Abstract:
A phase-shifting photomask with a self aligned undercut rim-shifting element and methods for its manufacture are provided. One embodiment of the invention provides a method of manufacturing a phase-shifting photomask having a self aligned rim-shifting element, the method comprising: applying a patterning film to a first portion of a transparent substrate; etching the substrate to a depth to remove a second portion of the substrate not beneath the patterning film; etching the first portion of the substrate to undercut an area beneath the patterning film; and removing the patterning film, wherein the etched substrate forms a self-aligned undercut rim-shifting element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of co-pending U.S. patent application Ser. No. 12/702,787, filed 9 Feb. 2010, which is hereby incorporated herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to semiconductor manufacturing and, more specifically, to chromeless phase-shifting photomasks having undercut rim-shifting elements and related methods for their manufacture. 
       BACKGROUND OF THE INVENTION 
       [0003]    Photolithography techniques have progressed to the point that the critical dimension (CD) is often smaller than the actinic wavelength of the radiation (e.g., ultraviolet light) employed, requiring the effects of optical diffraction to not only be accounted for, but often utilized for imaging today&#39;s sub-micron features. In response, various phase-shifting techniques have been employed to mitigate the detrimental effects of diffraction by taking advantage of the destructive interference caused by phase shifting. 
         [0004]    Photomasks incorporating rim-shifting elements are one type of phase-shifting device so employed. Some rim-shifting photomasks employ a pattern of opaque light blocking elements on a transparent substrate. A transparent phase shift material, such as quartz, covers the opaque light blocking elements and the transparent substrate, such that light passing through the thicker transparent phase shift material along the side walls of the opaque light blocking elements is phase shifted with respect to light passing through the thinner phase shift material between the opaque light blocking elements. The intersection of the phase-shifted light from the two regions forms a null on the wafer. This utilizes the effects of diffraction along the edges of the opaque light blockers and produces a sharpened image. The light transmission produced by such phase-shifting is binary, in that two transmissions are produced: a first passing through the thinner phase shift material and a second passing through the thicker phase shift material along the side walls of the opaque light blocking elements. Typically, the second transmission is phase-shifted 180° with respect to the first transmission. 
       SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the invention provide a phase-shifting photomask with a self aligned rim-shifting element and methods for its manufacture. 
         [0006]    One embodiment of the invention provides a method of manufacturing a phase-shifting photomask having a self aligned rim-shifting element, the method comprising: applying a patterning film to a first portion of a transparent substrate; etching the substrate to a depth to remove a second portion of the substrate not beneath the patterning film; etching the first portion of the substrate to undercut an area beneath the patterning film; and removing the patterning film, wherein the etched substrate forms a self-aligned undercut rim-shifting element. 
         [0007]    Another embodiment of the invention provides a photomask comprising: a transparent substrate including: a first portion; a second portion atop the first portion, the second portion including an undercut adjacent the first portion; and a third portion atop the first portion, the third portion including an undercut adjacent the first portion, wherein the first, second, and third portions of the transparent substrate are disposed to provide a plurality of transmission paths through the photomask. 
         [0008]    The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed, which are discoverable by a skilled artisan. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
           [0010]      FIGS. 1-4  show cross-sectional side views of steps in the manufacture of a photomask according to one embodiment of the invention; 
           [0011]      FIGS. 5 and 6  show transmission paths through photomasks according to illustrative embodiments of the invention; 
           [0012]      FIGS. 7 and 8  show photomasks according to other embodiments of the invention, as well as their transmission paths; 
           [0013]      FIGS. 9-11  show steps in the manufacture of a photomask according to another embodiment of the invention; 
           [0014]      FIG. 12  shows transmission paths through the photomask of  FIG. 11 ; and 
           [0015]      FIG. 13  shows a flow diagram of a method according to an embodiment of the invention. 
       
    
    
       [0016]    It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring now to the drawings,  FIG. 1  shows a transparent photomask substrate  10 , on top of which has been deposited a patterning film  20 ,  22 . Suitable transparent substrates will be recognized by one skilled in the art, but may include, for example, quartz (fused silica), modified or doped quartz, and crystalline fluorides, such as calcium fluoride, magnesium fluoride, barium fluoride, and lithium fluoride. Patterning film  20 ,  22  may include, for example, chromium, a refractory metal (e.g., tungsten), a metal silicide (e.g., molybdenum silicide), a metal nitride (e.g., tungsten nitride), amorphous silicon, or amorphous carbon. Patterning film  20 ,  22  may be formed or deposited by any known or later-developed techniques appropriate for the material to be deposited including but not limited to: chemical vapor deposition (CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) and high density plasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD (UHVCVD), limited reaction processing CVD (LRPCVD), metalorganic CVD (MOCVD), sputtering deposition, ion beam deposition, electron beam deposition, laser assisted deposition, thermal oxidation, thermal nitridation, spin-on methods, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical oxidation, molecular beam epitaxy (MBE), plating, and evaporation. After such deposition, the deposited material may be patterned using a mask and etched. 
         [0018]    In  FIG. 2 , substrate  10  has been etched to a first depth D. Substrate  10  may be etched using any known or later-developed technique, such as wet chemical etching. 
         [0019]    In  FIG. 3 , sidewall spacers  30 ,  32 ,  34 ,  36  have been applied to the pedestals  12 ,  14  of transparent substrate  10  beneath patterning film  20 ,  22 . Sidewall spacers  30 ,  32 ,  34 ,  36  may include, for example, quartz, photoresist, or polyimide, and may be applied by any known or later-developed technique, including conformal deposition and anisotropic etching, or selective deposition. It is understood that while four sidewall spacers have been delineated, the spacers may be connected as they extend about the pedestals  12 ,  14 . 
         [0020]      FIG. 4  shows substrate  10  following isotropic etching to form undercuts  40 A,  40 B,  40 C,  40 D of pedestals  12  and  14 . Etching may be discontinued upon reaching a chosen depth D′. Alternatively, as will be described in greater detail below with respect to  FIGS. 7 and 8 , substrate  10  may include a transparent etch stop layer  60  (shown in phantom) at depth D′ to which substrate  10  is etched. In cases where isotropic etching or another etching technique is used without transparent etch stop layer  60 , it typically yields a concavity  42 ,  44 ,  46  along the top surface of substrate  10 . 
         [0021]      FIG. 5  shows a completed photomask  100  according to an embodiment of the invention. Patterning film  20 ,  22  and sidewall spacers  30 ,  32 ,  34 ,  36  have been removed, yielding a photomask  100  having three transmission paths  50 ,  52 ,  56 . First path  50  (and similarly  50 ′) passes through pedestal  12  (and similarly  14 ) of substrate  10  and substrate  10 , yielding a relative phase shift of 0°. Third path  56 , passes through substrate  10  only, yielding a relative phase shift of 180° with respect to first path  50 . Thus, first path  50  and third path  56  yield phase shifts as might be achieved using known phase-shifting photomasks. However, in the embodiment shown in  FIG. 5 , second path  52  (and similarly  52 ′) passes through pedestal  12  (and similarly  14 ), the atmosphere within the undercut ( 40 B in  FIG. 4 ) of pedestal  12  (and similarly  14 ), and substrate  10 , and yields a phase shift intermediate that of first path  50  and third path  56 . In the embodiment shown in  FIG. 5 , the phase shift of second path  52  is 130° with respect to first path  50 . Other phase shifts intermediate those of first path  50  and third path  56  may be achieved by varying, for example, the extent and/or angle to which pedestal  12  is undercut. 
         [0022]      FIG. 6  shows a photomask  200  according to another embodiment of the invention, in which sidewall spacers  30 ,  32 ,  34 ,  36  remain on pedestals  12 ,  14  to yield a fourth transmission path  54  (and similarly  54 ′). Fourth path  54  passes through sidewall spacer  32 , the atmosphere beneath sidewall spacer  32 , and substrate  10 , yielding a phase shift between that of second path  52  and third path  56 . In the embodiment shown in  FIG. 5 , the phase shift of fourth path  54  is 150° with respect to first path  50 . 
         [0023]      FIGS. 7 and 8  show other embodiments of the invention in which a transparent etch stop layer  60  is included in substrate  10  at depth D′. Transparent etch stop layer  60  may include, for example, doped quartz, sapphire, or silicon. As can be seen, photomask  300  of  FIG. 7  yields transmission paths analogous to those of photomask  100  of  FIG. 5 , and photomask  400  of  FIG. 8  yields transmission paths analogous to those of photomask  200  of  FIG. 6 . However, transparent etch stop layer  60  prevents the formation of concavities  42 ,  44 ,  46  ( FIG. 4 ) in the top surface of substrate  10 . 
         [0024]      FIGS. 9-12  show yet another embodiment of the invention. In  FIG. 9 , prior to isotropically etching substrate  10 , an additional photoresist layer  70  is applied atop patterning film  22 , sidewall spacers  34  and  36 , and adjoining portions  16 ,  18  of substrate  10 . In  FIG. 10 , substrate  10  has been etched to the level of transparent etch stop layer  60 , yielding positive sidewalls  80 ,  82 . While substrate  10  is shown in this embodiment as including transparent etch stop layer  60 , as noted above, this is not essential.  FIG. 11  shows the finished photomask  500  with the patterned layer  20 ,  22  and additional photoresist  70  removed. 
         [0025]      FIG. 12  shows light transmission paths through photomask  500 . For the sake of clarity of description, most reference numbers of the photomask components have been removed in  FIG. 12 . The components are the same as, and will be described with respect to, those shown in  FIG. 11 . The photomask  500  shown in  FIG. 12  includes six different light transmission paths  50 ,  52 ,  54 ,  56 ,  57 ,  58 , each yielding a potentially distinct phase shift. The first path  50  (and similarly  50 ′) passes through pedestal  12  (and similarly  14 ), transparent etch stop layer  60 , and substrate  10 , yielding a relative phase shift of 0°. The fourth path  56  passes through transparent etch stop layer  60  and substrate  10 , yielding a relative phase shift of 180° with respect to first path  50 . 
         [0026]    The second path  52  passes through pedestal  12 , the atmosphere within the undercut ( 40 B in  FIG. 4 ) of pedestal  12 , transparent etch stop layer  60 , and substrate  10 , yielding a phase shift X between that of first path  50  and fourth path  56 . The third path  54  passes through sidewall spacer  32 , the atmosphere beneath sidewall spacer  32 , transparent etch stop liner  60 , and substrate  10 , yielding a phase shift X′ between that of second path  52  and fourth path  56 . 
         [0027]    The fifth path  57  passes through positive sidewall  80 , transparent etch stop layer  60 , and substrate  10 , yielding a phase shift X″ greater than that of fourth path  56 . The sixth path  58  passes through sidewall spacer  34 , positive sidewall  80 , transparent etch stop layer  60 , and substrate  10 , yielding a phase shift X″′ greater than that of fifth path  57 . 
         [0028]    As noted above with respect to other embodiments of the invention, removing one or more sidewall spacers  30 ,  32 ,  34 ,  36  would reduce the number of transmission paths. For example, still referring to  FIGS. 11 and 12 , removing sidewall spacer  32  would eliminate third path  54 , which becomes the same as fourth path  56 . 
         [0029]      FIG. 13  shows a flow diagram according to one embodiment of the invention. At S 1 , a patterning film ( FIG. 1 :  20 ,  22 ) is applied to a portion of a transparent substrate ( FIG. 1 :  10 ). At S 2 , the substrate ( 10 ) is etched to a depth ( FIG. 2 : D) to remove a portion of the substrate not beneath the patterning film ( 20 ,  22 ). At S 3 , a sidewall spacer ( FIG. 3 :  30 ,  32 ,  34 ,  36 ) may optionally be applied to an pedestal ( FIG. 3 :  12 ,  14 ) of the substrate ( 10 ) beneath the patterning film ( 20 ,  22 ). At S 4 , an additional photoresist ( FIG. 9 :  70 ) may optionally be applied over the patterning film ( 22 ) and, if applied at S 3 , the sidewall spacer ( 34 ,  36 ). 
         [0030]    At S 5 , the substrate ( 10 ) is etched to undercut ( FIG. 4 :  40 A,  40 B,  40 C,  40 D) a portion of the substrate ( 10 ) beneath the patterning film ( 20 ,  22 ) applied at S 1 . If applied at S 4 , the additional photoresist ( 70 ) is removed at S 6 . The patterning film ( 20 ,  22 ) applied at S 1  is removed at S 7  and, if applied, the sidewall spacer ( 30 ,  32 ,  34 ,  36 ) is removed at S 8 . 
         [0031]    The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.