Abstract:
A reticle for use in an extreme ultraviolet (euv) lithography tool includes a trench formed in the opaque border formed around the image field of the reticle. The trench is coated with an absorber material. The reticle is used in an euv lithography tool in conjunction with a reticle mask and the positioning of the reticle mask and the presence of the trench combine to prevent any divergent beams of radiation from reaching any undesired areas on the substrate being patterned. In this manner, only the exposure field of the substrate is exposed to the euv radiation. Pattern integrity in neighboring fields is maintained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/758,114, filed on Feb. 4, 2013, the contents of which is hereby incorporated by reference as if set forth in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to systems and methods for patterning semiconductor devices using extreme ultraviolet (“euv”) lithography. More particularly, the disclosure relates to a system, method and reticle mask used in euv lithography. 
     BACKGROUND 
     Extreme ultraviolet, euv, technology is a rapidly advancing lithography technology for patterning semiconductor and other devices. Euv lithography uses an extreme ultraviolet, euv, wavelength of radiation generally at about 13.5 nm. Euv lithography enables the creation of more accurate patterns with better resolution and smaller feature sizes. Euv lithography operations utilize reflective optics, including a reflective mask, also referred to as a reticle. 
     Reflective surfaces are used, instead of lenses, to guide the euv light beams from the euv light source to the reticle, because all matter absorbs euv radiation and glass lenses would immediately absorb euv photons. 
     The reticle used in euv lithography is a complex optical element with many parameters that affect the critical dimensions and precision of the features formed on the substrate. The reticle contains a reflecting multilayer that may be tuned to the wavelength of light used, and an absorber which defines the dark areas. A pattern of the reflective multilayer and absorber is formed on the reticle and this pattern is transferred to the substrate being patterned as the euv light radiation reflects off of the reticle. The euv light beam is incident upon the reticle at an oblique angle and reflects off of the reticle to reach the substrate. The euv light beams that reflect from the reticle should ideally have the same angle with respect to the reticle as the incident euv light beam. When there is a variance in the angle of the reflected euv light beam, this divergent, off-axis illumination is referred to as “flare.” Flare in EUV systems can be caused by surface roughness in the reflective surface which causes incident light to be scattered in multiple directions in addition to the specular direction. 
     The euv light beams that are reflected from the reticle and impinge upon the substrate surface chemically alter the exposed photoresist. The euv beam that reflects off of the reticle is scanned across the substrate to form patterns on the individual integrated circuits, i.e. die, of the substrate. A number of scans of the euv light beams are used to pattern the entire substrate. The portion of the pattern being scanned onto the substrate is known as the scanning field. A number of scanning fields are used to pattern the complete substrate. When a patterning operation is being carried out in one scanning field, it is critical that other portions of the substrate outside the present scanning field, are not subject to light reflected off of the reticle. Non-telecentric optics, imperfect absorber or reflective layers on the reticle and other conditions can result in flare and cause the euv radiation to be undesirably reflected onto neighboring fields not desired to be exposed. When such light is reflected outside the scanning field, this divergent light introduces patterning distortions and can alter or destroy features outside of the scanning field. The light that reaches areas outside the scanning field causes patterning problems such as CD (critical dimension) variation of device features on regions of the substrate that neighbor the field being scanned. This scattered light in the projection optics could result in several nanometers of on-wafer dimensional variation, if left uncorrected. 
     It would be desirable to reduce the divergent radiation associated with euv patterning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing. 
         FIG. 1  is a cross-sectional side view of an embodiment of a reticle including a trench, according to the disclosure; 
         FIG. 2  shows an exemplary arrangement of portions of an euv apparatus according to the disclosure; 
         FIG. 3  shows further details of a trench formed in a reticle according to an embodiment of the disclosure; and 
         FIGS. 4A-4C  are cross-sectional views that show a sequence of processing operations used to form a reticle according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure provides a reticle used in a lithography apparatus for uv or euv patterning operations. The disclosure also provides a method for forming the reticle. The disclosure also provides for carrying out a lithography operation using the reticle. Although the disclosure is described with respect to an euv lithography system, aspects of the disclosure are also applicable to other ultraviolet lithography systems. 
       FIG. 1  shows reticle  1  and reticle mask  21 . Reticle  1  is a reticle used in an euv lithography apparatus for patterning a substrate also positioned in the lithography apparatus. Reticle  1  includes two sections: image field  3  and opaque border  5 . Image field  3  includes pattern  15  that will be projected onto a substrate. Reticle  1  is formed of a base material  7 . Both opaque border  5  and image field  3  are formed of base material  7 . 
     Now turning to  FIG. 2 , components within an euv lithography apparatus are shown. It can be seen that opaque border  5  surrounds image field  3  on reticle  1  shown in cross-section. An euv light source within an euv lithography apparatus provides euv light beam  9  which is incident upon pattern side  11  of reticle  1  and is reflected as reflected beam  17  directed toward wafer  13 . Various sources of euv light generation are used to produce euv light beam  9  in various embodiments. Euv light is emitted by electrons that are bound to multicharged positive ions. The multicharged positive ions are produced in hot, dense plasmas in some embodiments. Xe or Sn plasma sources are used for euv lithography in some embodiments but other sources of euv radiation are used in other embodiments. The euv radiation is produced at various wavelengths and is about 13.5 nm in one embodiment. 
     Wafer  13  and reticle  1  are parallel to one another and in confronting relation. Wafer  13  is received on a stage (not shown) of the euv lithography apparatus. Wafer  13  will have been coated with a photoresist on surface  14  when undergoing the patterning operation. Reflected beam  17  forms a pattern in scanning field  19  of wafer  13  and the arrangement of the disclosure prevents portions of reflected beam  11  from reaching neighbor field portion  20  of wafer  13 . One aspect of the disclosure is the use of reticle mask  21  to block portions of euv light beam  9  from reaching pattern side  11  of reticle  1 . Reticle mask  21  is placed between the radiation source and reticle  1  in an attempt to prevent radiation such as euv light beam  9  from reflecting off of reticle  1  onto undesired areas on wafer  13 . The accurate placement of reticle mask  1  in the x- and y-directions with respect to euv light beam  9  and reticle  1  and also the x- and y-position of reticle mask  21  with respect to opaque border  5  around image field  3  formed on reticle  1 , is critical. Blocked beam portion  18  represents the location where a reflected beam would exist if not blocked by reticle mask  21 . As will be described, trench  25  further prevents divergent light beams from reaching neighbor field  20 . In the full-field scanning embodiment, scanning field  19  is the width of an entire die, but scanning field  19  includes multiple die or portions of a die, in other embodiments. The pattern of the reticle is reduced by a factor of 4:1 as produced on wafer  13  in one embodiment but other reduction factors are used in other embodiments. 
     Returning to  FIG. 1 , pattern  15  is formed on pattern side  11  of reticle  1 , which is disposed opposite back side  23  of reticle  1 . Base material  7  is formed of a material having a low coefficient of thermal expansion. A “low” coefficient of thermal expansion (CTE) is known and understood to be a CTE less than about 0±5 ppb/K. In various embodiments, glass, TiO 3  and other suitable materials are used for base material  7 . In some embodiments, base material  7  has an ultra low expansion coefficient (ULE) less than about 0±5 ppb/K. Trench  25  is formed within base material  7  of reticle  1  and extends inwardly from pattern side  11  of reticle  1 . Trench  25  is formed at or near the interface of opaque border  5  and image field  3  of reticle  1 . Trench  25  is bounded by bottom surface  27  and sides  29 . In the illustrated embodiment, one of the sides  29  of trench  25  forms an edge of image field  3 , but in other embodiments, trench  25  is located at other locations at or near the interface between opaque border  5  and image field  3 . Multilayer reflective material  31  is formed on pattern side  11  of reticle  1  in image field  3  portion and also in opaque border  5  portion. Multilayer reflective material  31  is formed on bottom  27  and sides  29  of trench  25 . Various materials are used for multilayer reflective material  31  and in some embodiments, molybdenum and silicon are used. In other embodiments, molybdenum, silicon and rubidium are used. Multilayer reflective material  31  is chosen to include a high reflectivity and the reflectivity varies from about 50% to about 90% in some embodiments. 
     Absorber layer  33  is formed over multilayer reflective material  31 . Absorber material  33  is a tantalum-boron-nitride composition in one embodiment, but other suitable absorber materials are used in other embodiments. Absorber layer  33  and multilayer reflective material  31  form pattern  15  on pattern side  11  in image field  3  portion of reticle  1 . Absorber layer  33  is dark and reflects only a very small portion of the incident light beam onto the substrate being patterned, but light beams that impinge upon multilayer reflective material  31  are reflected to the substrate. It should also be noted that the thickness of absorber layer  33  is expanded for clarity in  FIG. 1 . 
     Light beam  37  is incident on pattern side  11  of reticle  1  and even though absorber layer  33  is chosen to be minimally reflective, a portion of light beam  37  reflects as reflected beam  39  due to the high energy of the euv radiation. Reticle mask  21  is positioned to block light beams to the left of light beam  37  and which would fall outside of scan field  41 , i.e. to the left of scan field  41  in the illustrated embodiment. Distance  45  and the lateral position of reticle mask  21  is determined in conjunction with the incident angle of light beam  37 , which is one of half of angle  43  between incident light beam  37  and reflective light beam  39 . In some embodiments, distance  45  is about 5-15 mm, but other locations are used in other embodiments. Reflected light beam  39  reflects off bottom surface  27  of trench  25  and is disposed further to the right than a reflected beam would be in the absence of trench  25 . Dashed line  47  indicates the location of a beam that would be reflected from light beam  37 , if trench  25  was not present and also shows that reflected light beam  39  produced as a result of trench  25 , is directed further into scan field  41  by distance  48 , the therefore further distanced from being outside scan field  41 . Further details of the dimensions of trench  25  are shown in  FIG. 3 . 
       FIG. 3  shows trench  25  with sides  29  and bottom  27 . The dimensions, including depth  49  and width  51 , of trench  25  vary in various embodiments. In some embodiments, depth  49  is 60-75% of thickness  55  of reticle  1  and may be 50%-85% of thickness  55  of reticle  1  in some embodiments. In one embodiment, depth  49  is at least about 65% of thickness  55  of reticle  1 . Reticle  1  includes a thickness  55  of about 6 mm in some embodiments and about 5-10 mm in other embodiments but other thicknesses are used in still other embodiments. Various design factors are taken into account and used to calculate depth  49  and width  51  in various embodiments. In some embodiments, an alignment tolerance/accuracy for the placement of reticle  1  and an alignment tolerance/accuracy for placement of reticle mask  21  in a photolithography manufacturing apparatus, are determined. The tolerances are based on the accuracy of the positioning tools and measurement systems used to measure the positions of reticle  1  and parallel reticle mask  21  in a manufacturing tool. Angle  43  represents the angle between incident light beam  37  and reflected light beam  39  and ranges from about 4-15° in various embodiments. 
     In one embodiment, the dimensions of depth, d and width, e, bear the following relationship to other features:
 
( c+d )×tan  F °≧( a+b )
 
 e≧ 2( a+b )
 
     In the preceding equations, “a” represents the reticle position accuracy, “b” represents the reticle mask position accuracy, “c” represents distance  45  between the reticle mask  21  and reticle  1 , “d” represents depth  49  of trench  25 , “e” represents width  51  of trench  25 , and angle “F” represents angle  43 . The relationship of the preceding equations are useful for determining the dimensions “d” and “e” of trench  25  and are also useful for positioning the reticle mask  21  at the correct spacing “c” from reticle  1 . The preceding equations provide one embodiment for determining depth  49  and width  51  of trench  25  based on one set of parameters, but other techniques for calculating desirable dimensions of width  51  and depth  49  of trench  25  are used in other embodiments. In still other embodiments, depth  49  and width  51  are determined using other factors. 
       FIG. 4A  shows a portion of reticle  1 . Base material  7  includes pattern side  11  and back side  23 . Layer  61  is formed on back side  23  of reticle  1  and layer  61  is CrN in one embodiment, but other suitable materials are used on back side  23  as a backing material in other embodiments. Trench  25  is formed to extend downwardly from pattern side  11  and into base material  7 . Trench  25  is formed to depth  49  and width  51 . Various etching techniques are available and are used to form trench  25 .  FIG. 4B  shows multilayer reflective material  31  formed on pattern side  11  of reticle  1 , including within trench  25  that was shown more clearly in  FIG. 4A . Various deposition methods and epitaxial formation methods are used to form multilayer reflective material  31  in various embodiments. Molybdenum and silicon are used for multilayer reflective material  31  and in various embodiments several alternating layers of molybdenum and silicon are used. Other combinations of molybdenum and silicon are used in other embodiments. In some embodiments, rubidium is formed on surface  65  of multilayer reflective material  31 , but this is not shown in the embodiment of  FIG. 4B . Multilayer reflective material  31  includes a total thickness of about 40-400 nm in some embodiments, but other thicknesses are used in other embodiments. In some embodiments, multilayer reflective material  31  is formed of a repeating sequence of a 4 nm molybdenum layer and a 3 nm silicon layer and in one embodiment 40 pairs of these layers are used with an aggregate thickness of about 280 nm, but other material layers and thicknesses are used to form multilayer layers reflective material  31  in other embodiments. 
     In  FIG. 4C , absorber material  33  is formed on pattern side  11  of reticle  1  and, in particular, on multilayer reflective material  31 , including within trench  25 . Various formation methods and epitaxial formation methods are used to form absorber layer  33  in various embodiments. In some embodiments, absorber layer  33  is formed of tantalum boron nitride. In other embodiments, absorber layer  33  is formed of non-stoichiometric compositions of tantalum, boron, and nitrogen. Other suitable materials are used for absorber layer  33  in other embodiments. Absorber layer  33  includes a thickness of about 40-80 nm in some embodiments, but other thicknesses are used in other embodiments. 
     Another aspect of the disclosure is the use of reticle  1  such as shown with trench  25  in  FIG. 1 , in an euv lithography tool in an arrangement such as shown in  FIG. 2 , in which reticle  1  and wafer  13  are in confronting relation and parallel, and reticle mask  21  is positioned with respect to reticle  1  to prevent undesired light from reflecting onto areas other than scanning field  19 . Applicants have found that, as a result of the accurate placement of reticle mask  21  and the use of trench  25 , divergent light is reduced and the euv light radiation that reflects off of reticle  1  is directed to the scanning field  19  and not neighbor field  20 . 
     According to an aspect of the disclosure, a reticle for patterning a semiconductor device in an extreme ultraviolet (euv) lithography tool, is provided. The reticle comprises a reticle substrate having an image field and an opaque border at least partially surrounding the image field; the reticle substrate including a pattern side and an opposed back side and formed of a base material having a low coefficient of thermal expansion and including a trench formed in the base material in the opaque border or at an interface between the image field and the opaque border, the trench extending inwardly from the pattern side and lined with an absorber material. 
     According to an aspect of the disclosure, a method for forming a reticle used for patterning a semiconductor device using an extreme ultraviolet (EUV) lithography tool, is provided. The method comprises: providing a reticle substrate formed of a base material being a material having a coefficient of thermal expansion less than 0±5 ppb/K, the reticle substrate including a central image field and an opaque border at least partially surrounding the central image field; etching a trench into the base material in the opaque border or at an interface between the image field and the opaque border; forming a multilevel reflective structure of molybdenum and silicon on the reticle substrate, including on sides and a bottom of the trench; and, forming an absorber material over the multilayer reflective material. 
     According to another aspect of the disclosure, a method for patterning a semiconductor device in an extreme ultraviolet (euv) lithography apparatus, is provided. The method comprises: providing an extreme ultraviolet (euv) lithography apparatus with an euv radiation source; providing a substrate to be patterned on a stage in the euv lithography apparatus, the substrate coated with photoresist; positioning a reticle in confronting relation with the coated substrate; and directing a light beam from the euv radiation source, to a pattern side of the reticle at an oblique angle such that the light beam reflects off the reticle and onto the coated substrate. The reticle includes the pattern side and an opposed back side, is formed of a base material having a low coefficient of thermal expansion and includes a trench formed in the base material in the opaque border or at an interface between an image field and an opaque border of the reticle. The trench extends inwardly from the pattern side and is lined with an absorber material. 
     The preceding merely illustrates the principles of the disclosure. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. 
     Although the disclosure has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the disclosure, which may be made by those of ordinary skill in the art without departing from the scope and range of equivalents of the disclosure.