Patent Publication Number: US-2012025426-A1

Title: Method and system for thermal imprint lithography

Description:
FIELD 
     Embodiments according to the present invention generally relate to thermal imprint lithography. 
     BACKGROUND 
     Micro-fabrication involves the fabrication of very small structures, for example structures having features on the order of micro-meters or smaller. Lithography is a micro-fabrication technique used to create ultra-fine (sub-25 nm) patterns in thin film on a substrate. During lithography, a mold having at least one protruding feature is pressed into the thin film. The protruding feature in the mold creates a recess in the thin film, thus creating an image of the mold. The thin film retains the image as the mold is removed. The mold may be used to imprint multiple thin films on different substrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG. 1  is a cross-sectional view of thin film layers at an early stage of manufacture, and further showing an imprinter according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the imprinter forming features in the thin film layers of  FIG. 1  by electrically heating a thin heat layer according to an embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of the imprinter and the thin film layers after imprinting and separation, according to an embodiment. 
         FIG. 4  is a cross-sectional view of an imprinter forming features in a thin film layer by optically heating an optical absorbing layer according to an embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of an imprinter forming features in a thin film layer by optically heating an optical absorbing resist layer according to an embodiment of the present invention. 
         FIG. 6  is a flow diagram of a method of thermal imprint lithography according to an embodiment of the present invention. 
         FIG. 7  is a plan view of a disc drive data storage device. 
         FIG. 8  is a cross-sectional view of a perpendicular magnetic recording medium that may be used for the disc drive storage device ( FIG. 7 ), according to an embodiment. 
         FIG. 9  is a cross-sectional view of the perpendicular magnetic recording medium ( FIG. 8 ) with a head unit, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. While the embodiments will be described in conjunction with the drawings, it will be understood that they are not intended to limit the embodiments. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents. Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding. However, it will be recognized by one of ordinary skill in the art that the embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments. 
     For expository purposes, the term “horizontal” as used herein is defined as a plane parallel to the plane or surface of a substrate, regardless of its orientation. The term “vertical” refers to a direction perpendicular to the horizontal as just defined. Terms such as “above,” “below,” “bottom,” “top,” “side,” “higher,” “lower,” “upper,” “over,” and “under” are defined with respect to the horizontal plane. 
       FIG. 1  is a simplified cross-sectional view of thin film layers  100  at an early stage of manufacture, and further showing an imprinter  102  according to an embodiment of the present invention. At this stage, a resist layer  104  has been formed over a substrate  106  in preparation for thermal imprint lithography. In the current embodiment, the resist layer  104  is a thermal plastic, for example polymethyl methacrylate (PMMA), polystyrene (PS), or styrene acrylonitrile (SAN). 
     The imprinter  102  comprises a main body  108 , a layer of heating material  110  under the main body  108 , and an imprinting layer  112  under the layer of heating material  110 . Thus, the layer of heating material  110  is between the main body  108  and the imprinting layer  112 . 
     The main body  108  is comprised of a rigid material, for example Ni or Ni alloy. The layer of heating material  110  is a thin layer of an electrical heating material, for example an electrical resistance sheet material that converts electrical energy into thermal energy. The layer of heating material  110  may have electrical connectors or terminals (not shown) at opposite ends. 
     The imprinting layer  112  is comprised of a mechanically hard material, for example Ni or Ni alloy. In addition, the imprinting layer  112  includes a surface with a pattern  114  formed therein. The pattern  114  is a negative image of a pattern of sub-micron or nano-dimensioned features, for example lateral dimensions of about 60 nm and heights of about 40 nm, to be imprinted in the resist layer  104 . The pattern  114  can be densely packed, for example half pitch from 10 nm-100 nm or isolated features 10 nm-100 nm features with a few ums pitch between nano features. In the current embodiment, the pattern  114  is formed using conventional optical lithographic techniques. The imprinting layer  112  may be provided with a thin layer of an anti-sticking or release agent (not shown), for example a fluorinated polyether compound such as for example Zdol®, available from Ausimont, Thorofare, N.J. or a fluorine-based polymer with silane end group (self-assemble mono-layer structure). 
       FIG. 2  is a simplified cross-sectional view of the imprinter  102  during formation of features  202  in the thin film layers  200 , according to an embodiment of the present invention. The imprinter  102  and the perpendicular magnetic recording medium  100  are forcefully moved into contact, for example at a pressure of ˜0.2 to ˜10.0 MPa. An electric current is supplied from an electrical power supply (not shown) to the layer of heating material  110 . The layer of heating material  110  may be an electrical resistance heating material that quickly heats in as short an interval as practicable, for example ˜ 2  to ˜10 seconds, to a temperature above the glass transition temperature T g , for example at least about 180° C., of the resist layer  104 , causing the resist layer  104  to reflow. 
     The heating material  110  selectively heats the imprinting layer  112  and the resist layer  104 , without substantially altering the temperature of the substrate  106  and the main body  108 . Thus, heat is confined to the materials in interfacial contact (in the current embodiment, the resist layer  104 , the heating material  110 , and the imprinting layer  112 ) and their vicinity. After an appropriate interval, for example less than about 10 seconds, the supply of electrical power to the heating material  110  is terminated. Before separation (see  FIG. 3 ), the heating material  110 , the imprinting layer  112 , and the resist layer  104  are allowed to cool to a temperature below the glass transition temperature T g  of the resist layer  104 , for example about 130-140° C. for PMAA. 
     In the current embodiment, the relatively large main body  108  and substrate  106  are not heated or cooled. However, in alternate embodiments the thin film layers  100  and the imprinter  102  may be pre-heated and maintained at a preselected elevated temperature prior to heating the heating material  110 , thus reducing the processing interval. For example, the imprinter  102  may be pre-heated to and maintained at an elevated temperature close to the glass transition temperature T g  of the resist layer  104 , ˜105° C. for PMMA. Therefore, the heating material  110  quickly heats up to the glass transition temperature T g  of the resist layer  104  during formation of the features  202 . 
       FIG. 3  is a simplified cross-sectional view of the imprinter  102  and the thin film layers  100  after imprinting and separation, according to an embodiment of the present invention. The imprinter  102  and the thin film layers  100  have been separated after cooling. Thus, the resist layer  104  of the thin film layers  100  has been imprinted with the features  202 . 
       FIG. 4  is a simplified cross-sectional view of an imprinter  400  during formation of features  402  in thin film layers  404  at an early stage of manufacture, according to an alternate embodiment. Instead of electrically heating the heating material  112  ( FIG. 2 ), a heating material  406  is an optically absorbing layer between a main body  408  and an imprinting layer  410 . 
     The main body  408  may be transparent and, in the current embodiment, comprised of infra-red (IR) and visible light transmissive materials, for example quartz, Pyrex®, etc. The heating material  406  is comprised of an optically heated material that absorbs radiant/photonic energy, for example IR or visible light, and thus is selectively heated. The heating material  406  may comprise a thermoplastic polymer material that is inherently radiation absorbing, and/or the heating material  406  may include at least one radiation absorbing material for facilitating heating, for example a dye may be used. 
     A light source (not shown) delivers energy  414  to the heating material  406 . The energy  414  passes through the main body  408  and selectively heats the heating material  406 , without significantly heating the substrate  412  or the main body  408 . The temperature of the heating material  406  and the time to reach the appropriate temperature are controlled by regulating the intensity and wavelength of the energy  414 . 
     As in the previous embodiment, heating of the heating material  406  may stop when the temperature rises above the glass transition temperature T g  of a thermoplastic resist material  416 . The heating material  406 , the imprinting layer  410 , and the thermoplastic resist material  416  are then allowed to cool down to a temperature below the glass transition temperature T g  of the thermoplastic resist material  416 . The imprinter  400  and the thin film layers  404  are separated (not shown), leaving the thin film layers  404  ready for further processing (not shown). 
       FIG. 5  is a simplified cross-sectional view of an imprinter  500  during formation of features  502  in thin film layers  504  at an early stage of manufacture, according to an alternate embodiment of the present invention. In the present embodiment, there is no separate heating material ( 110  in  FIGS. 2 and 406  in  FIG. 4 ). Instead, an optically heated resist layer  506  is between a light transmissive main body  508  and the thin film layers  504 . 
     The optically heated resist layer  506  may be a thermoplastic resist layer over a substrate  512  that is inherently radiation absorbing and/or includes at least one radiation absorbing material. A light source (not shown) delivers energy  510  to the optically heated resist layer  506 . The energy  510  passes through the light transmissive main body, and selectively heats the optically heated resist layer  506 , without significantly heating the substrate  512  or the light transmissive main body  508 . 
     As in previous embodiments, heating of the optically heated resist layer  506  stops when the temperature of the optically heated resist layer  506  rises above the glass transition temperature T g . The optically heated resist layer  506  is then allowed to cool down to a temperature below the glass transition temperature T g . The imprinter  500  and the thin film layers  504  are separated (not shown), leaving the thin film layers  504  with an imprinted resist layer (not shown), ready for further processing (not shown). 
       FIG. 6  depicts a flowchart  600  of an exemplary method of thermal imprint lithography according to an embodiment of the present invention. Although specific steps are disclosed in the flowchart, such steps are exemplary. That is, embodiments of the present invention are well-suited to performing various other steps or variations of the steps recited in the flowchart. 
     In block  602 , an imprinter and a workpiece are pressed together. For example, in  FIG. 2  an imprinter comprises a main body, a layer of heating material under the main body, and an imprinting layer. The imprinter is moved against a surface of the workpiece to be imprinted. The workpiece is thin film layers, at an early stage of manufacture, comprising a substrate and a resist layer. 
     In block  604 , energy is supplied to the layer of heating material, causing a layer of material between the main body and the workpiece to heat and reflow. For example, in the embodiment of  FIG. 2  the layer of heating material is an electrical heating sheet and the material to reflow is a thermal plastic resist layer. Supplying electrical energy to the electrical heating sheet heats the electrical heating sheet, causing the thermal plastic resist layer to reflow, without substantially heating the imprinter and the main body. 
     In another example, in the alternate embodiments of  FIGS. 4 and 5 , the layer of heating material is an optically absorbing layer. Energy is supplied to the optically absorbing layer using a light source to deliver energy. In the embodiment of  FIG. 4 , energy from the light source causes an optically absorbing layer between the main body and the workpiece to heat. However, in the embodiment of  FIG. 5 , there is no separate optically absorbing layer. Instead, the resist layer is also the optically absorbing layer. Thus, the layer of heating material in  FIG. 5  is an optically absorbing resist layer over a substrate. Light energy is delivered to the optically absorbing resist layer, causing it to heat. 
     In a block  606 , the heating causes the workpiece to be imprinted by allowing the imprinting layer to form features in the thermal plastic resist layer. In a block  608 , the layer of material between the main body and the workpiece is cooled to a temperature where viscosity of the material between the main body and the workpiece is high, forming features in the surface of the workpiece. In a block  610 , the imprinter and the workpiece are separated. For example, in  FIG. 3  an imprinter and thin film layers have been separated after cooling of the resist layer. Features have been formed in the resist layer, and the thin film layers are ready for further processing. 
     Magnetic storage media are widely used in various applications, particularly in the computer industry for data storage and retrieval applications, as well as for storage of audio and video signals. Perpendicular magnetic recording media, for example hard disc drive storage devices, include recording media with a perpendicular anisotropy in the magnetic layer. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically by a layer of a magnetic material on a substrate. 
     A perpendicular recording disc drive head typically includes a trailing write pole, and a leading return or opposing pole magnetically coupled to the write pole. In addition, an electrically conductive magnetizing coil surrounds the yoke of the write pole. During operation, the recording head flies above the magnetic recording medium by a distance referred to as the fly height. To write to the magnetic recording medium, the magnetic recording medium is moved past the recording head so that the recording head follows the tracks of the magnetic recording medium, with the magnetic recording medium first passing under the return pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the return pole. In addition to providing a return path for the magnetic flux, the soft underlayer produces magnetic charge images of the magnetic recording layer, increasing the magnetic flux and increasing the playback signal. The current can be reversed, thereby reversing the magnetic field and reorienting the magnetic dipoles. 
     The perpendicular recording medium is a continuous layer of discrete, contiguous magnetic crystals or domains. Within the continuous magnetic layer, discrete information is stored in individual bits. The individual bits are magnetically oriented positively or negatively, to store binary information. The number of individual bits on the recording medium is a function of the areal density. As areal densities increase, the amount of information stored on the recording medium also increases. Manufacturers strive to satisfy the ever-increasing consumer demand for higher capacity hard drives by increasing the areal density. 
     High density perpendicular recording media use carefully balanced magnetic properties. These carefully balanced magnetic properties include sufficiently high anisotropy (perpendicular magnetic orientation) to ensure thermal stability, resist erasure, and function effectively with modern disc drive head designs; and grain-to-grain uniformity of magnetic properties sufficient to maintain thermal stability and minimum switching field distribution (SFD). 
     As recording densities increase, smaller grain structures help to maintain the number of magnetic particles in a bit at a similar value. Smaller grain structures are easier to erase, requiring higher anisotropy to maintain thermal stability, and making writability worse. Further, when individual storage bits within magnetic layers of magnetic recording media are reduced in size, they store less energy making it easier for the bits to lose information. Also, as individual weaker bits are placed closer together, it is easier for continuous read/write processes and operating environments to create interference within and between the bits. This interference disrupts the read/write operations, resulting in data loss. 
     The magnetic layers are designed as an ordered array of uniform islands, each island storing an individual bit. This is referred to as bit patterned media. By eliminating the continuous magnetic layer and restricting the bits to discrete magnetic islands, interference is reduced and areal densities are increased. However, high areal density bit patterned media (e.g., &gt;500 Gbpsi) demands high anisotropy of the magnetic material in the islands. 
     Methods and media structures are described herein, which embodiments of the present invention as described above, optimize anisotropy for bit patterned magnetic recording media. It is appreciated that magnetic recording media as discussed herein may be utilized with a variety of systems including disc drive memory systems, etc. 
       FIG. 7  is a data storage device in which embodiments of the present invention can be implemented to form bit-patterned media.  FIG. 7  is a plan view of a disc drive  700 . The disc drive  700  generally includes a base plate  702  and a cover (not shown) that may be disposed on the base plate  702  to define an enclosed housing for various disc drive components. The disc drive  700  includes one or more data storage discs  704  of computer-readable data storage media. Typically, both of the major surfaces of each data storage disc  704  include a plurality of concentrically disposed tracks for data storage purposes. Each data storage disc  704  is mounted on a hub or spindle  706 , which in turn is rotatably interconnected with the base plate  702  and/or cover. Multiple data storage discs  704  are typically mounted in vertically spaced and parallel relation on the spindle  706 . A spindle motor  708  rotates the data storage discs  704  at an appropriate rate. 
     The disc drive  700  also includes an actuator arm assembly  710  that pivots about a pivot bearing  712 , which in turn is rotatably supported by the base plate  702  and/or cover. The actuator arm assembly  710  includes one or more individual rigid actuator arms  714  that extend out from near the pivot bearing  712 . Multiple actuator arms  714  are typically disposed in vertically spaced relation, with one actuator arm  714  being provided for each major data storage surface of each data storage disc  704  of the disc drive  700 . Other types of actuator arm assembly configurations could be utilized as well, such as an “E” block having one or more rigid actuator arm tips or the like that cantilever from a common structure. Movement of the actuator arm assembly  710  is provided by an actuator arm drive assembly, such as a voice coil motor  716  or the like. The voice coil motor  716  is a magnetic assembly that controls the operation of the actuator arm assembly  710  under the direction of control electronics  718 . 
     A load beam or suspension  720  is attached to the free end of each actuator arm  714  and cantilevers therefrom. Typically, the suspension  720  is biased generally toward its corresponding data storage disc  704  by a spring-like force. A slider  722  is disposed at or near the free end of each suspension  720 . What is commonly referred to as the read/write head (e.g., transducer) is appropriately mounted as a head unit (not shown) under the slider  722  and is used in disc drive read/write operations. The head unit under the slider  722  may utilize various types of read sensor technologies such as anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), tunneling magnetoresistive (TuMR), other magnetoresistive technologies, or other suitable technologies. 
     The head unit under the slider  722  is connected to a preamplifier  726 , which is interconnected with the control electronics  718  of the disc drive  700  by a flex cable  728  that is typically mounted on the actuator arm assembly  710 . Signals are exchanged between the head unit and its corresponding data storage disc  704  for disc drive read/write operations. In this regard, the voice coil motor  716  is utilized to pivot the actuator arm assembly  710  to simultaneously move the slider  722  along a path  730  and across the corresponding data storage disc  704  to position the head unit at the appropriate position on the data storage disc  704  for disc drive read/write operations. 
     When the disc drive  700  is not in operation, the actuator arm assembly  710  is pivoted to a “parked position” to dispose each slider  722  generally at or beyond a perimeter of its corresponding data storage disc  704 , but in any case in vertically spaced relation to its corresponding data storage disc  704 . In this regard, the disc drive  700  includes a ramp assembly  732  that is disposed beyond a perimeter of the data storage disc  704  to both move the corresponding slider  722  vertically away from its corresponding data storage disc  704  and to also exert somewhat of a retaining force on the actuator arm assembly  710 . 
       FIG. 8  is a simplified cross-sectional view of a perpendicular magnetic recording medium  800 , which may be used for the data storage disc  704  ( FIG. 7 ). The perpendicular magnetic recording medium  800  is an apparatus including multiple layers established upon a substrate  802 . A seed layer  808  is a layer that is established overlying the substrate. A base layer  810  is a layer that is established overlying the seed layer  808 . Perpendicular magnetic recording islands  812  are recording areas that are established in the base layer  810  and on the seed layer  808 . 
     The substrate  802  can be fabricated from materials known to those skilled in the art to be useful for magnetic recording media for hard disc storage devices. For example, the substrate  802  may be fabricated from aluminum (Al) coated with a layer of nickel phosphorous (NiP). However, it will be appreciated that the substrate  802  can also be fabricated from other materials such as glass and glass-containing materials, including glass-ceramics. The substrate  802  may have a smooth surface upon which the remaining layers can be deposited. 
     In a further embodiment, a buffer layer  804  is established overlying the substrate  802 , a soft underlayer  806  is established overlying the buffer layer  804 , and the seed layer  808  is overlying the soft underlayer  806 . The buffer layer  804  can be established from elements such as Tantalum (Ta). The soft underlayer  806  can be established from soft magnetic materials such as CoZrNb, CoZrTa, FeCoB and FeTaC. The soft underlayer  806  can be formed with a high permeability and a low coercivity. For example, in an embodiment the soft underlayer  806  has a coercivity of not greater than about 10 oersteds (Oe) and a magnetic permeability of at least about 50. The soft underlayer  806  may comprise a single soft underlayer or multiple soft underlayers, and may be separated by spacers. If multiple soft underlayers are present, the soft underlayers can be fabricated from the same soft magnetic material or from different soft magnetic materials. 
     In the embodiment illustrated, the seed layer  808  is disposed on the soft underlayer  806 . The seed layer  808  can be established, for example, by physical vapor deposition (PVD) or chemical vapor deposition (CVD) from noble metal materials such as, for example, Ru, Ir, Pd, Pt, Os, Rh, Au, Ag or other alloys. The use of these materials results in desired growth properties of the perpendicular magnetic recording islands  812 . 
     The perpendicular magnetic recording islands  812  as described herein may be formed within the base layer  810  and on the seed layer  808  according to the embodiments of the present invention. The perpendicular magnetic recording islands  812  can be established to have an easy magnetization axis (e.g., the C-axis) that is oriented perpendicular to the surface of the perpendicular magnetic recording medium  800 . Useful materials for the perpendicular magnetic recording islands  812  include cobalt-based alloys with a hexagonal close packed (hcp) structure. Cobalt can be alloyed with elements such as chromium (Cr), platinum (Pt), boron (B), niobium (Nb), tungsten (W) and tantalum (Ta). 
     The perpendicular magnetic recording medium  800  can also include a protective layer (not shown) on top of the perpendicular magnetic recording islands  812  and/or the base layer  810 , such as a protective carbon layer, and a lubricant layer disposed over the protective layer. These layers are adapted to reduce damage from the read/write head interactions with the recording medium during start/stop operations. 
       FIG. 9  is a simplified cross-sectional view of a portion of the perpendicular magnetic recording medium  800  with a head unit  900 . During the writing process, a perpendicular write head  902  flies or floats above the perpendicular magnetic recording medium  800 . The perpendicular write head  902  includes a write pole  904  coupled to an auxiliary pole  906 . The arrows shown indicate the path of a magnetic flux  908 , which emanates from the write pole  904  of the perpendicular write head  902 , entering and passing through at least one perpendicular magnetic recording island  812  in the region below the write pole  904 , and entering and traveling within the soft underlayer  806  for a distance. The magnetically soft underlayer  806  serves to guide magnetic flux emanating from the head unit  900  through the recording island  812 , and enhances writability. As the magnetic flux  908  travels towards and returns to the auxiliary pole  906 , the magnetic flux  908  disperses. 
     The magnetic flux  908  is concentrated at the write pole  904 , and causes the perpendicular magnetic recording island  812  under the write pole  904  to magnetically align according to the input from the write pole  904 . As the magnetic flux  908  returns to the auxiliary pole  906  and disperses, the magnetic flux  908  may again encounter one or more perpendicular magnetic recording islands  812 . However, the magnetic flux  908  is no longer concentrated and passes through the perpendicular magnetic recording islands  812 , without detrimentally affecting the magnetic alignment of the perpendicular magnetic recording islands  812 . 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as may be suited to the particular use contemplated.