Abstract:
Water vapor plasma etching of metal surfaces facilitates removal of organic residues over metal surfaces. By plasma etching metal surfaces covered with an organic material, such as photoresist, in an atmosphere in which the water vapor to O 2  ratio exceeds 5:3 (such as about 5:1, for example), superior organic material removal results are observed, particularly over relatively wide metal surfaces. The duration of the water vapor plasma etch also may be increased, relative to conventional organic material-removing processes. The effectiveness of the high vapor etch according to the present invention allows the elimination of a subsequent dry organic material stripping step, reducing processing time and cost while increasing yields.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of semiconductor manufacturing processes. In particular, the present invention relates to methods and devices to enhance the reduction of organic residues over metals surfaces. 
     2. Description of the Related Art 
     Metal layers, in integrated circuits, carry signals and power from one area of the circuit to another. To form a metal structure in an integrated circuit, a metal layer may be deposited on the surface of the wafer, followed by a layer of photoresist material. The photoresist is used to coat and photosensitize the underlying surface. After deposition, the photoresist is patterned to cause the non-imaged areas of the underlying layer to resist subsequent modifications, such as etching or metal evaporation. When the etching step, for example, is finished, the photoresist is removed. 
     FIG. 1 is a flowchart outlining steps to remove photoresist material from a metal layer after an etching step. As shown therein, step S 0  is a metal etch step in which the metal layer exposed by the photoresist is etched, to form the desired metal structures, such as conductor lines, contact pads, buses etc. After the etching step S 0 , the wafer is subjected to a plasma etch removal of the photoresist, as shown in step S 1 . 
     The plasma etch step S 1  of FIG. 1 may be further broken down into four steps, in which the oxygen (“O 2 ”), water (“H 2 O”) vapor, and pressure remain constant while the Radio Frequency (“RF”) power within the plasma etch chamber and the position of the chuck or pedestal supporting the wafer within the plasma etch chamber varies from a lower position to an upper position. Table 1 outlines a process for plasma etching the photoresist and quantifies the aforementioned parameters. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Step 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
             
               
                   
                 Pressure 
                 1200  
                 1200  
                 1200  
                 1200  
               
               
                   
                 (milliTorr) 
               
               
                   
                 O 2  (sccm) 
                 180 
                 180 
                 180 
                 180 
               
               
                   
                 H 2 O (sccm) 
                 300 
                 300 
                 300 
                 300 
               
               
                   
                 Time (seconds) 
                  30 
                   3 
                 Endpt., 
                  15 
               
               
                   
                   
                   
                   
                  45 
               
               
                   
                 RF power (watts) 
                   0 
                   0 
                 1200  
                 1200  
               
               
                   
                 Chuck Position 
                 Down 
                 Up 
                 Up 
                 Up 
               
               
                   
                 (Up, Down) 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 1, the O 2  to H 2 O mixture of step S 1  of FIG. 1 is maintained at a 5:3 ratio (300 standard cubic centimeters per minute (“sccm”) H 2 O: 180 sccm O 2  at a pressure of 1200 milliTorr during all four steps. In a first step, lasting 30 seconds, the chuck is maintained at a lower position, further away from the plasma source, and the RF power is turned off. In a second step, lasting only 3 seconds, the same conditions prevail and the chuck is moved to an upper position, relatively closer to the source of plasma than the lower position thereof. In a third step, the same chuck position and O 2 :H 2 O ratio are maintained and the RF power is turned on to 1200 watts for 45 seconds. The same parameters are then maintained in a fourth step, lasting 15 seconds. 
     In step S 2  of FIG. 1, the metal layer, now etched, is then inspected to determine whether the photoresist has been properly removed from the surface of the metal layer. The photoresist is then subjected to a solvent strip to remove residual photoresist particulates from the metal layer as shown in step S 3 . The solvent strip of step S 3  is then followed by a dry strip step S 4 , in an effort to remove any last remaining particulates or residual photoresist left after the solvent strip step shown at S 3  in FIG.  1 . After the dry strip of step S 4 , the metal surfaces are again inspected, as shown at step S 5 , to determine the efficacy of the preceding steps in removing the photoresist from the metal layer. 
     Despite these steps, however, some residual photoresist particulates may still be present on the metal layer, which particulates can lead to device failures and decrease the overall effective yield of the process. Such remaining photoresist particulates are schematically shown in FIG.  2 . FIG. 2 shows a metal layer  100  comprising a wide area  110  and relatively narrower areas  130 . For example, the wide area  110  of the metal layer  100  may be, for example, a power bus, a contact pad or some other wide metallic structure, whereas the relatively narrower areas  130 , for example, represents thin conductor lines or some other narrow metal structure within an integrated circuit. It has been found that photoresist particulates, such as shown at reference numeral  120  in FIG. 2, tend to remain adhered to the wide area  110  of the metal layer  100 , even after the etch and cleaning steps of FIG.  1 . Indeed, while the steps shown in FIG. 1 are generally effective in removing the photoresist material from the relatively narrower areas  130  of metal layers, such as metal layer  100  of FIG. 2, the cleaning steps of FIG. 1 are somewhat less effective in removing residual photoresist over wide metal areas, such as shown at  110  in FIG.  2 . Such residual photoresist on wide metal surfaces leads to increased defect count per wafer and decreases the yield of the process. 
     What are needed, therefore, are methods and devices to enhance the reduction of organic material residues (such as photoresist residues) over metal surfaces. In particular, what are needed are methods and devices to enhance the reduction of photoresist and other organic residues over wide metal surfaces. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to provide methods and devices to enhance the reduction of photoresist and other organic residues over wide metal surfaces. 
     In accordance with the above-described objects and those that will be mentioned and will become apparent below, a method of removing organic residue from a metal surface, according to an embodiment of the present invention, comprises the steps of: 
     etching the metal surface with a plasma comprising H 2 O and an oxygen source (O 2 ), where a volume, flow rate or molar ratio of H 2 O:O 2  source exceeds 5:3; and 
     stripping the metal surface with a solvent to remove remaining organic residue from the metal surface. 
     According to other embodiments, the ratio of H 2 O vapor:O 2  source ratio exceeds 5:2 by flow rate, such as about 5:1 by flow rate. The RF power during the plasma etching step may be about 1200 watts. The plasma is in a chamber having a pressure of about 50 to about 10,000 milliTorr and preferably includes one or more (hydro)fluorocarbons, such as CF 4 , CHF 3 , C 2 F 6 , C 2 H 2 F 4  (e.g., Freon-134), c-C 4 F 8 , combinations thereof, or such other etchant gases as NF 3  and/or SiF 4 . Preferably, the water vapor etching step of the present invention is carried out for a period of time of at least about 60 seconds. For example, the present etching step may be carried out for a period of time ranging from about 60 seconds to about 120 seconds, such as for a period of time ranging from about 70 seconds to about 90 seconds. The organic residue may include photoresist. 
     The present invention may also be viewed as a device to remove organic residue from a metal surface, comprising: 
     a plasma etching chamber; 
     a pedestal for supporting the substrate, the pedestal being mounted within said plasma etching chamber, and 
     at least one gas inlet into the plasma etching chamber for introducing at least a source of H 2 O vapor and source of O 2  into the chamber at an H 2 O:O 2  ratio by volume and/or flow ratio (unit volume per unit time) or moles exceeding 5:3. According to a further embodiment, the H 2 O:O 2  vol./flow ratio is about 5:1. 
     The present invention is also a semiconductor device having at least one constituent metal layer processed by a method comprising the steps of: 
     forming first and second metal structures from the at least one constituent metal layer, the at least one constituent metal layer having a layer organic material thereon; and 
     etching the layer of organic material with a plasma having a ratio of H 2 O vapor to O 2  of greater than 5:3 by volume, flow rate or moles. 
     According to further embodiments, the method may further comprise stripping residual organic material with a solvent after the etching step. The ratio of H 2 O vapor to O 2  may be at least about 5:2. The organic material may include a photoresist. The metal layer may include at least one metal selected from the group consisting of Al, Cu, refractory metals, alloys, nitrides and silicides thereof The metal layer may comprise at least one member of the group consisting of Al, Cu, Ti, Ta, W, and alloys thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the objects and advantages of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying figures, in which: 
     FIG. 1 is a block diagram of a method to etch and to clean a metal layer of unwanted photoresist. 
     FIG. 2 is a representation of a metal layer and unwanted photoresist particulate matter thereon. 
     FIG. 3 is a flowchart of a method of enhancing the reduction of organic residues over metal surfaces, according to an embodiment of the present invention. 
     FIG. 4 is a flowchart of a method of enhancing the reduction of organic residues over metal surfaces, according to another embodiment of the present invention. 
     FIG. 5 is a block diagram of a device to enhance the reduction of organic residues over metal surfaces, according to an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows an embodiment of a method of enhancing the reduction of organic residue over metal layers, according to the present invention. Herein, the phrase “metal layer” refers to a layer of metal formed (deposited, for example) on a substrate, without regard to the relative magnitudes of the dimensions of the metal formation (i.e., the term “metal layer” is not intended to be restricted by the connotation associated with the word “layer”). Such metal layers (including for example, Al, Cu and/or refractory metals [such as Ti, W, Ta, alloys thereof and nitrides thereof] and/or alloys and silicides thereof) may be patterned (etched, for example) to form vias, contact pads, power and signal busses, local interconnects and the like on the semiconductor wafer to form structures on and/or within semiconductor devices. According to the present invention, after a conventional metal etch step (not shown), a water (H 2 O) vapor plasma etch step is carried out, to remove the layer of organic material (such as photoresist) from the metal layer, as shown in step S 0 . 
     However, such water vapor plasma etches benefit from the addition of a source of oxygen, such as O 2 , O 3 , CO 2 , H 2 O 2 , etc. According to the present invention, the ratio of H 2 O vapor to O 2  exceeds 5:3 by moles, volume or flow rate (unit vol./unit time), preferably by flow rate. Assuming the etchant gas behaves ideally, these bases for the H 2 O:O 2  ratio are equivalent. Preferably, the flow rate ratio of H 2 O to O 2  is at least about 5:2, but is less than or equal to about 20:1, and more preferably is about 5:1. Alternatively, the water vapor may be created in situ by the reaction of H 2  and O 2 , or decomposition of H 2 O 2 , but for safety reasons, these alternatives are not preferred. After the water vapor plasma etch step S 0 , the etched metal layer may be inspected, as shown in step S 1 . Step  1  is optional, and may be omitted. A solvent strip is then carried out, as shown in step S 2 , to remove any residual organic material (such as photoresist) from the metal layer, such as organic residue  120  on metal surface  100  in FIG.  2 . Another inspection step may then be carried out to inspect the metal surface, as shown in step S 3 . 
     FIG. 4 shows another embodiment of the present invention. As shown therein, the water vapor plasma etch step is carried out for an extended period of time, as compared to the process shown in FIG.  1 . For example, the water vapor plasma etch step S 0  of FIG. 4 may be carried out for a period of time ranging from about 60 seconds to about 120 seconds. Preferably, the extended water vapor etching step S 0  of FIG. 4 is carried out for a period of time ranging between about 70 seconds to about 90 seconds and more preferably between about 75 and about 80 seconds at an RF power of about 1200 watts. Different RF powers may, according to the present invention, necessitate corresponding variations in the duration of the water vapor plasma etching step S 0  of FIG.  4 . Steps S 1  through S 3  of FIG. 4 are similar to steps S 1  through S 3  of FIG. 3, respectively, and the corresponding disclosure thereof is incorporated herewith. 
     According to an embodiment of the present invention, the water vapor plasma etching step (steps S 0  of FIGS. 3 and 4) may be carried out in an atmosphere at a pressure of, for example, 1200 milliTorr, although other pressures may be freely chosen. Other pressures, however, may require corresponding changes in the RF power and/or the duration of the water vapor etching step. The atmosphere in which the water vapor etching step according to the present invention is carried out may include other process gases, such as inert gases (argon, nitrogen, or helium, for example) as well as other etchant gases ([hydro]fluorocarbons such as CF 4 , CHF 3 , C 2 F 6 , C 2 H 2 F 4 , c-C 4 F 8  and/or NF 3 , SiF 4 , for example). Table 2 summarizes the aforementioned parameters of the water vapor plasma etching step according to the present invention. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Step 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
             
               
                   
                 Pressure 
                 1200  
                 1200  
                 1200  
                 1200  
               
               
                   
                 (milliTorr) 
               
               
                   
                 O 2  (sccm) 
                 100 
                 100 
                 100 
                 100 
               
               
                   
                 H 2 O (sccm) 
                 500 
                 500 
                 500 
                 500 
               
               
                   
                 Time (seconds) 
                  30 
                   3 
                 Endpt., 
                  30 
               
               
                   
                   
                   
                   
                 60 
               
               
                   
                 RF power (watts) 
                   0 
                   0 
                 1200  
                 1200  
               
               
                   
                 Chuck Position 
                 Down 
                 Up 
                 Up 
                 Up 
               
               
                   
                 (Up, Down) 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 2, the O 2  to H 2 O mixture may be maintained at a 5:1 ratio (500 sccm H 2 O:100 sccm O 2 , for example) at a pressure of, for example, from 50 to 10,000 milliTorr, preferably from 200 to 6,000 milliTorr, more preferably 800 to 2,000 milliTorr (and in a specific example, about 1200 milliTorr) during all four steps. In a first step, which may last about 30 seconds, the chuck (e.g., pedestal within the plasma chamber) may be maintained in a lower position, further away from the plasma source, and the RF power may be turned off. In a second step, which may last about 3 seconds, the same conditions may be maintained and the chuck may be moved to an upper position, relatively closer to the source of plasma than the lower position thereof In a third step, the same chuck position and increased O 2 :H 2 O ratio are maintained and the RF power is turned on to, for example, about 1200 watts for about 60 seconds, for example. The same parameters may then maintained in a fourth step, which may last about 30 seconds. 
     According to the present invention, the organic material etch time may be increased by about 15 to about 20 seconds per wafer, as compared to the processes outlined in FIG.  1  and Table 1. However, as the water vapor plasma etching step, in combination with the subsequent solvent strip (shown at S 2  in FIGS.  3  and  4 ), is highly effective in removing organic materials from metal surfaces, a dry strip step (such as shown at S 4  in FIG. 1) is no longer necessary, saving both a process step and process time. As the dry strip step shown at S 4  in FIG. 1 lasts about 1 minute per wafer, implementation of the present invention saves about 40-45 seconds processing time per wafer while achieving superior organic material removal results over (relatively wide) metal surfaces. 
     The present invention may also be viewed as a semiconductor device in which at least one of the constituent metal layers thereof is processed according to the steps explained above and illustrated in FIGS. 3 and 4. 
     The present invention enjoys particular advantages when used to process a metal layer having a feature or structure at least 2×, preferably at least 3× and more preferably 4× the minimum feature width of the metal layer (as determined by the design rules for the device or technology containing the metal layer). Such features and/or structures included busses, contacts, “landing pads” for contacts, and input/output (bonding) pads. 
     Alternatively, the present invention may also be viewed as a device to remove organic residue, such as photoresist, from a metal layer on a substrate. An embodiment of such a device is shown in FIG.  5 . As shown therein, the device  500  includes a plasma etching chamber  510 , a chuck (pedestal)  530  for supporting a substrate such as a semiconductor wafer  540 , the chuck  530  being mounted within the plasma etch chamber  510 . An RF power source  520  is present within the plasma etch chamber  510 . One or more gas inlets  590  are configured to deliver process gases into the plasma etching chamber  510 . One or more gas outlets  580  may also be adapted to controllably evacuate the process gases from the chamber  510  and/or to maintain a desired pressure within the chamber  510 . Process gases may also be delivered (not shown) to the backside of the wafer through the chuck  530 , to regulate heat transfer between the semiconductor wafer  540  and the pedestal  530 . The device may include a plurality of gas reservoirs and gas flow regulators to introduce the process gases into the plasma etching chamber  510 . For example, the device  500  may include a water vapor source  550  in fluid communication with a regulator  555  to regulate the amount of water vapor introduced into the chamber  510 . Likewise, the device  500  may include a source of O 2  gas  560 , together with a corresponding regulator  565  to regulate the amount of O 2  gas introduced into the chamber  510 . As shown at  570 , the device  500  may also include one or more sources of other process gases, including inert gases and/or etchant gasses and a corresponding number of regulators  575  in fluid communication therewith. For example, inert gasses such as argon, helium and/or nitrogen and/or etchant gasses such as CF 4 , CHF 3 , C 2 F 6 , NF 3  and/or SiF 4 , for example, may also be introduced into the chamber  510  via the regulator(s)  575  and the inlet or inlets  590 . 
     According to the present invention, the regulators  555  and  565  are configured to deliver water vapor and O 2  at a ratio exceeding 5:3. Preferably, water vapor and O 2  are introduced into the chamber  510  at a ratio exceeding about 5:2 and more preferably at a ratio of about 5:1. For example, H 2 O vapor may be introduced into the plasma etching chamber  510  at a flow rate of about 500 sccm, whereas O 2  gas may be introduced into the chamber  510  at a flow rate of about 100 sccm. 
     Implementation of the (extended) water vapor plasma etch process to remove organics from metal surfaces according to the present invention has shown to be highly effective in removing organic materials from wide metal surfaces without, however substantially affecting narrower metal features. This result may be quantified, for example, by using an optical wafer surface inspection tool, such as the KLA-Tencor 2132 or 2138, commercially available from KLA-Tencor Corporation of San Jose, Calif., or such a tool as the WF-700 Series Wafer Defect Inspection System commercially available from Applied Materials of Santa Clara, Calif. Using a water vapor plasma etch according to the present invention, a defect reduction per wafer of about 25% was observed using such wafer inspection devices, as compared to other processes, such as shown in FIG.  1 . In one experiment involving two lots of wafers, the total yield of functional dies per wafer, following implementation of the water vapor extended etch of the present invention, increased by about 30 die per wafer (“dpw”), whereas virgin yield (full functionality upon first testing) increased by about 55 dpw, as compared to total and virgin yields of other processes (such as obtained by implementing the process shown in FIG.  1 ), respectively. 
     While the foregoing detailed description has described preferred embodiments of the present invention, it is to be understood that the above description is illustrative only and not limiting of the disclosed invention. Indeed, various modifications of the present invention may occur to those of skill in this art, and all such modifications are deemed to fall within the scope of the present invention. Thus, the present invention to be limited only by the claims as set forth below.