Abstract:
A sputtering target includes a backing plate, a copper target provided on the backing plate, and a protection layer formed of a corrosion-resistant metal on the surface of the copper target The protection layer depresses oxidation of the copper target and the adhesion of particles to a substrate due to the release of a deposited layer on the surface of the shielding plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority of Japanese Patent Application No. 2002-106602, filed on Apr. 9, 2002, the contents being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sputtering target of a metal, such as copper or others, which can be easily oxidized, and a method for fabricating the sputtering target. 
     As interconnection layers of the current highly integrated semiconductor devices, copper interconnection layers have taken the place of aluminum interconnection layers. 
     Generally, electroplating is widely used for forming copper interconnection layers. In forming copper interconnection layers by electroplating, a base conductor layer of a metal, such as copper or others, must be formed in advance. To form such base conductor layer, generally sputtering is used. 
       FIG. 5A  is a diagrammatic view of a magnetron sputtering system generally used in forming the above-described base conductor layer of copper or others. As shown, a substrate  104  of silicon or others to be sputtering processed is held substantially horizontal on a stage  102  disposed in a chamber  100 . A heater (not shown) for heating the substrate  104  as required is provided in the stage  102 . A sputtering target  106  is mounted on the inside of the chamber  100  above the substrate  104 , opposed to the surface of the substrate  104 . A shielding plate  108  is disposed surrounding the space between the substrate  104  and the sputtering target  106 . A direct current source  110  interconnects the sputtering target  106  as a cathode and the substrate  104 , so that a high negative voltage can be applied to the sputtering target. 
     In the sputtering processing, an internal pressure of the chamber  100  is decreased to a prescribed pressure, and a sputtering gas, such as argon gas, is fed into the chamber  100 . Then, a voltage is applied by the direct current source  110 . Thus, argon plasmas are generated between the sputtering target  106  and the substrate  104 . The thus dissociated argon ions impinge on the sputtering target  106 , which is the cathode to sputter the metal atoms. The sputtered metal atoms arrive at the substrate  104  to thereby form a metal layer on the substrate  104 . 
     To form the base conductor layer of copper by sputtering, a sputtering target of copper is necessary. In comparison with the sputtering target of aluminum, the surface of the copper sputtering target is very easily oxidized. That is, as shown in  FIG. 5B , usually an oxide layer  112  of copper oxide is formed on the surface of the copper sputtering target  106 . 
     When copper is applied to a substrate of silicon or others by the magnetron sputtering system by using a copper sputtering target with an oxide layer thus formed on the surface, it is necessary to prevent the oxide on the surface of the target from adhering to the substrate. To this end, conventionally, a prescribed amount of sputtering processing is performed on a dummy substrate to thereby sufficiently remove the oxide layer on the target surface, and sputtering processing is performed on a substrate to apply copper thereto. The processing for removing the oxide layer formed on the sputtering target surface before said sputtering processing is called burn-in processing or pre-sputtering processing. 
     In using the conventional copper sputtering target with an oxide layer formed on the surface, a disadvantage is that particles tend to adhere to a substrate to be sputtering processed. The adhesion of particles to a substrate in using the copper sputtering target will be explained with reference to  FIGS. 6A and 6B .  FIGS. 6A and 6B  are diagrammatic views of states of the shielding plate of the sputtering system after the burn-in processing and the sputtering processing. 
     In the burn-in processing, the oxide from an oxide layer on the surface of the copper sputtering target adheres to the surface of the shielding plate in the chamber to form a contaminant layer of the copper oxide. Then, in the intended sputtering process, the copper adheres to the contaminant layer formed on the surface of the shielding plate.  FIG. 6A  is a diagrammatic view of the contaminant layer  114  formed on the surface of the shielding plate  108  and the copper layer  116  formed on the contaminant layer  114 . 
     Generally, in the sputtering processing, depositing particles impinge at high energies on an object for the particles to adhere to, and it can be said that the formed deposited layer does not easily peel off. However, in the above-described burn-in processing, the contaminant layer  114  formed on the surface of the shielding plate  108  is formed of the oxide called copper oxide, which is exhibits poor adhesion to the surface of the shielding plate  108  of stainless steel or others. Then, as shown in  FIG. 6B , the contaminant layer  114  easily peels off the surface of the shielding plate  108  due to stresses of a copper layer  116  formed on the contaminant layer  114  or thermal stresses caused by the sputtering process. The deposits, which have grown on the surface of the shielding plate  108 , peel off, with the result that the particles adhere to a substrate in the sputtering processing. 
     The influence of the above-described particle adhesion to substrates on semiconductor devices is becoming an unignorable and critical problem as the high integration goes on. 
     As a method for preventing the surface of the sputtering target from being oxidized, an adhesive tape or the like is adhered to the surface until a time immediately before the sputtering target is mounted on the sputtering system. This method is effective in shielding the sputtering target surface from the atmospheric air to prevent the oxidation. However, the deposited layer formed by using the sputtering target with an adhesive tape or the like applied thereto is contaminated with organic substances. Because of the sputtering target surface contaminated with an organic substance, a contaminant layer is formed between the shielding plate and the deposited layer, and resultantly the deposited layer on the shielding plate surface tends to easily peel off. As a result, in the sputtering processing, particles tend to adhere to a substrate due to the release of the deposits on the shielding plate surface. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a sputtering target which can depress adhesion of particles to a substrate when the sputtering target is formed of copper which is easily oxidized, and a method for fabricating the sputtering target. 
     According to one aspect of the present invention, there is provided a sputtering target comprising: a target base of copper; and a protection layer formed on the surface of the target base and formed of corrosion-resistant metal. 
     According to another aspect of the present invention, there is provided a method for fabricating a sputtering target comprising the step of: forming a protection layer of a corrosion-resistant metal on the surface of a target base of copper. 
     According to another aspect of the present invention, there is provided a sputtering method for depositing copper on a substrate by using a sputtering target comprising a target base of copper, and a protection layer formed on the surface of the target base and formed of a corrosion-resistant metal, comprising the steps of: pre-sputtering the sputtering target to remove the protection layer; and sputtering the target base to deposit copper on the substrate. 
     According to another aspect of the present invention, there is provided a method for storing a sputtering target comprising a target base of copper, and a protection layer formed on the surface of the target base and formed of a corrosion-resistant metal in a vacuum package. 
     According to a further aspect of the present invention, there is provided a method for transporting a sputtering target comprising a target base of copper, and a protection layer formed on the surface of the target base and formed of a corrosion-resistant metal in a vacuum package. 
     As described above, the sputtering target according to the present invention comprises a target base of copper and a protection layer of a corrosion-resistant metal, whereby the oxidation of the surface of the target base of copper, which is easily oxidized, can be suppressed. Thus, the formation of a contaminant layer on the shielding plate of the sputtering system can be suppressed, and accordingly the adhesion of particles to a substrate due to the release of a deposited layer on the surface of the shielding plate can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are diagrammatic views of the sputtering target according to one embodiment of the present invention. 
         FIGS. 2A ,  2 B and  2 C are views of the sputtering target in the steps of the method for fabricating the sputtering target and the method for storing and transporting the sputtering target, which explain the methods. 
         FIGS. 3A ,  3 B and  3 C are views of the sputtering target according to the embodiment in the steps of the method for sputtering processing the sputtering target, which explain the method. 
         FIGS. 4A ,  4 B,  4 C, and  4 D are sectional views of a semiconductor device in the steps of the method for forming a copper interconnection layer of the semiconductor device, to which the sputtering method according to the embodiment of the present invention. 
         FIGS. 5A and 5B  are diagrammatic views of the sputtering system and the surface of the conventional sputtering target. 
         FIGS. 6A and 6B  are diagrammatic views of states of the shielding plate of the sputtering system using the conventional sputtering target after the burn-in processing and the sputtering processing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The sputtering target according to one embodiment of the present invention and a method for fabricating the sputtering target will be explained with reference to  FIGS. 1A ,  1 B,  2 A,  2 B,  2 C,  3 A,  3 B,  3 C,  4 A,  4 B,  4 C, and  4 D.  FIGS. 1A and 1B  are diagrammatic views of the sputtering target according to the present embodiment.  FIGS. 2A ,  2 B,  2 C,  3 A,  3 B, and  3 C are views of the sputtering target according to the present embodiment in the steps of the method for fabricating the same, the method for storing and transporting the same and the sputtering method, which explain the methods.  FIGS. 4A ,  4 B,  4 C, and  4 D are sectional views of a semiconductor device in the steps of the method for forming a copper interconnection layer by using the sputtering method according to the present embodiment, which explains the method. 
     First, the sputtering target according to the present embodiment will be explained with reference to  FIG. 1 . 
     As shown in  FIG. 1A , on a disc-shaped backing plate  10  to be connected to the cathode of a sputtering system, a disc-shaped copper target  12  of a smaller diameter than the backing plate  10  is disposed. A protection layer  14  of aluminum is formed on the surface of the copper target  12 . 
       FIG. 1B  is a diagrammatic view of the sputtering target mounted on the sputtering system, which shows the mounted state. 
     The sputtering target  20  according to the present embodiment is mounted above a stage  18  which is disposed in a chamber  16  and supports a substrate. A shielding plate  22  is disposed surrounding the space between the stage  18  and the sputtering target  20 . The sputtering target  20  is connected to a direct current source  24 . 
     As described above, the sputtering target  20  according to the present embodiment is characterized mainly in that the protection layer  14  of aluminum is formed on the surface-to-be-sputtered of the target  12  of copper, which is easily oxidized. Aluminum which is a material of the protection layer  14  has an oxidized coating of good corrosion resistance, which is called a passive coating formed on the surface, and has a property that oxidation does not advance inside. Accordingly, the protection film  14  of aluminum is formed on the surface of the target  12  of copper, which is easily oxidized, whereby the oxidation of the surface of the copper target  12  can be depressed. As a result, the formation of a contaminant layer of the oxide on the shield plate of the sputtering system can be depressed, and accordingly deposits on the surface of the shielding plate peeling off to adhere to a substrate to be sputtering processed are minimized. 
     A thickness of the protection layer  14  on the surface of the copper target  12  can be, e.g., 50 μm but is not limited to 50 μm. However, in case of a thickness of the protection layer  14  being below 0.05 μm, oxygen in the atmosphere passes through the protection layer and arrives at the surface of the copper target  12 , and the surface of the copper target  12  is oxidized. On the other hand, when a thickness of the protection layer  14  is above 1000 μm, it is difficult to sufficiently remove the protection layer  14  by the burn-in processing. Thus, a thickness of the protection layer  14  is preferably in 0.05–1000 μm, more preferably 50–300 μm. 
     When the copper target  12  is sputtered by the magnetron sputtering system, plasmas contact the shielding plate  22 , and electrons go away to the earth, which makes the generation of plasmas unstable. Accordingly, in the magnetron sputtering, an outer circumferential vicinity of the copper target  12  is little abraded by the plasmas. Accordingly, in a case that the protection layer  14  is formed up to the outer circumferential vicinity of the copper target  12 , the protection layer  14  formed in the outer circumferential vicinity cannot be easily removed. 
     Then, the protection layer  14  is not formed in the outer circumferential vicinity of the surface of the disc-shaped copper target  12 . Specifically, for the copper target  12  of a 340 mm-diameter, the protection layer  14  may not be formed in the 15 mm-width region from the outer circumference. 
     Even when the surface of the outer circumferential vicinity of the copper target  12  with the protection layer  14  not formed in is oxidized, a ratio of an area of the outer circumferential vicinity is small to a entire surface of the copper target  12 . Specifically, in the case that the protection layer  14  is not formed in a 15 mm-width region from the outer circumferential vicinity of the copper target of a 340 mm-diameter, an area with the protection layer  14  not formed in is 17% to the entire area. Accordingly, the generation amount of oxides of copper from the copper target  12  is little. Thus, even when copper oxides from the outer circumferential region adhere to the shielding plate  22 , the adhesion amount is little, and the deposited copper does not peel off from the shielding plate  22 . 
     Next, the method for fabricating the sputtering target according to the present embodiment, the method for storing and transporting the same, and a string of the steps of the method for sputtering processing using the sputtering target according to the present embodiment will be explained with reference to  FIGS. 2A ,  2 B,  2 C,  3 A,  3 B, and  3 C. 
     First, the method for fabricating the sputtering target will be explained. 
     Copper plates are machined by a precision lathe to form the backing plate  10  and a copper target  12  ( FIG. 2A ). For example, the copper target  12  is formed of a copper plate of a high purity (6N) for semiconductor devices, and the backing plate  10  is formed of a copper plate of the usual grade. The copper target  12  and the backing plate  10  are combined with each other by bonding. 
     Then, the protection layer  14  of aluminum is formed on the surface of the copper target  12  within 12 hours after the machining by the precision lathe is finished, so that the surface of the formed copper target  12  is not oxidized ( FIG. 2B ). As a method for forming the protection film  14 , the flame spraying, for example, in which an aluminum wire is continuously fed while being melted with gas flames, and pulverized globules of aluminum are blasted with compressed air, can be used. 
     Thus, the sputtering target  20  according to the present embodiment is formed. 
     Next, the sputtering target  20  is sealed in a vacuum pack  26  having the air replaced by an inert gas, such as nitrogen gas or others to be vacuum-packaged so that the side surfaces of the backing plate  10  and the copper target  12  are not oxidized ( FIG. 2C ). 
     Thus, being vacuum-sealed, the sputtering target  20  is stored or transported to a site or others. 
     When the conventional copper target, whose surface is exposed to the atmosphere, is exposed to the atmosphere for more than 24 hours, i.e., a period of time from the lathe machining of the target to the package of the target with the use of an inert gas and a period of time from unpackaging the target to mounting of the target on a sputtering system, the surface of the copper is oxidized and discolored. 
     In contrast to the conventional sputtering target, the sputtering target  20  according to the present embodiment is vacuum-packaged in the vacuum pack  26 , and furthermore, the protection layer  14  is formed on the surface of the copper target  12 , whereby the surface of the copper target  12 , which is easily oxidized, is not exposed to the atmosphere. Thus, the sputtering target  20  can be stored or transported with the surface of the copper target  12  kept from being oxidized. As described above, the sputtering target  20  according to the present embodiment can keep the surface of the copper target  12  from being oxidized, which facilitates handling the sputtering target  20  in comparison with the conventional sputtering target. 
     Then, the sputtering method using the sputtering target  20  according to the present embodiment, which is fabricated as described above and stored and transported in the vacuum package as shown in  FIG. 2C  will be explained. 
     First, the vacuum package  26  is opened to take out the sputtering target  20 . Then, the sputtering target  20  is mounted in the chamber  16  of the sputtering system ( FIG. 3A ). 
     Next, a dummy substrate  28  is held on the stage  18 . The interior of the chamber  16  is depressurized to a prescribed pressure, and argon, for example, as a sputtering gas is supplied into the chamber  16 . Then, a voltage is applied from the direct current source  24  to excite argon plasmas to sputter the protection layer  14  of aluminum formed on the surface of the copper target  12  by argon ions. Thus, the burn-in processing is performed to remove the protection layer  14  of aluminum formed on the surface of the copper target  12  ( FIG. 3B ). 
     In this burn-in processing, the aluminum of the protection layer  14  adheres to the shielding plate  22 . Aluminum has good adhesion to the shielding plate  22  and copper. Accordingly, when copper adheres to the aluminum layer staying on the surface of the shielding plate  22  in the following copper sputtering, the copper does not easily peel off. Resultantly, in the copper sputtering processing after burn-in processing, adhesion of particles to a substrate can be suppressed. 
     After the protection layer  14  is sufficiently removed by the burn-in processing, the dummy substrate  28  is unloaded, and a substrate  30  to be copper sputtering processed is held on the stage  18 . Then, the interior of the chamber  16  is depressurized to a prescribed pressure, and argon, for example, as a sputtering gas is supplied into the chamber  16 . Then, a voltage is applied from the direct current source  24  to excite argon plasmas, and the surface of the copper target  12  is sputtered by argon ions. The thus-sputtered copper atoms of the copper target  12  arrive at the substrate  30 , and a copper layer is formed on the substrate  30  ( FIG. 3C ). 
     The above-described sputtering method according to the present embodiment is applicable, e.g., to forming a seed layer for forming copper interconnection layers of semiconductor devices by electroplating. The method for forming a copper interconnection layer of a semiconductor device, to which the sputtering method according to the present embodiment is applied, will be explained with reference to  FIGS. 4A ,  4 B,  4 C, and  4 D. 
     First, grooves  34  of a prescribed depth and width are formed in an oxide film  32  formed on a substrate of silicon or others (not shown) in a prescribed interconnection layer pattern by the usual lithography and etching ( FIG. 4A ). 
     Then, a tantalum nitride layer  36  of, e.g., a 25 nm-thickness is formed as a barrier metal on the entire surface of the insulation film  34  with the grooves  34  formed by the usual sputtering method. Then, the copper layer  38  of, e.g., a 150 nm-thickness which functions as a seed layer for electroplating is formed on the entire surface by the sputtering method according to the present embodiment ( FIG. 4B ). 
     At this time, with the oxidation of the surface of the copper target used for the sputtering processing suppressed, the formation of a contaminant layer of the oxides on the shielding plate of the sputtering system, whereby the adhesion of particles to the substrate can be suppressed. 
     Then, with the copper layer  38  formed by the sputtering method according to the present embodiment as a seed layer, the copper layer  40  of, e.g., a 1 μm-thickness is formed on the entire surface by electroplating. Thus, the grooves  34  of the interconnection layer pattern are filled with the copper layer  40  ( FIG. 4C ). 
     Next, the entire surface is polished by CMP (Chemical Mechanical Polishing) to remove the excessive copper layer  40  except that in the grooves  34  ( FIG. 4D ). 
     Thus, the copper interconnection layer of the copper layer  40  buried in the grooves  34  in the insulation film  32  is formed. 
     As described above, according to the present embodiment, the protection layer  14  of aluminum is formed on the surface of the copper target  12 , which is easily oxidized, whereby the surface of the copper target  12  is not exposed to the atmosphere, and the oxidation of the surface of the copper target  12  can be suppressed. Thus, the adhesion of oxides to the shielding plate  22  of the sputtering system can be suppressed. Resultantly, the adhesion of particles to a substrate, which is caused by deposits on the surface of the shielding plate  22  peeling off, can be suppressed. 
     [Modifications] 
     The present invention is not limited to the above-described embodiment and can cover other various modifications. 
     For example, in the above-described embodiment, the protection layer  14  is formed of aluminum, but the material of the protection layer  14  is not limited to aluminum. A material of the protection layer  14  can be, other than aluminum, a metal such as tin zinc, nickel, magnesium, titanium or others. These metals have merits that they are not easily oxidized in the atmosphere, and that when they are used in forming copper interconnection layers of semiconductor devices, they provide longer usefulness of the copper interconnection layers. 
     A metal, such as, e.g., zirconium, vanadium, molybdenum, cobalt or others, which has good adhesion to both copper and the shielding plate  22  and does not easily form oxide films, can be used as a material of the protection layer  14 . A metal which is good adhesion not only to the shielding plate  22  but also other members incorporated in the chamber  16  of the sputtering system is used as a material of the protection layer  14 , whereby the adhesion of particles to substrates can be further suppressed. 
     In the above-described embodiment, the protection layer  14  of aluminum is formed on the surface of the copper target  12  by flame spraying, but the method for forming the protection layer  14  is not limited to the flame spraying. For example, the protection layer  14  may be formed by sputtering, vapor deposition, chemical growth or others. 
     In the above-described embodiment, the protection layer  14  is not formed in an outer circumferential vicinity of the copper target  12 . However, the protection layer  14  may be formed in the outer circumferential vicinity, and in this case, it is preferable that the protection layer  14  formed in the outer circumferential vicinity is thin enough to be removed by the burn-in processing and is formed in a smaller thickness than the protection layer  14  formed in a central vicinity. The protection film  14  of, e.g., a 2 μm-thickness aluminum is formed also in the outer circumferential vicinity, whereby the sputtering target can be stored in the atmosphere for several months without the surface of the copper target being oxidized. 
     In the above-described embodiment, the sputtering system using the sputtering target according to the present invention is explained by means of the magnetron sputtering system, but the sputtering system is not limited to the magnetron sputtering system. 
     In the above-described embodiment, the sputtering method according to the present invention is used in forming the seed layer for the step of forming the copper interconnection layer. However, the application of the sputtering method according to the present invention is not limited to said step and is applicable various steps.