Abstract:
High Hc (&gt;4,000 Oe) and high Hk (&gt;1 Tesla) has been achieved in FePt films as thin as 70 Angstroms. This was accomplished by starting with a relatively thick film having the required high coercivity, coating it with a suitable material such as Ta, and then using ion beam etching to remove surface material until the desired thickness was reached.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the general field of magnetic thin films with particular emphasis on films that simultaneously have low thickness and high coercivity. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ultra-thin films with ultra-high Hc (coercivity) or Hk (uniaxial anisotropy) are needed for certain magnetic recording applications. However, material structures of this type are not readily available. Conventional hard magnetic materials such as CoPt or CoPtX (X=Cr, Ta, Ru, SiO2 etc) typically have Hc values in the 2000 to 2,500 range Oe range, which is not high enough for these special applications. 
         [0003]    Ordered Co 50 Pt 50  or Fe 50 Pt 50  film structures have been used to achieve higher Hc or Hk films. However, these require a strenuous annealing treatment and a relatively thick film to form the ordered structure. To achieve a higher Hc in thinner films of this type, would demand even more strenuous annealing conditions which would create major problems for device integration. 
         [0004]    Much effort has been expended on attempts to lower the temperature required to produce these ordered FePt films. However, none of these showed much promise until recently [1]. The present invention builds on this work and discloses a novel method for producing ultra-thin FePt films (&lt;60 Å) having an Hc of about of 4,000 Oe or more. 
         [0005]    It has been suggested that, in order to form an ordered structure in FePt, the FePt layer should be deposited on a Pt seed layer and then capped with a Pt capping layer. Following this, the structure needs to be annealed at a temperature in the range of from 250 to 400° C. for 4 hours or longer. In this way it has been possible to achieve a Hc of 5,000 Oe or more in FePt films 200 or more Angstroms thick. The present invention discloses how the magnetic properties of an ordered structure can be preserved in even thinner films having thicknesses less than 150 Å. 
       REFERENCES 
       [0006]    J. Qiu , et al, “hard magnet with cap and seed layers and data storage device read/write head incorporating the same”, US20090274931A1 
         [0007]    A routine search of the prior art was performed with the following references of interest being found: 
         [0008]    In U.S. Patent Application 2009/0274931, Qiu et al. disclose depositing a FePt layer on a Pt seed layer, then a Pt cap layer, and annealing. The thin film has a thickness of &lt;=200 Angstroms. In U.S. Patent Application 2006/0188743, Seki et al. disclose a FePt thin film&lt;100 nm thick formed by sputtering on an undercoat layer. U.S. Pat. No. 6,007,623 (Thiele et al.) teaches forming a FePt and CoPt layer of 50 nm sputtered onto an etched (and roughened) seed layer while U.S. Patent Application 2001/0009119 (Murray et al.) teaches depositing FePt and annealing. The resulting coercivity is 500-6500 Oe. None of these references teaches both a low thickness and a high coercivity in the same film. 
       SUMMARY OF THE INVENTION 
       [0009]    It has been an object of at least one embodiment of the present invention to describe a magnetic film that simultaneously possesses low thickness and high coercivity. 
         [0010]    Another object of at least one embodiment of the present invention has been for this magnetic film to have a thickness that is no greater than 55 Å and a coercivity of at least 3680 Oe. 
         [0011]    Still another object of at least one embodiment of the present invention has been to provide a method for the preparation of this film. 
         [0012]    A further object of at least one embodiment of the present invention has been for said method to be easy to incorporate as part of current methods for manufacturing magneto-resistive devices 
         [0013]    These objects have been achieved through a process that begins by first depositing a relatively thick FePt film and then annealing it for an acceptable time at a moderate temperature (for example below 300° C. for less than 10 hrs) until it acquires an ordered structure having a high Hc. We then deposit an ion beam protection layer (IBPL) using a suitable material such as Ta. We follow this with the application of ion beam etching to remove surface material until a particular desired thickness is reached. 
         [0014]    Through use of this process we have been able to form FePt films having, simultaneously a Hc in excess of 4000 Oe and a thickness as low as 50 Å. 
         [0015]    As an alternative, the Ta protection layer deposition and the ion beam etching removal can be done in situ in an IBD system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows how even a small amount of etch-back of a 200 Å FePt film reduces its coercivity to zero. 
           [0017]      FIG. 2  shows results similar to  FIG. 1  for a FePt film that was protected with 40 Å of Ru prior to starting etch-back. 
           [0018]      FIG. 3  is a flow diagram for the process of the invention. 
           [0019]      FIG. 4  shows how a FePt film that was protected with 40 Å of Ta prior to starting etch-back can be thinned down to as low as 38 Å with a Hc as high as 3,250 Oe. 
           [0020]      FIG. 5 . Shows MH curves after etch back processes associated with the second embodiment of the invention 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    It is well known that magnetic ordering temperature is very sensitive to film thickness—when the film thickness is larger, a lower annealing temperature is required to form an ordered structure. Thus, when the film becomes thinner, the annealing temperature becomes much higher and the annealing time becomes much longer. For example, in order to achieve an ordered structure in a 100 Å FePt (FP) film, the annealing temperature would have to be at least 450° C. Such a high annealing temperature cannot be tolerated for most device applications. 
         [0022]    To overcome this problem the possibility of using an etch-back process was investigated. First, several relatively thick FePt film samples were annealed in the usual manner until they formed well ordered structures capable of acting as high Hc FePt films. An etch-back process, such as low energy ion-beam sputter etching, was then initiated to reduce the thickness of the initial FePt film. As will become apparent below, this work yielded some very unexpected results related to the specifics of how the etch back process was performed. 
         [0023]    Work began with the Sample 1 structure (Pt20/FP200/Pt20) since it offered the best initial Hc (5150 Oe after deposition and annealing). The outcome following the next step turned out to be very sensitive to the specifics of the ion beam etching: 
         [0024]    Following even a very slight etch, the Hc of Sample 1 (x-axis value when y-axis value is zero) was found, surprisingly, to have been reduced to zero. This is reflected in the data shown in  FIG. 1 : 
         [0025]      FIG. 1(   a ) shows the MH curve for Sample 1 after 280° C./6 h anneal and prior to any etch-back.  FIGS. 1(   b ) through  1 ( e ) are MH curves for Sample 1 after removal of 30 Å, 60 Å, 90 Å, and 120 Å of FePt, respectively. 
         [0026]    For the record, we note here that, as a matter of routine following completion of the etch-back process, a 50 Å Ta capping layer was applied to protect the sample from oxidation during subsequent handling. 
         [0027]    Tests were then performed on the Sample 2 structure (Pt20/FePt200/Pt20/Ru40), which included a 40 Å Ru cap on top of the existing Pt capping layer. Although adding the reduced the pre-etch-back value of Hc, it was useful as a further test on capping material selection. As shown in  FIG. 2 , after etch-back Hc for Sample 2 deteriorated completely in a similar manner to Sample 1. Details for  FIG. 2  are MH curves, as follows 
         [0028]      2 ( a ) prior to etch-back; 
         [0029]      2 ( b ) after ion beam etching (IBE) removal of 30 Å FePt; 
         [0030]      2 ( c ) after IBE removal of 60 Å FePt; 
         [0031]      2 ( d ) after IBE removal of 90 Å FePt; and 
         [0032]      2 ( e ) after IBE removal of 120 Å FePt. 
         [0033]    This data made it clear that a Ru layer does not offer significant protection from ion beam damage during etch-back so additional materials needed to be investigated as possible candidates for protecting FePt during ion beam etching. 
         [0034]    In sample 3 the 40 Å layer of Ru was replaced by an ion beam protection layer (IBPL) comprising a 40 Å layer of Ta, giving Sample 3 a (Pt20/FePt200/Pt20/Ta40) structure. This yielded much more encouraging results. The procedure followed for Sample 3 is summarized in flow chart form in  FIG. 3  while the results are displayed in  FIG. 4  as follows: 
         [0035]      4 ( a ) Prior to etch-back; 200 Å of FePt remaining; Hc=5150 Oe 
         [0036]      4 ( b ) After etch-back; 160 Å of FePt remaining; Hc=4791 Oe 
         [0037]      4 ( c ) After etch-back; 122 Å of FePt remaining; Hc=4855 Oe 
         [0038]      4 ( d ) After etch-back; 51 Å of FePt remaining; Hc=4230 Oe 
         [0039]      4 ( e ) After etch-back; 38 Å of FePt remaining; Hc=3250 Oe 
         [0040]    The process used with Sample 3 constituted a first embodiment of the invention. It is a matter of designer&#39;s choice whether deposition of the IBPL, in this case a 40 Å layer of Ta, and performance of the etch back process are implemented in the same system or in separate systems. However, regardless of what choice was made, it is clear that providing an IBPL , such as Ta, prior to initiating etch back is critical to retaining an ordered FePt structure during ion beam etching. 
       Embodiment 2 
       [0041]    As an alternative to pure Ta, the IBPL could comprise a bilayer of Ta and a second material selected from the group consisting of Cu, Ru, Rh, NiCr, Hf, and their alloys or the IBPL could comprise a bilayer of either Hf or NiCr together with a second material selected from the group consisting of Cu, Ru, Rh, NiCr, Ta, and their alloys. MH curves in which a Ru20/Ta40 IBPL was used are shown in  FIG. 5 . Details are as follows: 
         [0042]      5 ( a ) Prior to etch-back; 240 Å of FePt remaining; Hc=4400 Oe 
         [0043]      5 ( b ) After etch-back; 180 Å of FePt remaining; Hc=4600 Oe 
         [0044]      5 ( c ) After etch-back; 134 Å of FePt remaining; Hc=4530 Oe 
         [0045]      5 ( d ) After etch-back; 83 Å of FePt remaining; Hc=4300 Oe 
         [0046]      5 ( e ) After etch-back; 55 Å of FePt remaining; Hc=3680 Oe 
         [0047]    Although we have been unable to provide a satisfactory explanation for the foregoing results (which, as noted above, were largely unexpected) we did amass the following additional information related to the basic process:
       a. A low power ion beam etch is preferred, for example, power ranges from 50 w to 300 w, with Ar flow from 3 sccm to 15 sccm. Our preferred thickness for an IBPL of pure Ta was 30 to 50 Å layer thickness but a range of from 20A to 100 Å still provided acceptable results. When a Ru/Ta IBPL was used, the preferred thickness range for the Ru portion was 20 to 50 Å and 20 to 100 Å for the Ta portion.   b. The invention was found to produce comparable results (to those detailed above for FePt) in other high Hc magnetic thin films such as CoPt   c. For both CoPt and FePt having either in-plane or perpendicular-to-plane anisotropy.   d. High Hc(&gt;4000 Oe) and high Hk (&gt;1 Tesla) has been achieved even with a 70 Å thick FePt film