sets is provided in the optical system as an ensemble of multiple counterpart optical-element sets that are individually selectable for positioning and use in performing the optical function. For each ensemble, a respective storage-and-exchange mechanism is situated and configured to hold the ensemble and to move a selected optical-element set of the ensemble from an off-line position for placement at the operational position and to move another counterpart optical-element set of the ensemble from the operational position to an off-line position. By way of example, the lithography system is an EUV lithography system, wherein the optical system is an illumination-optical system in which each optical-element set comprises at least one respective EUV-reflective mirror.  
         [0023]     In an embodiment of an optical system the first optical-element set can comprise the ensemble, and the second optical-element set can be stationary. Each counterpart optical-element set of the ensemble can comprise a respective collimator-mirror set, wherein the second optical-element set can comprise a fly-eye-mirror set and a condenser-mirror set. Alternatively, each counterpart optical-element set of the ensemble can comprise a collimator-mirror set and a fly-eye-mirror set, wherein the second optical-element set can comprise a condenser-mirror set.  
         [0024]     In another embodiment of an optical system the first optical-element set can comprise a first ensemble, wherein the second optical-element set can comprise a second ensemble. Each counterpart optical-element set of the first ensemble can comprise a respective collimator-mirror set, and each counterpart optical-element set of the second ensemble can comprise a respective fly-eye-mirror set and a respective condenser-mirror set.  
         [0025]     Another embodiment of the optical system further comprises a third optical-element set, wherein the first optical-element set comprises the ensemble, and the second and third optical-element sets are stationary. In such a system each counterpart optical-element set of the ensemble can comprise a respective collimator-mirror set, the second optical-element set can comprise a fly-eye-mirror set, and the third optical-element set can comprise a condenser-mirror set. Alternatively, each counterpart optical-element set of the ensemble can comprise a respective fly-eye-mirror set, the second optical-element set can comprise a collimator-mirror set, and the third optical-element set can comprise a condenser-mirror set.  
         [0026]     Yet another embodiment of an optical system further can comprise a third optical-element set. In such a system the first optical-element set can comprise a first ensemble, the second optical-element set can comprise a second ensemble, and the third optical-element set can be stationary. In this system each counterpart optical-element set of the first ensemble can comprise a respective collimator-mirror set, each counterpart optical-element set of the second ensemble can comprise a respective fly-eye-mirror set, and the third optical-element set can comprise a condenser-mirror set. Alternatively, the first optical-element set can comprise a first ensemble, the second optical-element set can comprise a second ensemble, and the third optical-element set can comprise a third ensemble. In this system each counterpart optical-element set of the first ensemble can comprise a respective collimator-mirror set, each counterpart optical-element set of the second ensemble can comprise a respective fly-eye-mirror set, and each counterpart optical-element set of the third ensemble can comprise a respective condenser-mirror set.  
         [0027]     According to another aspect, illumination-optical systems are provided for EUV lithography systems. An embodiment of such an illumination-optical system comprises at least a first mirror set and a second mirror set collectively configured to perform an optical function in which the first and second mirror sets receive an EUV light flux from an EUV source and direct the EUV light flux to a pattern master so as to illuminate a selected region of the pattern master. At least one of the first and second mirror sets is provided in the illumination-optical system as an ensemble of multiple counterpart mirror sets that are individually selectable for positioning and use at an operational position for performing the optical function. For each ensemble, a respective exchange mechanism is situated and configured to hold the ensemble and to move a selected counterpart mirror set of the ensemble from an off-line position for placement at the operational position and to move another counterpart mirror set of the ensemble from the operational position to an off-line position. In this system the first mirror set can be located closer to the EUV source, the second mirror set can be located more distantly from the EUV source than the first mirror set, and the ensemble can be of multiple selectable counterpart mirror sets of the first mirror set. The exchange mechanism can comprise a housing configured to hold the ensemble, wherein the housing can define, for example, a respective branch for each of the selectable counterpart mirror sets of the ensemble. In this configuration the exchange mechanism can be configured to move the respective branch of the selected counterpart mirror set into the operational position. The branches of the housing can be arranged as a turret that is rotatable about an axis by the exchange mechanism to place a selected branch of the turret in the operational position. The respective counterpart mirror set in each branch of the turret can comprise, for example, a respective collimator mirror.  
         [0028]     In an illumination-optical system as summarized above, the first mirror set can comprise a mirror situated closest to the EUV source of all the mirrors of the illumination-optical system. In such a configuration the ensemble can be of multiple selectable counterpart mirror sets of the first mirror set.  
         [0029]     If the first mirror set is a collimator-mirror set, then the second mirror set can comprise at least one fly-eye mirror and at least one condenser mirror, wherein the ensemble is of multiple selectable counterpart mirror sets of the first mirror set.  
         [0030]     Each counterpart mirror set in the ensemble can consist of a single respective mirror. By way of example, each single respective mirror in the ensemble can be a respective collimator mirror. By way of another example, each single respective mirror in the ensemble can have identical optical properties. By way of another example, at least two of the single respective mirrors in the ensemble can have different optical properties. By way of yet another example, each counterpart mirror set in the ensemble can consist of multiple respective mirrors including a respective collimator mirror.  
         [0031]     In another embodiment of an optical system the first mirror set is provided in the illumination-optical system as a first ensemble of multiple selectable counterpart mirror sets that are individually selectable for positioning and use in performing the optical function, and the second mirror set is provided in the illumination-optical system as a second ensemble of multiple selectable counterpart mirror sets that are individually selectable for positioning and use in performing the optical function. In this embodiment the first mirror set can be contained in a respective housing that holds the multiple selectable counterpart mirror sets of the first mirror set in respective branches of the housing, and the multiple selectable counterpart mirror sets of the second mirror set can be contained in individual respective housings. The respective exchange mechanism for the first mirror set moves a selected branch to a respective operational position, and the respective exchange mechanism for the second mirror set moves a selected counterpart mirror set in its respective housing to a respective operational position. The housing for the first mirror set can be configured so as not to contact the selected housing of the second mirror set. The system further can comprise a tracking interface situated between the respective operational position of the first mirror set and the respective operational position of the second mirror set. A respective actuator mechanism can be associated with at least one of the first and second mirror sets and configured to align the respective mirror set based on data, regarding positions of the selected counterpart mirror sets at their respective operational positions, produced by the tracking interface. The tracking interface can be configured to produce the position data in the absence of physical contact of the first mirror set with the second mirror set. The tracking interface can be configured to track, in an active manner, the position of one of the first and second mirror sets relative to the position of the other of the first and second mirror sets.  
         [0032]     In another embodiment of an optical system each of the counterpart mirror sets of the first mirror set can comprise a respective collimator mirror, and each of the counterpart mirror sets of the second mirror set can comprise at least one mirror selected from the group consisting of fly-eye mirrors and condenser mirrors. For example, each of the counterpart mirror sets of the second mirror set can comprise a respective pair of fly-eye mirrors, and each of the counterpart mirror sets of the second mirror set further can comprise a respective pair of condenser mirrors.  
         [0033]     Each of the counterpart mirror sets in the first mirror set can have identical optical properties, wherein at least two of the counterpart mirror sets in the second mirror set have different optical properties.  
         [0034]     In another embodiment of an optical system at least two of the counterpart mirror sets in the first mirror set have different optical properties, and at least two of the counterpart mirror sets in the second mirror set have different optical properties. Alternatively, each of the counterpart mirror sets in the second mirror set has identical optical properties.  
         [0035]     In another embodiment of an optical system each counterpart mirror set of the ensemble is housed in a respective housing, wherein the exchange mechanism can be operable to store the housings of off-line counterpart mirror sets and to move the housing of the selected counterpart mirror set into the operational position. By way of example, the exchange mechanism can comprise a rotary portion situated and configured to store the housings of off-line counterpart mirror sets, and the exchange mechanism can comprise a robot situated and configured to move the selected counterpart mirror set from the rotary portion to the operational position.  
         [0036]     In another embodiment the first mirror set is provided in the illumination-optical system as a first ensemble of multiple selectable counterpart mirror sets that are individually selectable for positioning and use in performing the optical function. The second mirror set can be provided in the illumination-optical system as a second ensemble of multiple selectable counterpart mirror sets that are individually selectable for positioning and use in performing the optical function. Each counterpart mirror set of the first mirror set can comprise a respective collimator mirror, and each counterpart mirror set of the second mirror set can comprise a respective pair of fly-eye mirrors. Each counterpart mirror set of the second mirror set further can comprise a respective pair of condenser mirrors.  
         [0037]     Another embodiment of an optical system can include a tracking interface situated between the first mirror set and the second mirror set. An exemplary tracking interface includes a position-sensing device that detects position of the first and second mirror sets relative to each other. This embodiment further can comprise at least one actuator system situated with at least one of the first and second mirror sets. The at least one actuator system can be selectively actuatable to perform an adjustment of the position of at least one of the first and second mirror sets relative to each other, based on data from the position-sensing device, to align the first and second mirror sets. The actuator system can comprise one or more actuators sufficient to move the respective mirror set in multiple degrees of freedom relative to the other mirror set.  
         [0038]     According to another aspect, optical systems for an EUV lithography system are provided. An embodiment of such an optical system comprises first reflective-optical-element means and second reflective-optical-element means that are collectively configured to perform an optical function in which the first and second reflective-optical-element means receive EUV light from an EUV-source means and direct the EUV light for performing lithographic imaging. At least one of the reflective-optical-element means comprises an ensemble of multiple selectable counterpart reflective-optical-element sets that are individually selectable for positioning and use in performing the optical function. The optical system embodiment also includes exchange means, associated with each ensemble, for holding the ensemble, for moving a selected reflective-optical-element set of the ensemble from an off-line position for placement at the operational position, and for moving another counterpart reflective-optical-element set of the ensemble from the operational position to an off-line position.  
         [0039]     In the optical system summarized above the optical system can comprise illumination-system means for illuminating a pattern-master means. In this configuration the optical function can comprise illuminating selected portions of the pattern-master means with the EUV light. At least the first reflective-optical-element means can comprise a respective ensemble and has an associated exchange means. In this configuration the first reflective-optical-element means can be situated proximally the EUV-source means to receive the EUV light from the EUV-source means. The first reflective-optical-element means can comprise at least collimating means for receiving the EUV light from the EUV-source means and collimating the EUV light as the EUV light reflects from said collimating means.  
         [0040]     In another embodiment of the optical system both the first and second reflective-optical-element means can comprise a respective ensemble and an associated exchange means.  
         [0041]     Another aspect is directed to methods set forth in the context of a lithography system. The subject methods are for maintaining on-line operation of an optical system of the lithography system. The optical system includes at least a first optical-element set and a second optical-element set that collectively perform an optical function in which the first and second optical-element sets receive a light flux from a light source and direct the light flux for a lithographic-imaging purpose. An embodiment of the method comprises configuring at least the first optical-element set as a respective ensemble of multiple counterpart optical-element sets that are individually selectable for positioning for use in performing the optical function. A desired counterpart optical-element set of the ensemble is selected. The desired counterpart optical-element set is placed at an operational position at which the selected counterpart optical-element set works cooperatively with the second optical-element set to perform the optical function. A currently selected counterpart optical-element set can be moved into the operational position upon removing a previously selected counterpart optical-element set from the operational position. The currently selected and previously selected counterpart optical-element sets can be moved using a storage-and-exchange robot.  
         [0042]     In the methods, if the first optical-element set is vulnerable to contamination during use, then a currently selected counterpart optical-element set can be moved into the operational position to replace a previously selected counterpart optical-element set that has become contaminated. For example, if the microlithography system is an EUV lithography system, and if the optical system is situated downstream of an EUV source that produces particulate contamination, then the first optical-element set can be situated closer to the EUV source than the second optical-element set.  
         [0043]     The first optical-element set can have a parameter that, when changed, correspondingly changes an operational aspect of the optical system. In this situation a currently selected counterpart optical-element set, having a first value of the parameter, can be moved into the operational position to replace a previously selected counterpart optical-element set having a second value of the parameter, wherein the first value of the parameter is more desired than the second value in performing the optical function.  
         [0044]     In the methods summarized above, if the microlithography system is an EUV lithography system and the optical system is an illumination-optical system, the first optical system can comprise a collimator mirror. Alternatively, the first optical system can comprise at least one mirror selected from the group consisting of collimator mirrors, fly-eye mirrors, and condenser mirrors.  
         [0045]     In another embodiment of the subject methods, the second optical-element set is configured as a respective ensemble of multiple counterpart optical-element sets that are individually selectable for positioning for use in performing the optical function. The methods include selecting a desired counterpart optical-element set of the respective ensemble of the second optical-element set and placing the desired counterpart optical-element set at a respective operational position at which the selected counterpart optical-element set works cooperatively with the selected counterpart optical-element set of the first optical-element set to perform the optical function. By way of example, the microlithography system can be an EUV lithography system, in which the optical system is an illumination-optical system situated downstream of an EUV source, the first optical-element set is closer to the EUV source than the second optical-element set, and the first optical-element set comprises at least a collimator mirror. In this example the second optical-element set can comprise at least one mirror selected from the group consisting of fly-eye mirrors and condenser mirrors.  
         [0046]     Another embodiment of the methods further comprises actively tracking the first and second optical-element sets relative to each other, without contacting each other, to maintain alignment of the first and second optical-element sets with each other for performing the optical function.  
         [0047]     Thus, using systems and methods as disclosed herein, it is possible, for example, to change the numerical aperture of an optical system without sacrificing productivity of the lithography system. Similarly, to maximize productivity for various field sizes, a portion of the illumination system can be exchanged so that illumination light is not wasted. (For EUV lithography, various field sizes include 26×31 mm, currently the maximum standard field size, and 25×25 mm.)  
         [0048]     The foregoing and additional features and advantages of apparatus and methods as disclosed herein will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]      FIG. 1  is a schematic diagram of an exemplary illumination-optical system for an EUV microlithography system, illustrating certain relationships of the components with one another.  
         [0050]      FIG. 2  is a schematic diagram of certain consequences of changing sigma (a;) of the illumination-optical system of  FIG. 1  from 0.3 to 0.8.  
         [0051]      FIG. 3  is a perspective view of an illumination-optical system according to a first representative embodiment that comprises a rotary exchanger holding multiple collimator (COL) frames each comprising a respective collimator mirror that can be selected, by rotation of the exchanger, for use in the illumination-optical system.  
         [0052]      FIG. 4  is a schematic diagram showing control relationships of the first representative embodiment, in which the collimator (COL) frame and fly-eye/condenser (FE/CON) frame are independently mounted to the system base by respective mountings providing respective six degrees of freedom of adjustability as well as active vibration isolation and attenuation.  
         [0053]      FIG. 5  is a schematic diagram showing control relationships of an alternative configuration of the first representative embodiment, in which the COL frame and FE/CON frame are mounted to an illumination-unit main frame that is mounted to the system base by a mounting providing six degrees of freedom of adjustability as well as active vibration isolation and attenuation.  
         [0054]      FIG. 6  is a perspective view of an illumination-optical system according to a second representative embodiment that comprises a first rotary exchanger holding multiple COL frames, a second rotary exchanger holding multiple fly-eye (FE) frames, and a linear robot for selecting a FE frame and moving the selected FE frame to an operational position relative to the selected COL frame and the condenser (CON) frame.  
         [0055]      FIG. 7  is a schematic diagram showing control relationships of the second representative embodiment, in which the collimator (COL) frame, the FE frame, and the CON frame are independently mounted to the system base by respective mountings providing respective six degrees of freedom of adjustability as well as active vibration isolation and attenuation.  
         [0056]      FIG. 8  includes two perspective views of the illumination-optical system of the second representative embodiment contained in a housing.  
         [0057]      FIG. 9  is a perspective view of an illumination-optical system according to the third representative embodiment that comprises multiple fly-eye/collimator (FEC) frames and a rotary exchanger for holding the FEC frames, wherein a selected FEC frame is moved into an operation position from the rotary exchanger.  
         [0058]      FIG. 10  includes two perspective views of the illumination-optical system of the third representative embodiment, including a housing and vacuum pumps.  
         [0059]      FIG. 11  provides a perspective view and several orthogonal views of an illumination-optical system according to a fourth representative embodiment, in which a selected one of multiple fly-eye/collector (FEC) frames can be placed in an operational position.  
         [0060]      FIG. 12  is a perspective view of an illumination-optical system according to the fifth representative embodiment, in which a selected one of multiple FEC frames can be placed in an operational position.  
         [0061]      FIG. 13  is a perspective view of an illumination-optical system according to the sixth representative embodiment, in which a selected one of multiple FEC frames can be placed in an operational position.  
         [0062]      FIG. 14  is a perspective view of an illumination-optical system according to the seventh representative embodiment, in which one of multiple FEC frames (selected from a linear array of FEC frames) can be placed in an operational position.  
         [0063]      FIG. 15  is a perspective view of an illumination-optical system according to the eighth representative embodiment, in which one of multiple FEC frames (selected from the FEC frames on a rotary exchanger) can be placed in an operational position.  
         [0064]      FIG. 16  is a perspective view of an illumination-optical system according to the ninth representative embodiment, in which one of multiple fly-eye (FE) frames (selected from multiple such frames on a rotary exchanger) can be placed in an operational position.  
         [0065]      FIG. 17  is a perspective view of an alternative configuration of the ninth representative embodiment, in which one of multiple FE frames (in a linear array) can be selected and placed in an operational position.  
         [0066]      FIG. 18  is a block diagram of an exemplary semiconductor-device fabrication process that includes wafer-processing steps comprising a microlithography step performed using a microlithography system as described herein.  
         [0067]      FIG. 19  is a block diagram of a wafer-processing process as referred to in  FIG. 18 .  
         [0068]      FIG. 20  is a schematic diagram of certain optical components of a conventional EUVL system. 
     
    
     DETAILED DESCRIPTION  
       [0069]     This disclosure is set forth in the context of representative embodiments that are not intended to be limiting in any way.  
         [0000]     General Considerations:  
         [0070]     A layout of an exemplary illumination-optical system, with EUV source, is shown in  FIG. 1 . The system  10  is depicted with certain approximate dimensions, in mm. The system  10  includes an EUV source  12 , a collimator mirror  14 , a first fly-eye mirror  16 , a second fly-eye mirror  18 , a first condenser mirror  20 , a second condenser mirror  22 ; and a planar grazing-incidence mirror  24 . Also depicted are representative ray traces  26  from the collimator mirror  14  to the reticle  28 . The wafer (substrate)  30  is situated optically downstream from the reticle  28 .  
         [0071]     The EUV source  12  includes the following features that are not detailed but are well-understood: a plasma zone where a target material is converted into a plasma (e.g., by intense laser illumination or electrical discharge), a collector mirror (for collecting EUV radiation radiating from the plasma zone and reflecting the collected EUV radiation in a desired direction as indicated by the trace  32 ), and a spectral filter (through which the beam of collected EUV radiation passes, to remove unwanted wavelengths of light) as the beam exits the EUV source  12 . These components are contained in a housing  34  in which the atmosphere can be evacuated to a desired level.  
         [0072]     Exemplary dimensions of the respective mirrors are as follows: collimator  14 : 200 mm diameter; fly-eye mirrors  16 ,  18 : each 200 mm diameter; first condenser  20 : 100 mm diameter; and second condenser  22 : 325 mm×150 mm. Exemplary heat loads (due principally to radiative heating from the incident EUV radiation) on the mirrors range from 80 W (for the collimator  14 ) to about 6 W (for the second condenser  22 ) if the spectral filter in the EUV source  12  is not used, and range from 40 W for the collimator  14 ) to about 3 W (for the second condenser  22 ) if the spectral filter in the EUV source is used. In either event, the greatest heat load is experienced by the collimator mirror  14  (because it is nearest the EUV source  12 ), and the least heat load is experienced by the grazing-incidence mirror  24 . Conventionally, the collimator mirror  14 , the fly-eye mirrors  16 ,  18 , and the condenser mirrors  20 ,  22  are mounted on a rigid frame that is situated relative to the EUV source  12 , grazing-incidence mirror  24 , and reticle  28  and housed in a chamber in which the atmosphere can be evacuated to a desired level (&lt;1 mTorr or &lt;6.66 Pa).  
         [0073]      FIG. 1  also depicts an outline of a projection-optics “box”  38  containing a projection-optical system  36 . Because of the opposing positions of the reticle  28  and wafer  30 , the projection-optical system  36  desirably comprises an even number of mirrors, such as two, four, or six mirrors (not detailed but understood in the art). The projection-optics box  38  desirably includes a reticle stage  40  (on which the reticle  28  is mounted by chucking, facing downward in the figure) and a wafer stage  42  (on which the wafer  30  is mounted by chucking, facing upward in the figure). The grazing-incidence mirror  24  for convenience is situated within the housing  38 .  
         [0074]     The two fly-eye mirrors  16 ,  18  can be regarded as a fly-eye (FE) “set”  44 . Similarly, the two condenser mirrors  20 ,  22  can be regarded as a condenser (CON) “set”  46 .  
         [0075]     As noted above, it is desirable that the illumination-optical system  10  provide variable sigma (variable σ, wherein σ=NA illum /NA proj , and NA illum  is numerical aperture of the illumination-optical system and NA proj  is numerical aperture of the projection-optical system), for example three different a settings within a range of 0.3 to 0.8. (Variable sigma is useful for enhancing imaging quality for various types of pattern geometrics.) As shown in  FIG. 2 , changing (y from 0.3 to 0.8 involves a change in position and diameter of at least two mirrors, namely the collimator mirror (“COL”)  14  and the first fly-eye mirror (“FE1”)  16 , and a change in diameter also of the second fly-eye mirror  18 . As a result, in this example, the collimator mirror  14  and the first fly-eye mirror change position by as much as 200 mm, and the collimator mirror  14  and first and second fly-eye mirrors change diameter by as much as 150 mm. These dimensions represent substantial changes to the illumination-optical system  10 . It also is desirable that the illumination-optical system  10  provide at least two different field settings. Providing the illumination-optical system  10  with two different field settings and three different a settings requires six different sets of fly-eye mirrors (each set having different sizes and/or positions of the two fly-eye mirrors  16 ,  18 ), and three different collimator mirrors  14  (each having a different size and/or position).  
         [0076]     Each of the mirrors in the illumination-optical system desirably has very high stability. Exemplary stability specifications include: 
        Shape: ˜100 μm     Position: ˜20 μm     Tilt: ˜35 μrad 
 
 In achieving the foregoing performance criteria, exemplary adjustment-resolution specifications are: 
    Position: 20 μm     Tilt: 15 grad (30 μm at 200 mm diameter)        
 
         [0082]     To provide the foregoing features while meeting the stated specifications, various representative embodiments are set forth below that provide one or more “exchangers” allowing rapid exchange of a subject mirror or mirror set to meet the demands imposed by one or more of the following: (a) replacement of one or more excessively contaminated mirrors with clean or new mirrors; (b) exchanging one or more mirrors configured and positioned to meet a first optical setting with corresponding mirrors configured and positioned to meet a second optical setting.  
       First Representative Embodiment  
       [0083]     This embodiment, depicted in  FIG. 3 , provides an illumination-optical system  100  in which the collimator mirror can be exchanged. The embodiment of  FIG. 3  depicts the following components: the collector mirror  102  of the EUV source (the remainder of the EUV source is not shown, but see  FIG. 1 ), a four-way exchanger (turret)  104  housing and ensemble of four individually selectable collimator mirrors, a housing  106  containing a set of first and second fly-eye mirrors and a set of first and second condenser mirrors (see  FIG. 1 ), and a grazing-incidence mirror  108 . The exchanger  104  includes and/or serves as a “frame” and housing for the collimator mirrors, and hence also is termed a “COL frame.” Similarly, the housing  106  includes and/or serves as a “frame” for the mirrors inside it, and hence also is termed an “FE/CON frame.” 
         [0084]     The COL frame  104  includes four independently selectable branches  104   a - 104   d  each containing a respective collimator mirror. The COL frame  104  is rotatable about an axis A 1  to position a selected branch  104   a - 104   d  relative to the FE/CON frame  106 . The respective collimator mirrors in the branches  104   a - 104   d  can be the same or different. For example, the branches  104   a - 104   d  can provide four different respective collimator mirrors each having a different size, a different surface profile, and/or a different position to provide four different respective optical settings. Alternatively, two or more of the collimator mirrors can be identical, in which configuration the COL frame  104  allows exchange of a contaminated collimator mirror for a clean or new one by simply rotating the turret to a different branch containing the clean or new mirror. Note that, in this embodiment, only one selected branch  104   b  can be in an operational position  110  at which the respective mirror can receive EUV light  112  from the collector mirror  102 . EUV light  114  reflected from the collimator mirror in the selected branch  104   b  exits the COL frame  104  and enters the FE/CON frame  106  as the light propagates to the first fly-eye mirror situated in the FE/CON frame  106 . In the FE/CON frame  106 , the EUV light reflects, in succession, from the first fly-eye mirror, the second fly-eye mirror, the first condenser mirror, and the second condenser mirror. EUV light  107  reflected from the second condenser mirror exits the FE/CON frame  106  as the light propagates to the grazing-incidence mirror  108 .  
         [0085]     Desirably, the COL frame  104  does not physically contact the FE/CON frame  106 , either during rotation of the COL frame or during use of the selected branch thereof. Turning now to  FIG. 4 , the COL frame  104  is mounted on a rotary stage  116  mounted to a system base  118 . The rotary stage  116  is rotatable relative to the base  118  to rotate the COL frame  104  relative to the base  118  as required to select a particular branch  104   a - 104   d.  Between the stage  116  and the COL frame  104  is a mounting  120  providing an active vibration-isolation system (AVIS) and six degrees of freedom (6 DOF) of actuation motion (x, y, z, θ x , θ y , θ z ) of the COL frame (for positional-adjustment and vibration-isolation purposes) relative to the stage  116 . Thus, the mounting  120  serves as a “fine stage” for the rotary stage  116 , wherein the rotary stage  116  includes and serves as a coarse stage (not detailed). In  FIG. 4  the branch  104   d  containing “collimator  4 ” (“COL 4 ”) has been selected for use. Thus, the 6DOF mounting  120  imparts active vibration isolation and 6 DOF of positional adjustability of the branch  104   d,  and hence of the collimator mirror COL 4 . Since the 6DOF mounting  120  serves the entire COL frame  104 , individual 6DOF mountings are not required for each collimator mirror COL 1 -COL 4 . This alleviates system complexity and simplifies routing of flexible hoses (for hydraulics) and electrical cables  122  extending between the base  118  and the rotary stage  116  and of hoses and cables  124  extending between the rotary stage  116  and the collimator mirrors COL 1 -COL 4  in the COL frame  104 . (Hoses and cables can introduce vibrations and moments to movable objects to which the hoses and cables are connected, so reducing the number of cables and hoses is advantageous.)  
         [0086]     The FE/CON frame  106  is mounted to the base  118  via a mounting  126  providing an active vibration-isolation system (AVIS) and six degrees of freedom (6DOF) of actuation motion (x, y, z, θ x , θ y , θ z ) of the FE/CON frame  106  (for positional-adjustment and vibration-isolation purposes) relative to the system base  118  and relative to the projection-optics box (“POB”)  128 . Determinations of the positions of the FE/CON frame  106  and COL frame  104  relative to each other are made by an appropriate sensor  130  across a tracking interface  132 . The sensor  130  provides feedback to the 6DOF mounting  120 . Thus, the COL frame  104  actively tracks the position of the FE/CON frame  106  using the 6DOF mounting  120 . The sensor  130 , served by a flexible cable (not shown) desirably is mounted to the FE/CON frame  106  rather than the COL frame  104  because the FE/CON frame  106  is stationary, which eliminates problems otherwise associated with attaching a cable to a movable object such as the COL frame  104 .  
         [0087]     For alignment purposes, at least the first fly-eye mirror (FE 1 )  134 , the second fly-eye mirror (FE 2 )  136 , and first condenser mirror (CON 1 )  138  include respective adjusters  140 ,  142 ,  144  for adjusting the position of the respective mirror relative to each other and to the FE/CON frame  106 . Each of these adjusters  140 ,  142 ,  144  provides 6 DOF of positional adjustability of the respective mirror and typically is a “set-and-forget” device that, once set, can be left in its respective adjusted position (at least until adjustment is again required). The second condenser mirror (CON 2 )  146  can have a respective 6DOF adjuster (not shown) if desired. Determinations of the positions of the FE/CON frame  106  and projection-optics box  128  relative to each other are made by an appropriate sensor  148  across a tracking interface  150 . The sensor  148  provides feedback to the 6DOF mounting  126 . Thus, using the 6DOF mounting  126 , the FE/CON frame  106  actively tracks the position of the projection-optics box  128 . The sensor  148 , served by a flexible cable (not shown) desirably is mounted to the FE/CON frame  106  rather than to the projection-optics box  128  to simplify routing of cables and the like (not shown).  
         [0088]     A “tracking interface” comprises two principal surfaces, one on each body to be aligned with each other. The tracking interface includes at least one sensor (e.g., interferometer, capacitative sensor, inductive sensor, laser scale, or optical sensor) used for measuring the relative distance between the two principal surfaces of the tracking interface. Typically, the sensor is attached to one of the principal surfaces. In some instances, multiple components of the sensor need to be attached to both principal surfaces. For example, in the case of an interferometric sensor, mirrors usually need to be placed on both principal surfaces.  
         [0089]     The configuration shown in  FIG. 4  is an example of a two-body active-stabilization configuration, in which the FE/CON frame  106  actively tracks the projection-optical box (POB)  128  using the 6DOF mounting  126 , and the COL frame  104  actively tracks the FE/CON frame  106  using the 6DOF mounting  120 .  
         [0090]     An alternative configuration is depicted in  FIG. 5 , in which components that are similar to respective components shown in  FIG. 4  have the same reference designators. In  FIG. 5  the COL frame  104  (configured as a turret containing multiple collimator mirrors) and the FE/CON frame  106  (containing a set of fly-eye mirrors and a set of condenser mirrors) are mounted to an illumination-optical-system (IU) “main frame”  152 . The IU main frame  152  is mounted to the system base  118  via a 6DOF mounting  153  that includes active vibration isolation (AVIS) and 6 DOF of positional adjustability. The rotary stage  116 , in turn, is mounted to the IU main frame  152 , and the COL frame  104  is mounted to the rotary stage  116 . Each collimator mirror COL 1  -COL 4  is mounted to its respective branch of the COL frame  104  using a respective adjuster  154 ,  156 ,  158 ,  160  each providing 6 DOF of positional adjustability of the respective mirror as well as a mounting for the mirror. Each adjuster  154 ,  156 ,  158 ,  160  can be a respective “set-and-forget” device.  
         [0091]     The FE/CON frame  106  for the fly-eye mirrors  134 ,  136  and condenser mirrors  138 ,  146  is also mounted to the IU main frame  152 . Each of the mirrors  134 ,  136 ,  138 ,  146  includes a respective adjuster  140 ,  142 ,  144 ,  162  each providing 6 DOF of positional adjustability of the respective mirror. Each adjuster  140 ,  142 ,  144 ,  162  desirably can be a respective “set-and-forget” device.  
         [0092]     Determinations of the positions of the IU main frame  152  (and thus of the FE/CON frame  106  and COL frame  104 ) and projection-optics box  128  relative to each other are made by the sensor  148  across the tracking interface  150 . The sensor  148  provides feedback to the 6DOF mounting  153 . Thus, using the 6DOF mounting  153 , the IU main frame  152  (and thus the FE/CON frame  106  and COL frame  104 ) actively track the position of the projection-optics box  128 .  
         [0093]     The configuration of  FIG. 5  is an example of a one-body active-stabilization configuration, in which the FE/CON frame  106  and the COL frame  104  are mounted to the IU main frame  152 , and the IU main frame  152  tracks the projection-optical box  128  using the 6DOF mounting  153 . An advantage of the configuration of  FIG. 5  is simplicity of routing of cables and hoses  164 ,  166  from stationary portions to movable portions of the system.  
         [0094]     As noted above, each of the collimator mirrors COL 1 -COL 4  in the embodiment of  FIG. 5  desirably are mounted to the COL frame  104  via a respective adjuster  154 ,  156 ,  158 ,  160  providing an individual mounting for the mirror as well as 6 DOF of positional adjustability of the mirror. An especially desirable adjuster for this purpose is a “KALM” kinematic mounting as described in U.S. Published Patent Application No. U.S. 2002/0163741 A1, incorporated herein by reference. A KALM mounting can utilize any of various types of actuators, including but not limited to, piezoelectric (PZT) actuator with strain gauge, pico motor with encoder, stepper motor with micrometer (μmeter) and encoder, and voice-coil motor (VCM) with inductive sensor. Various parametric data concerning these actuators are set forth in Table 1, below. These data are based on the following assumptions: 
        mirror mass=10 kg     frame mass=100 kg        
 
         [0097]     first mode=150 Hz  
                                   TABLE 1                           Requirement   PZT + strain           VCM + inductive       Parameter   for mirror   gauge   Pico motor + encoder   Step motor + μmeter + encoder   sensor                   Resolution   1 μm   4 nm   65 nm   0.1 μm   10 nm       Range   5 mm   180 μm   10 mm   20 mm   5 mm       Force (vert)   100 N   4500 N   22 N   120 N   100 N       Force   10 N   500 N   22 N   120 N   1000 N       (horiz)       Stiffness*   10 N/μm   35 N/μm   ˜15 N/μm   ˜15 N/μm   &gt;10 N/μm       Hold Power   zero   low   zero   zero   high       Actuator       0.4 kg   0.1 kg   0.6 kg   1 kg       mass       Actuator       Ø25 × 185 mm   ˜50 × 50 × 50 mm   Ø50 × 170 mm   50 × 50 × 30 mm       size       Manually   yes   no   yes   yes   no       adjustable?       Mfr       PI   Newfocus   PI   Micro-                           Epsilon                 *An actuator&#39;s effective stiffness could be increased at the expense of the effective stroke of the actuator by employing an appropriate transmission element (e.g., lever with flexural hinge).             
 
         [0098]     As noted above, the IU main frame  152  is mounted to the system base  118  via a 6DOF mounting  153  providing 6 DOF of positional adjustability of the IU main frame  152  as well as vibrational isolation. The 6DOF mounting  153  can include any of several types of actuators, including but not limited to, stepper motor with micrometer (μmeter), encoder, and air bellows; and voice-coil motor (VCM) with inductive sensor and air bellows. Various parametric data concerning these actuators are set forth in Table 2, below. These data are based on the following assumptions: 
        mirror mass=10 kg     frame mass=100 kg        
 
         [0101]     first mode=150 Hz  
                           TABLE 2                           Requirement   Stepper motor +   VCM + inductive           for   μmeter + encoder + air   sensor +       Parameter   frame   bellows   air bellows                   Resolution   1 μm   0.1 μm   10 nm       Range   5 mm   20 mm   5 mm       Force (vert)   1000 N   1000 N   100 N       Force (horiz)   100 N   120 N   1000 N       Stiffness*   100 N/μm   ˜15 N/μm   &gt;100 N/μm       Hold Power   zero   zero   low       Actuator mass       0.6 kg   5 kg       Actuator size       Ø50 × 170 mm   50 × 50 × 30 mm       Manually   yes   yes   no       adjustable?       Mfr       PI   Micro-Epsilon                 *An actuator&#39;s effective stiffness could be increased at the expense of the effective stroke of the actuator by employing an appropriate transmission element (e.g., lever with flexural hinge).             
 
         [0102]     Advantages of the stepper motor plus micrometer plus encoder plus air bellows are: (a) no holding power, and (b) usability as a “set-and-forget” device; disadvantages are inability to provide vibration isolation. Advantages of the VCM plus inductive sensor plus air bellows are: (a) ability to provide vibration isolation if required, and (b) adequate stiffness; disadvantages are requirement for active control, and inability to be manually adjusted. Hence, vibration isolation is the primary distinguishing performance-based characteristic between these actuators.  
         [0103]     This embodiment provides an illumination-optical system including an exchanger providing multiple selectable collimator mirrors. Thus, a collimator mirror currently in use can be replaced with a new one, clean one, or different one quickly, without having to shut the system down. Indeed, the collimator frame can be configured to replace the collimator mirror automatically such as after a defined operation time or upon an excessive amount of debris or other contamination has accumulated on the collimator mirror currently in use. The collimator frame also provides multiple collimator mirrors in a small operating volume of space. Furthermore, because there is no physical contact of, for example, the collimator frame with the FE/CON frame, exchanges of the collimator mirror can be performed with minimal disturbance to other optical components that are not being replaced, which improves alignment time after the mirror exchange and facilitates mirror exchanges being performed with less particle generation. Thus, system contamination is reduced and system productivity is increased. The actuators and sensors associated with the collimator mirrors and with the collimator frame serve to position and stabilize the mirrors and frame relative to a reference frame such as the projection-optics box. Finally, the collimator frame can be readily housed in a vacuum chamber that can be provided easily with an opening providing access to the collimator mirrors.  
       Second Representative Embodiment  
       [0104]     This embodiment, depicted in  FIG. 6 , provides an illumination-optical system  200  in which the collimator mirror as well as the fly-eye-mirror set (consisting of the first and second fly-eye mirrors) can be exchanged in order to provide a range in sigma (σ) from, for example, 0.3 to 0.8. To provide this performance, the respective positions of the first fly-eye mirror and the collimator mirror change as much as 200 mm, and the respective diameters of the first fly-eye mirror, second fly-eye mirror, and collimator mirrors change as much as 150 mm. The embodiment of  FIG. 6  depicts the following components: the collector mirror  202  of the EUV source (the remainder of the EUV source is not shown, but see  FIG. 1 ); a four-way exchanger or turret (termed the COL frame)  204  containing an ensemble of four selectable collimator mirrors (similar to the first representative embodiment); a “CON frame”  206  containing a set of first and second condenser mirrors; a rotary frame-storage device  208  holding an ensemble of six selectable “fly-eye frames” (FE frames); an FE-frame exchanger  210  that moves a selected FE frame  212  from the frame-storage device  208  to an operational position  214  at which the selected FE frame is aligned with the selected collimator mirror and with the CON frame  206 ; and a grazing-incidence mirror  216 . Thus, this embodiment differs from the first representative embodiment in providing an ensemble of multiple fly-eye-mirror sets that can be individually selected as desired or required. Each of the fly-eye-mirror sets, although termed a respective “fly-eye frame” herein, is configured as a separate fly-eye-mirror “module” including a respective housing for the respective fly-eye-mirror set.  
         [0105]     The COL frame  204  includes four independently selectable branches  204   a - 204   d  each containing a respective collimator mirror. The COL frame  204  is rotatable about an axis A 1  to position a selected branch  204   a - 204   d  relative to the selected FE frame  212 . The respective collimator mirrors in the branches  204   a - 204   d  can be the same or different. For example, the branches  204   a - 204   d  can provide four different respective collimator mirrors each having a different size, a different surface profile, and/or a different position to provide four different respective optical settings. Alternatively, two or more of the collimator mirrors can be identical, in which configuration the COL frame  204  allows exchange of a contaminated collimator mirror for a clean or new one by simply rotating the COL frame  204  to a different branch containing the clean or new mirror. Note that, in this embodiment, only one selected branch  204   a - 204   d  can be in an operational position  218  at which the respective mirror can receive EUV light  220  from the collector mirror  202 . EUV light  222  reflected from the collimator mirror in the selected branch exits the COL frame  204  and enters the selected FE frame  212  as the light propagates to the first fly-eye mirror situated in the selected FE frame  212 . In the FE frame  212  the EUV light reflects, in succession, from the first fly-eye mirror and the second fly-eye mirror. The EUV light then passes from the selected FE frame  212  to the CON frame  206 , in which the EUV light reflects, in succession, from the first condenser mirror and the second condenser mirror. EUV light  224  reflected from the second condenser mirror exits the CON frame  206  as the light  224  propagates to the grazing-incidence mirror  216 .  
         [0106]     Referring to  FIG. 7 , the COL frame  204  is mounted on a rotary stage  260  mounted to a system base  230 . The rotary stage  260  is rotatable relative to the base  230  to rotate the COL frame  204  relative to the base  230  as required to select a particular branch  204   a - 204   d.  Between the stage  260  and the COL frame  204  is a mounting  262  providing an active vibration-isolation system (AVIS) and six degrees of freedom (6 DOF) of actuation motion (x, y, z, θ x , θ y , θ z ) of the COL frame (for positional-adjustment and vibration-isolation purposes) relative to the stage  260 . Thus, the mounting  262  serves as a “fine stage” for the rotary stage  260 , wherein the rotary stage  260  includes and serves as a coarse stage (not detailed). In  FIG. 7 , the branch  204   c,  containing “collimator  3 ” (“COL 3 ”), has been selected for use. Thus, the 6DOF mounting  262  imparts active vibration isolation and 6 DOF of positional adjustability of the branch  204   c,  and hence of the collimator mirror COL 3 . Since the 6DOF mounting  262  serves the entire COL frame  204 , individual 6DOF mountings are not required for each collimator mirror COL 1 -COL 4  (COL  4  not shown). This alleviates system complexity and simplifies routing of flexible hoses (for hydraulics) and electrical cables  264  extending between the base  230  and the rotary stage  260  and of hoses and cables  266  extending between the rotary stage  260  and the collimator mirrors COL 1 -COL 4  in the COL frame  204 .  
         [0107]     The rotary frame-storage device  208  holds an ensemble of multiple (e.g., six as shown) independently selectable FE frames  226   a - 226   f  each containing a respective counterpart set of first and second fly-eye mirrors. Each counterpart set is contained in a respective housing. The rotary frame-storage device  208  is rotatable about an axis A 2  to position a particular FE frame (e.g.,  226   f ) for movement by the FE-frame exchanger  210 . To achieve this rotation, the rotary frame-storage device  208  is mounted on a rotary stage  228  that is mounted to the system base  230 . The respective sets of fly-eye mirrors in the FE frames  226   a - 226   f  can be the same or different. For example, the FE frames  226   a - 226   f  can provide six different respective sets of fly-eye mirrors having different shapes and different distances between them to provide six different respective optical settings. Alternatively, two or more of the fly-eye sets can be identical, in which configuration the rotary frame-storage device  208  allows exchange of a contaminated set of fly-eye mirrors for a clean or new set by simply rotating the frame-storage device  208  to a different position providing the FE frame containing the clean or new mirrors.  
         [0108]     The FE-frame exchanger  210  in this embodiment comprises a linear stage  232  that moves a link  234  from a first position  236   a  (at the rotary frame-storage device  208 ) to a second position  236   b  (at the operational position  214 ). The link  234  is configured to interact with the selected FE frame  212  in a manner by which the selected FE frame is moved, with corresponding motion of the link  234 , from the first position  236   a  to the second position  236   b.  The link  234  is mounted to the linear stage  232  via a 6DOF mounting  238  that desirably also provides active vibration isolation (AVIS) between the linear stage  232  and the link  234 . Upon the selected FE frame  212  being placed at the operational position  214 , the link  234  remains attached to the selected FE frame  212 ; thus, the 6DOF mounting  238  provides motion of the selected FE frame  212  as required to align the selected FE frame with the selected branch of the COL frame  204  and with the CON frame  206 .  
         [0109]     Desirably, the COL frame  204  does not physically contact the selected FE frame  212 , either during rotation of the COL frame, during use of the selected branch of the COL frame, during placement of the selected FE frame  212  at the operational position  214 , or during use of the selected FE frame  212 . Each branch of the COL frame  204  has a respective sensor  240   a - 240   d  (only three,  240   a - 240   c,  are shown in  FIG. 7 ) that senses positional alignment, across a tracking interface  242 , of the respective branch with the selected FE frame  212  at the operational position  214 . Data from the sensor  240  is used as feedback to the 6DOF mounting  238 .  
         [0110]     In each FE frame  226   a - 226   f,  the first respective fly-eye mirror FE 1  can be mounted to a respective 6DOF adjuster  244  that provides 6 DOF of positional adjustment of that fly-eye mirror FE 1  relative to the FE frame  226  and to the second respective fly-eye mirror FE 2  ( FIG. 7 ). The second respective fly-eye mirror FE 2  need not have such an adjuster, but can include an adjuster, if desired, providing six or fewer degrees of freedom of motion. Each 6DOF adjuster  244  can be configured as a “set-and-forget” type of adjuster.  
         [0111]     The CON frame  206  is mounted to the system base  230  via a respective 6DOF mounting  246  that desirably also provides active vibration isolation (AVIS) of the CON frame  206  relative to the system base  230 . Thus, the CON frame can move as required to achieve accurate alignment with the selected FE frame  212  at the operational position  214  and with the projection-optics box (POB)  248 . The CON frame  206  has a first sensor  250  and a second sensor  252  for sensing these alignments. Specifically, the first sensor  250  senses alignment with the selected FE frame  212  across a first tracking interface  254 , and the second sensor  252  senses alignment with the projection-optics box  248  across a second tracking interface  256 . Data from the sensors  250 ,  252  is fed back to the 6DOF mounting  246 .  
         [0112]     In the CON frame, the first respective condenser mirror CON 1  can be mounted to a respective 6DOF adjuster  258  that provides 6 DOF of positional adjustment of that condenser mirror CON 1  relative to the CON frame  206  and to the second respective condenser mirror CON 2  ( FIG. 7 ). The second respective condenser mirror CON 2  need not have such an adjuster, but can include an adjuster, if desired, providing six or fewer degrees of freedom of motion. Each 6DOF adjuster  258  can be configured as a “set-and-forget” type of adjuster.  
         [0113]     Based on the foregoing description, this embodiment exhibits three-body active stabilization, achieved by the 6DOF mountings  238 ,  246 ,  262  (for the FE frame  226 , CON frame  206 , and COL frame  204 , respectively). This particular configuration also minimizes the number of flexible hoses and cables that must be extended to each of the frames  204 ,  206 ,  226  (in  FIG. 7  see cables and hoses  264 ,  266 ,  268 ,  270 ,  272 ,  274 ).  
         [0114]     Referring now to  FIG. 8 , a housing  280  is shown that encloses substantially the entire illumination-optical system  200 . Exemplary dimensions also are shown. The housing  280  defines a first opening  282  for accessing the FE frames on the rotary stage  228 . Thus, one or more of the FE frames  226   a - 226   f  can be removed for cleaning, replacement, or adjustment, for example. The housing  280  also defines a second opening  284  for accessing the COL frame  204  for cleaning, replacement, or adjustment of any of the collimator mirrors COL 1 -COL 4 . (To such end, each of the branches  204   a - 204   d  of the COL frame  204  can be independently detachable from the COL frame  204  for ease of removal without disturbing any of the remaining branches.) The housing  280  also defines a third opening  286  providing access to the CON frame  206  and through which the beam  224  propagates to the grazing-incidence mirror  216  and then to the reticle (not shown). The housing also defines a fourth opening  288  through which, for example, the wafer stage (not shown) can extend, and a fifth opening  290  for access to, for example, the linear stage  232 . The housing  280  is compact and space-efficient, and thus facilitates efficient evacuation (using appropriate vacuum pumps) of the atmosphere inside, while retaining accessibility to the components inside for quick service, cleaning, replacement, or the like.  
       Third Representative Embodiment  
       [0115]     Relevant features of this embodiment are depicted in  FIG. 9 , which depicts an illumination-optical system  300  that receives EUV radiation  302  from a source (only the collector mirror  304  is shown). The illumination-optical system includes a condenser frame (CON frame)  306  that includes a set of first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  308  propagates to the grazing-incidence mirror  310  and then to a reticle (not shown). The illumination-optical system  300  also includes a rotary frame-storage device  312  comprising an ensemble of multiple selectable fly-eye/collimator (FEC) frames  314   a - 314   f.  Each FEC frame  314   a - 314   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A selected FEC frame (e.g., frame  314   f ) is moved from the frame-storage device  312  to an operational position  316  by a linear FEC-frame exchanger (not shown, but similar to the FE-frame exchanger  210  of the second representative embodiment). The rotary frame-storage device  312  is configurationally and operationally similar to the rotary frame-storage device  208  of the second representative embodiment.  
         [0116]     This embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets.  
         [0117]     Turning to  FIG. 10 , an exemplary housing  318  for the CON frame  306  and selected FEC frame  314  in the operational position  316  is shown. The housing  318  also can contain the EUV source (note position of collector mirror  304 ) desirably supported on a separate structure (not detailed) in the housing  318 . The housing  318  includes space  320  for the linear FEC-frame exchanger and defines an opening  322  through which the selected FEC frame  314  is moved from the rotary frame-storage device  312  to the operational position  316 . An advantage of this embodiment is its ability to accommodate vacuum pumps close to the optical path in the illumination-optical system. For example, attached to a side wall  324  of the housing  318  are vacuum pumps  326   a - 326   d  (e.g., turbomolecular pumps each having a pump rate of 3600 L/s) for evacuating the illumination-optical system and projection-optics box (not shown), and vacuum pumps  328   a - 328   c  (e.g., turbomolecular pumps each having a pump rate of 3600 L/s) for evacuating the EUV source.  
       Fourth Representative Embodiment  
       [0118]     Relevant features of this embodiment are depicted in  FIG. 11 , which depicts an illumination-optical system  350  that receives EUV radiation  352  from a source (only the collector mirror  354  is shown). The illumination-optical system  350  includes a condenser frame (CON frame)  356  that includes a set of first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  358  propagates to the grazing-incidence mirror  360  and then to a reticle (not shown). The illumination-optical system  350  also includes an ensemble of multiple selectable fly-eye/collimator (FEC) frames  364   a - 364   f  that are storable on both sides of the CON frame  356 . Each FEC frame  364   a - 364   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A desired FEC frame (e.g., frame  364   f ) is selected and moved into an operational position  366  by a combination of linear motions  368   a,    368   b  in the X and Z directions, respectively. These motions  368   a,    368   b  can be effected, for example, by respective linear actuators (not shown, but similar to the FE-frame exchanger  210  in the second representative embodiment) or by an X, Z robot. Similar to the third representative embodiment, this embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets. Advantages of this embodiment include compactness and simple robotics.  
       Fifth Representative Embodiment  
       [0119]     Relevant features of this embodiment are depicted in  FIG. 12 , which depicts an illumination-optical system  400  that receives EUV radiation  402  from a source (only the collector mirror  404  is shown). The illumination-optical system  400  includes a condenser frame (CON frame)  406  that includes a set of first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  408  propagates to the grazing-incidence mirror  410  and then to a reticle (not shown). The illumination-optical system  400  includes an ensemble of multiple selectable fly-eye/collimator (FEC) frames  414   a - 414   f  that are storable on one side of the CON frame  406 . Each FEC frame  414   a - 414   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A desired FEC frame (e.g., frame  414   f ) is selected and moved into an operational position  416  by a combination of linear motions (see fourth representative embodiment) in the X and Z directions. These motions can be effected, for example, by respective linear actuators (not shown, but similar to the FE-frame exchanger  210  in the second representative embodiment) or by an X-Z robot. Similar to the third representative embodiment, this embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets.  
         [0120]     Advantages of this embodiment include the ability to accommodate vacuum pumps placed close to the optical path of the illumination-optical system, compactness, and simple robotics.  
       Sixth Representative Embodiment  
       [0121]     Relevant features of this embodiment are depicted in  FIG. 13 , which depicts an illumination-optical system  450  that receives EUV radiation from a source (not shown). The illumination-optical system  450  includes a condenser frame (CON frame)  456  that includes a set of first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  458  propagates to the grazing-incidence mirror  460  and then to a reticle (not shown). The illumination-optical system  450  includes an ensemble of multiple selectable fly-eye/collimator (FEC) frames  464   a - 464   f  that are storable on one side of the CON frame  456 . Each FEC frame  464   a - 464   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A desired FEC frame (e.g., frame  464   f ) is selected and moved into an operational position  466  by a combination of rotary motion (arrows  468 ) and linear motion (arrows  470 ). The rotary motion  468  can be effected by a rotary stage or the like (see rotary frame-storage device  208  in the second representative embodiment), and the linear motion  470  can be effected by a linear exchanger (see FE-frame exchanger in the second representative embodiment). Similar to the third representative embodiment, this embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets.  
         [0122]     Advantages of this embodiment include the ability to accommodate vacuum pumps close to the optical path of the illumination-optical system, compactness, and rapid exchange time.  
       Seventh Representative Embodiment  
       [0123]     Relevant features of this embodiment are depicted in  FIG. 14 , which depicts an illumination-optical system  500  that receives EUV radiation  502  from a source (not shown). The illumination-optical system  500  includes a condenser frame (CON frame)  506  that includes a set of the first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  508  propagates to the grazing-incidence mirror  510  and then to a reticle (not shown). The illumination-optical system  500  includes an ensemble of multiple selectable fly-eye/collimator (FEC) frames  514   a - 514   f  that are storable on one side of the CON frame  506 . Each FEC frame  514   a - 514   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A desired FEC frame (e.g., frame  514   f ) is selected and moved into an operational position  516  by a combination of linear motions (see fourth representative embodiment) in the X and Z directions. These motions can be effected, for example, by respective linear actuators (not shown, but similar to the FE-frame exchanger  210  in the second representative embodiment) or by an X-Z robot. Similar to the third representative embodiment, this embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets.  
         [0124]     Advantages of this embodiment include the ability to accommodate vacuum pumps placed close to the optical path of the illumination-optical system, a simple exchange mechanism (X-Z robot), and rapid exchange time.  
       Eighth Representative Embodiment  
       [0125]     Relevant features of this embodiment are depicted in  FIG. 15 , which depicts an illumination-optical system  550  that receives EUV radiation  552  from a source (only the collector mirror  554  is shown). The illumination-optical system  500  includes a condenser frame (CON frame)  556  that includes a set of the first and second condenser mirrors (not detailed). From the second condenser mirror the EUV light flux  558  propagates to the grazing-incidence mirror  560  and then to a reticle (not shown). The illumination-optical system  550  also includes a rotary frame-storage device  562  comprising an ensemble of multiple selectable fly-eye/collimator (FEC) frames  564   a - 564   f.  Each FEC frame  564   a - 564   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A selected FEC frame (e.g., frame  564   f ) is moved from the frame-storage device  562  to an operational position  566  by a linear FEC-frame exchanger (not shown, but similar to the FE-frame exchanger  210  of the second representative embodiment). The rotary frame-storage device  562  is analogous to the rotary frame-storage device  208  of the second representative embodiment, but since the FEC frames all remain in the same orientation, the mechanism of the rotary frame-storage device  562  is more complex than of the frame-storage device  208 .  
         [0126]     Similar to the third representative embodiment, this embodiment can be used, for example, whenever (1) the number of selectable collimator mirrors can be equal to the number of selectable fly-eye-mirror sets, and (2) it is unnecessary that the collimator mirrors be selectable independently of the fly-eye-mirror sets.  
       Ninth Representative Embodiment  
       [0127]     Relevant features of this embodiment are depicted in  FIG. 16 , which depicts an illumination-optical system  600  that receives EUV radiation  602  from a source (only the collector mirror  604  is shown). The illumination-optical system  600  includes a condenser frame (CON frame)  606  that includes a set of first and second condenser mirrors (not detailed). The CON frame  606  is supported, relative to a system base (not shown) by a 6DOF mounting including AVIS. This 6DOF mounting is not shown, but see, for example, item  126  in  FIG. 4 . From the second condenser mirror the EUV light flux propagates to the grazing-incidence mirror  610  and then to a reticle (not shown). The illumination-optical system  600  also includes a rotary frame-storage device  612  comprising an ensemble of multiple selectable fly-eye (FE) frames  614   a - 614   f.  Each FE frame  614   a - 614   f  is configured as a separate housing or “module” containing the respective set of fly-eye mirrors and collimator mirror. A selected FE frame (e.g., frame  614   f ) is moved from the frame-storage device  612  to an operational position  616  by a linear FE-frame-exchanger robot  618  (similar to the FE-frame exchanger  210  of the second representative embodiment). The FE-frame-exchanger robot  618  has a long stroke in the X direction and is supported by a 6DOF mounting including AVIS. At the operational position  616  the selected FE frame is situated between the CON frame  606  and the COL frame  620 . A respective tracking interface  622   a  is situated between the COL frame  620  and the selected FE frame  614   f,  a respective tracking interface  622   b  is situated between the selected FE frame  614   f  and the CON frame  606 , and a respective tracking interface  622   c  is situated between the CON frame  606  and the projection-optics box. Thus, this embodiment provides non-contacting, active kinematic coupling of the frames (including the selected FE frame) with each other and with the main IU frame.  
         [0128]     If desired, this embodiment can be provided with an ensemble of multiple selectable COL frames associated with a suitable exchanger  624  such as any of the various exchangers described above. This embodiment also provides good management of hoses and cables, allows placement of vacuum pumps near the EUV optical pathways, is compact and scalable, allows fast exchange time, and utilizes a low number of actuators.  
         [0129]     An alternative configuration is shown in  FIG. 17 , in which the rotary frame-storage device  612  is eliminated, and a selected one FE frame  614   b  (of multiple such frames, three of which  614   a,    614   b,    614   c  are shown) is placed in the operational position  616  using the FE-frame-exchanger robot  618  only. Advantages of this configuration are: (a) compact and scalable, (b) fast exchange time, (c) simpler exchange mechanism than the embodiment shown in  FIG. 16 , (d) simpler management of hoses and cables than the embodiment shown in  FIG. 16 .  
         [0130]     An EUVL system including the above-described 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 and projection-optical system) 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.  
         [0131]     Semiconductor devices can be fabricated by processes including microlithography steps performed using a microlithography system as described above. Referring to  FIG. 18 , in step  701  the function and performance characteristics of the semiconductor device are designed. In step  702  a reticle defining the desired pattern is designed according to the previous design step. Meanwhile, in step  703 , a substrate (wafer) is made and coated with a suitable resist. In step  704  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 the 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.  
         [0132]     Representative details of a wafer-processing process including a microlithography step are shown in  FIG. 19 . In step  711  (oxidation) the wafer surface is oxidized. In step  712  (CVD) an insulative layer is formed on the wafer surface. 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.  
         [0133]     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  (development) 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.  
         [0134]     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.  
         [0135]     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.