Abstract:
An enhanced process forming a material pattern on a substrate deposits the material anisotropically on resist material patterned to correspond to an image of the material pattern. The material is etched isotropically to remove a thickness of the material on sidewalls of the resist pattern while leaving the material on a top surface of the resist pattern and portions of the surface of the substrate. The resist pattern is removed by dissolution thereby lifting-off the material on the top surface of the resist pattern while leaving the material on the substrate surface as the material pattern. Alternately, a first material layer is deposited on the resist pattern and a second material layer is deposited and planarized. The second material layer is etched exposing the first material while leaving the second material in features of the resist pattern. The first material and the resist are removed leaving the first material pattern.

Description:
[0001]    This application for patent claims priority to U.S. Provisional Patent Application Ser. No. 60/826,124 and U.S. Provisional Patent Application Ser. No. 60/824,762, which are hereby incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The United States government has a paid-up license in this invention and the right in limited circumstance to require the patent owner to license others on reasonable terms as provided by the terms of N66001-66-C2003 awarded by the Space and Naval Warfare Systems Center San Diego (SPAWAR). 
     
    
     TECHNICAL FIELD 
       [0003]    The field of the invention relates generally to nano-fabrication of structures. More particularly, the present invention is directed to a method of patterning a substrate. 
       BACKGROUND 
       [0004]    Nano-fabrication involves the fabrication of very small structures, e.g., having features on the order of nanometers or smaller. One area in which nano-fabrication has had a sizeable impact is in the processing of integrated circuits. As the semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate, nano-fabrication becomes increasingly important. Nano-fabrication provides greater process control while allowing increased reduction of the minimum feature dimension of the structures formed. Other areas of development in which nano-fabrication has been employed include biotechnology, optical technology, mechanical systems and the like. 
         [0005]    An exemplary nano-fabrication technique is commonly referred to as imprint lithography. Exemplary imprint lithography processes are described in detail in numerous publications, such as United States patent application publication 2004/0065976 filed as U.S. patent application Ser. No. 10/264,960, entitled, “Method and a Mold to Arrange Features on a Substrate to Replicate Features having Minimal Dimensional Variability”; United States patent application publication 2004/0065252 filed as U.S. patent application Ser. No. 10/264,926, entitled “Method of Forming a Layer on a Substrate to Facilitate Fabrication of Metrology Standards”; and U.S. Pat. No. 6,936,194, entitled “Functional Patterning Material for Imprint Lithography Processes,” all of which are assigned to the assignee of the present invention and all of which are incorporated by reference herein. 
         [0006]    An imprint lithography technique disclosed in each of the aforementioned United States patent application publications and United States patent includes formation of a relief pattern in a layer suitable for polymerization and transferring a pattern corresponding to the relief pattern into an underlying substrate. The substrate may be positioned upon a motion stage to obtain a desired position to facilitate patterning thereof. To that end, a template is employed spaced-apart from the substrate with a formable liquid present between the template and the substrate. The liquid is solidified to form a solidified layer that has a pattern recorded therein that is conforming to a shape of the surface of the template in contact with the liquid. The template is then separated from the solidified layer such that the template and the substrate are spaced-apart. The substrate and the solidified layer are then subjected to processes to transfer, into the substrate, a relief image that corresponds to the pattern in the solidified layer. 
       SUMMARY 
       [0007]    It may be difficult to produce a vertical or re-entrant resist profile when patterning at a high resolution; i.e., the resist profile may contain some slope. This may be true for patterns with high areal density. As a result, it has been difficult to create high resolution structures by lift-off patterning. Since it is difficult to solve the problem of resist profile, it may be desired to determine a solution to degrade or substantially remove the material that is deposited on the resist sidewall. Reactive ion etching (RIE) is effective for isotropic removal of a variety of materials that are patterned by the lift-off approach. Dry etch processes generally operate with much lower defect levels than lift-off processes. To reduce the process defects while maintaining the overall result of a lift-off process, a planarizing coating may be applied to allow selective etching of extraneous material from on top of the resist patterns. Existing approaches for high-resolution lift-off patterning may create moderate physical damage to the material that is deposited on the resist sidewalls. The present invention improves upon the existing practices by substantially removing the sidewall material. These and other embodiments are described more fully below. 
         [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a simplified side view of a lithographic system having a template spaced-apart from a substrate; 
           [0010]      FIG. 2A  is a side view of a substrate, as shown in  FIG. 1 , having a resist pattern with re-entrant sidewalls; 
           [0011]      FIG. 2B  is a side view of the substrate, shown in  FIG. 2A , the resist pattern having an anisotropic deposition material positioned thereon creating a discontinuous film; 
           [0012]      FIG. 2C  is a side view of the substrate, shown in  FIG. 2B , after immersion in a solvent, the resist material is dissolved and the extraneous material is removed by lift-off into solution, with the material that remains on the substrate forming a reverse image of the original resist pattern; 
           [0013]      FIG. 3A  is a side view of the substrate, shown in  FIG. 1 , having a resist pattern with sloped sidewalls; 
           [0014]      FIG. 3B  is a side view of the substrate, shown in  FIG. 3A , with deposition of material on the resist pattern with sloped sidewalls resulting in a continuous film that is connected to the substrate, the continuous film inhibiting the lift-off liquid from dissolving the resist and being difficult to remove because it is tethered to the substrate; 
           [0015]      FIG. 4A  is a side view of the substrate, shown in  FIG. 1 , with deposition of material on a resist pattern with sloped sidewalls resulting in sidewall coverage; as a result of the anisotropic nature of the deposition process, the sidewall coverage is thinner than the substrate coverage, wherein additional material is deposited at this step to compensate for the material that will be removed during a subsequent etch step; 
           [0016]      FIG. 4B  is a side view of the substrate, shown in  FIG. 4A , wherein an isotropic etch is employed to substantially remove the material from the sidewall of the pattern, enabling the lift-off liquid to penetrate and dissolve the resist material, resulting in a thinning of the material on the substrate; 
           [0017]      FIG. 4C  is a side view of the substrate, shown in  FIG. 4B , after immersion in a solvent, the resist material is dissolved and the extraneous material is removed by lift-off into a solution, with the material that remains on the substrate forming a reverse image of the original resist pattern; 
           [0018]      FIG. 5A  is a side view of the substrate, shown in  FIG. 1 , after deposition of material on a resist pattern with sloped sidewalls resulting in sidewall coverage, wherein the anisotropic nature of the deposition process results in the sidewall coverage is thinner than the substrate coverage; 
           [0019]      FIG. 5B  is a side view of the substrate, shown in  FIG. 5A , after a planarizing coating is applied which provides a top surface which is substantially planar; 
           [0020]      FIG. 5C  is a side view of the substrate, shown in  FIG. 5B , after the planarizing coating is partially etched, such that the material on top of the resist features is removed, while a substantial portion of the material remains between the resist features; 
           [0021]      FIG. 5D  is a side view of the substrate, shown in  FIG. 5C , after the deposited material is etched from the top of the resist feature; and 
           [0022]      FIG. 5E  is a side view of the substrate, shown in  FIG. 5D , after the resist is dissolved away, wherein the deposited material remains on the substrate, forming a reverse image of the original resist pattern. 
       
    
    
       [0023]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0024]    Referring to  FIG. 1 , a system  8  to form a relief pattern on a substrate  12  includes a stage  10  upon which substrate  12  is supported and a template  14 , having a patterning surface  18  thereon. In a further embodiment, substrate  12  may be coupled to a substrate chuck (not shown), the substrate chuck (not shown) being any chuck including, but not limited to, vacuum and electromagnetic. 
         [0025]    Template  14  and/or mold  16  may be formed from such materials including but not limited to, fused-silica, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, and hardened sapphire. As shown, patterning surface  18  comprises features defined by a plurality of spaced-apart recesses  17  and protrusions  19 , with recessions  17  extending along a direction parallel to protrusions  19  that provide a patterning surface  18  with a shape of a battlement. However, recess  17  and protrusions  19  may correspond to virtually any feature desired, including features to create an integrated circuit and may be as small as a few nanometers. However, in a further embodiment, patterning surface  18  may be substantially smooth and/or planar. Patterning surface  18  may define an original pattern that forms the basis of a pattern to be formed on substrate  12 . 
         [0026]    Template  14  may be coupled to an imprint head  20  to facilitate movement of template  14 , and therefore, mold  16 . In a further embodiment, template  14  may be coupled to a template chuck (not shown), the template chuck (not shown) being any chuck including, but not limited to, vacuum and electromagnetic. A fluid dispense system  22  is coupled to be selectively placed in fluid communication with substrate  12  so as to deposit monomeric material  24  thereon. It should be understood that monomeric material  24  may be deposited using any known technique, e.g., drop dispense, spin-coating, dip coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and the like. Monomeric material  24  is polymerized upon ultra violet (UV) exposure. 
         [0027]    A source  26  of energy  28  is coupled to direct energy  28  along a path  30 . Imprint head  20  and stage  10  are configured to arrange mold  16  and substrate  12 , respectively, to be in superimposition and disposed in path  30 . Either imprint head  20 , stage  10 , or both vary a distance between mold  16  and substrate  12  to define a desired volume there between that is filled by monomeric material  24 . 
         [0028]    An exemplary source  26  may produce ultraviolet energy. Other energy sources may be employed, such as thermal, electromagnetic and the like. The selection of energy employed to initiate the polymerization of monomeric material  24  is known to one skilled in the art and typically depends on the specific application which is desired. 
         [0029]    Referring to  FIG. 1 , typically, monomeric material  24  is disposed upon substrate  12  before the desired volume is defined between mold  16  and substrate  12 . However, monomeric material  24  may fill the volume after the desired volume has been obtained. After the desired volume is filled with monomeric material  24 , source  26  produces energy  28 , e.g., broadband energy that causes monomeric material  24  to solidify and/or cross-link conforming to the shape of a surface  25  of substrate  12  and patterning surface  18 . 
         [0030]    The broadband energy may comprises an actinic component including, but not limited to, ultraviolet wavelengths, thermal energy, electromagnetic energy, visible light and the like. The actinic component employed is known to one skilled in the art and typically depends on the material from which imprinting layer  12  is formed. Control of this process is regulated by a processor  32  that is in data communication with stage  10 , imprint head  20 , fluid dispense system  22 , source  26 , operating on a computer readable program stored in memory  34 . 
         [0031]    The above-mentioned may be further be employed in imprint lithography processes and system referred to in U.S. Pat. No. 6,932,934 entitled “Formation of Discontinuous Films During an Imprint Lithography Process;” United States patent application publication 2004/0124566, filed as U.S. patent application Ser. No. 10/194,991 entitled “Step and Repeat Imprint Lithography Processes;” and United States patent application publication 2004/0188381, filed as U.S. patent application Ser. No. 10/396,615, entitled “Positive Tone Bi-Layer Imprint Lithography Method”; and United States patent application publication 2004/0211754, filed as U.S. patent application Ser. No. 10/432,642, entitled Method of Forming Stepped Structures Employing Imprint Lithography,” all of which are incorporated by reference herein. 
         [0032]    To that end, lithographic techniques for creating patterns in material may be generally characterized as either “additive” or “subtractive”. Subtractive processes typically work by patterning a resist on top of the material and then using the resist as a mask during an etch process. The resist protects the material during the etching process, and thus the resist pattern is transferred into the underlying material. The resist may then be removed. Subtractive processes frequently require a highly tuned, anisotropic reactive ion etching (RIE) process. 
         [0033]    Referring to  FIGS. 2A-2C , additive processes do not generally use RIE etching processes. The most widely used “additive” technique is termed “lift-off patterning” which involves imaging the resist  50  and then depositing the material  52  over the resist pattern  50 . Material  52   a  is thus deposited directly on the substrate  112  in the regions where no resist is present. Material  52   b  is also deposited on top of the resist  50 . This extra material  52   b  is removed by dissolving the underlying resist  50  in a liquid (usually an organic solvent), which causes the overlying material  52   b  to “lift-off” into the liquid solution. The lift-off process patterns the deposited material  52   a , with a “negative-tone” image of the resist pattern, as shown in  FIG. 2C . 
         [0034]    Referring to  FIGS. 3A-3B , when performing a lift-off patterning process, it is preferable that the material  52  is deposited with high directionality (usually by vacuum evaporation). This ensures that the material  52   a  is mainly deposited on the underlying substrate  112  and the top of the resist features, as opposed to coating the resist sidewalls  56 . Deposition of materials  52   b  on the sidewalls of the resist feature may cause several problems. First, the deposited material may form a continuous layer that prevents the liquid, as mentioned above with respect to  FIGS. 2A-2C , from reaching the resist  50  during the lift-off dissolution process. The material  52   b  also provides a physical tether that may inhibit the material  52   b  from releasing into the liquid solution. Material  52   b  that is deposited on resist sidewalls  56  is also more likely to persist after the lift-off process, resulting in irregular patterning or unwanted residues. 
         [0035]    One approach to minimize, if not prevent, deposition of material  52   b  on the resist sidewalls  56  is to incorporate a re-entrant profile in the resist pattern  50 . The re-entrant portion of the resist sidewall  56  is “shadowed” such that the deposited material  52  forms a discontinuous film. Thus, the material  52   a  on the substrate  112  is not connected to the material  52   b  that is to be lifted-off. Re-entrant profiles have been created by using with multilayer resists, chemical surface treatments that alter the resist composition, and preferential exposure of the resist surface. However, it may be difficult to incorporate a re-entrant resist profile. It is difficult to create a re-entrant profile when very small features are required or when the resist pattern is created by imprinting. 
         [0036]    Further referring to  FIG. 3A , the shown profile may be created by electron beam lithography using processes that are well known in the field. In one embodiment, resist  50  is an electron beam resist coated on substrate  112  by spin coating and baking The substrate may comprise silicon, fused silica, or other materials and may include a coating or a plurality of coatings. In the embodiment, a layer of tantalum is coated to 5 nm thickness on the top surface of a fused silica wafer to provide electrical conductivity. The resist  50  may be poly methyl methacrylate (PMMA), ZEP520A (Nippon Zeon Corporation), or any other electron beam resist material. The thickness of the resist  50  film may be in the range of 10 nm to 150 nm. Selected regions of the resist  50  corresponding to a desired pattern are exposed to a finely focused beam of electrons. The resist material  50  is developed by immersing the substrate in a solvent solution. One process for developing PMMA is immersion in a solution of 70% isopropanol and 30% water at 0 degrees Celsius for 60 seconds followed by drying under nitrogen. One process for developing the ZEP520A resist is immersion in amyl acetate at 0 degrees Celsius for 60 seconds followed by rinsing in isopropanol and drying under nitrogen. Following this processing, a resist profile similar to that shown in  FIG. 3A  is formed. A brief “descum” reactive ion etch (RIE) process may be used to remove resist residues from the now-exposed surface of the substrate. Descum is defined as a plasma operation after develop that ensures full removal of resist from uncovered areas. One descum process comprises processing at 50 to 200 volts DC (direct current) bias with a gas feed mixture of argon (75-100%) and oxygen (0-25%), at a pressure of 5 to 50 mT, for a duration of 0-60 seconds 
         [0037]    Referring to  FIGS. 4A-4C , this contribution describes an advanced lift-off technique that is compatible with a resist process that does not produce a re-entrant profile. First, a resist pattern  50  is created on the substrate  112 . This resist pattern  50  may have a sloped sidewall  56 . Material  52  is deposited on top of the resist pattern  50  via a directional deposition technique (e.g., vacuum evaporation). Directional evaporation ensures that the thickness of the material  52   b  on the resist sidewalls  56  is less than the thickness of the material  52   a  that has been deposited directly on the substrate  112 . The deposited material  52  is then etched with an etch process that is substantially isotropic, such that all exposed material  52  is etch at a similar rate. This results in removal of the material  52   b  that was deposited on the resist sidewalls  56 . The isotropic etch also etches the material  52   a  that was deposited on the substrate  112 , but the thickness of this material  52   a  is thicker and thus material  52   a  will remain after the sidewall material  52   b  has been substantially removed. (The isotropic etch may be also tuned to substantially degrade or etch the resist material. For example, radiation-induced resist damage or RIE may be utilized to degrade the resist material and thus further promote lift-off). The RIE process may also be tuned to remove material from the substrate surface at a slower rate than material is removed from the feature sidewalls or the top surface of the resist. After the isotropic etch, the resist  50  is removed with liquid, as mentioned above with respect to  FIGS. 2A-2C , which may be agitated by stirring and/or sonication. The extra material  52   a  positioned on resist pattern  50  is lifted-off into solution, and the material  52   a  that remains on the substrate is a negative-tone image of the original resist pattern  50 , as shown in  FIG. 4C . 
         [0038]    Referring to  FIGS. 3B and 4A , one method for anisotropic deposition is electron-beam evaporation in a high vacuum system. This process deposits 3 nm to 20 nm Cr (chromium) metal at a rate of 0.1 to 0.5 nm/second. 
         [0039]    Referring to  FIG. 4B , one process for isotropic etching comprises RIE etching at 15 to 150 volts DC bias with a gas feed mixture of chlorine (60-80%) and oxygen (20-40%), at a pressure of 50 to 150 mT, for a duration of 30-120 seconds. 
         [0040]    Referring to  FIG. 4C , one process for lift-off processing comprises immersing the substrate in a solvent that is known to rapidly dissolve the resist material. For PMMA, the one such solvent is dichloromethane. One solvent for ZEP520A is dimethylacetamide. The lift-off process may be performed with the substrate in an inverted orientation, such that solid particles released from the resist surface during lift-off are not deposited on the substrate. In one implementation, this process is performed in an ultrasonic bath to facilitate the lift-off process. 
         [0041]    Referring to  FIGS. 5A-5C , a further embodiment is shown. First, a resist pattern  50  is created on the substrate  112 . The resist pattern  50  may have a sloped sidewall  56 . Material  52  is deposited on top of the resist pattern  50 . Furthermore, a thickness of material  52   b  on sidewalls  56  need not be thinner than a thickness of material  52   a  positioned directly on substrate  112 . Next, a coating  65  is applied, in an example, by spin-coating, which provides a top surface  66  that is substantially planar. The planarizing coating  65  has thickness defined between surface  66  and a top portion of resist features  50  that is less than a thickness defined between the resist features  50 . An etch-back step removes portions the coating  65  from on top of the resist features  50 , while a substantial thickness of the coating  65  remains in between the resist features  50 . An etching step, in an example either wet or dry/plasma etching, may then be used to remove the deposited material  52  from on top of the resist features  50 , which exposes the underlying resist pattern  50 . The resist  50  may then be removed, i.e., via dissolution in solvent, or chemical etching. The material  67  that remains on the substrate is a negative-tone image of the original resist pattern  50 . One advantage of this approach is that the extraneous material is removed with a process that is much less prone to defect creation than a solvent lift-off process. 
         [0042]    The embodiments of the present invention described above are exemplary. Many changes and modifications may be made to the disclosure recited above, while remaining within the scope of the invention. Therefore, the scope of the invention should not be limited by the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalent.