Abstract:
A method of fabricating a device including imprinting a mold having a protrusion against a substrate having a resist layer such that the protrusion engages the resist layer. The mold further has a mask member positioned generally adjacent the resist layer. Radiation energy is then transmitted through the mold and into the resist layer; however, the mask member substantially prevents transmission of the radiation energy therethrough, thereby defining an unexposed area in the resist layer. Once the mold is removed from the substrate, which consequently forms a first feature from nanoimprinting, the unexposed area of resist layer is removed through dissolving in a developer solution.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/447,116, filed on Feb. 13, 2003. The disclosure of the above application is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENTAL SUPPORT  
       [0002]     This invention was made with Government support under Grant No. N00014-02-1-0899 awarded by the Office of Naval Research and Grant No. N66001-02-C-8039 awarded by Defense Advanced Research Projects Agency (DARPA). The government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention generally relates to the fabrication of nano and microstructures and, more particularly, relates to nano-scale patterning for nano and microstructures as well as potential applications.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0004]     Nano-patterning is an essential part of nanotechnology research and is used to fabricate nanostructures to harness their unique properties. However, in order for nano-device and nanostructure fabrication to have significant practical value, a low-cost and high-throughput nano-patterning technique is needed. Among many new emerging lithography techniques that are aimed at addressing this issue, nanoimprinting techniques are regarded as one of the most promising.  
         [0005]     Nanoimprint lithography (NIL) is a nano-scale lithography technique where a surface relief pattern on a hard mold is physically imprinted into a thermal plastic polymer film at elevated temperature and pressure. Nanoimprint lithography has attracted more and more attention in both research and commercial applications due to its potential application in nano-scale patterning. It is often desirable because of its sub-10 nm resolution and simple equipment setup requirements. Nanoimprint lithography is further a relatively simple process that has high throughput, thus enabling low-cost, large-scale patterning of nano-structures.  
         [0006]     Although nanoimprint lithography has proved to be very successful in nano-patterning, especially in replicating nano-scale features with uniform sizes, it still suffers from several limitations as a flexible lithographic technique. A preferred lithographic technique should be capable of producing both large and small features in various combinations and distributions, which is a typical requirement in micro- and nano-fabrication processes. For example, in the case of imprint lithography (such as nanoimprint lithography), mold features on the mold are physically pressed into a polymer. Larger features on the mold must displace more polymer material over larger distances. Thus, patterns with large features are much more difficult to imprint than smaller features (also known as nano-patterns). Furthermore, defects or pattern failures in the form of incomplete pattern transfer can occur due to the high viscosity of the polymer melt and the mold pattern complexity.  
         [0007]     On the other hand and separate from nanoimprint lithography, photolithography is a well-developed process and has been pushed towards its limit to maintain its role in future microelectronic fabrication. In most cases, the cost of these next generation photolithography systems is prohibitive, except for large-scale production runs.  
         [0008]     The present invention combines the processing steps of nanoimprint lithography and photolithography to provide a new technique that provides many new advantages. That is, the present invention enables patterning of both large-scale and sub-micron size structures in a single step. With many advantages, the present invention may be used in the fabrication of a wide range of nano-scale electronic, photonic, and biological devices where patterns of various sizes and densities are needed.  
         [0009]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0011]     FIGS.  1 ( a )-( c ) is a series of cross sectional schematic views illustrating non-uniform pattern height according to the prior art;  
         [0012]     FIGS.  1 ( d )-( f ) is a series of cross sectional schematic views illustrating non-uniform residual layer thickness according to the prior art;  
         [0013]     FIGS.  1 ( g )-( i ) is a series of cross sectional schematic views illustrating incomplete nano-pattern replication according to the prior art;  
         [0014]     FIGS.  2 ( a )-( d ) is a series of cross sectional schematic views illustrating the method steps of a first embodiment of the present invention;  
         [0015]      FIG. 3 ( a ) is a perspective view illustrating an SEM micrograph of a resist pattern formed according to the prior art;  
         [0016]      FIG. 3 ( b ) is an enlarged perspective view illustrating an SEM micrograph of relief beams in the resist pattern formed according to the prior art;  
         [0017]      FIG. 3 ( c ) is an enlarged perspective view illustrating an SEM micrograph of 200 μm squares in the resist pattern formed according to the prior art;  
         [0018]      FIG. 4 ( a ) is a perspective view illustrating an SEM micrograph of a resist pattern formed according to the present invention;  
         [0019]      FIG. 4 ( b ) is an enlarged perspective view illustrating an SEM micrograph of relief beams in the resist pattern formed according to the present invention;  
         [0020]      FIG. 4 ( c ) is a further enlarged perspective view illustrating an SEM micrograph of the relief beams in the resist pattern formed according to the present invention;  
         [0021]     FIGS.  5 ( a )-( d ) is a series of cross sectional schematic views illustrating the method steps of a second embodiment of the present invention;  
         [0022]      FIG. 6  is an SEM micrograph illustrating a portion of the hybrid mold formed according to the second embodiment;  
         [0023]      FIG. 7 ( a ) is an SEM micrograph illustrating the residual layer of resist layer remaining following fabrication according to the prior art;  
         [0024]      FIG. 7 ( b ) is an SEM micrograph illustrating the lack of a residual layer of resist layer remaining following fabrication according to the present embodiment;  
         [0025]      FIG. 8 ( a ) is an SEM micrograph of a portion of the hybrid mold used according to the present embodiment;  
         [0026]      FIG. 8 ( b ) is an SEM micrograph of a resultant pattern formed according to the prior art;  
         [0027]      FIG. 8 ( c ) is an SEM micrograph of a resultant pattern formed according to the present embodiment;  
         [0028]      FIG. 9 ( a ) is a cross sectional schematic view illustrating the hybrid mold according to the second embodiment;  
         [0029]      FIG. 9 ( b ) is a cross sectional schematic view illustrating a mold according to a prior art method;  
         [0030]      FIG. 9 ( c ) is a cross sectional schematic view illustrating a mold according to a prior art method;  
         [0031]      FIG. 9 ( d ) is a cross sectional schematic view illustrating a mold according to a prior art method; and  
         [0032]      FIG. 9 ( e ) is a cross sectional schematic view illustrating a mold according to a prior art method. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0034]     In order to appreciate the advantages and concepts of the present invention, it is believed that a brief explanation of nanoimprinting is beneficial. Therefore, with reference to  FIG. 1 , several disadvantages of a conventional nanoimprinting method are discussed. A primary disadvantage of convention nanoimprinting is its inability to pattern a wide variety of feature sizes simultaneously.  
         [0035]     Briefly, conventional nanoimprinting employs a mold  100  having of a combination of nano-scale mold features  102  adjacent to large-scale mold features  104 . If a low pressure is used for imprinting (see FIGS.  1 ( a )-( c )), there will be no bending in either mold  100  or a substrate  106 . Mold features  102 ,  104  penetrate into a resist layer or polymer film  108  on substrate  106  in a parallel fashion as shown. However, because of viscous polymer flow, large-scale mold feature  104  cannot fully penetrate the polymer film  108  in a period that is practical for nanoimprint lithography. In other words, large-scale mold feature  104  can not displace sufficient portions of polymer film  108  to achieve the desired penetration within a reasonable time period. Not only does this lead to incomplete polymer molding of large-scale patterns  110 , but nano-scale mold features  102  next to large-scale mold features  104  are also strongly affected, thereby yielding a shallower nano-scale pattern  112  than desired. This creates a reduced aspect ratio for the nano-patterns  112 , which significantly increases the difficulty in the subsequent fabrication processing steps—such as metal lift-off or pattern transfer into substrates.  
         [0036]     However, if a high pressure is used in nanoimprinting, either mold  100  or substrate  106  may bend such that they have conformal contact at polymer resist layer  108 , as shown in FIGS.  1 ( d )-( f ). In this case, the same feature height can be achieved for both large-scale and nano-scale patterns. However, due to mold bending, the nano-scale relief patterns  112  penetrate deeper into resist layer  108  than large-scale patterns  110 . This leads to a thinner residue layer, T′, in the nano-scale pattern region than that in the large-scale pattern region, T. This non-uniform thickness T, T′ across the sample makes it difficult to determine how much etching is necessary to remove the residual layer. Some patterns might be lost if not enough of the residual layer is removed, while over removal can reduce the aspect ratio of the nano-scale resist feature.  
         [0037]     Due to the mechanical molding nature of nanoimprint lithography, the displaced polymer melt must be accommodated by recessed regions on the mold. Therefore, the location and size of recessed areas on the mold can affect nanoimprint lithography results. Complex mask patterns will give rise to a random distribution of the recessed areas on the mold and some patterns cannot be fully replicated in the imprinting process. FIGS.  1 ( g )-( i ) illustrate an example where nano-patterns are next to the large structures on the mold. The displaced polymer (i.e. resist layer) cannot reach the center of the nano-pattern region, leaving the center of the nano-patterns incompletely formed.  
         [0038]     The aforementioned pattern failures are related to the mold pattern itself and are inherent in nanoimprint lithography process. In real applications, micro- and nano-fabricated devices usually require a mixture of arbitrary patterns of various sizes. Therefore, the pattern-related defects and failure can limit the versatility of nanoimprint lithography technique in general microfabrication.  
         [0039]     The previous approach to address the issue of replicating patterns of various sizes was to use a mix-and-match method that is carried out in two separate steps. In a typical mix-and-match approach, the nano-scale pattern is defined by nanoimprint lithography first, and the large-scale pattern is added afterwards by conventional photolithography. In this approach, not only is alignment needed between the two steps, but also the accuracy is limited in the photolithography step. This extra photolithography step adds complexity and cost to the overall process.  
         [0040]     Therefore, according to a first embodiment of the present invention, a novel method of combined nanoimprint and photolithography is provided that integrates the benefits of nanoimprinting with the benefits of photolithography to achieve new micro and nano fabrication capability not otherwise possible using the techniques individually.  
         [0041]     As best seen in FIGS.  2 ( a )-( d ), the method according to the present invention is illustrated. The present invention employs a hybrid mold configuration, generally indicated at  12 , which acts as both a nanoimprint lithography mold and a photolithography mask—an example of hybrid mold  12  is illustrated at  FIG. 2 ( a ). Hybrid mold  12  is preferably made of an ultraviolet (UV) transparent material, such as fused silica. Protrusions  14  are formed on hybrid mold  12  for physically imprinting nano-scale features  16  within a resist layer  20  on a substrate  24 . Hybrid mold  12  further includes mask members  18 , in the form of metal pads, embedded therein to serve as ultraviolet radiation masks for photolithography of large patterns  22  in resist layer  20 . In other words, metal pads  18  prevent the free transmission of ultraviolet radiation through hybrid mold  12  and into resist layer  20 . It should be understood that mask members  18  are not limited to metal members, but can be any material that will inhibit the free transmission of radiation energy therethrough. Resist layer  20  may be either a negative tone photoresist or a UV-curable monomer.  
         [0042]     The processing steps of the present invention are very simple, convenient, and inexpensive. Specifically, and with continued reference to FIGS.  2 ( a )-( d ), hybrid mold  12  first imprints protrusions  14  into resist layer  20  via applied pressure to form nano-scale features  16 . This is often done when resist layer  20  is at about its glass transition temperature. However, depending on the resist layer material chosen, the imprinting could be done at any temperature, such as room temperature. While hybrid mold  12  remains within or engaged with resist layer  20 , hybrid mold  12 , resist layer  20 , and a substrate  24  are exposed to ultraviolet (UV) radiation (see  FIG. 2 ( b )). As described above, hybrid mold  12  is generally transmissive to ultraviolet (UV) radiation except through those portions that are obstructed by metal pads  18 . Therefore, ultraviolet radiation passes through those transmissive portions not obstructed by metal pads  18  and into resist layer  20  thereby exposing portions  25  of resist layer  20 . Hybrid mold  12  is then removed from resist layer  20  (see  FIG. 2 ( c )) and resist layer  20  is developed within a developing solution. According to the present embodiment, the developing of resist layer  20  causes those portions  26 , which are unexposed to the ultraviolet radiation, to be removed through photolithography while those portions  25 , which were exposed to the ultraviolet radiation, to remain intact (see  FIG. 2 ( d )). Therefore, according to the present invention, both large-scale patterns  22  and nano-scale patterns  16  are created simultaneously without suffering from the disadvantages of the prior art.  
         [0043]     The effectiveness of the present invention is illustrated through a comparison of a resultant product formed according to the present invention and a resultant product formed according to conventional nanoimprint lithography, as illustrated in FIGS.  3 ( a )-( c ) and  4 ( a )-( c ). According to the present comparison, a test pattern consisting of two major components—200 μm squares and closely spaced 350 nm wide relief beams—was used.  
         [0044]     In fabricating hybrid mold  12 , a thin layer of polymethyl methacrylate (PMMA) with an average molecular weight of 15,000 was spun on a fused silica substrate. A grating mold with 700 nm pitch and 50% duty cycle was then used to pattern the PMMA resist on the substrate by conventional nanoimprint lithography at 175° C. and 50 kg/cm 2 . After a Ni evaporation and lift-off, the fused silica is etched by reactive ion etching (RIE) to form nano-scale protrusions  14  on hybrid mold  12 . Next, 200-μm size pixel patterns are added to hybrid mold  12  by photolithography. After photolithography, the exposed fused silica region is etched by RIE using the patterned photoresist as a mask, which creates 200 μm size shallow cavities. These shallow cavity patterns are back-filled with metals such as Cr or Ni that have high absorption coefficient for UV light by e-beam evaporation and lift-off. SU-8, a common negative tone photoresist, was used in this example. In this case, hybrid mold  12  is thus formed and may be used to form a plurality of resultant products. It should be appreciated that the above description only illustrates one of many ways to fabricate hybrid mold  12 .  
         [0045]     In both cases, the imprinting of the mold in the resist layer is done at 80° C. under a pressure of 50 kg/cm2. In the method according to the present invention, hybrid mold  12  is exposed to a 365 nm UV light while disposed within resist layer  20 . Hybrid mold  12  and resist layer  20  are then separated after baking at 80° C. on a hotplate for 1 minute. Substrate  24  and resist layer  20  are developed in SU-8 developer solvent for 1 minute to remove unexposed resist portions  26 .  
         [0046]     Referring now to FIGS.  3 ( a )-( c ), SEM images are provided that illustrate the SU-8 patterns obtained by conventional nanoimprint lithography. It can be clearly seen that each 200 μm square (FIGS.  3 ( a ) and  3 ( c )) has a large void defect  200  in the middle of the resist pattern, which is due to insufficient SU-8 flow during nanoimprint lithography. Additionally, because the SU-8 melt is not easily displaced, the 350 nm beam protrusion features of the mold cannot completely penetrate into the SU-8 resist film. This leads to a grating pattern  202  that is much shallower than that on the mold.  
         [0047]     In contrast, the problems of conventional nanoimprint lithography illustrated in FIGS.  3 ( a )-( c ) are solved by the present invention, which is illustrated in FIGS.  4 ( a )-( c ).  FIG. 4 ( a ) specifically illustrates the results obtained by the present invention under the same imprinting conditions, followed by UV exposure and resist developing. As can be seen, no defects are observed in any of the 200-μm square patterns. Similarly, the 350 nm beam pattern is well replicated and has the same height as protrusions  14  on hybrid mold  12 .  
         [0048]     According to a second embodiment of the present invention, yet another new method of combined nanoimprint and photolithography is provided wherein a metal layer is placed on an end of the mold protrusion feature, which eliminates the separate residual removal step in nanoimprint lithography.  
         [0049]     According to the present embodiment as illustrated in FIGS.  5 ( a )-( d ), hybrid mold  12  is modified, generally designated as hybrid mold  12 ′, such that it includes a light-blocking metal layer  50 , such as nickel, disposed at an end of nano-scale protrusions  14 . Metal layer  50 , similar to metal layer  18 , acts as an embedded photomask and prevents ultraviolet radiation from penetrating into portions of resist layer  20  (specifically, a residual layer of resist disposed at a lowermost portion of the feature on top of substrate  24 ).  
         [0050]     More particularly, hybrid mold  12 ′ is first imprinted into resist layer  20 , such as a negative tone UV resist (see  FIG. 5 ( a )). The entire assembly is then flood-exposed with ultraviolet (UV) radiation having a wavelength of 365 nm, preferably (see  FIG. 5 ( b )). However, it should be appreciated that radiation energy have other wavelengths may be used. In fact, it has been found that shorter wavelengths provided enhanced resolution. During this exposure, metal layers  18 ,  50  block the ultraviolet (UV) radiation from entering portions of resist layer  20 . After exposure, hybrid mold  12 ′ is separated from resist layer  20  and substrate  24  (see  FIG. 5 ( c )), thereby leaving exposed portions  42  and unexposed portions  44 . Unexposed portions  44  of resist layer  20  are then easily removed using a developer solution. Hence, according to the present embodiment, resist patterns without residual layer can be obtained in a single step, which eliminates the separate O 2  RIE step that is necessary in both conventional nanoimprint lithography and conventional S-FIL.  
         [0051]     The effectiveness of the present invention is illustrated through a comparison of a resultant product formed according to the present invention and a resultant product formed according to conventional nanoimprint lithography.  
         [0052]     To fabricate hybrid mold  12 ′, patterns are first defined in resist layer on a fused silica substrate by using any appropriate lithography technique. Titanium/Nickel double layer is then deposited on top of the resist template, where the titanium enhances the adhesion of the nickel on silica substrate and increases the durability of metal layer for repeated imprinting cycles. After a lift-off process, the nickel pattern is used as etching mask for the RIE of the silica substrate. The nickel film remains on the etched oxide protrusion after the etching process. The fabricated silica mold substrate is then treated with a brief O 2  RIE to oxidize the metal surface, and a surfactant coating process similar to that used in nanoimprint lithography completes the fabrication of hybrid mold  12 ′.  FIG. 6  is an SEM micrograph illustrating an example of hybrid mold  12 ′ wherein metal layer  50  is disposed upon nano-scale protrusions  14 .  
         [0053]     Hybrid mold  12 ′ is then used as similarly described in connection with hybrid mold  12  to form a resultant product for comparison with a product formed through conventional nanoimprint lithography. This comparison can be seen in FIGS.  7 ( a ) and ( b ).  FIG. 7 ( a ) illustrates the resultant product produced using conventional nanoimprint lithography, while  FIG. 7 ( b ) illustrates the resultant product produced according to the second embodiment of the present invention. In both cases, an imprinting pressure of 50 kg/cm 2  and a temperature of 80° C. were used. A residual layer  300  is clearly seen in the SU-8 pattern after conventional nanoimprint lithography imprinting, while no residual layer  302  is left following completion of the method of the present invention.  
         [0054]     This complete residual layer removal of the present invention is further demonstrated in sub-micron structures as shown in FIGS.  8 ( a )-( c ), while illustrate that according to the present invention it is possible to create resultant features that actually have higher aspect-ratios than those defined in hybrid mold  12 ,  12 ′. This is illustrated in the following figures:  FIG. 8 ( a ) illustrates the SEM micrograph of hybrid mold  12 ′ having 700 nm period grating features;  FIG. 8 ( b ) illustrates the 700 nm pitch SU-8 grating obtained by conventional nanoimprint lithography imprinting; and  FIG. 8 ( c ) illustrates the grating obtained through the method of the present invention. As can be seen, the height of the grating  400  produced by the present invention ( FIG. 8 ( c )) can actually be higher than that on hybrid mold  12 ,  12 ′ ( FIG. 8 ( a )). In other words, the present invention can create pattern features with higher aspect ratio than that on the mold. This is in strong contrast to conventional nanoimprint lithography where the required residual removal step by oxygen RIE usually reduces the aspect ratio of the imprinted structure. This feature is advantageous for nano-scale patterning where patterns with higher aspect ratios can greatly ease the subsequent processing steps. When compared with S-FIL, a variation of nanoimprint lithography technique that uses transparent mold and UV curable monomer as resist, the method of the present invention is able to achieve all of the benefits of S-FIL, while also providing additional advantages in residual removal and increased pattern aspect ratio. In addition, the metal layer on the hybrid mold used in this technique serves as improved registration marks as compared with the mere surface relief structures used in S-FIL or NIL. That is, the latter is unable to create much refractive index contrast upon intimate contact with a polymer layer that has a similar refractive index, and therefore makes the registration marks hardly visible.  
         [0055]     From the above, it is clear that the method of the present invention effectively addresses the issues that conventional nanoimprint lithography fails to overcome. A further benefit of the present invention is that the resist layer, being perhaps photoresist or UV-curable material, can have improved chemical formulation to provide higher etching durability as compared with thermal plastic polymers that are commonly used in conventional nanoimprint lithography.  
         [0056]     As should now be appreciated, there are many distinct advantages of the present invention relative to the prior art. First, the present invention enables one-step lithography of arbitrary patterns containing both large-scale and nano-scale structures. Second, because there are only nano-scale mold protrusion features on hybrid mold  12 , the present invention allows low imprinting pressure to be used since only a very small amount of polymer needs to be displaced. Third, by forming the large patterns as a photomask (i.e. making them as metal pads), it reduces the complexity of the relief pattern on the hybrid mold. This simplifies residue layer thickness distribution, which can ease the step for residue removal significantly. Finally, this hybrid mask-mold scheme retains all the advantages of conventional nanoimprint lithography and step-and-flash imprint lithography (S-FIL), such as low-cost, high-throughput, simple process, and capable of alignment.  
         [0057]     The present invention further provides a number of distinct advantages over other lithographic techniques that have been used to enhance the resolution of conventional contact photolithography. These other techniques may involve the use of metal-embedded masks (MEM) (see  FIG. 9 ( d )), light-coupling masks (LCM) (see  FIG. 9 ( e )), or traditional metal-protrusion mask (MPM) (see  FIG. 9 ( c )). These schemes typically employ either optical near-field effect or light coupling technique for resolution enhancement. However, in MPM, MEM and LCM, the resolution is limited by the thickness of the resist layer due to either light diffraction or near field attenuation. With reference to the present invention (see  FIG. 9 ( a )), because protrusion  14  of hybrid mold  12 ′ penetrates into the resist layer to create the desired resist patterns, the metal layer merely needs to block the very thin resist underneath from being exposed by the ultraviolet radiation. Therefore, the resolution of the present invention is greatly enhanced compared with the prior.  
         [0058]     It has been demonstrated that the present invention overcomes the limitations of conventional nanoimprint lithography and contact photolithography by integrating their strengths. With the present invention, no residual layer is left after processing and the patterned structure may have a higher aspect ratio than that on the mold, which simplifies subsequent processing steps and greatly enhances the throughput by eliminating the oxygen RIE step. Compared with contact photolithography techniques, the present invention achieves much higher resolution by reducing the effective resist thickness down to tens of nanometers.  
         [0059]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.