Abstract:
The present invention is directed to an apparatus for patterning a liquid on a substrate, with the apparatus including, a template having a pair of spaced-apart recessions with a protrusion disposed therebetween, with the protrusion being spaced-apart from the substrate a first distance and each of the pair of spaced-apart recessions being spaced-apart from the substrate a second distance, with the second distance being greater than the first distance; and a source of voltage in electrical communication with the template to produce an electric field between the template and the substrate, with a strength of the electrical field being inversely proportional to the first and second distances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional of U.S. patent application Ser. No. 09/905,718 filed on May 16, 2001 entitled “Method and System for Fabricating Nanoscale Patterns in Light Curable Compositions using an Electric Field,” which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention generally relates to the area of low cost, high-resolution, high-throughput lithography with the potential to make structures that are below 100 nm in size. 
         [0004]    2. Description of the Relevant Art 
         [0005]    Optical lithography techniques are currently used to make microelectronic devices. However, these methods are reaching their limits in resolution. Sub-micron scale lithography has been a critical process in the microelectronics industry. The use of sub-micron scale lithography allows manufacturers to meet the increased demand for smaller and more densely packed electronic components on chips. The finest structures producible in the microelectronics industry are currently on the order of about 0.13 μm. It is expected that in the coming years, the microelectronics industry will pursue structures that are smaller than 0.05 μm (50 nm). Further, there are emerging applications of nanometer scale lithography in the areas of opto-electronics and magnetic storage. For example, photonic crystals and high-density patterned magnetic memory of the order of terabytes per square inch require nanometer scale lithography. 
         [0006]    For making sub-50 nm structures, optical lithography techniques may require the use of very short wavelengths of light (for instance 13.2 nm). At these short wavelengths, few, if any, materials are optically transparent and therefore imaging systems typically have to be constructed using complicated reflective optics [ 1 ]. Furthermore, obtaining a light source that has sufficient output intensity at these wavelengths of light is difficult. Such systems lead to extremely complicated equipment and processes that appear to be prohibitively expensive. High-resolution e-beam lithography techniques, though very precise, typically are too slow for high-volume commercial applications. 
         [0007]    One of the main challenges with current imprint lithography technologies is the need to establish direct contact between the template (master) and the substrate. This may lead to defects, low process yields, and low template life. Additionally, the template in imprint lithography typically is the same size as the eventual structures on the substrate (1X), as compared to 4X masks typically used in optical lithography. The cost of preparing the template and the life of the template are issues that may make imprint lithography impractical. Hence there exists a need for improved lithography techniques that address the challenges associated with optical lithography, e-beam lithography and imprint lithography for creating very high-resolution features. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to an apparatus for patterning a liquid on a substrate, with the apparatus including, a template having a pair of spaced-apart recessions with a protrusion disposed therebetween, with the protrusion being spaced-apart from the substrate a first distance and each of the pair of spaced-apart recessions being spaced-apart from the substrate a second distance, with the second distance being greater than the first distance; and a source of voltage in electrical communication with the template to produce an electric field between the template and the substrate, with a strength of the electrical field being inversely proportional to the first and second distances. These and other embodiments are discussed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1A-1E  illustrate a version of the imprint lithography process according to the invention; 
           [0010]      FIG. 2  is a process flow diagram showing the sequence of steps of the imprint lithography process of  FIGS. 1A-1E ; 
           [0011]      FIG. 3  is a side view of a template positioned over a substrate for electric field based lithography; 
           [0012]      FIG. 4  is a side view of a process for forming nanoscale structures using direct contact with a template; 
           [0013]      FIG. 5  is a side view of a process for forming nanoscale structures using non-direct contact with a template; 
           [0014]      FIG. 6  is a side view of a substrate holder configured to alter the planarity of the substrate; and 
           [0015]      FIG. 7  is a side view of an apparatus for positioning a template over a substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]      FIGS. 1A through 1E  illustrate an imprint lithography process according to the invention, denoted generally as  10 . In  FIG. 1A , a template  12  is orientated in spaced relation to a substrate  14  so that a gap  16  is formed in the space separating template  12  and substrate  14 . A surface  18  of template  12  is treated with a thin layer  20  that lowers the template surface energy and assists in separation of template  12  from substrate  14 . The manner of orientation including devices for controlling of gap  16  between template  12  and substrate  14  are discussed below. Next, in  FIG. 1B , gap  16  is filled with a substance  22  that conforms to the shape of surface  13 . Preferably, substance  22  is a liquid so that it fills the space of gap  16  rather easily without the use of high temperatures and gap  16  can be closed without requiring high pressures. 
         [0017]    A curing agent  24 , shown in  FIG. 1C , is applied to template  12  causing substance  22  to harden and to assume the shape of the space defined by gap  16  between template  12  and substrate  14 . In this way, desired features  26 , shown in FIG. ID, from template  12  are transferred to the upper surface of substrate  14 . A transfer layer  28  is provided directly on the upper surface of substrate  14  which facilitates the amplification of features transferred from template  12  onto substrate  14  to generate high aspect ratio features. 
         [0018]    In  FIG. 1D , template  12  is removed from substrate  14  leaving the desired features  26  thereon. The separation of template  12  from substrate  14  must be done so that desired features  26  remain intact without shearing or tearing from the surface of substrate  14 . 
         [0019]    Finally, in  FIG. 1E , features  26  transferred from template  12 , shown in  FIG. 1D , to substrate  14  are amplified in vertical size by the action of transfer layer  28 , as is known in the use of bi-layer resist processes. The resulting structure can be further processed to complete the manufacturing process using well-known techniques.  FIG. 2  summarizes the imprint lithography process, denoted generally as 30, of the present invention in flow chart form. Initially, at step  32 , course orientation of a template and a substrate is performed so that a rough alignment of the template and the substrate is achieved. The advantage of course orientation at step  32  is that it allows pre-calibration in a manufacturing environment where numerous devices are to be manufactured with efficiency and with high production yields. For example, where the substrate comprises one of many die on a semiconductor wafer, course alignment (step  32 ) can be performed once on the first die and applied to all other dies during a single production run. In this way, production cycle times are reduced and yields are increased. 
         [0020]    Next, at step  34 , the spacing between the template and the substrate is controlled so that a relatively uniform gap is created between the two layers permitting the type of precise orientation required for successful imprinting. The present invention provides a device and a system for achieving the type of orientation (both course and fine) required at step  34 . At step  36 , a liquid is dispensed into the gap between the template and the substrate. Preferably, the liquid is a UV curable organosilicon solution or other organic liquids that become a solid when exposed to UV light. The fact that a liquid is used eliminates the need for high temperatures and high pressures associated with prior art lithography techniques. 
         [0021]    At step  38 , the gap is closed with fine orientation of the template about the substrate and the liquid is cured resulting in a hardening of the liquid into a form having the features of the template. Next, the template is separated from the substrate, step  40 , resulting in features from the template being imprinted or transferred onto the substrate. Finally, the structure is etched, step  42 , using a preliminary etch to remove residual material and a well-known oxygen etching technique to etch the transfer layer. 
         [0022]    As mentioned above, recent imprint lithography techniques with UV curable liquids [ 2 ,  3 ,  4 ,  5 ] and polymers [ 6 ] have been described for preparing nanoscale structures. These techniques may potentially be significantly lower cost than optical lithography techniques for sub-50 nm resolution. Recent research [ 7 ,  8 ] has also investigated the possibility of applying electric fields and van der Waals attractions between a template that possesses a topography and a substrate that contains a polymeric material to form nanoscale structures. This research has been for systems of polymeric material that may be heated to temperatures that are slightly above their glass transition temperature. These viscous polymeric materials tend to react very slowly to the electric fields (order of several minutes) making them less desirable for commercial applications. 
         [0023]    The embodiments described herein may potentially create lithographic patterned structures quickly (in a time of less than about 1 second). The structures may have sizes of tens of nanometers. The structures may be created by curing a poiymerizable composition (e.g., a spin-coated UV curable liquid) in the presence of electric fields. Curing the polymerizable composition then sets the pattern of structures on the substrate. The pattern may be created by placing a template with a specific nanometer-scale topography at a carefully controlled nanoscale distance from the surface of a thin layer of the liquid on a substrate. If all or a portion of the desired structures are regularly repeating patterns (such as an array of dots), the pattern on the template may be considerably larger than the size of the desired repeating structures. The template may be formed using direct write e-beam lithography. The template may be used repeatedly in a high-throughput process to replicate nanostructures onto substrates. In one embodiment, the template may be fabricated from a conducting material such as Indium Tin Oxide that is also transparent to UV light. The template fabrication process is similar to that of phase shift photomasks for optical lithography; phase shift masks require an etch step that creates a topography on the template. 
         [0024]    The replication of the pattern on the template may be achieved by applying an electric, field between, the template and the substrate. Because the liquid and air (or vacuum) have different dielectric constants and the electric field varies locally due to the presence of the topography of the template, an electrostatic force may be generated that attracts regions of the liquid toward the template. At high electric field strengths, the polymerizable composition may be made to attach to the template and dewet from the substrate at certain points. This polymerizable composition may be hardened in place by polymerization of the composition. The template may be treated with a low energy self-assembled monolayer film (e.g., a fluorinated surfactant) to aid in detachment of the template the polymerized composition. 
         [0025]    It may be possible co control the electric field, the design of the topography of the template and the proximity of the template to the liquid surface so as to create a pattern in the poiymerizable composition that does not come into contact with the surface of the template. This technique may eliminate the need for mechanical separation of the template from the polymerized composition. This technique may also eliminate a potential source of defects in the pattern. In the absence of contact, however, the liquid may not form sharp, high-resolution structures that are as well defined as in the case of contact. This may be addressed by first creating structures in the poiymerizable composition that are partially defined at a given electric field. Subsequently, the gap may be increased between the template and substrate while simultaneously increasing the magnitude of the electric field to “draw-out” the liquid to form clearly defined structures without requiring contact. 
         [0026]    The polymerizable composition may be deposited on top of a hard-baked resist material to lead to a bi-layer process. Such a bi-layer process allows for the formation of low aspect ratio, high-resolution structures using the electrical fields followed by an anisotropic etch that results in high-aspect ratio, high-resolution structures. Such a bi-layer process may also be used to perform a “metal lift-off process” to deposit a metal on the substrate such that the metal is left behind after lift-off in the trench areas of the originally created structures. 
         [0027]    By using a low viscosity polymerizable composition, the pattern formation due to the electric field may be fast (e.g., less than about 1 sec), and the structure may be rapidly cured. Avoiding temperature variations in the substrate and the polymerizable composition may also avoid undesirable pattern distortion that makes nano-resolution layer-to-layer alignment impractical. In addition, as mentioned above, it is possible to quickly norm a pattern without contact with the template, thus eliminating defects associated with imprint methods that require direct contact. 
         [0028]      FIG. 3  depicts an embodiment of the template and the substrate designs. Template  12  may be formed from a material that is transparent to activating light produced by curing agent  24  to allow curing of substance  22 , with substance  22  being a polymerizable composition, by exposure to activating light. Forming template  12  from a transparent material may also allow the use of established optical techniques to measure gap  16  between template  12  and substrate  14  and to measure overlay marks to perform overlay alignment and magnification correction during formation of the structures. Template  12  may also be thermally and mechanically stable to provide nano-resolution patterning capability. Template  12  may also include an electrically conducting material to allow electric fields to be generated at the template-substrate interface. 
         [0029]    In one embodiment, depicted in  FIG. 3 , a thick blank of fused silica has been chosen as the base material for template  12 . Indium Tin Oxide (ITO) may be deposited onto the fused Silica. ITO is transparent to visible and UV light and is a conducting material. ITO may be patterned using high-resolution e-beam lithography. Thin layer  20  (for example, a fluorine containing self-assembly monolayer) may be coated onto template  12  to improve the release characteristics between template  12  and substance  22 . Substrate  14  may include standard wafer materials, such as Si, GaAs, SiGeC and InP. A UV curable liquid may be used as substance  22 . Substance  22  may be spin coated onto substrate  14 . An optional transfer layer  28  may be placed between substrate  14  and substance  22 . Transfer layer  28  may be used for bi-layer process. Transfer layer  23  material properties and thickness may be chosen to allow for the creation of high-aspect ratio structures from low-aspect ratio structures created in substance  22 . An electric field may be generated between template  12  and substrate  14  by connecting the ITO to a voltage source. 
         [0030]    In  FIGS. 4 and 5 , two variants of the above-described process are presented. In each variant, it is assumed that a desired uniform gap  16  may be maintained between template  12  and substrate  14 . An electric field of the desired magnitude may be applied resulting in the attraction of substance  22  towards the raised portions of template  12 . In  FIG. 4 , gap  16  and the field magnitudes are such that substance  22  makes direct contact and adheres to template  12 . A UV curing process may be used to harden substance  22  in that configuration. Once the structures have been formed, template  12  is separated from substrate  14  by either increasing gap  15  till the separation is achieved, or by initiating a peel and pull motion wherein template  12  is peeled away from substrate  14  starting at one edge of template  12 . Prior to its use, template  12  is assumed to be treated with thin layer  20  that assists in the separation step. 
         [0031]    In  FIG. 5 , gap  16  and the field magnitudes are chosen such that substance  22  achieves a topography that is essentially the same as that of template  12 . This topography may be achieved without making direct contact with template  12 . A UV curing process may be used to harden substance  22  in that configuration. In both the processes of  FIGS. 4 and 5 , a subsequent etch process may be used to eliminate the residual layer of the UV cured material. A further etch may also be used if transfer layer  23  is present between substance  22  and substrate  14 , as shown in  FIGS. 4 and 5 . As mentioned earlier, transfer layer  28  may be used to obtain a high-aspect ratio structure from a low aspect ratio structure created in substance  22 . 
         [0032]      FIG. 6  illustrates mechanical devices that may increase the planarity of the substrate. The template may be formed from high-quality optical flats of fused-silica with Indium Tin Oxide deposited on the fused silica. Therefore, the template typically possess extremely high planarity. The substrates typically have low planarity. Sources of variations in the planarity of the substrate include poor finishing of the back side of the wafer, the presence of particular contaminants trapped between the wafer and the wafer chuck, and wafer distortions caused by thermal processing of the wafer. In one embodiment, the substrate may be mounted on a chuck whose top surface shape may be altered by a large array of piezoelectric actuators. The chuck thickness may be such that accurate corrections in surface topography of up to a few microns may be achieved. The substrate may be mounted to the chuck such that it substantially conforms to the shape of the chuck. Once the substrate is loaded on to the chuck, a sensing system (e.g., an optical surface topography measurement system) may be used to map the top surface of the substrate accurately. Once the surface topology is known, the array of piezoelectric actuators may be actuated to rectify the topography variations such that the upper surface of the substrate exhibits a planarity of less than about 1μum. Since the template is assumed to be made from an optically flat material, this leads to template and substrate that are high quality planar surfaces. 
         [0033]    The mechanical device in  FIG. 7  may be used to perform a high-resolution gap control at the template-substrate interface. This device may control two tilting degrees of freedom (about orthogonal axes that lie on the surface of the template) and the vertical translation degree of freedom of the template. The magnitude of the gap between the template and the substrate may be measured in real-time. These real-time measurements may be used to identify the corrective template motions required about the tilting degrees of freedom and the vertical displacement degree of freedom. The three gap measurements may be obtained by using a broadband optical interferometric approach that is similar to the one used for measuring thicknesses of thin films and thin film stacks. This approach of capacitive sensing may also be used for measuring these three gaps. 
         [0034]    Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein arc to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
       REFERENCES 
       [0035]    The following references are specifically incorporated herein by reference: 
         [0036]    1. “Getting More from Moore&#39;s,” Gary Stix, Scientific American, April 2001. 
         [0037]    2. “Step and Flash Imprint Lithography: An alternative approach to high resolution patterning,” M. Colburn, S. Johnson, M. Stewart, S. Damle, B. J. Choi, T. Bailey, M. Wedlake, T. Michaelson, S. V. Sreenivasan, J. Skerdt, C. G. Nillson, Proc. SPIE Vol.3676, 379-383,1999. 
         [0038]    3. “Design of Orientation Stages for Step and Flash Imprint Lithography,” B. J. Choi, S. Johnson, M. Colburn, S. V. Sreenivasan, C. G. Willson, To appear in J. of Precision Engineering. 
         [0039]    4. U.S. patent application Ser. No. 09/266,663 entitled “Step and Flash Imprint Lithography” to Grant Willson and Matt Colburn. 
         [0040]    5. U.S. patent application Ser. No. 09/698,317 entitled “High Precision Orientation Alignment and Gap Control Stages for Imprint Lithography Processes” to B. J. Choi, S.V. Sreenivasan and Steve Johnson. 
         [0041]    6. “Large area high density quantized magnetic disks fabricated using nanoimprint lithography,” W. Wu, B. Cui, X. Y. Sun, W. Zhang, L. Zhunag, and S. Y. Chou., J. Vac Sci Technol B 16 (6) 3825-3829 Nov-Dec 1998 
         [0042]    7. “Lithographically- induced Self-assembly of Periodic Polymer Micropillar Arrays,” S. Y. Chou, L. Zhuang, J Vac Sci Tech B 17 (6), 3197-3202, 1999 
         [0043]    8. “Large Area Domain Alignment in Block Copolymer Thin Films Using Electric Fields,” P. Mansky, 1. DeRouchey, J. Mays, M. Pitsikalis, T. Morkved, H. Jaeger and T. Russell, Macromolecules 13,4399 (1998).