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Top 20 Most Read Articles
December 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Degradation of superhard nanocomposites by built-in impurities J. Vac. Sci. Technol. B 22, L5 (2004); doi:10.1116/1.1689305 (5 pages) Online Publication Date: 24 March 2004
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Impurities such as oxygen and chlorine can strongly decrease the hardness of superhard nc-TiN/a-Si3N4 and similar nanocomposites when incorporated into the coatings during their deposition. It is shown that 1–1.5 at. % of oxygen causes a hardness decrease to about 30 GPa, as compared to 45–55 GPa for the pure material. This may explain some of the contradictory results found by other authors, particularly for coatings deposited by physical vapor deposition at relatively low nitrogen pressure, deposition temperature, and deposition rates. © 2004 American Vacuum Society. |
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J. Vac. Sci. Technol. B 28, 1195 (2010); doi:10.1116/1.3503612 (7 pages) Online Publication Date: 25 October 2010
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A surface micromachining technique for the release of high length-to-thickness aspect ratio (800:1) bridge structures is presented. During a timed etch release, the remaining side wall geometry of the sacrificial layer provides intrinsic support for the structural layer. The micromachining process itself is an equipment limited procedure in which the wet etchant for the sacrificial layer is replaced in solution (i.e., in situ) with a supportive photoresist layer. Once in solid form, the photoresist is removed via ashing in an oxygen plasma. This combination of controlling the sidewall etch profile of the sacrificial layer and its removal technique results in the successful release of bridge structures, which are 4000 μm long and 5 μm thick, with a 2 μm suspension gap.
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Band offsets of wide-band-gap oxides and implications for future electronic devices J. Vac. Sci. Technol. B 18, 1785 (2000); doi:10.1116/1.591472 (7 pages)
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Wide-band-gap oxides such as SrTiO3 are shown to be critical tests of theories of Schottky barrier heights based on metal-induced gap states and charge neutrality levels. This theory is reviewed and used to calculate the Schottky barrier heights and band offsets for many important high dielectric constant oxides on Pt and Si. Good agreement with experiment is found for barrier heights. The band offsets for electrons on Si are found to be small for many key oxides such as SrTiO3 and Ta2O5 which limit their utility as gate oxides in future silicon field effect transistors. The calculations are extended to screen other proposed oxides such as BaZrO3. ZrO2, HfO2, La2O3, Y2O3, HfSiO4, and ZrSiO4. Predictions are also given for barrier heights of the ferroelectric oxides Pb1−xZrxTiO3 and SrBi2Ta2O9 which are used in nonvolatile memories. © 2000 American Vacuum Society. |
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Fabrication and performance of graphene nanoelectromechanical systems J. Vac. Sci. Technol. B 29, 050801 (2011); doi:10.1116/1.3623419 (10 pages) Online Publication Date: 9 September 2011
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As a result of the recent progress in fabricating large-area graphene sheets, graphene-based mechanical devices have become vastly easier to manufacture and now show even greater promise for a range of applications. This article reviews the progress of resonant graphene nanoelectromechanical systems and the possible applications of this technology to signal processing, sensing, and other areas. After discussing recent advances in fabrication and measurement techniques that make graphene resonators a viable technology, the article presents what is known about the performance of graphene mechanical systems. The authors also highlight unresolved questions, such as the source of the dissipation in graphene resonators, and discuss the progress made on these issues to date. The authors conclude with a discussion of important future directions for graphene research and the applications for which graphene nanomechanical devices may be well suited.
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J. Vac. Sci. Technol. B 10, 1237 (1992); doi:10.1116/1.585897 (30 pages)
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The status of research on both wurtzite and zinc‐blende GaN, AlN, and InN and their alloys is reviewed including exciting recent results. Attention is paid to the crystal growth techniques, structural, optical, and electrical properties of GaN, AlN, InN, and their alloys. The various theoretical results for each material are summarized. We also describe the performance of several device structures which have been demonstrated in these materials. Near‐term goals and critical areas in need of further research in the III–V nitride material system are identified. |
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Large-area suspended graphene on GaN nanopillars J. Vac. Sci. Technol. B 29, 060601 (2011); doi:10.1116/1.3654042 (5 pages) Online Publication Date: 26 October 2011
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The authors have demonstrated large-area suspended graphene on GaN nanopillars predefined by nanosphere lithography and inductively coupled plasma etching. The graphene was successfully suspended over large areas without ripples and corrugations. Scanning electron microscopy, atomic force microscopy and micro-Raman spectroscopy were used to characterize the properties of the suspended graphene on nanopillars. The thermal properties of the suspended and supported graphene were investigated by varying the underlying GaN nanopilllar geometries from flat-top to sharp-cone morphologies and heating the resulting structures via irradiation with laser powers of 1.53 mW, 8.03 mW, and 16.19 mW. The heat transfer was effective even when the contact area between the suspended graphene and the supporting substrate was small, due to the high thermal conductivities of graphene and GaN. The extremely high thermal conductivity of the graphene can improve the thermal management in GaN-based high power electronic and optoelectronics devices, a critical factor for their long-term reliability.
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Recent developments and design challenges in continuous roller micro- and nanoimprinting J. Vac. Sci. Technol. B 30, 010801 (2012); doi:10.1116/1.3661355 (28 pages) Online Publication Date: 1 December 2011
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As an emerging technology for the manufacture of micro- and nano-scale patterns, continuous imprinting; otherwise known as roll-to-roll or roller imprinting, is attracting interest from researchers around the world because of its inherent advantages of low cost, high throughput, large area patterning. This technology is an evolutionary advance on the more traditional nanoimprint lithography developed in the 1990s, which is considered a batch mode, or dis-continuous patterning approach. In recent years, a number of commercial applications have been discovered which require low cost, large area patterning, particularly displays, optical coatings and films, and biological applications such as anti-fouling surfaces and micro-fluidic devices. This review covers a variety of continuous imprinting approaches, highlights challenges, and surveys progress towards high speed production of micro- and nanoscale features for these applications and others using this platform technology.
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Monolithic integration of silicon CMOS and GaN transistors in a current mirror circuit J. Vac. Sci. Technol. B 30, 02B101 (2012); doi:10.1116/1.3665220 (6 pages) Online Publication Date: 6 December 2011
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GaN high electron mobility transistors (HEMTs) were monolithically integrated with silicon CMOS to create a functional current mirror circuit. The integrated circuit was fabricated on 100 mm diameter modified silicon-on-insulator (SOI) wafers incorporating a resistive (111) silicon handle substrate and a lightly doped (100) silicon device layer. In a CMOS-first process, the CMOS was fabricated using the (100) device layer. Subsequently GaN was grown by plasma molecular beam epitaxy in windows on the (111) handle substrate surface without wire growth despite using gallium-rich growth conditions. Transmission lines fabricated on the GaN buffer/SOI wafer exhibited a microwave loss of less than 0.2 dB/mm up to 35 GHz. Direct current measurements on GaN HEMTs yielded a current density of 1.0 A/mm and transconductance of 270 mS/mm. At 10 GHz and a drain bias of 28 V, 1.25 mm long transistors demonstrated a small signal gain of 10.7 dB and a maximum power added efficiency of 53% with a concomitant power of 5.6 W. The silicon and GaN transistors were interconnected to form high yield test interconnect daisy chains and a monolithic current mirror circuit. The CMOS output drain current controlled the GaN transistor quiescent current and consequently the microwave gain.
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J. Vac. Sci. Technol. B 29, 010801 (2011); doi:10.1116/1.3532949 (35 pages) Online Publication Date: 14 January 2011
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Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasma etch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasma etch is a formation mechanism of rough sidewalls. The authors next summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresist materials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surface roughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin ( ∼ 1–3 nm) dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymer surface roughness formation has been proposed by
Bruce et al. [J. Appl. Phys. 107, 084310 (2010)]
. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS) polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measured surface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth of about 100 nm can either soften (scission) or stiffen (cross-linking) this region, which produce enhanced or reduced surface roughness.
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Method to pattern etch masks in two inclined planes for three-dimensional nano- and microfabrication J. Vac. Sci. Technol. B 29, 061604 (2011); doi:10.1116/1.3662000 (8 pages) Online Publication Date: 5 December 2011
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The authors present a method to pattern etch masks for arbitrary nano- and microstructures on different, inclined planes of a sample. Our method allows standard CMOS fabrication techniques to be used in different inclined planes; thus yielding three-dimensional structures with a network topology. The method involves processing of the sample in a first plane, followed by mounting the prepared sample in a specially designed silicon holder wafer such that the second, inclined plane is exposed to continued processing. As a proof of principle we demonstrate the fabrication of a patterned chromium etch mask for three-dimensional photonic crystals in silicon. The etch mask is made on the 90° inclined plane of a silicon sample that already contains high aspect ratio nanopores. The etch mask is carefully aligned with respect to these pores, with a high translational accuracy of <30 nm along the y-axis and a high rotational accuracy of 0.71° around the z-axis of the crystal. Such high alignment precisions are crucial for nanophotonics and for sub-micrometer applications in general. Although we limit ourselves to processing on two planes of a sample, it is in principle possible to repeat the presented method on more planes. The authors foresee potential applications of this technique in, e.g., microfluidics, photonics, and three-dimensional silicon electronics.
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Mechanical properties of suspended graphene sheets J. Vac. Sci. Technol. B 25, 2558 (2007); doi:10.1116/1.2789446 (4 pages) Online Publication Date: 11 December 2007
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Using an atomic force microscope, we measured effective spring constants of stacks of graphene sheets (less than 5) suspended over photolithographically defined trenches in silicon dioxide. Measurements were made on layered graphene sheets of thicknesses between 2 and 8 nm, with measured spring constants scaling as expected with the dimensions of the suspended section, ranging from 1 to 5 N/m. When our data are fitted to a model for doubly clamped beams under tension, we extract a Young’s modulus of 0.5 TPa, compared to 1 TPa for bulk graphite along the basal plane, and tensions on the order of 10−7 N.
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Nanoimprint lithography: An old story in modern times? A review J. Vac. Sci. Technol. B 26, 458 (2008); doi:10.1116/1.2890972 (23 pages) Online Publication Date: 27 March 2008
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Nanoimprint lithography (NIL) is a high throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional material displacement. This can be done by shaping a liquid followed by a curing process for hardening, by variation of the thermomechanical properties of a film by heating and cooling, or by any other kind of shaping process using the difference in hardness of a mold and a moldable material. The local thickness contrast of the resulting thin molded film can be used as a means to pattern an underlying substrate on wafer level by standard pattern transfer methods, but also directly in applications where a bulk modified functional layer is needed. Therefore it is mainly aimed toward fields in which electron beam and high-end photolithography are costly and do not provide sufficient resolution at reasonable throughput. The aim of this review is to play between two poles: the need to establish standard processes and tools for research and industry, and the issues that make NIL a scientific endeavor. It is not the author’s intention to duplicate the content of the reviews already published, but to look on the NIL process as a whole. The author will also address some issues, which are not covered by the other reviews, e.g., the origin of NIL and the misconceptions, which sometimes dominate the debate about problems of NIL, and guide the reader to issues, which are often forgotten or overlooked.
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Multitip atomic force microscope lithography system for high throughput nanopatterning J. Vac. Sci. Technol. B 29, 06FD03 (2011); doi:10.1116/1.3662396 (6 pages) Online Publication Date: 22 November 2011
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An atomic force microscope (AFM) system with multiple parallel lithography probes of equal heights on a single cantilever was created in order to improve the throughput of AFM lithography. The multitip probe was fabricated by electron-beam (e-beam) lithography and a dry silicon etching process. Several carbon islands were made on a single cantilever in a straight line by e-beam lithography and were used as an etch mask, whereas the silicon pedestal structure of the multitip probe was fabricated by reactive ion etching (RIE). Finally the carbon islands were sharpened by a RIE process using oxygen gas. The multitip probe was successfully applied to form multidot pattern arrays on a negative resist film coated on silicon by low electric field induced AFM lithography. A pedestal nanopillar structure was utilized as a convenient support feature that enabled better control of multiple nanotip arrays for AFM writing. The authors fabricated such a nanopedestal array with extremely sharp nanoneedle tips.
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Functional semiconductor nanowires via vapor deposition J. Vac. Sci. Technol. B 29, 060801 (2011); doi:10.1116/1.3641913 (21 pages) Online Publication Date: 27 September 2011
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More than a decade’s worth of research has led to significant progress toward an understanding of the growth behavior of nanowires (NWs). Among all of the different bottom-up and top-down strategies, vapor deposition has unique advantages in producing high quality NW structures. This paper reviews the current understandings of the thermodynamics and kinetics of NW nucleation and growth behaviors using a vapor deposition approach. NW deposition from the vapor phase is divided into two general categories: that with and that without foreign metal catalysts. The distinct crystal nucleation and growth mechanisms, NW morphologies, and controlling parameters of these two categories are presented in detail and compared. In addition, ways to apply these strategies in order to realize complex NW structures such as NW heterojunctions and 3D NW networks are also discussed. The information about NW vapor deposition reviewed in this paper provides a comprehensive background for understanding NW growth phenomena, ways of achieving morphology and property control, and how to eventually pave the road toward industrial-level NW manufacturing.
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InGaN laser diodes operating at 450–460 nm grown by rf-plasma MBE J. Vac. Sci. Technol. B 30, 02B102 (2012); doi:10.1116/1.3665223 (5 pages) Online Publication Date: 6 December 2011
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This work demonstrates the first true blue laser diodes (LDs) grown by plasma assisted molecular beam epitaxy that operate at the region of 450–460 nm. The single quantum well LDs were grown on several types of c-plane bulk GaN substrates, with threading dislocation densities varying from 104 to 108cm−2. The key factors that allowed the authors to achieve lasing in true-blue wavelengths are improvements in the growth technology of the InGaN quantum wells attributed to the high nitrogen flux used and the design of the LD structure, which reduced the light losses in the cavity. The authors discuss the influence of the diodes’ design on the parameters of LDs.
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J. Vac. Sci. Technol. B 16, 558 (1998); doi:10.1116/1.589862 (3 pages)
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Etching results using the solution system of citric acid/H2O2 and de-ionized H2O/buffered oxide etch are shown to provide good selective wet etching of AlGaAs/GaAs structures. For AlxGa1−xAs(x<0.5) layers the selective characteristics of each Al composition strongly depend on the volume ratio of the citric acid/H2O2 solution. The turning volume ratio of the solution, at which etching starts, sensitively depends on Al composition. Additionally, the etch rate of AlyGa1−yAs (y>0.7) quickly decreases with decreasing Al composition in de-ionized H2O/buffered oxide etch solution, providing a high degree of etching selectivity. These simple selective etching processes have been applied to define AlAs/GaAs distributed Bragg reflector mesas in a vertical-cavity surface-emitting laser structure. © 1998 American Vacuum Society. |
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High aspect ratio fine pattern transfer using a novel mold by nanoimprint lithography J. Vac. Sci. Technol. B 29, 06FC15 (2011); doi:10.1116/1.3662080 (5 pages) Online Publication Date: 18 November 2011
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To conduct our research, a Si mold with a high aspect nano trench pattern was fabricated using a new edge lithography process, and the pattern was replicated into PMMA films on Si wafer by thermal nanoimprint lithography. By using edge lithography, a SiO2 circular line pattern of 35 nm width and 3.5 μm height was obtained and the aspect ratio became 100. The Cr patterns were fabricated by a lift-off process by using the high aspect SiO2 nano patterns, and the Si substrate was etched by the advanced plasma etching of the gas switching process. A Si trench pattern of 25 nm width and 1.0 μm depth was obtained, and these Si trench patterns were used for nanoimprint molds. The aspect ratios of the nano trench patterns were about 10. The nano trench patterns were replicated into PMMA films with various molecular weights by thermal nanoimprint lithography. The pattern replication failed when the PMMA resins of 50 and 120 k molecular weights were used. It is found that the PMMA pattern was often broken during the demolding process and its strength was very important for the successful pattern replication of the high aspect pattern. A PMMA line pattern of 30 nm width and 230 nm height was successfully fabricated when the PMMA resin of 996 k molecular weight was used.
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J. Vac. Sci. Technol. B 29, 020602 (2011); doi:10.1116/1.3556962 (5 pages) Online Publication Date: 7 March 2011
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Cone-shaped subwavelength structures (SWSs) were fabricated on a GaAs substrate by utilizing a confined convective self-assembly process followed by inductively coupled-plasma reactive-ion etching. A self-assembled polystyrene monolayer was used as an etch mask for pattern transfer onto the GaAs substrate. The fabricated SWS, having a cone profile with an aspect ratio of 1.5 and a 300 nm pitch, exhibited very low reflectance throughout the solar spectrum range and exhibited wide tolerance to different optical incidence angles. Reflectance of the cone-shaped SWS on the GaAs surface was less than 4% in a spectral range of 300–1000 nm under a normal incidence condition.
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J. Vac. Sci. Technol. B 24, 2350 (2006); doi:10.1116/1.2353844 (6 pages) Online Publication Date: 21 September 2006
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Al2O3 and TiO2 deposited by atomic layer deposition are evaluated as etch masks for dry etch processes in an inductively coupled plasma reactor using the Bosch process. In the inductively coupled plasma chamber during deep silicon etching, because of the chemical nature of the etch process and the inert nature of Al2O3, the result is exceptional selectivity for silicon over as-deposited Al2O3, particularly at relatively low bias and high pressures used for through-wafer etching. TiO2 is less resistant and appears to suffer more from chemical attack. In both cases, etch rate increases slowly with increasing rf bias. However, there is a sharp discontinuity in the etch rate of Al2O3 when the bias power is operated in a pulsed low-frequency mode. This is thought to be due to increased sputtering from heavier ions. Preliminary studies indicate the etching conditions for Al2O3 may be extended into a dielectric etch regime requiring more study.
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Fabrication of fluidic devices with 30 nm nanochannels by direct imprinting J. Vac. Sci. Technol. B 29, 06F801 (2011); doi:10.1116/1.3662886 (7 pages) Online Publication Date: 23 November 2011
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In this work, we propose an innovative approach to the fabrication of a complete micro/nano fluidic system, based on direct nanoimprint lithography. The fabricated device consists of nanochannels connected to U-shaped microchannels by triangular tapered inlets, and has four large reservoirs for liquid input. A master silicon stamp with the multilevel structures is fabricated first, and then a negative replica is made, to be used as a stamp for ultraviolet nanoimprint lithography (UV-NIL). Afterwards, just one single UV-NIL step is necessary for patterning all the the micro and nanostructures. Furthermore, the devices are made of all-transparent materials, and the method allows flexibility for the type of substrates used. The active material (an inorganic-organic hybrid polymer) used for the fabrication of the device has been carefully chosen, so it has adequate surface properties (inert and hydrophilic) for its direct use for biological applications. Devices having 30 nm wide, 30 nm deep nanochannels have been fabricated, and the successful performance of the fluidic system and the continuity of the nanochannels have been proven by flow tests.
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