Patent Application
20220244635
The invention relates to the field of micro-imprinting. More specifically, the invention relates to a method for imprinting micropatterns on a flat or curved chalcogenide glass body.
A chalcogenide glass contains one or more chalcogens, such as sulfur, selenium and tellurium, but not oxygen. Chalcogenide glasses are covalently bonded materials, and may be classified as covalent network solids.
The chalcogenide glasses are attractive materials for many infrared optical applications, thanks to their high transmittance and refractive index, which can be tuned by varying the glass composition. Furthermore, its pronounced 3rd order non-linearity makes it an appealing material for all-optical switching devices. The photon-induced transition between the amorphous and crystalline phases of the chalcogenide glass also makes it a promising material for memory devices and tunable photonics.
The realization of optical devices that are based on chalcogenide glasses is often technologically challenging. For example, sub-wavelength light-manipulating structures, such as diffraction gratings or waveguides, are commonly found in many optical devices. However, the formation of such structures on chalcogenide glass substrates requires fabrication approaches that are different from those applicable in conventional optical materials, such as silicon-based glasses.
Chalcogenide glasses naturally reflect up to 30% of mid-infrared light. Thus, high-end optical components (such as lenses or windows) made of these materials usually require an antireflective coating. Traditional thin-film-based antireflective coatings require use of expensive vacuum deposition technology which is hardly applicable to chalcogenide glasses due to a lack of materials with appropriate refractive index, pure adhesion of deposited films, and a mechanical stress these films generate. This type of coating often results in cracking and delamination, especially under harsh environmental conditions. An emerging alternative to thin-film based antireflective coating are sub-wavelength micro or nano-structures (hereinafter, both referred to as “micro-structures”). The micro-structures produce a highly omnidirectional and broadband antireflective effect. These micro-structures can also provide a surface with super-hydrophobicity and self-cleaning properties, that are commonly referred to as the “lotus leaf effect”. Anti-reflective and self-clearing micro-structures have been successfully demonstrated on other commonly used materials, such as silicon or glass.
The surface of chalcogenide glasses can be directly patterned by means of an electron beam or by laser writing. However, these techniques are serial, provide low-throughput patterning, and are unsuitable for scalable fabrication. Chalcogenide glasses can be thermally imprinted with a soft elastomeric stamp.
Nanoimprint is a technique which is widely used for shaping in a nano-scale (or micro-scale) surfaces of bodies, such as, optical components, electronic devices, photonic nanostructures, etc. Soft nanoimprinting is a versatile, high-throughput, and cost-effective nanolithography technique in which a nanoscale pattern is mechanically transferred onto a resist by an elastomeric mold. In view of the mechanical flexibility of soft molds, soft imprint can produce high-resolution nanostructures in UV curable polymer films deposited on substrates with unconventional geometry, such as lenses and optical fibers. However, soft imprinting has been demonstrated so far only on thin films of chalcogenide glasses, that were in turn deposited on solid substrates such as silicon. Direct surface imprint of bulk chalcogenide glasses has still been fundamentally challenging: applying high imprint pressure and temperature necessarily deforms the imprinted substrate, whereas imprinting with a reduced pressure and temperature results in incomplete pattern transfer. Moreover, many important applications of chalcogenide glasses, such as lenses, require micro-patterning of curved optical surfaces, for example, to provide antireflection, and superhydrophobic layer. However, a scalable pattering of curved surfaces of chalcogenide glasses has not been demonstrated yet.
It is an object of the present invention to provide a method for direct surface patterning of micro-structures on a chalcogenide surface.
It is another object of the invention to provide a soft nanoimprint technique for patterning chalcogenide bodies.
It is still another object of the invention to provide a soft nanoimprint technique for patterning chalcogenide bodies, which is simple, scalable, and which can be applied in a high-throughput manner.
It is still another object of the invention to provide a soft nanoimprint technique for patterning chalcogenide bodies, having curved or flat surfaces.
It is still another object of the invention to provide a soft nanoimprint technique for patterning surfaces of chalcogenide bodies, in order to obtain non-reflective and super-hydrophobic surfaces.
Other objects and advantages of the invention will become apparent as the description proceeds.
In a first embodiment, the invention relates to a method for nanoimprinting a pattern on a chalcogenide-glass substrate, comprising: (A) preparing a soft operational mold, the operational mold comprising an elastomeric matrix and a reinforcement, wherein the matrix is transparent to IR radiation, and the reinforcement is opaque to IR radiation, and the mold further includes a pattern to be replicated to the substrate; (B) placing the mold on a top surface of a chalcogenide-glass substrate to form a structure, and simultaneously applying (i) IR radiation to heat an area at a top surface of the substrate to a temperature T>Tg, where Tg is the glass transition temperature of chalcogenide-glass, and (ii) applying a controlled pressure on the mold to effect penetration to the top surface of the chalcogenide-glass substrate, thereby to replicate the pattern of the mold to the top surface of the substrate; and (C) separating the operational mold from the patterned substrate.
In a second embodiment, the invention relates to a method for nanoimprinting a pattern on a chalcogenide-glass substrate, comprising: (A) providing said chalcogenide-glass substrate; (B) creating on a top surface of the chalcogenide-glass substrate a layer of softened chalcogenide-glass, said softened layer having a glass transition temperature Tsg which is lower than a respective glass transition temperature Tg of the rest of the substrate; (c) placing a soft operational mold which includes a patter on the top surface of the chalcogenide-glass substrate to form a structure, and simultaneously (i) heating the structure to a temperature Tsg<T<Tg, where Tg is the glass transition temperature of chalcogenide-glass, and (ii) applying a controlled pressure on the mold to effect penetration to the top surface of the chalcogenide-glass substrate, thereby to replicate the pattern of the mold within said softened layer; and (D) separating the operational mold from the patterned substrate.
In a third embodiment, the invention relates to a method for nanoimprinting a pattern on a chalcogenide-glass substrate, comprising: (A) preparing a soft operational mold, the operational mold comprising a pattern to be replicated to the substrate; (B) soaking the operational mold in a solvent to produce diffusion of solvent to the mold; (C) removing the operational mold from the solvent, and placing it on a top surface of the chalcogenide-glass substrate to form a structure, and simultaneously (i) heating the structure to a temperature Tsg<T<Tg, where Tg is the glass transition temperature of chalcogenide-glass, and Tsg is a glass transition temperature of the top surface of the substrate, which results to be lower than Tg due to diffusion with the solvent in the mold, and (ii) applying a controlled pressure on the mold to effect penetration to the top surface of the chalcogenide-glass substrate, thereby to replicate the pattern of the mold to the top surface of the substrate; and (D) separating the operational mold from the patterned substrate.
In the drawings:
a shows 3D and z-section AFM of the PDMS stamp and of the imprinted As2Se3 moth eye structure, for both tested geometries of the diffraction grating;
The invention provides three techniques for a soft nano-imprinting on a chalcogenide-glass surface of a body, which is either flat or curved. The chalcogenide-glass is characterized by both having a melting temperature (in the order of 350°), substantially lower compared to a typical silica glass, and by having a low glass-transition temperature Tg—in the order of 165°-180° (163° C. for GeSe4 or 185° for As2S3). The chalcogenide-glass is also highly transparent to infra-red radiation, making it a very attractive material for components of optical devices.
To facilitate an effective mold heating by radiation, the inventors produced the operational mold from a composite material of polydimethylsiloxane (PDMS) with multi-wall carbon-nanotubes. The operational mold was prepared by casting the PDMS-nanotube mixture onto a photolithographically fabricated master mold. The operational mold was used to directly imprint a surface of As2Se3—a chalcogenide glass body, whose glass transition temperature is about 185° C. The operational mold and the chalcogenide-glass substrate were sandwiched between two transparent membranes, and then heated by infrared radiative source, while simultaneously they were pneumatically pressurized. This imprint technique, together with appropriately chosen process conditions, ensured that only a thin layer at the mold-glass interface was sufficiently heated above the As2Se3 glass transition point. As a result of this highly localized heating, a viscous flow of As2Se3 was developed at the interface between the mold and the surface of the body, whereas the rest of the glass substrate was not deformed during the imprint. A full pattern transfer from the mold to the surface of chalcogenide glass was obtained, while maintaining the glass substrate undistorted at all. The inventors verified that the composition and structure of the chalcogenide glass body were both maintained throughout the imprint process. The inventors performed a series of surface analysis tests of the imprinted product, including Raman Spectroscopy, Energy-dispersive X-ray Spectroscopy (EDS), X-Ray Photoelectron Spectroscopy (XPS), and X-Ray Diffraction. During imprint on a flat substrate, the inventors found that a complete flatness of the imprinted substrate can be assured by use of a mechanical support to its back side. The inventors produced a 2D diffraction grating, and characterized it in both reflection and transmission modes.
The inventors also successfully demonstrated an imprint of chalcogenide-glass bodies having a non-planar geometry. The inventors successfully produced a diffraction grating on a spherical lens of As2Se3.
The pattern that was used to demonstrate the invention consisted of a 2D diffraction grating with a periodicity of 10 μm. To fabricate soft molds with this pattern, a master mold was prepared using a photolithography technique. The inventors patterned a film of photoresist on a Si substrate, and used it directly as a 3D master mold. The inventors patterned a photoresist film whose thickness was 1.6 microns, to obtain relief features at the operational mold with 1.6 μm height.
An important component of a radiative imprint process is the radiative source. Radiative heating in a nanoimprint process requires that either the operational mold or the imprinted substrate would absorb the radiation. Notably, both As2Se3 (as most chalcogenide glasses) and PDMS—the material of choice for the soft imprint mold—are transparent to the wavelength range (IR) of the radiative heating source used by nanoimprint equipment. To address this constraint, and to allow effective heat absorption of the soft operative mold, the PDMS mold was reinforced with multiwall carbon-nanotubes. Carbon-nanotubes are ideal adsorbing-medium candidates, for several reasons: (i) their radius is a few-orders of magnitude smaller than the relief features of the master mold, so they can easily fill these features without distorting the produced pattern; (ii) carbon-nanotubes are mechanically flexible; and, (iii) they effectively absorb light in the visible and near IR spectra.
A mixture of PDMS and multiwall carbon-nanotubes was directly casted onto a master mold in the form a silicon wafer with patterned photoresist. Then, the master mold was baked. When cooled down, the operational mold was formed, and was mechanically peeled off the master mold. The dimensions of the relief features on the obtained operational mold precisely replicated the pattern at the master mold.
A remarkable advantage of the radiative heating in the imprint process of the first technique is that it allows fast heating
to the desired temperature, usually within a few seconds. In the setup used by the inventors, the heating source faced the back side of the operational mold. Since carbon nanotube-PDMS composite is an effective thermal conduction, it was assumed that the AS2Se3 surface reached the imprint temperature immediately at the beginning of the heating. The total imprint time was kept equal to 4 minutes. This imprint time was found to be sufficient to achieve full pattern transfer, but short enough to prevent deformation of the bulk of the As2Se3 substrate. Such a tight control over the imprint period enabled maintaining the original shape and dimensions of the As2Se3 substrate. The result of the chalcogenide-glass imprint is shown in
To quantitatively assess the possible impact of imprint on the global shape of the As2Se3 substrate, the inventors characterized its flatness by profilometry and 2D laser scanning. It was found that the As2Se3 developed a bow of about 150 microns, which was due to the creep that As2Se3 undergrows at the temperature and pressure used by the process. To prevent this creep, the inventors attached a flat (flatness <1 μm) BK7 glass substrate to the backside of the As2Se3 substrate (
Maintaining the structure and composition of chalcogenide glasses during their imprint is very significant for their optical applications. It is known, for example, that chalcogenide glasses (such as As2S3) crystalize upon their imprint. As for AsxSe1-x glasses, their bulk-nucleation and crystallization that occurs during the thermal cycles was fundamentally investigated, and it was found to depend on the As content and the impurities present in the glass. The crystallization of a nanoimprinted chalcogenide glass is highly undesirable for optical applications, because of the high scattering loss caused by the crystalline domains. To assess whether the imprint process caused any crystallization of As2Se3, the inventors characterized the imprinted surface by X-ray Electron Diffraction. The measured spectrum showed a broad-peaks characteristic of a completely amorphous structure of As2Se3, and clearly demonstrated that the nanoimprint process of the invention did not cause crystallization.
The inventors used additional characterization techniques to assess any possible effect of the nanoimprint process on the structure, and on the composition of the As2Se3 final product. Those techniques have confirmed that no crystallization took place on the imprinted final product.
The inventors also characterized bare and imprinted As2Se3 surfaces by X-ray Photoelectron Spectroscopy (XPS), and found that in both cases the surface contained As and Se in stochiometric ratio (2:3). The inventors also found a certain amount of oxygen. An XPS analyses at varying depths using Ar sputtering revealed that both in the bare As2Se3 substrate and in the imprinted substrate, oxygen was present only down to about 20 nm depth. Since the binding energies of As and Se peaks did not vary with the sampling depth, it was concluded that the oxygen signals were originated from contaminations rather than from oxidized As and Se. Finally, a presence of silicon was seen on the imprinted As2Se3 surface. The binding energy of Si was found to be 103 eV, which corresponds to a known measured value for Si2p in PDMS. It was thus concluded that both the observed O and the Si signals were originated from a minor contamination caused by the contact with PDMS during the imprint process. It was also confirmed that Si contamination is present only on the surface and not deeper in the bulk of As2Se2, based on EDS of bare and imprinted substrates.
To demonstrate the applicability of the imprint process in the fabrication of optical devices and components, the inventors characterized the imprinted diffraction grating in two modes, reflective and transmit. Since As2Se3 is reflective in the visible region, the inventors used a HeNe laser (632.8 nm) as a light source for characterizing the reflective diffraction. The characterization setup consisted of a HeNe laser, whose beam passed through two apertures, a standard optical aperture was used to reduce the beam diameter, and another aperture was used within a black board. The board, in turn, was used to visualize a 2D diffraction pattern reflected form the imprinted As2Se3. The sample tilt and rotation were aligned to ensure that the beam of the 0-order diffraction returned exactly into the aperture in the board. By measuring the distances between the laser spots in the obtained diffraction pattern, it was concluded that the diffraction angles are in a good agreement with the theoretical angles that were calculated from the relation between the diffraction angle and the grating geometry: d sin θ=nλ, (n=0, ±1, ±2 . . . ). The good agreement between the calculated diffraction-angles and the diffraction-angles measured in both x and y directions confirmed that the grating geometry was faithfully reproduced from the master mold to the imprinted surface. Such a high pattern fidelity indicates that the used technique holds a significant potential for the fabrication of precision-optics-components based on chalcogenide glasses.
The inventors also demonstrated the applicability of the nanoimprint process of the invention to patterning of non-planar optical surfaces of chalcogenide glasses geometry. The inventors have successfully produced a diffraction grating on a lens of As2Se3, with a diameter of 50 mm with a radius of curvature of 43 mm.
The production of the PDMS-nanotube composite mold (the operational mold): Multiwall Carbon-nanotubes (Cheep Tubes Inc.) were first dispersed in toluene using a probe sonicator. Simultaneously, PDMS (Sylgard 184, Dow Corning) was diluted in toluene (2:1) and was placed in an ultrasonic bath for 1 hour. The two solutions were mixed and sonicated in a probe sonicator for 1 hour. The mixture was then placed in a rotary evaporator to cause evaporation of the toluene from the solution. Finally, a curing agent was added to the PDMS-MWCNT solution and manually mixed for 10 minutes. The solution was then casted onto a master mold, degassed and baked.
The nanoimprint procedure: 2.5 cm circular substrates of As2Se3 were imprinted in a commercial nanoimprint tool (Nanonex NX-B200). The mold was placed on the bottom, facing the radiative source. The imprint temperature was 220° C. (which was monitored throughout the imprint process by a thermocouple, touching the membrane on the mold side). The imprint pressure was 50 psi, and the imprint time 4 minutes. The convex lens was imprinted using the same conditions as was used with the flat substrates.
Characterization of imprinted As2Se3: The flatness of the bare and imprinted substrates was measured by profilometry (Veeco Dektak 8), and laser profiler OLS5000. XRD was measured by use of Rigaku, D/max-2100, Cu(kα), 40 keV, 30 mA. Raman Spectroscopy was measured using Horiba LabRam HR evolution micro-Raman system, equipped with a Synapse Open Electrode CCD detector air-cooled to −60° C. The excitation source was a 532 nm laser with power on the sample of 0.05 mW. The laser was focused with an ×50 objective to a spot of about 2 μm. The measurements were taken with a 600 gmm−1 grating and a 100 μm confocal microscope hole. Typical exposure time was 180 sec. XPS data were collected using an X-ray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1*10−9 bar) apparatus with an AlKα X-ray source and a monochromator. The X-ray beam size was 500 μm and survey spectra were recorded with pass energy (PE) 150 eV and high energy resolution spectra were recorded with a pass energy (PE) of 20 eV. To correct for charging effects, all spectra were calibrated relative to a carbon C is peak positioned at 284.8 eV. Processing of the XPS results was carried out using AVANTGE program.
To verify the feasibility of the second embodiment, the inventors plasticized As2Se3. The inventors systematically studied the impact of the annealing conditions on the Tg of As2Se surface layer formed from solution, and found that the Tg can be controllably lowered by almost 40° C. when compared to that of bulk chalcogenide-glass, without any substantial change to glass structure, composition, and optical properties. By a serial of chemical analyses, the inventors found that the controlled reduction of Tg scales with the amount of residual solvent, and concluded that the solvent functions as a plasticizing agent that facilitates the thermoforming of the glass, similarly to commercial plasticizers in organic polymers. The inventors harnessed this controlled plasticizing format to a surface imprint of As2Se, with nanoscale features sized down to 20 nm, and applied this imprint approach to the fabrication of several functional microstructures including diffraction gratings and moth-eye antireflective coating for mid-infrared spectrum. The imprinted antireflective microstructure of the invention produced superhydrophobic effect—the first of its type on a surface of chalcogenide glass. The superhydrophobic effect was characterized by use of a Cassie-Baxter mechanism. The nanoimprint approach of the invention opens a route for a scalable-nanoscale surface patterning of chalcogenide glasses, and their numerous applications.
To produce As2Se3 substrates, the inventors mixed As and Se within a quartz ampoule, fused the mixture in vacuum, quenched in air, and molded the obtained glass to form discs of 25 mm in diameter and 2 mm in thickness. To form surface layers of plasticized As2Se3, either on Si substrates or on As2Se3 substrates, the inventors first grinded As2Se3 to obtain a fine powder and dissolved the powder in ethylene diamine (EDA). The inventors then applied the obtained solution onto either Si or As2Se3 substrates by spin-coating and prebaked the formed film for 2 hours at 80° C. The thickness of the obtained film ranged from 1 to 3 microns, depending on As2Se3 concentration and the spinning parameters. The prebaked films were then annealed during a period of 7 hours at different temperatures, to controllably evacuate the excess EDA from the films. All the steps were performed in an inert atmosphere inside a glove-box, to prevent oxidation of As2Se3 and formation of crystalline defects at its surface. Before the direct imprint of As2Se3 films on As2Se3 substrates, the inventors optimized the imprint parameters at which the film could be softened by heating above its Tsg. This was done while keeping the bulk As2Se3 substrate below its own Tg to prevent its deformation. The inventors found that the Tsg of the plasticized film depends of the amount of the residual solvent, and thus can be precisely tuned by the annealing conditions. To verify this, the inventors spin coated As2Se3 films on silicon substrates, annealed them at different temperatures, and measured their Tsg by nanoindentation. The inventors placed the substrates with the films on a nano-indenter stage with a controlled heating, and measured the indentation depth using a constant force rate of 1 mN/s until the force reached 10 mN, held the indenter at this force for 5 s, and unloaded the indenter with a constant unloading rate of 1 mN/s. The inventors repeated the measurements at different temperatures for each sample and assessed the Tsg in each case, based on the temperature at which the indentation depth increased abruptly. The inventors obtained a general trend by which Tsg gradually increases with the annealing temperature. The inventors obtained similar Tsg values for the annealing temperatures in the range of 140° C.-160° C., while the inventors believe that there are minor differences between these values. The highest Tsg (150° C.) was obtained for a film that was annealed at 170° C. This Tsg is, however, still lower than the Tg of a bulk As2Se3, which is typically about 185° C. The inventors believe that a higher Tsg of a solution deposited As2Se3 films can be obtained by annealing at a higher temperature and for a longer time, which will cause further removal of EDA and densification of As2Se3. Yet, in the context of a lithographic imprint of a plasticized As2Se3 film, it is important to keep the Tsg of the film below that of the substrate, thereby to enable thermal imprint of the film without deformation of the substrate. Based on the obtained data, a As2Se3 film with no annealing whose Tsg is 135° C., can be imprinted at around 150° C., which completely addresses the requirements of the invention. All the obtained films were very uniform and with no visible defects, most probably due to the fact that they were annealed in an inert atmosphere that prevented oxidation and crystallization of the As2Se3. These results provide a process window to yield high-quality plasticized As2Se3 films with precisely tuned Tsg, which, in turn, opens a route for a soft direct imprint of bulk As2Se3.
The performance of a functional structure imprinted on the surface of a chalcogenide-glass substrate depends not only on the shape of the structure, but also on the composition and properties of the imprinted material itself. To ensure that plasticized As2Se3 films have a composition and optical properties close to those of pristine As2Se3, the inventors performed a series of chemical, structural and optical characterizations. The inventors verified the absence of macroscopic crystallites by use of an optical microscope Then, the inventors performed a more detailed morphological study using X-ray Diffraction (XRD).
As discussed above, the inventors have demonstrated the precise tuning of the Tsg of plasticized chalcogenide glass films, while keeping their composition and optical properties similar to that of pristine chalcogenide glass. This enabled the production of direct and maskless surface patterning with functional microstructures via soft imprinting. As an example of such fabrication, the inventors imprinted a diffraction grating onto a plasticized surface of As2Se3 substrate. For this purpose, the inventors first produced a master mold by photolithography on a Si substrate followed by plasma etching and resist removal. The inventors then replicated the etched structures into hybrid hard-soft PDMS stamp, and used it to imprint a plasticized As3Se3 film deposited from solution onto a bulk As2Se3 substrate and baked at 80° C. for two hours in nitrogen atmosphere, with no further annealing. The inventors then imprinted As3Se3 using a custom-made imprinting tool, which is based on conductive heating of the imprinted substrate and anisotropic pneumatic pressure applied onto the attached soft stamps through a flexible membrane. The inventors used the following imprint parameters: pressure of 4 bar, time of 20 min, and temperature of 155° C. The value of the imprinting temperature was deliberately chosen between the Tsg of non-annealed As2Se3 film, previously found to be 135° C., and the Tg of the bulk As2Se3 that was equal to 185° C. The inventors imprinted two diffraction gratings with periodicities of 10 μm and 20 μm.
So far, the inventors demonstrated a direct imprint of a chalcogenide glass with features sized in the micron scale. However, imprinting of much smaller features, sized down to the sub-micron scale, is often required for some optical applications, such as high-performance wave-guides for near IR. To further explore the resolution that can be obtained by the nanoimprint approach of the invention, the inventors produced a master mold with a series of patterns of sub-100 nm feature size using electron-beam lithography. The inventors then replicated a soft stamp from this master mold and used it for direct thermal imprinting of As2Se3.
An important application of direct imprinting is the fabrication of antireflective microstructures. The inventors produced antireflective structures of periodic bumps with a periodicity of 2 μm, a duty cycle of 0.75, and a height of 1.4 μm, to provide an optimal reduction in surface reflection for a wavelength range of 8-13 μm. For this purpose, the inventors first produced a master mold by self-assembly of 2 μm polystyrene microspheres on a silicon substrate, followed by trimming of the microspheres in oxygen plasma, and etching the underlying Si through the mask formed by the microspheres. The diameter of the microspheres defined the periodicity of the moth-eye structure, and a trimming time was used to control the duty cycle. The inventors then replicated a PDMS stamp from the Si master mold and used it to imprint an As2Se3 substrate coated with a plasticized As2Se3 film, in a same manner as described above.
Besides the attractive antireflective properties, micro-structured surfaces possess fascinating superhydrophobic properties, and are often termed as “lotus leave effect”. This effect is particularly important for optical applications due to its self-cleaning potential: microstructures that repel water prevent surface contamination, and thus contribute to the long-term reliability and high performance of optical components. For this reason, patterned microstructures have often been produced for two purposes—antireflection and self-cleaning. However, superhydrophobic microstructures on chalcogenide glasses have not been demonstrated up to date. The inventors have used the directly imprinted moth-eye microstructures described above as a superhydrophobic coating on As2Se3. The inventors characterized the wetting properties of imprinted chalcogenide glass by measuring advancing contact angle (θ) of water-ethanol mixtures at different ratios, and compared these to the angles on pristine flat As2Se3. Interestingly, for most of the water-ethanol ratios, the advancing contact angle on the patterned surface was only slightly higher than that on the flat surface. However, the contact angle of pure water on the micropatterned surface was 150°, compared to 95° on the bare surface, indicating a pronounced superhydrophobic behavior of the imprinted moth-eye pattern. The results are shown in
Preparation of plasticized As2Se3 layer: Bulk As2Se3 chalcogenide glass was grinded into a powder and mixed with EDA in a 2:3 mass ratio. The mixture was stirred at 80° c. for 12 hours until complete dissolution. After transfer to a glove box with Nitrogen atmosphere. The solution was spin coated either on Si or As2Se3 substrates, followed by soft baking at 80° c. for 2 hours. Spinning at 1000 rpm for 15 s produced a film thickness of about 2.5 μm.
Compositional, structural and mechanical characterizations of plasticized As2Se3 layer: XRD spectra were measured using Rigaku Spectrometer, D/max-2100, Cu(kα) source, Pass energy of 40 keV. XPS data were collected using an X-ray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1×10−9 bar) apparatus with an Al (Kα) X-ray source and a monochromator. The X-ray beam size was 500 μm and survey spectra were recorded with pass energy (PE) 150 eV and high energy resolution spectra were recorded with a pass energy (PE) 20 eV. To correct for charging effects, all spectra were calibrated relative to a carbon C is peak positioned at 284.8 eV. Processing of the XPS results was carried out using AVANTAGE software. EDS measurements were performed using a scanning electron microscope fitted with EDS detector with 15 kV for 1 μm depth. A nano-indenter (MFP, Asylum Research) was used for Tg measurement of the annealed thin layers. For each measurement, the samples were heated up by 10° C. steps, from 130° C. to 170° C., and the indentation was carried out under a constant load of 10 mN for 5 sec. Force Indentation curves were plotted for each measurement.
Optical measurement of As2Se3 plasticized layers: Refractive index of the As2Se3 plasticized layers was measured using a Woollam IR VASE spectroscopic ellipsometer. Data were collected in the 2-40 μm range. The fitting was performed using the WVASE software. The As2Se3 film was assumed to be isotropic, and a non-absorbing Cauchy model was fitted in the 2-13 μm range. The reflection measurements of the As2Se3 films were obtained using a Perkin Elmer Frontier optics FTIR spectrometer using a 8° reflection accessory and a Ge wedge for reference. Reflection spectra of the antireflection subwavelength structures were simulated used the OPTILAYER thin films software.
Fabrication of imprint stamps: First, masters for the stamp cast were prepared. For the diffraction grating, the master was prepared using photolithography of Az2020 negative resist on silicon substrate, followed by electron-beam evaporation of Ni (100 nm), lift-off in hot acetone, Si dry etching in SF6/C4F8 plasma (36 sccm SF6, 15 sccm C2H4, RF=15 W, LF=250 W, 25 min) though the Nickel mask, and NI strip using wet Ni etch (piranha solution). The master with antireflective and superhydrophobic pattern was prepared by colloidal lithography using polystyrene microspheres of 2 μm diameter in a Langmuir-Blodgett trough. Then, the microsphere diameter was reduced to 1.5 μm by dry etching in O2 plasma (100 sccm O2, RF=15W, LF=200W, 30 sec). The microsphere pattern was transferred to Si by dry etch as described above, and the remaining microspheres were removed by sonication in hot chlorobenzene. The master mold with nanometric features was fabricated by electron beam lithography (Raith eLine) using PMMA as positive resist. No pattern transfer to Si was done in this case, and patterned PMMA was directly used for the replication of the soft stamp. Hybrid soft stamps were replicated from the fabricated masters using previously reported protocol.
Direct thermal imprinting: Imprint was done in a custom-built tool (
A PDMS mold was prepared substantially according to the procedure as discussed with respect to mold 230 of
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IL2020/050574 | 5/26/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62853730 | May 2019 | US |
