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Top 20 Most Read Articles
June 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Postfabrication resistance trimming of a superconducting tunnel junction using a focused ion beam J. Vac. Sci. Technol. B 13, 318 (1995); http://dx.doi.org/10.1116/1.588372 (3 pages)
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A high current density NbN/MgO/Nb superconducting tunnel junction has been modified using a focused ion beam to trim the junction area following fabrication and measurement. After milling, the exposed edge of the junction was cleaned of resputtered material using a low current ion beam combined with gas assisted etching. Some remaining shorts across the barrier were then removed by anodization. The modified junction showed a resistance increase correlating exactly with the decrease in area and very little change in quality. This technique may be used on fully fabricated circuits to trim accurately the resistance of selected junctions in order to overcome variations in barrier conductance and junction area. © 1995 American Vacuum Society |
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Photoluminescence from ZnO nanowires J. Vac. Sci. Technol. B 27, 1688 (2009); http://dx.doi.org/10.1116/1.3130159 (5 pages) Online Publication Date: 29 May 2009
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Self-organized ZnO nanowires were grown by metal-organic chemical vapor deposition on sapphire substrate. Steady-state photoluminescence (PL) from the samples with different densities of the nanowires was studied in wide range of temperatures and excitation intensities. At 13 K the PL spectrum consisted of sharp exciton lines at 3.354, 3.357, and 3.363 eV and a weak red band with a maximum at ∼ 1.75 eV. The peak intensity of the red band was four orders of magnitude lower than that of the strongest exciton line. The authors investigated also the effect of ambient on PL at room temperature. In vacuum the PL intensity increased linearly with the excitation power density in the range from 3×10−4 to 0.3 W/cm2, whereas in air the PL intensity increased superlinearly. Such behavior is attributed to photoinduced adsorption of species from air which increases the near-surface barrier and depletion region.
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J. Vac. Sci. Technol. B 10, 1237 (1992); http://dx.doi.org/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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Band offsets of wide-band-gap oxides and implications for future electronic devices J. Vac. Sci. Technol. B 18, 1785 (2000); http://dx.doi.org/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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Nanoimprint lithography: An old story in modern times? A review J. Vac. Sci. Technol. B 26, 458 (2008); http://dx.doi.org/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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Deep reactive ion etching as a tool for nanostructure fabrication J. Vac. Sci. Technol. B 27, 1520 (2009); http://dx.doi.org/10.1116/1.3065991 (7 pages) Online Publication Date: 27 May 2009
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Deep reactive ion etching (DRIE) is investigated as a tool for the realization of nanostructures and architectures, including nanopillars, silicon nanowires or carbon nanotubes on Si nanopillars, nanowalls, and nanonetworks. The potential of combining top-down fabrication methods with the bottom-up synthesis of one-dimensional nanocomponents is assessed. The field-emission properties of carbon nanotubes/Si pillars hybrid structures are measured, as well as the transport properties of large-area nanowires obtained via nanowire lithography. The potential of DRIE for the fabrication of three-dimensional nanostructures is also revealed.
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J. Vac. Sci. Technol. B 27, 1693 (2009); http://dx.doi.org/10.1116/1.3086719 (5 pages) Online Publication Date: 29 May 2009
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ZnO p-n junctions were grown by two-step pulsed laser deposition (PLD) on a-plane sapphire substrates using a Ga-doped ZnO thin film as n-type conducting material. On top of these n-type films, phosphorous-doped ZnO (ZnO:P) nanowires were prepared by high-pressure PLD. Rectifying I-V curves with threshold voltage of about 3.2 V and a forward/reverse current ratio of 100 at ±3.5 V were measured reproducibly on these junctions. There are three independent indications for reproducible and about 1 year stable p-type conductivity of the ZnO:P wires: (1) Low-temperature cathodoluminescence of single ZnO:P nanowires exhibits phosphorus acceptor-related peaks: (A0,X), (e,A0), and donor-acceptor pair [
B. Q. Cao et al., Nanotechnology 18, 455707 (2007)
], (2) bottom-gate field effect transistors using undoped (n-type) ZnO and ZnO:P wires showed opposite transfer characteristics [
B. Q. Cao et al., Phys. Status Solidi (RRL) 2, 37 (2008)
], and (3) the rectifying I-V characteristics of the ZnO:P nanowire/ZnO:Ga-film junctions as shown here.
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Alternative high-k dielectrics for semiconductor applications J. Vac. Sci. Technol. B 27, 209 (2009); http://dx.doi.org/10.1116/1.3025855 (5 pages) Online Publication Date: 4 February 2009
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Although the next generation high-k gate dielectrics has been defined for the 45 nm complementary metal oxide semiconductor technology node, threshold voltage control and equivalent oxide thickness (EOT) scaling remain concerns for future devices. Therefore, the authors explored the effect of incorporating dysprosium in the gate stack. Results suggest that improved EOT-leakage scaling is possible by adding Dy to the interfacial SiO2 layer in a 1:1 ratio or by adding 10% Dy to bulk HfO2. The deposition of a 1 nm Dy2O3 cap layer lowered the threshold voltage by ∼ 250 mV. In addition, for future dynamic random access memory capacitor applications, dielectrics with ε of 50–130 are projected by the International Technology Roadmap for Semiconductors, unachievable with standard high-k dielectrics. Theoretical modeling can help direct the experimental work needed for extensive screening of alternative dielectrics. Moreover, materials such as perovskites only exhibit a sufficiently high-k value when properly crystallized. Therefore, control over the crystalline phase of the material might become a necessity to obtain the proper material characteristics as shown for SrTiOx. After crystallization, the permittivity was observed to increase from 20 to 135. In addition, material and gate stack optimization to limit leakage current densities for these higher-k dielectrics will be needed.
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Recent advances in processing of ZnO J. Vac. Sci. Technol. B 22, 932 (2004); http://dx.doi.org/10.1116/1.1714985 (17 pages) Online Publication Date: 26 April 2004
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A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures. © 2004 American Vacuum Society. |
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J. Vac. Sci. Technol. B 21, 2231 (2003); http://dx.doi.org/10.1116/1.1622676 (31 pages) Online Publication Date: 3 November 2003
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Atomic layer deposition (ALD) has been studied for several decades now, but the interest in ALD of metal and nitride thin films has increased only recently, driven by the need for highly conformal nanoscale thin films in modern semiconductor device manufacturing technology. ALD is a very promising deposition technique with the ability to produce thin films with excellent conformality and compositional control with atomic scale dimensions. However, the applications of metals and nitrides ALD in semiconductor device processes require a deeper understanding about the underlying deposition process as well as the physical and electrical properties of the deposited films. This article reviews the current research efforts in ALD for metal and nitride films as well as their applications in modern semiconductor device fabrication. © 2003 American Vacuum Society. |
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Sub-10 nm imprint lithography and applications J. Vac. Sci. Technol. B 15, 2897 (1997); http://dx.doi.org/10.1116/1.589752 (8 pages)
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New developments, further details, and applications of imprint lithography are presented. Arrays of 10 nm diameter and 40 nm period holes were imprinted not only in polymethylmethacrylate (PMMA) on silicon, but also in PMMA on gold substrates. The smallest hole diameter imprinted in PMMA is 6 nm. All the PMMA patterns were transferred to a metal using a liftoff. In addition, PMMA mesa’s of a size from 45 nm to 50 μm were obtained in a single imprint. Moreover, imprint lithography was used to fabricate the silicon quantum dot, wire, and ring transistors, which showed the same behavior as those fabricated using electron (e)-beam lithography. Finally, imprint lithography was used to fabricate nanocompact disks with 10 nm features and 400 Gbits/in.2 data density—near three orders of magnitude higher than current critical dimensions (CDs). A silicon scanning probe was used to read back the data successfully. The study of wear indicates that due to the ultrasmall force in tapping mode, both the nano-CD and the scanning probe will not show noticeable wear after a large number of scans. © 1997 American Vacuum Society. |
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Fabrication of TiO2 and Ag wires and arrays using opal polystyrene crystal templates J. Vac. Sci. Technol. B 27, 1484 (2009); http://dx.doi.org/10.1116/1.3054299 (5 pages) Online Publication Date: 27 May 2009
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Monodispersed polystyrene (PS) colloids suspended in an aqueous phase with diameters of 46±2.0 and 903±9 nm were employed to deposit opal templates on electrically conductive diamond substrates using a drop-dry method. Ordered close-packed polystyrene opals were produced in the case of the larger PS spheres. In contrast the smaller spheres exhibited a more disordered packing and produced a periodic arrangement of linear voids as a result of the capillary forces operating during the drying process. The PS arrays produced in this way were infiltrated with titania colloidal solution (particle size of 230 nm), followed by drying, dissolution of the PS and calcination. This produced inverse opal titania in the anatase phase for the 903 nm PS spheres, whereas it is shown that the structure using the smaller PS spheres is dominated by microwire arrays as a result of the linear voids present in the original template. It is shown that electrochemically deposited Ag also adopts a similar microstructure when using this template.
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Gas-assisted focused electron beam and ion beam processing and fabrication J. Vac. Sci. Technol. B 26, 1197 (2008); http://dx.doi.org/10.1116/1.2955728 (80 pages) Online Publication Date: 11 August 2008
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Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.
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Pd-catalyzed hydrogen sensing with InN nanobelts J. Vac. Sci. Technol. B 27, L8 (2009); http://dx.doi.org/10.1116/1.3125267 (3 pages) Online Publication Date: 5 May 2009
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The use of Pd coatings on multiple InN nanobelts is shown to enhance their sensitivity for hydrogen sensing at hundreds of ppm level at 25 °C. Without the metal coating to catalyze dissociation of the hydrogen molecules, the InN nanobelts with Ohmic contacts at either end showed no detectable change in current when exposed to hydrogen under the same conditions. Moreover, the Pd-coated InN showed no response to CO2, C2H6, NH3, and O2 (all in N2 ambient). The relative resistance change in the Pd-coated sensors was not linearly dependent on the hydrogen concentration at dilute levels, i.e., 8% at 100 ppm H2 and 9.5% at 1000 ppm H2. The recovery characteristics of the sensors at room temperature after hydrogen sensing were also examined and ∼ 50% of the initial InN resistance was recovered 10 min after sensor exposure to air. At higher temperatures, larger resistance changes and faster response and recovery were obtained. Pd-coated InN nanobelt sensors displayed much higher relative response than Pt-coated sensors.
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Field emission properties of nanocrystalline chemically vapor deposited-diamond films J. Vac. Sci. Technol. B 17, 1970 (1999); http://dx.doi.org/10.1116/1.590857 (17 pages)
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We have deposited nanocrystalline diamond films on p-type Si(100) substrates using plasma enhanced chemical vapor deposition (CVD). The diamond films were deposited at substrate temperatures between 950 and 980 °C using a high methane concentration of 5% in H2. The films obtained showed good field emission properties with threshold fields of around 5 V μm−1 (for 1 nA emission current). X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy showed that the nanocrystalline films still exhibit the basic electronic features of diamond with a band gap of 5.5 eV and a negative electron affinity when the surface is hydrogen plasma treated. The Fermi level position in these films is found to be 1±0.2 eV above the valence band maximum. The energy resolved field emission measurements show the typical asymmetric peak shape of Fowler-Nordheim (FN) tunneling through a surface potential barrier. The electrons emitted originate from a continuum of electronic states at the Fermi energy of the emitter. From a combined measurement of the field emitted electron energy distribution and the field emission I–V characteristic of an emitter we could independently determine the work function and the local electric field present at the emission site. In the case of nanocrystalline CVD diamond emitters we determined work function values around 5.7 eV and local fields in the range of 2000–3000 V μm−1 (for emission currents of 10–1000 pA). The corresponding field enhancement factors can range from 250 to 1700. Simultaneous field and photoelectron emission spectroscopy showed no indication of field penetration. Deviations from the FN law in the high current regime of the I–V plots may be related to an internal resistance of the emitter. © 1999 American Vacuum Society. |
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Degradation of superhard nanocomposites by built-in impurities J. Vac. Sci. Technol. B 22, L5 (2004); http://dx.doi.org/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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Contrast reversal effect in scanning electron microscopy due to charging J. Vac. Sci. Technol. B 27, 1039 (2009); http://dx.doi.org/10.1116/1.3114486 (4 pages) Online Publication Date: 15 April 2009
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In semiconductor manufacturing, accurate measurement of shapes and sizes of fabricated features is required. These measurements are carried out using critical dimension scanning electron microscope (CD-SEM). Positions of edges are often unclear because of charging. Depending on the SEM setup and the pattern under measurement, the effect of charging varies. The influence of measurement conditions can be simulated and optimized. A Monte Carlo electron beam simulation tool was developed, which takes into account electron scattering and charging. CD-SEM imaging of silicon dioxide lines on silicon was studied. In the experiment, changes in the beam voltage were found to result in contrast tone reversal. The same effect was also found in simulations considering charging. The time dependence of contrast variation was studied. A good agreement between simulation and measurement was found. The simulation software proved reliable in predicting SEM images, which makes it an important instrument to optimize settings of electron beam systems.
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Interface stability in advanced high-κ-metal-gate stacks J. Vac. Sci. Technol. B 27, 1021 (2009); http://dx.doi.org/10.1116/1.3112629 (5 pages) Online Publication Date: 15 April 2009
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Thin film reactions and interface formation in advanced high-κ-metal-gate stacks containing rare-earth oxides have been studied. In particular, interfacial reactions of Dy2O3 and HfSiO4 with TiN- or TaCN-based metals were studied. It is shown that Dy diffusion is considerably stronger than Hf diffusion. It is further demonstrated that for TaCN-based materials, the material density and the microstructure are of much larger influence than the chemical composition.
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Focused ion beam technology and applications J. Vac. Sci. Technol. B 5, 469 (1987); http://dx.doi.org/10.1116/1.583937 (27 pages)
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Ions of kiloelectron volt energies incident on a solid surface produce a number of effects: several atoms are sputtered off, several electrons are emitted, chemical reactions may be induced, atoms are displaced from their equilibrium positions, and ions implant themselves in the solid, altering its properties. Some of these effects, such as sputtering and implantation are widely used in semiconductor device fabrication and in other fields. Thus the capability to focus a beam of ions to submicrometer dimensions, i.e., dimensions compatible with the most demanding fabrication procedures, is an important development. The focused ion beam field has been spurred by the invention of the liquid metal ion source and by the utilization of focusing columns with mass separation capability. This has led to the use of alloy ion sources making available a large menu of ion species, in particular the dopants of Si and GaAs. The ability to sputter and to also induce deposition by causing breakdown of an adsorbed film has produced an immediate application of focused ion beams to photomask repair. The total number of focused ion beam fabrication systems in use worldwide is about 35, about 25 of them in Japan. In addition, there are many more simpler focused ion beam columns for specialized uses. The interest is growing rapidly. The following range of specifications of these systems has been reported: accelerating potential 3 to 200 kV, ion current density in focal spot up to 10 A/cm2, beam diameters from 0.05 to 1 μm, deflection accuracy of the beam over the surface ±0.1 μm, and ion species available Ga, Au, Si, Be, B, As, P, etc. Some of the applications which have been demonstrated or suggested include: mask repair, lithography (to replace electron beam lithography), direct, patterned, implantation doping of semiconductors, ion induced deposition for circuit repair or rewiring, scanning ion microscopy, and scanning ion mass spectroscopy. |
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ZnO nanorod arrays on n-type Si(111) substrates for pH measurements J. Vac. Sci. Technol. B 27, 1684 (2009); http://dx.doi.org/10.1116/1.3098503 (4 pages) Online Publication Date: 29 May 2009
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ZnO nanorod arrays were grown directly on n-Si (111) substrates using an aqueous solution method aiming at integration with Si-based device technology. The growth of the ZnO nanorod arrays was found to be influenced by the thickness of the native oxide layer on Si substrates. A dense array of about 200 nm diameter nanorods was obtained from the thick equimolar aqueous solution containing 100 mM of zinc nitrate hexahydrate and hexamethylenetetramine, while the solution with lower concentrations resulted in scattered corn-shaped ones. Electrochemical potential in electrolyte of the ZnO nanorod arrays on Si (111) substrates was studied and found to have a slope of −50 mV/pH, suggesting a potential application to future sensing nanosystems.
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