Abstract:
The present invention discloses a method for fabricating concentration-gradient high-frequency ferromagnetic film, wherein the primary material target is arranged exactly below the sputter-coated substrate to achieve the on-substrate concentration uniformity of the components coming from the primary material target; at least one doping target is arranged at a position deviating from the center of the substrate to create a doping concentration gradient on the substrate along a direction, and a stress gradient is thus created on the substrate along the direction of concentration variation. Thus, the as-deposited ferromagnetic material fabricated at ambient temperature can possess the uniaxial anisotropy that a high-frequency ferromagnetic material needs.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for fabricating a high-frequency ferromagnetic film, particularly to a method for fabricating a concentration-gradient high-frequency ferromagnetic film. 
       BACKGROUND OF THE INVENTION 
       [0002]    The communication-related IC has persistently advanced with the development of the communication market, and more and more manufacturers adopt SOC (system-on-chip) to fabricate products. However, not all elements are suitable be integrated into a single chip. 
         [0003]    As a ferromagnetic planar inductor can increase the inductance, reduce the size and promote the performance, the integration technology of a high-frequency ferromagnetic film and a planar inductor has been extensively used in the integration process of passive elements recently. At present, ferromagnetic planar inductors have been widely applied to DC-DC converters, filters, modulators and phase shifters. 
         [0004]    In applications mentioned above, the high-frequency performance of a ferromagnetic film is a critical factor in the integration process. The ferromagnetic material usually needs high magnetization intensity and a uniaxial anisotropy. Thus, an iron-cobalt-based material is usually adopted as the ferromagnetic material and heat-treated at a temperature of between 300 and 600□ to create a uniaxial anisotropy. However, in addition to increasing the fabrication cost, the higher temperature of the heat treatment is incompatible with the conventional integration process of a silicon substrate because the temperature of the heat treatment will damage other elements or even make the entire fabrication process impossible. 
       SUMMARY OF THE INVENTION 
       [0005]    The primary objective of the present invention is to provide a method for fabricating a uniaxial-anisotropy high-frequency ferromagnetic film at ambient temperature without using a heat treatment. 
         [0006]    Another objective of the present invention is to provide a method for fabricating a high-frequency ferromagnetic film at ambient temperature to solve the conventional problem that the process of fabricating a ferromagnetic film is incompatible with the processes of fabricating other elements. 
         [0007]    To achieve objectives mentioned above, the present invention proposes a method for fabricating a composition-gradient high-frequency ferromagnetic film, which comprises the following steps: arranging the primary material target exactly below the sputter-coated substrate to achieve the on-substrate concentration uniformity of the components coming from the primary material target; and arranging at least one doping target at a position deviating from the center of the substrate to create a doping concentration gradient on the substrate along a direction. 
         [0008]    The components of the primary material target are selected from ferromagnetic materials. The ferromagnetic material may be iron, cobalt or nickel. Alternatively, the ferromagnetic material may be a ferromagnetic alloy, such as an alloy of magnetic transition metals or an alloy of transition metals and rare earth elements. Further, the components of the primary material target may be selected from non-metal magnetic materials. The non-metal magnetic materials are magnetic oxides, such as ferrite. The components of the doping target are selected from metals, oxides, nitrides and borides. 
         [0009]    The present invention utilizes the gradient sputtering technology, which arranges the doping target at a position deviating from the center of the substrate, to perform the sputtering of a doping (such as a metal, an oxide, a nitride or a boride) and create a doping concentration gradient on the substrate along a direction. Thus, a stress gradient is created on the substrate along the direction of concentration variation. Thereby, the as-deposited ferromagnetic material fabricated at ambient temperature can possess the uniaxial anisotropy that a high-frequency ferromagnetic material needs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram schematically showing the method for fabricating concentration-gradient high-frequency ferromagnetic film according to the present invention. 
           [0011]      FIG. 2  is a diagram schematically showing a concentration-gradient high-frequency ferromagnetic film fabricated according to the present invention. 
           [0012]      FIG. 3  is a diagram showing the Al concentration distribution on a substrate. 
           [0013]      FIG. 4  is a diagram showing the magnetization intensity of a sample having a composition Fe 39.3 CO 43.2 Al 2.4 O 15.1 . 
           [0014]      FIG. 5  is a diagram showing the relationship between the magnetic response and the frequency of a sample having a composition Fe 39.3 CO 43.2 Al 20.4 O 15 . 
           [0015]      FIG. 6  is a diagram showing the Hf concentration distribution on a substrate. 
           [0016]      FIG. 7  is a diagram showing the magnetization intensity of a sample having a composition Fe 48.5 CO 20.7 Hf 10.9 N 19.9 . 
           [0017]      FIG. 8  is a diagram showing the relationship between the magnetic response and the frequency of a sample having a composition Fe 48.5 Co 20.7 Hf 10.9 N 19.9 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The technical contents of the present invention will be described in detail with the embodiments. It is to be noted that those embodiments are only to exemplify the present invention but not to limit the scope of the present invention. 
         [0019]    The present invention is realized with a vacuum sputtering process. Refer to  FIG. 1 . A substrate  20 , which is to be sputter-coated, is arranged on a sputtering table  10 . A target holder  11  holding a primary material target  30  is arranged exactly below the substrate  20  to achieve the on-substrate concentration uniformity of the components coming from the primary materials. A target holder  12  holding a doping target  40  is arranged at a position deviating from the center of the substrate  20  to create a doping concentration gradient on the substrate  20  along a direction. Refer to  FIG. 2 . The concentrations of the components coming from the doping target  40  increase along a direction in the sputter-coated film  50  (containing the components of the primary material target  30  and the doping target  40 ) on the substrate  20 . 
         [0020]    The components of the primary material target  30  are selected from ferromagnetic materials. The ferromagnetic materials may be iron, cobalt or nickel. Alternatively, the ferromagnetic material may be a ferromagnetic alloy, such as an alloy of magnetic transition metals or an alloy of transition metals and rare earth elements. Further, the components of the primary material target may be selected from non-metal magnetic materials. The non-metal magnetic materials are magnetic oxides, such as ferrite. The components of the doping target  40  are selected from metals, oxides, nitrides and borides. 
         [0021]    Below, the existing high-frequency ferromagnetic materials FeCo-oxides and FeCoMN are used to exemplify the present invention. The composition expression of FeCo-oxides is Fe x Co y -Q z , wherein x=40˜80; y=20˜50; x+y+z=100; Q=Al 2 O 3 , SiO 2 , TiO 2 , or Ta 2 O 5 , which is attained via sputtering the doping target  40 . However, the concentration of an oxide in the sputter-coated film is not necessarily equal to the concentration of the oxide in the doping target  40 . The composition expression of FeCoMN is Fe x Co y M z N v , wherein x=40˜80; y=20˜50; v=2˜40; x+y+z+v=100; M=Hf, Zr, Nb, V, Mo, W, Cr, or Ta. The numerical values mentioned above refer to atom percentage. 
         [0022]    According to the method of the present invention, the primary material target  30 —FeCo is arranged exactly below the substrate  20  to achieve the concentration uniformity of iron and cobalt on the substrate  20 . At least one doping target  40 , such as a metal target or an oxide target, is arranged at a position deviating from the center of the substrate  20  to create a doping concentration gradient on the substrate  20  along a direction. If a nitride is to be sputtered, an appropriate proportion of the mixture gas of nitrogen and argon can be added into the reaction chamber. 
         [0023]    Below, the characteristics and properties of two samples having typical compositions are used to demonstrate the efficacy of the present invention. 
         [0024]    Composition 1—the (Fe x Co y ) z —(Al u O v ) w  system: The material system adopts a Fe 50 CO 50  target as the primary material target  30  and an Al 2 O 3  target as the doping target  40 , and the film is formed via RF (Radio Frequency) gradient sputtering under an Ar-containing environment. In this example, the sputtering powers of the two targets are respectively 60 W and 60 W. The concentration gradient on the substrate  20  is detected with FE-EPMA (Field Emission Electron Probe Micro-Analyzer), and the measurement result is shown in the table below. 
         [0000]                                                                  Distance                       from S to E (mm)   Fe (at. %)   Co (at. %)   Al (at. %)   O (at. %)                                2.5   36.38   42.55   3.29   17.78       7.5   37.64   42.08   3.00   17.28       12.5   39.63   42.05   2.50   15.83       17.5   38.91   42.69   2.43   15.97       22.5   38.92   43.70   2.29   15.09       27.5   40.11   43.10   2.06   14.73       32.5   40.72   42.86   1.90   14.52       37.5   40.85   43.25   1.81   14.10       42.5   40.04   44.48   1.74   13.74       47.5   40.72   44.03   1.65   13.60                    
The distance from S to E is measured on the substrate  20  along the direction from the edge of the sputtering table  10  toward the center of the sputtering table  10 .
 
         [0025]    Refer to  FIG. 3  a diagram showing the distribution of Al concentribution from S to E (measured along the direction from the edge of the sputtering table to the center of the sputtering table). From  FIG. 3 , it is observed: Al concentration decreases continuously from the edge to the center, i.e. there is an Al concentration gradient on the substrate  20 . The as-deposited sample having a typical composition Fe 39.3 CO 43.2 Al 2.4 O 15.1  is tested with a VSM (Vibration Sample Magnetometer) and a high-frequency permeameter. The test results are respectively shown in  FIG. 4  and  FIG. 5 . From  FIG. 4 , it is observed: The as-deposited substrate  20  has an obvious uniaxial anisotropy, i.e. there is a very great anisotropic field (over 100 Oe) along the radial direction, and saturation magnetization is easily attained in the direction vertical to the radius direction.  FIG. 5  is a diagram showing the magnetic spectrum of the as-deposited sample having a typical composition Fe 39.3 CO 43.2 Al 2.4 O 15.1 . From  FIG. 5 , it is observed that the sample can reach a resonance frequency over 3 GHz, and the relative permeability thereof is over 100. The test results indicate that the material of the (Fe x Co y ) z —(Al u O v ) w  system fabricated according to the present invention has superior magnetic properties and high-frequency magnetic performance. 
         [0026]    Composition 2—the (Fe x Co y ) z Hf u N v  system: The material system adopts a Fe 70 CO 30  target as the primary material target  30  and an Hf target as the doping target  40 , and the film is formed via RF (Radio Frequency) gradient sputtering under an N 2 /Ar-containing environment. In this example, the sputtering powers of the two targets are respectively 60 W and 100 W. The concentration gradient on the substrate  20  is detected with FE-EPMA (Field Emission Electron Probe Micro-Analyzer), and the measurement result is shown in the table below. 
         [0000]                                                                  Distance                       from S to E (mm)   Fe (at. %)   Co (at. %)   Hf (at. %)   N (at. %)                                10   44.24   19.22   14.71   21.82       15   45.40   19.20   13.20   22.20       20   46.20   20.60   11.30   21.90       25   48.45   20.74   10.87   19.94       30   49.60   20.84   9.12   20.44       35   49.25   22.12   8.81   19.82       40   51.50   22.00   7.90   18.60       45   51.95   22.52   7.61   17.92       50   51.70   21.24   7.11   19.94                    
The distance form S to E is measured on the substrate  20  along the direction from the edge of the sputtering table  10  toward the center of the same.
 
         [0027]    Refer to  FIG. 6  a diagram showing the distribution of Hf concentribution from S to E (measured along the direction from the edge of the sputtering table to the center of the sputtering table). From  FIG. 6 , it is observed: Hf concentration decreases continuously from the edge to the center, i.e. there is an Hf concentration gradient on the substrate  20 . The as-deposited sample having a typical composition Fe 48.5 CO 20.7 Hf 10.9 N 19.9  is tested with a VSM (Vibration Sample Magnetometer) and a high-frequency permeameter. Test results are respectively shown in  FIG. 7  and  FIG. 8 . From  FIG. 7 , it is observed: The as-deposited substrate  20  has an obvious uniaxial anisotropy, i.e. there is a very great anisotropic field (over 150 Oe) along the radial direction, and saturation magnetization is easily attained in the direction vertical to the radius direction.  FIG. 8  is a diagram showing the magnetic spectrum of the as-deposited sample with a typical composition Fe 48.5 Co 20.7 Hf 10.9 N 19.9 . From  FIG. 8 , it is observed: The sample can reach a resonance frequency over 3 GHz, and the relative permeability thereof is over 100 at near 3 GHz. Test results indicate that the material of the (Fe x Co y ) z Hf u N v  system fabricated according to the present invention has superior magnetic properties and high-frequency magnetic performance. 
         [0028]    From the experimental results of two material systems mentioned above, it is known that the concentration-gradient high-frequency ferromagnetic film fabricated with the method of the present invention has a saturation magnetization (Ms) of between 12 and 25 kG, a uniaxial anisotropic field of between 50 and 400 Oe or more and a self-resonance frequency of near 3 GHz or more according to the concentration of the doping. 
         [0029]    Those described above are only the preferred embodiments to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.