Abstract:
A spectral purity filter arrangement is disclosed. The spectral purity filter arrangement includes a film configured for filtering out at least a portion of input light and a support structure coupled to the film along at least one edge of the film. The spectral purity filter arrangement further includes a gas control subsystem configured to direct a gas at the film to support the film at least when the film is disposed in an operational position to perform the filtering.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Spectral purity filters have long been employed in UV (ultra-violet) lithography systems that are designed for integrated circuit (IC) fabrication. In certain UV lithography systems (which, as the term is employed herein, also includes deep UV or extreme UV lithography systems), one or more spectral purity filters may be employed to filter out out-of-band radiation (i.e., light that is outside of the range of wavelengths of interest). 
         [0002]    Although the examples herein discuss UV light, it should be understood that a spectral purity filter may be designed to filter out light of any wavelength, ranging from UV to infrared (IR) for example. In a typical example, a spectral purity filter may be created using a thin-film filter made of a material that permits light of certain wavelengths to pass through while blocking light of other wavelengths. Example materials that may be employed for such spectral purity filters (for 13.5 nm central wavelength) include, for example, Zirconium (Zr) and Silicon (Si). 
         [0003]    One of the more important considerations in the design of a spectral purity filter is transmission efficiency. All things being equal, a spectral purity filter that permits more of the in-band radiation (i.e., light that is in the range of wavelengths of interest) to pass through is more desirable than a spectral purity filter that blocks more of the in-band radiation. 
         [0004]    Due to the thinness of the thin film material with which spectral purity filters are fabricated, mechanical support is also a critical design consideration. For horizontally disposed spectral purity filters, for example, it has been estimated that the limited inherent strength of the spectral purity filter material limits the filter size to a few (such as 10-12) square millimeters. 
         [0005]    To construct a larger spectral purity filter, mechanical support structures have been proposed. In a prior art configuration, a spectral purity filter may be disposed in a frame that is made of a suitable supporting material, such as metal or some form of composite material. The frame provides mechanical strength around the periphery of the spectral purity filter. 
         [0006]    Further, a mesh may be attached to the frame such that the thin film material of the spectral purity filter may be supported by the mesh material. The openings in the mesh would allow light that is within the range of wavelengths of interest (i.e., in-band radiation) to pass through. By using a mesh to support the thin film material, it is possible to create a larger spectral purity filter out of an inherently fragile thin film. 
         [0007]    However, the presence of the mesh material reduces the transmission efficiency of the spectral purity filter assembly. As discussed, in-band radiation may pass through the openings in the mesh of a spectral purity filter that is mesh-supported. However, a non-trivial portion of the in-band radiation is blocked by the mesh material itself. This blockage reduces transmission efficiency and is thus undesirable. 
       SUMMARY OF INVENTION 
       [0008]    The invention relates, in an embodiment, to a spectral purity filter arrangement that includes a film configured for filtering out at least a portion of input light and a support structure coupled to the film along at least one edge of the film The spectral purity filter arrangement further includes at least a first gas port configured to provide gas flow support for the film at least when the film is disposed in an operational position to perform the filtering. 
         [0009]    In another embodiment, the invention relates to a method for implementing spectral purity filtering in a lithography system. The method includes providing a spectral purity filter arrangement that includes providing a film and a support structure, the film configured for filtering out at least a portion of input light and coupled to the support structure along at least one edge of the film. The method further includes providing gas flow support for the film at least when the film is disposed in an operational position to perform the filtering. 
         [0010]    In yet another embodiment, the invention relates to an ultraviolet (UV) lithography system configured for processing substrates. The UV lithography system includes a spectral purity filter film configured for filtering out at least a portion of input light and a support structure coupled to the spectral purity filter film along al least one edge of the spectral purity filter film. The UV lithography system also includes means for providing a gas to create gas flow support for the spectral purity filter film al least when the spectral purity filter film is disposed in an operational position  10  perform the filtering. 
         [0011]    These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0013]      FIG. 1  shows, in accordance with an embodiment of the present invention, a spectral purity filter having a thin film and a support structure. 
           [0014]      FIG. 2  shows, in accordance with an embodiment, a spectral purity filter disposed in an example operational position. 
           [0015]      FIG. 3  shows, in accordance with an embodiment, a spectral purity filter in the pre-operational position. 
           [0016]      FIG. 4  shows, in accordance with an embodiment of the present invention, a spectral purity filter having a thin film and a support structure that includes a structural member and a flexible member. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0017]    The present invention will now be described in detail with reference to a few embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
         [0018]    One or more embodiments of the invention relate to spectral purity filters for use in lithography systems. In an embodiment, a spectral purity filter having one edge attached to a supporting member is proposed. During use, gas flow, and preferably laminar gas flow, is furnished to provide gas flow support for the spectral purity filter and to counteract gravity. By way of example, hydrogen is often present in EUV lithography systems and may be employed to provide gas flow support for the thin film material of the spectral purity filter. 
         [0019]    As the term is employed herein, gas flow support refers to the support for the film that otherwise would not exist in the absence of such gas flow. Within this broad definition, the support itself may be accomplished by any number of gas ports (such as a single gas port or multiple gas ports) directed at or along or across one or both surfaces of the film. Furthermore, the direction and/or volume and/or velocity of the gas exiting each port may be the same or may differ as desired. 
         [0020]    By using a gas flow to support the spectral purity filter, support is provided more evenly for the thin film. This is in contrast to the mesh approach, whereby the thin film is supported mostly in the vicinity of the mesh, necessitating small mesh openings (and concomitantly a greater ratio of mesh vs. transmittance area) to support the fragile thin film material. In an embodiment, no mesh material in employed in the area of the thin film that is employed for filtering purposes, thereby maximizing transmittance efficiency. 
         [0021]    In one or more embodiments, the gas flow is substantially parallel to the thin film surface. In one or more other embodiments, the gas flow may be directed at an angle to the thin film to achieve proper gas flow support to maintain the spectral purity filter at the desired operational position. In one or more embodiments, different gas volumes and/or velocities may be employed at different points and/or on different sides of the thin film to achieve proper and stable gas flow support for the thin film. 
         [0022]    In one or more embodiments, the remaining edges of the spectral purity filters are unsupported. In this configuration, there are no structures interrupting the laminar gas flow along the thin film surfaces). In one or more embodiments, one or both of the edges that are parallel to the gas flow may be attached to one or more supporting members to stiffen the thin film. Since the gas flow is parallel to the edge supporting members, there are also no structures in the gas flow path that may disrupt the laminar flow even though additional stiffness is achieved. 
         [0023]    In one or more embodiments, the supporting member that is attached to the thin film is a relatively inflexible supporting member that is capable of providing mechanical strength to permit coupling of the spectral purity filter to the rest of the lithography system. A flexible connecting member, such as a Mylar® film, may optionally be coupled between the inflexible supporting member and the more fragile thin film material. By providing a flexible connecting member, stress fractures due to, for example, vibration or other forms of mechanical stress may be reduced during use. 
         [0024]    In one or more embodiments, a start-up procedure may be provided. The spectral purity filter may initially rest in a vertical direction and may be supported by gravity. As part of the start-up procedure, gas flow may be gradually provided and may be varied in volume and/or velocity and/or direction to smoothly rotate the thin film to a more horizontal position. The gas flow may be provided in gradually increasing volumes and/or velocities and/or direction to avoid blasting the thin film with a sudden blast of gas. In an embodiment, a mechanical rotating arrangement (such as a motor) may be provided, alternatively or additionally, to assist the cantilevered thin film to rotate to the operational position, which may be more horizontal. 
         [0025]    The features and advantages of various embodiments of the present invention may be better understood with reference to the figures and discussions that follow.  FIG. 1  shows, in accordance with an embodiment of the present invention, a spectral purity filter  100  having a thin film  102 . Thin film  102  is attached along one edge  104  to a support structure  106 . For discussion purpose, edge  104  of thin film  102  that is attached to support structure  106  is referred to herein as the primary supported edge. 
         [0026]    Thin film  102  may represent, for example, a thin film in the range of 10-100 nm thick and may be formed of a suitable material such as, for example, Zr or Si. In one or more embodiments, thin film  102  may be a planar structure having parallel planar surfaces. In other embodiments, thin film  102  may be a tapered structure such that the primary supported edge  104  is either thicker or thinner than the opposite edge or may be in the shape of a lens. 
         [0027]    In an embodiment, thin film  102  is rectangular or square in shape, and only one edge is attached to a support structure. In another embodiment, thin film  102  may have a different shape than shown (such as for example a non-polygonal shape or a polygon with more than three or four edges). 
         [0028]      FIG. 2  shows, in accordance with an embodiment, spectral purity filter  100  disposed in an example operational position, which is substantially horizontal and perpendicular to the direction of gravity. However, such position is not a limitation of the present invention, and the operational position may be at any desired angle. If the operational position is other than vertical, i.e., other than parallel to gravity, support is provided to spectral purity filter  100  in the form of gas flow support provided by gas flow directed along the planar surface or planar surfaces of spectral purity filter  100 . For example, a gas may be flowed out of gas port  120   a  along the lower side  110  of spectral purity filter  100  to provide gas flow support for spectral purity filter  100 . 
         [0029]    In some embodiments, gas may also be flowed along the upper side  112  of spectral purity filter  100 . This gas is shown flowing from gas port  120   b  in the example of  FIG. 2 . The angle of incidence of the gas flow may be zero (in which case the gas is flowed substantially parallelly to the plane of the thin film) or may be at some other angle to provide the desired gas flow support. Gas may be flowed from a single slit disposed near the primary-supported edge as shown in the example of  FIG. 2  or may be flowed from one or more jets directed along or at the planar surface or surfaces of the thin film. 
         [0030]    In one or more embodiments, the gas employed to provide gas flow support for the thin film of the spectral purity filter may be hydrogen, which is present in many EUV lithography systems. However, any gas may be employed without limitation. 
         [0031]    In  FIG. 2 , edge  120  that is opposite primary supported edge  104  is preferably left free, i.e., unattached to a supporting member, so as to avoid disrupting the laminar air flow. Furthermore, the edges that are parallel to the air flow are also left unattached. In this cantilevered arrangement, the spectral purity filter is supported mechanically along the primary supported edge  104  and is supported by gas flow elsewhere. In other embodiments, however, one or more of the remaining edges may be attached to a supporting member, which provides mechanical stiffness for the attached edge(s). 
         [0032]    In the example of  FIG. 2 , support structure  106  is mechanically coupled to a gas manifold  130  that is configured for flowing gas along the lower and upper surfaces of thin film  102 . Manifold  130  may in turn be coupled to other support structures of the lithography system. In an embodiment, the gas flow is directed along the axis that connects primary supported edge  104  with opposite edge  120 . 
         [0033]      FIG. 3  shows spectral purity filter  100  in the pre-operational position. In this position, supporting structure  106 , which is coupled with primary supported edge  104 , is connected to the gas manifold (or to other supporting structures of the lithography system). However, since thin film  102  hangs vertically downward and is supported by gravity, gas flow support is unnecessary. As a start-up sequence, gas flow may be provided to facilitate rotating thin film  102  to its operational position (e.g., the horizontal position of  FIG. 2 ). The volume and/or velocity of gas employed for rotating thin film  102  may be varied over time and/or direction (using for example rotatable nozzles) and/or differentiated between the upper and lower surfaces to facilitate smoothly rotating thin film  102  into its operational position. In this case, support structure  106  may be permitted to free pivot to enable the rotation of spectral purity filter  100  into the operational position. 
         [0034]    In another example, a rotational force (using for example a motor) may be exerted on support structure  106  to assist in the rotation of spectral purity filter  100  into its operational position. It is preferred, however, that gas flow be provided to provide continuous gas flow support to thin film  102  while rotating and while spectral purity filter  100  is disposed at its operational position. 
         [0035]      FIG. 4  shows, in accordance with an embodiment of the present invention, a spectral purity filter  400  having a thin film  402 . Thin film  402  has an edge  404  coupled to a support structure  410  that comprises a flexible member  408  and a structural member  406 . In the example of  FIG. 4 , edge  104  is coupled to flexible member  408 , which is in turn coupled to structural member  406 . Flexible member  408 , which is more flexible than structural member  406 , absorbs mechanical vibration and shocks to better protect thin film  402  against mechanical stress. In an example, flexible member  408  represents a flexible membrane such as Mylar and may be attached to thin film  402  using an appropriate adhesive, for example. 
         [0036]    While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents, which fall within the scope of this invention. Although various examples are provided herein, it is intended that these examples be illustrative and not limiting with respect to the invention. 
         [0037]    For example, although the gas that provides gas flow support is shown flowing from slits, it is possible to flow such gas from one or more jets directed along or at the surface(s) of the thin film. As another example, although the gas is shown flowing along the axis that connects the primary supported edge with its opposite edge, the gas that provides the support may be flowed cross-wise or in any other direction as suitable for providing such support. As another example, although Zr and Si are discussed as examples of spectral purity filter thin film material, any suitable spectral purity filter material may be employed. 
         [0038]    Also, the title and summary are provided herein for convenience and should not be used to construe the scope of the claims herein. Further, the abstract is written in a highly abbreviated form and is provided herein for convenience and thus should not be employed to construe or limit the overall invention, which is expressed in the claims. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.