Patent Number: 
Section: 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 xe2x80x9chollowxe2x80x9d 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 xe2x80x9cEMR-flux collimator,xe2x80x9d 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 xe2x80x9cBackgroundxe2x80x9d 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 xe2x80x9creductionxe2x80x9d (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 xe2x80x9cilluminatesxe2x80x9d 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 xc2xc) by passage through a projection-optical system 510. I.e., in this embodiment, the projection-optical system 510 produces a xc2xc-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 25xc3x9725 mm on the substrate 511. An exemplary pattern resolution achievable with the apparatus of FIG. 5 is a line spacing of 0.07 xcexcm. 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.