Abstract:
An improved method of forming a semiconductor device including an interconnect layer formed using multilayer hard mask comprising metal mask and dielectric mask is provided. To form the second opening pattern being aligned to the first pattern, after the multilayer hard mask is used at the first step, then the dielectric mask is used to form a damascene structure in an insulator layer at the second step followed by removing the metal mask.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to methods of making integrated circuits. More specifically it relates to methods of fabricating interconnect structures in semiconductor devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    The need for lower resistance and capacitance in interconnect dielectric films caused by the ever-increasing miniaturization of semiconductor devices has led to the use of copper to form interconnects and vias rather than aluminum. When those structures are formed from copper a dual damascene process is typically used, in view of the difficulty in dry etching copper. 
         [0003]    As the line width of interconnects continues to decrease, additional measures must be taken to guarantee the reliability of damascene interconnects that include trenches and vias. Brain et. Al., “Low-k Interconnect Stack with a Novel Self-Aligned Via Patterning Process for 32 nm High Volume Manufacturing,” IITC2009, session 13.1 (pp. 249-251), discloses a hardmask process for making the tightest pitch layers in the interconnect stack, to enable production of Self-Aligned Vias (SAV). In this and other conventional dual damascene processes, the vias are first created in the ILD, followed by the trenches, and then the vias and trenches are lined with a metallic Cu barrier and then filled with bulk Cu, followed by planarization. 
         [0004]    U.S. Pat. No. 7,067,919 discloses a damascene interconnect method in which a metal mask having a trench pattern is formed on an oxide film overlying an interconnect film. The via pattern is defined in a layer of photo resist overlying the metal mask, and the interconnect film is etched to form the vias. After via formation, the photo resist film is removed and the trenches are formed using the metal mask, followed by filling the trenches and vias with copper. 
         [0005]    U.S. Pat. No. 7,524,752 discloses removing a metal mask after forming trenches and vias, filling the trenches and vias with metal, followed by chemical mechanical processing (CMP). Dimensional variation that could occur if the metal mask were removed by CMP after filling the trenches and vias is said to be reduced. 
         [0006]    The present inventor has however found that these techniques have various problems. Low-k film is typically used for the interconnect dielectric layers, so as to reduce unwanted interlayer capacitance. On the other hand, a metal mask is used to form an opening such as a via or through hole or trench, in order for the feature to be self-aligned. However, when using metal mask to form fine patterns, there is a difference in the stresses between the low-k film and the metal mask, which causes strain at the interfaces between these layers, and which makes it difficult to obtain a desired pattern with a high degree of precision. 
       SUMMARY 
       [0007]    The present invention provides novel methods for making semiconductor devices, in which a first pattern is formed in both a metal film and an underlying dielectric layer that overlie an insulator layer above a semiconductor substrate. A second pattern is formed in the insulator layer, the second pattern being defined at least in part by a first mask positioned above the metal film and the dielectric layer. The metal film is then removed, and the first pattern is transferred to the insulator layer using the dielectric layer as a second mask. The first pattern preferably extends into the insulator layer to a depth different than that of the second pattern in the insulator layer. 
         [0008]    In preferred embodiments, the second pattern is defined by the overlapping profiles of the mask and the first pattern in the metal film. The first mask may comprise a patterned layer of photoresist. 
         [0009]    The first pattern preferably comprises a series of elongated openings arranged generally parallel to one another, corresponding to trenches to be formed in the insulator layer. The second pattern preferably comprises an array of openings corresponding to vias to be formed through the insulator layer. In such embodiments the first pattern is transferred to the insulator layer as a series of trenches whose depth is less than the thickness of the insulator layer, and the second pattern is formed in the insulator layer as an array of vias that preferably pass entirely through the insulator layer. 
         [0010]    The trenches and vias formed according to these embodiments of the invention are preferably then filled with copper by a damascene method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
           [0012]      FIGS. 1A and 1B  are cross-sectional and top views respectively, through a semiconductor device at a first stage of processing according to a preferred embodiment of the method according to the present invention; 
           [0013]      FIGS. 2A and 2B  are cross-sectional and top views respectively, through the semiconductor device of  FIGS. 1A and 1B  at a subsequent stage of processing; 
           [0014]      FIGS. 3A and 3B  are cross-sectional and top views respectively, through the semiconductor device of  FIGS. 2A and 2B  at a subsequent stage of processing; 
           [0015]      FIGS. 4A and 4B  are cross-sectional and top views respectively, through the semiconductor device of  FIGS. 3A and 3B  at a subsequent stage of processing; 
           [0016]      FIGS. 5A and 5B  are cross-sectional and top views respectively, through the semiconductor device of  FIGS. 4A and 4B  at a subsequent stage of processing; 
           [0017]      FIGS. 6A and 6B  are cross-sectional and top views respectively, through the semiconductor device of  FIGS. 5A and 5B  at a subsequent stage of processing; 
           [0018]      FIG. 7A  is a cross sectional view, through the semiconductor device of  FIG. 6A  at a subsequent stage of processing; 
           [0019]      FIG. 7B  is a cross sectional view, through the semiconductor device of  FIG. 7A  at a subsequent stage of processing; 
           [0020]      FIG. 8  is an overall schematic cross sectional view of a semiconductor device according to an embodiment of the present invention; 
           [0021]      FIGS. 9A through 9D  show the chemical formulae of exemplary materials that may be utilized in preferred embodiments of the present invention; 
           [0022]      FIG. 10  show the chemical formula of a further exemplary material that may be utilized in preferred embodiments of the present invention; 
           [0023]      FIGS. 11A and 11B  conceptually illustrate the improved dimensional tolerances that may be achieved by preferred embodiments of the present invention; 
           [0024]      FIGS. 12A and 12B  are pictures of experimental results corresponding to  FIGS. 11A and 11B ; and 
           [0025]      FIG. 13  illustrates measured etching selectivity of embodiments according to the invention in relation to a conventional technique. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
         [0027]    In  FIG. 1   a , an insulator layer  173  has been formed on a substrate  100 , and dielectric layer  175  is formed on the insulator layer  173 . Metal film  178  is formed on the dielectric layer  175 , and overlying metal film  178  is a photoresist tri-mask composed of photoresist layer  188 , SiARC layer  184  (Si-based Anti-Reflection Coating layer) or LTO (Low Temperature Silicon Oxide) and an organic layer  181 , which is preferably an organic planarization layer (OPL), which acts like an unexposed resist. 
         [0028]    As can be seen in  FIG. 1   b , photoresist layer  188  bears a trench pattern, with the openings in the pattern exposing the underlying SiARC layer  184 . 
         [0029]    The semiconductor device beneath the interconnect layers described above is in this example a transistor  110 , which includes device isolation regions  106 , insulator layer  130  and contact holes  135  formed on a substrate  100 , such as silicon substrate. Another interconnect layer  160  including interconnect  165  may overlie the insulator layer  130 . Etch stop film  170 , for example a silicon and nitrogen-containing film, is formed on the interconnect layer  160 . 
         [0030]    The insulator layer  173  is formed on the etch stop film  170 . Dielectric layer  175  and metal film  178  are formed in that order as a hard mask layer on the insulator layer  173 . 
         [0031]    The insulator layer  173  may include a porous SiOCH material. SiO 2  is preferably used as the dielectric layer  175 , although SiC, SiN or SiCN may also be used to form the dielectric layer  175 . Metal film  178  may include TiN, TaN or WN. As it is desirable to remove this dielectric layer  175  selectively from the insulator layer  173  when forming a mask, the carbon content in the insulator layer  173  maybe more than 40 atomic percent, as shown in the  FIG. 13 . 
         [0032]    Turning now to  FIGS. 2A and 2B , Si-ARC layer  184 , OPL layer  181 , metal film  178  and dielectric layer  175  are dry-etched so as to expose the insulator layer  173 , through photo resist mask  188 . After removal of the residual tri-mask layers e.g. by ashing, the metal film  178  and underlying dielectric layer  175  now bear the pattern transferred from mask  188 , exposing the underlying insulator layer  173  through the trench openings (see  FIG. 2B ). 
         [0033]    When the metal film  178  includes TiN, TaN or WN, carbon fluoride-based gases, such as CF 4 /C 4 F 8 /Ar/N 2 /CO, are preferably utilized to etch the metal film  178 . According to the present embodiment, the dry-etching selectivity between the insulator layer  173  and the dielectric layer  175  can be controlled to be in a range from 5-20 by choosing the materials and the etching condition. 
         [0034]    When the dielectric layer  175  is made of SiO 2 , the main etching gas for etching the dielectric layer will be selected from O 2 , N 2 , H 2 , N 2 /H 2 , NH 3 , CO and CO 2 . To avoid residual SiO, at least one additional gas selected from CHF 3 , CH 2 F 2 , C 4 F 8 , CHF 3 , CF 3 I, CF 4  and NF 3  is preferably added to the main etching gas. The ratio of additional gas to main gas may be 0-20 volume %, preferably 5-10 volume %. To obtain a high selectivity, O 2 /CH 2 F 2  is preferably used for this etching step. To reduce the damage to the insulator layer  173 , N 2 /CH 2 F 2 , N 2 /H 2 /CH 2 F 2  or CO 2 /CO/CH 2 F 2  is preferably used for this etching step. 
         [0035]    When the dielectric layer  175  is made of SiC, SiN or SiCN, the same gases as for SiO 2  may be used. It is also possible to use etching gas selected from O 2 /C 4 F 8 , N 2 /C 4 F 8 , N 2 /H 2 /C 4 F 8  and CO 2 /CO/C 4 F 8 . C 4 F 8  as additional gas may be added in a quantity of 0-20 volume %. Ar may be added to generate plasma. 
         [0036]    The etching chamber pressure is preferably set to about 6.7 Pa (50 mT) and the bias power is preferably set on source power of about 500 W and bias power of about 100 W, for example. 
         [0037]    Next, as shown in  FIGS. 3A and 3B , the via lithography is performed. A tri-layer via mask is composed of new OPL layer  182  formed on the metal film/mask  178 , new SiARC layer  185  and new resist film  189 . As shown in  FIG. 3B , resist film  189  has openings defining a via pattern, exposing the underlying SiARC layer  185 . The broken lines in  FIG. 3B  show the location of the underlyinh trench openings in the metal mask  178  and dielectric layer  175 , from which it can be seen that the vias to be formed in the insulator layer  173  will be defined in part by photo resist mask  189  and in part by the trench hardmask  178 / 175 , which promotes self-alignment of the vias to the trenches. 
         [0038]    The via tri-mask is then dry etched as described above in connection with the trench lithography. In this step, however, etching is continued to actually form the vias  190  extending through dielectric layer  173  and reaching etch stop layer  170 , as shown in  FIG. 4A . After removal of the residual photo resist  189 , SiARC layer  185  and the OPL layer  182  by ashing, the plan view of  FIG. 4B  is again of metal film  178 , but now through its openings there can be seen the partially etched insulator layer  173  as well as the regions of etch stop layer  170  that are exposed at the bottom of the vias  190 . 
         [0039]    At this stage of the process according to the present embodiment, the metal layer  178  is removed from the dielectric layer  175  by dry etching or wet etching, as shown in  FIGS. 5A and 5B . The trench pattern that had been transferred to metal layer  178  is preserved in the dielectric layer  175 , as shown in  FIG. 5B , but the trenches themselves are not yet formed in the insulator layer  173 . 
         [0040]    Cl 2  may be used as an etching gas to remove the metal film  178  from the dielectric layer  175 , because it has good selectivity for the metal film when TiN is used for the metal film  178 . Alternatively or in addition, H 2 O 2  or a mixed solution of H 2 O 2  and an alkaline additive may be used as a wet etching solution when TiN is used for the metal film  178 . 
         [0041]    Referring to  FIGS. 6A and 6B , trenches  186  are then formed in the insulator layer  173 , using dielectric layer  175  as a mask. This etching serves also to remove the etch stop film  170  exposed at the bottoms of the vias  190 , to reveal the underlying interconnects  165 . Trenches  186  and vias  190  are thus aligned, as shown in  FIG. 6B . 
         [0042]    When the metal film  178  is etched by dry-etching, it is possible to perform the above-described process in the same dry-etching chamber, from the step of patterning the photoresist  188  through formation of the trenches in the insulator layer  173 . However, it is also possible to use equipment having multiple chambers through which the device is transported between the steps of patterning photoresist  188  and forming the trench pattern in insulator layer  173 . 
         [0043]    After forming trenches  186  in the insulator layer  173 , a wet clean process may be performed using conventional cleaning solutions such as dilute hydrofluoric acid or an organic amine solution. Next, as shown in  FIG. 7A , a barrier film  177  such as TaN is formed on the insulator layer  173 . Then, a layer  180  of a metal, preferably Cu, is formed by plating following seed metal PVD, for example, and excess metal film is removed by CMP, as shown in  FIG. 7B . 
         [0044]      FIG. 8  schematically depicts an overall semiconductor device as may be formed by the methods according to the present invention. A variety of interconnects  155  are depicted, each composed of at least one via  140  and at least one trench  150  formed in an insulator layer  173 . A next interconnect layer comprises etch stop layer  192  and insulator layer  194  formed on the insulator layer  173  (after forming the interconnects  155 ). Interconnect  196  is formed in the insulator layer  194  in the same way as the interconnect  155 . A semiconductor device having the multilayer interconnects illustrated in the  FIG. 8  is formed by repeatedly forming interconnects as described above. 
         [0045]      FIGS. 9A-9D  illustrate examples of compounds that form porous materials that are well-suited for use as the insulator layers  173 ,  194 , etc. These compounds are ring shaped organo-siloxanes. It is also possible to use MPS (Molecular Pore Silica), which is a material formed by mixing ring shaped organo-siloxane with a compound as illustrated in  FIG. 10 . These technologies are disclosed for example in U.S. Published Patent Appln. No. 2010/0219512, the entirety of which is hereby expressly incorporated by reference. 
         [0046]      FIG. 11A  shows schematically and for purposes of comparison the effect when trenches  186  are formed with the metal film  178  still in place, as is conventional.  FIG. 11B  shows by contrast when the metal film  178  is removed prior to forming trenches  186  in insulator film  173 , as per various embodiments of the present invention. Low-k film such as porous SiOCH is usually used for the insulator layer  173 . The stress difference between metal film  178  and the insulator layer  173  becomes more critical as the pattern size is reduced. This stress difference causes pattern “wigging” or “flop over”, as shown in  FIGS. 11A and 12A . 
         [0047]    On the other hand, in the  FIG. 11B , the metal mask  178  is removed before forming the trenches  186  in the insulator layer  173 , using only dielectric layer  175  as a mask. In this case, the pattern “wigging” or “flop over” does not occur, even if the pattern size is reduced.  FIG. 12B  shows an example of these results. Fine trench patterns are formed without “wigging” or “flop over.” 
         [0048]    Additional advantages of these embodiments include that, as the metal mask  179  has not remained on the dielectric film mask  176  during etching the trenches  186 , the aspect ratio of the interconnect  155  is reduced when filling the metal into the via and trench by plating or PVD. This enables filling the trenches and vias with less chance of voids. 
         [0049]    Furthermore, as a top corner of the dielectric layer  175  is rounded during the etching of trenches  186 , when forming the PVD barrier or seed films before copper plating, overhang at the top corner of the dielectric pattern is reduced. Conventional methods typically require a separate etching step to ensure this rounded shape at the top corner of the dielectric pattern, which damages the surface of the insulator layer  173 . The embodiments described above provide the desired contour without an extra etching step and hence without the attendant damage to the insulator layer  173 . 
         [0050]      FIG. 13  shows results of the measured etching selectivity between the insulator layer  173  and the dielectric layer  175  or the metal film  178  as a function of the carbon content of the insulator layer  173 . In particular, the etching selectivity relationship between the insulator layer  173  and the metal film  178  is almost constant when changing the carbon contents in the insulator layer  173 . On the other hand, the carbon content of the insulator layer  173  significantly affects the etching selectivity between a silicon dioxide-containing dielectric layer  175  and the insulator layer  173 . To ensure an etching selectivity more than 5 between the insulator layer  173  and the dielectric layer  175 , a porous SiOCH material which includes more than 40 atomic % carbon is preferred for use as the insulator layer  173 . 
         [0051]    The embodiments of the present invention were described above with reference to the drawings. However, these embodiments are illustrative of the present invention and it is possible to adopt various configurations other than those described above.