Abstract:
Apparatus having a chamber with an interior wall and a region within the chamber from which a contaminating material emanates when the apparatus is in operation. A plurality of vanes is positioned on a portion of the interior wall, each of the vanes having a first surface which is oriented along a direction between the vane and the region and a second surface adjacent the first surface which is oriented to deflect the contaminating material striking the second surface away from the region, the second surfaces being dimensioned and juxtaposed with respect to one another such that the second surfaces substantially prevent the contaminating material from striking the portion of the interior wall. At least part of each of the vanes may be covered with a mesh. The vanes may be heated, and may be heated at least to a melting point of the contaminating material. The apparatus is especially applicable to protecting multilayer mirrors serving as collectors in systems for generating EUV light for use in semiconductor photolithography.

Description:
FIELD 
     The present disclosure relates to protection of optical elements that operate in environments in which they are subject to contamination. An example of such an environment is the vacuum chamber of an apparatus for generating extreme ultraviolet (“EUV”) radiation from a plasma created through discharge or laser ablation of a source material. In this application, the optical elements are used, for example, to collect and direct the radiation for utilization outside of the vacuum chamber, e.g., for semiconductor photolithography. 
     BACKGROUND 
     EUV light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere, it will be understood that the term “light” is used to encompass electromagnetic radiation outside of the visible part of the spectrum. 
     Methods for generating EUV light include converting a source material from a liquid state into a plasma state. The source material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a source material having the required line-emitting element. 
     One LPP technique involves generating a stream of source material droplets and irradiating at least some of the droplets with laser light pulses. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10&#39;s of eV. 
     The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. 
     In the EUV portion of the spectrum reflective optics are usually used for the collector. At the wavelengths involved, the collector is advantageously implemented as a multi-layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material over a foundation or substrate. 
     The optical element must be placed within the vacuum chamber with the plasma to collect and redirect the EUV light. The environment within the chamber is inimical to the optical element and so limits its useful lifetime, for example, by reducing its reflectivity. An optical element within the environment may be exposed to high energy ions or particles of source material. The particles of source material can contaminate the optical element&#39;s exposed surface. Particles of source material can also cause physical damage and localized heating of the MLM surface. The source materials may be particularly reactive with a material making up at least one layer of the MLM, e.g., molybdenum and silicon. Temperature stability, ion-implantation and diffusion problems may need to be addressed even with less reactive source materials, e.g., tin, indium, or xenon. 
     Several techniques have been employed to increase optical element lifetime despite these harsh conditions. For example, protective layers or intermediate diffusion barrier layers may be used to isolate the MLM layers from the environment. The collector may be heated to an elevated temperature of, e.g., up to 500° C., to evaporate debris from its surface. The collector surface may be cleaned using hydrogen radicals. An etchant may be employed e.g., a halogen etchant, to etch debris from the collector surfaces and create a shielding plasma in the vicinity of the reflector surfaces. 
     Another technique which may be employed is to reduce the likelihood that contaminating source material reaches the collector surface. Source material may accumulate on the interior surfaces of the vessel. This source material may reach the collector through the influence of gravity. There is a need to protect the system from this material. For example, some systems use vanes to protect the collector from micro-droplets of source material created during plasma generation. It is possible in such a system, however, for source material to accumulate on the vanes. This in turn creates the possibility that accumulated source material will detach from the vanes and impinge on the surface of the collector, especially when the system for dispensing source material into the vessel is deployed at a large angle from vertical. 
     There remains a need to extend collector lifetime by protecting the surfaces of optical elements from source material in systems for generating EUV light. With this in mind, applicants disclose arrangements for improved protection of surfaces of optical elements. 
     SUMMARY 
     The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. 
     In one aspect, there is provided an apparatus having a chamber with an interior wall and a region within the chamber from which a contaminating material emanates when the apparatus is in operation. A plurality of vanes is positioned on a portion of the interior wall, each of the vanes having a first surface which is oriented along a direction between the vane and the region and a second surface adjacent the first surface which is oriented to deflect the contaminating material striking the second surface away from the region, the second surfaces being dimensioned and juxtaposed with respect to one another such that the second surfaces substantially prevent the contaminating material from striking the portion of the interior wall. The vanes may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. At least part of each of the vanes may be covered with a mesh. The mesh may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. 
     The vanes may be heated, and may be heated at least to a melting point of the contaminating material. Each of the vanes may include an electrical resistive element, and the vane may be heated by supplying current to the electrical resistive heater. Alternatively or in addition each of the vanes may include an internal fluid channel and the vane may be heated by causing heated fluid to flow in the internal fluid channel. 
     At least part of each of the vanes may be covered with a mesh and is heated at least to a melting point of the contaminating material. Each of said vanes may be in fluid communication with a vessel for collecting the contaminating material and the mesh may be arranged to direct liquid contaminating material to the vessel. 
     In another aspect, there is provided an apparatus for generating light useful for semiconductor fabrication by generating a plasma from a source material, the apparatus having a chamber with an interior wall and a region within the chamber from which source material emanates when the apparatus is in operation. A plurality of vanes may be positioned on a portion of the interior wall, each of the vanes having a first surface which is oriented along a direction between the vane and the region and a second surface adjacent the first surface which is oriented to deflect the source material striking the second surface away from the region, the second surfaces being dimensioned and juxtaposed with respect to one another such that the second surfaces substantially prevent the source material from striking the portion of the interior wall. The vanes may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. At least a portion of each of the vanes may be covered with a mesh. The mesh may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. 
     The vanes may be heated and, in particular, may be heated at least to a melting point of the source material. Each of the vanes may include an electrical resistive element, in which case the vane may be heated by supplying current to the electrical resistive heater. Each of the vanes may include an internal fluid channel, in which case the vane may be heated by causing heated fluid to flow in the internal fluid channel. Each of the vanes may be in fluid communication with a vessel for collecting the source material and the mesh may be arranged to direct liquid source material to the vessel. 
     In yet another aspect there is provided an apparatus for generating light useful for semiconductor fabrication by generating a plasma from a source material, the apparatus having a chamber with an interior wall and a region within the chamber from which source material will emanate when the apparatus is in operation. A first vane may be positioned on a first portion of the interior wall, the vane having a first vane tangential surface which is oriented along a first direction between the first vane and the region and a first vane shielding surface adjacent the first vane tangential surface which is oriented to deflect the source material striking the first vane shielding surface away from the region. A second vane may be positioned on a second portion of the interior wall, the second vane having a second vane tangential surface which is oriented along a second direction different from the first direction between the second vane and the region and a second vane blocking surface adjacent the second vane tangential surface which is oriented to deflect the source material striking the second vane shielding surface away from the region. The vanes may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. The first vane shielding surface and the second vane shielding surface are dimensioned and juxtaposed with respect to one another such that the first vane shielding surface and the second vane shielding surface substantially prevent the source material from striking the first and second portions of the interior wall. 
     At least a portion of the first and second vanes may be covered with a mesh. The mesh may be made of a material such as molybdenum or stainless steel and may be covered with an inert material such as gold. Each of the first and second vanes may be heated. At least a portion of the first and second vanes may be covered with a mesh and each of said first and second vanes may be heated at least to a melting point of the source material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic, not to scale, view of an overall broad conception for a laser-produced plasma EUV light source system according to an aspect of the present invention. 
         FIG. 2  is a not-to-scale side view of an optical element protection system according to one aspect of the present invention. 
         FIG. 3  is a not-to-scale perspective view of an optical element protection system according to one aspect of the present invention. 
         FIG. 4  is not-to-scale cutaway side view of some of the elements shown in  FIG. 2  and  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram foam in order to facilitate description of one or more embodiments. 
     With initial reference to  FIG. 1  there is shown a schematic view of an exemplary EUV light source, e.g., a laser produced plasma EUV light source  20  according to one aspect of an embodiment of the present invention. As shown, the EUV light source  20  may include a pulsed or continuous laser source  22 , which may for example be a pulsed gas discharge CO 2  laser source producing radiation at 10.6 μm. The pulsed gas discharge CO 2  laser source may have DC or RF excitation operating at high power and high pulse repetition rate. 
     The EUV light source  20  also includes a source delivery system  24  for delivering source material in the form of liquid droplets or a continuous liquid stream. The source material may be made up of tin or a tin compound, although other materials could be used. The source delivery system  24  introduces the source material into the interior of a vessel or chamber  26  to an irradiation region  28  where the source material may be irradiated to produce plasma. In some cases, an electrical charge is placed on the source material to permit the source material to be steered toward or away from the irradiation region  28 . It should be noted that as used herein an irradiation region is a region where source material irradiation may occur, and is an irradiation region even at times when no irradiation is actually occurring. 
     Continuing with  FIG. 1 , the light source  20  may also include one or more optical elements such as a collector  30 . The collector  30  may be a normal incidence reflector, for example, implemented as a multilayer mirror (“MLM”), that is, a silicon carbide (SiC) substrate coated with a Mo/Si multilayer with additional thin barrier layers deposited at each interface to effectively block thermally-induced interlayer diffusion. Other substrate materials, such as Al or Si, can also be used. The collector  30  may be in the form of a prolate ellipsoid, with an aperture to allow the laser light to pass through and reach the irradiation region  28 . The collector  30  may be, e.g., in the shape of a ellipsoid that has a first focus at the irradiation region  28  and a second focus at a so-called intermediate point  40  (also called the intermediate focus  40 ) where the EUV light may be output from the EUV light source  20  and input to, e.g., an integrated circuit lithography tool  50  which uses the light, for example, to process a silicon wafer workpiece  52  in a known manner. The silicon wafer workpiece  52  is then additionally processed in a known manner to obtain an integrated circuit device. 
     As described above, one of the technical challenges in the design of an optical element such as the collector  30  is extending its lifetime. The surface of the collector, which is usually a coating, becomes contaminated with source material, e.g., tin. One source of this contaminating source material is source material detaching from surfaces within the vessel  26  where it has accumulated. It is thus desirable to prevent this source material from accumulating on the interior surfaces of the vessel  26 . 
     To achieve this end, in one embodiment, an array of vane-like structures is arranged on the interior surface of the vessel  26 . Such an arrangement is shown in  FIG. 2 .  FIG. 2  shows a collector  30  arranged to redirect light from an irradiation region  28  as in  FIG. 1 . Also shown is an optical axis  35  for the collector  30 .  FIG. 2  shows a segment of the wall  100  of the vessel  26  on which an array  110  of vanes  120  is arranged.  FIG. 2  shows only the lowermost portion of the vessel  26 . One of ordinary skill in the art will readily appreciate, however, that the array  110  may be placed additionally or alternatively on the side and upper portions of the vessel  26 . If the vessel  26  has a cylindrical shape with the axis of the cylinder parallel to the optical axis  35 , the array  110  may cover the entire interior surface of the vessel  26 . Also, although the arrangement of vanes  120  is referred to herein as an array, this term is not intended to connote that the spacing between and the sizes of the vanes  120  are regular. It will be readily appreciated by one of ordinary skill in the art that the number, spacing, and size of the vanes  120  may be altered without departing from the principles of the invention. 
     The vanes  120  preferably made from a material that is resistant to corrosion such as stainless steel or molybdenum. The vanes  120  can also be coated with an inert material such as gold. 
       FIG. 3  is a perspective view of an arrangement such as that shown in  FIG. 2  where like numerals refer to like elements. As can be seen, the vanes  120  extend laterally across the wall  100  is such a way that microdroplets of contaminating material necessarily strike the vanes  120  and are deflected away from irradiation region  28  and collector  30 . 
     As can be seen, in a presently preferred embodiment each vane  120  has a substantially trapezoidal cross-section. Referring to  FIG. 4 , the vane  120  has a first edge  120   a  that points generally towards the irradiation region  28 . Each vane  120  also has a back side  120   b  that extends in a direction towards the irradiation region  28 . Each vane also has a front side  120   f  that is tilted so that source material from the irradiation region  28 , typically in the form of micro-droplets of source material, will be deflected in a direction away from the irradiation region  28  and, hence, away from the collector  30 . 
     The front surface  120   f  is dimensioned and positioned so that it extends generally laterally at least until it touches an imaginary line  130  extending from the center of the irradiation region  28  and tangential to a back surface  120   b  of an adjacent vane. This is shown in  FIG. 2 . In other words, the front surface  120   f  of a first vane  120  and back surface  120   b  of an adjacent vane  120  are arranged and dimensioned so that droplets of source material emanating from the irradiation region  28  can strike only the front surface  120   f  of a vane  120  and cannot strike the portion of the interior surface of the wall  100 . The physical gap between adjacent vanes  120  is thus covered by the extension of the front surface  120   f . This is because the back surface  120   b  is arranged to be substantially tangential to the path of the droplets, and the front surfaces together occlude the interior surface of wall  100  of the vessel  26 . 
     As shown in  FIG. 4 , in a presently preferred embodiment the vanes  120  have a substantial thickness as measured between the front surface  120   f  and an opposing surface  120   g . This thickness can be between about 3 mm and about 50 mm or even larger. In a presently preferred embodiment, this thickness is preferably in the range of about 6 mm to about 25 mm and more preferably in the range of about 10 mm to about 25 mm. 
     Also, as shown in  FIG. 4 , the vane  120  is preferably provided with one or more heating elements  150 . These heating elements  150  are placed within the vane  120  to permit heating of the vane  120 , preferably at least to the melting point of the source material. In instances in which the source material is tin, it is preferred to heat the vane  120  at least up to the melting point of tin (231.9° C.) and more preferably to a temperature in the range of about 250° C. to about 350° C. The heating elements  150  may be electric or may be conduits for the flow of a heated gas or liquid. In the case of electric heating elements  150 , the heating elements  150  may be supplied with power through a feedthrough  160 . In the case of heating elements  150  in the form of conduits, the heated fluid may be supplied through the feedthrough  160 . As shown, the feedthrough  160  may also support the vane  120 . Alternatively, the vane  120  can be supported by separate supporting hardware (not shown). 
     According to another aspect of a preferred embodiment of the invention, one or more of the vanes  120  are also provided with a wire mesh  170 . The purpose of the mesh  170  is to trap and retain source material that strikes the vanes  120 . Another purpose of the mesh  170  is to direct the flow of source material that strikes the vanes  120  to a desired location within the vessel  26  such as a source material collection sump  190  as shown in  FIG. 2 . The source material collection sump  190  may be configured as a port supplied with a freeze valve. 
     The materials for the mesh  170  are chosen to optimize these functions. For example, the mesh  170  is preferably made from a material that is resistant to corrosion such as stainless steel or molybdenum. The mesh  170  can also be coated with an inert material such as gold. In the instances in which the mesh  170  is made up of wires, the diameter of the wire making up the mesh  170  is preferably in the range of about 100 microns to 1 mm. The mesh opening size is preferably in the range of about 100 microns to about 2 mm. The percentage opening of the wire mesh  170  is preferably equal to or greater than 50%. 
     As shown in  FIG. 4 , the mesh  170  can be attached to the vane  120  at several points using attachment members  180 . The attachment members  180  may be dimensioned to maintain a gap of constant width between the mesh  180  and the opposing surface of the vane  120 , or may be dimensioned so that the gap width varies in a desired fashion. 
     The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.