Abstract:
An apparatus includes a storage media having a surface coated with a lubricant, and a plurality of probes having tips contacting the lubricant, wherein the probes are coated with one of a fluorocarbon, perfluoropolyether, polytetrafluoroethylene, fluorinated ethylene propylene, polyethylene, or a hydrocarbon polymer.

Description:
FIELD OF THE INVENTION 
     This invention relates to apparatus for reducing meniscus forces between a probe and a data storage media. 
     BACKGROUND OF THE INVENTION 
     In probe recording devices, compliant probes are mounted such that tips of the probes contact the surface of a storage media. Some form of actuator is included to provide relative movement of the probes and the storage media so that the probes can be positioned with respect to the media surface. The storage media is coated with a liquid lubricant to reduce the wear rate of the probes and the media at the contacting surfaces. 
     Due to the contact of the probes and the media lubricant, a meniscus is formed between the lubricant and the sides of the probes. This creates a force that pulls the probes toward the media and increases the contact stress between these surfaces and thus the wear rate, static friction and dynamic friction, which need to be minimized for tracking performance. The meniscus develops because of the non-zero interfacial energy between the probes and the liquid lubricant. In addition, due to the relative motion of the probes and media and the non-zero wear rate, debris can accumulate along the sides of the probes and impact the mechanical function of the probes. 
     It would be advantageous to provide a probe storage apparatus in which the meniscus force is minimized between the probes and the liquid lubricant. 
     SUMMARY OF THE INVENTION 
     This invention provides an apparatus comprising a storage media having a surface coated with a lubricant, and a plurality of probes having tips contacting the lubricant, wherein the probes are coated with one of a fluorocarbon, perfluoropolyether, polytetrafluoroethylene, fluorinated ethylene propylene, polyethylene, or a hydrocarbon polymer. 
     In another aspect, the invention provides an apparatus comprising a storage media having a surface coated with a lubricant, and a plurality of probes having tips contacting the lubricant, wherein the probes are coated with a self-assembled monolayer. 
     In yet another aspect, the invention provides an apparatus comprising a storage media having a surface coated with a lubricant, and a plurality of probes having tips contacting the lubricant, wherein the probes are coated with a low surface energy coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a probe storage device that can be constructed in accordance with an embodiment of the invention. 
         FIG. 2  is a schematic cross-sectional view of a probe storage device. 
         FIG. 3  is a schematic representation of a probe coated with a low surface energy coating that includes a tip positioned adjacent to a storage media. 
         FIG. 4  is a schematic representation of a tilted probe with a meniscus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of a probe storage device  10  that can be constructed in accordance with an embodiment of the invention. In the storage device  10  of  FIG. 1 , an array  12  of probes  14 , also called tips or heads, are positioned adjacent to a storage media  16 . In the configuration shown in  FIG. 1  the probes  14  and the media  16  lie in planes that are generally parallel with each other. The probes  14  are electrically connected to connectors  18 . The storage media  16  is coupled to at least one actuator  20 , which is configured to move the media  16  relative to probes  14 . This movement causes individual storage locations or domains on the media  16  to be moved relative to the heads. Each head can include one or more electrodes. The storage media in the example of  FIG. 1  can be, for example, a ferroelectric or magnetic storage media. However, the invention is not limited to any particular type of storage media. 
     Probe storage devices may include actuators and suspension assemblies for providing relative movement between the storage media and the array of probes.  FIG. 2  is a schematic cross-sectional view of a probe storage device  30 . The device includes an enclosure  32 , also referred to as a case, base, or frame, which contains a substrate  34 . An array of probes  36  is positioned on the substrate. The probes extend upward to make contact with a storage media  38 . The storage media  38  is mounted on a movable member, or sled  40 . In this example, coils  42  and  44  are mounted on the movable member. Magnets  46  and  48  are mounted in the enclosure near the coils. Springs  50  and  52  form part of a suspension assembly that supports the movable member. The enclosure  32  can be formed of, for example, injection molded plastic. 
     The combination of coils and magnets forms actuators that are used to move the movable member. Electric current in the coils creates a magnetic field that interacts with the magnetic field produced by the magnets to produce a force that has a component in the plane of the movable member and causes linear movement of the movable member. 
       FIGS. 1 and 2  show one example of a probe storage device. However, other probe storage devices are known to include other types of actuators (such as electrostatic actuators) that provide relative movement between the probes and the storage media. Furthermore, the relative movement between the probes and the storage media can result from actuation of the probes toward or away from the media surface. While the present invention can be used in probe storage devices, it is not limited to any particular configuration of probe storage device components. 
     To reduce mechanical wear, the surface of the storage media is coated with a liquid lubricant. When the probes make contact with the media surface, the lubricant extends along the sides of the probes, forming a meniscus adjacent to the probe tip. The meniscus can cause forces that resist movement of the probe relative to the storage media. This invention provides an apparatus that reduces meniscus forces. 
     In one embodiment of the invention, the sides of the probes are coated with a low surface energy coating. As used herein, a low surface energy coating is a coating that provides a spreading coefficient, between the coating and the lubricant, of the proper sign and magnitude sufficient to yield a desired increase in the contact angle with the probe. The contact angle is the angle formed by the liquid lubricant at the three-phase boundary where a liquid lubricant, air, and the side of the probe intersect. The contact angle is determined by drawing a tangent at the contact point where the liquid and the solid intersect. It provides a quantitative measure of the wetting of the probe by the liquid lubricant. The desired contact angle may be determined by its effect on the functioning of the device, for example, by increasing the contact angle to minimize tracking errors. 
     Low surface energy coatings are readily available in solid and liquid form. Applying these coatings to sides of the probes will lower the interfacial energy between the probes and the liquid lubricant on the storage media, and will decrease the meniscus force and the adhesion of accumulated debris. 
       FIG. 3  is a schematic representation of a probe  60  coated with a low surface energy coating. The probe includes a tip  62  positioned adjacent to a storage media.  FIG. 3  depicts a released probe  60  wherein the tip  62  is adjacent to, but not in contact with, a surface  64  of a storage media  66  that has a thin layer of lubricant  68 . The probe in this example includes a first layer  70 , which can be for example Ta, Ti, V, or other metal, and a second layer  72 , which is nonconductive and can be, for example, Al 2 O 3 , Si, Si 3 N 4 , or SiO 2 . In addition, the probe has a narrow conductive layer  73  that is patterned at the tip, which can be made of Ru, PT, Rh, W, or other wear-resistant metals. The conductive layer at the tip provides the ability to apply an electrical potential to the media. 
     In the example of  FIG. 3 , the probe has a rectangular cross-sectional shape. While  FIG. 3  only shows two sides  74  and  76  of the probe, all sides of the probe have been coated with a low surface energy coating  78 . The coating should have a thickness of between 1 nm and 100 nm, enough to provide sufficient adhesion to the probe sides  74  and  76 . 
     The lubricant on the media can be, for example, any pure (neat) or modified liquid that provides a significant decrease in the wear rate of the probe over the unlubricated media. Examples include, but are not limited to, perfluoropolyethers, nonfinctionalized hydrocarbons, polyester-based lubricants, and polyphenol ethers. 
     The angle of contact of the probe with the surface plays an important role in the meniscus force.  FIG. 4  is a schematic representation of a tilted probe  80 , having a tip  82  in contact with a surface  84  of a storage media  86 . Lubricant  88  on the surface of the media forms a meniscus  90 . 
     The friction force F f  between the probe and media can be written as:
 
 F   f =μ( F   N   +F   m )+ F   vis  
 
where μ is the static friction coefficient, F N  is the normal force, F m  is the meniscus force, and F vis  is the viscous force.
 
     For a rectangularly shaped probe, the meniscus force can be written as: 
               F   m     =     2   ⁢           ⁢     γ   ⁡     (     w   +     t     sin   ⁢           ⁢   θ         )       ⁢     (     1   +     cos   ⁢           ⁢   ϕ       )             
where γ is the surface energy, w is the width of the probe, t is the thickness of the probe, θ is the angle the probe makes the media surface, and φ is the angle of contact between the lubricant and the lower side of the probe. These relationships are illustrated in  FIG. 4 , with the width w of the probe being in a direction perpendicular to the plane of the figure. While this example assumes that the probe has a generally rectangular cross-section, other probe shapes can be used. For example, the probes can have a triangular, elliptical, or trapezoidal cross-section.
 
     For the case of a probe that is 15 μm wide and 1 μm thick, contacting the media surface at θ≈45°, a fully wetting media lubricant, such as perfluoropolyether (PFPE), which has φ≈0° and γ≈25 mN/m 2 , would give a meniscus force of 1.7 μN in a direction that attracts the probe toward the media. The angle of the probe with respect to the surface of the media would be designed to meet system requirements, but is currently expected to be in the range of 15 to 90 degrees. 
     The meniscus force is independent of the normal force. However, if the probe sides are coated with a low surface energy film of, for example, a PFPE lubricant with a contact angle of ˜90°, then a ˜50% decrease in this force is expected, thereby lessening its deleterious effects. 
     It is widely known that mechanical interactions of contacting bodies, which take place in a fluid, result in a film build-up along the sides of the contacting members. This fluid build-up can affect the ability to hold the mechanical members in contact. The work of adhesion measures the strength of interaction between, for example, an accumulating film and a surface. As shown above, the strength of this interaction is related to cos φ. Thus by coating the sides of the probe with a low surface energy film, the adhesion of debris will be lessened and its rate of build-up will be lowered. 
     There are several commercially available low surface energy materials, such as liquid fluorocarbons that can be applied to the probe. In addition, solid fluorocarbon films can be used for the low surface energy coating material. Solid fluorocarbon films can be deposited using a number of deposition methods. For example, plasma enhanced chemical vapor deposition (PECVD) may be advantageous because it conformally coats complex geometries and is compatible with envisioned probe processing. The low surface energy coating can also be formed using physical vapor deposition. 
     Sputtering is another method that may be used to deposit low surface energy fluorocarbon films. Sputter-deposited thin polytetrafluoroethylene (PTFE, also known as Teflon®) films have low surface energy, as measured by the water contact angle. Because of the low surface energy of PTFE films, meniscus forces resulting from the lubricant on the storage media will be smaller. Another method of creating a low surface energy solid fluorocarbon film is via liquid deposition. Flurad™ FC-732, produced by 3M, is a commercially available liquid fluorocarbon that has been used to reduce the meniscus force between probes and PFPE lubricants. Self-assembled monolayers (SAMs), such as n-trichlorosilanes (with n=8 to 30), can also be used. 
     In one embodiment, deposition of low surface energy film could be incorporated into the current probe fabrication procedure. This would be an alternate approach to vapor deposition after the probes had been fabricated. In this approach, PTFE solid films could be deposited on one or both sides of the probe. 
     Additionally, it is possible that certain hydrocarbon films (such as high density polyethylene (HDPE), polyetheretherketone (PEEK), or polyimide could achieve the desired reduction in meniscus forces. The hydrocarbon coatings would be applied to the same probe surfaces, but any deposition process would be specific to the coating material. 
     This invention also reduces the amount of debris that can accumulate in the vicinity of the probe tips. Debris has been observed to accumulate at probe tips during operation of probe storage devices. Debris generation increases with the normal force on the probe. Coating the probe will reduce the meniscus force and will lower the amount of wear (or debris generation). Additionally, with a coating, the lubricant would have a lower work of adhesion and will not bond to the tip as readily. The rate of debris accumulation is thereby decreased and the overall amount of debris on the tip is lessened. 
     While the invention has been described in terms of several examples, it will be apparent to those skilled in the art that various changes can be made to the described examples without departing from the scope of the invention as set forth in the following claims.