WO2012167076A2

WO2012167076A2 - Nanochannel-guided patterning for polymeric substrates

Info

Publication number: WO2012167076A2
Application number: PCT/US2012/040454
Authority: WO
Inventors: Jong G. OK; Se Hyun AHN; Lingjie Jay Guo
Original assignee: The Regents Of The University Of Michigan
Priority date: 2011-06-01
Filing date: 2012-06-01
Publication date: 2012-12-06
Also published as: WO2012167076A3

Abstract

Novel continuous high speed techniques are provided to fabricate high aspect-ratio microscale structures by using curable liquid materials on a deformable polymeric substrate by a mold-guided pattern formation process with a rigid mold. Methods comprise contacting a patterned surface of a rigid mold with a curable liquid material disposed on a moldable polymeric substrate. The patterned surface has one or more cavities defining at least one microscale feature, so that the curable liquid material flows into the cavity and the major surface is at least partially deformed during the contacting. The liquid is cured to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

Description

NANOCHANNEL-GUIDED PATTERNING FOR POLYMERIC SUBSTRATES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61/492,283, filed on June 1, 2011. The entire disclosure of the above application is incorporated herein by reference.

GOVERNMENT RIGHTS

[0002] This invention was made with government support under CMMI1000425 awarded by the National Science Foundation. The government has certain rights in the invention.

FIELD

[0003] The present disclosure relates to methods of patterning microfeatures and nanofeatures onto deformable substrates, which can be used to form microelectronic devices.

BACKGROUND

[0004] This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0005] Nanoscale grating structures can be utilized in a variety of device applications such as optics, organic optoelectronics, and biosensors. Several techniques can be used to fabricate nanograting structures. Current methods for forming such structures suffer from the inability to form high resolution, high aspect ratio, nanoscale features for use in various microelectronic devices. The teachings of the present disclosure provide rapid and high throughput methods of forming microscale and nanoscale features on a surface of a substrate in a continuous process (e.g., a steady-state process) with high-aspect ratios and high fidelity to a mold pattern, overcoming shortcomings associated with conventional nanoinscribing and nanopatterning processes.

SUMMARY

[0006] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0007] In various aspects, the present disclosure provides methods of forming at least one microscale feature on a substrate. In certain variations, the method may comprise contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate. The patterned surface of the rigid mold defines a cavity defining at least one microscale feature, so that the curable liquid material flows into the cavity. Further, the major surface is at least partially deformed during the contacting. A contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method further comprises curing the curable liquid material after the contacting to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

[0008] In other aspects, the present disclosure provides a method for continuously patterning at least one microscale feature that comprises continuously imprinting a major surface of a moldable polymeric substrate having a curable liquid material disposed thereon. The continuous imprinting is achieved by applying pressure to the major surface by contacting with a stationary rigid mold. The stationary rigid mold has a patterned surface that defines a cavity to form at least one microscale feature, so that the curable liquid material flows into the cavity. The major surface is at least partially deformed during the applying of pressure. An advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method also comprises curing the curable liquid material after the contacting to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

[0009] In yet other aspects, a method of forming at least one nanoscale feature on a substrate. The method comprises applying a curable liquid material comprising ultraviolet radiation-curable epoxy-silsesquioxane (SSQ) to a major surface of a moldable polymeric substrate having a surface energy of greater than or equal to about 10 mN/m and less than or equal to about 50 mN/m. An advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method additionally comprises continuously passing the major surface having the curable liquid material disposed thereon into contact with a heated rigid mold having a patterned surface. The patterned surface defines a cavity to form at least one nanoscale feature. The curable liquid material flows into the cavity during the contact and the major surface is at least partially deformed by the patterned surface. The curable liquid material is cured with ultraviolet radiation after the contact to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

[0010] In yet other aspects, the present disclosure provides a method of forming a waveguide structure. In one variation, such a method comprises contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate. The patterned surface defines a plurality of nanochannels, so that the curable liquid material flows into the plurality of nanochannels. The major surface is at least partially deformed during the contacting. A contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The curable liquid material is thus cured after the contacting to form a cured polymer having at least one nanoscale feature so as to define a waveguide structure over the moldable polymeric substrate.

[0011] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

[0012] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0013] Figure 1 shows an exemplary measurement of a static contact angle between a curable liquid material and an underlying plastically deformed polymeric substrate;

[0014] Figures 2A-2D. Figures 2 A and 2D show schematics of an embodiment of an inventive process according to certain aspects of the present teachings. Figures 2B-2C show patterned surfaces of molds for forming gratings in the curable liquid materials, where Figure 2B has a 700 nm period and Figure 2C has a 200 nm period. Figure 2D shows a schematic of a curing and coating module for nano-channel guided patterning according to certain variations of the present teachings;

[0015] Figures 3A-3B shows scanning electron microscopy (SEM) images of 200 nm period nanogratings formed on a perfluoroalkoxy (PFA) substrate at 80°C with a curable liquid epoxy-silsequioxane (SSQ) coating (Figure 3A) and without liquid SSQ coating (Figure 3B) formed via a conventional dynamic nanoinscribing process. The insets are the counterprofiles of each grating structure, showing that the aspect ratio of resulting nanogratings is significantly improved by the use of SSQ layer; [0016] Figures 4A-4J show comparative schematics, diagrams, and SEM images of nanograting formations by certain variations of an inventive process according to the present disclosure, where a PFA substrate is coated with a curable SSQ liquid resin in accordance with the inventive process in Figures 4A-4E, as compared with a polyethylene terephthlate (PET) substrate similarly coated with an SSQ liquid resin in comparative Figures 4F-4J;

[0017] Figures 5A-5H compare 700 nm period SSQ nanograting formations on a normal PET substrate (Figures 5A-5D) as compared to a fluorosilane-treated F-PET substrate in accordance with certain aspects of the present teachings (Figures 5E-5H);

[0018] Figure 6 shows viscosity of a curable liquid epoxy- silsequioxane (SSQ) coating as a function of shear rate. Viscosity remains constant and shear stress increases linearly along the shear rate sweep, indicating that SSQ behaves as a Newtonian fluid;

[0019] Figure 7 shows viscosity versus temperature for a curable liquid epoxy-silsequioxane (SSQ) resin;

[0020] Figure 8 shows grating depths of curable SSQ-coated structures and mold cavity filling capability at different processing temperatures;

[0021] Figures 9A-9D show SEM images of 700 nm period nanogratings formed on PFA or PET substrates. Figure 9A has an SSQ coating on a PFA substrate, while the SSQ coating is absent on the PFA substrate in Figure 9B. An SSQ coating is formed on a PET substrates in Figure 9C, while Figure 9D has no SSQ coating on the PET substrate. All are processed at 80°C. The insets are the counter profiles of each grating structure, showing that the aspect ratio of resulting nanogratings can be significantly enhanced by the use of SSQ layer;

[0022] Figure 10 shows an SEM image of the exposed underlying PFA surfaces after certain variations of the inventive methods are conducted showing deformed morphology along the mold transfer direction. The sample is fabricated by applying the 200 nm period mold on the SSQ-coated PFA substrate at 80°C, followed by full curing under intense UV light. To make a boundary, the fully cured SSQ/PFA nanograting sample is first vapor-treated with an epoxy- functional silane adhesion promoter (SILQUEST™ A-187) at 90°C for 5 min. Next, epoxysilicone is partially applied (via drop-casting) and cured, then is quickly delaminated by a razor blade. The SSQ/PFA grating at the boundary appears to be somewhat fused because of capillary infiltration of residual epoxysilicone;

[0023] Figures 11A-11E: Figures 11A-11D show SEM images of 700 nm period nanogratings formed on the SSQ-coated PET substrates at different processing temperatures: Figure 11 A at room temperature, Figure 11B at 50°C, Figure 11C at 80°C, and Figure 11D at 100°C. The viscosity of SSQ as a function of temperature is shown in Figure HE with the marks at which Figures 11A-11D are processed. The grating depth appears to increase with more faithful profiles from room temperature up to 80°C, as the SSQ viscosity decreases, while it appears to become shallower when processed at 100°C at which point the SSQ viscosity increases because of curing effect. The viscosity measurement becomes unstable after 90°C presumably due to the "stick and slip" motions caused by the SSQ curing, but overall an increasing trend is demonstrated; and

[0024] Figures 12A-12D: Figure 12A shows a schematic of a process for nano-channel imprinting according to certain aspects of the present teachings; Figures 12B-12C show waveguide arrays formed in accordance with certain aspects of the present teachings.

[0025] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0026] Example embodiments will now be described more fully with reference to the accompanying drawings. Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. Further, the present disclosure contemplates that any particular feature or embodiment can be combined with any other feature or embodiment described herein. In some example embodiments, well-known processes, well-known device structures, and well- known technologies are not described in detail.

[0027] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0028] The terms "comprises," "comprising," "including," and "having," are inclusive and therefore specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. [0029] As referred to herein, the word "substantially," when applied to a characteristic of a composition or method of this disclosure, indicates that there may be variation in the characteristic without having substantial effect on the chemical or physical attributes of the composition or method.

[0030] As used herein, the term "about," when applied to the value for a parameter of a composition or method of this disclosure, indicates that the calculation or the measurement of the value allows some slight imprecision without having a substantial effect on the chemical or physical attributes of the composition or method. If, for some reason, the imprecision provided by "about" is not otherwise understood in the art with this ordinary meaning, then "about" as used herein indicates a possible variation of up to 5% in the value.

[0031] When an element or layer is referred to as being "on," "contacting," "engaged to," "connected to," or "coupled to" another element or layer, it may be directly on, engaged, contacting, connected, or coupled to the other element or layer, or intervening elements or layers may be present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0032] Although the terms first, second, third, and the like may be used herein to describe various components, moieties, elements, regions, layers and/or sections, these components, moieties, elements, regions, layers and/or sections are not exclusive and should not be limited by these terms. These terms may be only used to distinguish one component, moiety, element, region, layer or section from another component, moiety, element, region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first component, moiety, element, region, layer or section discussed below could be termed a second component, moiety, element, region, layer or section without departing from the teachings of the example embodiments.

[0033] Spatially relative terms, such as "inner," "outer," "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0034] Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term "about" whether or not "about" actually appears before the numerical value. "About" indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by "about" is not otherwise understood in the art with this ordinary meaning, then "about" as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints given for the ranges.

[0035] The present disclosure includes methods related to a high throughput and high speed continuous processing of microscale and/or nanoscale features (referred to herein as "microscale features"). In certain aspects, such methods enable the creation of microstructure features for creating a component of a microelectronic device, such as a polarizer or an electrode having such microstructures, for example. Thus, the phrase "electronic device" includes micro- and nano-electronic devices including one or more microscale structural features, such as, for example, micro- and nano-sized transistors, diodes, electromagnetic field polarizers, solar cells and the like. The methods of the present invention are applicable to the fabrication of various electronic devices, including, by way of example, organic thin film transistors (OTFTs), organic photo-voltaics or solar cells for solar electricity or photo-detectors, organic light emitting devices (OLEDs), organic solid state lasers or organic solid state lighting, organic thin film memory for data storage, organic sensors for bio- application and chemical detection, optical waveguides and optical polarizers, such as bilayer grid polarizers for light emitting applications, like those used in flat panel LCD display devices. Micro and nano-photonic devices, such as micro-resonators, communication network multiplexers, and optical buffers, require on-chip interconnects using optical waveguides. Chip-to-chip interconnects using waveguide or fibers are also becoming necessary to solve the bandwidth challenges. The present teachings provide in certain aspects, processes for scalable waveguide fabrication techniques.

[0036] In certain aspects, the disclosure provides methods of forming a structure or feature that is of a microscale on a substrate surface. In some aspects, the methods of the present disclosure are capable of forming structures that are smaller than a microstructure, such as a nanoscale structure or feature. As used herein, "microscale" refers to a structure having at least one dimension that is less than about 100 μιη, optionally less than about 10 μηι and in some aspects, less than about 1 μιη. A "nanoscale" structure or feature has at least one dimension that is less than about 500 nm (0.5 μιη), optionally less than about 100 nm (0.1 μιη), optionally less than about 50 nm, and optionally less than about 10 nm. Optionally, a micro or nanoscale structure or feature may have two or more dimensions in the above-listed ranges, e.g., such as height and width of a channel. As used herein, reference to a microscale, microstructure, microchannel, or microfeatures, encompasses smaller structures, such as the equivalent nanoscale structures or nanofeatures, as well.

[0037] Thus, in certain variations, a method of forming at least one microscale feature on a substrate is provided. The method optionally comprises contacting a patterned surface of a rigid stamp with a curable liquid material disposed on a major surface of a moldable polymeric substrate. The curable liquid comprises at least one curable precursor. Furthermore, the use of "liquid" encompasses materials that are capable of flowing and can include semi-solid and gel materials. Heat may be applied during the contacting and continuous imprinting process to optimize the viscosity and flowability of the liquid polymer material, as appreciated by those of skill in the art.

[0038] The patterned surface of the rigid stamp defines a cavity that will create and define at least one microscale feature after the contacting. During the contacting, the curable liquid material flows into the cavity. In certain aspects, the curable liquid material has a viscosity such that it can rapidly flow into the one or more cavities of the patterned surface of the rigid stamp, yet is high enough that the curable liquid material retains an imprinted structure (defined by the cavity of the rigid stamp) for at least a short duration of time. In certain variations, the viscosity of the curable liquid material is greater than or equal to about 1 Pa-s to less than or equal to about 100 Pa-s during the contacting, and in certain variations, optionally greater than or equal to about 5 Pa-s to less than or equal to about 50 Pa-s.

[0039] Furthermore, during the contacting of the rigid stamp with the curable liquid material, the major surface of the moldable polymeric substrate can desirably be at least partially deformed. The partial deformation of the major surface of the moldable polymeric substrate can further serve to retain the structure imprinted from the stamp for a duration sufficient to cure and form a microscale feature. Suitable rigid materials for forming the stamp (having a patterned surface comprising one or more cavities that correspond to the one or more microscale features to be patterned on the polymeric substrate) include dielectric materials, such as silicon dioxide, silicon nitride; semiconductors, such as silicon; and metals, such as nickel (Ni), chromium (Cr); and cross-linked polymers.

[0040] After contact and applied pressure with the rigid stamp, the curable liquid material is thus cured on the major surface of the moldable polymeric substrate. Rapid curing mechanisms and processes are particularly advantageous. In certain variations, curing is initiated within a very short duration of time, on the order of milliseconds or seconds, after the contacting with the rigid stamp ceases. For example, in certain variations, the curing begins within 2 seconds after contacting with the rigid stamp is ceased to solidify the liquid material imprinted with the patterned surface structure(s) on the major surface of the moldable polymeric substrate. In certain variations, the curing of the curable material (meaning the curable material is initially exposed to the radiation source) occurs within 1 second after contact with the rigid stamp ceases, preferably within 500 milliseconds, and in certain aspects, optionally within 100 milliseconds, and in certain variations, optionally within 10 milliseconds. [0041] Depending upon the curing mechanisms for the polymeric precursors in the curable liquid polymeric material, the curing may include applying thermal energy and/or actinic radiation energy. In certain other variations, electron beam (e-beam) curing may be employed. After curing, at least one microscale feature (transferred from the cavity/cavities of the patterned surface of the rigid stamp) is formed from the cured polymer on the major surface of the moldable polymeric substrate.

[0042] As noted above, the major surface of the moldable polymeric substrate is selected to be such that the curable liquid material can be deposited and remain on the surface; however, is low enough in surface energy to maximize a contact angle between the liquid material and the major surface (to minimize wetting of the liquid material onto the major surface and therefore to maximize retention of structure formed during imprinting with the patterned stamp in the liquid material). This will be described in more detail below in the context of Figures 4A-4J and 5A-5H. To reduce wetting behavior, a surface energy of the polymeric substrate (selected for use as the moldable polymeric substrate) may be less than or equal to about 50 mN/m, optionally less than or equal to about 40 mN/m, optionally less than or equal to about 30 mN/m. The simplest measure of wetting on a smooth (non-textured) surface is the equilibrium contact angle Θ, given by the Young's equation as,

COS 0 (Equation 1)

where, the surface tension of the liquid is γΐν, the surface energy of the solid is y_Sv, and the solid-liquid interfacial energy is ysi. Surfaces that display contact angles Θ greater than about 90° with water are considered to be hydrophobic and surfaces that display contact angles greater than 90° with oil are considered to be oleophobic. For example, in certain variations, a major surface of the moldable polymeric substrate has a surface energy of less than or equal to about 50 mN/m. In yet other variations, the surface energy of the major surface of the moldable polymeric substrate is greater than or equal to about 10 mN/m and less than or equal to about 25 mN/m and optionally greater than or equal to about 15 mN/m and less than or equal to about 20 mN/m.

[0043] As used herein, a contact angle generally refers to an average of a static contact angle Θ between the liquid material and the underlying solid (see Figure 1 and also Figures 4A-4J and 5A-5H). In various aspects, a contact angle Θ formed between the curable liquid material and the major surface of the polymeric substrate is preferably maximized (see, e.g., Figures 1, 4A-4J and 5A- 5H). For example, a suitable contact angle Θ is optionally greater than or equal to about 65°, optionally greater than or equal to about 70°, optionally greater than or equal to about 75°, optionally greater than or equal to about 80°, optionally greater than or equal to about 90°, and in certain variations, may be greater than or equal to about 100°.

[0044] In yet other aspects, the major surface of the moldable polymeric substrate is pretreated prior to applying the curable liquid material to reduce its surface energy and therefore to increase a contact angle between the liquid material and the major surface. For example, the polymeric substrate can be treated to impart a reduced surface energy, such as treating with a fluorine containing material to facilitate formation of fluorine-containing groups on the surface of the polymeric substrate. In certain variations, a fluorine-containing composition, such as a fluorosilane, can be reacted with the polymeric material to form a surface bearing fluorine-containing groups. Other treatments known in the art for lowering surface energy of polymeric substrates are likewise contemplated.

[0045] Additionally, the polymeric substrate preferably comprises a moldable polymer so that it at least partially deforms during contact with the rigid stamp (and the concurrent application of pressure that occurs). Such deformation of the moldable polymeric substrate is shown in Figures 2A-2D, 3A- 3B, 5A-5H, and 9A-9D. Materials having a relatively lower modulus of elasticity are preferred, because they tend to plastically deform more readily and therefore do not elastically recover to the original shape of the polymer after the imprinting process. Deformation of the underlying substrate serves to further minimize wetting of the liquid material over the major surface (as described in greater detail below (see Figure 10) and shown in comparative Figures 4A-4J and 5A-5H). Suitable materials typically have a modulus of elasticity (E) of less than or equal to about 3.5 GPa; optionally less than or equal to about 3 GPa; optionally less than or equal to about 2.5 GPa; optionally less than or equal to about 2 GPa; optionally less than or equal to about 1.5 GPa; and in certain aspects, less than or equal to about 1 GPa. In certain embodiments, a suitable modulus of elasticity (E) is about 0.5 GPa (e.g., for a perfluoralkoxy polymer).

[0046] Thus, in certain variations, the moldable polymeric substrate comprises perfluoroalkoxy (PFA). In other variations, a polyethylene terephthalate (PET) can be subsequently treated to reduce its surface energy to desired levels described above. For example, in certain aspects, the moldable polymeric substrate may comprise a fluorine-treated PET (PET treated with a fluorosilane agent). In yet other alternative embodiments, the moldable substrate can be formed from a non-polymeric material, such as a malleable deformable metal.

[0047] The curable liquid material applied to the major surface of the moldable polymeric substrate may comprise one or more polymer precursors. Such a curable liquid material may comprise epoxysilicone, epoxy precursor (SU-8), polydimethylsiloxane, PDMS, or thermal or photocurable silsesquioxane. In certain variations, a particularly suitable curable liquid material comprises a silsesquioxane (SSQ), such as an ultraviolet light-curable epoxy-silsesquioxane (SSQ). UV light curable epoxy-silsesquioxane polymers may comprise UV curable groups, such as epoxy groups and/or methacrylate groups, for example. SSQ polymers, like poly(methyl-co-3-glycidoxypropyl) silsesquioxanes (T^MeT^EP), poly(phenyl-co-3-glycidoxypropyl) silsesquioxanes (T^phT^EP), and poly(phenyl-co- 3-glycidoxypropyl-coperfluorooctyl) silsesquioxanes

_can ¾_e precisely designed and synthesized by incorporating the necessary functional groups onto the SSQ backbone and are contemplated for use as curable liquid materials in accordance with certain aspects of the present teachings. Such materials are described in Pina-Hernandez et al., "High-Resolution Functional Epoxysilsesquioxane-Based Patterning Layers for Large-Area Nanoimprinting," ACS Nano, vol. 4, no. 8, 2010, pp. 4776-4784, incorporated herein by reference in its entirety. One particularly suitable curable liquid material comprises an epoxy-SSQ (T^pheny1o.4Qo.iT^Epoxyo.5) that includes a tri-functional unit comprising a phenyl (F^SiOs^, where R¹ is a phenyl), a quaternary functional unit (S1O2), and a tri-functional epoxy (R²SiO3/2, where R² is an epoxy containing group), which are mixed with 3 wt. % photoacid generator (UV-9820, Dow Corning Corp.) and diluted with propylene glycol monomethyl ether acetate (PGMEA) to make a SSQ resist solution containing 10-20 wt. % SSQ.

[0048] Other suitable curable liquid materials are described in Pina- Hernandez et al., "High-Throughput and Etch-Selective Nanoimprinting and Stamping Based on Fast-Thermal-Curing Poly(dimethylsiloxane)s," Adv. Mater. Comm., DOI: 10.1002/adma.200601905 (2007), pp. 1-7 (and published as Pina- Hernandez, C, et al., "High-Throughput and Etch- Selective Nanoimprinting and Stamping Based on Fast-Thermal-Curing Poly(dimethylsiloxane)s," Adv. Mater. 19(9) (2007), pp. 1222-1227), incorporated herein by reference in its entirety. Yet other suitable curable liquid materials include those described in Cheng, Xing, et al., "Room-Temperature, Low-Pressure, Nano-imprinting Based on Cationic Photopolymerization of Novel Epoxysilicone Monomers," Adv. Mater. 17 (2005), pp. 1419-1424, incorporated herein by reference in its entirety. Other curable polymeric materials, especially photoresist materials, known or to be discovered in the art are contemplated. The curable liquid material may be applied to the major surface of the moldable polymeric substrate prior to contacting the stamp by a process selected from spin casting, spin coating, ink jetting, spraying, and/or by gravure application methods.

[0049] In certain preferred variations, the moldable polymeric substrate comprises perfluoroalkoxy and the curable liquid material comprises ultraviolet light-curable epoxy-silsesquioxane (SSQ).

[0050] In certain alternative variations, the liquid or semi-liquid material applied to a major surface of the moldable polymeric substrate comprises a functional material, such as a conductive paste (which may comprise a resin or other material and a plurality of conductive particles). When such a functional liquid or semi-liquid material is applied, it can form conductive lines. Such a functional liquid or semi-liquid material can be optionally cured and optionally subsequently annealing. Such an embodiment can be used to form a transparent electrode, by way of non-limiting example.

[0051] Accordingly, the present teachings provide high-throughput continuous patterning or imprinting of microstructural features on a major surface of a deformable substrate, thus providing a commercially viable and scalable method of producing such nanoscale devices, such as nanograting and waveguide structures for transparent conductive electrodes or optical polarizers. The disclosure provides methods of forming a microscale or nanoscale structure within or on a major surface of a substrate, where each microscale structure has an elongate axis (or length), a height, and a width between respective microscale features. A major elongate axis refers to a shape having a prominent elongate dimension or length. In certain aspects, the methods of the present disclosure form nanofeatures or microfeatures of the present disclosure having desirably high aspect ratios with regard to height and width dimensions of each respective structure. For example, an aspect ratio is optionally defined as AR = H/W where H and W are the height and the width of the nanofeature or microfeature. Desirably, the present teachings enable high throughput production of microfeatures having an AR of greater than or equal to about 1, optionally greater than or equal to about 1.5, optionally greater than or equal to about 2, optionally greater than or equal to about 2.5, optionally greater than or equal to about 3, optionally greater than or equal to about 4, optionally greater than or equal to about 5, and in certain aspects, optionally greater than or equal to about 10. In this manner, the present teachings provide the ability to provide desirably high aspect ratio structures with a desirably short period or physical distance/interval between adjacent structures.

[0052] In certain variations, such techniques are used to fabricate waveguides for transparent electrodes or polarizers that comprise conductive/metallic wire grids or grating structures by using the inventive roller-based optical lithography methods. A linear structure comprises at least one row feature. A "grating structure" is a material forms a structure that comprises one or more openings therethrough to permit certain wavelength(s) of light to pass through. For example, in certain preferred aspects, a grating structure for a waveguide may comprise a plurality of material rows or discrete regions spaced apart, but that are substantially parallel to one another. The spacing between adjacent rows defines a plurality of openings through which certain wavelengths of light may pass. The grating may also comprise a second plurality of rows having a distinct orientation from the first plurality of rows that are likewise spaced apart, but substantially parallel to one another. The first and second plurality of rows may intersect or contact one another at one or more locations to form a grid or mesh structure. It should be noted that in preferred aspects, the grating comprises at least two rows to form at least two openings, but that the number of rows and layers of distinct grating structures are not limited to only two, but rather may comprise multiple different designs and layers. Further, as described below, while the adjacent rows or other regions of the plurality are preferably distanced at a sub-wavelength distance from one another (a distance of less than the target wavelength or range of wavelengths), each respective pair of rows may define a distinct distance for each opening (or slit diameter) there between and thus will permit different wavelengths of light to travel there through. Thus, a grating structure where the rows, optionally comprising a conductive metal or graphene, may be employed as a waveguide for a transparent conductive electrode and/or as an optical polarizer, by way of non-limiting example.

[0053] In certain variations, a grating pattern of rows formed on a major surface of the substrate defines a period "p" (a distance defined from a side of a first row or linear feature to a side of a second adjacent row or linear feature). A distance "d" between adjacent rows is considered an opening (or aperture or slit). It should be noted that distance "d" may vary through the grating pattern, where d is represented by d=p-a. A row or linear feature has a height "h" and a width of each row is "a." A duty cycle is defined by f=a/p. Periodicity refers to at least one period (p) between a pair of rows in the grating pattern, but where there are more than two openings typically to a repeating period (p) in the grating pattern. In certain exemplary embodiments, a period (p) between rows is less than or equal to about 700 nm, optionally less than or equal to 600 nm, optionally less than or equal to about 500 nm, optionally less than or equal to about 400 nm, optionally less than or equal to about 300 nm, and in certain variations, optionally less than or equal to about 200 nm. Preferably the periodicity of such a grating structure also coincides with the high aspect ratios for each respective row in the grating structure, described just above. In certain embodiments, a width (a) of each row is less than or equal to about 500 nm, optionally less than or equal to about 400 nm, optionally less than or equal to about 300 nm, optionally less than or equal to about 200 nm, optionally less than or equal to about 100 nm, optionally less than or equal to about 75 nm, and in certain variations, optionally less than or equal to about 50 nm. Similarly, a height (h) of each row can be less than or equal to about 300 nm, optionally less than or equal to about 200 nm, optionally less than or equal to about 100 nm, optionally less than or equal to about 75 nm, optionally less than or equal to about 50 nm, and in certain variations, optionally less than or equal to about 40 nm. Thus, a waveguide structure can be designed by adjusting row width (a) and period (p) so that different wavelengths of light can be transmitted through openings (d) between rows. High conductance can likewise be achieved by adjusting the thickness (h) of the material forming rows. Such a grating pattern provides a highly flexible design that can be readily tailored for different performance criteria.

[0054] With reference to Figures 2A-2D, the present methods can be employed as a continuous process that can optionally include the applying of a curable liquid material 120 onto a major surface 122 of a moldable polymeric material substrate 124, then contacting the curable liquid material 120 and the moldable polymeric material substrate 124 with a rigid mold or stamp 130. The rigid mold 130 comprises a patterned surface 126 that comprises one or more channels or cavities 154 that will define microfeatures in the curable liquid material 120 as contact is made therebetween. Then, the method comprises curing the curable liquid material 120, which is accomplished by passing the moldable polymeric substrate 124 (on a carrier or conveyor 132) through an application module (not shown), then a contacting or coating module 140, and then a curing module (142). See, e.g., Figures 2A and 2D showing the contacting and the curing modules 140, 142 (not showing upstream application module). The curing module 140 comprises a source of UV radiation 150, although in alternative embodiments, this may be an alternative source for generating energy (like e-beam generator) for alternative types of curing. The mold 130 comprises an optional heater 152 that can maintain a desired viscosity of the curable liquid material 120, so that the curable liquid material 120 can flow into one or more cavities 154 of the mold 130 and yet be imprinted with the desired pattern.

[0055] In various embodiments, the mold 130 is angled with respect to the major surface 120 of the moldable polymeric substrate 124, so as to establish two-dimensional contact along a contact line with the major surface 120 of the moldable polymeric substrate 124 and the curable liquid material 120 to facilitate imprinting and extrusion. In various aspects, an angle formed between the mold 130 and the major surface 120 of the moldable polymeric substrate 124 is about 5° to about 50°; and optionally about 10° to about 30°. In certain embodiments, the angle formed between the mold 130 and the major surface 120 is about 15°, which facilitates two-dimensional contact in a contact region established therebetween. The rigid mold 130 at least partially deforms (e.g., at least partially plastically deforms) the deformable polymeric substrate 124 and further creates a pattern (see 160 in Figures 2A and 2D) in the curable liquid material 120 as it encounters the patterned surface of the rigid stamp (see Figure 2D). More specifically, the curable liquid material fills the cavities in the patterned surface of the rigid stamp. As discussed, shortly after contact with the rigid stamp ceases, the deformable polymeric substrate and the curable liquid material are subjected to a curing process, which solidifies that curable liquid material on the deformable polymeric substrate to create at least one microscale feature (corresponding to the filled cavities in the patterned surface).

[0056] In certain alternative variations, a continuous process is used that involves a configuration where the rigid stamp is instead stationary and disposed at an angle with respect to the major surface of the moldable polymeric substrate. A carrier supports and transports the moldable polymeric substrate, which moves past the stationary rigid stamp.

[0057] In yet other aspects, the methods of the present teachings may further comprise treating of the major surface with another surface modification process, which are well known to those of skill in the art. For example, such a treatment process may include a subsequent deposition process or etching process, as are well known in the microelectronic arts. In certain variations, the methods of the present disclosure apply a metal material after the curing process over at least a portion of the major surface having the at least one microscale feature formed thereon. The metal material comprises at least one metal, but may comprise a plurality of metals and optionally may further contain non- metals, as well. Such a metal material can optionally be applied by at least one process selected from chemical vapor deposition and/or physical vapor deposition, by way of non-limiting example.

[0058] The following discussion further elaborates on the improvements of the inventive technology over certain processes of forming microscale features in polymeric materials. By way of background, one process for forming nanoscale channels and grating structures is Dynamic Nanoinscribing (DNI). DNI processes can create seamless large-area nanogratings on any materials softer than the molds at high speed and low cost by master molds without area limitation. DNI uses a slice of rigid grating molds directly pressed into a solid substrate to mechanically inscribe a solid metal or polymer surface on a two dimensional contact to create nanograting structures in a dynamic fashion.

[0059] While Dynamic Nanolnscription (DNI) can realize seamless fabrication of nanogratings, it can suffer from elastic recovery that prevents formation of structures having high aspect-ratio structures, especially for grating or channel structures having smaller periodicity. During DNI, the plastically deformed solid surfaces can elastically recover after release of mechanical force, which hinders forming final structures having high aspect-ratio profiles, which are highly desired in many applications such as metal wire-grid polarizers. Further, since the nature of DNI relies on mechanical deformation, which can exponentially reduce as the scale lowers down, it typically becomes quite challenging to inscribe high aspect-ratio nanopatterns on solid substrates at smaller grating periods.

[0060] In contrast, the inventive technology provides a novel nanopatterning technique that achieves continuous fabrication of higher aspect- ratio micro-grating or nano-grating structures by employing in various aspects, a curable liquid polymer material, such as a cross-linkable liquid resist coating that is patterned. Under a slight mechanical force, the liquid resist can readily infiltrate and fill the cavities or channels defined by the patterned surface of the rigid mold or stamp (e.g., mold gratings) upon contact. Under the guidance of nano-channels in the patterned surface of a rigid stamp/mold, the UV-curable liquid resist is smoothly extruded and self-stabilized along the slightly inscribed (e.g., deformed) solid substrate (major surface of a moldable polymeric material). Self-stabilization is related to the liquid resist wettability (the curable resist material wettability) to the underlying substrate, as well as the major surface topography. The final nanograting geometry can be readily tuned by surface modification of the major surface of the moldable polymeric substrate that adjusts its non-wetting property (reduces wettability) to the liquid curable resist material, and also by the processing temperature during contact with the stamp that controls the viscosity of the curable liquid resist material and therefore the nanochannel filling height during the process.

[0061] Therefore, the stamp pattern, such as a grating profile, created in the as-formed liquid resist is retained by designing the system to have proper non- wetting behavior (of the liquid resist along the major surface of the solid polymeric material). The as-formed curably liquid resist material is promptly cured to form cross-linked nanograting patterns in a seamless and continuous manner. An aspect ratio of the pattern formed can be tuned by the processing temperature at which the viscosity of liquid resist changes, thereby regulating the mold cavity (gap) filling height at contact. While the temperatures will range depending on the liquid resist material employed, in certain variations, a curable liquid material is heated to a temperature of greater than or equal to room temperature (about 20°C), optionally greater than or equal to about 60°C, optionally greater than or equal to about 70°C, optionally greater than or equal to about 75°C, optionally greater than or equal to about 80°C, and optionally greater than or equal to about 90°C. In certain variations, the temperature is less than 100°C, and in certain variations, greater than or equal to about 75°C to less than or equal to about 90°C.

[0062] The inventive technology thus provides a nano-patterning technique that employs curable liquid polymer materials, such as liquid resist materials, in a high-throughput nanoinscribing process, that achieves high-speed and low-cost fabrication of continuous nanograting structures for large-area optoelectronic applications. In certain embodiments, a suitable liquid resist comprises a UV-curable epoxy-silsesquioxane (SSQ), which is a viscous liquid polymer that solidifies into a cross-linked high-modulus material upon curing by ultraviolet radiation. In certain aspects, the UV-curable epoxy-silsequioxane has a viscosity between 5 and 50 Pa-s. Whereas a relatively large force is required to "inscribe" nanopatterns on a solid substrate by plastically deforming the material (like in the conventional DNI processes), a liquid resist can readily "infiltrate" the openings and channels formed in a patterned surface of a mold (e.g., a grating pattern) upon contact under slight mechanical force. These nanochannel-guided liquid streaks are continuously extruded from the contact region as the rigid mold translates along the surface, enabling continuous formation of nanograting patterns without elastic recovery. To ensure the success of the nanopatterning process by the inventive technology (referred to herein as "NanoChannel-guided Lithography (NCL)" for brevity), the polymeric substrate material desirably has a non-wetting property with respect to the liquid resist used in the process. The shallow, but plastically deformed channel features on the polymeric substrate, along with the non-wetting characteristics of the surface of the polymeric substrate prevent the immediate reflow of the as- formed liquid nanograting structures, until the pattern can be fully cured (e.g., by exposure to UV light) to form a solid having the shape of the nanopattern cavities (microscale features).

[0063] Furthermore, an aspect ratio of the pattern can be tuned by the processing temperature during contacting of the stamp with the curable liquid material, which changes the viscosity of the liquid material, thereby regulating the mold cavity/gap filling height at contact. Small period and high aspect-ratio patterns are obtained when the polymeric substrate surface is slightly inscribed (at least partially plastically deformed) by the edge of the hard mold. The resultant periodic topography, albeit shallow, helps to retain the shape of the liquid resist walls before it is cured by UV light. The ability to retain the shape of the pattern while in liquid phase depends on the wettability of the liquid material on the underlying material and the effective contact angle with the shallow surface morphology. In general, the polymer having smaller elastic modulus can be more easily patterned, leading to surface topography with larger depth (so that the effective contact angle is greater). This combined effect is verified by conducting experiments on two types of polymer substrates of different modulus and with different SSQ wettability.

[0064] The setup of an embodiment of a process according to certain aspects of the present technology (referred to herein as a nano-channel guided lithography process or "NCL") is schematically shown with renewed reference to Figure 2 A and is merely exemplary and non-limiting. First, an edge 170 of the stationary rigid grating mold 130 makes contact with the UV-curable liquid resist 120 coated on a softer polymer substrate 124 (e.g., perfluoroalkoxy (PFA) or a comparative polyethylene terephthalate (PET), both of which are deformable). The stationary rigid grating mold 130 is inclined at an angle of approximately 15° with respect to the moving substrate 124 (disposed on the moving carrier 132) and the contact force is about 5 N. The contact point can be maintained at ambient or elevated temperature, controlled by localized heating by using the conductive heater 152 attached to the backside of the mold. This method of heating is non-limiting and other methods of heating at the contact point known to those of skill in the art are likewise contemplated. The heating is used to adjust the viscosity of the liquid resist for optimal filling of the nanochannel features 154 on the patterned surface 126 of the mold 130 within the processing time. The polymer substrate 124 is placed on a silicone rubber film (moving carrier) 132, which prevents the substrate 124 from slipping during the contacting/inscribing process, and ensures conformal contact to the mold edge 170 with the support of a multi-axial tilting stage 172. As the polymeric substrate 124 is moved at a controlled speed with respective to the mold 130, the liquid resist material 120 on top of the substrate 124 is delineated into the shape of patterned surface channels 160 as it is extruded from the end of the nanochannels 154 on the mold 130, as illustrated in Figures 2B-2D. A curing module 142 comprises a UV light 150 that is placed in front of the mold 130 to promptly cure the liquid resist 120 to form the microscale features (e.g., nanogratings) with well-retained profile so the channel profile 160 is retained. Such a continuous NCL process produces a seamless nanograting in cured liquid resist material.

[0065] Figure 2A is a schematic showing the NCL process, where the liquid resist material 120 is extruded from the nanochannels or cavities 154 in the patterned surface 126 of the mold 130 and promptly or rapidly cured by UV light from UV light source 150 to retain the desired profile or shape. A slice of the Si02/Si grating molds, having either (Figure 2B) 700 nm or (Figure 2C) 200 nm period, is used at ambient or heated condition, and the liquid SSQ resist is coated on the polymer substrate (e.g., PFA or PET) under conformal contact, while the UV light cures after contact with the patterned surface of the mold. An enlarged perspective view of the process (Figure 2D) illustrates the liquid lines are extruded from the openings of the channels in the mold at the contact region.

[0066] One particularly advantageous benefit of the inventive methods are that they can produce nanograting structures in a UV curable liquid resist with higher aspect-ratio profiles than conventional processes (e.g., as compared to conventional structures inscribed on the solid plastic surface by the DNI process). This is demonstrated by comparing the 200 nm period gratings patterned on a SSQ-coated PFA surface by the inventive NCL techniques with the grating formed by a similar process, but instead formed directly onto a solid PFA by a DNI imprinting process, as shown in Figures 3A-3B.

[0067] Figures 3A-3B thus show SEM images of 200 nm period nanogratings formed on the PFA substrates at 80°C with (Figure 3A) and without liquid SSQ coating (Figure 3B) formed via a conventional nanoinscribing type of process. The insets are the counterprofiles of each grating structure, showing that the aspect ratio of resulting nanogratings in Figure 3A prepared in accordance with certain aspects of the present teachings is significantly improved by the use of SSQ layer and the inventive techniques. Similar results are observed in gratings of different periods (e.g., 700 nm) and substrate materials (e.g., PET), as can be found in Figures 9A-9D.

[0068] In either case, it is noted that the inventive patterning process employs continuous deformation of the contacted deformable polymeric substrate's surface, rather than material removal. Furthermore, for conventional DNI patterning on solids, the plastic deformation by mechanical inscribing that forms the nanograting is inherently limited by the elastic recovery process. This effect hampers reliable and faithful patterning of nanograting with small periods (e.g., 200 nm period) even under a very large mechanical load. In comparison, in the new inventive processes, the liquid resist material readily fills the nanochannels on the patterned surface of the mold (Si02/Si) due to viscous flow. Excessive liquid is continuously swept away by the mold until the process terminates. This is confirmed by the experiments, where different thicknesses of an initial SSQ layer have the same geometry in the resulting nanograting structures, despite different thicknesses of the initial SSQ resist applied. The final pattern appears to have slightly rounded corners due to the reflow of as- patterned polymer liquid grating before it is fully cured. This behavior is similar to previous reports. However, the striated/as-extruded liquid on the underlying surface can mostly sustain the structures without significant immediate reflow by the control of wetting properties, as will be further explained below.

[0069] In certain aspects of the inventive processes, the liquid-solid interaction plays an important role in determining the final profile of the nanostructures. This is because the as-formed liquid SSQ microstructures or nanostructures extruded from the nanochannels or cavities of the patterned surface of the rigid mold can maintain its profile rather than immediate reflowing before it is cured, which is directly related to the wetting property of the solid polymeric surface on which the liquid material is disposed. The solid substrate surface underneath the liquid SSQ layer appears to be simultaneously inscribed (partially plastically deformed) during the inventive process (see Figure 10), providing periodic topology to SSQ wetting above. The inscribed profile on PFA substrate is more pronounced than that of PET; and the SSQ grating is more faithfully replicated in profile on the PFA substrate as compared to the PET substrate. The ability to form such nanograting in the NCL process appears to be related to two main factors: (1) the wetting characteristics of curable liquid material on the solid polymeric surfaces, and (2) the deformation characteristics of the solid polymeric surface. Therefore both the surface energy and elastic moduli of the solid substrate are important parameters in such processes.

[0070] Figures 4A-4J compare the nanograting formation characteristics on solid polymeric materials (e.g., PFA and comparative PET) having different elastic moduli; and SSQ liquid resist also has different wetting behaviors behavior on the two substrates due to the different surface energies of the two substrates. The contact angles of a liquid SSQ droplet on the flat PFA surface and on a comparative PET surface at room temperature are 67.9° and 20.1°, respectively (Figures 4C and 4H).

[0071] The large contact angle and the resultant non-wetting behavior of liquid SSQ on PFA will stabilize the as-formed liquid grating against reflow, allowing time for it to be cured and solidified (Figure 4A). In fact, it is observed that the grating profile remains almost static regardless of the UV curing time, confirming the self-stabilization of the as-extruded liquid lines from the nanochannels on the mold on PFA. In contrast, the small contact angle between liquid SSQ on PET dictates a good wetting behavior of the SSQ on PET that results in quick reflow of the as-formed nanograting structure thus preventing formation of the desired profile of microstructures (Figures 4F-4G). Furthermore, the underlying PFA surface has a smaller modulus of elasticity (£ about 0.5 GPa), which plastically deforms more readily during the NCL processing than PET (£ about 3 GPa). The large contact angle and the non-wetting behavior of liquid SSQ on PFA prevents immediate reflow of the as-formed liquid grating, allowing time for it to be cured and form stable patterns (Figures 4A-4B). Whereas the small contact angle between liquid SSQ on PET dictates a wetting behavior of the SSQ on PET that results in quick reflow of the as-formed nanograting patterns in Figures 4F-4G. Thus, as schematically shown in Figure 4D, the as-formed liquid SSQ grating sitting on top of the inscribed PFA grating benefits from the large contact angle at SSQ-PFA interfaces and maintains a vertical profile, which helps to stabilize the liquid grating pattern and allows time for it to be fully cured that is important for obtaining high aspect-ratio nanograting structures. On the other hand, the shallow profile in PET due to its high modulus and the low contact angle of SSQ cannot efficiently prevent the lateral reflow of the liquid resist pattern before it is fully cured (Figure 41).

[0072] Figures 4E and 4J show diagrams of nanograting formations by an inventive NCL process on SSQ-coated PFA substrate (Figure 4E) and on a comparative PET (Figure 4J) substrate. At contact with the patterned surface of the rigid mold, liquid resist is filled into the nanochannels on the mold and solid substrate is plastically inscribed by the sharp edge of the grating mold. The inscribed solid surfaces each undergo a different extent of elastic recovery, while the as-formed liquid experiences reflow depending on its wettability on the solid surface beneath. Final grating geometry is determined by the cooperative effect of liquid wetting and substrate topography. As can be seen, the contact angle of SSQ droplets is much larger on PFA than on PET (Figure 4C) and (Figure 4H). Meanwhile, PFA is deformed more than PET due to its smaller modulus, which helps to maintain a vertical profile of the SSQ ridges on top. This is comparatively depicted in (Figure 4D) and (Figure 41) where the subsequent reflow directions are marked with arrows. The SSQ reflow on PFA is effectively restricted by the local contact with SSQ-repellant PFA deformed grooves, whereas the as-formed liquid SSQ lines on the PET surface shortly collapse due to the better wettability on PET along with the insufficient local deformation of the PET surface. Accordingly, the resulting nanogratings processed at 80°C ((Figure 4E) and (Figure 4J)) show that more faithful, higher aspect-ratio nanogratings can be created on the SSQ-coated PFA surface (inset to (Figure 4E) shows the counterprofile of a cross-section).

[0073] Consistent with this wetting-dependent mechanism, the aspect ratio of final nanograting structures can be controlled by surface modification of the major surface of the deformable polymeric substrate. This enables high aspect-ratio nanogratings on hard surfaces to be formed, which cannot usually form very deep patterns under normal circumstances. Figures 5A-5H compare 700 nm period SSQ nanograting formations on a normal PET substrate (Figures 5A-5D) and a fluorosilane-treated F-PET substrate (Figures 5E-5H), exemplifying how the pattern profiles can be tuned by different wetting conditions on the same material. Here, a much larger contact angle of SSQ to the fluoro-PET allows the as-formed SSQ grating to be fully cured before it is relaxed by the wetting-induced reflow process.

[0074] Figures 5A-5H show comparison of nanogratings formed on the surfaces of the same material with different surface properties: Figures 5A- 5D is a normal PET. Figure 5 A shows a contact angle of 20.1 of SSQ resist on bare PET polymer, while Figures 5B-5C show SEM images of the PET polymer having an SSQ applied thereto, while Figure 5D shows a schematic of the SSQ (including a contact angle) on the deformed PET substrate. Figures 5E-5H are a fluorosilane-treated PET (F-PET). Figure 5E shows a contact angle of 70.1 of SSQ resist on a F-PET, while Figures 5F-5G show SEM images of the deformed F-PET polymer having an SSQ applied thereto, while Figure 5H shows a schematic of the SSQ (including a contact angle) on the deformed F-PET substrate. A significant increase in contact angle is observed in a F-PET surface (Figures 5E and 5H), resulting in the nanograting (processed at 80°C) with a higher aspect ratio, which is attributed to the improved non-wetting characteristic of the substrate surface that mitigates the reflow of the as-formed liquid resist grating. [0075] Temperature-controlled grating heights

[0076] Understanding the flow characteristics of the curable liquid material (in this embodiment, an exemplary SSQ resist material), such as viscosity can be important for determining the channel filling behavior in the inventive processes prior to curing. The viscosity of the liquid SSQ decreases with the increasing temperature from room temperature up to around 80°C, as presented in Figure 7. Liquid SSQ behaves as a Newtonian fluid since the viscosity remains constant and shear stress increases linearly with the sweeping shear rate (see Figure 6). In Figure 7, viscosity of liquid SSQ as a function of temperature is shown, where the SSQ viscosity decreases as the temperature increases up to about 80-90°C. SSQ starts curing after around 90°C, causing increase in viscosity. For a Newtonian liquid such as liquid SSQ, and assuming laminar flow, shear stress is defined as the applied force divided by channel entrance area can be simply written as τ = μ^- , where τ is shear stress, L is the channel width, and μ is the viscosity of the SSQ liquid material The velocity, u, at which a mold channel is filled is proportional to τ (L/μ), where again τ is shear stress, L is the channel width, and μ is the viscosity of the SSQ liquid material. In the inventive processes, this relationship suggests that for the same applied pressure, and given a similar process speed, a grating pattern with a greater depth can be obtained for a lower viscosity SSQ due to a more complete filling of the nanochannels or cavities of the rigid mold by the liquid SSQ, which can be achieved by employing a higher processing temperature (Figure 8). The pattern depth becomes smaller at 100°C than at 80°C, because the heat causes partial curing of the SSQ resist and therefore an increase in SSQ viscosity. In Figure 8, the grating depths measured from the counterprofiles of 200 nm period nanogratings formed on SSQ-coated PFA substrates at different temperatures. The values are averaged over three different positions for each case. A similar trend is observed when using PET as substrates, as shown in Figures 11A-11E.

[0077] Thus, the present teachings provide in various aspects, novel techniques to fabricate continuous high aspect-ratio nanograting structures by using curable liquid materials on a deformable polymeric substrate by a mold- guided pattern formation process with a rigid mold. In certain variations, the methods include the following steps: (1) a slice of grating molds, optionally heated by a heater, contacts a liquid-coated major surface of a deformable polymeric substrate at an angle. In the second step (2), at mold-substrate contact, the liquid layer (e.g., SSQ) infiltrates and substantially fills the nanochannels or cavities defined in a patterned surface of the mold, while the edge of the mold simultaneously inscribes the plastic substrate to form shallow grooves. In the third step (3) as the process proceeds, the liquid SSQ grating is extruded from the nanochannels or cavities. In the fourth step (4), the non- wetting behavior of the liquid resist against the topographically deformed plastic substrate prevents the immediate reflow of the liquid grating pattern, allowing time for it to be fully cured and become solid nanogratings having superior aspect ratios and well defines structures.

[0078] The final geometry of liquid features created from the patterning can be tailored by the processing temperatures, as well as the surface characteristics of the underlying surfaces of the solid polymeric substrates. Such a "direct-write" nanopatterning of liquid resists is a gentler process than conventional DNI and results in much more faithful pattern replication especially for small period and high aspect ratio structures. Thus, the present methods are capable of being employed to mass produce large-area, high-quality nanostructures at low cost. Such grating structures can be utilized in a variety of applications such as metal wire-grid polarizers and plasmonic color filters, by way of non-limiting example. [0079] As noted above, in certain variations, optical waveguide structures can be formed in accordance with certain aspects of the present teachings. Such waveguides can be used in micro- and nano-photonic devices, such as micro-resonators, communication network multiplexers, and optical buffers, require on-chip interconnects using optical waveguides. For example, developing optical waveguides having low propagation loss is particularly desirable. Waveguide loss is the sum of many factors including material absorption, Rayleigh scattering, substrate leakage and sidewall roughness; among which sidewall roughness is a dominant one in the micro-fabricated waveguides. In certain aspects, the processes of the present teachings can create seamless linear line structures or grating structures at high speed in a continuous fashion under two dimensional (2-D) contact with a rigid mold. For example, in certain variations, to fabricate polymer waveguides using the inventive techniques, a well-cleaved waveguide mold (one with trenches or cavities fabricated on a surface of Si) typically heated (to approximately 80°C) and tilted (about 15-45°) slides over a liquid-coated substrate under conformal contact with slight pressure (Figure 12A) to mechanically extrude the rib arrays to form linear structures in a continuous manner.

[0080] Such a process can produce seamless and infinite long waveguide structures have ideally smooth sidewalls without scallops or jags (Figures 12C-12D), regardless of the roughness existing on the original trench mold. This can otherwise significantly reduce the waveguide loss. Also, aided by the use of a liquid resist layer, the inventive processes can enable formation of higher aspect-ratio structures for certain applications. In one embodiment, a curable liquid epoxy-silsesquioxane (SSQ, refractive index of about 1.5) is employed as a waveguide material that is formed on a perfluoroalkoxy (PFA, refractive index of about 1.34) substrate that functions as undercladding for the SSQ waveguide. In certain aspects, a metal material can be applied over at least a portion of the major surface having the at least one microscale feature (e.g., nanochannel) formed thereon in the cured material. The metal material may be applied by at least one process selected from chemical vapor deposition and/or physical vapor deposition.

[0081] Thus, in certain aspects, the present teachings provide processes involving NanoChannel-guided Lithography (NCL) that can produce seamless and infinite long waveguide structures having ideally smooth sidewall regardless of the roughness existing on the original trench mold. In Figure 12A, a cleaved Si trench mold is typically heated (up to about 80°C) and is tilted to make contact to the moldable polymeric substrate, so as to continuously create rib waveguide structures at high speed in the liquid material. In Figure 12B, waveguide arrays fabricated by certain variations of the inventive processes are shown. Enlarged views of the individual waveguides fabricated by a (Figure 12C) DNI process and (Figure 12D) an inventive NCL process, show an ideally smooth sidewall surface, which helps reduce optical loss in waveguide structures. Use of a liquid resist in an NCL provides waveguides with higher aspect-ratios, which further improves quality.

[0082] Examples

[0083] Preparations of molds and substrates

[0084] The patterned (e.g., grating) molds are fabricated by nanoimprint lithography on a thermally grown S1O2 layer on silicon (Si) substrate, as described in Kang, M. G.; et al., "Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes," Advanced Materials, 20(23) (2008), pp. 4408-4413, incorporated herein by reference in its entirety. For certain variations of the inventive process, only a thin slice of well-cleaved SiO2/Si grating mold is needed, because the patterning process is based on line contact with the liquid resist on the polymer substrate. The cleaving direction of the Si mold is perpendicular to the grating pattern. The molds have two different grating periods: 700 and 200 ran, as shown in Figures 2B and 2C, respectively. To prepare the substrates, a sheet of perfluoroalkoxy (PFA) or polyethylene terephthalate (PET) is first cleaned by acetone and isopropyl alcohol (IP A) followed by nitrogen drying, and then is treated by O2 plasma (17 seem, 80 W, 30 s) to remove residual moisture and increase the surface energy by forming -OH surface groups for the ease of resist coating. Fluorine treatment on the PET surface is performed by vapor deposition of a fluorinated surfactant, (tridecafluoro-l,l,2,2-tetrahydrooctyl)trichlorosilane (GELEST, Inc.), for 15 minutes at 90°C. To coat the substrates with UV-curable liquid resist, an epoxy- SSQ

mixed with 3 wt.% photoacid generator (UV-9820, Dow Corning Corp.) is diluted with propylene glycol monomethyl ether acetate (PGMEA) to make a SSQ resist solution containing 10-20 wt% SSQ. The SSQ solution is spin-cast onto the substrate sheet at 500-1,000 rpm. PGMEA is then completely dried to leave a thin layer of SSQ with a thickness ranging from 400 nm - 2 μιη. The spin-casting process can be replaced by a continuous coating method, such as die or micro-gravure coating.

[0085] Nano-Channel Guided Lithography Processing

[0086] A well-cleaved grating mold is mounted to a heater-attached holder, which is inclined at an angle of about 15° with respect to the substrate plane. A substrate is placed on a silicone rubber film attached to a tilting stage. By adjusting the 5-DOF tilting stage (Newport Corp.), the mold edge and the substrate surface are positioned to be parallel along the contact line. The conductive heater attached to the backside of the mold is then turned on to control the process temperature, maintained by the feedback controller (Yokogawa Corp.) throughout the process. Once the temperature is in equilibrium, the edge of a mold makes contact with a substrate under a slight mechanical force (about 5 N, monitored by a flexible force sensor (Tekscan Inc.)). Then, the substrate is transferred at a controlled speed (-0.5-2 cm/s) under a conformal contact with the mold edge, to create NCL nanograting. A UV light source (7.2 W/cm², EXFO Inc.) mounted in front of the mold at 10 cm distance promptly cures the liquid resist extruding from the end of the nanochannels on the mold, to complete the fabrication of nanograting structures with well- retained profile.

[0087] Characterizations

[0088] SEM imaging is performed using a Philips XL30-FEG, operating at 20-30 kV, after sputtering a thin Au film (approximately 2-3 nm) to avoid electron charging. To prepare the cross sections, because of the difficulty to cleanly cleave the grating samples fabricated on flexible polymers (e.g., PFA or PET), they are stamped to cleaned Si substrates using epoxysilicone as a resist, followed by UV curing and cleaving. To ensure good adhesion between epoxysilicone and Si surfaces during demolding, the Si substrates are cleaned with acetone and IPA and dried by nitrogen blow are preheated with O2 plasma (17 seem, 150 W, 120 s) and then treated with epoxy-adhesion promoters (Silquest A-187, Momentive Performance Materials). Thus, all cross-sectional views in the article reveal the counterprofiles of the patterned resist gratings. The viscosity and shear stress of liquid SSQ are measured using an ARES Rheometer (TA Instruments), monitored and recorded by TA Orchestrator.

[0089] To determine both the viscosity and shear stress at different shear rates of the liquid SSQ material, a shear rate sweep is performed at room temperature by increasing the shear rate of a pair of stainless steel disk plates (1 inch diameter, 0.4 mm gap) uniformly filled with liquid SSQ. Viscosity as a function of temperature is then measured by sweeping the temperature at the fixed shear rate of 1 rad/s under identical configurations. The static contact angles of liquid SSQ are measured by a home-made contact angle analyzer with real-time imaging by gently placing an SSQ droplet (about 6 μΐ) on the targeted surface. The values presented are averaged over at least three measurements in each case. The results are shown in Figures 4A-4J and 5A-5H.

[0090] In certain aspects, methods of forming at least one microscale feature on a substrate are provided. The methods optionally comprise contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate. The patterned surface of the rigid mold defines a cavity defining at least one microscale feature. The curable liquid material thus flows into the cavity of the rigid mold. Further, the major surface is at least partially deformed during the contacting. In certain aspects, a contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method further comprises curing the curable liquid material after the contacting to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate. In certain variations, the curing begins within 500 milliseconds after contacting with the rigid mold is ceased. In certain variations, the curing occurs by applying thermal energy and/or actinic radiation energy. In certain embodiments, the curable liquid material has a viscosity of greater than or equal to about 1 Pa-s to less than or equal to about 100 Pa-s during the contacting. In certain aspects, the rigid mold is angled with respect to the major surface of the moldable polymeric substrate, so that an angle formed between the rigid mold and the major surface is about 5° to about 50°.

[0091] In certain variations, the methods further comprise applying a metal material over at least a portion of the major surface having the at least one microscale feature formed thereon. The metal material may be applied by at least one process selected from chemical vapor deposition and/or physical vapor deposition. In certain aspects, the major surface of the moldable polymeric substrate has a surface energy of less than or equal to about 50 mN/m. In certain embodiments, the surface energy is greater than or equal to about 10 mN/m and less than or equal to about 50 mN/m. In yet other variations, the major surface of the moldable polymeric substrate is pretreated prior to applying the curable liquid material to increase the contact angle therebetween. The moldable polymeric substrate may optionally comprise fluorine-treated polyethylene terephthalate or perfluoroalkoxy (PFA). In certain aspects, the curable liquid material comprises epoxysilicone, epoxy precursor (SU-8), polydimethylsiloxane, PDMS, or thermal or photocurable silsesquioxane. In certain embodiments, the curable liquid material comprises an ultraviolet light- curable epoxy-silsesquioxane (SSQ).

[0092] In certain variations, the methods further comprise applying the curable liquid material prior to the contacting, wherein the applying is by spin casting, ink jetting, spraying, and/or by gravure application methods. The methods may optionally comprise a continuous process of applying, contacting, and curing steps, which are accomplished by passing the moldable polymeric substrate through an application module, a contacting module, and a curing module.

[0093] In yet other aspects, the present disclosure provides a method for continuously patterning at least one microscale feature. In certain variations, the method comprises continuously imprinting a major surface of a moldable polymeric substrate having a curable liquid material disposed thereon by applying pressure to the major surface by contact with a stationary rigid mold. The stationary rigid mold has a patterned surface that defines a cavity to form at least one microscale feature, so that the curable liquid material flows into the cavity and the major surface is at least partially deformed during the applying of pressure. An advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method further comprises curing the curable liquid material after the contacting to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

[0094] In such methods, the moldable polymeric substrate may be disposed on a moving carrier that retains the substrate during the continuous imprinting with the stationary rigid mold. A metal material may be applied over at least a portion of the major surface having the at least one microscale feature formed thereon prior to the continuous imprinting. In certain aspects, the moldable polymeric substrate comprises perfluoroalkoxy and the curable liquid material comprises ultraviolet light-curable epoxy-silsesquioxane (SSQ).

[0095] In yet other aspects, a method of forming at least one nanoscale feature on a substrate comprises applying a curable liquid material comprising ultraviolet radiation-curable epoxy-silsesquioxane (SSQ) to a major surface of a moldable polymeric substrate having a surface energy of greater than or equal to about 10 mN/m and less than or equal to about 50 mN/m. An advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The major surface having the curable liquid material disposed thereon continuously passes into contact with a heated rigid mold having a patterned surface that defines a cavity to form at least one nanoscale feature. In this manner, the curable liquid material flows into the cavity during the contact and the major surface is at least partially deformed by the patterned surface.

[0096] The method further comprises curing the curable liquid material with ultraviolet radiation after the contact to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate. In certain variations, the methods further comprise applying a metal material over at least a portion of the major surface having the at least one microscale feature formed thereon. The metal material may be applied by at least one process selected from chemical vapor deposition and/or physical vapor deposition. Any of the variations described above may be employed with these methods of forming at least one nanoscale feature on a polymeric substrate.

[0097] The present disclosure further provides methods for forming a waveguide structure. Such methods may comprise contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate. The patterned surface defines a plurality of nanochannels, so that the curable liquid material flows into the plurality of nanochannels and the major surface is at least partially deformed during the contacting. A contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°. The method further comprises curing the curable liquid material after the contacting to form a cured polymer having at least one nanoscale feature so as to define a waveguide structure over the moldable polymeric substrate. In certain variations, the methods further comprise applying a metal material over at least a portion of the major surface having the at least one microscale feature formed thereon to form the waveguide structure. Any of the other variants described above are contemplated for use in conjunction with the present methods of forming waveguide structures.

[0098] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

CLAIMS What is claimed is:

1. A method of forming at least one microscale feature on a substrate, the method comprising:

contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate, wherein the patterned surface defines a cavity defining at least one microscale feature, so that the curable liquid material flows into the cavity and the major surface is at least partially deformed during the contacting, wherein a contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°; and

curing the curable liquid material after the contacting to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

2. The method of claim 1, wherein the curing begins within 500 milliseconds after contacting with the rigid mold is ceased.

3. The method of claim 1, wherein the curing occurs by applying thermal energy and/or actinic radiation energy.

4. The method of claim 1, wherein the curable liquid material has a viscosity of greater than or equal to about 1 Pa-s to less than or equal to about 100 Pa-s during the contacting.

5. The method of claim 1, wherein the rigid mold is angled with respect to the major surface of the moldable polymeric substrate, so that an angle formed between the rigid mold and the major surface is about 5° to about 50°.

6. The method of claim 1, further comprising applying a metal material over at least a portion of the major surface having the at least one microscale feature formed thereon.

7. The method of claim 6, wherein the metal material is applied by at least one process selected from chemical vapor deposition and/or physical vapor deposition.

8. The method of claim 1, wherein the major surface of the moldable polymeric substrate has a surface energy of less than or equal to about 50 mN/m.

9. The method of claim 1, wherein the surface energy is greater than or equal to about 10 mN/m and less than or equal to about 50 mN/m.

10. The method of claim 1 wherein the major surface of the moldable polymeric substrate is pretreated prior to applying the curable liquid material to increase the contact angle therebetween.

11. The method of claim 1, wherein the moldable polymeric substrate comprises fluorine-treated polyethylene terephthalate or perfluoroalkoxy (PFA).

12. The method of claim 1 wherein the curable liquid material comprises epoxysilicone, epoxy precursor (SU-8), polydimethylsiloxane, PDMS, or thermal or photocurable silsesquioxane.

13. The method of claim 1, wherein the curable liquid material comprises an ultraviolet light-curable epoxy-silsesquioxane (SSQ).

14. The method of claim 1, further comprising applying the curable liquid material prior to the contacting, wherein the applying is by spin casting, ink jetting, spraying, and/or by gravure application methods.

15. The method of claim 14, which comprises a continuous process of applying, contacting, and curing steps, which are accomplished by passing the moldable polymeric substrate through an application module, a contacting module, and a curing module.

16. A method for continuously patterning at least one microscale feature, the method comprising:

continuously imprinting a major surface of a moldable polymeric substrate having a curable liquid material disposed thereon by applying pressure to the major surface by contacting a stationary rigid mold, wherein the stationary rigid mold has a patterned surface that defines a cavity to form at least one microscale feature, so that the curable liquid material flows into the cavity and the major surface is at least partially deformed during the applying of pressure, wherein an advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°; and

17. The method of claim 16, wherein the moldable polymeric substrate is disposed on a moving carrier that retains the substrate during the continuous imprinting with the stationary rigid mold.

18. The method of claim 16, further comprising applying a metal material over at least a portion of the major surface having the at least one microscale feature formed thereon prior to the continuous imprinting.

19. The method of claim 16, wherein the moldable polymeric substrate comprises perfluoroalkoxy and the curable liquid material comprises ultraviolet light-curable epoxy-silsesquioxane (SSQ).

20. A method of forming at least one nanoscale feature on a substrate, the method comprising:

applying a curable liquid material comprising ultraviolet radiation- curable epoxy-silsesquioxane (SSQ) to a major surface of a moldable polymeric substrate having a surface energy of greater than or equal to about 10 mN/m and less than or equal to about 50 mN/m, wherein an advancing contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°;

continuously passing the major surface having the curable liquid material disposed thereon into contact with a heated rigid mold having a patterned surface that defines a cavity to form at least one nanoscale feature, wherein the curable liquid material flows into the cavity during the contact and the major surface is at least partially deformed by the patterned surface; and

curing the curable liquid material with ultraviolet radiation after the contact to form a cured polymer having the at least one microscale feature over the moldable polymeric substrate.

21. A method of forming a waveguide structure comprising:

contacting a patterned surface of a rigid mold with a curable liquid material disposed on a major surface of a moldable polymeric substrate, wherein the patterned surface defines a plurality of nanochannels, so that the curable liquid material flows into the plurality of nanochannels and the major surface is at least partially deformed during the contacting, wherein a contact angle between the curable liquid material and the major surface of the moldable polymeric substrate is greater than or equal to about 65°; and

curing the curable liquid material after the contacting to form a cured polymer having at least one nanoscale feature so as to define a waveguide structure over the moldable polymeric substrate.