Abstract:
A damascene structure includes a conductive layer, a first dielectric layer, a first barrier metal layer, a barrier layer, and a second barrier metal layer sequentially formed on the conductive layer. The first dielectric layer having a via therein. The barrier layer is comprised of a material different with that of the first barrier metal layer. A bottom of the barrier layer disposed on the via bottom is not punched through. The accomplished barrier layers will have lower resistivity on the via bottom in the first dielectric layer and they are capable of preventing copper atoms from diffusing into the dielectric layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/626,925 filed on Nov. 29, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 11/646,387 filed on Dec. 28, 2006, which is a continuation of U.S. patent application Ser. No. 10/841,562, filed on May 10, 2004, which is a divisional of U.S. patent application Ser. No. 10/461,346, filed Jun. 16, 2003, all of which are commonly assigned. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for the manufacture of semiconductor devices and more particularly to the method for forming a barrier layer in a damascene structure. 
         [0004]    2. Description of the Prior Art 
         [0005]    In the processes for the manufacture of semiconductor devices, when the active elements of these semiconductor devices are constructed, the following work will be the manufacture of the metal conductive layers above these active elements to complete the electrical interconnection inside the semiconductor devices. The processes for the manufacture of the metal conductive layers are usually as follows: first, forming a metal layer above the active regions of the semiconductor devices, second, proceeding with photoresist coating, developing, and etching to complete the manufacture of a first metal layer, third, depositing a dielectric layer on the first metal layer, and finally proceeding with the manufacture of multiple metal layers dependent on the needs of the different semiconductor devices. 
         [0006]    For many years, materials of metal conductive layers of semiconductors are mainly aluminum and aluminum alloys. However, as sizes of semiconductor devices get more and more smaller, operating speeds of semiconductor devices get more and more faster, and power consumptions of semiconductor devices get more and more lower, it is necessary to use metal materials of lower resistivity and dielectric materials of low dielectric constant to complete the electrical interconnection inside semiconductor devices. U.S. Pat. No. 6,489,240 B1 cites using copper and dielectric materials of dielectric constant lower than 4 to complete the electrical interconnection inside semiconductor devices. When copper is used as the material of metal conductors of semiconductors, as shown in  FIG. 1A , considering that copper is difficult to be vaporized after etching processes, a dual damascene structure  10  is often used to proceed with copper forming processes inside the dual damascene structure  10 . U.S. Pat. No. 6,492,270 B1 mentions the details of forming copper dual damascene. A dual damascene structure  10  comprises a first etch-stop layer  120 , a first dielectric layer  160 , a second etch-stop layer  140 , and a second dielectric layer  180 . Before copper processes inside the dual damascene structure  10  above the copper metal layer  100  are performed, as shown in  FIG. 1B , a barrier layer  190  has to be formed to prevent copper atoms from diffusing into surrounding dielectric layers. 
         [0007]    In order to prevent copper atoms from diffusing into dielectric layers in the prior art, titanium nitride (TiN) or tantalum nitride (TaN) is usually used to form a barrier layer. U.S. Pat. No. 6,541,374 B1 mentions details of forming a barrier layer with TiN. Practically, when the barrier layer  190  is deposited, as a result of the direction of depositing is about perpendicular to the wafer surface, the thickness of the sidewall of the dual damascene structure  10  will be about one-fifth to a half of the thickness above the via bottom in the first dielectric layer  160  and above the trench bottom in the second dielectric layer  180 , easily causing that the deposition of the sidewall of the dual damascene structure  10  is incomplete and copper atoms formed later in the dual damascene structure  10  diffuse into surrounding dielectric layers. Consequently the electric property of the surrounding dielectric layers will be affected and then the semiconductor devices will be damaged. Accordingly there is a need for completely depositing a barrier layer of the sidewall of a dual damascene structure  10  to prevent copper atoms from diffusing into surrounding dielectric layers. 
         [0008]    In the other hand, the resistivity of nitrided metal materials in the prior art is far more higher than the resistivity of metal materials. Hence if TiN or TaN is used as the material of the barrier layer  190  in the dual damascene structure  10 , the resistivity between metals in the dual damascene structure  10  will be so high that the operating speed and the power consumption of the semiconductor devices will be influenced. Therefore there is a need for reducing the resistivity of the barrier layer  190  above the via bottom in the first dielectric layer  160 . 
       BRIEF SUMMARY 
       [0009]    One main purpose of the present invention is to use the barrier layer formed by at least two metal layers and a barrier layer of metallized materials to fully prevent copper atoms from diffusing into surrounding dielectric layers. 
         [0010]    The other main purpose of the present invention is to reduce the resistivity of the barrier layer above the via bottom in the dielectric layer of a dual damascene structure and to make a good ohmic contact between the barrier layer and the copper layer below the barrier layer and the copper layer later formed above the barrier layer. 
         [0011]    In one embodiment, a damascene structure is disclosed. The damascene structure includes a conductive layer, a first dielectric layer, a first barrier metal layer, a barrier layer, a second barrier metal layer and a third barrier metal layer. The first dielectric layer is disposed on the conductive layer, and has a via therein. The first barrier metal layer is disposed on the via bottom and the via sidewall in the first dielectric layer. The first barrier metal layer covers the conductive layer on the via bottom. The barrier layer is comprised of a material different with that of the first barrier metal layer. The second barrier metal layer covers the barrier layer on the via sidewall, and exposes the barrier layer on the via bottom. The third barrier metal layer covers the second barrier metal layer on the via sidewall, and covers the first barrier metal layer on the via bottom. A bottom of the barrier layer disposed on the via bottom is not punched through. 
         [0012]    The present invention uses chemical vapor deposition processes or physical vapor deposition processes to form a barrier layer on a conductive layer of a semiconductor device and then uses ion-bombardment to remove metallized materials of higher resistivity to reduce the resistivity of the barrier layer neighboring to the conductive layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
           [0014]      FIG. 1A  shows an illustrative chart of a dual damascene structure of the prior art; 
           [0015]      FIG. 1B  shows an illustrative chart of forming a barrier layer on a dual damascene structure of the prior art; 
           [0016]      FIGS. 2A-2E  shows an illustrative chart of the steps for forming multi-barrier layers on a dual damascene structure of a first embodiment in the present invention; 
           [0017]      FIGS. 3A-3E  shows an illustrative chart of the steps for forming multi-barrier layers on a damascene structure of a second embodiment in the present invention; 
           [0018]      FIG. 4  shows an illustrative chart of proceeding with physical vapor deposition processes in a plasma reactor in the present invention; 
           [0019]      FIG. 5  shows an illustrative chart of proceeding with ion-bombardment processes in a plasma reactor in the present invention; 
           [0020]      FIGS. 6D-6E  shows an illustrative chart of the steps for forming multi-barrier layers on a dual damascene structure of a third embodiment in the present invention; 
           [0021]      FIGS. 7D-7E  shows an illustrative chart of the steps for forming multi-barrier layers on a damascene structure of a fourth embodiment in the present invention; 
           [0022]      FIGS. 8B-8E  shows an illustrative chart of the steps for forming multi-barrier layers on a dual damascene structure of a fifth embodiment in the present invention; 
           [0023]      FIGS. 9B-9E  shows an illustrative chart of the steps for forming multi-barrier layers on a damascene structure of a sixth embodiment in the present invention; 
           [0024]      FIGS. 10E  shows an illustrative chart of forming multi-barrier layers on a dual damascene structure of a seventh embodiment in the present invention; and 
           [0025]      FIGS. 11E  shows an illustrative chart of forming multi-barrier layers on a damascene structure of an eighth embodiment in the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Some embodiments of the invention will be described exquisitely as below. Besides, the invention can also be practiced extensively in other embodiments. That is to say, the scope of the invention should not be restricted by the proposed embodiments. The scope of the invention should be based on the claims proposed later. 
         [0027]    In a first embodiment of the present invention, as shown in  FIGS. 2A-2E , a dual damascene structure  20  has been already formed on a metal layer  200  of a wafer. The dual damascene structure  20  comprises a first etch-stop layer  220 , a first dielectric layer  260  on the first etch-stop layer  220 , a second etch-stop layer  240  on the first dielectric layer  260 , and a second dielectric layer  280  on the second etch-stop layer  240 , wherein the metal layer  200  is a copper layer. A material of the first etch-stop layer  220  and the second etch-stop layer  240  is the one which can prevent copper atoms from diffusing into surrounding dielectric layers, such as silicon nitride (Si.sub.3N.sub.4). The material of the first dielectric layer  260  and the second dielectric layer  280  can be silicon dioxide or any other material whose dielectric constant is lower than 4, such as fluorinated silicate glass (FSG), organo silicate glass, fluorinated amorphous carbon, hydrogenated amorphous carbon, and tetrafluoropoly-p-xylylene. The first dielectric layer  260  and the second dielectric layer  280  of these materials can be formed by chemical vapor deposition processes. The material of the first dielectric layer  260  and the second dielectric layer  280  can also be hydrogenated silsesquioxane (HSQ), poly arylene ethers (PAE), co-polymer of divinylsiloxane and bis-Benzocyclobutene, aerogel, and xerogel. The first dielectric layer  260  and the second dielectric layer  280  can be formed by spin coating. 
         [0028]    As shown in  FIG. 2A , a first tantalum layer  300  is formed on the dual damascene structure  20  and the first tantalum layer  300  can be formed by chemical vapor deposition (CVD) processes or physical vapor deposition (PVD) processes. The first tantalum layer  300  is formed by PVD processes in the present embodiment. A plasma reactor  60  is shown in  FIG. 4 , a wafer  62  is secured to a wafer supporter  61  and the wafer supporter  61  is connected to a direct current (DC) bias  65 . A tantalum target  64  is secured to a metal target base  63  and the metal target base  63  is grounded. In the PVD processes, argon ions will bombard the tantalum target  64  and the tantalum atoms or ions bombarded out by argon ions will be attracted by the DC bias  65  and deposited on the wafer  62  thereby forming the first tantalum layer  300 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torrs to 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0029]    As shown in  FIG. 2B , a tantalum nitride layer  320  is formed on the first tantalum layer  300  and the tantalum nitride layer  320  can be formed by CVD processes or PVD processes. The tantalum nitride layer  320  is formed by PVD processes in the present embodiment. Similar to the way of forming the first tantalum layer  300 , nitrogen gas is introduced into the plasma reactor  60  and the nitrogen molecules will react with the tantalum atoms  67  or tantalum ions  66  from the tantalum target  64 , which is bombarded by argon ions to form the tantalum nitride layer  320  on the wafer  62 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torrs to 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0030]    The resistivity of the tantalum nitride layer  320  varies with the proportion of the nitrogen atoms. Generally, the resistivity of the tantalum nitride layer  320  is about between 95 micro-ohms centimeter and 14800 micro-ohms centimeter. The resistivity of the tantalum nitride layer  320  is far more than the resistivity of a tantalum layer. The resistivity of an α-phase tantalum layer is about between 15 micro-ohms centimeter and 30 micro-ohms centimeter and the resistivity of a β-phase tantalum layer is about between 150 micro-ohms centimeter and 220 micro-ohms centimeter. However, the resistivity of a copper layer is about 1.7 micro-ohms centimeter. Accordingly, in order to reduce the resistivity above the via bottom in the first dielectric layer  260 , the tantalum nitride layer  320  above the via bottom in the first dielectric layer  260  has to be removed. 
         [0031]    As shown in  FIG. 2C , in order to remove the tantalum nitride layer  320  above the via bottom in the first dielectric layer  260 , a method of ion-bombardment is taken. As shown in  FIG. 5 , a plasma reactor  80  is connected with a plasma generating power  84  and an alternating current bias power  83 . A wafer  82  is secured to a wafer supporter  81  in the plasma reactor  80 . When an ion-bombardment process is proceeded with, a self-direct current bias produced by the alternating current bias power  83  attracts argon ions  86  in the plasma  85  to bombard the wafer  82 . Then the tantalum atoms  360 , which escape from the tantalum nitride layer  320  above the via bottom in the first dielectric layer  260 , will be deposited on the via sidewall in the first dielectric layer  260 . The tantalum nitride layer  320  above the via bottom in the first dielectric layer  260  is removed. Because a moving direction of the argon atoms  86  is perpendicular to a surface of the wafer  82 , the tantalum nitride layer  320  deposited on the via sidewall in the first dielectric layer  260  sustains less ion-bombardment than the tantalum nitride layer  320  deposited above the via bottom in the first dielectric layer  260 . In the present embodiment, the self-direct current bias produced on the wafer supporter  81  is higher than the direct current bias in the PVD processes for deposition of the tantalum layer or the tantalum nitride layer. 
         [0032]    After the tantalum nitride layer  320  above the via bottom in the first dielectric layer  260  is removed by the method of ion-bombardment, the structure above the metal layer  200  is shown in  FIG. 2D . Only the first tantalum layer  300  exists above the via bottom in the first dielectric layer  260 . The tantalum atoms  360  sputtered from the via bottom in the first dielectric layer  260  and from the trench bottom in the second dielectric layer  280  will then separately be deposited on the sidewall of the downside of the via in the first dielectric layer  260  and on the sidewall of the downside of the trench in the second dielectric layer  280 . The figure of the structure is shown in  FIG. 2D . Further, as shown in  FIG. 2E , a second tantalum layer  340  is formed on the tantalum nitride layer  320  by the method such as the aforementioned method used for forming the first tantalum layer  300 . The second tantalum layer  340  can be formed by PVD processes or CVD processes. The second tantalum layer  340  is formed by PVD processes in the embodiment. A plasma reactor  60  is shown in  FIG. 4 , a wafer  62  is secured to a wafer supporter  61  and the wafer supporter  61  is connected to a direct current (DC) bias  65 . A tantalum target  64  is secured to a metal target base  63  and the metal target base  63  is grounded. In the PVD processes, argon ions will bombard the tantalum target  64  and the tantalum atoms or ions bombarded out by argon ions will be attracted by the DC bias  65  to be deposited on the wafer  62  thereby forming the second tantalum layer  340 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torr to 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0033]    After completing the aforementioned steps, the barrier layers of the dual damascene structure  20  are shown in  FIG. 2E . Only the tantalum layer, which consists of the first tantalum layer  300  and the second tantalum layer  340 , exists above the via bottom in the first dielectric layer  260  of the dual damascene structure  20 , however, all the three barrier layers exist on all the other portions of the dual damascene structure  20  except the via bottom. The three barrier layers are the first tantalum layer  300 , the tantalum nitride layer  320 , and the second tantalum layer  340  respectively. The tantalum is used because it has good adhesion to copper. The tantalum nitride is capable of preventing copper atoms from diffusing into surrounding dielectric layers. The barrier structure of the three barrier layers is thicker than the barrier layer at the side wall portion of a dual damascene structure in the prior art and thus the three barrier layers prevent copper atoms from diffusing into surrounding dielectric layers more efficiently. Besides, a portion of the barrier layer structure above the via bottom has a 30% lower resistance than of the prior art. Therefore, the tantalum layer has better ohmic contact with the copper layer below and the copper layer formed inside the dual damascene structure later. 
         [0034]    In another embodiment of the present disclosure, as shown in  FIGS. 3A-3E , a damascene structure  40  has been already formed on a metal layer  400  of a wafer. The damascene structure  40  comprises an etch-stop layer  420  and a dielectric layer  440  on the etch-stop layer  420 . The metal layer  400  is a copper layer. The etch-stop layer  420  consists of a material which can prevent copper atoms from diffusing into surrounding dielectric layers such as silicon nitride (Si.sub.3N.sub.4). The material of the dielectric layer  440  can be silicon dioxide or any other material whose dielectric constant is lower than 4, such as fluorinated silicate glass (FSG), organo silicate glass, fluorinated amorphous carbon, hydrogenated amorphous carbon, and tetrafluoropoly-p-xylylene. The dielectric layer  440  of these materials can be formed by chemical vapor deposition processes. The material of the dielectric layer  440  can also be hydrogenated silsesquioxane (HSQ), poly arylene ethers (PAE), co-polymar of divinylsiloxane and bis-Benzocyclobutene, aerogel, and xerogel, and dielectric layer  440  of these materials can be formed by spin coating. 
         [0035]    As shown in  FIG. 3A , a first tantalum layer  460  is formed on the damascene structure  40  and the first tantalum layer  460  can be formed by chemical vapor deposition (CVD) processes or physical vapor deposition (PVD) processes. The first tantalum layer  460  is formed by PVD processes in the present embodiment. A plasma reactor  60  is shown in  FIG. 4 , a wafer  62  is secured to a wafer supporter  61  and the wafer supporter  61  is connected to a direct current (DC) bias  65 . A tantalum target  64  is secured to a metal target base  63  and the metal target base  63  is grounded. In the PVD processes, argon ions will bombard the tantalum target  64  and the tantalum atoms or ions bombarded out by the argon ions will be attracted by the DC bias  65  to be deposited on the wafer  62  thereby forming the first tantalum layer  460 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torr tos 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0036]    As shown in  FIG. 3B , a tantalum nitride layer  480  is formed on the first tantalum layer  460  and the tantalum nitride layer  480  can be formed by CVD processes or PVD processes. The tantalum nitride layer  480  is formed by PVD processes in the present embodiment. Similar to the way of forming the first tantalum layer  460 , nitrogen gas is introduced into the plasma reactor  60  and the nitrogen molecules will react with the tantalum atoms  67  or tantalum ions  66  from the tantalum target  64  to form the tantalum nitride layer  480 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torrs to 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0037]    The resistivity of the tantalum nitride layer  480  varies with the proportion of the nitrogen atoms. Generally, the resistivity is about between 95 micro-ohms centimeter and 14800 micro-ohms centimeter. The resistivity of the tantalum nitride layer  480  is far more than the resistivity of a tantalum layer. The resistivity of the α-phase tantalum layer is about between 15 micro-ohms centimeter and 30 micro-ohms centimeter and the resistivity of the β-phase tantalum layer is about between 150 micro-ohms centimeter and 220 micro-ohms centimeter. However, the resistivity of a copper layer is about 1.7 micro-ohms centimeter. Accordingly, to reduce the resistivity above the via bottom in the dielectric layer  440 , the tantalum nitride layer  480  above the via bottom in the dielectric layer  440  has to be removed. 
         [0038]    As shown in  FIG. 3C , in order to remove the tantalum nitride layer  480  above the via bottom in the dielectric layer  440 , a method of ion-bombardment is taken. As shown in  FIG. 5 , a plasma reactor  80  is connected with a plasma generating power  84  and an alternating current bias power  83 . A wafer  82  is secured to a wafer supporter  81  in the plasma reactor  80 . When an ion-bombardment process is proceeded with, a self-direct current bias produced by the alternating current bias power  83  attracts argon ions  86  in the plasma  85  to bombard the wafer  82 , and then tantalum atoms  520 , which escape from the tantalum nitride layer  480  above the via bottom in the dielectric layer  440 , will be deposited on the via sidewall in the dielectric layer  440 . The tantalum nitride layer  480  above the via bottom in the dielectric layer  440  is removed. Because a moving direction of the argon atoms  86  is perpendicular to a surface of the wafer  82 , the tantalum nitride layer  480  deposited on the via sidewall in the dielectric layer  440  sustains less ion-bombardment than the tantalum nitride layer  480  deposited above the via bottom in the dielectric layer  440 . In the present embodiment, the self-direct current bias produced on the wafer supporter  81  is higher than the direct current bias in the PVD processes for deposition of the tantalum layer or the tantalum nitride layer. 
         [0039]    After the tantalum nitride layer  480  above the via bottom in the dielectric layer  440  is removed by the method of ion-bombardment, the structure above the metal layer  400  is shown in  FIG. 3D . Only the first tantalum layer  460  exists above the via bottom in the dielectric layer  440 . The tantalum atoms  520  that escape from the via bottom in the dielectric layer  440  is deposited on the sidewall of the downside of the via in the dielectric layer  440 . Then, the figure of the structure is shown in  FIG. 3D . Further, as shown in  FIG. 3E , a second tantalum layer  500  is formed on the tantalum nitride layer  480  by a method such as the aforementioned method used for forming the first tantalum layer  460 . The second tantalum layer  500  can be formed by PVD processes or CVD processes. The second tantalum layer  500  is formed by PVD processes in the present embodiment. A plasma reactor  60  is shown in  FIG. 4 , the wafer  62  is secured to the wafer supporter  61  and the wafer supporter  61  is connected to the direct current (DC) bias  65 . The tantalum target  64  is secured to the metal target base  63  and the metal target base  63  is grounded. In the PVD processes, argon ions will bombard the tantalum target  64  and the tantalum atoms or ions will be attracted by the DC bias  65  to be deposited on the wafer  62  thereby forming the second tantalum layer  500 . In the PVD processes, the process pressure in the plasma reactor  60  is about from 0 torrs to 50 millitorrs and the process temperature in the plasma reactor  60  is about from 0 degrees centigrade to 400 degrees centigrade. 
         [0040]    After completing the aforementioned steps, the barrier layers of the damascene structure  40  are shown in  FIG. 3E . Only the tantalum layer consisting of the first tantalum layer  460  and the second tantalum layer  500  exists above the via bottom in the dielectric layer  440  of the damascene structure  40 , however the three barrier layers exist on all the other portions of the damascene structure  40  except the via bottom. The three barrier layers are the first tantalum layer  440 , the tantalum nitride layer  480 , and the second tantalum layer  500  respectively. The tantalum is used because it has good adhesion to copper. The tantalum nitride is capable of preventing copper atoms from diffusing into surrounding dielectric layers. The barrier structure of the three barrier layers is thicker than the barrier layer of the side wall portion of a dual damascene structure in the prior art, and thus the barrier structure prevent copper atoms from diffusing into surrounding dielectric layers more efficiently. Besides, the portions of the tantalum layers directly above the via bottom of the dielectric layer has 30% lower resistance than that of the prior art. Therefore, the tantalum layer will have better ohmic contact with the copper layer below and the copper layer formed inside the damascene structure later. 
         [0041]    It is noted that the barrier layer of metallized materials disposed on the via bottom may be punched through in the above-mentioned embodiments, and may just be thinned in other embodiments.  FIGS. 6D-6E  illustrate a method for forming multi-barrier layers on a dual damascene structure of a third embodiment in the present disclosure. Compared with the first embodiment discussed previously, same labels will be carried forward through  FIGS. 6D-6E . 
         [0042]    As shown in  FIG. 6D , a dual damascene structure  70  is formed on the metal layer  200  of a wafer, the first tantalum layer  300  is formed on the dual damascene structure  70 , a tantalum nitride layer  320   a  is formed on the first tantalum layer  300 , and an ion-bombardment process may be performed on the tantalum nitride layer  320   a  through the steps shown in  FIG. 2A-2C . One difference between the first embodiment and the third embodiment is that the ion-bombardment process does not punch through the tantalum nitride layer  320   a  disposed on the via bottom in the third embodiment. In other words, only portions of the tantalum nitride layer  320   a  on the via bottom in the first dielectric layer  260  are removed. Portions of the tantalum nitride layer  320   a  may still remain on the via bottom and the via sidewall in the first dielectric layer  260  without removing the first tantalum layer  300  on the via bottom. In the ion-bombardment process, a self-direct current bias attracts argon ions  86  to bombard the tantalum nitride layer  320   a,  and the tantalum atoms  360  that escape from the tantalum nitride layer  320  on the via bottom move toward the via sidewall. Therefore, the tantalum nitride layer  320   a  may still remain on the whole via bottom in the first dielectric layer  260 , and portions of the tantalum nitride layer  320   a  disposed on the via bottom is thinned by the ion-bombardment process. 
         [0043]    As shown in  FIG. 6E , the second tantalum layer  340  is formed on the tantalum nitride layer  320   a.  After completing the aforementioned steps, the tri-layer barrier structure including the first tantalum layer  300 , the tantalum nitride layer  320   a  and the second tantalum layer  340  may be disposed on both the via bottom and the whole via sidewall. Portions of the tantalum nitride layer  320   a  disposed on the via bottom may be thinner than portions of the tantalum nitride layer  320   a  disposed on the via sidewall. After the second tantalum layer  340  is formed, a conductive layer, such as copper layer, (not shown) may be formed on the second tantalum layer  340  and filling the dual damascene structure  70 . Since the resistivity of the tantalum nitride layer  320   a  varies with the proportion of the nitrogen atoms within the tantalum nitride layer  320   a,  and the tantalum nitride layer  320   a  may be thinned, the resistance above the via bottom in the first dielectric layer  260  can also be effectively reduced. 
         [0044]    The ion-bombardment process without punching through the tantalum nitride layer may also be applied to a damascene structure.  FIGS. 7D-7E , a method for forming multi-barrier layers on a damascene structure of a fourth embodiment in the present disclosure is illustrated. In order to compare to the second embodiment discussed previously, same labels will be carried forward through  FIGS. 7D-7E . 
         [0045]    As shown in  FIG. 7D , a damascene structure  90  is formed on a metal layer  400  of a wafer, the first tantalum layer  460  is formed on the dual damascene structure  90 , the tantalum nitride layer  480   a  is formed on the first tantalum layer  460 , and an ion-bombardment process may be performed on the tantalum nitride layer  480   a  through the steps shown in  FIG. 3A-3C . One difference between the second embodiment and the fourth embodiment is that the ion-bombardment process does not punch through the tantalum nitride layer  480   a  disposed on the via bottom in the fourth embodiment. In other words, only portions of the tantalum nitride layer  480   a  on the via bottom in the dielectric layer  440  are removed. Portions of the tantalum nitride layer  480   a  may still remain on the whole bottom and the whole via sidewall in the dielectric layer  440  without removing the first tantalum layer  460  on the via bottom. 
         [0046]    As shown in  FIG. 7E , the second tantalum layer  500  is formed on the tantalum nitride layer  480   a.  After completing the aforementioned steps, the tri-layer barrier structure including the first tantalum layer  300 , the tantalum nitride layer  480   a  and the second tantalum layer  500  may be disposed on both the via bottom and the whole via sidewall. Portions of the tantalum nitride layer  480   a  disposed on the via bottom may be thinner than portions of the tantalum nitride layer  480   a  disposed on the via sidewall. Since the resistivity of the tantalum nitride layer  480   a  varies with the proportion of the nitrogen atoms within the tantalum nitride layer  480   a,  and the tantalum nitride layer  480   a  may be thinned, the resistance above the via bottom in the dielectric layer  440  can also be effectively reduced. 
         [0047]    Moreover, the multi-barrier layers formed on the damascene structure or on the dual damascene structure may include more than three barrier layers in other embodiments. Please refer to  FIGS. 8B-8E , a method for forming multi-barrier layers on a dual damascene structure of a fifth embodiment in the present invention is illustrated. In order to compare to the first embodiment discussed previously, same labels will be carried forward through  FIGS. 8B-8E . 
         [0048]    As shown in  FIG. 8B , a dual damascene structure  30  is first formed on the metal layer  200  of a wafer, the first tantalum layer  300  is formed on the damascene structure  50 , and the tantalum nitride layer  320  is formed on the first tantalum layer  300  through the steps shown in  FIG. 2A-2B . One difference between the first embodiment and the fifth embodiment is that a second tantalum layer  340  is further formed on the tantalum nitride layer  320  before the ion-bombardment process in the fifth embodiment. 
         [0049]    As shown in  FIG. 8C-8D , an ion-bombardment process may be performed next on both the second tantalum layer  340  and the tantalum nitride layer  320 . The ion-bombardment process may first remove the second tantalum layer  340 , and may subsequently remove the tantalum nitride layer  320  after the second tantalum layer  340  is punched through. In this embodiment, the ion-bombardment process may punch through both the second tantalum layer  340  and the tantalum nitride layer  320  disposed on the via bottom. Only the first tantalum layer  300  exists above the via bottom in the first dielectric layer  260 . The ion-bombardment process may leave the second tantalum layer  340  and the tantalum nitride layer  320  remaining on the whole via sidewall in the first dielectric layer  260  without removing the first tantalum layer  300  on the via bottom. 
         [0050]    As shown in  FIG. 8E , the third tantalum layer  350  is formed on the second tantalum layer  340  and the tantalum nitride layer  320 . After completing the aforementioned steps, both the first tantalum layer  300  and the third tantalum layer  350  may be disposed on the via bottom; and the first tantalum layer  300 , the tantalum nitride layer  320 , the second tantalum layer  340  and the third tantalum layer  350  may be disposed on the whole via sidewall. In other words, there are four barrier layers, which include the first tantalum layer  300 , the tantalum nitride layer  320 , the second tantalum layer  340  and the third tantalum layer  350 , on the via sidewall to prevent copper atoms from diffusing into surrounding dielectric layers. Portions of the tantalum nitride layer  320  disposed on the via bottom is punched through or thinned. After the third tantalum layer  350  is formed, a conductive layer, such as copper layer, (not shown) may be formed on the third tantalum layer  350  and filling the dual damascene structure  30 . Since the resistivity of the tantalum nitride layer  320  varies with the proportion of the nitrogen atoms within the tantalum nitride layer  320 , and the tantalum nitride layer  320  may be punched through or thinned, the resistance above the via bottom in the first dielectric layer  260  can also be effectively reduced. 
         [0051]    It can be understood that portions of the tantalum nitride layer  320  and/or portions of the second tantalum layer  340  may still remain on the via bottom in other embodiments, as shown in  FIG. 10E . 
         [0052]    The four-barrier layers may also be applied to a damascene structure. Please refer to  FIGS. 9B-9E , a method for forming multi-barrier layers on a damascene structure of a sixth embodiment in the present invention is illustrated. In order to compare to the second embodiment discussed previously, same labels will be carried forward through  FIGS. 9B-9E . 
         [0053]    As shown in  FIG. 9B , a damascene structure  50  is first formed on a metal layer  400  of a wafer, the first tantalum layer  460  is formed on the damascene structure  50 , and the tantalum nitride layer  480  is formed on the first tantalum layer  460  through the steps shown in  FIG. 3A-3B . One difference between the second embodiment and the sixth embodiment is that the second tantalum layer  500  is further formed on the tantalum nitride layer  480  before the ion-bombardment process in the sixth embodiment. 
         [0054]    As shown in  FIG. 9C-9D , an ion-bombardment process may be performed on both the second tantalum layer  500  and the tantalum nitride layer  480  on the via bottom. The ion-bombardment process may first remove the second tantalum layer  500 , and may subsequently remove the tantalum nitride layer  480 . In this embodiment, the ion-bombardment process may punch through both the second tantalum layer  500  and the tantalum nitride layer  480  disposed on the via bottom. The ion-bombardment process may leave both the second tantalum layer  500  and the tantalum nitride layer  480  remaining on the whole via sidewall in the dielectric layer  440  without removing the first tantalum layer  460  on the via bottom. 
         [0055]    As shown in  FIG. 9E , the third tantalum layer  510  is formed on the second tantalum layer  500  and the tantalum nitride layer  480 . After completing the aforementioned steps, the first tantalum layer  460  and the third tantalum layer  510  may be disposed on the via bottom; and the first tantalum layer  460 , the tantalum nitride layer  480 , the second tantalum layer  500  and the third tantalum layer  510  may be disposed on the whole via sidewall. In other words, there are four barrier layers, which include the first tantalum layer  460 , the tantalum nitride layer  480 , the second tantalum layer  500  and the third tantalum layer  510 , on the via sidewall to prevent copper atoms from diffusing into surrounding dielectric layers. Portions of the tantalum nitride layer  480  disposed on the via bottom is punched through or thinned. Since the resistivity of the tantalum nitride layer  480  varies with the proportion of the nitrogen atoms within the tantalum nitride layer  480 , and the tantalum nitride layer  480  may be punched through or thinned, the resistance above the via bottom in the dielectric layer  440  can also be effectively reduced. 
         [0056]    It can be understood that portions of the second tantalum layer  500  and portions of the tantalum nitride layer  480  may still remain on the via bottom in other embodiments, as shown in  FIG. 11E . 
         [0057]    The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including configurations ways of the recessed portions and materials and/or designs of the attaching structures. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments. As with the operating sequence of the present invention, many variations are possible, and any rearrangement of the operating sequence for achieving same functionality is still within the spirit and scope of the invention.