Abstract:
A method of cleaning an imprint template is disclosed. The method includes exposing the imprint template to a reductive fluid.

Description:
FIELD 
       [0001]    The present invention relates to a cleaning method, e.g. a method of cleaning a template for use in imprint lithography. 
       BACKGROUND 
       [0002]    In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are however rather costly. 
         [0003]    A potentially less costly road to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a “stamp” to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the wavelength of a radiation source or the numerical aperture of a projection system as in photolithography, but mainly just by the pattern density on the stamp (also referred to as a template). For example, the template may have nanometer and/or micrometer features. There are three main approaches to imprint lithography, examples of which are schematically depicted in  FIGS. 1   a  to  1   c.    
         [0004]      FIG. 1   a  shows an example of a type of imprint lithography that is often referred to as micro-contact printing. Micro-contact printing involves transferring a layer of molecules  11  (typically an ink such as a thiol) from a template  10  (for example a polydimethylsiloxane template) onto a resist layer  13  which is supported by a substrate  12  and planarization and transfer layer  12 ′. The template  10  has a pattern of features on its surface, the molecular layer being disposed upon the features. When the template is pressed against the resist layer, the layer of molecules  11  are transferred onto the resist. After removal of the template, the resist is etched such that the areas of the resist not covered by the transferred molecular layer are etched down to the substrate. For more information on micro-contact printing, see for example, U.S. Pat. No. 6,180,239. 
         [0005]      FIG. 1   b  shows an example of so-called hot imprint lithography (or hot embossing). In a typical hot imprint process, a template  14  is imprinted into a thermosetting or a thermoplastic polymer resin  15  (or more generally an imprintable medium), which is on the surface of a substrate  12 . The resin may for instance be spin coated and baked onto the substrate surface or, as in the example illustrated, onto a planarization and transfer layer  12 ′. When a thermosetting polymer resin is used, the resin is heated to a temperature such that, upon contact with the template, the resin is sufficiently flowable to flow into the pattern features defined on the template. The temperature of the resin is then increased to thermally cure (crosslink) the resin so that it solidifies and irreversibly adopts the desired pattern. The template may then be removed and the patterned resin cooled. In hot imprint lithography employing a layer of thermoplastic polymer resin, the thermoplastic resin is heated so that it is in a freely flowable state immediately prior to imprinting with the template. It may be necessary to heat a thermoplastic resin to a temperature considerably above the glass transition temperature of the resin. The template is pressed into the flowable resin and then cooled to below its glass transition temperature with the template in place to harden the pattern. Thereafter, the template is removed. The pattern will consist of the features in relief from a residual layer of the resin which residual layer may then be removed by an appropriate etch process to leave only the pattern features. Examples of thermoplastic polymer resins used in hot imprint lithography processes include poly (methyl methacrylate), polystyrene, poly (benzyl methacrylate) or poly (cyclohexyl methacrylate). For more information on hot imprint, see for example, U.S. Pat. Nos. 4,731,155 and 5,772,905. 
         [0006]      FIG. 1   c  shows an example of ultraviolet (UV) imprint lithography, which involves the use of a transparent template and a UV-curable liquid as resist and imprintable medium (the term “UV” is used here for convenience but should be interpreted as including any suitable actinic radiation for curing the resist). A UV curable liquid is often less viscous than the thermosetting and thermoplastic resins used in hot imprint lithography and consequently may move much faster to fill template pattern features. A quartz template  16  is applied to a UV-curable resin  17  in a similar manner to the process of  FIG. 1   b . However, instead of using heat or temperature cycling as in hot imprint, the pattern is frozen by curing the resin with UV radiation that is applied through the quartz template onto the resin. After removal of the template, the resist is etched such that the areas of the resist not in relief are etched down to the substrate. A particular manner of patterning a substrate through UV imprint lithography is so-called step and flash imprint lithography (SFIL), which may be used to pattern a substrate in small steps in a similar manner to optical steppers conventionally used in IC manufacture. For more information on UV imprint, see for example, United States patent application publication US 2004-0124566, U.S. Pat. No. 6,334,960, PCT patent application publication WO 02/067055, and the article by J. Haisma entitled “Mold-assisted nanolithography: A process for reliable pattern replication”, J. Vac. Sci. Technol. B14(6), November/December 1996. 
         [0007]    Combinations of the above imprint techniques are also possible. See, for example, United States patent application publication US 2005-0274693, which mentions a combination of heating and UV curing a resist. 
       SUMMARY 
       [0008]    As described above, an imprint template may be provided with a layer of molecules that are brought into contact with, for example, a layer of resist. Alternatively or additionally, the imprint template may be imprinted into an imprintable medium, for example a resin. When the molecules have been applied to the resist, or the imprint template has been imprinted into the imprintable medium, the imprint template is released from the resist and/or imprintable medium. It is possible that, during the release, molecules, resist or other material (for example imprintable medium) remains on the imprint template. The material which remains on the imprint template could be a thin layer, or could be particles of material or flakes of material or the like. If the material which remains on the imprint template after release is not removed, it may introduce an error into any subsequent patterns imprinted using the imprint template. This is because the material which remains on the imprint template may itself pattern, for example, the imprintable medium which it is brought into contact with during subsequent imprints. The introduced error could be so significant as to render the subsequent imprinted patterns defective, and even useless. A solution to this problem would be to replace an imprint template every time it becomes too contaminated to be used to imprint further patterns. However, this solution is undesirable due to the high costs associated with the fabrication of replacement imprint templates. 
         [0009]    It is therefore desirable, for example, to provide a method of cleaning an imprint template, and an imprint template cleaning apparatus, which obviates or mitigates at least one of the disadvantages of the prior art, whether identified herein or elsewhere. 
         [0010]    According to an aspect of the present invention, there is provided a method of cleaning an imprint template, comprising exposing the imprint template to a reductive fluid. 
         [0011]    According to an aspect of the present invention, there is provided an imprint template cleaning apparatus, comprising a device which, in use, is arranged to expose an imprint template to a reductive fluid. 
         [0012]    According to an aspect of the present invention, there is provided a method of cleaning a patterned surface, the patterned surface comprising one of glass, quartz or fused silica, the method comprising exposing the patterned surface to a reductive fluid. 
         [0013]    According to an aspect of the present invention, there is provided a patterned surface cleaning apparatus comprising a device which, in use, is arranged to expose a patterned surface to a reductive fluid, wherein the patterned surface comprises one of glass, quartz or fused silica. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIGS. 1   a - c  schematically show examples of, respectively, micro-contact printing, hot imprint, and UV imprint; 
           [0015]      FIG. 2  schematically shows an imprint template having contamination attached to it; 
           [0016]      FIG. 3  depicts an imprint lithography method according to an embodiment of the present invention; 
           [0017]      FIG. 4  depicts an imprint lithography method according to an embodiment of the present invention; 
           [0018]      FIGS. 5   a  and  5   b  depict imprint lithography methods according to an embodiment of the present invention; 
           [0019]      FIGS. 6   a  and  6   b  schematically show the effects of one or more embodiments of the present invention; and 
           [0020]      FIG. 7  schematically shows an imprint template after application of a method according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    According to an embodiment of the present invention, the imprint template is cleaned in a reductive environment. In other words, the imprint template is exposed to a reductive fluid, for example a reductive liquid or a reductive gas. In an example, resist or other material (for example, material containing organic matter) is removed from an imprint template using hydrogen radicals, its isotope deuterium, and/or combinations thereof. 
         [0022]    Hydrogen radicals are produced in the dissociation of hydrogen molecules into atomic hydrogen radicals. This may be achieved in a number of ways, for example by passing hydrogen gas over a hot filament, or by introducing a microwave discharge or radio frequency (RF) discharge in the hydrogen gas. The reaction which takes place in the disassociation of hydrogen molecules into atomic hydrogen radicals is as follows: H 2 (g)→2H. 
         [0023]    The atomic hydrogen atoms react with, for example, resist, under the formation of methane (CH 4 ) from carbon, water (H 2 O) from (organically bound) oxygen, and silane (SiH 4 ) from (organically bound) silicon. In an embodiment, all the reaction products are gaseous and will therefore not remain attached to, deposited on, etc. the imprint template. This results in cleaning of the imprint template. The imprint template may be formed from one or more materials which are substantially inert to the hydrogen radicals, or whichever reductive fluid is used to clean the imprint template. Suitable examples of materials which may be used to form the imprint template are glass, fused silica, and quartz. 
         [0024]    An advantage of a method according to an embodiment of the present invention, and in particular the use of hydrogen radicals, is the speed of the cleaning process. A cleaning (or in other words etching) rate, though dependent on the exact conditions of the imprint template and contamination on the imprint template, is typically in the range of greater than 1 nm-2 nm per second. Applying this cleaning rate to a resist defect on the template with, for example, a depth of 50 nm results in a clean time of less than a minute. Furthermore, cleaning using a reductive fluid is gentler, and is less likely to damage the imprint template than, for example, a plasma. 
         [0025]    Implementation of a method according to an embodiment of the present invention will now be described with reference to  FIGS. 2 to 6 . 
         [0026]      FIG. 2  depicts an imprint template  20 . The imprint template  20  is formed from fused silica, but could be formed from any material which is relatively inert to a reductive fluid used to clean the imprint template. For example, the imprint template  20  may be formed from glass, fused silica, or quartz. The imprint template  20  has just been released from a layer of imprintable medium. It can be seen that after the release process, the imprint template  20  has some imprint medium  21  attached to it. As described above, it is desirable to remove the imprint medium  21  which has become stuck to the imprint template  20  in order to reduce or eliminate the possibility of introducing defects into later imprinted patterns. 
         [0027]      FIG. 3  depicts an apparatus which may be used to clean the imprint template  20  of  FIG. 2 .  FIG. 3  depicts a hot-filament  30  disposed adjacent to the surface of the imprint template  20  to be cleaned of imprint medium  21 . In an embodiment, the hot-filament is made from tungsten, but can also be made from other materials such as Mo and Ni. A tube  31  is disposed adjacent to the hot-filament  30 , and on the opposite side of the hot-filament  30  to the imprint template  20 . The tube  31  is used to transport hydrogen  32  toward and over (and/or around, etc.) the hot-filament  30 . It will be appreciated that the hydrogen may be transported using any suitable conduit, and not necessarily a tube. 
         [0028]    In use, hydrogen  32  is passed through the tube  31  at a flow rate between 20 sccm and 300 sccm. The hydrogen  32  is passed over the hot-filament  30 . The hot-filament  30  is maintained at a temperature of between 1750° C. and 2250° C. When the hydrogen  32  is passed over the hot-filament  30 , it disassociates into atomic hydrogen radicals  33 . The higher the temperature of the hot-filament  30 , the higher the yield of hydrogen radicals  33  and thus the greater the etch (or cleaning) rate. 
         [0029]    The cleaning of the imprint template  20  may be undertaken in any suitable environment, and maybe undertaken in an enclosed chamber (not shown). An optimum pressure within the chamber depends on the configuration of apparatus within the chamber. If there are no, for example metallic obstructions between the imprint template  20  and the hydrogen  32  emitted from the tube  31 , a high pressure may be used, in order to limit wall recombination of hydrogen radicals  33 . However, the pressure should not be too high as this may promote recombination of hydrogen radicals. A typical pressure along a path between the source of the hydrogen radicals  33  (e.g. around the hot-filament  30 ) and the imprint template  20  may be in the range of 0.1 to 20 kNm −2 . 
         [0030]    As described above, all of the reaction products formed when using hydrogen radicals to clean the imprint template  20  may be, for example, gaseous, and evaporate away from the imprint template  20  leaving it with a clean surface. 
         [0031]    It is possible that the hot-filament  30  may heat the imprint template  20 . Heating of the imprint template  20  is undesirable, since the heat may distort the imprint template  20  and any pattern with which it is provided. Therefore, it is desirable to prevent heat from the hot-filament  30  reaching the imprint template  20 .  FIG. 4  depicts an apparatus which can be used to clean the imprint template  20  of imprint medium  21 , while reducing the heating of the imprint template  20 .  FIG. 4  depicts a chamber  40 . Located in the chamber is a hot-filament  41 . The chamber  40  is provided with an outlet port  42 . The outlet port  42  leads into a tube  43  which is arranged to extend toward and open adjacent to a surface of the imprint template  20  to be cleaned of contamination  21 . The imprint template  20  is shielded from the heat generated by the hot-filament  41  by a heat shield  44 . The tube  43  extends through the heat shield  44 . 
         [0032]    The heat shield  44  may be formed from any suitable material, and in particular any material which is known to absorb or reflect heat. For example, the heat shield  44  may be formed from a ceramic material or metal, and/or or may be formed with a reflective surface. The heat shield  44  may also be cooled using a fluid flowing alongside or in the heat shield  44 , or in a conduit in contact with the heat shield  44 . 
         [0033]    In use, hydrogen  45  is passed over the hot-filament  41 , which causes the hydrogen  45  to disassociate into atomic hydrogen radicals  46 . The atomic hydrogen radicals travel along the tube  43  and onto the surface of the imprint template  20  to be cleaned. The imprint template  20  is then cleaned as described above. 
         [0034]    The tube  42  which transports the hydrogen radicals  46  to the imprint template  20  desirably has a low surface recombination coefficient for hydrogen radicals (or whichever reductive fluid is used to clean the imprint template  20 ). Quartz, borosilicate glass (for example, Pyrex™), fused silica and glass are suitable materials for transporting the hydrogen radicals  46 , since the surface recombination coefficient of these materials for hydrogen radicals is low (for example 4×10 −3  to 7×10 −4 ) when compared to, for example, platinum (which has a recombination coefficient for hydrogen radicals of 1). Similarly, if a chamber or any other device is arranged to, in use, expose the imprint template to hydrogen radicals (or any other reductive fluid), the device should have a low surface recombination coefficient with respect to the reductive fluid used. For example, the device may be formed from quartz, borosilicate glass (for example, Pyrex™), fused silica or glass. 
         [0035]    It is desirable to clean the imprint template as quickly as possible, and it is therefore desirable that the speed at which the reductive fluid (e.g. hydrogen radicals) reacts with or etches the imprint medium is also as high as possible. The reaction of hydrogen radicals with the imprint medium (for example, resist) is generally an exothermic reaction. That is, heat is liberated during the reaction. Thus, higher reaction speed results in increased heating of the imprint template. The heating of the template can be a limiting factor in the cleaning process when the imprint template has reached the maximum allowable temperature (which corresponds to the pattern on the imprint template becoming too distorted for immediate use, and may be around, for example, 50° C.).  FIGS. 5   a  and  5   b  depict apparatuses which may be used to reduce or prevent excessive heating of the imprint template  20 , and/or an imprint template holder  50  which is used to hold the imprint template  20 . To reduce or prevent excessive heating of the imprint template  20 , the imprint template  20 , and/or imprint template holder  50  can be actively conditioned, e.g. cooled. Active cooling can be achieved by passing a fluid  60  via a conduit  61  which is in contact with or in close proximity to the imprint template  20  and/or the imprint template holder  50 . Heat from the imprint template  20  and/or the imprint template holder  50  is dissipated into the conduit  61  and the fluid  60  which is passing through it. This dissipated heat heats up the fluid  60 . The fluid  60  flows through the conduit  61 , and thus takes heat which it contains away from the imprint template  20  and/or the imprint template holder  50 . The fluid  60  maybe, for example, water. 
         [0036]    It is desirable that reaction products which are formed in the cleaning of the imprint template are removed as quickly as possible from the vicinity of the imprint template. This is because it is desirable to reduce or eliminate the probability of these reaction products being deposited elsewhere on the imprint template, or elsewhere in and around the apparatus used to clean the imprint template. The reaction products may therefore be removed using an exhaust, or other pumping or extraction apparatus. The reactions products could then be vented to atmosphere, or to a scrubbing apparatus, for example. 
         [0037]    It is known that hydrogen radicals formed near the hot-filament are not stable and may eventually recombine to form molecular hydrogen. It is therefore advantageous to reduce the time between radical formation and reaction with the imprint medium on the imprint template, since this will lead to a higher hydrogen radical concentration on the surface of the imprint template. Reduction in the time between radical formation and contact with the imprint template can be achieved by using a carrier gas. Desirably, the carrier gas is an inert gas, and this inert gas may be used to transport the hydrogen radicals to the imprint template after they have been formed at or in the vicinity of the hot-filament. A suitable carrier gas is, for example, Ar, He, Ne, Xe, Kr, and/or Rn. 
         [0038]    In the above embodiment, a heat shield and active cooling have been described as being suitable to reduce the heating of the imprint template. These solutions can be replaced or supplemented by using a pulsed cleaning scheme. For example, rather than allowing the hot-filament to continuously emit heat, the hot-filament may be repeatedly turned on and off to allow the imprint template to cool down when the hot-filament is not emitting heat. For example, the hot-filament may be turned on for 10 seconds, and then turned off for 10 seconds in a repetitious pulsed manner. This process may be repeated until the imprint template is clean of imprint medium. 
         [0039]      FIGS. 6   a  and  6   b  illustrate examples of the possible effectiveness of the cleaning methods described above.  FIGS. 6   a  and  6   b  depict dark field microscope images of (part of) an imprint template with four fields of pattern features  70  increasing in size from left to right.  FIG. 6   a  shows the pattern features after the imprint template has been used to imprint a number of patterns. During the imprints, material (e.g. resist) becomes stuck to the imprint template, and it can be seen that some of the patterns feature  70  have at least partially disappeared in the dark field microscope image. In other words, the material which has stuck in-between and onto the features  70  have resulted in a loss of optical contrast in the microscope image. The imprint template is then cleaned using a reductive fluid (e.g. hydrogen radicals) as described above.  FIG. 6   b  shows that the pattern features  70  are now clearly visible, and are defect free. This shows that the cleaning step was successful in removing resist from the pattern features  70 . 
         [0040]      FIG. 7  schematically depicts an imprint template  20  which has been cleaned using the methods and apparatuses described above. It can be seen that, after the cleaning process, there is substantially no imprint medium remaining on the imprint template. 
         [0041]    Instead of using a hydrogen gas to generate the hydrogen radicals, a hydrogen halide gas may be used. An advantage of using a halide gas is that halogens may be used to remove materials that cannot be removed (either at all, or as easily) using hydrogen radicals alone. Alternatively or additionally, a metal material serving as a catalyst for hydrogen radical formulation may be provided in the vicinity of the hot-filament, or may form at least a part of the hot-filament. The metal catalyst may be selected from the group consisting of Ti, Pt, Ni, V, Mg, Mn, Ru, W, and Ta (and alloys and combinations thereof). 
         [0042]    The cleaning apparatus as described above may be housed in a cleaning chamber. Alternatively, the cleaning apparatuses described above may be part of another system, for example, an imprint lithography system or the like. That is, the imprint template may be cleaned in-situ or ex-situ of the imprint lithography system. 
         [0043]    The imprint template may be cleaned at specific times, for example after a batch of patterns has been imprinted on a substrate. Alternatively or additionally, the imprint template could be periodically tested to determine whether it has too much contamination (e.g. imprint medium) attached to it, and the imprint template could then be cleaned if it is too contaminated. 
         [0044]    In the above embodiments, the cleaning of an imprint template has been described. However, in an embodiment, the methods described above may be used to clean other objects and/or surfaces of those objects. For example, in an embodiment, the embodiments described above may be used to clean the patterned surfaces of objects having or being formed from glass, fused silica, or quartz. The use of reductive fluid to clean such patterned surfaces is a quick cleaning solution, as described above. Examples of patterned surfaces may include the burled surface of a substrate table or carrier, e.g. a wafer or reticle table/carrier, or gratings, e.g. diffraction gratings for use in lithography, etc. 
         [0045]    As mentioned above, the reductive fluid used to clean the imprint template does not need to be or comprise hydrogen or deuterium radicals. Other reductive fluids may be used. 
         [0046]    It will be appreciated that the above embodiments have been described by way of example only. It can be appreciated by one of ordinary skill in the art that various modifications may be made to these and other embodiments without departing from the invention as defined by the claims that follow.