Abstract:
A method for fabricating a mask which includes a printable contact and/or line area which is aligned with a phase-shifter. The method includes preparing a mask-in-process comprising a substrate underlying a first layer, an opaque layer overlying the first layer, and a first resist material overlying the opaque layer, and subjecting the mask-in-process to a plurality of exposures and at least one etching to create a phase-shifter and to open a printable contact and/or line area surrounded by a second resist material, wherein the printable contact and/or line area is aligned with the phase-shifter.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to optical lithography used for fabricating semiconductor devices, and more particularly to optical lithographic phase-shifting masks and a method for fabricating such masks.  
         BACKGROUND  
         [0002]    Photolithography methods are well-known for producing fine patterns on integrated circuits and other electronic devices. Typically, photosensitive resist material is deposited upon a substrate and a portion of the material is exposed in a predetermined pattern. The pattern is then developed by selective removal or retention, depending upon whether the resist material is a positive or a negative resist.  
           [0003]    Exposure of the resist material is typically accomplished by transmitting light, e.g. ultraviolet light, through a mask. Exposure through a mask causes diffraction, image spreading, and/or other interference effects at the boundaries of opaque areas of the masks. Such effects may cause ghost patterns or lobes in the exposed pattern. This is partly because the masks must be at a distance from the resist material during exposure in order to ensure that the size of the pattern at the resist surface is reduced compared to the size of the pattern in the mask. Until fairly recently, these effects were relatively dimensionally small. However, recent increases in the integration density of integrated circuits has pushed minimum feature sizes of patterns such that the effects are now becoming significant.  
           [0004]    To improve the clarity of the exposure patterns, phase-shifting masks have been developed to limit the image spreading effects. One type of phase-shifting mask, a rim type phase-shifting mask, assists in limiting image spreading in the exposure of features having a closed shape, such as contacts. Another type is a Levenson-type phase-shifting mask, which assists in limiting image spreading in the exposure of periodically repeated patterns, like parallel lines, such as arrays of parallel conductors.  
           [0005]    The fabrication of phase-shifting masks generally has been difficult and expensive due to the need to form extremely small regions having differing optical lengths at the edges of opaque regions. Thus, either patterning must be done within the mask pattern or the opaque regions of the mask must be recessed from the regions of differing optical path length. Examples of known phase-shifting mask fabrication methods can be found in U.S. Pat. No. 5,747,196 (Chao et al.), U.S. Pat. No. 5,633,103 (DeMarco et al.), U.S. Pat. No. 5,532,089 (Adair et al.), and U.S. Pat. No. 5,484,672 (Bajuk et al.). Known phase-shifting mask fabrication methods have had difficulty with ensuring a symmetric exposure of either a printable contact area on the mask or a printable line area on the mask without image spreading effects.  
         SUMMARY  
         [0006]    The invention provides a fabrication process for a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed symmetric to an adjacent phase shifting feature.  
           [0007]    In one aspect, the invention provides a method of forming a mask. The method includes forming a first layer of material over a substrate and forming an opaque layer overlying the first layer of material. The opaque material layer has at least one opening filled with a second material, the second material residing over the first layer of material and defining areas of the first layer of material which are to be removed. The method also includes using the second material as a mask to remove the areas of the first layer of material, and then removing the second material. The result is a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed aligned to an adjacent phase shifting feature.  
           [0008]    In another aspect, the invention provides a method of forming a mask, which includes forming an opaque layer over a substrate, the opaque layer having at least one opening therein filled with a first material, the first material defining areas of the substrate which are to be removed. The method also includes using the first material as a mask to remove the areas of the substrate, and removing the first material. The result is a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed aligned to an adjacent phase shifting feature.  
           [0009]    These and other advantages and features of the invention will be more readily understood from the following detailed description which is provided in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a top view of a mask-in-process in accordance with an embodiment of the invention.  
         [0011]    [0011]FIG. 2 is a cross-sectional view of the mask-in-process of FIG. 1 taken along line II-II.  
         [0012]    [0012]FIG. 3A is a top view like FIG. 1 showing an opening in a first resist layer.  
         [0013]    [0013]FIG. 3B is a cross-sectional view of the mask-in-process of FIG. 3A taken along line IIIB-IIIB.  
         [0014]    FIGS.  4 - 5  are cross-section views, like FIG. 3B, showing subsequent processing steps to the mask-in-process of FIG. 1.  
         [0015]    [0015]FIG. 6A is a top view like FIG. 1 showing a ring of resist material.  
         [0016]    [0016]FIG. 6B is a cross-sectional view of the mask-in-process of FIG. 6A taken along line VIB-VIB.  
         [0017]    FIGS.  7 - 9  are cross-section views, like FIG. 6B, showing subsequent processing steps to the mask-in-process of FIG. 1.  
         [0018]    [0018]FIG. 10 is a cross-sectional view of a mask constructed in accordance with the process steps of FIGS.  1 - 9 .  
         [0019]    [0019]FIG. 11A is a top view of a mask-in-process in accordance with an embodiment of the invention.  
         [0020]    [0020]FIG. 11B is a cross-sectional view of the mask-in-process of FIG. 11A taken along line XIB-XIB.  
         [0021]    FIGS.  12 - 19  are cross-sectional views, like FIG. 10, showing subsequent processing steps to a mask-in-process in accordance with another embodiment of the invention.  
         [0022]    [0022]FIG. 20 is a cross-sectional view of a mask constructed in accordance with the process steps of FIGS.  12 - 19 .  
         [0023]    FIGS.  21 - 22  are cross-sectional views, like FIG. 10, showing alternative subsequent processing steps to a mask-in-process in accordance with another embodiment of the invention.  
         [0024]    [0024]FIG. 23 is a cross-sectional view of a mask constructed in accordance with the process steps of FIGS.  12 - 18  and  21 - 22 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]    The invention is directed to a mask fabrication process which may be used to ensure that printable contact areas and/or printable line areas will be laid symmetrical relative to an adjacent phase-shifter. A ring-like opening is exposed in a mask-in-process which leaves the printable contact area fully covered by an opaque layer. A resist is applied on a top surface and filling in the ring-like opening. The mask-in-process is exposed to ultraviolet light through a lower transparent material which leaves the resist only within the ring-like opening. Another resist is applied on an upper surface and covering the ring-like opening. Selective etching opens up the printable contact areas and/or printable line areas.  
         [0026]    Through this process, openings can be exposed which are larger than the desired final printable contact areas. Thus, possible misalignments can be accounted for during the processing. Further, asymmetric rims can be laid and rims with different rim widths can be laid in different locations.  
         [0027]    FIGS.  1 - 10  illustrate the processing of a mask  50  (FIG. 10) from a mask-in-process  10  in accordance with an aspect of the invention. The mask  50  will include a phase-shifter feature, which may be a rim type phase-shifter, once fabricated. The process described with reference to FIGS.  1 - 10  allows for printable contact areas or printable line areas to be opened up symmetrical to the adjacent phase-shifter feature.  
         [0028]    The mask-in-process  10  includes a substrate  12 , a layer  16 , an opaque layer  22 , and a first resist layer  28 . The substrate  12  is formed of a transparent material, preferably quartz. A portion of the substrate  12  may eventually become either a printable contact area or a printable line area. A portion of the layer  16  adjacent to the printable contact area of the substrate  12  will eventually be fabricated into the phase-shifter. The layer  16  is preferably formed of a material or materials which allow for a one-hundred and eighty degree (180°) phase shift with respect to the open areas of the substrate  12 . The material or materials may be formed of molybdenum-silicide, chromium flouride, silicon nitride-titanium nitride, tantalum silicide, zirconium silicon oxide or other like material. The opaque layer  22  may be formed of chromium or other material suitable for blocking out ultraviolet light.  
         [0029]    The first resist layer  28  is positioned on a first surface  24  of the opaque layer  22 . The opaque layer  22  is itself positioned on the layer  16  such that a second surface  26  of the opaque layer  22  is in contact with a first surface  18  of the layer  16 . The layer  16  is positioned on the substrate  12  such that a second surface  20  of the layer  16  is in contact with a first surface  14  of the substrate  12 .  
         [0030]    As shown in FIG. 2, a first exposure  32  is directed toward portions of the first resist layer  28 . A preferred exposure method utilizes an electron or laser beam  32  from, respectively, an electron beam or laser writing tool  3 , such as, for example, MEBES 4500 or ALTA 3500. The exposed portions of the first resist layer  28  are removed, and the underlying portions of the opaque layer  22  are then etched (FIG. 3). The resist removal and opaque layer etching steps leave openings  30  within the mask-in-process  10 . If, as shown in FIGS.  1 - 10 , a rim type phase-shifter is being fabricated on the mask  50 , the openings  30  are a single ring-shaped opening (FIGS.  3 A- 3 B). The remaining first resist layer  28  is then removed. However, it should be understood that this technique is not limited by feature geometry.  
         [0031]    The opening  30  is then filled with a second resist material  34  (FIG. 4), which covers the entire surface, including the exposed portions of the opaque layer  22 . Preferably, the second resist material  34  is a positive-tone resist which can be made to image reverse to a negative-tone by way of a post-exposure bake process. One such resist material is AZ5200, supplied by Hoechst Celanese Corporation. Alternatively, any negative-tone resist compatible with a positive-tone resist could be used as the second resist material  34 .  
         [0032]    As shown in FIG. 4, a second ultraviolet exposure  36  is directed at the mask-in-process  10 . Unlike the first exposure  32 , the second is a flood exposure  36 , directed through the substrate  12  toward the resist materials  28 ,  34 . The opaque layer  22  serves as a mask to prevent exposure of some of the second resist material  34 , and only the portion of the second resist material  34  within the ring-shaped opening  30  (shown within the opposing dotted lines in FIG. 4) is exposed by the second ultraviolet light  36 . The mask-in-process  10  is then baked for a sufficient period of time to reverse tone and harden the exposed second resist material  34 .  
         [0033]    [0033]FIG. 5 illustrates a third ultraviolet exposure  38  directed at the mask-in-process  10 . The third exposure  38  is an ultraviolet flood exposure directed at the resist material  34 . Since the second resist material  34  within the openings  30  has been reversed tone (to negative) and hardened, the third ultraviolet exposure  38  will only expose the remainder of the second resist material  34 . As shown in FIGS. 6A and 6B, the portions of the second resist material  34  not within the openings  30  are rendered soluble in, e.g. tetramethyl ammonium hydroxide (TMAH), and then removed, leaving only a ring of the second resist material  34 . Alternatively, the mask-in-process  10  may be subjected to a chemical bath to remove the previously unexposed portions of the second resist material  34 .  
         [0034]    A third resist material  40  is overlaid over the opaque layer  22  and the ring of the second resist material  34  (FIG. 7). The third resist material  40  is preferably a positive-tone resist material. To initiate the opening of printable contact areas within the ring of the second resist material  34 , the third resist material  40  is subjected to a lithography step, removing portions of the third resist material  40  to create an opening  42  bounded by the ring of the second resist material  34  (FIG. 8). The lithography step may be done with a larger than necessary opening  42 , so misalignment is not a factor in the lithography step.  
         [0035]    The opening  42  will eventually be extended down to the substrate  12  to become the printable contact area of the mask  50 . Specifically, a portion of the opaque layer  22  and the layer  16  underlying the opening  42  is etched (FIG. 9) to open up a printable contact area  13 . Once the printable contact area  13  is completely etched to the substrate  12 , any remaining second and third resist materials  34 ,  40  are removed (FIG. 10) finishing the mask  50 .  
         [0036]    Through this process, a printable contact area  13  is opened interior to a ring of resist material, thereby ensuring alignment between the printable contact area  13  and the rim type phase-shifter formed by the portion of the layer  16  bounded by the printable contact area  13  and the opaque layer  22 . It is to be understood that the above method is equally capable of creating a mask having printable line areas adjacent to and symmetrical with a Levenson-type phase-shifter. It is further to be understood that asymmetrical printable areas may be created through the above method. Finally, it is to be understood that the phase-shifters of the mask  50  may be zero degrees while the printable contact area  13  may be 180 degrees, or the phase-shifters may be 180 degrees while the printable contact area  13  may be zero degrees, or the phase-shifters and the printable contact area  13  may be somewhere in between zero and 180 degrees.  
         [0037]    With reference to FIGS.  11 - 20 , next will be described an alternative method for forming a mask  150  (FIG. 20) from a mask-in-process  110 . The mask-in-process  110  includes a substrate  112  having a first surface  114 , an opaque layer  122  having a first surface  124  and a second surface  126 , and a resist layer  128 . The opaque layer  122  is positioned relative to the substrate  112  such that the second surface  126  of the opaque layer  122  is in contact with the first surface  114  of the substrate  112 . Further, the resist material  128  is positioned such that it contacts the first surface  124  of the opaque layer  122 . Selective portions of the resist material  128  are exposed and subsequently removed, leaving generally parallel openings  130  and a generally ring-like opening  130 ′ (FIG. 11A).  
         [0038]    The mask-in-process  110  is then etched (FIG. 12). Specifically, portions of the opaque layer  122  underlying the openings  130  and the opening  130 ′ are etched, thereby deepening the openings  130 ,  130 ′ to opening extensions  132  and  132 ′ which are each contiguous with, respectively, the first openings  130 ,  130 ′. Then, the first resist material  128  is completely removed (FIG. 13) and another layer of the first resist material  128  is deposited over the opaque layer  122  and the opening extensions  132  and  132 ′ (FIG. 14). Then a portion  128   a  of the first resist material  128  overlying the opening extensions  132  is exposed. As shown in FIG. 15, an etching process is employed, which causes the opening extensions  132  to be further deepened into the substrate  112 . A portion of the first resist material  128  is left to protect the opening extension  132 ′ during the etching process.  
         [0039]    The remainder of the first resist material  128  is removed from the mask-in-process  110 . After removal of the remaining first resist material  128 , a second resist material  134  is deposited over the opaque layer  122  and within the opening extensions  132 ,  132 ′. The second resist material is similar to the resist material  34  of FIGS.  4 - 9 , in that preferably the second resist material  134  is a positive-tone resist which can be made to image reverse tone as a negative-tone would by way of a post-exposure bake process. By exposing the second resist material  134  through the substrate  112 , baking the mask-in-process  110 , and then exposing the second resist material  134  a second time (the second time not being through the substrate  112 ), portions of the second resist material  134  can be removed to leave a pair of walls of the second resist material  134  within and extending from the opening extensions  132  and a ring within and extending from the opening extension  132 ′ (FIG. 16).  
         [0040]    A third resist material  140  is then deposited over the opaque layer  122  and the second resist material  134 . The third resist material  140  is subjected to a lithography step (FIG. 17). The openings may be larger than necessary, since overlay misalignment is not an issue. After the addition of the third resist material  140 , portions of the opaque layer  122  which are bounded by the second resist material  134  are removed (FIG. 18).  
         [0041]    A fourth resist material  142  is then added and patterned through another lithography step to leave an opening bounded by the ring of the second resist material  134 . Again, the openings may be larger than necessary, since overlay misalignment is not an issue. The portion of the substrate  112  underlying the opening bounded by the ring of the second resist material  134  is etched leaving an opening  144  within the substrate  112  (FIG. 19), the base of which is a printable contact area  113 ′. Finally, as shown in FIG. 20, the resist materials  134 ,  140 ,  142  are all removed, leaving the mask  150 , and opening up printable line areas  113  and the printable contact area  113 ′.  
         [0042]    The resulting mask  150  includes printable line areas  113  with a phase shifter area between the areas  113 . The mask  150  further includes a printable contact area  113 ′ within a surround phase shifter area.  
         [0043]    With reference to FIGS.  21 - 23 , an alternative set of processing steps are described. After processing a mask-in-process  110  as shown in FIGS.  12 - 18 , the portions of the substrate  112  underlying the openings bounded by the second resist material  134  are etched leaving openings  144  and  144 ′, the bases of which are, respectively, printable contact areas  113 ′ and  113 ″. The fourth resist material  142  is then added and patterned through another lithography step to leave an opening bounded by the ring of the second resist material  134 . Again, the openings may be larger than necessary, since overlay misalignment is not an issue. An etching process is then performed on the opening  144 ′, creating an opening  146  which is deeper than and bounded by the openings  132  (FIGS. 15, 23). Finally, as shown in FIG. 23, the resist materials  134 ,  140 ,  142  are all removed, leaving the mask  150 , and opening up printable line areas  113  and the printable contact area  113 ′.  
         [0044]    While the foregoing has described in detail preferred embodiments known at the time, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.