Abstract:
A system and method for forming servo patterns on magnetic media is disclosed. A magnetic film coated with a layer of polystyrene is stamped with a nickel stamper reproducing the negative image of the stamped pattern on the polystyrene. Ions are then accelerated towards the surface of the polystyrene, which stopps the ions in the areas where the polystyrene is thick and allows the ions to penetrate through to the magnetic layer in the areas where the polystyrene is thin. The ions, which penetrate through to the magnetic layer, interact with the magnetic layer altering the magnetic layer&#39;s structure reducing its coercivity (Hc) and remnant moment (Mrt). This reproduces the stamped polystyrene pattern on the magnetic layer. The polystyrene is then removed by oxygen plasma etching the surface leaving behind a patterned magnetic media.

Description:
This application claims priority from U.S. provisional application Ser. No. 60/337,259, filed on Nov. 30, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to patterned magnetic media, and more particularly to resist used for nano-imprint lithography and subsequent ion implantation in patterned magnetic media. 
     2. Description of the Related Art 
     In the microelectronics industry, a conventional patterning process usually consists of two parts. The first part includes patterning a polymeric resist layer by lithographic methods, such as photolithography, e-beam or X-ray lithography, for mask definition. The second part includes subsequently transferring the pattern into a hard material using a process such as dry etching, wet etching, lift-off, or electroforming. As the feature size approaches sub-100 nm, there is an urgent need for fast reliable and cost effective nano-lithography. Nano-imprint lithography, developed in recent years has shown promise in meeting this need. Nano-imprint lithography creates pattern in a thermoplastic resist layer by hot embossing a rigid mold with a negative image of the desired pattern, such as a nickel stamper, into the resist. The embossing process creates a thickness contrast between the crests and troughs of the pattern. Most of the work in this field has been done using poly(methyl methacrylate) (PMMA) as the resist material. 
     Conventional methods of making patterned media with nano-imprint lithography use PMMA as a resist.  FIG. 1A  is an illustration showing the layers of a conventional magnetic media structure overlaid with resist and ready for patterning. Conventional magnetic media structure includes a substrate  105 , seed layer  110 , a magnetic layer  115  and a protective layer  117 . The resist used to overlay the conventional magnetic media includes a PMMA resist layer  120 . The first layer of the media structure is the substrate  105 , which is typically made of nickel-phosphorous plated aluminum or glass that has been textured. The seed layer  110 , typically made of chromium, is a thin film that is deposited onto the substrate  105  creating an interface of intermixed substrate layer  105  and seed layer  110  molecules. The magnetic layer  115 , typically made of a magnetic alloy containing cobalt (Co), platinum (Pt) and chromium (Cr), is a thin film deposited on top of the seed layer  110  creating a second interface of intermixed seed layer  110  molecules and magnetic layer  115  molecules between the two. The protective layer  117 , typically made of carbon or diamond like carbon (DLC) is a thin film deposited on top of the magnetic layer  115  creating a third interface of intermixed magnetic layer  115  molecules and protective layer  117  molecules. Finally the resist layer  120 , typically made of PMMA, is deposited on top of the protective layer  117  using spin-coating techniques. 
       FIG. 1B  is a flow chart showing the typical steps used for nano-imprint lithography and subsequent ion implantation for servo pattern media with PMMA as a resist. The process begins with step  150  by transferring a partially complete media with substrate  105 , seed layer  110 , magnetic layer  115  and protective layer  117  to a conventional resist application station. In step  155 , a PMMA resist layer is applied using a conventional spin-coating technique. In a spin coating technique, the PMMA resist is dropped on to the disk as the disk spins and gets spread out over the surface of the disk by the centrifugal force on the liquid as the disk spins. Temperature, speed of spinning and time are typically used to adjust the coating uniformity across the disk. 
     Next in step  160 , conventional nano-imprint lithography is used to create a servo pattern on the resist layer. Conventional nano-imprint lithography creates patterns in a thermoplastic PMMA resist layer by hot embossing a rigid mold with a negative image of the desired pattern, such as a nickel stamper, into the resist. The pattern produced on the PMMA creates a thickness contrast. 
     Next in step  165 , ion implantation is used to transfer the servo pattern to the underneath magnetic layer. By using the patterned resist layer as a mask the pattern embossed on the PMMA in step  160  is transferred to the underneath magnetic layer by ion implantation. The ion implantation produces magnetic properties difference, such as coercivity (Hc) and (remnant moment×thickness) (Mrt), between the protected and unprotected area. Ion beam irradiation reduces the Hc and Mrt by damaging the magnetic layer structure and consequently generates a magnetic pattern on the magnetic layer identical to the pattern on the resist layer. The protective layer  117 , which separates the magnetic layer  115  from the resist layer  120 , is not affected significantly to impact conventional processes. 
     After the pattern has been transferred to the magnetic layer  115  the PMMA resist  120  is removed in step  170  using conventional PMMA removal processes such as oxygen plasma etching. Since the oxygen plasma etching process removes organics, both the PMMA resist and the carbon protective layer  117  are removed in step  170 . Finally in step  175  the patterned magnetic media is transferred to the next manufacturing operation, which typically includes re-depositing protective layer  117  and lubricating the disk. 
     This method of producing servo pattern media with nano-imprint lithography and subsequent ion implantation is unreliable as is further discussed with reference to  FIGS. 1C and 1D , below. 
       FIG. 1C  shows the magnetic media stack with a PMMA resist layer, before being exposed to ions, while  FIG. 1D  shows the magnetic media stack with a PMMA resist layer, after being exposed to ions.  FIG. 1C  includes the magnetic media structure with PMMA resist having a substrate  105 , seed layer  110 , a magnetic layer  115 , a protective layer  117  and stamped PMMA resist  122  as well as an ion source  125 .  FIG. 1D  includes the substrate  105 , the seed layer  110 , a magnetic layer after ion implantation  112 , a protective layer after ion implantation  119 , a PMMA resist after ion implantation  123  as well the ion source  125  and ions  130 . A comparison of  FIGS. 1C and 1D  shows a reduction in thickness of the PMMA resist caused by ion implantation. The reduction of the PMMA thickness during ion implantation affects the magnetic properties of the entire magnetic disk instead of just the portion of the disk designated according to the pattern on the resist. 
       FIG. 1C  shows the magnetic media stack with the PMMA nano-imprinted resist layer waiting to be ion implanted.  FIG. 1D  shows an altered magnetic media stack, after undergoing ion implantation, having an altered magnetic layer after ion implantation  112 , an altered protective layer after ion implantation  119  and an altered PMMA nano-imprinted resist layer ion implantation  123 . Ion implantation decomposes the PMMA resist  122  and reduces its thickness by as much as  75  percent as shown by comparing the PMMA resist after ion implantation  123  with the PMMA resist before ion implantation  122 . Additionally, ion implantation damages the magnetic layer  115 , transforming the magnetic layer  115  into a different magnetic layer  112  having different magnetic properties including reduced coercivity (Hc). Although the intention is to use ion implantation to alter the magnetic layer according to the nano-imprint pattern, the ion implantation alters the entire magnetic layer reducing the Hc of the entire layer. The poor ion stopping effectiveness of the PMMA resist  120  along with its reduction in thickness when exposed to ions is the cause for the damage that ion implantation has on the magnetic properties of the magnetic layer. 
     Therefore what is needed is a system and method which overcomes these problems and makes it possible to use nano-imprint lithography and subsequent ion implantation to reliably create servo pattern media. Additionally, a system and method, which only alters the properties of the magnetic media according to a predetermined and defined pattern, is needed. 
     SUMMARY OF THE INVENTION 
     This limitation is overcome by using Polystyrene as a resist material for nano-imprint lithography and subsequent ion implantation for servo pattern media. 
     Polystyrene, which is deposited over a magnetic media, is first stamped with a stamper that creates a nano-imprint pattern on the polystyrene. The pattern consists of thinner and thicker regions of polystyrene, which are the negative image of the stamp pattern, over the magnetic media. The patterned structure is then exposed to ions. Since polystyrene has excellent ion stopping properties, the thicker polystyrene stops the incoming ions. However, the thinner portions of the stamped polystyrene surface is incapable of stopping the incoming ions because it is too thin and consequently allows the ions to pass through to the magnetic layer. Since the ions penetrate through the polystyrene layer and the protective layer to the magnetic layer according to the stamped pattern, the same stamped pattern is reproduced on the magnetic layer in the form of reduced coercivity (Hc) and reduced Mrt. 
     The present invention also can be implemented as a computer-readable program storage device which tangibly embodies a program of instructions executable by a computer system to perform a system method. In addition, the invention also can be implemented as a system itself. 
     These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         FIG. 1A  is a block diagram showing a prior art conventional magnetic media structure covered with resist; 
         FIG. 1B  is a flowchart illustrating the prior art method of using PMMA resist for nano-imprint lithography and subsequent ion implantation for servo pattern media; 
         FIG. 1C  is an illustrating showing the conventional magnetic media structure covered with patterned PMMA resist before ion implantation; 
         FIG. 1D  is an illustrating showing the conventional magnetic media structure covered with patterned PMMA resist after ion implantation; 
         FIG. 2  is a block diagram showing the polystyrene layer in a magnetic media environment in accordance with an embodiment of the invention; 
         FIG. 3  is a flowchart showing the preferred method of using polystyrene in conjunction with nano-imprint lithography and subsequent ion implantation to create servo pattern media in accordance with an embodiment of the invention; 
         FIG. 4A  is an illustration showing the conventional magnetic media structure covered with polystyrene before being patterned with a stamper; 
         FIG. 4B  is an illustrating showing the conventional magnetic media structure covered with polystyrene after being patterned with a stamper; 
         FIG. 5A  is an illustrating showing the conventional magnetic media structure covered with patterned polystyrene before ion implantation; and 
         FIG. 5B  is an illustrating showing the conventional magnetic media structure covered with patterned polystyrene after ion implantation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a system and method for nano-imprint lithography and subsequent ion implantation for servo pattern media. 
       FIG. 2  is a block diagram showing a magnetic medium, ready for patterning, having a substrate  105 , a seed layer  110 , a magnetic layer  115 , a protective layer  117  and a polystyrene resist layer  210 . Polystyrene resist layer  210  overlays the protective layer  117  in accordance with one embodiment of the invention. Although the preferred embodiment is described using polystyrene as the thermoplastic resist material, the invention is not limited to polystyrene. Other forms of polymers, can any styrene co-polymers, can also be used as a thermoplastic resist material. The resist materials used in the preferred embodiment all have thermoplastic properties. 
     Although, the preferred embodiment of  FIG. 2  shows the seed layer  110  deposited between the substrate  105  and the magnetic layer  115  the invention is not dependent on the presence of the seed layer  110 . Since the purpose of the seed layer is to define the structure of the magnetic layer  115 , which is deposited on of it, it may be advantages to leave out the seed layer  110  and deposit the magnetic layer  115  directly on top of the substrate  105 , in some cases. Additionally, the magnetic layer  115  can consist of more than just a single layer such as CoCrPt. For example, in another embodiment the magnetic layer  115  may consist of a series of magnetic layers separated by buffers such as chromium. 
       FIG. 3  is a flow chart showing the preferred steps used to make servo pattern media using polystyrene resist  210 , nano-imprint lithography and subsequent ion implantation. The process begins with step  305  by transferring a ready for patterning magnetic medium having substrate  105 , seed layer  110 , magnetic layer  115  and protective layer  117  into a conventional resist spin coating apparatus such as a spin coater built by SUSS MicroTec Inc. (228 SUSS Dr., Waterbury Center, Vt. 05677.) 
     Next in step  310 , conventional spin-coating techniques are used to spin-coat polystyrene  210  onto the magnetic medium. Polystyrene  210  is spin-coated onto the magnetic medium by applying a mixture of polystyrene and a solvent onto the magnetic medium and spinning the magnetic medium at a high rate. Typically, a conventional solvent, such as methoxybenzene, benzene, toluene or metaletherketone, is mixed in with the polystyrene. Although there are no restrictions on the revolutions per minute (rpm) that the disk should be spun, it is typically spun at approximately 3000 rpm. 
     Next in step  315  a servo pattern is created on the polystyrene layer  210 , using nano-imprint lithography. Nano-imprint lithography creates patterns in the polystyrene resist layer  210  by hot embossing a rigid mold, having a desired pattern, into the polystyrene resist layer  210  as is further discussed with reference to  FIG. 4B  below. This creates a thickness contrast pattern on top of the polystyrene resist layer  210 . In the preferred embodiment, a nickel stamper imprints the polystyrene resist layer at temperatures of approximately 135° C. The nickel stamper is then separated from the polystyrene at temperatures of approximately 115° C. 
     Next in step  320  the stamped pattern is transferred to the underneath magnetic layer  115 , using ion implantation. During ion implantation, ions are accelerated towards the top of the polystyrene resist layer  210  using an ion source as is further discussed with reference to  FIG. 4B  below. The ions can be ionized molecules or atoms, such as argon, nitrogen, helium and chromium. The patterned polystyrene resist layer  210  acts as a mask stopping the ions which hit the thicker part of the polystyrene resist layer  210  and allowing ions which hit the thinner part of the polystyrene resist layer  210  to pass through to the underneath magnetic layer  115 . The ion implantation produces magnetic properties differences, such as coercivity (Hc) and (remnant moment x thickness) (Mrt), between the protected and unprotected areas. Ion beam irradiation reduces the Hc and Mrt by damaging the magnetic layer  115  structure. Consequently a magnetic pattern, identical to the pattern on the stamped polystyrene resist layer  210 , is generated on the magnetic layer  115 . There are many possible ion energies and dosages which can be used for ion implantation. For example, in one embodiment ion implantation is done using an ion energy of 30 KeV and an ion dosage of 3×10 15  ions/cm 2 , while in another embodiment ion implantation is done using an ion energy of 21 KeV and an ion dosage of 5×10 15  ions/cm 2 . The invention is not limited to any particular ion energy or ion dosage. 
     Next in step  325  the polystyrene resist layer is removed from the magnetic media structure leaving the servo pattern in the underlying magnetic layer. Polystyrene resist is removed using a conventional oxygen plasma etch process similar to that used for removing conventional PMMA resist. Since the oxygen plasma etching process removes organics, both the polystyrene resist and the carbon protective layer  117  are removed in step  325 . Step  325  can be adjusted to completely remove all of the polystyrene and protective layer  117  or it can be adjusted to leave a small amount of protective layer  117 . Those skilled in the art will recognize that this is accomplished by determining the etch rate of the combination of polystyrene and protective layer  117  and setting the etching time to be less than the time needed to completely etch away all of the polystyrene and protective layer  117 . Finally in step  375  the patterned magnetic media is transferred to the next manufacturing operation, which typically includes re-depositing protective layer  117  and lubricating the disk. 
       FIG. 4A  shows a magnetic media, which includes a substrate  105 , a seed layer  110 , a magnetic layer  115  and a protective layer  117  covered with the polystyrene resist layer  210 , ready to be nano-imprinted with a stamper  420 , in accordance with one embodiment of the invention. Stamper  420  is made of nickel and has a pattern  430  whose negative image is transferred onto the polystyrene resist layer  210  by pressing the stamper  420  into the polystyrene resist layer  210 . In one embodiment the imprinting is done by holding stamper  420  at 135° C. and pressing it onto the polystyrene resist layer  210 . After the image has been imprinted onto the polystyrene, the stamper is separated from the polystyrene resist layer  210  at 115° C. leaving an imprint on the polystyrene as shown in FIG.  4 B. 
       FIG. 4B  is an illustration of the magnetic media with a polystyrene resist layer after it has been imprinted. The patterned polystyrene magnetic media  480  includes a substrate  105 , a seed layer  110 , a magnetic layer  115 , a protective layer  117 , a polystyrene resist layer after imprinting  410  and a nano-imprinted pattern  440  on the polystyrene resist film in accordance with one embodiment of the invention. The imprinting creates a thickness contrast in the polystyrene pattern  440  that follows the negative image of the nano-imprinted pattern  430  of the stamper  420 . The thicker polystyrene resist  450  blocks ions during the ion implantation process whereas the thinner polystyrene pattern  455  permits the transmission of ions through the protective layer  117  to the magnetic under layer  115  as is further discussed with reference to  FIG. 5B  below. The thinner polystyrene pattern  455  can be as thin as the residue layer left over after stamper  420  has been pressed down as far as possible without damaging the protective layer  117 . 
       FIG. 5A  shows the patterned polystyrene covered magnetic media  480  of  FIG. 4B , which includes a substrate  105 , a seed layer  110 , a magnetic layer  115 , a protective layer  117 , a polystyrene resist layer after imprinting  410 , a nano-imprinted pattern  440 , thick polystyrene resist  450  and thin polystyrene resist  455 , ready to undergo ion implantation. Patterned polystyrene covered magnetic media  480  is in chamber  510  with an ion source  525  ready to be ion implanted.  FIG. 5A  is positioned next to  FIG. 5B , which shows the patterned polystyrene covered magnetic media  480  of  FIG. 4B  undergoing ion implantation. 
       FIG. 5B  includes the substrate  105 , the seed layer  110 , a magnetic layer after ion implantation  515 , a protective layer after ion implantation  517 , a polystyrene resist after ion implantation  520 , an ion source  525 , ions  530 , a nano-imprinted pattern after ion implantation  540 , thick polystyrene resist after ion implantation  550  and thin polystyrene resist after ion implantation  555 . The magnetic layer after ion implantation  515  differs from the magnetic layer  115 , before ion implantation, in that its magnetic properties, including coercivity (Hc), have been altered according to the nano-imprinted pattern  540 . Typically, the portions of the magnetic layer  515 , which has been exposed to ions  530 , have lower coercivity than those portions of the magnetic layer  515  that have not been exposed to ions  530 . Ions  530 , which are ejected from the ion source  525  and are accelerated towards the patterned polystyrene surface, penetrate the thinner portions  555  of stamped polystyrene surface, reach the magnetic layer  115 , react with the magnetic layer  115  and alter the magnetic layer  115  transforming it into a different magnetic layer  515 . Since thicker polystyrene  550  prevents ions from penetrating through to the magnetic layer  515  while the thinner polystyrene  555  permits the transmission of ions  530 , the patterned polystyrene  540  prevents the magnetic layer  515  from being altered in areas other than those designated by the stamped nano-imprinted pattern  540 . Therefore, a pattern of high and low coercivity is produced on the magnetic layer  515 , which matches the stamped polystyrene pattern  540 . 
     Protective layer  117  changes into protective layer  517  during the ion implantation process. Since the changes in protective layer  117  do not effect the overall performance of the magnetic media a detailed discussion of its changes are omitted. 
     Additionally, the ion implantation decomposes the polystyrene resist  520  and  540  and reduces its thickness by about 25 percent as shown by comparing the thickness of the polystyrene in  FIG. 5A  before ion implantation to the thickness of the polystyrene in  FIG. 5B  after ion implantation. This reduction in the polystyrene thickness is much less than occurs to PMMA resist  120  under the same ion implantation conditions as was discussed with reference to FIG.  1 C. Consequently the magnetic layer after ion implantation  515  covered by polystyrene  520  and  540  is altered only according to the polystyrene stamped pattern  540  whereas the magnetic layer  115  covered with protective layer  117  and PMMA  120  is entirely altered reducing the Hc and Mrt of the entire magnetic layer. 
     It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment and for particular applications, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be utilized in any number of environments and implementations.