Patent Publication Number: US-2006013956-A1

Title: Method and apparatus for providing shear-induced alignment of nanostructure in thin films

Description:
RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application Ser. No. 60/563,652 filed Apr. 20, 2004, the entire disclosure of which is expressly incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTERESTS  
      The present invention was made under a grant of the National Science Foundation, Grant No. DMR-0213706. Accordingly, the Government may have certain interests in the present invention. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the alignment of nanostructures in thin films. More specifically, the present invention relates to shear-induced alignment of spherical nanodomains in block copolymer films.  
      2. Related Art  
      Nanofabrication is witnessing a rapid trend towards self-assembled templates as a cost-effective method of generating densely-patterned surfaces. Such surfaces are particularly desirable in forming high-density memory arrays and devices. Templating with block copolymer (BCP) thin films, until recently an area of essentially academic interest, has become increasingly popular in the semiconductor industry to form such densely-patterned surfaces. BCPs are macromolecules composed of two or more (“diblock copolymer” or “diBCP”) chemically distinct, covalently connected, polymer chains which are typically immiscible in bulk. In these polymers, molecular connectivity prohibits macroscopic phase separation. Instead, BCPs “microphase separate” to form nanoscale domains.  
      In diBCPs where one block is much shorter than the other, the minority blocks self-assemble into spherical nanodomains within a matrix of the majority block. If the length disparity is less pronounced, the nanodomains are cylindrical or lamellar. The self-assembled polymeric patterns obtained in this fashion can be used as templates for lithography, enabling economical and versatile patterning techniques that are capable of creating arrays of dielectric, metallic, quantum, or magnetic dots spaced tens of nanometers apart. Such techniques are fully compatible with silicon semiconductor processing and are presently being investigated for fabrication of memory devices, including magnetic hard disks and nanocrystal flash memories.  
      A significant shortcoming with existing template fabrication techniques is an inability to accurately and consistently align nanostructures in thin films. This results in a lack of order, and thus addressability, of nanodomains in the film, which limits data storage density to well below the theoretical maximum of one bit per nanodomain. While the resulting arrays typically display excellent short-range order, due to the existence of topological defects, only limited-range order can be achieved by traditional self-assembly, even when coupled with annealing. Recent research efforts have been directed at guiding the self-assembly process in order to induce long-range, in-plane order in the array of BCP nanodomains which define the template structures. However, the high degree of isotropy imposed by the hexagonal packing of the spherical nanodomains complicates the alignment problem. For example, electric fields, which are highly successful for in-plane alignment of cylinder-forming BCPs for defining templates of stripe arrays, have not been profitably applied to arrays of BCP spheres.  
      Accordingly, what would be desirable, but has not yet been provided, is a method and apparatus for providing shear-induced alignment of nanostructures in thin films, wherein uniform alignment of nanostructures is achieved with long-range order.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method and apparatus for aligning nanostructures in thin films. The method comprises the steps of forming, on a substrate, a thin film having nanostructures; annealing the thin film; applying a shear stress to the thin film during annealing; and allowing the nanostructures to align. The thin film could comprise a block copolymer (BCP) with spherical nanodomains formed therein, such as a poly(ethylene-alt-propylene) (PEP) matrix with polystyrene (PS) nanodomains formed therein, or a PS matrix with polyisoprene (PI) particles formed therein, or any other type of sphere-forming copolymer.  
      The present invention also provides an apparatus for aligning nanostructures in thin films. The apparatus comprises a substrate for receiving a thin film containing nanostructures to be aligned; means for annealing the thin film; and means for imparting a shear stress on the thin film. The means for imparting a shear stress comprises, in one embodiment of the present invention, a flexible or rigid pad positioned on the thin film and means for moving the pad with respect to the substrate. A weight could be placed on the pad to ensure uniform contact between the pad and the thin film. In another embodiment of the present invention, a thin fluid layer is provided between the pad and the thin film, wherein shear stress is transmitted through the thin fluid layer to the thin film. The fluid layer could comprise a viscous silicone, hydrocarbon oil, or other suitable fluid. Optionally, the pad and weight could be replaced with a rolling apparatus, and shear applied to the thin film using a rolling process to align nanostructures in the film. Further, shear could be applied to the thin film using a confined channel and a fluid flowing through the channel.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other important objects and features of the invention will be apparent from the following Detailed Description of the Invention taken in connection with the accompanying drawings in which:  
       FIG. 1  is a cross-sectional view showing a thin film having nanostructures to be aligned, wherein the thin film is positioned in the apparatus of the present invention and a shear stress is applied to the film.  
       FIG. 2  is a cross-sectional view of another embodiment of the present invention, wherein a thin fluid layer is positioned between the pad and the thin film.  
       FIG. 3  is a top view showing alignment of two layers of spherical nanodomains in a BCP thin film achieved by the present invention.  
       FIGS. 4   a - 4   b  are tapping-mode atomic force microscopy (TM-AFM) images of sheared bilayer films of a polystyrene-poly(ethylene-alt-propylene) diblock copolymer (PS-PEP) produced by the present invention, taken at two separate locations on a single cm 2  sample.  
       FIG. 5  is a TM-AFM image showing a disordered PS-PEP film resulting from shearing films that are slightly thicker or thinner than a bilayer.  
       FIG. 6  is a scanning electron microscopy (SEM) image of a sheared thin film bilayer of polystyrene (PS)/polyisoprene (PI) produced by the present invention.  
       FIG. 7  is a diagram showing an alternate embodiment of the present invention, wherein shear stress is applied to a thin film using a rolling apparatus.  
       FIG. 8  is a diagram showing an alternate embodiment of the present invention, wherein shear stress is applied using a confining channel and a fluid flowing therethrough.  
    
    
     DETAILED DESCRIPTION  
      The present invention relates to a method and apparatus for providing shear-induced alignment of nanostructures in thin films, such as block co-polymer (BCP) thin films. The term “nanostructure,” as used herein, includes but is not limited to, spherical nanodomains, self-assembled nanodomains, and particles. A substrate is provided, and a thin film layer having nanostructures therein is formed on the substrate. The thin film could be formed by spin coating, flow coating, or other suitable technique. A flexible or rigid pad, such as an elastomer pad, polished metal plate, or silicon wafer, is positioned on the thin film layer. A weight could be positioned on the pad to ensure uniform contact between the pad and the film. The film is annealed and the pad moved with respect to the substrate to impart a shear stress to the film during annealing. The shear stress aligns the nanostructures in the film. After annealing and application of the shear stress, the pad is removed, and the nanostructures remain uniformly aligned. Optionally, a thin fluid layer could be provided between the pad and the thin film to transmit shear stress from the pad to the film. Further, the pad and the weight could be replaced with a rolling apparatus, and shear stress applied to the thin film using a rolling process. Additionally, the shear stress could be applied using a confining channel with a fluid flowing therethrough.  
       FIG. 1  is a cross-sectional view showing a thin film  30  having nanostructures to be aligned and positioned in the apparatus  10  of the present invention. The apparatus  10  comprises a substrate  20  with a uniformly flat surface and a pad  40  applied to the thin film  30 . The substrate  20  could comprise a silicon wafer, or any other suitable material. The pad  40  could comprise a pad made of an elastomer, a rigid solid, or other suitable material. A weight  50  could be positioned on the pad  40  to ensure uniform contact between the pad  40  and the film  30 . To align nanostructures in the film  30 , the silicon substrate  20  is first polished to form a clean and uniform surface. The substrate  20  is approximately 400 microns in thickness, but any suitable thickness could be used. Then, the film  30  is formed on the substrate  20 . The film  30  could comprise a block copolymer (BCP), which could be produced by living anionic polymerization, and optionally, with subsequent chemical modification such as hydrogenation. Examples of suitable BCP materials include, but are not limited to, a polystyrene-b-poly(ethylene-alt-propylene) (PS-PEP 3-23) layer with block molecular weights of 3 kg/mol (PS) and 23 kg/mol (PEP), and a polystyrene-b-polyisoprene (PS-PI 68-12) layer with block molecular weights of 68 kg/mol (PS) and 12 kg/mol (PI). The bulk morphology of the film  30  comprises spherical nanodomains wherein the minority copolymer block is embedded in a matrix comprising the majority copolymer block in a body-centered cubic (bcc) structure. For thin films, the spherical nanodomains assemble into hexagonally-packed arrays.  
      Thin films of the BCP could be cast from a dilute (1-2%) solution using spin coating, flow coating, or any other suitable technique known in the art. Suitable thicknesses of the film  30  can range from a few nanometers to several hundred nanometers. However, as will be discussed later with reference to  FIG. 5 , the thickness of the film  30  must be carefully controlled in order to ensure uniform alignment of nanostructures in the film  30 .  
      After formation of the film  30 , the pad  40  is positioned on the film  30 . The film  30  is then annealed by heating to a temperature between the glass transition temperature and the order-disorder transition temperature (T g &lt;T&lt;T ODT ) of the polymer forming the layer  30 . Optionally, a weight  50  could be positioned on the pad  40 . The pad  40  could be formed of a polydimethylsiloxane (PDMS) elastomer or other suitable material. In the examples disclosed herein, the pad  40  has an area of approximately 1 cm 2 , but of course, other dimensions could be provided without departing from the spirit or scope of the present invention. Further, the pad  40  has a thickness of approximately 0.5 to 4 millimeters, but other thicknesses could be used. Weight  50 , if utilized, has a mass of approximately 2 kilograms, but other masses could be provided. It should be noted that the present invention can be practiced without the pad  40  and weight  50 , wherein the pad  40  and weight  50  are replaced with a rolling apparatus and shear is imparted to the film using a rolling process. Additionally, the stress could be imparted using a confining channel with a fluid flowing therethrough. Any suitable means for imparting shear to the film can be used without departing from the spirit and scope of the present invention.  
      As shown in  FIG. 1 , a shear stress is applied to the film while the film  30  is annealed. The shear stress is applied to the film  30  by slowly moving the pad  40  and weight  50  (if utilized) in a lateral direction (e.g., a direction parallel to the substrate  10 ), indicated generally by the arrow A. The stress is applied for the duration of the annealing process, which typically lasts between a few minutes to a few hours, but could vary. The magnitude of the stress applied to the film  30  depends on the lateral pulling force applied to the pad  40  and the area of contact between the pad  40  and the film  30 . The pad  40  moves at a rate of approximately 40 nm/sec with respect to the substrate  20 , but this velocity can vary according to the shear stress applied, temperature, composition of the film  30 , and the mass of weight  50  (if utilized). The shearing process creates alignment of nanostructures in thin films. After annealing and movement of the pad, the pad  40  is removed from the film  30  and temperature is reduced to ambient temperature. During cooling, the nanostructures remain aligned.  
       FIG. 2  is a cross-sectional view of another embodiment of the present invention, wherein a thin fluid layer  35  is positioned between the pad  40  and the thin film  30 . The fluid layer  35  could comprise a viscous silicone or hydrocarbon oil, or other suitable material. Similar to the embodiment discussed earlier with respect to  FIG. 1 , the weight  50  (if utilized) and the pad  40  are moved in the lateral direction A. Shear stress is transmitted through the fluid layer  35  and to the thin film  30 , to align nanostructures in the film  30 . Importantly, the fluid layer  35  allows the pad  40  to be easily removed from the film  30  after alignment has been achieved. If the fluid layer  35  is utilized, the pad  40  need not be manufactured from an elastomer and could comprise a hard, solid surface such as a metal sheet or a silicon wafer.  
      An explanation of the alignment achieved by the present invention can be appreciated with reference to  FIG. 3 . The film  30  includes a number of spherical nanodomains which are intrinsic to the BCP material and which are stacked in different layers. For purposes of illustration, the film  30  comprises a first layer  30   a  having a plurality of spherical nanodomains  35   a , and a second layer  30   b  having a plurality of spherical nanodomains  35   b . When the stress A is applied to the pad  40 , the layer  30   a  moves with respect to the layer  30   b , and consequently, the nanodomains  35   a  move with respect to the nanodomains  35   b . The shear stress A must be sufficiently strong to break the lattice formed by the nanodomains in the film  30 . The critical shear stress depends upon the chemical composition and thickness of the film  30 . For PS-PEP 3-23, the critical stress required to break the lattice is approximately 400 Pascals.  
      The spherical nanodomains  35   a ,  35   b  are dynamic in that they can be broken and re-assembled. The nanodomains  35   a  of the top layer  30   a  can easily slide in the spaces between the nanodomains  35   b  of the bottom layer  30   b  when the shear stress A is applied. When the shear stress A is applied, nanodomains having lattice axes normal to the direction of shear (“perpendicular” configuration) break under the shear stress, and re-assemble into nanodomains having lattice axes parallel to the direction of shear (“parallel” configuration, as shown in  FIG. 3 ). Fluctuations in orientations of the nanodomains are thereby reduced, resulting in a highly-ordered (aligned) arrangement of nanodomains. Further, the sheared film  30  strongly inhibits the formation of dislocations in the layer. This is advantageous in that a single, isolated dislocation would result in several nanodomains assembled in the perpendicular configuration, which would cause spheres to jam into each other rather than sliding through the troughs of nanodomains in the adjacent layer.  
      The alignment of nanostructures achieved by the present invention can be appreciated with reference to the following Examples, which are supplied for purposes of illustration only and are not intended to limit the spirit or scope of the present invention:  
     EXAMPLE 1  
       FIGS. 4   a - 4   b  are tapping-mode atomic force microscopy (TM-AFM) images of a sheared PS-PEP 3-23 BCP bilayer film produced by the present invention, taken at two separate locations on a single cm 2  sample. The BCP film was approximately 50 nm in thickness, which corresponds to the thickness of two layers of nanodomains. The substrate thickness was approximately 500 microns, and a PDMS pad having a thickness of approximately 1 mm and an area of approximately 1 cm 2  was utilized. A shearing force of approximately 0.8 N was applied to the pad, and a weight of approximately 9.8 N was provided on the pad. The sample was heated to a temperature of approximately 100 degrees C., and the BCP layer was sheared for approximately 20 minutes. As can be seen in both images, TM-AFM imaging revealed an aligned hexagonal lattice over the full extent of the sheared region of the BCP layer, with one of the lattice directions coinciding with the shear direction. The inserts in  FIGS. 4   a - 4   b  show Fourier transforms, wherein the six dots indicate the uniform orientation of the samples in the direction of shear.  
      To investigate the quality of the alignment, the distribution of topological defects (disclinations and dislocations) were determined using computerized image analysis tools. The orientational order of the lattice was perfect over the entire sheared region, and no disclinations were identified anywhere in the sample (which consisted of a single grain). Translational order was good, but was limited by the presence of occasional isolated dislocations, which appeared, on average, 6 times per square micrometer. For purposes of comparison, samples annealed at similar temperatures for much longer time (e.g., 4 hours), but without shear, developed a multigrain structure with high topological defect densities (an average of 18 disclinations and 150 dislocations per square micron). Thus, the present invention achieves significant alignment of nanostructures, with high order.  
     EXAMPLE 2  
      As mentioned earlier, the thickness of the BCP layer significantly affects the quality of alignment produced by the present invention. This can be appreciated with reference to  FIG. 5 , which is a TM-AFM image showing a disordered BCP bilayer film resulting from shear stress applied to slightly thicker and thinner films, as well as monolayer films. The BCP film was approximately 49 nm in thickness. The substrate thickness was approximately 500 microns, and a PDMS pad having a thickness of approximately 1 mm and an area of approximately 1 cm 2  was utilized. A shearing force of approximately 0.8 N was applied to the pad, and a weight of approximately 9.8 N was provided on the pad. The sample was heated to a temperature of approximately 100 degrees C., and the BCP layer was sheared for approximately 20 minutes.  
      The image of  FIG. 5  shows a completely disordered lattice, with no translational or orientational order. The density of topological defects is also very high, with approximately 40 disclinations and 300 dislocations occurring per square micron. To study the dependence of alignment quality on film thickness, a flow-coating technique was used to create films with a thickness gradient along one direction, which were then sheared in a direction normal to the thickness gradient using an elastomer pad in direct contact with the film. The resulting samples showed alignment within a band corresponding to approximately 1 nm thickness variation, or 2% of the bilayer thickness. Consistently, for films created by spin-coating, a change of +/−3% in spin speed resulted in disordered samples. Thus, shear-alignment in bilayer BCP films is dependent upon film thickness. The insert in the top-right corner of  FIG. 5  shows a Fourier transform in the form of a circle, indicating random orientation of the nanodomain lattice.  
      Secondary ion mass spectrometry studies of a similar PS-PEP BCP thin film that forms PS cylinders revealed the existence of a PS wetting layer between the silicon substrate and the BCP thin film, which can facilitate rearrangement of the nanostructures. As such, a wetting layer can optionally be present between the substrate  20  and the film  30  to facilitate the rearrangement of nanostructures. Such a wetting layer is shown in the insert in  FIG. 5 , and is indicated generally as  22 .  
     EXAMPLE 3  
       FIG. 6  is a scanning electron microscope (SEM) image of an aligned BCP thin film of polyisoprene (PI) spheres in a polystyrene (PS) matrix taken with staining of the film using osmium tetroxide. It has been found that good alignment can be achieved with a PS-PI 68-12 BCP layer having a thickness of 110 nm. At room temperature, PI is a rubber and PS is a glass. The PS-PI 68-12 layer is heated to 180° C., wherein the PI and PS materials are fluids. The PS-PI 68-12 BCP layer is formed on the substrate in similar fashion as the PS-PEP 3-23 BCP layer discussed earlier, and an elastomer pad applied to the PS-PI 68-12 BCP layer. The substrate thickness was approximately 400 microns, and a PDMS pad having a thickness of approximately 1 mm and an area of approximately 1 cm 2  was utilized. A shearing force of approximately 0.07 N was applied to the pad, and a weight of approximately 9.8 N was provided on the pad. The sample was heated to a temperature of approximately 180 degrees C., and the BCP layer was sheared for approximately 9 hours.  
       FIG. 7  is a diagram showing an alternate embodiment of the present invention, indicated generally at  100 , wherein shear stress is applied to a thin film using a rolling apparatus  110 . The rolling apparatus could comprise a pair of cylindrical rollers  110  having an adjustable gap therebetween. The substrate  120  and the BCP film  130  are fed through the rollers  110  in the general direction indicated by arrow B. Optionally, a fluid layer could be used between the rollers  110  and the BCP film  130 . The rolling action of the rollers  110  imparts a shear force on the film  130  sufficient to align nanostructures in the film.  
       FIG. 8  is a diagram showing an alternate embodiment of the present invention, indicated generally at  200 , wherein shear stress is applied using a confining channel  240  and a fluid  250  flowing therethrough. The fluid  250  generates a shear force sufficient to align nanostructures in the film  230 .  
      In conclusion, the present invention provides a method and apparatus capable of creating BCP templates for arrays of spherical nanodomains (dots) that are well-aligned over cm 2  regions. It should be noted that the invention could be expanded to provide arrays of well-aligned regions of any desired size by increasing the area of the thin film to which the shear stress is applied (e.g., by increasing the area of the pad). In this manner, mass-fabrication of ultradense memory devices can be achieved.  
      Having thus described the invention in detail, it is to be understood that the foregoing description is not intended to limit the spirit and scope thereof. What is desired to be protected by letters patent is set forth in the appended claims.