Abstract:
A thin-film capacitor comprising a first thin-film electrode, a second thin-film electrode, and a thin dielectric film arranged therebetween and formed of a tantalum oxide layer and an aluminum oxide layer neighboring thereto. A method of producing such a thin-film capacitor is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a thin-film capacitor, to be incorporated in a circuit board used for semiconductor packages and the like, and to a method of producing the capacitor.  
         [0003]     2. Description of the Related Art  
         [0004]     When capacitor parts are necessary in a semiconductor package, it has been known to incorporate them in the circuit board of the semiconductor package (see, for example, Japanese Unexamined Patent Publications (Kokai) NOs. 2001-110675 and 2002-260960, corresponding to U.S. Pat. No. 6,498,714 B1 and U.S. 2002/0122283 A1, respectively.). The capacitor incorporated in the circuit board has a thin film stacked structure in which a dielectric thin film is arranged between thin conductor films forming two electrodes. It is also called thin-film capacitor.  
         [0005]     As a conductor for forming electrodes of the thin-film capacitor, there can be used titanium, platinum, copper, aluminum or the like. A capacitor using copper electrodes is known as a low-resistance capacitor. As a dielectric, Ta 2 O 5 , which is a material having a high dielectric constant, is usually used. A capacitor constituted by copper (Cu) electrodes and the Ta 2 O 5  dielectric is fabricated by anodically treating (anodizing) a Ta film formed on a Cu film that serves as one electrode to thereby form a Ta 2 O 5  dielectric film, and forming a Cu film thereon to serve as another electrode.  
         [0006]     Another thin film may also be provided between the thin conductor film and the thin dielectric film in order to improve capacitor characteristics or to improve adhesion between the thin films. For example, it has been known to provide a thin Ti film as a barrier metal layer between the Cu film and the Ta film to prevent the interruption of anodic treatment that results from the leakage of a treating current as Cu dissolves in the treating solution (electrolyte) at the time of anodically treating the Ta film on the Cu film. It has further been known to arrange, for example, a thin Cr film between the Ta 2 O 5  film and the Cu film to improve the adhesion between them. Further, it has been known to insert thin alumina (Al 2 O 3 ) films between the two Cu electrodes and the Ta 2 O 5  film for improving the quality of the Ta 2 O 5  film in the capacitor of the Cu/Ta 2 O 5 /Cu stacked structure for LSIs for wireless communication equipment (T. Ishikawa, et al, High-Capacitance Cu/Ta 2 O 5 /Cu MIM Structure for SoC Applications Featuring a Single-Mask Add-on Process, 2002 International Electron Devices Meetings, IEEE (2002)), though this is not for the circuit boards.  
       SUMMARY OF THE INVENTION  
       [0007]     As described above, in the thin-film capacitor, a Ta 2 O 5  film is usually used as a dielectric material. However, a capacitor using the Ta 2 O 5  film has a disadvantage of a small withstand voltage (about 5 V) and it permits a large leakage current to flow.  
         [0008]     In conducting the anodic treatment, further, it is difficult to precisely convert the whole Ta film into the Ta 2 O 5  film. Usually, therefore, the anodic treatment is finished while leaving part of the Ta film. In this case, as Ta is a material having a high electric resistance, Ta that is remaining raises the electric resistance of the capacitor.  
         [0009]     In a case where Cu is used as the electrode, when a Ti film is inserted between the Cu film and the Ta film to prevent the elution of Cu at the time of anodically treating Ta, Ti, which is also a material having a high electric resistance, increases the electrode resistance, making it difficult to utilize properties of Cu which is a low-resistance material to a sufficient degree.  
         [0010]     It is therefore an object of the present invention to provide a thin-film capacitor which has a small leakage current and makes it easy to lower the resistance of the electrodes, as a result of solving the above-mentioned problems.  
         [0011]     A thin-film capacitor of the present invention comprises a first thin-film electrode, a second thin-film electrode, and a thin dielectric film arranged therebetween and formed of a tantalum oxide layer and an aluminum oxide layer neighboring thereto.  
         [0012]     The thin-film capacitor of the invention can be produced by a method of producing a thin-film capacitor by forming a first thin electrode layer, successively forming an aluminum layer and a tantalum layer or a tantalum nitride layer thereon, anodically treating the tantalum layer or the tantalum nitride layer and the aluminum layer to form a thin dielectric film comprising a tantalum oxide layer and an aluminum oxide layer, and forming a second thin electrode layer on the tantalum oxide layer.  
         [0013]     In the thin-film capacitor of this invention, the dielectric portion between the electrodes forms the structure of a series connection of the tantalum oxide film and the aluminum oxide film, and can decrease a potential difference applied across the tantalum oxide portion, which has a high dielectric constant but has poor withstand voltage, while it can improve the withstand voltage of the capacitor owing to the presence of the aluminum oxide portion having an excellent withstand voltage. The aluminum oxide film is effective in preventing the diffusion of highly migrating metals such as Cu. According to the present invention, therefore, even when Cu is used for the electrode material, there can be realized a capacitor having a low resistance without the need of providing a barrier metal layer formed by Ti having a high resistance. In conducting the anodic treatment, all the tantalum is oxidized, and no high resistance tantalum remains, which contributes to lowering the resistance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  schematically illustrates a thin-film capacitor according to the present invention;  
         [0015]      FIGS. 2A and 2B  illustrate potential differences applied to the dielectric portions in a conventional capacitor and in the capacitor of the invention, respectively;  
         [0016]      FIG. 3  illustrates a circuit board incorporating a thin-film capacitor of the present invention;  
         [0017]      FIG. 4  illustrates another circuit board incorporating a thin-film capacitor according of the present invention;  
         [0018]      FIG. 5  illustrates another embodiment of the thin-film capacitor of the present invention;  
         [0019]      FIGS. 6A  to  6 D illustrate the production of the capacitor of Example 1;  
         [0020]      FIG. 7  illustrates the capacitor produced in Example 2; and  
         [0021]      FIGS. 8A and 8B  are graphs showing leakage current characteristics of a capacitor of the comparative example and the capacitor of Example 1, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     A thin-film capacitor  10  of the present invention is schematically illustrated in  FIG. 1 . The thin-film capacitor  10  is constituted by a first thin electrode film  12 , a dielectric film  14  located thereon and formed of an aluminum oxide layer  14   a  and a tantalum oxide layer  14   b,  and a second thin electrode film  16  located thereon.  
         [0023]     The two thin electrode films  12  and  16  are formed by using a conductor material. Representative conductor materials include platinum (Pt), gold (Au), ruthenium (Ru), copper (Cu) and nickel (Ni). Use of Cu which is a low-resistance material makes it possible to obtain a capacitor having a small resistance. The two thin electrode films  12  and  16  may be made of the same material or different materials.  
         [0024]     The thin dielectric film  14  is formed of an aluminum oxide (hereinafter often expressed as Al 2 O 3 ) layer  14   a  and a tantalum oxide (hereinafter often expressed as Ta 2 O 5 ) layer  14   b.  The Ta 2 O 5  layer  14   b  has a high dielectric constant, making it possible to obtain a capacitor of a large capacity, but its withstand voltage is not favorable. The Al 2 O 3  layer  14   a  compensates for the lack of withstand voltage of the Ta 2 O 5  layer  14   b  but has an inferior dielectric constant. In the capacitor of the invention, it is preferred that the thicknesses of the two layers are determined by taking the properties of the two layers into consideration so as to realize the characteristics required for a certain capacitor. Usually, it is preferred that the thickness of the Al 2 O 3  layer  14   a  is decreased to as small as possible so that the total dielectric constant of the capacitor does not decrease unnecessarily. For instance, the Ta 2 O 5  layer  14   b  may have a thickness of about 0.1 to about 0.5μ, and the Al 2 O 3  layer  14   a  may have a thickness of about 0.01 to about 0.1 μm.  
         [0025]     The oxide layers  14   a  and  14   b  can be formed by anodically treating the aluminum (Al) layer and the tantalum (Ta) layer, that have been successively formed on the first thin electrode film  12 , together. The Ta 2 O 5  layer can also be obtained by anodically treating a tantalum nitride (TaN) layer that is formed instead of the Ta layer. The thickness of the metal layer oxidized by the treatment can be controlled by the applied voltage. The layers to be subjected to the anodic treatment are very thin. In oxidizing the Al layer following the oxidation of the Ta layer, therefore, it is difficult to so control that only the Al layer is entirely oxidized without affecting the underlying thin electrode layer  12 . Usually, therefore, the applied voltage is so controlled that the oxidation ends before the oxidation reaches the interface between the Al layer and the thin electrode film  12 . As a result, there remains a thin Al film  13  as shown in  FIG. 1  between the Al 2 O 3  layer  14   a  and the first thin electrode film  12 . Nevertheless, a capacitor of a structure with no remaining Al film  13  as a result of completely oxidizing the Al layer, is included in the scope of the invention as a matter of course.  
         [0026]     In a conventional thin-film capacitor in which a dielectric portion between the electrodes is constituted by a Ta 2 O 5  film, a potential difference Va−Vb which is a difference between a potential Va on one electrode and a potential Vb on the other electrode is applied to the dielectric portion as illustrated in  FIG. 2A  when the capacitor operates. In the thin-film capacitor of the invention illustrated in  FIG. 1 , on the other hand, the dielectric portion between the electrodes  12  and  16  has a structure in which the Al 2 O 3  film  14   a  and the Ta 2 O 3  film  14   b  are connected in series. Therefore, when the same potential difference Va−Vb as that of the prior art of  FIG. 2A  is applied across the two electrodes, the potential difference Va−Vc applied to the tantalum oxide portion having a high dielectric constant but an inferior withstand voltage can be decreased to be smaller than Va−Vb as illustrated in  FIG. 2B . Accordingly, the leakage current can be decreased.  
         [0027]     The thin-film capacitor  10  of the invention can also include thin films or layers other than those described, as required. For example, when the thin electrode film is formed by using Cu, a thin Cr film (not shown) may be included between the Ta 2 O 5  layer  14   b  and the thin electrode layer  16  of Cu to improve adhesion between them.  
         [0028]     The capacitor  10  of the invention is incorporated in the circuit board used in a semiconductor package or the like, and is positioned on any insulating support member  11  which is a member of the circuit board. A representative example of the circuit board incorporating the capacitor of the invention is a printed wiring board. A build-up board formed by alternately stacking the wiring layers and the insulating layers on one surface or on both surfaces of the printed wiring board which is a core substrate is also included in the circuit board that is referred to here. The capacitor of the invention may be used as one incorporated in the rerouted wiring layer of chip-size packages, or for any other application.  
         [0029]      FIG. 3  illustrates an example of incorporating the capacitor of the invention in the uppermost layer of a build-up wiring board  20 . The build-up wiring board  20  is formed by alternately stacking the wiring layers  22  and the insulating layers  24  on the upper and lower surfaces of the printed board  21  which is a core substrate having a through hole  23  formed in a predetermined portion. The upper and lower wiring layers  22  of the printed board  21  are communicated together through a wiring conductor deposited on the inner wall of the through hole  23 . The thin-film capacitor of the invention (stacked structure of lower electrode  12 , remaining Al layer  13 , Al 2 O 3  layer  14   a,  Ta 2 O 5  layer  14   b  and upper electrode  16 ) is incorporated above the through hole  23 . The lower electrode  12  of the capacitor is connected to the lower wiring layer  22 , and the upper electrode  16  is connected to a semiconductor chip or the like by forming, for example, a bump (not shown), in the opening provided in the uppermost insulating layer  24  covering the upper electrode  16 .  
         [0030]      FIG. 4  illustrates an example of incorporating the capacitor of the invention within a build-up board  30 . This build-up board  30  is also formed by alternately stacking wiring layers  32  and an insulating layers  34  on the upper and lower surfaces of a printed board  31  which is a core substrate. The upper and lower wiring layers  32  of the printed board  31  are connected together through a wiring conductor filled in a through hole  35  formed in the insulating material that penetrates the printed board  31 . A thin-film capacitor  10   a  of the invention is formed on the wiring layer  32  directly provided on the printed board  31  and, in addition, a thin-film capacitor  10   b  of the invention is formed on the upper wiring layer  32 . The upper thin-film capacitor  10   b  is connected to the uppermost wiring layer  32  through an opening formed in the insulating layer  34  covering the upper thin-film capacitor  10   b,  the uppermost wiring layer being protected by a protection film  37 .  
         [0031]     The build-up boards, such as those illustrated in  FIGS. 3 and 4 , incorporating the capacitor or capacitors and methods of their production are well known (see, for example, Japanese Unexamined Patent Publications (Kokai) NOs. 2001-110675 and 2002-260960), and are not described here in detail.  
         [0032]     The thin-film capacitor  10  ( FIG. 1 ) of the present invention can be easily produced by forming the stacked structure constituting the capacitor on any support member  11  by known methods of depositing and patterning a material. For example, the deposition of the materials for the respective films can be carried out by any process, such as sputtering, evaporation, plating or chemical vapor deposition (CVD). The dielectric film of the capacitor can be easily obtained by anodically treating Ta and Al of the stack together. The anodic treatment itself is widely known.  
         [0033]     In the boards incorporating the capacitor or capacitors of the invention illustrated in  FIGS. 3 and 4 , the upper electrode and the lower electrode of the capacitor are connected to members which overlay and underlie the capacitor. Although the two electrodes  12  and  16 , and the Al film  13 , Al 2 O 3  layer  14   a  and Ta 2 O 5  layer  14   b  interposed therebetween, have the same size, they need not necessarily have the same size.  
         [0034]     For instance, both the upper electrode and the lower electrode may be connected to members located at one side of the capacitor. In this case, the two electrodes have different sizes and shapes.  FIG. 5  illustrates a capacitor in which both electrodes are connected to the upper wiring lines. In the capacitor of  FIG. 5 , the lower electrode  12  and the Al film  13  are formed in sizes larger than the Al 2 O 3  layer  14   a,  Ta 2 O 5  layer  14   b  and upper electrode  16 , which are located thereon, and are extending toward the right side. An insulating layer  18  is formed to cover the capacitor, and the upper electrode  16  and the lower electrode  12  are connected to the upper wiring lines (not shown) through an opening  19   a  formed in the insulating film  18  and reaching the upper electrode  16  and through an opening  19   b  reaching the lower electrode  12 , respectively. The Al film  13  may not be formed on the portion of the lower electrode  12  extending toward the right, so that the lower electrode  12  can be connected to the upper wiring line without through the Al film  13 .  
         [0035]     Although the above-mentioned thin-film capacitor of the invention has the Al 2 O 3  film  14   a  formed only between the Ta 2 O 5  film  14   b  of a high dielectric constant and the one electrode  12 , another Al 2 O 3  film may be formed between the Ta 2 O 5  film  14   b  and another electrode  16 .  
       EXAMPLES  
       [0036]     The invention will now be described making reference to examples which, however, are not intended to limit the invention.  
       Example 1  
       [0037]     On a support member  50  of an insulating material (epoxy resin) shown in  FIG. 6A , there were successively formed a Cu film  52  (10 μm thick), an Al film  54  (1 μm thick) and a Ta film  56  (0.15 μm thick) by sputtering. Next, the support member  50  having the above films formed was immersed in an aqueous solution of citric acid and was subjected to the anodic treatment by the application of a DC voltage of 200 V to convert the Ta film  56  into a Ta 2 O 5  film  56   a  ( FIG. 6B ) and to convert part of the Al film  54  into an Al 2 O 3  film  54   a  ( FIG. 6B ). Subsequently, as shown in  FIG. 6C , a Cu film  58  (10 μm thick) was formed by sputtering on the Ta 2 O 5  film  56   a.  Then, the films were successively patterned by dry etching to produce a thin-film capacitor  60  illustrated in  FIG. 6D .  
       Example 2  
       [0038]     Example 1 was repeated with the exception of further forming an Al film (0.1 μm thick) on the Ta film  56  of  FIG. 6A , and anodically treating three films, i.e., the Al film, and the underlying Ta film and Al film together (at an applied voltage of 250 V), to produce a capacitor having a dielectric film of a three-layer structure of Al 2 O 3  film  54   a /Ta 2 O 5  film  56   a /Al 2 O 3  film  57  illustrated in  FIG. 7 . Of the two Al films, the one on the Ta film was completely oxidized, and no Al film existed between the Al 2 O 3  film  57  and the upper electrode  58 .  
       Comparative Example  
       [0039]     The same thin-film capacitor as that of Example 1 was prepared but it had a Ti film of 1 μm thick instead of the Al 2 O 3  film and the Al film between the lower electrode and the Ta 2 O 5  film.  
         [0040]     The capacitor obtained in Comparative Example and the capacitor obtained in Example 1 were measured for their leakage current characteristics (I−V measurement) to obtain the results as shown in  FIGS. 8A and 8B , respectively. The capacitor of Comparative Example has a practicable withstand voltage in a range of about ±5 V ( FIG. 8A ), whereas the capacitor of the present invention has a practicable withstand voltage over a range of from about −10 to about 12 V ( FIG. 8B ) owing to only the provision of the Al 2 O 3  layer between the one electrode and the Ta 2 O 5  dielectric layer. In this range, the amount of leakage current of the capacitor of the invention is about 10 −9  A/cm 2 , exhibiting favorable leakage current characteristics as compared to the amount of leakage current of 10 −8  A/cm 2  of the capacitor for LSIs used in the wireless communication equipment, as taught in T. Ishikawa et al. referred to above.