Abstract:
Sources are disclosed for producing short-wavelength electromagnetic radiation (EMR) such as extreme ultraviolet (“EUV” or “soft X-ray”) radiation useful in microlithography. The sources collect a greater amount of the EMR produced by a plasma than conventional sources and form the collected EMR into an illumination EMR flux having higher intensity than conventionally. The EMR flux desirably has a rotationally symmetrical intensity distribution. The plasma is produced by two electrodes contained in a vacuum chamber. A high-voltage pulsed power supply applies a plasma-creating potential across the electrodes. EMR produced by the plasma is collected, typically by a reflective element configured to form a collimated beam of EMR. The electrodes are configured and oriented such that, as the collimated beam passes by the electrodes, the electrodes exhibit minimal blocking of the EMR flux. The electrodes can include a center electrode and a surrounding hollow cylindrical electrode separated from the center electrode by an insulating member. The axis of rotational symmetry of the electrodes desirably is substantially parallel to the propagation axis of the EMR flux.

Description:
FIELD OF THE INVENTION 
     The present invention relates to sources of electromagnetic radiation (EMR) that can produce EMR in the extreme ultraviolet (soft X-ray) range of the electromagnetic spectrum. EMR from such a source can be used for microlithography, which is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. 
     BACKGROUND OF THE INVENTION 
     As noted above, a key technique in the manufacture of microelectronic devices such as integrated circuits is microlithography. Most conventional microlithography is performed using deep ultraviolet (DUV) light. The pattern to be transferred is defined on a reticle or mask that is illuminated by a beam of DUV light. A downstream image of the illuminated portion of the reticle is projected (usually with demagnification) by a beam of DUV light onto a suitable substrate (e.g., semiconductor wafer) coated with a resist that is “sensitive” to exposure by the DUV light. Microlithography performed using DUV light still is within the realm of “optical microlithography.” 
     With ever-increasing miniaturization and density of microelectronic devices, the need has become acute for a microlithography method offering greater resolution than optical microlithography. In fact, optical microlithography now is being conducted at or very nearly at the diffraction limit of DUV light, which means that substantially greater resolution than currently obtainable is probably not possible with optical microlithography. As a result of this dilemma, considerable research and development effort currently is underway to develop a practical “next generation” microlithography apparatus. Among top contenders are charged-particle-beam microlithography and “extreme ultraviolet” (also termed “EUV” or “soft X-ray”) microlithography. The EUV wavelength range receiving the most current attention is 11 to 13 nm. 
     Unfortunately, EUV light and EMR of neighboring wavelengths are strongly absorbed by most known substances, and no optical materials are currently known that are transmissive to such EMR. Hence, with such EMR as used for microlithography, there is no known way in which to provide a refracting system that can be used for reticle illumination and/or projection of an image onto a substrate. Consequently, illumination-optical systems and projection-optical systems for use in microlithography performed using these short-wavelength EMRs must be made of reflecting optical elements. 
     Another difficulty with EUV radiation and related short-wavelength EMR is that reflectance of such radiation from ordinary reflective mirrors is extremely low. To obtain maximal reflectance, the mirrors are configured with reflecting surfaces made of a multilayer-film structure. For example, EUV-reflective mirrors have been produced with multilayer reflective films of molybdenum (Mo) and silicon (Si) for reflecting 13-nm EUV light, and multilayer reflective films of Mo and beryllium (Be) for reflecting 11-nm EUV light. However, even with the most efficient mirrors of these types, reflectance of EUV light is at most about 70%. The resulting loss of EMR at each mirror in the illumination-optical system and projection-optical system has led to considerable difficulty in achieving satisfactory imaging performance and throughput. 
     EUV radiation used in the technologies summarized above typically is produced from a highly specialized source such as an undulator, a laser-plasma source, or a discharge-plasma source. The latter two are attractive because of their relatively small size. In a laser-plasma source, a high-intensity pulsed laser light is converged on a target material to cause the target material to produce a high-temperature plasma from which EUV radiation is emitted. In a discharge-plasma source, the plasma is produced by electrical discharge between electrodes. 
     An exemplary plasma-focused source (a type of discharge-plasma source) is disclosed in Japan Kôkai Patent document no. Hei 10-319195 and shown in FIG.  8 . The source includes an anode  1 , a cathode  2 , and a base member  3  situated inside a vacuum chamber  8 . The electrodes  1 ,  2  are connected to and energized by high-voltage pulses produced by a pulse generator  7 . A working-gas mixture (consisting of a buffer gas and a working gas that produces a desired transition when exposed to an electrical discharge) is introduced into the vacuum chamber  8  via a conduit  10 . Specifically, the working-gas mixture is introduced by the conduit  10  to a space above the base member  3  and between the anode  1  and cathode  2 . The cathode  2  surrounds the anode  1  in the manner of a cylinder. High-voltage pulses from the high-voltage pulse generator  7  are applied across the electrodes  1 ,  2  to create a discharge between the electrodes  1 ,  2 . The discharge begins on the surface of the base member  3  and produces an “initial” plasma. The initial plasma is formed by ionization of the working gas in the region between the electrodes  1 ,  2  and above the base member  3 . 
     Upon creation of the initial plasma, electrons and ions in the initial plasma move relative to each other under the influence of the electric-field produced by the voltage gradient between the electrodes  1 ,  2 , thereby forming a current in the plasma. The current in the plasma, in turn, generates a magnetic field in the plasma. The ions and electrons moving through the plasma interact with the magnetic field and move upward. As a result, the plasma becomes concentrated at the distal end of the anode  1 . The concentrated plasma has elevated temperature and density, sufficient to produce EUV light that radiates from the plasma. 
     In these sources, the material that actually forms the plasma is material situated at the electrode member excited by the concentrated plasma. Typically, the material includes not only the electrode member itself but also molecules of the working gas situated in the immediate vicinity of the electrode. The wavelength of EMR produced by the plasma corresponds to specific transitions in ions of the electrode member and of the working gas. The plasma region in which the desired EMR is produced is situated substantially within a volume having a diameter of about 1 mm at the distal tip of the electrode  1 . Because plasma production is pulsatile, release of radiation from the plasma is pulsatile. Each pulse of released EMR has a duration in the range of about 0.1 μs to 1 μs. By way of example, if the working-gas mixture surrounding the distal end of the electrode  1  contains lithium vapor, then the resulting line spectrum of the produced EUV radiation is about 13.5 nm, which is attributable to the transition in the lithium ions in the plasma. 
     The amount of EMR produced per pulse by the plasma-focused source of FIG. 8 is greater than from a laser-plasma light source. Also, with this plasma-focused source, EMR can be produced having a relatively high pulse rate, e.g., of up to several kilohertz. Increasing the pulse rate yields an increase in the net amount of EMR that can be obtained from the source and allows the amount of radiation produced per unit time from the source to be controlled with higher precision. 
     Japan Kôkai Patent Document No. Hei 11-312638 discloses use of an EUV light source, as described above, in an EUV microlithography apparatus. The optical system downstream of the source is depicted in FIG. 9 herein, wherein the rays  6  are propagating from the source. The optical elements  11   a  and  11   b  are “fly-eye” (compound) mirrors having respective surfaces such as shown in FIGS.  10 (A) or FIG.  10 (B). Upstream of the mirrors  11   a,    11   b  are other mirrors that collect and collimate the EUV radiation produced by the source. Further with respect to FIG. 9, item  12  is a reflective reticle, item  13  is a reticle stage, items  14   a - 14   f  are mirrors, item  15  is the substrate, and item  16  is a wafer stage. The mirrors  14   a,    14   b,  along with the mirrors  11   a,    11   b  and mirrors situated between the mirror  11   b  and the source, constitute the “illumination-optical system.” The mirrors  14   c - 14   f  constitute the “projection-optical system” that projects a reduced (demagnified) image of the illuminated portion of the reticle  12  onto the substrate  15 . 
     The maximal achievable reflectance of each of the multilayer mirrors used in the illumination and projection systems is about 70%. In other words, at least about 30% of incident EMR on each mirror is lost. Consequently, after reflection from multiple mirrors to produce the demagnified images at the substrate  15 , the maximal amount of EMR initially produced that actually participates in making an exposure on the substrate  15  is only a few percent. Since throughput is a function of the intensity of exposure light, to obtain more rapid exposure and correspondingly improved throughput, every bit of the EMR generated from the source must be gathered and utilized for exposure. 
     With an illumination-optical system configured as shown in FIG. 9, the respective areas of the reticle  12  and substrate  15  undergoing illumination and imaging, respectively, receive uniform illumination intensity. This is due in part to the uniformizing effects of the mirrors  11   a ,  11   b  (FIGS.  10 (A) and  10 (B)). As a result, imaging performance tends to be independent of the position or direction of the elements of the pattern being projected from the reticle  12  to the substrate  15 . For even better imaging performance, it is desirable that the intensity of the EMR flux incident on the mirror  11   b  have a rotationally symmetrical (relative to the optical axis) distribution of intensity. 
     However, whenever a plasma-focused light source such as shown in FIG. 8 is used as a source of short-wavelength EMR, substantial limitations are imposed on the configuration of the illumination-optical system. As a result, it is very difficult to form an EMR flux, for illumination purposes, having a rotationally symmetrical intensity distribution with respect to the optical axis. I.e., from a plasma-focused source, the generated EMR propagates radially outward from the plasma. To be useful for microlithographic illumination purposes, the EMR flux  6  from the source must be collimated, as shown in FIG.  9 . One possible way in which the EMR from the source can be collimated is to place a mirror, configured as a paraboloid of revolution having a focal point, relative to the source such that the EMR-producing plasma is at the focal point of the mirror. Hence, EMR produced by the plasma reflects from the mirror as a collimated beam. Unfortunately, in conventional configurations of this nature that have been considered to date, the electrodes of the plasma-focused source undesirably block propagation of some of the EMR reflected from the mirror. This blocking limits the solid angle at which the EMR can be utilized from the source and used to form the collimated beam. 
     Therefore, there is a need for improved devices and methods for forming a collimated flux of short-wavelength EMR, for illumination purposes, from a plasma-focused light source, wherein the amount of EMR not utilized from the source (due to blockage by electrodes) is reduced compared to conventional sources, and wherein the produced EMR flux has a rotationally symmetrical distribution of intensity. There also is a need for microlithography apparatus and methods including use of such improved sources. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, and according to a first aspect of the invention, devices are provided for generating a flux of electromagnetic radiation (EMR). An embodiment of such a device comprises a vacuum chamber, first and second electrodes located within the vacuum chamber, an insulating member, and an EMR-flux collimator. The first electrode has an axis of rotational symmetry. The second electrode is situated coaxially with but spaced apart from the first electrode. The first and second electrodes are connectable to a power supply configured to apply a high-voltage pulse across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode. The insulating member is attached to the respective proximal ends of and extending between the first and second electrodes so as to support the first electrode relative to the second electrode. The EMR-flux collimator is situated in the vacuum chamber relative to the first and second electrodes such that EMR produced by the plasma is collected and collimated by the EMR-flux collimator to produce a collimated EMR flux. The EMR-flux collimator is situated and configured to direct the collimated EMR flux along a propagation axis, extending parallel to the axis of rotational symmetry of the first electrode, past the first and second electrodes. 
     The device can include a power supply connected to the first and second electrodes and configured to apply high-voltage pulses across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode. 
     The EMR-flux collimator can include an EMR-reflective element. The EMR-reflective element desirably is a concave mirror having an EMR-reflective surface configured as a paraboloid of revolution about a mirror axis. The mirror axis desirably is parallel to the propagation axis, more desirably the mirror axis extends along the axis of rotational symmetry. 
     The second electrode can be a unitary cylindrical electrode surrounding the first electrode. Alternatively, the second electrode comprises multiple electrode portions commonly connectable to the power supply and collectively surrounding the first electrode about the axis of rotational symmetry. The first electrode can be, for example, a solid or hollow cylinder in conformation. 
     The insulating member desirably is configured with spokes or mesh extending between the proximal ends of the first and second electrodes. The spokes desirably extend radially from the proximal end of the first electrode to the proximal end of the second electrode. In any event, the insulating presents a minimal obstacle to the EMR flux propagating past the electrodes from the EMR-flux collimator. 
     The EMR-flux-generating device also can include a supply of a gas comprising a working gas. The gas supply is connected to the vacuum chamber so as to supply the gas between the first and second electrodes and thus allow the working gas to become ionized in the plasma sufficient to contribute to the EMR flux produced by the plasma. The working gas can be formulated so that the plasma produces EMR including EUV radiation. 
     According to another embodiment, a device for generating a flux of electromagnetic radiation (EMR) comprises a vacuum chamber, first and second electrodes located in the vacuum chamber, and an EMR-flux former. The first electrode has an axis of rotational symmetry as summarized above. The second electrode has an inner wall that is separated from and in coaxial radial symmetry with the first electrode. The first and second electrodes are connectable to a power supply configured to apply a high-voltage pulse across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode. The inner wall of the second electrode has at least a region thereof comprising a multilayer film that is reflective to the EMR. The EMR-flux former is situated in the vacuum chamber relative to the first and second electrodes, and is situated and configured to collect and reflect EMR, from the plasma, into an EMR flux propagating along a propagation axis past the first and second electrodes. 
     As summarized above, the second electrode can be a unitary cylindrical electrode surrounding the first electrode, or can comprise multiple electrode portions collectively surrounding the first electrode about the axis of symmetry. In the latter instance, each electrode portion comprises a respective inner wall that comprises a respective portion of the inner wall of the second electrode. Similarly, the first electrode can be a solid or hollow cylinder in conformation. 
     The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a paraboloid of revolution, a spheroid of revolution, an ellipsoid of revolution, or a hyperboloid of revolution about the axis of rotational symmetry. In any of such configurations, the region comprising the EMR-reflective multilayer film has a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located. 
     The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a concave reflective surface having a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located. In this configuration, the concave reflective surface is situated to reflect EMR from the plasma to the EMR-flux former. The concave reflective device can be situated to reflect EMR from the plasma back to the plasma and then to the EMR-flux former. In such a configuration, the EMR-flux former can comprise a concave mirror having an EMR-reflective surface configured as a paraboloid of revolution about a mirror axis, wherein the mirror axis extends along the axis of rotational symmetry. The EMR-reflective surface can be configured to form, by reflection, the EMR flux that propagates along the mirror axis past the electrodes. 
     The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a concave reflective surface having a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located. In this configuration, the concave reflective surface is situated to reflect EMR from the plasma axially past the first and second electrodes. 
     According to another aspect of the invention, microlithography apparatus are provided that include a device, such as any of the embodiments summarized above, for generating an EMR flux. Such an apparatus also includes an illumination-optical system situated and configured to illuminate a reticle with an EMR flux produced by the device, wherein the reticle defines a pattern to be transferred to a sensitive substrate. The apparatus also includes a projection-optical system situated downstream of the illumination-optical system and configured to transfer the pattern from the reticle to the sensitive substrate. 
     The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of an electromagnetic radiation (EMR) source according to a first representative embodiment of the invention. 
     FIG.  2 (A) is a plan view (along the axis) of the electrodes and insulating member of a conventional EMR source. 
     FIG.  2 (B) is a plan view (along the axis) of the electrodes and insulating member of the EMR source of the embodiment shown in FIG.  1 . 
     FIG. 3 is an elevational view of an EMR source according to a second representative embodiment of the invention. 
     FIG. 4 is an elevational view of an EMR source according to a third representative embodiment of the invention. 
     FIG. 5 is a simplified schematic diagram of an EUV microlithography apparatus according to the fourth representative embodiment of the invention. 
     FIG. 6 is a flow chart of a microelectronic-device manufacturing process including use of an EUV microlithography apparatus according to the invention. 
     FIG. 7 is a flow chart of key steps in the microlithography step of the process of FIG.  6 . 
     FIG. 8 is an elevational view of a conventional dense-plasma-focus (DPF) EMR source as disclosed in Japan Kôkai Patent Document No. Hei 10-319195. 
     FIG. 9 is an optical diagram of portions of the illumination-optical and projection-optical systems of a conventional EUV microlithography apparatus as disclosed in Japan Kôkai Patent Document No. Hei 11-312638. 
     FIGS.  10 (A)- 10 (B) are plan views of respective configurations of fly-eye mirrors used in the illumination-optical system of a conventional EUV microlithography apparatus, as disclosed in Japan Kôkai Patent Document No. Hei 11-312638. 
    
    
     DETAILED DESCRIPTION 
     The invention is described below in the context of representative embodiments that are not to be regarded as limiting in any way. 
     A first representative embodiment of an electromagnetic radiation (EMR) source according to the invention is depicted in FIG.  1 . This embodiment is configured as a dense-plasma-focused (DPF) source that can be used as a source of EUV light for use in, e.g., the illumination-optical system of an EUV microlithography apparatus (see FIG.  9 ). In the configuration of FIG. 1 a center electrode  21  (made of, e.g., tungsten if the source is to be used to generate EUV light) is situated in a vacuum chamber  28  that can be evacuated, during operation, to a pressure of 10 Pa or less. The center electrode  21  desirably is configured as an axially extended member situated on an axis A of rotational symmetry. For example, the center electrode  21  can be configured as a solid or hollow cylinder. The center electrode  21  is surrounded by a coaxial “hollow” electrode  22  that is desirably cylindrical in configuration. In this embodiment, the center electrode  21  is the anode and the cylindrical electrode  22  is the cathode. The electrodes  21 ,  22  are positioned relative to each other by an insulating member  23 , described further below. 
     Although the cylindrical electrode  22  is depicted in the figure as a single electrode, it alternatively may be configured as a plurality of electrode portions collectively surrounding the center electrode  21  in the general manner shown. In such a configuration, each electrode portion contributes its respective share to surrounding the center electrode  21  about the axis A. Whenever the cylindrical electrode  22  is configured with multiple electrode portions, all the portions typically are energized with the same electrical potential and polarity. It also is possible for the center electrode  21  to comprise multiple electrode portions each contributing its respective share to the overall configuration of the center electrode  21 . However, the center electrode  21  usually consists of only one electrode member (as shown) since the plasma  24  is concentrated at the distal end of the center electrode  21 . 
     During plasma generation, to generate EMR within the EUV band, a mixed gas of helium and lithium vapor (serving as the working gas) is introduced into the vacuum chamber  28  through a gas-supply conduit (not shown, but well understood in the art; see FIG.  8 ). A high-voltage pulse power supply  27  is connectable to the electrodes  21 ,  22 . The power supply  27  is configured to apply a pulsed (e.g., 1 kHz) potential (e.g., 1 kV) across the electrodes  21 ,  22 . In any event, the potential applied across the electrodes  21 ,  22  results in production of a concentrated plasma  24  of high temperature and density at the distal end of the center electrode  21 . Molecules of the working gas are drawn into the concentrated plasma. If the working gas includes lithium vapor, then the concentrated plasma produces EMR of a wavelength (about 13.5 nm) suitable for EUV microlithography. 
     The configuration of FIG. 1 includes, as a representative “EMR-flux collimator,” a concave mirror  25  of which the concave reflective surface is configured as a paraboloid of revolution (about a respective mirror axis). So as to be reflective to the EMR produced by the plasma  24 , the concave reflective surface of the mirror  25  includes a multilayer film  29  (too thin to detail in the figure, but having a configuration as summarized above in the “Background” section) that is especially configured to reflect the EMR produced by the plasma  24  (e.g., EUV radiation of 13.5 nm). The mirror  25  has a focal point, and the mirror  25  is positioned relative to the electrodes  21 ,  22  such that the concentrated plasma  24  is situated at the focal point of the mirror  25 . Hence, EMR from the concentrated plasma  24  is reflected by the paraboloidal concave surface of the multilayer mirror  25  as a collimated beam (rays  26 ) of illumination EMR. As shown in FIG. 1, the collimated beam  26  propagates along a propagation axis P past the electrodes  21 ,  22 . 
     The propagation axis P desirably is parallel to the axis A, and more desirably extends along the axis A. In the latter instance, the transverse section presented by the electrodes  21 ,  22  and insulating member  23  is rotationally symmetrical relative to the EMR flux and is as small as possible. Hence, more EMR flux in the collimated beam  26  is available for downstream illumination purposes, such as for EUV microlithography. 
     The EMR source of FIG. 1 is configured especially to prevent significant blocking of the collimated beam  26  by the electrodes  21 ,  22  and insulating member  23 . To such end, as noted above, the electrodes  21 ,  22  are oriented such that their axis A is oriented parallel to (desirably oriented along) the propagation axis P of the collimated beam  26 . According to another aspect of the configuration, the insulating member  23  (attached to the respective proximal ends of the electrodes  21 ,  22 ) has a spoked configuration, for example as shown in FIG.  2 (B), wherein the spokes extend radially relative to the axis of the center electrode  21 . This spoked configuration is compared with a conventional EUV source (FIG.  2 (A)) in which the insulating member  3  extends across all the space between the center electrode  1  and the surrounding electrode  2 . Hence, with the spoked configuration as shown in FIG.  2 (B), the insulating member  23  and electrodes  21 ,  22  of the EMR source of this embodiment exhibit substantially reduced blocking of the collimated EMR flux reflected from the mirror  25 , compared to the conventional configuration shown in FIG.  2 (A). 
     Although the insulating member  23  in this embodiment is depicted in FIG.  2 (B) as having a spoked configuration, the insulating member  23  alternatively can have any of various other configurations. For example, the insulating member  23  can have a meshed configuration or the like that imparts minimal blocking of the collimated EMR flux reflected from the mirror  25  and propagating past the electrodes  21 ,  22 . In general, the insulating member  23  presents a surface area to the reflected collimated beam that is maximally reduced while still providing adequate support for the electrodes  21 ,  22  and while still serving as a situs for generating the initial plasma. 
     A second representative embodiment of an EMR source according to the invention is depicted in FIG.  3 . This embodiment is configured as a DPF source that can be used, for example, as an EUV source for an illumination-optical system of an EUV microlithography apparatus. 
     This embodiment comprises a center electrode  31  and surrounding electrode  32  spaced from the center electrode  31  by an insulating member  33  to which the proximal ends of the electrodes are attached. The electrodes  31 ,  32 , and insulating member  33  are situated inside a vacuum chamber  38 , and the electrodes  31 ,  32  are connectable to a pulse power supply  37  as described above. The center electrode  31  and insulating member  33  have substantially the same respective structures as the respective components  21 ,  23  in the first representative embodiment. However, the surrounding electrode  32  is configured differently in the instant embodiment. Specifically, the inner wall surface of the distal end of the surrounding electrode  32  is configured as a paraboloid of revolution having a focal point situated at the position of a concentrated plasma  34  produced adjacent the distal end of the center electrode  31 . So as to be maximally reflective to the EMR produced by the concentrated plasma  34 , the paraboloidal surface of the inside wall of the surrounding electrode comprises a multilayer film  39  configured to reflect the EMR produced by the concentrated plasma  34 . The paraboloid of revolution is oriented such that EMR radiated from the plasma  34  reflects from the paraboloidal surface of the inside wall of the surrounding electrode  32  and thus forms the collimated EMR flux  36 . 
     With a surrounding electrode  32  configured as shown in FIG. 3, EMR rays that conventionally are blocked by the surrounding electrode now are guided in directions allowing the rays to be collected into the collimated EMR flux  36  and used for downstream illumination purposes. 
     A third representative embodiment of an EMR source according to the invention is depicted in FIG.  4 . This embodiment is configured as a DPF source that can be used, for example, as an EUV source for an illumination-optical system of an EUV microlithography apparatus. 
     This embodiment comprises a center electrode  41  and surrounding electrode  42  spaced from the center electrode  41  by an insulating member  43 . Similar to the first representative embodiment, the instant embodiment also includes a concave mirror  45  (with paraboloidal reflective surface) situated and configured in a manner similar to the paraboloidal mirror  25  in the first representative embodiment. The electrodes  41 ,  42 , insulating member  43 , and paraboloidal mirror  45  are situated inside a vacuum chamber  48 , and the electrodes  41 ,  42  are connectable to a pulse power supply  47  as described above. The center electrode  41  and insulating member  43  have substantially the same respective structures as the respective components  21 ,  23  in the first representative embodiment. However, the surrounding electrode  42  is configured differently in the instant embodiment. Specifically, the distal end of the surrounding electrode  42  is configured as a spheroid having a center located at the concentrated plasma  44  produced adjacent the distal end of the center electrode  41 . So as to be maximally reflective to EMR produced by the concentrated plasma  44 , the inner surface (concave surface) of the spherical portion of the surrounding electrode  42  comprises a multilayer film  49  configured to reflect the EMR produced by the concentrated plasma  44 . EMR produced by the plasma  44  and radiating downward (in the figure) reflects from the inner surface of the spheroid and propagates through the plasma  44  to the paraboloidal mirror  45 . EMR produced by the plasma  44  and radiating upward (in the figure) reflects from the multilayer surficial film of the paraboloidal mirror  45  in the same manner as in the first representative embodiment. Substantially all EMR reflected from the multilayer surficial film of the paraboloidal mirror  45  comprises a collimated flux  46  of EMR that can be used for microlithographic illumination purposes. 
     Since the paraboloidal mirror  45  collects EMR propagating directly from the plasma  44 , as well as EMR reflected from the concave spheroidal portion of the surrounding electrode  42 , this embodiment is able to collect and utilize more of the EMR produced by the plasma  44  than the first representative embodiment. 
     The concave reflective surface of the mirror  45  need not be configured as a paraboloid of revolution. Alternatively, the concave reflective surface can have any of various other profiles that is rotationally symmetrical and that can function in concert with the distal end of the surrounding electrode  42  to produce the desired EMR flux  46 . 
     Although the EMR-source embodiments are described above in the context of producing EUV light (as a representative EMR), it will be understood that the wavelength of light produced by any of these embodiments is not limited strictly to the EUV band of electromagnetic radiation. As noted above, the wavelength of EMR produced by these sources depends upon the composition of the working gas and/or the material of the center electrode. By changing one or more of these materials, it is possible to produce an EMR flux having a wavelength outside the EUV band, and the materials are selected according to the particular wavelength desired to be produced. 
     FIG. 5 is a schematic diagram of a fourth representative embodiment of the invention, directed specifically to a “reduction” (demagnifying) EUV (soft X-ray) microlithograpy apparatus that includes an EMR source such as any of the embodiments described above. The apparatus of FIG. 5 can be used, e.g., for performing a lithography step in the wafer processing used to produce any of various microelectronic devices. For example, the embodiment shown in FIG. 5 utilizes the DPF EMR source of the first representative embodiment. Hence, for details of the EMR source not provided below, reference should be made to the discussion above pertaining to the first representative embodiment. 
     In the instant embodiment, the EMR source  501  comprises an anode electrode (center electrode) made of tungsten, and the target substance is lithium crystals. Hence, the EMR source  501  produces soft X-rays (EUV radiation) having a wavelength in the vicinity of 13 nm. The EMR source  501  is affixed inside a vacuum chamber  500  by a support column  502 . The EMR source  501  includes a paraboloidal reflective mirror  503  for reflecting and collimating EUV radiation produced by the dense plasma. The concave reflective surface of the paraboloidal reflective mirror  503  includes a Mo/Si multilayer film suitable for reflecting EUV radiation of the desired wavelength band. I.e., the multilayer film is constructed of alternating layers of Mo and Si, and the period length of the Mo/Si multilayer structure is established so as to be maximally reflective to EUV radiation of about 13 nm. Hence, although the EMR produced by the source  501  may contain various wavelengths, only EUV radiation having a wavelength of about 13 nm is reflected from the concave surface of the paraboloidal mirror  503 . 
     The collimated beam of EUV radiation reflected from the mirror  503  is transmitted through a filtering window  506  configured to block visible light. To such end, the filtering window  506  is made of, e.g., zirconium (Zr) with a thickness of 0.15 nm. The transmitted EUV radiation is incident on an illumination-optical system  507 . The illumination-optical system  507  in this embodiment forms an illumination beam having an arc-shaped transverse section for illumination purposes. The illumination beam is incident on a reflecting reticle  508  that defines a pattern for a microelectronic device (e.g., a pattern for a layer in an integrated circuit). Thus, the illumination beam “illuminates” a region on the reticle  508 . EUV radiation reflected from the reticle  508  is demagnified (e.g., by a factor of 4, yielding a demagnification factor of ¼) by passage through a projection-optical system  510 . I.e., in this embodiment, the projection-optical system  510  produces a ¼-sized image of the illuminated portion of the reticle  508  on the surface of a substrate (e.g., silicon wafer)  511  coated with a suitable resist. 
     The reflecting reticle  508  and substrate  511  are mounted on a reticle stage  513  and wafer stage  514 , respectively. Since only a portion of the reticle  508  is illuminated (by the arc-shaped illumination beam) at any one instant during exposure, the reticle  508  and substrate  511  must be moved as exposure proceeds to achieve exposure of the entire pattern. Hence, during exposure, the stages  513 ,  514  are moved relative to each other in a synchronous scanning manner to complete exposure of, for example, an integrated-circuit pattern measuring 25×25 mm on the substrate  511 . An exemplary pattern resolution achievable with the apparatus of FIG. 5 is a line spacing of 0.07 μm. 
     It will be understood that the apparatus of FIG. 5 can employ, as an EMR source, any of the other embodiments described above. Any of the EMR sources according to the invention can be operated continuously for extended periods of time, thereby allowing, in a microlithographic context, microelectronic devices to be manufactured at high yield. In addition, because the EMR sources according to the invention collect and collimate a larger percentage of plasma-produced EMR than conventional EMR sources, throughput of any microelectronic-device-manufacturing process performed using a microlithography apparatus including an EMR source according to the invention exhibit higher throughput than corresponding conventional apparatus. 
     FIG. 6 is a flowchart of an exemplary microelectronic-fabrication method in which apparatus and methods according to the invention can be applied readily. The fabrication method generally comprises the main steps of wafer production (wafer manufacturing or preparation), reticle (mask) production or preparation; wafer processing, device (chip) assembly (including dicing of chips and rendering the chips operational), and device (chip) inspection. Each step usually comprises several sub-steps. 
     Among the main steps, wafer processing is key to achieving the smallest feature sizes (critical dimensions) and best inter-layer registration. In the wafer-processing step, multiple circuit patterns are layered successively atop one another on the wafer, forming multiple chips destined to be memory chips or main processing units (MPUs), for example. The formation of each layer typically involves multiple sub-steps. Usually, many operative microelectronic devices are produced on each wafer. 
     Typical wafer-processing steps include: (1) thin-film formation (by, e.g., sputtering or CVD) involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires or electrodes; (2) oxidation step to oxidize the substrate or the thin-film layer previously formed; (3) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (4) etching or analogous step (e.g., dry-etching) to etch the thin film or substrate according to the resist pattern; (5) doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (6) resist stripping to remove the remaining resist from the wafer; and (7) wafer inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired microelectronic devices on the wafer. 
     FIG. 7 provides a flowchart of typical steps performed in microlithography, which is a principal step in the wafer processing step shown in FIG.  6 . The microlithography step typically includes: (1) resist-application step, wherein a suitable resist is coated on the wafer substrate (which an include a circuit element formed in a previous wafer-processing step); (2) exposure step, to expose the resist with the desired pattern by microlithography; (3) development step, to develop the exposed resist to produce the imprinted image; and (4) optional resist-annealing step, to enhance the durability of and stabilize the resist pattern. 
     The process steps summarized above are all well known and are not described further herein. 
     Whereas the invention has been described in connection with a representative embodiment, it will be understood that the invention is not limited to that embodiment. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.