Patent Publication Number: US-7722997-B2

Title: Holographic reticle and patterning method

Description:
This application is a divisional of patent application Ser. No. 10/792,084, entitled “Holographic Reticle and Patterning Method,” filed on Mar. 3, 2004, now U.S. Pat. No. 7,312,021 which application is incorporated herein by reference. 
    
    
     REFERENCE TO COMPUTER PROGRAM LISTING APPENDIX 
     The material on two identical compact disks, Copy 1 and Copy 2, is incorporated herein by reference. Each compact disc contains a file entitled “TSM02-0658, Appendix A”, created on Sep. 18, 2003, having a size of 9 KB. 
     TECHNICAL FIELD 
     The present invention relates generally to lithography for semiconductor devices, and more particularly to a reticle design including a holographic pattern and methods of patterning a semiconductor wafer using a holographic reticle. 
     BACKGROUND 
     In semiconductor device manufacturing, features and geometric patterns are created on various layers of semiconductor wafers using optical photolithography. Typically, optical photolithography involves projecting or transmitting energy or light through a mask or reticle having a pattern made of optically opaque areas and optically clear areas. Alternatively, a mask or reticle may be reflective rather than transmissive. A phase-shifting mask (PSM) is a type of mask or reticle that uses a phase difference rather than a transmittance difference to generate patterns. 
     A mask is generally used to pattern an entire wafer at a time, while a reticle is used to pattern a portion of a wafer, e.g., in step-and-repeat projection systems. The term “reticle” as used herein refers to any patterning device having a pattern thereon that may be transferred to the entire surface of a semiconductor wafer, or a portion of a surface of a semiconductor wafer or target. 
     A prior art reticle  10  used to pattern a target such as a semiconductor wafer is shown in  FIG. 1 . The reticle  10  may comprise a binary chrome-on-glass mask, for example. A transparent substrate  12  comprising silicon quartz, for example, is provided. An opaque layer  14  is deposited over the substrate  12 . The opaque layer  14  typically comprises chrome, for example. The opaque layer  14  is patterned with a desired pattern so that light may pass through transparent regions  16  of the opaque layer  14 . The opaque layer  14  of the reticle  10  may be patterned by depositing a photoresist, and patterning the photoresist directly using an electron beam or laser to expose the resist, as examples. The photoresist pattern is then transferred into the opaque layer, e.g., by wet etching. 
     The reticle  10  may be used to pattern a photoresist layer on a target such as a semiconductor wafer  20 , shown in  FIGS. 2 and 3 .  FIG. 2  shows a top view of the wafer  20  and  FIG. 3  shows a cross-sectional view of the wafer  20  at  3 - 3 ′ of  FIG. 2 . The wafer  20  may comprise a substrate or workpiece  21  having a material layer  23  disposed on the top surface that will be patterned. A photoresist layer  22  is deposited on the top surface of the wafer  20  over the material layer  23  to be patterned. The photoresist layer  22  is patterned by illuminating the photoresist layer  22  of the wafer  20  with energy, e.g., light, through the reticle  10  of  FIG. 1 . The photoresist layer  22  is then developed, and portions of photoresist layer  22  are removed, leaving a pattern in the photoresist layer  22  that corresponds with the pattern on the reticle  10 , shown in  FIG. 1 . The optically opaque areas  14  of the reticle  10  block the light, thereby casting shadows and creating dark areas, while the optically clear areas  16  allow the light to pass, thereby creating light areas on the wafer  20 . The light areas and dark areas may be projected onto and through an optional lens (not shown), and subsequently onto the photoresist layer  22  of the wafer  20 . 
     When the wafer photoresist layer  22  is developed, exposed areas of the photoresist may be removed, leaving a positive image of the reticle  10  in the photoresist layer  22 , e.g., for a positive photoresist. Alternatively, unexposed areas of the wafer photoresist layer  22  may be removed, leaving a negative image of the reticle in the photoresist, e.g., for a negative photoresist (not shown). 
     The patterned photoresist  22  is then used as a mask to pattern the underlying material layer  23  of the wafer  20 . For example, the photoresist  22  may be left in place on the wafer  20  while the wafer  20  is exposed to a dry or wet etchant to remove exposed portions of the material layer  23 . The photoresist  22  is removed either in a separate etch step, or at the same time the material layer  23  is etched. The patterned material layer  23  is left remaining over the workpiece  21  top surface. Semiconductor wafers  20  are typically manufactured by the deposition and patterning of multiple layers of insulating, conductive and semiconductive materials, in the manner described above. Another way to form the desired layout on the wafers  20  is to process the lithography and developing process, and then deposit a metal or other material layer over the patterned material layer  23 , using a damascene process. 
     The original image of prior art reticles  10  is typically duplicated on the wafer  20 , either in the pattern original size, in a 1× magnification scheme, or alternatively, a 4-5× magnification reduction may be used for projection lithography systems to produce a wafer having a material layer  23  pattern that is ¼ or ⅕ smaller than the reticle  10  pattern, for example. Thus, a one-to-one corresponding relationship exists in prior art reticle  10  patterns and images produced on the wafer  20 . 
     A disadvantage of prior art lithography is that this one-to-one relationship between the reticle  10  and the wafer  20  can result in a reticle defect  18 , particularly if the defect is large, inducing a flaw  23   a  on a wafer  20 . Hard defects and/or soft defects can be formed during the manufacturing process or handling of a reticle  10 . Soft defects refer to pattern defects that may be removed by cleaning, whereas hard defects generally refer to pattern defects that cannot be removed by a cleaning process. Reticles  10  having relatively large reticle defects  18  are unacceptable because the defect may be transferred to the target  20 . 
     Because reticles  10  are typically expensive and time-consuming to manufacture, attempts are usually made to repair them, rather than scrapping them. Larger hard opaque defects  18  are often removed using a laser to evaporate unwanted material. However, reticle  10  defect inspection and repair are difficult, time-consuming tasks. Also, laser repair of a reticle  10  can damage the reticle substrate, leaving a laser burn and possibly creating a printable defect on the substrate  21 . The repair of some reticle  10  defects is often impossible to achieve. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention, in which a layout pattern or image to be transferred to a target is converted into a holographic representation of the image, and a hologram reticle is manufactured that includes the holographic representation. The hologram reticle is then used to pattern a wafer. Advantageously, imperfections or defects on the hologram reticle are not transferred to the wafer. The original image is partitioned and encoded across the entire hologram reticle, which breaks the one-to-one corresponding relationship between defects on the reticle to the wafer. A defect on the hologram reticle does not directly induce a flaw on a wafer, but rather, the defect influence is spread into the entire hologram reticle image, and merely affects the intensity or contrast of the hologram reticle slightly. 
     In accordance with a preferred embodiment of the present invention, a lithography reticle includes a material having a pattern, the pattern including opaque regions and transparent regions, the pattern comprising a holographic representation of an image, wherein the holographic representation of the image is formed using a Computer-Generated Holography encoding technique. 
     In accordance with another preferred embodiment of the present invention, a method of manufacturing a lithography reticle includes providing an image, creating a holographic representation of the image using a local encoding technique (LET), providing a material, and patterning the material with the holographic representation of the image, wherein the patterned material comprises transparent regions and opaque regions. 
     In yet another preferred embodiment of the invention, a method of patterning a target includes providing a target, the target having a top surface, the target top surface having a material layer disposed thereon, a first photoresist layer disposed over the material layer, a transparent spacer material disposed over the first photoresist layer, and a second photoresist layer disposed over the spacer material. The method includes patterning the second photoresist layer of the target with a holographic fringe representation of an image. 
     Another embodiment of the invention is a method of patterning a target. The method includes providing a target, the target having a top surface, the target top surface having a photoresist layer disposed thereon, and providing a lithography reticle, the lithography reticle comprising a holographic representation of an image to be patterned on the target. The photoresist layer is patterned with a three-dimensional pattern using the lithography reticle, and depositing a material layer over the photoresist layer. The photoresist layer is removed, leaving three-dimensional structures comprised of the material layer disposed over the target. 
     An advantage of preferred embodiments of the present invention includes providing a defect-withstanding reticle for patterning a target. Defects on the hologram reticle do not result in the formation of defects on the patterned target surface. Thus, there is a reduced need for repair of defects on the reticle, resulting in a cost savings. The depth of focus (DOF) may be increased to extend the lithography process window, particularly on a topographic substrate. A further advantage of preferred embodiments of the present invention is the ability to precisely control and decrease the DOF, and form three-dimensional (3-D) structures in the photoresist layer on the target. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a top view of a prior art reticle having a defect thereon; 
         FIG. 2  shows a top view of a prior art wafer upon which the reticle defect has been transferred; 
         FIG. 3  shows a cross-sectional view of the wafer shown in  FIG. 2 ; 
         FIG. 4  is a flow chart showing a method of manufacturing and using a hologram reticle in accordance with embodiments of the present invention; 
         FIG. 5  shows a top view of a hologram reticle having a holographic fringe pattern in accordance with an embodiment of the present invention; 
         FIG. 6  illustrates a cross-sectional view of the hologram reticle shown in  FIG. 5 , including a substrate and an opaque material patterned with a holographic representation of an image to be transferred to a target disposed over the substrate; 
         FIG. 7  illustrates an alternative embodiment of a hologram reticle including a phase-shifting material formed over portions of the holographic fringe pattern; 
         FIG. 8  shows a side view of the hologram reticle shown in  FIG. 5 ; 
         FIG. 9  illustrates a top view of a hologram reticle in accordance with an embodiment of the invention; 
         FIG. 10  shows a cross-sectional view of the hologram reticle shown in  FIG. 9 ; 
         FIGS. 11A and 11B  illustrate visual illustrations of light sources that may be used in a look-up table having a plurality of templates of holographic fringes in accordance with an embodiment of the present invention; 
         FIG. 12  illustrates a top view of a hologram reticle in accordance with an embodiment of the invention; 
         FIG. 13  shows a cross-sectional view of the hologram reticle shown in  FIG. 12 ; 
         FIG. 14A  illustrates a top view of a hologram reticle in accordance with an embodiment of the invention; 
         FIG. 14B  shows a more detailed view of a portion of the reticle shown in  FIG. 14A ; 
         FIG. 15  shows an embodiment of the invention, wherein a hologram reticle is directly illuminated to transfer or reconstruct the image to a photoresist layer of a target; 
         FIG. 16  shows another embodiment of the present invention, wherein a hologram reticle is illuminated with an oblique beam to transfer the image to a photoresist layer of a target; 
         FIG. 17  shows a reconstruction scheme in accordance with an embodiment of the invention, wherein a holographic representation of an image is duplicated on a top layer of photoresist on a target; 
         FIG. 18  illustrates the target of  FIG. 17 , wherein the top layer of photoresist having the holographic representation of an image is illuminated to reconstruct the image on a bottom layer of photoresist disposed below the top layer of photoresist on the target; 
         FIG. 19  shows a hologram reticle in accordance with embodiments of the present invention having defects thereon; 
         FIG. 20  shows a semiconductor wafer having a material layer that has been patterned using the hologram reticle of  FIG. 19 , wherein the hologram reticle defects are not transferred to the wafer pattern; 
         FIG. 21  illustrates a cross-sectional view of photoresist formed over a workpiece, illustrating that the depth of focus may be varied to pattern at a predetermined vertical depth within the photoresist in accordance with an embodiment of the present invention; 
         FIG. 22  shows a prior art cross-sectional view of photoresist patterned completely in the vertical direction; 
         FIG. 23  shows an embodiment of the invention, wherein the depth of focus is varied at a plurality of locations, to create a 3-D pattern in the photoresist layer; 
         FIGS. 24A through 24D  illustrate cross-sectional views of a workpiece at several stages of manufacturing, wherein three-dimensional patterns are formed in a photoresist, and the patterns are filled with a material to form 3-D structures in a material layer; 
         FIG. 25  illustrates a cross-sectional view of a semiconductor wafer in accordance with an embodiment of the invention, wherein multiple layers of interconnect are patterned within a single resist layer; 
         FIG. 26  show a prior art multi-layer device formed by a plurality of sequential deposition steps and patterning steps of photoresist and interconnect layers; and 
         FIGS. 27 and 28  illustrate examples of 3-D pattern formation within a photoresist in accordance with an embodiment of the invention. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely, to a lithography reticle and method for patterning semiconductor wafers. Embodiments of the invention may also be applied, however, to other fields of lithography and lithography for other types of targets. 
       FIG. 4  is a flow chart  124  illustrating methods of manufacturing and using a hologram reticle in accordance with embodiments of the present invention. First, the desired pattern layout is prepared (step  126 ). The desired pattern layout, also referred to herein as an image, preferably comprises a pattern that will be transferred to a material layer of a target or wafer  120 / 220 / 420  (see  FIGS. 15 ,  18  and  23 , respectively). The pattern layout is converted into a holographic fringe pattern (step  128 ), preferably using an encoding technique such as computer generated holography (CGH), as an example. The encoding hologram imaging technique may utilize software adapted to implement a local encoding technique (LET) in one embodiment. Alternatively, the encoding hologram imaging technique may comprise a fast Fourier transform (FFT) method of generating a holographic fringe pattern, as examples. A preferred embodiment of the programming code in C for the encoding of the pattern layout into a holographic fringe pattern is disclosed in Appendix A, which may be found in the file “TSM02-0658, Appendix A” of the computer programming listing appendix provided on compact disc submitted with this patent application, which is incorporated by reference. However, the C code disclosed herein is exemplary, and the embodiments described herein may be implemented in other types of code and form. 
     Referring again to the flow chart  124  of  FIG. 4 , next, in some embodiments of the invention, a hologram reticle  140 / 240 / 340 / 440  is manufactured having the holographic fringe pattern patterned into an opaque layer (step  130 ). The hologram reticle  140 / 240 / 340 / 440  is then used to pattern a first photoresist layer  122  on a target (step  133 ) (see  FIGS. 15-16 ). Portions of the target first photoresist layer are then removed (step  134 ), and then the target first photoresist layer  122  is used as a reticle to pattern a material layer of the target (step  135 ). 
     In another embodiment, the target comprises a first photoresist layer and a second photoresist layer formed over the first photoresist layer, as shown in  FIGS. 17 and 18 . The hologram reticle having a holographic fringe pattern is used to pattern the second photoresist layer (step  132 ). The second photoresist layer of the target is then used to pattern the first photoresist layer of the target (step  136 ) with the image, and portions of the first photoresist layer of the target are removed (step  137 ). The first photoresist layer is then used as a reticle to pattern the image onto a material layer of the target (step  138 ). 
     In yet another embodiment, no hologram reticle is required, and the target comprises a first photoresist layer and a second photoresist layer formed over the first photoresist layer. The second photoresist layer of the target is patterned directly with the holographic fringe pattern (step  131 ). The target second photoresist layer is then used to pattern the first photoresist layer of the target (step  136 ), and portions of the first photoresist layer are removed (step  137 ). The first photoresist layer is then used as a reticle to pattern a material layer of the target (step  138 ). 
     Details of the preferred embodiments illustrated in the flow chart  124  will be described further herein. Embodiments of the present invention may be used to pattern periodic (e.g., repeating) patterns, or general patterns having no particular repetition. After the hologram reticle is manufactured, it may be stored, e.g., on a shelf, until it is time to manufacture wafers with the image that is patterned in holographic form onto the hologram reticle. The hologram reticle may be used and stored in the same manner as traditional masks and reticles of the past. Advantageously, embodiments of the hologram reticles described herein are compatible with existing exposure tools and lithography systems currently in use. The holograph reticles and methods of patterning using holographic techniques described herein may be used to pattern a plurality of different types of material layers on a target, such as conductors, insulators and semiconductors, as examples. 
       FIG. 5  shows a top view of a hologram reticle  140  having a holographic fringe pattern  142  formed in an opaque layer  146  (see  FIG. 6 ), in accordance with an embodiment of the present invention. The original pattern layout to be patterned on the target (such as the pattern of opaque layer  14  shown in the prior art drawing of  FIG. 1 ) and the encoded holographic fringe pattern  142 , shown in  FIG. 5 , are quite different, in accordance with embodiments of the invention. The holographic fringe pattern  142  is preferably computer generated, and may appear visually to the eye of a viewer as a plurality of random dots or apertures, as shown. The image to be patterned on the target is generally not visibly recognizable in the pattern  142  formed on the hologram reticle. However, the holographic fringe pattern correlates to the desired pattern layout or image that will be transferred to the target  120  (see  FIG. 20 ). The original image will appear after the reconstruction process on the target  120 . 
       FIG. 6  illustrates a cross-sectional view of the hologram reticle  140  shown in  FIG. 5 . The hologram reticle  140  includes a substrate  144  and an opaque material  146  disposed over the substrate  144 . The substrate  144  preferably comprises a transparent material, such as quartz, and alternatively may comprise other transparent materials, for example. The substrate  144  preferably comprises a thickness of about 1 mm, for a commercially available diffraction optical element, to about one-quarter inch for a conventional 6 inches quartz mask, as examples. Alternatively, the substrate  144  may comprise other thicknesses. 
     The opaque material  146  preferably comprises a metal such as chrome, and may alternatively comprise other metals and opaque materials, for example. The opaque material  146  is preferably about 700 nm to 1000 nm thick, for example, although the opaque material  146  may alternatively comprise other thicknesses. The opaque material  146  is preferably patterned by depositing a photoresist over the opaque material  146 , patterning the photoresist using an electron beam or laser, as examples, (although other patterning methods may be used) removing exposed (or unexposed) portions of the photoresist, and then using the photoresist as a mask to remove portions of the opaque material  146 . Alternatively, rather than using a photoresist, the opaque material  146  may be directly patterned by a reactive ion etch (RIE) process or by ion milling, as examples. The hologram reticle  140  preferably comprises a transmissive, thin-film, binary hologram reticle, as shown in  FIG. 6 . 
     In another embodiment, shown in  FIG. 7 , the hologram reticle  240  may comprise phase shifting regions  248  disposed proximate the substrate  244  between portions of the opaque regions  246 , in particular, disposed or formed over portions of the holographic fringe pattern  242 . The phase shifting regions  248  may comprise an additional layer of transparent material, as shown, or alternatively, portions of the substrate  244  may be removed to reduce the thickness of the substrate  244  and create phase-shifting regions, for example (not shown). The hologram reticle may be implemented in any other reticle configuration, such as a reflective volume, thin-film, binary, phase, or imprint with physical contact reticle, as examples. 
       FIG. 8  shows a cross sectional view of the hologram reticle  140  shown in  FIG. 5 . In one embodiment of the present invention, a Computer-Generated Holography encoding technique, such as an FFT encoding technique, is used to encode the entire image, the entire image being represented by the letter “S” in  FIGS. 5 and 8 , into a pattern for a hologram reticle. FFT is a technology used in the fields of mathematics, physics, and digital imaging processes, for example. With FFT, for each image S shown in  FIGS. 5 and 8 , holographic fringe must be calculated across the entire hologram. 
     In accordance with another embodiment of the present invention, an LET encoding technique is used to encode only a portion of the image into a pattern for a hologram reticle. The portion of the image encoded to the hologram reticle is represented by the letter “S” in  FIGS. 9 and 10 , which illustrate a top view and a cross-sectional view, respectively, of a hologram reticle  340  in accordance with an embodiment of the invention. The portion of the image S of  FIGS. 9 and 10  has a width m and a height n. Thus, in the LET encoding technique, the image is encoded into an m×n area on the hologram reticle  340 . A plurality of image portions S are encoded using LET in accordance with an embodiment of the present invention, until the entire surface of the image is encoded into a holographic representation. 
     By using an LET, for each image portion S, only the area A which is defined by m×n needs to be calculated. This is advantageous in that the calculation time is greatly reduced. For example, a six inch reticle may have a total area of about 132,000 μm by 132,000 μm, and an LET area A may be, for example, about 100 μm×100 μm. That is, approximately over 0.5 million points of data may need to be calculated. The use of LET in accordance with embodiments of the present invention advantageously reduces the time required for the calculation process to encode the image into a holographic pattern, and reduces the use of computer resources required for the calculation process. Using an LET not only saves time but also makes the intensity, phase, and DOF of individual points in the patterning process more controllable. 
     A “look-up table” concept may be used to further reduce the time required for the large number of calculations required. The look-up table may comprise a plurality of visual illustrations of light sources that may be selected. For example, the visual illustrations of light sources may comprise a fringe pattern from a 1×1 matrix single point light source such as the one shown in  FIG. 11A , or a fringe pattern from a 3×3 matrix three point light source such as the one shown in  FIG. 11B . Alternatively, the visual illustrations of light sources may comprise other numbers and arrangements of light source matrixes, such as 1×2, 1×3, 1×4, . . . 2×1, 2×2, 2×3, 2×4 . . . 4×4, 4×1, 4×2, 4×3, 4×4, 4×5, etc., as examples, not shown. The look-up table may include a plurality of templates of fringe patterns such as the ones shown in  FIGS. 11A and 11B , for example. The number of fringe patterns in the look-up table depends on the complexity of the layout, for example. The intensity of the individual image portion S of  FIGS. 9 and 10  may be controlled by adjusting the size of the area A. The reconstruction characteristics of the image portion S can also be controlled by modifying the dimensions m and n. 
     The encoding of portions of the image is repeated until the entire reticle is encoded, as shown in  FIGS. 12 and 13 . Each portion encoded S 1  has an associated area m 1 ×n 1 , and associated area m 2 ×n 2 , for portion S 2 , for example. Note that if the area of m 2 ×n 2  is larger than the area of m 1 ×n 1 , then the intensity of S 2  will be higher than the intensity of S 1 . However, the intensity of S 1  and S 2  can be controlled individually by controlling the size of the encoding area. 
     S 1  and S 2  denote two individual patterns or image portions, and can be considered as two points of light sources. The arrows represent the light paths coming from the hologram reticle  340  and focusing to the target (to form image portion S) during the reconstruction process. The area m by n denotes how large the area or how many pixels contribute to reconstruct the image portion S during the imaging or reconstruction process. The unit of m and n may comprise length (for example, several μm or nm) or simply pixels, and may alternatively comprise a minimum component size on the reticle  340 . The size or dimension of m and n may be approximately between several to hundreds of μm in one embodiment. 
     The individual intensity will be decreased if there is overlapping of the encoded area. An illustration of this phenomena is shown in a top view of a hologram reticle  340  in  FIG. 14A . The intensity decreasing coefficient (IDC) may be expressed by Equation 1: 
                       1   2     ×     1   mn     ×     {     [       A   1     ⊗     A   2       ]     }       ;           Eq   .           ⁢   1               
where A 1   A 2  represents the area of overlapping, with A 1  being the amount of horizontal overlap and A 2  being the amount of vertical overlap, wherein m 1 =m 2 =m and n 1 =n 2 =n.  FIG. 14B  shows a more detailed view of the area of overlap.
 
     Equation 1 expresses the IDC for two overlapping image portions S 1  and S 2 . Similarly, the IDC for three overlapping image portions may be represented by Equation 2: 
                       1   2     ×     1   mn     ×     {       [       A   1     ⊗     A   2       ]     +     [       A   1     ⊗     A   3       ]       }       -       1   3     ×     1   mn     ×     {     [       A   1     ⊗     A   2     ⊗     A   3       ]     }               Eq   .           ⁢   2               
and the IDC for four overlapping image portions may be represented by Equation 3:
 
     
       
         
           
             
               
                 
                   
                     
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     If two reconstruction image portions S 1  and S 2  are located too close together, their A 1  and A 2  will overlap on the hologram reticle, as shown in  FIG. 14B . The size of A will affect the intensity of S; therefore, when there is overlap, the intensity of image portions S 1  and S 2  are both decreased because by sharing the overlapping area [A 1   A 2 ] with each other, the “effective” areas of A 1  and A 2  are decreased. In this case, the IDC quantifies the effect of the decrease area. The IDC depends on the location and distance of the overlapping. According to the IDC, the loss of intensity can be compensated by increasing the size of the area A. 
       FIG. 15  illustrates an embodiment of the invention, wherein a hologram reticle  140  is directly illuminated with illumination or a beam from energy source  154  to transfer or reconstruct the image to a target  120 . The target  120 , shown in cross-sectional view, may comprise a semiconductor wafer, as an example, including a workpiece  121 , a material layer  155  disposed over the workpiece, and a layer of photoresist  122  disposed over the material layer  155 . The workpiece  121  may include a semiconductor substrate comprising silicon or other semiconductor materials covered by an insulating layer, for example. The workpiece  121  may also include other active components or circuits, not shown. The workpiece  121  may comprise silicon oxide over single-crystal silicon, for example. The workpiece  121  may include other conductive layers or other semiconductor elements, e.g. transistors, diodes, etc. Compound semiconductors, GaAs, InP, Si/Ge, or SiC, as examples, may be used in place of silicon. The material layer  155  may comprise a conductive material, insulative material, semiconductive material, or other materials, as examples. The photoresist  122  comprises an organic or polymer resist typically used in lithographic techniques. 
     In this reconstruction method, the target or wafer  120  is directly illuminated, with a hologram reticle  140  (or  240  or  340 ) described herein being disposed between the source of illumination (e.g., energy source  154 ) and the target  120 . The energy source  154  may comprise a coherent or partial coherent light source such as a laser source, as examples, although other energy sources may alternatively be used. In this embodiment of the invention, energy is directed substantially perpendicular to the surface of the target  120 . The energy may comprise a beam, such as an electron beam or an ion beam, as examples. The holographic fringe pattern of the hologram reticle  140  is adapted to expose the photoresist  122  and form the desired image on the photoresist  122  of the target  120 . The photoresist  122  is then developed, and portions of the photoresist  122  are removed. The photoresist is then used as a mask to etch the material layer  155  of the target  120  and form the image within the material layer  155 . 
     The reconstruction processes for the hologram imaging technique described herein preferably comprise a combination of constructive and destructive interference. The incident angle of illumination also affects the hologram imaging technique reconstruction. 
     In another embodiment, shown in  FIG. 16 , a hologram reticle  140  is illuminated with an oblique beam (e.g., a beam that is directed non-perpendicular to the surface of the workpiece  121 ) to transfer the image to the target  120 . The oblique beam is used to illuminate the target  120  through the hologram reticle  140 . In this embodiment, a wafer blade  150  may be disposed between the target  120  and the hologram reticle  140 , and/or a reticle blade  152  may be disposed between the energy source  154  and hologram reticle  140 . The wafer blade  150  and/or reticle blade  152  may be moved while the target  120  is illuminated, in the same or opposing directions, for example. The incident illumination light or oblique beam is directed at an angle to project a shadow on the wafer blade  150 . The reticle blade  152  controls the size of the slit, to optimize the performance of the hologram reticle  140  lithography system. The reticle blade  152  is adapted to block the unwanted incident illumination. Similarly, the wafer blade  150  is adapted to block unwanted diffraction lights, such as high order diffraction beams, as an example. The angle θ may comprise 30 degrees, as an example, and more preferably, angle θ ranges from 15 to 45 degrees, for example. 
     Embodiments of the invention may also be implemented on a target having multiple layers of photoresist, also referred to herein as top surface imaging (TSI), as shown in  FIGS. 17 and 18 . TSI involves patterning features on a target using two photoresist layers  257  and  256  deposited over the target  220 , with a spacer  258  separating the two photoresist layers  257  and  256 . The top photoresist layer  257  preferably comprises a thickness of about 500 Å, and the bottom photoresist layer  256  preferably comprises a thickness of about 3000 Å, as examples, although the top and bottom photoresist layer  257  and  256  may alternatively comprise other thicknesses. The top photoresist  257  is used to pattern the underlying photoresist layer  256 . The spacer  258  or buffer layer preferably comprises a transparent material that is adapted to separate the top photoresist  257  from the bottom photoresist layer  256  (often referred to as a “button” layer) by a predetermined distance; e.g., the spacer may comprise a thickness between about 3,000 to 10,000 Å, as examples. The spacer  258  may comprise spin-on glass (SOG) or borophosphosilicate glass (BPSG), as examples, although alternatively, other optically transparent materials may be used. In this embodiment, the top photoresist  257  functions as a secondary hologram reticle, implemented in TSI on the target  220 . The secondary hologram reticle  257  is also defect-withstanding and CD-error insensitive, as is the hologram reticle  240 , to be described further herein. 
       FIG. 17  shows a reconstruction scheme in accordance with an embodiment of the invention, wherein the holographic representation of the hologram reticle is duplicated on the top layer of photoresist  257  on a target  220 . Using an exposure tool  260 , energy from an energy source  254  is passed through a hologram reticle  240  to pattern the top photoresist layer  257  with the holographic representation of an image. The exposure tool  260  may comprise, as examples, an optical exposure tool such as a stepper, scanner or imprint tool, although other exposure tools may alternatively be used. The target top photoresist layer  257  is then developed, preferably using a gas or dry development, as examples, without destroying the bottom layer of photoresist  256 . Preferably, during the exposure step and subsequent development step of the top photoresist layer  257 , the remaining layers of the target  220 , such as workpiece  221 , material layer to be patterned  255 , photoresist layer  256 , and spacer  258 , remain substantially unaffected. 
     In one embodiment, the TSI latent pattern having the holographic fringe representation of the desired image to be patterned may be formed in the top photoresist layer  257  by a lithography process with a hologram reticle as shown in  FIG. 17 , or alternatively, the TSI latent pattern be formed by direct-writing a holographic representation of the desired image onto the top photoresist layer  257  (not shown). Alternatively, the holographic representation of the desired image may be formed into the top photoresist layer  257  by another maskless lithography process such as by RIE or by ion milling, as examples (also not shown). 
       FIG. 18  illustrates the target  220  of  FIG. 17 , wherein a reconstructed image is formed on the bottom photoresist layer  256  that is substantially the same as the original image. The patterned top layer of photoresist  257  is illuminated with a beam of energy from the energy source  254  to reconstruct the desired image on the bottom layer of photoresist  256 . The photoresist  257  that is patterned with a holographic representation of the desired image is removed, and the spacer  258  is removed. The bottom layer of photoresist  256  is developed, and then the bottom photoresist layer  256  is used as a mask to pattern the material layer  255  of the target  220 . The material layer  255  then comprises the desired image. The bottom photoresist layer  256  is then removed, and subsequent processing steps may then be performed on the target  220  to complete the manufacturing process. 
       FIG. 19  shows a top view of a hologram reticle  140  in accordance with embodiments of the invention having defects  118  disposed thereon. Due to the nature of a holographic pattern, any defects  118  on the hologram reticle  140  will not be transferred to a target such as the semiconductor wafer  120  shown in  FIG. 20  when the hologram reticle  140  is used to pattern the wafer  120 . The holographic representation of the image breaks the one-to-one relationship between the reticle pattern and the image in the material layer  155  patterned. Advantageously, any defects  118  on the hologram reticle  140  merely affect the intensity of the holographic image, and are not transferred to the desired image that is patterned within the material layer  155  of the wafer  120 . 
       FIG. 21  illustrates a cross-sectional view of photoresist  322 , showing how the depth of focus (DOF) may be varied to pattern at a particular vertical depth within the photoresist  322  perpendicular to the target  320  surface, in accordance with an embodiment of the present invention. The target or wafer  320  comprises a workpiece  321  and a material layer  355  to be patterned. A layer of photoresist  322  has been formed over the material layer  355 . The depth of focus, using the hologram reticle  340 , may be varied to pattern at any depth within the photoresist  322 , as shown. 
     In one embodiment of the present invention, a two-photon process is used to pattern a photoresist layer. A two-photon process is a non-linear quantum photochemical reaction. With suitable situation and material, a molecule can absorb two low energy photons (for example, having a wavelength of about 800 nm) rather than one high energy photon (having a wavelength of about 400 nm). This non-linear phenomenon happens only in an area with a very high density of photons. In accordance with embodiments of the present invention, a very high density area is created by virtual light sources of the holographic reconstruction images. More specifically, a 3-D structure pattern may be created or reconstructed in a photoresist layer using a holographic reticle with a laser beam. 
     With sufficient illumination intensity, the resist  322  will be exposed by absorbing two low energy photons such as infrared (IR) photons rather than one high energy photon such as the ultraviolet (UV) photon. This two-photon process occurs close to the focal point  366  with high photon density. In this manner, one spot (e.g., focal point  366 ) may be exposed at a time using a two-photon process. In contrast, prior art photolithography techniques involve using an traditional imaging system with a conventional mask  10  to pattern resist  22  on a target  20  completely in the vertical direction, as shown in  FIG. 22 . The condensed energy beam induces the vertical cylinder shape of resist  22  that is exposed, in prior art photolithography techniques. 
     In accordance with an embodiment of the present invention, a target  420  comprises a workpiece  421  and a material layer  455  deposited over the workpiece  421 , as shown in  FIG. 23 . A photoresist layer  422  is deposited over the material layer  455 . The photoresist  422  may be patterned at two or more depths, or a range of depths, as shown in  FIG. 23 , to create a 3-D pattern, e.g., using a two-photon illumination process. A direct write scheme may be used to construct 3-D structures in the photoresist layer  422 , for example. By using the holographic imaging processes described herein, a plurality of locations within the layer of photoresist  422  may be exposed in a single exposure. 
     For example, generally, a semiconductor manufacturing process contains a front-end and back-end, referring to periods of time in the manufacturing flow. The frond-end is the portion of the manufacturing process in which silicon and polysilicon processes occur, such as deposition, doping and implanting processes, to create active devices such as transistors and other circuit elements. The back-end is the portion of the manufacturing process after the front-end, in which the metallization layers, contact hole layers and other connecting layers for the front-end devices are formed. For a typical foundry chip, there are about two silicon-type layers formed in a front-end, and about eight contact hole layers and eight metallization layers formed in a back-end: a total of 18 layers, for example. It is notable that 16 of these 18 layers are processed only for connecting purposes. Because traditional lithography can only form an image on a two-dimensional plane across a wafer surface, these 16 connection layers have to be processed one by one, requiring a different mask for each layer. By using a holographic reticle combined with a two-photon process as described herein, three-dimensional images can be reconstructed in a photoresist layer, thus patterning more than one layer in a single exposure. 
       FIGS. 24A through 24D  show cross-sectional views of a semiconductor device  520  in which a holographic reticle and two-photon process are used to pattern a 3D image of a semiconductor device  520 , saving one level of lithography patterning. A workpiece  512  is provided, and a photoresist layer  570  is deposited over the workpiece  512 . The photoresist  570  in this embodiment is preferably relatively thick, comprising a thickness of two or more material layers, for example. The photoresist  570  is patterned using a holographic reticle  440  such as the one shown in  FIG. 23 , and a two-photon process is used to illuminate the photoresist  570  through the holographic reticle  440 , for example. The pattern  572  formed in the photoresist  570  in this embodiment comprises a dual damascene pattern, including a contact pattern (the thin regions) and conductive line pattern (the thicker regions disposed over the thin regions). Patterned portions of the photoresist  570  are removed, as shown in  FIG. 24B , and a material  574  such as a conductive material (although other materials may also be used) is deposited over the patterned photoresist  570 , as shown in  FIG. 24C . Excess material  574  may be removed from over a top surface of the photoresist  570 , using a chemical-mechanical polishing (CMP) process or other methods. The photoresist  570  is then removed, leaving a three-dimensional structure formed from the material  574 , as shown in  FIG. 24D . This is advantageous, because prior art damascene patterning methods for metal layers require two separate mask levels. Thus, embodiments of the present invention reduce the number of patterning steps and reduce the number of masks required for semiconductor fabrication. 
       FIG. 25  illustrates an embodiment of the invention, wherein multiple layers of interconnect are patterned within a single resist layer disposed over a workpiece  421 . Active areas  462  may be formed over the workpiece  421 , as shown, for example. In this example, seventeen or more layers of metallization (M 1 -M 9 ) and contact or via levels (V 1 -V 8 ) may be replaced by a single holographic imaging process, combined with a two photon process. In comparison,  FIG. 26  shows a prior art semiconductor device  20  having a multi-layer interconnect, formed by sequential depositions and patterning of photoresist and interconnect layers. Thus, in accordance with embodiments of the present invention, many sequential patterning steps and lithography masks may be eliminated, saving processing time and expense of designing and making the masks. 
       FIGS. 27 and 28  illustrate examples of 3-D pattern formation within a photoresist in accordance with an embodiment of the invention. In  FIG. 27 , a 3-D image  464  which may comprise a pyramid, for example, is converted into a holographic representation of the image, using an LET in one embodiment, although other methods may be used. The holographic representation is patterned onto a mask  440 , and the mask  440  is used to pattern a single relatively thick layer of photoresist  470 . Top, middle and bottom views of the photoresist  470  are shown. A material layer is deposited over the photoresist  470  to fill the 3-D pattern, and the photoresist is removed, leaving a 3-D structure formed over a workpiece (not shown in  FIG. 27 ). Similarly,  FIG. 28  illustrates a 3-D image  464  comprising the letters A and Z that are transferred to a holographic pattern (not shown), and the three-dimensional image of the letters is reconstructed using a hologram reticle and two-photon process into a photoresist layer  470  of a target, thus illustrating the capability of forming two layers of an image in one exposure, in accordance with embodiments of the present invention. 
     A short DOF and small focal point are required to induce exposed spots with the two photon process described herein. The hologram reticles  140 / 240 / 340 / 440  described herein produce a short DOF because of the characteristics of the holographic representation of the image. High order diffraction beams have a larger incident angle. The larger angle represents a higher numerical aperture (NA) and produce a shorter depth of focus (DOF). In accordance with embodiments of the present invention, a holographic pattern can be modified or designed so that it contains more or less higher order fringes in order to control the individual DOF. 
     Embodiments of the present invention are useful and have application in static holographic reticles, as described herein, wherein the holographic representation of the image is fixedly patterned into a reticle. However, embodiments of the present invention also have application in dynamic reticles, wherein the holographic fringe representation of the image may be altered or erased after being patterned into the reticle. Masks or reticles in which a dynamic holographic representation of an image may be patterned include electric-optical modulation devices such as a liquid crystal display (LCD), or micro-machined devices such as a special light modulator (SLM), as examples. 
     The holographic reticles described herein may comprise binary reticles, phase-shifted reticles, and/or volume reticles, for example. Fringes on a phase holographic reticle represent phase information rather than intensity information, for example. A volume holographic reticle comprises a holographic fringe representation of a 3-D structure, for example. 
     Embodiments of the present invention achieve technical advantages as a novel hologram reticle and lithography process that is defect-withstanding. Defects on the hologram reticle  140 / 240 / 340 / 440  are not transferred to the image produced on a target. Due to the characteristics of the holographic representation of the image, defects that are not too large in size or number may be left remaining on the hologram reticle  140 / 240 / 340 / 440 , yet defect-free targets may be produced using the hologram reticle  140 / 240 / 340 / 440 . This is advantageous because defect repair may be reduced or eliminated, resulting in a cost savings and increased yield for manufactured hologram reticles  140 / 240 / 340 / 440 . If repair is needed, the hologram reticle  140 / 240 / 340 / 440  described herein is more durable for the repair process. Any defects on the hologram reticle  140 / 240 / 340 / 440  do not directly induce a flaw on a target, but rather, the defect influence is spread into the entire image, affecting intensity or contrast, or both, only slightly. 
     The multi-layer 3-D imaging technique using a holographic reticle described herein may be used with existing lithography tools, advantageously. No special design schemes or tools are required for the holographic reticle and patterning methods described herein. 
     Another advantage is that the hologram reticle  140 / 240 / 340 / 440  is CD error insensitive: critical dimension (CD) errors on holographic reticle will not affect the final image on the wafer or other target substantially. For a diffraction element, pitch is more important than the CD. 
     A further advantage is the ability to precisely control and decrease the DOF and form 3-D structures in the photoresist layer on the target using a two photon process. Combining the holographic reticle with a two photon process produces an extremely short DOF, allowing three-dimensional reconstruction in thick photoresist layers. Alternatively, the DOF may be increased to extend the lithography process window, which is particularly advantageous on a topographic substrate. 
     Furthermore, an LET can be used to reduce the calculation time required to convert the image to be patterned to a holographic representation on the hologram reticle  140 / 240 / 340 / 440 . An LET can also be used to control the intensity of the individual image. A look-up table of fringes may be used to further reduce the time required for the conversion calculations. 
     Additional advantages include compatibility with existing exposure tools and lithography systems, and the ability to implement the hologram reticle  140 / 240 / 340 / 440  with TSI. Furthermore, the CGH algorithm used to convert the image to a holographic representation of the image may be modified to improve the image quality, including contrast, corner rounding, and depth of focus, as examples. 
     The defect-withstanding hologram reticle  140 / 240 / 340 / 440  described herein is particularly advantageous for use with X-ray Lithography (XRL), SCattering with Angular Limitation in Projection Electron beam Lithography (SCALPEL), Extreme Ultra-Violet Reflective Projection Lithography (EUVL), Ion-beam Projection Lithography (IPL), and E-beam Lithography processes, as examples 
     Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, while the hologram reticle  140 / 240 / 340 / 440  is described herein as a transmissive reticle, (e.g. light is passed through the reticle towards the target), the hologram reticle  140 / 240 / 340 / 440  may alternatively comprise a reflective reticle. As another example, it will be readily understood by those skilled in the art that the selection of the target to be patterned using the hologram reticle described herein may be varied while remaining within the scope of the present invention. As examples, the target may comprise a semiconductor wafer, or may alternatively comprise an optical device, or an organic material, as examples. Embodiments of the present invention include targets such as semiconductor devices that have been patterned using the holographic reticles and methods of patterning described herein. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.