Patent Publication Number: US-6703709-B1

Title: Structures formed using silicide cap as an etch stop in multilayer metal processes

Description:
This application is a division of application Ser. No. 09/146,744 filed Sep. 3, 1998 now U.S. Pat. No. 6,117,793. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a layered trace configuration which prevents the formation of metal polymer residues and allows for removal of oxide polymer residues from a via with substantially no damage to the via or underlying structures carried on a semiconductor substrate. 
     2. State of the Art 
     Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. One commonly used technique in the fabrication of integrated circuits involves stacking of multiple layers of active and passive components one atop another to allow for multilevel electrical interconnection between devices formed on each of these layers. This multilevel electrical interconnection is generally achieved with a plurality of metal-filled visa (“contacts”) extending through dielectric layers which separate the component layers from one another. These visa are generally formed by etching through each dielectric layer by etching methods known in the industry, such as plasma etching and reactive ion etching. A fluorinated gas, such as CF 4 , CHF 3 , C 2 F 6 , CH 2 F 2 , SF 6 , or other freons, and mixtures thereof, in combination with a carrier gas, such as Ar, He, Ne, Kr, O 2 , or mixtures thereof, are usually used as the etching gas for these etching methods. A problem with such etching methods is that at least one layer of residue forms in the visa as a result of the etching process. 
     An exemplary method for forming a via through a dielectric layer is illustrated in FIGS. 10-13. It should be understood that the figures presented in conjunction with this description are not meant to be actual cross-sectional views of any particular portion of an actual semiconductor device, but are merely idealized representations which are employed to more clearly and fully depict the process of this typical method than would otherwise be possible. 
     FIG. 10 illustrates an intermediate structure comprising a semiconductor substrate  200  bearing a dielectric or insulating layer  202  (such as an oxide-silicon dioxide, etc.) having a metal-containing trace or pad  204  of aluminum, copper, aluminum/copper alloys, or the like, formed thereon. The term “semiconductor substrate” is used herein to denote any solid semiconductor surface, such as is provided by a silicon or gallium arsenide wafer, or a layer of such material formed on glass, ceramic, sapphire, or other supporting carrier, as known in the art, and includes such semiconductor surfaces bearing an insulating layer thereon. The term “trace” is used herein to denote any metallized structure in a semiconductor device including, but not limited to, conductive traces and conductive pads. 
     A barrier layer  206  (such as titanium nitride) is deposited over the metal-containing trace or pad  204  and an interlayer dielectric  208  (such as silicon dioxide) is disposed over the barrier layer  206 . As shown in FIG. 11, the interlayer dielectric  208  is masked with a resist material  212 , which is then patterned to define a via location. A partial via  214  is then selectively etched with a fluorinated gas down to the barrier layer  206 , which acts as an etch stop. The etching of the partial via  214  results in a first residue layer  216  of a carbon-fluorine based polymer containing residue of the interlayer dielectric  208  (“oxide polymer”) coating the sidewall  218  of the partial via  214 , as shown in FIG.  12 . 
     The barrier layer  206  at the bottom of partial via  214  is then etched to expose the metal-containing trace or pad  204  and form a full via  222 , as shown in FIG.  13 . However, due to the variation in the thickness of the interlayer dielectric  208  from the center of a wafer to the edge (usually between 4000 and 5000 Å), an over-etch is applied, such that the via will usually extend through the barrier layer  206  and into the metal-containing trace or pad  204 . When the barrier layer  206  and metal containing trace or pad  204  are etched, a second residue layer  224  (“metal polymer”) of a carbon-fluorine based polymer including metal etched from the metal-containing trace or pad  204 , as well as any metal components in the barrier layer  206 , such as the titanium in a titanium nitride barrier layer, is formed over the first residue layer  216  and the exposed surface  226  of the metal-containing trace or pad  204 , also shown in FIG.  13 . 
     It is, of course, understood that a single etch could be performed to expose the metal-containing trace or pad  204 , which etch would result in a single residue layer. However, even if a single etch were performed, the single residue layer would still have a portion of the residue layer adjacent the via sidewall  218  containing predominantly oxide polymer and a portion adjacent the via aperture and the bottom of the via containing predominantly metal polymer. 
     Residue layers, such as first residue layer  216  and second residue layer  224 , which coat the full via, are very difficult to remove. These residue layers may be removed by dipping the structure in a phosphoric acid solution; and, although this technique is effective in removing most of the residue layers, the residue layers are still not completely removed. The portion of the residue still remaining after the phosphoric acid dip adversely affects the conductivity of contacts subsequently formed in the full via  222 . It is noted, that although extending the residence time of the semiconductor substrate structure in the phosphoric acid will effectively remove all of the residue layer(s), the increased residence time also results in damage to the metal-containing trace or pad  204 . 
     Thus, it can be appreciated that it would be advantageous to develop a technique to form a via which prevents the formation of metal polymer residues and allows for removal of oxide polymer residues from the via without substantial damage to the metal-containing trace or pad while using commercially-available, widely-practiced semiconductor device fabrication techniques. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a layered trace comprising a conductive trace capped with a silicide material. When such a layered trace is used in a multilayer semiconductor structure, it allows for non-damaging removal of any oxide polymer residues forming in visa used to electrically connect the various layers through dielectric layers separating them, and eliminates or greatly reduces the formation of metal polymer residues in the visa. This results in better contact reliability. 
     One embodiment of forming an interlayer contact according to the present invention involves providing a conductive layer deposited over a semiconductor substrate. A substrate dielectric or insulating layer preferably separates the semiconductor substrate from the conductive layer. A silicide layer, such as tungsten silicide, cobalt silicide and the like, is disposed over the conductive layer. An optional barrier layer, such as a thin film of titanium, may be disposed between the conductive layer and the silicide layer to prevent silicon molecules from the silicide layer from migrating into and contaminating the metals in the conductive layer. 
     A first resist material is patterned over the silicide layer and the silicide layer, the barrier layer, and the conductive layer are etched and any remaining first resist material is removed to form a layered trace or pad. An interlayer dielectric is deposited over the layered trace and the substrate dielectric. A second resist material is then patterned over the interlayer dielectric layer such that an opening in the second resist material is positioned over the layered trace. 
     The interlayer dielectric layer is then etched, preferably using an oxide etch selectively stopping on the silicide layer, through the opening in the second resist material to form a via through the interlayer dielectric layer to the silicide layer of the layered trace. The etching of the via through the interlayer dielectric layer creates an oxide polymer residue layer on sidewalls of the via. As used herein, the term “sidewall” of a via encompasses both a single, continuous sidewall such as may define a round or circular via, as well as a plurality of sidewalls defining a via of other cross-section. The presence of the oxide polymer residue layer is a natural consequence of the etching of the via and, since the etching gas generally comprises a fluorinated gas, the oxide polymer residue layer is usually a carbon-fluorine based polymer containing residue of the interlayer dielectric. 
     The oxide polymer residue layer and any remaining second resist material are preferably removed by any suitable known technique. At this point, an upper-layer trace or contact can be formed over the interlayer dielectric and into the via to make electrical contact with the silicide layer of the layered trace. However, if electrical resistance is a concern, a highly selective etch can be used to subsequently remove the silicide layer in the via, which will reduce contact resistance by bringing subsequently deposited conductive material into direct contact with the barrier layer or the conductive layer. An upper-layer trace or a contact may be completed by depositing a conductive material into the via. 
     It is, of course, understood that the layered trace may be formed on an interlayer dielectric above the semiconductor substrate with another interlayer disposed over the layered trace and the interlayer dielectric, with a contact being formed to the layered trace in a manner described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
     FIGS. 1-9 are side cross-sectional views of a method of forming an electrical trace according to a technique of the present invention; and 
     FIGS. 10-13 are side cross-sectional views of a via formation process according to a known technique. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-9 illustrate one embodiment for layered trace and contact via formation according to the present invention. It should be understood that the figures presented in conjunction with this description are not meant to be actual cross-sectional views of any particular portion of an actual semiconductor device, but are merely idealized representations which are employed to more clearly and fully depict the process of the invention than would otherwise be possible. 
     FIG. 1 illustrates a conductive layer  104 , preferably an aluminum/copper alloy, deposited over a substrate dielectric or insulating layer  102 , such as silicon dioxide, atop a semiconductor substrate  100 . A silicide layer  108 , preferably tungsten silicide between about 400 and 600 Å thick, is disposed over the conductive layer  104 . An optional barrier layer  106 , preferably titanium or titanium nitride about 100 Å thick, is disposed between the conductive layer  104  and the silicide layer  108 . The optional barrier layer  106  is used to prevent silicon molecules from the silicide layer  108  from migrating into and contaminating the metals in the conductive layer  104 , if the metals are susceptible to such contamination. 
     A first resist material  112  is patterned over the silicide layer  108 , as shown in FIG.  2 . The silicide layer  108 , the barrier layer  106 , and the conductive layer  104  are etched and any remaining first resist material  112  is removed to form a layered trace or pad  114 , as shown in FIG.  3 . An interlayer dielectric  116 , such as silicon dioxide, spin-on-glass, or the like, is deposited over the layered trace  114  and the substrate dielectric  102 , as shown in FIG.  4 . The interlayer dielectric layer  116  is preferably planarized such as by chemical mechanical planarization. A second resist material  118  is then patterned over the interlayer dielectric layer  116  such that an opening  122  in the second resist material  118  is positioned over the layered trace  114 , as shown in FIG.  5 . 
     As shown in FIG. 6, the interlayer dielectric layer  116  is then etched, preferably using an oxide etch selectively stopping on the silicide layer  108 , such as plasma etching and reactive ion etching with a fluorinated gas (CF 4 , CHF 3 , C 2 F 6 , CH 2 F 2 , or other freons, and mixtures thereof) in combination with a carrier gas, such as Ar, He, Ne, Kr, O 2 , or mixtures thereof, through the opening  122  in the second resist material  118  to form a via  124  through the interlayer dielectric layer  116  to the silicide layer  108  of the layered trace  114 . The etching of the via  124  through the interlayer dielectric layer  116  creates an oxide polymer residue layer  126  on sidewalls  128  of the via  124 . As used herein, the term “sidewall” of a via encompasses both a single, continuous sidewall such as may define a round or circular via, as well as a plurality of sidewalls defining a via of other cross-section. The presence of the oxide polymer residue layer  126  is a natural consequence of the etching of the via  124  and, since the etching gas generally comprises a fluorinated gas, the oxide polymer residue layer  126  is usually a carbon-fluorine based polymer containing residue of the interlayer dielectric layer  116 . 
     The oxide polymer residue layer  126  and any remaining second resist material  118  are preferably removed using a hydrofluoric acid dip, ammonia/peroxide mixture (APM), tetramethyl ammonium hydroxide (TMAH), or the like, as shown in FIG.  7 . If electrical resistance in the via  124  is a concern, a highly selectively etch consisting of trifluoronitride (NF 3 ) in a helium/oxygen carrier gas can be used to remove the silicide layer  108  in the via  124 , as shown in FIG.  8 . 
     Once a clean via  124  is achieved, an electrical trace  130  may be formed by depositing a conductive material  132  into the via  124  and over the interlayer dielectric layer  116  in a predetermined pattern, as shown in FIG.  9 . The conductive material  132  is preferably a metal, including but not limited to titanium, copper, silver, gold, aluminum, and alloys thereof. However, conductive polymers may be used. The deposition of the conductive material  132  may be effected by methods including, but not limited to, hot sputter/reflow, ionized plasma, hot pressure fill, as well as physical vapor deposition and chemical vapor deposition combinations. 
     EXAMPLE 
     An experiment was conducted with an aluminum/copper alloy layer (approximately 4 KÅ thick) deposited over a silicon dioxide material layer atop a semiconductor substrate. A titanium nitride barrier layer (approximately 100 Å thick) was disposed over the aluminum/copper alloy layer and a tungsten silicide layer (approximately 600 Å thick) was disposed over the tungsten silicide layer to form a layered structure. A silicon dioxide interlayer dielectric (approximately 5.5 KÅ thick) was deposited over the layered structure. A via was then etched through the silicon dioxide interlayer dielectric with an oxide etch (60% overetch), preferably with an etch gas comprising 15 sccm of CHF 3  and 60 sccm of CF 4 . The oxide polymer residue formed during the oxide etch was removed with a stripper followed by a 20:1 (H 3 PO 4 :H 2 O 2 ) wash. Scanning electron micrographs taken of a cross-section of the via formed showed that no oxide polymer residue was present. 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope thereof.