Abstract:
A via-first dual damascene process is disclosed. When forming trench lines directly above two small pitched, dense via openings having diameter that is substantially equal to the line width of the trench lines, the trench photoresist is biased on the via openings to partially mask the sidewalls of the two dense via openings. By doing this, via-to-via bridging defects can be avoided.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to copper interconnects. More particularly, the present invention relates to a via-first dual damascene process capable of avoiding bridging defects. 
   2. Description of the Prior Art 
   Damascene processes incorporated with copper interconnect technique are known in the art, which are also referred to as “copper damascene processes” in the semiconductor industry. The continuous miniaturization of copper chip wiring and consequently shrinkage of line width and via size/space poses significant challenges. 
   In a via-first approach, the vias are defined first in the inter-layer dielectric, followed by patterning the trenches. The sequence of forming the damascene recesses in the via-first approach begins by exposing the via patterns with the first mask. After etching the vias completely through the entire dielectric stack (except not through the barrier layer at the bottom of the dielectric stack) and stripping the resist, a second mask is used to pattern the trenches. The trenches are then created by etching the dielectric down to the embedded etch-stop layer. 
   Typically, the barrier layer at the bottom of the vias is protected from further etching during the trench-etch by a resist layer that floods the vias. After the resist is stripped and the etch-stop layer at the bottom of the via is removed by dry-etching, the metal that fills both the vias and the trenches can be deposited. After deposition, it is polished back to create the dual-damascene structure. 
     FIGS. 1–5  are schematic, cross-sectional diagrams showing several typical intermediate phases of a semiconductor wafer during the via-first dual damascene process according to the prior art method. As shown in  FIG. 1 , conductive structures  111  and  112  such as damascened copper wirings are provided in a device layer  101  of a semiconductor substrate  100 . A capping layer  115  such as silicon nitride is deposited to cover the exposed conductive structures  111  and  112 , and the device layer  101 . A dielectric stack  120  is then deposited on the capping layer  115 . The dielectric stack  120  is composed of a first dielectric layer  121 , a second dielectric layer  123 , and an etch stop layer  122  interposed between the first dielectric layer  121  and the second dielectric layer  123 . A silicon oxy-nitride layer  130  is then deposited on the first dielectric layer  121 . 
   A first photoresist layer  140  having via openings  141  and  142  is formed on the silicon oxy-nitride layer  130 , assuming that the via opening  141  is an isolated via pattern, i.e. there is no other via opening located in the proximity of the via opening  141 , and the via opening  142  is a dense via pattern. Using the first photoresist layer  140  as a etching mask, an etching process is performed to etch away, in the order of, the silicon oxy-nitride layer  130 , the dielectric stack  120 , to the capping layer  115 , through the via openings  141  and  142 , thereby forming via holes  151  and  152   a/b.    
   As shown in  FIG. 2 , after stripping the first photoresist layer  140  off the silicon oxy-nitride layer  130 , a gap-filling polymer (GFP) layer  200  is coated on the semiconductor substrate  100  and fills the via holes  151  and  152   a/b . The GFP layer  200  is typically composed of resist materials known in the art. Coating of the GFP layer  200  is known in the art and an additional post-baking step may be carried out if desired. 
   As shown in  FIG. 3 , the GFP layer  200  is then etched back to a predetermined depth so as to form plug  201  in the isolated via hole  151  and plugs  202   a / 202   b  in the dense via holes  152 . The top surface of the plugs  201  and  202   a/b  is lower than the top surface of the silicon oxy-nitride layer  130 , forming recesses  301 ,  302   a  and  302   b . As shown in  FIG. 4 , a second photoresist layer  400  is coated on the semiconductor substrate  100  and fills the recesses  301  and recesses  302   a/b  using methods known in the art such as spin coating. 
   As shown in  FIG. 5 , following the coating of the second photoresist layer  400 , a lithographic process is carried out. The exposed second photoresist layer  400  is developed using a proper developer. Trench  411  is formed above the recess  301 , trench  412   a  is formed directly above the recess  302   a , and trench  412   b  is formed directly above the recess  302   b.    
   Please refer to  FIG. 6  and briefly back to  FIG. 5 , wherein  FIG. 6  is a plan view of the via holes and trench patterns of the second photoresist layer  400  of  FIG. 5 , and  FIG. 5  is a cross-sectional view taken along line I—I of  FIG. 6 . As shown in  FIGS. 5 and 6 , the line width L of the trench  412   a  is equal to the diameter of the underlying via hole  152   a . Likewise, the line width of the trench  412   b  is equal to the diameter of the underlying via hole  152   b . The line width of the trench pattern  411  is larger than the diameter of the underlying via hole  151 . 
   One drawback of the above-described prior art method is that when etching trench lines into the dielectric stack  120  in the following trench forming step, the exposed first dielectric layer  121  of the dielectric stack  120  in the recesses  301 ,  302   a  and  302   b  are also laterally etched. Since the via  152   a  and via  152   b  are very close to each other, such lateral etch of the exposed first dielectric layer  121  between the dense via  152   a  and  152   b  usually causes bridging defect after copper CMP. 
   SUMMARY OF THE INVENTION 
   It is the primary object of the present invention is to provide an improved via-first dual damascene process to alleviate or eliminate the via-to-via bridging problem. 
   To achieve the above object, a via-first dual damascene process is provided. The via-first dual damascene process includes the following steps: 
   providing a semiconductor substrate having a dielectric layer deposited over the semiconductor substrate, wherein the dielectric layer has a via opening; 
   filling the via openings with a gap-filling polymer to form a gap-filling polymer (GFP) layer on the dielectric layer; 
   etching the GFP layer back to a predetermined depth to form a GFP plug in the via opening, wherein an exposed surface of the GFP plug is lower than a top surface of the dielectric layer, thereby forming a recess above the via opening; 
   coating a photoresist layer over the dielectric layer, the photoresist layer filling the recess; 
   performing a lithographic process to form a trench line pattern in the photoresist layer above the via opening, wherein the trench line pattern has a first section that has a substantially constant line width L and does not overlap with the via opening, and a second section that is directly above the via opening and has a tapered line width smaller than L, wherein the line width L is substantially equal to diameter of the via opening; and 
   etching the dielectric layer and the GFP layer through the trench line pattern using the photoresist layer as an etching mask. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  to  FIG. 5  are cross-sectional schematic diagrams showing several typical intermediate phases of a semiconductor wafer during the via-first dual damascene process according to the prior art method; 
       FIG. 6  is a top view of  FIG. 5 ; 
       FIG. 7  to  FIG. 12  are cross-sectional schematic diagrams illustrating the via-first dual damascene process according to one preferred embodiment of this invention; 
       FIG. 13  is a plan view of the via holes previously formed in the dielectric stack and trench patterns of the second photoresist layer of  FIG. 11 ; 
       FIG. 14  depicts the layout of a photo mask for printing a pattern of photoresist corresponding to the interconnection area depicted in  FIG. 13 ; and 
       FIG. 15  is a plan view of the dual-damascene structures of  FIG. 12 . 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 7  to  FIG. 12 .  FIG. 7  to  FIG. 12  are schematic, cross-sectional diagrams illustrating the via-first dual damascene process according to the preferred embodiment of this invention. As shown in  FIG. 7 , a semiconductor substrate  700  is provided. Conductive structures  711  and  712  such as damascened copper wirings are provided in a device layer  701  of the semiconductor substrate  700 . The device layer  701  may be a low-k dielectric, but not limited thereto. Subsequently, a capping layer  715  such as silicon nitride is deposited to cover the exposed conductive structures  711  and  712 , and the device layer  701  over the semiconductor substrate  700 . 
   Likewise, a dielectric stack  720  is formed on the capping layer  715 . The dielectric stack  720  is composed of a first dielectric layer  721 , a second dielectric layer  723 , and an etch stop layer  722  interposed between the first dielectric layer  721  and the second dielectric layer  723 . Preferably, both of the first dielectric layer  721  and the second dielectric layer  723  have a dielectric constant of less than 3.0. For example, suitable low-k material for the first dielectric layer  721  and the second dielectric layer  723  may be selected from the group including, but not limited to, FLARE™, SiLK™, poly(arylene ether) polymer, parylene, polyimide, fluorinated polyimide, HSQ, BCB, FSG, silicon dioxide, and nanoporous silica. 
   Still referring to  FIG. 7 , a silicon oxy-nitride layer  730  is then deposited on the first dielectric layer  721 . A first photoresist layer (Via Photo)  740  having via openings  741  and  742  is formed on the silicon oxy-nitride layer  730 , assuming that the via opening  741  is an isolated via pattern, i.e. there is no other via opening located in the proximity of the via opening  741 , and the via openings  742  are dense via pattern. Using the first photoresist layer  740  as an etching mask, an etching process is performed to etch away, in the order of, the silicon oxy-nitride layer  730 , the stacked layer  720 , to the capping layer  715 , through the via openings  741  and  742 , thereby forming deep via holes  751 ,  752   a  and  752   b . The average diameter of via holes  751 ,  752   a  and  752   b  is about 0.08–0.2 micrometers. 
   As shown in  FIG. 8 , the first photoresist layer  740  is stripped off from the silicon oxy-nitride layer  730  by methods known in the art such as oxygen plasma ashing. A gap-filling polymer (GFP) layer  800  is then coated on the semiconductor substrate  700  and fills the via holes  751 ,  752   a  and  752   b . The GFP layer  800  may be composed of an i-line resist such as novolak, poly hydroxystyrene (PHS) or acrylate-based resins. Spin coating of the GFP layer  800  is known in the art and optional post-baking step may be carried out if desired. 
   As shown in  FIG. 9 , the GFP layer  800  is then etched back to a predetermined depth so as to form GFP plugs  801 ,  802   a  and  802   b  within the via holes  901 ,  902   a  and  902   b , respectively. The top surface of the GFP plugs  801 ,  802   a  and  802   b  is lower than the surface of the silicon oxy-nitride layer  730 , thereby forming recesses  901 ,  902   a  and  902   b . The recesses  901 ,  902   a  and  902   b  are defined by the respective sidewalls  911 ,  912   a  and  912   b  and the corresponding exposed top surfaces of the GFP plugs  801 ,  802   a  and  802   b . As shown in  FIG. 10 , a second photoresist layer (Trench Photo)  1000  is coated on the semiconductor substrate  700  and fills the treated recesses  901 ,  902   a  and  902   b  using methods known in the art such as spin coating. 
   As shown in  FIG. 11 , following the coating of the second photoresist layer  1000 , a photolithographic process is carried out. In the photolithographic process (or trench photo-lithographic process), a photo mask having a predetermined trench pattern thereon (shown in  FIG. 14 ) is provided, which is positioned over the semiconductor substrate  700  in an exposure tool. Light such as deep UV is projected on the photo-mask and passes through clear areas of the photo-mask to irradiate the underlying second photoresist layer  1000 , thereby forming latent trench images, which is soluble in a developer, over the respective recesses  901 ,  902   a  and  902   b  in the second photoresist layer  1000 . Thereafter, the exposed second photoresist layer  1000  is developed using a proper developer. The latent trench images are removed to form trench patterns  1011 ,  1012   a  and  1012   b  directly above the recesses  901 ,  902   a  and  902   b , respectively. 
   It is the main feature of the present invention that after development the sidewalls  912   a  and  912   b  of the neighboring recesses  902   a  and  902   b  are partially masked and protected by the second photoresist layer  1000 , and are thus not exposed to etchant used in the subsequent trench etch step. 
   Please now refer to  FIG. 13  and briefly back to  FIG. 11 , wherein  FIG. 13  is a plan view of the via holes previously formed in the dielectric stack  720  and trench patterns of the second photoresist layer  1000  of  FIG. 11 , and  FIG. 11  is a cross-sectional view taken along line II—II of  FIG. 13 . As shown in  FIGS. 11 and 13 , the line width of the trench pattern  1011  is larger than the diameter of the underlying via hole  751 . According to the preferred embodiment, each of the trench patterns  1012   a  and  1012   b  includes a first section  1200  that does not overlap with the underlying via hole and has a substantially constant line width of L, and a tapered second section  1300  that is situated directly above the via hole thereof and has a tapered line width that is less than L. 
   The layout of the photo mask for printing a pattern of photoresist corresponding to the interconnection area depicted in  FIG. 13  is illustrated in  FIG. 14 . The photo mask  500  includes a dark region  525 , and bright line region  511  for printing trench  1011  in the second photoresist layer  1000 , a bright line region  512   a  for printing trench  1012   a , and a bright line region  512   b  for printing trench  1012   b . The line width of the bright line region  512   a  is equal to the diameter of the underlying via hole  752   a  and line width of the bright line region  512   b  is equal to the diameter of the underlying via hole  752   b . The via hole  752   a  and via hole  752   b  are close to each other (i.e., small pitched, dense via holes). The bright line region  512   a  is biased with a pair of dark regions  532   a  at the area that is directly above the via hole  752   a . The bright line region  512   b  is biased with a pair of dark regions  532   b  at the area that is directly above the via hole  752   b.    
   According to the preferred embodiment, the biasing dark regions  532   a  and  532   b  are equal in size, and each of which is defined by a width w and length l. Preferably, the length l of the biasing dark regions  532   a  and  532   b  is equal to or greater than the diameter of the via hole, and the width w is about 5%–30% of the length l. For example, for a via hole with a diameter of about 0.2 micrometers, the dimension of each biasing dark region will be 200 nanometers (minimum length)×10–60 nanometers (width). 
   As shown in  FIGS. 12–13 , using the patterned second photoresist layer  1000  as an etching hard mask, a dry etching process is carried out to etch trenches into the first dielectric layer  721  through the trench patterns  1011 ,  1012   a  and  1012   b  directly above the recesses  901 ,  902   a  and  902   b , respectively. After the resist is stripped and the etch-stop layer at the bottom of the via is removed by dry-etching, the metal that fills both the vias and the trenches can be deposited. After deposition, it is polished back to create the dual-damascene structures  1410 ,  1412   a  and  1412   b . The dual-damascene structure  1410  comprises via plug  1401 . The dual-damascene structure  1412   a  comprises via plug  1402   a . The dual-damascene structure  1412   b  comprises via plug  1402   b.    
     FIG. 15  is a plan view of the dual-damascene structures  1410 ,  1412   a  and  1412   b  of  FIG. 12 . As shown in  FIG. 15 , each of the dual-damascene structures  1412   a  and  1412   b  includes a first section  1600  that does not overlap with the underlying via plug and has a substantially constant line width of L, and a notched second section  1700  that is situated directly above the via hole thereof. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.