Patent Publication Number: US-2011070373-A1

Title: Method for forming an ordered alloy

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese application No. 098132055, filed on Sep. 23, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for forming an ordered alloy, more particularly to a method involving forming layers of a first metal and layers of a second metal alternately disposed with the layers of the first metal under an elevated temperature. 
     2. Description of the Related Art 
     In the magnetic recording technology, perpendicular magnetic recording (PMR) materials have been successfully developed to achieve a high recording density. It is known in the art that FePt alloy having an ordered phase (or L1 0  phase), i.e., a face-centered tetragonal (FCT) crystal structure, exhibits a high magnetocrystalline anisotropy (MCA) and thus can be used in a PMR medium to enhance the thermal stability of the PMR medium. In addition, the ordered FePT alloy has an out-of-plane coercive field (Hc ⊥ ) and an out-of-plane squareness (S ⊥ ) higher than the requirements of a satisfactory PMR medium, which are respectively required to be higher than 1.0 kOe and 0.5. FePt alloy films used in the PMR medium are usually formed by sputtering techniques. The FePt alloy films thus formed normally have a disordered phase, i.e., a face-centered cubic (FCC) structure. Conventionally, the disordered FePt alloy is required to be annealed under a temperature of above 400° C. so as to be transformed from the FCC structure into the FCT structure and to be used in the PMR medium. However, the annealing temperature is too high and can result in problems, such as damage to semiconductor components to which the PMR medium is integrated, and an increase in the capital cost for making the PMR medium. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide a method for forming an ordered alloy that can overcome the aforesaid drawback of the prior art. 
     According to this invention, there is provided a method for forming an ordered alloy that comprises: (a) forming a layer of a first metal with a layer thickness of less than 0.3 nm over a substrate; (b) forming a layer of a second metal with a layer thickness of less than 0.3 nm on the layer of the first metal under an elevated temperature sufficient to cause interdiffusion of atoms of the first and second metals between the layer of the first metal and the layer of the second metal so as to form the ordered alloy; and (c) repeating steps (a) and (b) until a predetermined layer thickness of the ordered alloy is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates consecutive steps of the preferred embodiment of a method for making an ordered alloy according to this invention; 
         FIG. 2  is a fragmentary partly sectional view of a sputtering chamber to illustrate a state in which a substrate is disposed at a first position in the sputtering chamber; 
         FIG. 3  is a fragmentary partly sectional view of the sputtering chamber to illustrate a state in which the substrate is disposed at a second position in the sputtering chamber; 
         FIG. 4  is a sectional view taken along line IV-IV of  FIG. 2 ; 
         FIG. 5  is a sectional view taken along line V-V of  FIG. 3 ; 
         FIG. 6  illustrates a lattice structure of a unit cell of an ordered FePt alloy; 
         FIG. 7  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 1 (E1); 
         FIG. 8  is anX-Ray Diffraction (XRD) plot to illustrate the crystal structure of the FePt alloy of Example 1 (E1); 
         FIG. 9  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 2 (E2); 
         FIG. 10  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 3 (E3); 
         FIG. 11  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 4 (E4); 
         FIG. 12  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 5 (E5); 
         FIG. 13  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 6 (E6); 
         FIG. 14  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 7 (E7); 
         FIG. 15  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 8 (E8); 
         FIG. 16  is an XRD plot to illustrate the crystal structures of the FePt alloy of Examples 4˜8 (E4˜E8); 
         FIG. 17  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of Example 9 (E9); and 
         FIG. 18  is a hysteresis-loop plot to illustrate the magnetic property of a FePt alloy of the comparative example (CE). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , the preferred embodiment of a method for forming an ordered alloy  33  is included in a process for making a perpendicular magnetic recording medium. The process includes: forming a soft magnetic underlayer  30  on a substrate  3 ; forming a first interlayer  31  made from Cr, MgO, or Cr alloy on the soft magnetic underlayer  30 ; forming a second interlayer  32  made from Pt on the first interlayer  31 ; forming a layer  331  of a first metal with a layer thickness of less than 0.3 nm on the second interlayer  32 ; forming a layer  332  of a second metal with a layer thickness of less than 0.3 nm on the layer  331  of the first metal under an elevated temperature sufficient to cause interdiffusion of atoms of the first and second metals between the layer  331  of the first metal and the layer  332  of the second metal so as to form the ordered alloy  33 ; repeating the steps of forming the layers  331 ,  332  alternately until a predetermined layer thickness of the ordered alloy  33 , which is suitable as a magnetic recording material, is achieved; and forming a protective layer  35  on the ordered alloy  33 . 
     Preferably, the layer thickness of each of the layers  331  of the first metal and the layers  332  of the second metal is greater than 0.01 nm and less than 0.3 nm. 
     Preferably, the first metal is Fe, Co, Ni, Mn, Au, or Cu, and the second metal is Pt, Pd, Cr, Al, or Cu with the proviso that the first metal and the second metal cannot be Cu at the same time. More preferably, the first metal is Fe, Co, or Ni, and the second metal is Pt, Pd, or Cr. 
     Preferably, formation of the ordered alloy  33  is performed by sputtering techniques. 
     Referring to  FIGS. 2 to 5 , the sputtering is conducted in a manner that the substrate  3  is disposed movably in a sputtering chamber  4  so as to be alternately disposed between a first position (see  FIG. 2 ), in which the substrate  3  is aligned with a target  41  of the first metal, and a second position (see  FIG. 3 ), in which the substrate  3  is aligned with a target  42  of the second metal, thereby alternately forming layers  331  of the first metal and layers  332  of the second metal on the second interlayer  32 . Preferably, the substrate  3  is carried by a rotatable carrier  43  provided in the sputtering chamber  4  so as to be rotatable and disposable alternately between the first and second positions. 
     When the first metal and the second metal are respectively Fe and Pt, the following conditions are preferable: the elevated temperature ranges from 300° C. to 350° C., the carrier  43  has a rotation speed ranging from 1 rpm to 20 rpm during the sputtering, and the sputtering is conducted by applying an output power ranging from 20 W to 70 W to the target  41  of the first metal (Fe) and an output power ranging from 10 W to 36 W to the target  42  of the second metal (Pt). When the elevated temperature is 350° C., the following conditions are more preferable: the carrier  43  has a rotation speed ranging from 10 rpm to 20 rpm during the sputtering, and the layer thickness of each of the layers  331  of the first metal (Fe) and the layers  332  of the second metal (Pt) ranges from 0.014 nm to 0.12 nm. 
     When the output power applied to each of the targets  41 ,  42  is increased, the deposition rate of each of the layers  331 ,  332  is increased correspondingly. In addition, when the rotation speed of the carrier  43  is increased, the layer thickness of each of the layers  331 ,  332  is decreased correspondingly due to a shorter time period for the substrate  3  to stay at the first and second positions. Hence, the layer thickness of each of the layers  331 ,  332  can be controlled by adjusting the rotation speed of the carrier  43  and/or the output power applied to the targets  41 ,  42 . 
       FIG. 6  illustrates the lattice structure of a unit cell of an ordered FePt alloy (the L1 0  phase). The lattice structure includes an upper layer of Pt atoms, a middle layer of Fe atoms, and a lower layer of Pt atoms. The upper, middle and lower layers are aligned along the z-axis, i.e., [001] direction. It is noted herein that the elevated temperature can be reduced by decreasing the layer thickness of each of the layers  331  of the first metal (Fe) and the layers  332  of the second metal (Pt), i.e., by decreasing the interdiffusion distance for Fe and Pt atoms to move to the lattice sites of Fe atoms and the lattice sites of Pt atoms of the L1 0  phase. Because of the reduced interdiffusion distance, the magnetic recording material formed of the aforesaid ordered alloy  33  exhibits superior [001]-preferred orientation (texture), thereby resulting in a high out-of-plane squareness. 
     The following examples and comparative example are provided to illustrate the merits of the preferred embodiments of the invention, and should not be construed as limiting the scope of the invention. 
     Example 1 (E1) 
     A substrate  3  made from glass and having a size of 7 cm×7 cm was placed in a sputtering chamber. The substrate  3  was deposited with a first interlayer  31  of CrRu alloy with a layer thickness of 90 nm thereon under a working pressure of 5 mTorr and a substrate temperature of 300° C. A second interlayer  32  made from Pt and having a layer thickness of 2 nm was then formed on the first interlayer  31  by sputtering under a working pressure of 10 mTorr and a substrate temperature of 350° C. The substrate  3  formed with the first and second interlayers  31 ,  32  thereon was then mounted to a rotatable carrier  43  (which is in the form of a disc with a diameter of 50 cm) in a magnetron sputtering chamber  4  with targets of Fe and Pt (which have a size approximate to the size of the substrate  3 ) mounted diametrically therein. Sputtering was performed by applying an output power of 20 W to the target  41  of Fe and an output power of 10 W to the target  42  of Pt under an elevated temperature (i.e., annealing temperature) of 300° C. and a working pressure of 10 mTorr. The carrier  43  was operated at a rotation speed of 10 rpm to move the substrate  3  to the first and second positions alternately during sputtering. After sputtering, a total layer thickness of 20 nm of a FePt alloy  33  was obtained. The layer thickness of each of the layers  331  of Fe and the layers  332  of Pt of the FePt alloy was calculated based on the total layer thickness and the rotation speed of the carrier  43 , and is shown in Table 1. The FePt alloy thus formed has a composition of Fe 48 Pt 52  as determined by using energy dispersive spectrometry (EDS). 
     Example 2 (E2) 
     A FePt alloy of Example 2 (E2) was prepared by steps and operating conditions similar to those of Example 1 (E1), except that the rotation speed of the carrier  43  was 5 rpm, and the elevated temperature was 350° C. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0578 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 3 (E3) 
     A FePt alloy of Example 3 (E3) was prepared by steps and operating conditions similar to those of Example 2 (E2), except that the rotation speed of the carrier  43  was 20 rpm. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0145 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 4 (E4) 
     A FePt alloy of Example 4 (E4) was prepared by steps and operating conditions similar to those of Example 1 (E1), except that the elevated temperature was 350° C. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0289 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 5 (E5) 
     A FePt alloy of Example 5 (E5) was prepared by steps and operating conditions similar to those of Example 4 (E4), except that the output powers applied to the target  41  of Fe and target  42  of Pt were 40 W and 20 W, respectively. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0589 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 6 (E6) 
     A FePt alloy of Example 6 (E6) was prepared by steps and operating conditions similar to those of Example 4 (E4), except that the output powers applied to the target  41  of Fe and target  42  of Pt were 50 W and 25 W, respectively. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0749 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 7 (E7) 
     A FePt alloy of Example 7 (E7) was prepared by steps and operating conditions similar to those of Example 4 (E4), except that the output powers applied to the target  41  of Fe and target  42  of Pt were 60 W and 31 W, respectively. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.0938 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 8 (E8) 
     A FePt alloy of Example 8 (E8) was prepared by steps and operating conditions similar to those of Example 4 (E4), except that the output powers applied to the target  41  of Fe and target  42  of Pt were 70 W and 36 W, respectively. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.1113 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Example 9 (E9) 
     A FePt alloy of Example 9 (E9) was prepared by steps and operating conditions similar to those of Example 4 (E4), except that the rotation speed of the carrier  43  was 1 rpm. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.289 nm (see Table 1). The FePt alloy thus formed has a composition of Fe 48 Pt 52 . 
     Comparative Example (CE) 
     A FePt alloy of the comparative example was prepared by steps and operating conditions similar to those of Example 9 (E9), except that the output powers applied to the target  41  of Fe and target  42  of Pt were 40 W and 20 W, respectively. Each of the layers  331  of Fe and the layers  332  of Pt thus formed has a layer thickness of 0.5895 nm (see Table 1). 
     &lt;Analysis of the FePt Alloy&gt; 
     The relationship between the applied magnetic field (H a ) and magnetization (M), i.e., hysteresis-loops, of the FePt alloy of each of Examples (E1˜E9) and the comparative example (CE) was measured using a vibrating sample magnetometer (VSM). 
     From the hysteresis-loop shown in  FIG. 7 , the coercive field (Hc ┘ ) and the squareness (S ⊥ ) of Example 1 (E1) are respectively 1740 Oe and 0.61 (see Table 1) under the elevated temperature of 300° C. 
     The XRD plot shown in  FIG. 8 , with reference to No. 43-1395 of JCPDF card (not shown), demonstrates that the FePt alloy of Example 1 (E1) has an L1 0  phase and a (001) preferred orientation texture. 
     From the hysteresis-loop shown in  FIG. 9 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 2 (E2) are 1575 Oe and 0.56 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 10 , the coercive field (Hc ⊥ ) of Example 3 (E3) is 2125 Oe, and the squareness (S ⊥ ) of Example 3 (E3) is increased to 0.79 (see Table 1) as compared to Example 2. 
     From the hysteresis-loop shown in  FIG. 11 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 4 (E4) are 3320.59 Oe and 0.7935 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 12 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 5 (E5) are 2567.30 Oe and 0.9387 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 13 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 6 (E6) are 1842.40 Oe and 0.8993 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 14 , the coercive field (H ⊥ ) and the squareness (S ⊥ ) of Example 7 (E7) are 1576.96 Oe and 0.8584 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 15 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 8 (E8) are 1430.86 Oe and 0.7725 (see Table 1), respectively. 
     The XRD curves shown in  FIG. 16 , with reference to No. 43-1395 of JCPDF card (not shown), demonstrates that the FePt alloy of each of Examples 4˜8 (E4-E8) has an L1 0  phase and a (001) preferred orientation texture. 
     From the hysteresis-loop shown in  FIG. 17 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Example 9 (E9) are 1923.35 Oe and 0.5570 (see Table 1), respectively. 
     From the hysteresis-loop shown in  FIG. 18 , the coercive field (Hc ⊥ ) and the squareness (S ⊥ ) of Comparative Example (CE) are only 798.54 Oe and 0.3340 (see Table 1), respectively. This indicates that the metal alloy composed of layers  331  of Fe and layers  332  of Pt (each of the layers  331  and layers  332  having a layer thickness greater than 0.3 nm) cannot provide satisfactory Hc ⊥  and S ⊥  properties. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Output power 
                   
                   
                   
               
               
                   
                 Thickness 1   
                 Speed 2   
                 (W) 
                 Temp. 3   
                 Hc⊥ 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 (nm) 
                 (rpm) 
                 Fe 
                 Pt 
                 (° C.) 
                 (Oe) 
                 S⊥ 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 E1 
                 0.0289 
                 10 
                 20 
                 10 
                 300 
                 1740.00 
                 0.6100 
               
               
                 E2 
                 0.0578 
                 5 
                 20 
                 10 
                 350 
                 1575.00 
                 0.5600 
               
               
                 E3 
                 0.0145 
                 20 
                 20 
                 10 
                 350 
                 2125.00 
                 0.7900 
               
               
                 E4 
                 0.0289 
                 10 
                 20 
                 10 
                 350 
                 3320.59 
                 0.7935 
               
               
                 E5 
                 0.0589 
                 10 
                 40 
                 20 
                 350 
                 2567.30 
                 0.9387 
               
               
                 E6 
                 0.0749 
                 10 
                 50 
                 25 
                 350 
                 1842.40 
                 0.8993 
               
               
                 E7 
                 0.0938 
                 10 
                 60 
                 31 
                 350 
                 1576.96 
                 0.8584 
               
               
                 E8 
                 0.1113 
                 10 
                 70 
                 36 
                 350 
                 1430.86 
                 0.7725 
               
               
                 E9 
                 0.2890 
                 1 
                 20 
                 10 
                 350 
                 1923.35 
                 0.5570 
               
               
                 CE 
                 0.5895 
                 1 
                 40 
                 20 
                 350 
                 798.54 
                 0.3440 
               
               
                   
               
               
                   1 layer thickness of each layer of Fe and Pt 
               
               
                   2 rotation speed of the carrier 
               
               
                   3 elevated temperature of the sputtering operation 
               
            
           
         
       
     
     In conclusion, by forming alternately each layer of Pt with the layer thickness of less than 0.3 nm and each layer of Fe with the layer thickness of less than 0.3 nm, the annealing temperature in the method of this invention for forming the ordered alloy can be reduced to below 400° C. so that the aforesaid drawback of requiring a high annealing temperature for forming an ordered alloy in the prior art can be eliminated. Moreover, the ordered alloy thus obtained exhibits satisfactory and good Hc ⊥  and S ⊥  properties. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.