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Top 20 Most Read Articles
November 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Novel method for cleaning a vacuum chamber from hydrocarbon contamination J. Vac. Sci. Technol. A 28, 1413 (2010); http://dx.doi.org/10.1116/1.3484242 (8 pages) Online Publication Date: 1 November 2010
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A novel method for cleaning a high vacuum chamber is presented. This method is based on concurrent in situ high-energetic UV light activation of contaminants located in the residual gas and at the vacuum chamber surfaces as well as the in situ generation of highly reactive ozone. Ozone oxidizes the contaminants to volatile species. Investigations by energy-dispersive x-ray analysis of residual gas depositions and mass-spectroscopy measurements of the residual gas in the vacuum chamber identify the contaminant species as hydrocarbons. After a cleaning period of 8 h, a decrease in measured chamber contamination by about 90% could be achieved according to atomic force microscope analysis. Mass spectroscopy measurements using a residual gas analyzer indicate the creation of volatile, carbonaceous species during the cleaning process.
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J. Vac. Sci. Technol. A 28, 1338 (2010); http://dx.doi.org/10.1116/1.3491036 (6 pages) Online Publication Date: 27 September 2010
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Cu2O thin films were deposited on ZnO coated glass substrates by metal organic chemical vapor deposition from copper(II) hexafluoroacetylacetonate [Cu(C5HF6O2)2], oxygen gas, and water vapor. The dependence of the structural and electrical properties of Cu2O films on deposition temperature and film thickness was investigated. X-ray diffraction showed that Cu2O thin films grow on ZnO with preferred (220)Cu2O∥(0002)ZnO orientation. The grain size and stress in Cu2O films increase with increasing substrate temperature but decrease with increasing film thickness. The carrier mobility increases with increasing grain size indicating that the carrier transport is limited by scattering from the grain boundaries. Single-phase epitaxial p-type Cu2O films with hole mobilities exceeding 30 cm2/V s are obtained at a deposition temperature of 400 °C.
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J. Vac. Sci. Technol. A 28, L15 (2010); http://dx.doi.org/10.1116/1.3491182 (3 pages) Online Publication Date: 12 October 2010
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Adsorption and penetration of Al and Ni atoms into Ni(111) and Al(111), respectively, are investigated through first principles calculations, shedding light into the driving forces impacting Al/Ni interfaces produced during multilayer deposition. The authors show that Ni deposition follows an exothermic path toward penetration associated with small activation barriers while Al on Ni(111) path is endothermic accompanied with high activations. Moreover, Ni and Al penetrations proceed through interstitial and substitutional sites, respectively. These differentiated behaviors at early deposition stages illustrate that dual processing conditions are required to achieve the growth of specific Ni/Al interfaces during multilayer deposition processes and that a local melting process at the interface is mandatory to arrive at the formation of a proper barrier layer.
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Microstructured optical fiber UHV integration for cold-atom experiments J. Vac. Sci. Technol. A 28, 1421 (2010); http://dx.doi.org/10.1116/1.3497028 (2 pages) Online Publication Date: 1 November 2010
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Numerical simulation of turbomolecular pump over a wide range of gas rarefaction J. Vac. Sci. Technol. A 28, 1312 (2010); http://dx.doi.org/10.1116/1.3484139 (4 pages) Online Publication Date: 23 September 2010
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A gas flow through a turbomolecular pump is modeled by the direct-simulation Monte Carlo method over a wide range of gas rarefaction covering the free-molecular, transitional, and hydrodynamic regimes. Several values of the rotor speed are considered. The main characteristics of the pump, such as maximum pumping speed and compression ratio, are reported. It is pointed out that in the range between the free-molecular and transitional regimes, the pump characteristics change slightly, but they vary significantly in the hydrodynamic regime. It is shown that the pumping speed increases in the hydrodynamic regime when the rotor speed is high, but it decreases for a small value of the speed. The compression ratio always decreases when approaching the hydrodynamic regime.
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J. Vac. Sci. Technol. A 25, 1317 (2007); http://dx.doi.org/10.1116/1.2764082 (19 pages) Online Publication Date: 30 July 2007
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Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.
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Scaling of hollow cathode magnetrons for ionized metal physical vapor deposition J. Vac. Sci. Technol. A 24, 1955 (2006); http://dx.doi.org/10.1116/1.2335864 (15 pages) Online Publication Date: 6 September 2006
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Ionized metal physical vapor deposition is being increasingly used to deposit diffusion barriers and Cu seed layers into high aspect ratio trenches for microelectronics fabrication. Hollow cathode magnetrons (HCMs) represent a technology capable of depositing metal over large areas at pressures of a few millitorrs. The fundamental mechanisms of these devices are not well understood and so their optimization is difficult. In this article, results from a two-dimensional computational investigation of HCMs are discussed to illuminate scaling issues. The hybrid model incorporates algorithms whereby transport coefficients for use in fluid equations are derived using a kinetic simulation. The goal is to enable the fluid algorithms in the model to be able to more accurately represent low pressure operation. The consequences of power, pressure, and magnitude and orientation of applied magnetic fields were investigated. The authors found that the magnetic field configuration significantly affects the magnitude and distribution of fluxes incident on the substrate. A study of the Cu seed layer deposition process, carried out using a feature scale model, correlates changes in plasma properties with conformal deposition into trenches.
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J. Vac. Sci. Technol. A 28, 1404 (2010); http://dx.doi.org/10.1116/1.3504600 (9 pages) Online Publication Date: 1 November 2010
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A comparison of the performance of nonevaporable getter (NEG) films deposited using two different types of targets has been made to find the one that has the best pumping properties. For the first time, the NEG coating was deposited using a preformed Ti-Zr-V alloy target. The NEG film characterization and pumping properties have been studied in comparison with a film deposited using the commonly used three-wire twisted target. It was demonstrated that the alloy target produces a NEG coating with uniform composition both laterally and in depth. The composition of the film was found to be the same as the target. Film topography and microstructure with 5 nm grain sizes were found to be the same for both targets. The main result is that the activation temperature of the NEG coating deposited using the Ti-Zr-V alloy target is 160 °C, which is 20 °C lower than for NEG coatings deposited using three twisted wires.
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Interfacial organic layers: Tailored surface chemistry for nucleation and growth J. Vac. Sci. Technol. A 28, 1033 (2010); http://dx.doi.org/10.1116/1.3480920 (27 pages) Online Publication Date: 2 September 2010
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The interfaces between inorganic and organic materials are important to a wide variety of technologies. A significant challenge concerns the formation of these interfaces when the inorganic layer must be grown on a pre-existing organic layer. In this review the authors focus on fundamental aspects of inorganic-organic interface formation using transition metal coordination complexes and atomic layer deposition. First, the authors discuss aspects of the synthesis and characterization of ultrathin interfacial organic layers, formed mostly on SiO2 and possessing a variety of functional groups, including layers with a branched microstructure. The authors go on to discuss the reactions of transition metal coordination complexes with these layers. A number of factors control the uptake of the transition metal complex and the composition of the adsorbed species that are formed. These include the identity, density, and dimensionality or spatial distribution of the functional groups. At room temperature, adsorption on layers that lack functional groups results in the penetration of the organic layer by the transition metal complex and the reaction with residual OH at the organic/SiO2 interface. Adsorption on layers with a mostly two-dimensional arrangement of reactive functional groups results in the formation of molecular “bipods,” where the surface bound functional groups react with the complex via two ligand exchange reactions. In contrast, for layers that possess a high density of functional groups arranged three dimensionally, the transition metal complex can be virtually stripped of its ligands. Atomic layer deposition on interfacial organic layers also depends strongly on the density and accessibility of reactive functional groups. On surfaces that possess a high density of functional groups, deployed two dimensionally, growth via atomic layer deposition is initially weakly attenuated, mostly uniform and smooth, and eventually evolves to growth characteristic of unmodified SiO2. Growth on layers that lack sufficient densities of functional groups is initially strongly attenuated, in contrast, and the resulting films are rough, severely islanded and three dimensional. As a consequence, there is a correlation between the strength of the initial attenuation in the rate of growth and the thin film morphology. Correlations between the initial uptake of the transition metal complex by the organic layer and the initial rate of thin film growth are less direct, however, as the composition and structure of the chemisorbed species must also be considered.
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Dry etching of polydimethylsiloxane for microfluidic systems J. Vac. Sci. Technol. A 20, 975 (2002); http://dx.doi.org/10.1116/1.1460896 (8 pages) Online Publication Date: 7 May 2002
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A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches. © 2002 American Vacuum Society. |
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J. Vac. Sci. Technol. A 28, 1393 (2010); http://dx.doi.org/10.1116/1.3504596 (6 pages) Online Publication Date: 27 October 2010
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The rarefied gas flow through a short channel into a vacuum has been investigated computationally using the direct simulation Monte Carlo method. Taking into account the gas-surface scattering and the gas molecule-molecule interaction, the mass flow rate is calculated as a function of gas rarefaction and the length to height ratio. This study demonstrates that the effects of the gas molecule-molecule interaction and the gas-surface scattering can make a noticeable impact on the mass flow rate of the rarefied gas through a short channel into a vacuum. The maximum manifestation of these effects was evaluated.
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Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces J. Vac. Sci. Technol. A 13, 1553 (1995); http://dx.doi.org/10.1116/1.579726 (6 pages)
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In this article nanosphere lithography (NSL) is demonstrated to be a materials general fabrication process for the production of periodic particle array (PPA) surfaces having nanometer scale features. A variety of PPA surfaces have been prepared using identical single‐layer (SL) and double‐layer (DL) NSL masks made by self‐assembly of polymer nanospheres with diameter, D=264 nm, and varying both the substrate material S and the particle material M. In the examples shown here, S was an insulator, semiconductor, or metal and M was a metal, inorganic ionic insulator, or an organic π‐electron semiconductor. PPA structural characterization and determination of nanoparticle metrics was accomplished with atomic force microscopy. This is the first demonstration of nanometer scale PPA surfaces formed from molecular materials. © 1995 American Vacuum Society |
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Material selection for hard coatings J. Vac. Sci. Technol. A 4, 2661 (1986); http://dx.doi.org/10.1116/1.573700 (9 pages)
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Multicomponent refractory material systems can provide opportunities for specific materials for wear resistant coatings. The multitude of potential hard coating materials can be subdivided into three groups according to variations in chemical bonding character of the compounds. Many fundamental relations between the position of coating material components in the Periodic Table of the elements and the properties can be used to optimize these material selections. However, restrictions exist because of increasing hardness and strength which primarily decrease toughness and adherence. Multicomponent boride, carbide, nitride, and oxide systems are discussed in view of their potential as coating materials. Additional options for materials selection and optimization arise from the possibility of adjusting specific microstructures in the layers, especially in multilayer and multiphase coatings. |
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Unique cryogenic pumping array for low sticking coefficient gas flows J. Vac. Sci. Technol. A 28, 1356 (2010); http://dx.doi.org/10.1116/1.3497029 (7 pages) Online Publication Date: 7 October 2010
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A radial fin cryogenic pumping array design is described which enhances the pumping efficiency of high enthalpy and high kinetic energy flows in vacuum. In general, these flows have relatively low sticking coefficients due to the large temperature differences between the particles in the flow and the cryogenic pumping surfaces. A Monte Carlo numerical model has been developed to investigate the pumping efficiency of the radial fin array. A comparison of the pumping efficiency of the radial fin array with a flat plate pumping surface has shown that particles with a high sticking coefficient will be pumped better with a simple flat panel, whereas particles with low sticking coefficients will be more efficiently pumped with the radial fin array. A set of experiments has been performed to investigate the pumping efficiency of the radial fin array as manufactured. These experiments compared the radial fin results to a more traditional flat plate pumping surface with a neutral gas flow. The experimental results indicate that there are flow regimes in which the radial fins are more efficient at pumping incident particles than a flat surface.
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Imaging characterization techniques applied to Cu(In,Ga)Se2 solar cells J. Vac. Sci. Technol. A 28, 665 (2010); http://dx.doi.org/10.1116/1.3358303 (6 pages) Online Publication Date: 29 June 2010
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The authors present examples of imaging characterization on Cu(In,Ga)Se2 (CIGS) solar cell devices. These imaging techniques include photoluminescence imaging, electroluminescence imaging, illuminated lock-in thermography, and forward- and reverse-bias dark lock-in thermographies. Images were collected on CIGS devices deposited at the National Renewable Energy Laboratory with intentional spatial inhomogeneities of the material parameters. Photoluminescence imaging shows brightness variations, which correlate to the device open-circuit voltage. Photoluminescence and electroluminescence imaging on CIGS solar cells show dark spots that correspond to bright spots on images from illuminated and forward-bias lock-in thermography. These image-detected defect areas are weak diodes that conduct current under solar cell operating conditions. Shunt defects are imaged using reverse-bias lock-in thermography. The authors show how imaging can be used to detect structural defects detrimental to solar cell performance. The images provide defect locations that are analyzed in more detail by scanning electron microscopy techniques using top view and cross section imaging.
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J. Vac. Sci. Technol. A 24, 1197 (2006); http://dx.doi.org/10.1116/1.2167077 (6 pages) Online Publication Date: 21 June 2006
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Ultrathin coatings of fluorosilane films for silicon and polydimethylsiloxane (PDMS) nanochannels are desirable to control the hydrophobicity of the surface and reduce or prevent undesired protein adsorption or cell interactions critical for the performance of most biomedical micro/nanodevices. Surface modifications using vapor-phase deposition become increasingly important for some biomedical nanodevices and have advantages over liquid-phase deposition since the vapor phase can permeate more efficiently into silicon nanochannels. In this study, vapor-phase deposition was used to deposit ultrathin films of four fluorosilanes on silicon and PDMS and identify deposition conditions for an optimal process. The films were characterized by means of a contact angle analyzer for hydrophobicity, an ellipsometer for film thickness, and an atomic force microscope for surface roughness of these films. Results of this study and relevant mechanisms are the subject of this article.
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The microstructure of sputter‐deposited coatings J. Vac. Sci. Technol. A 4, 3059 (1986); http://dx.doi.org/10.1116/1.573628 (7 pages)
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Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This paper reviews some of the recent developments in understanding the fundamental aspects of the relationship between the deposition conditions and the microstructure of sputter‐deposited thin films, with particular emphasis on the origin of the growth structure and its suppression through energetic particle bombardment. |
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Control of bombardment energy and energetic species toward a superdense titanium nitride film J. Vac. Sci. Technol. A 28, 1326 (2010); http://dx.doi.org/10.1116/1.3490018 (4 pages) Online Publication Date: 24 September 2010
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TiN deposited by dc magnetron sputtering has been widely used as a hard mask material for dielectric patterning in multilevel Cu interconnects. Typically inside a “poison-mode” regime, the film density is 4.5–4.9 g/cm3. The microstructure, varying from columnar structure to nanocrystalline, is controlled by both thermodynamics and surface kinetics through ionization, substrate bias, target voltage, etc. A relatively low density film can be correlated with a porous columnar structure, low mechanical robustness, and weak resistance to plasma etching. However, with controlled growth, an applied substrate bias does not create resputtering and crystal defects. Instead, the authors create film with a maximum density of 5.3 g/cm3. In this high density film, carrier scatterings through grain boundary are greatly suppressed and the film resistivity is as low as 95 μΩ cm, which brings additional benefits as a conductive capping layer. As it is deposited at room temperature, the process minimizes the thermal budget to the underlying low-k dielectric materials to be patterned.
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Deep GaN etching by inductively coupled plasma and induced surface defects J. Vac. Sci. Technol. A 28, 1226 (2010); http://dx.doi.org/10.1116/1.3478674 (8 pages) Online Publication Date: 3 September 2010
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GaN etching was studied in Cl2/Ar plasmas as a function of process parameters. In addition, for a better understanding of the etching mechanisms, Langmuir probe measurements and optical emission spectroscopy were carried out. Etch rate was found to depend strongly on bias power. After optimization, an etch rate greater than 1000 nm/min was achieved. A second part of this work is dedicated to the etched surface defects. An original method to estimate GaN dislocation density and to localize nanopipes in the material is presented. Columnar defects could also appear with impurities in the etching reactor. The authors also present a possible formation mechanism of those columnar defects.
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J. Vac. Sci. Technol. A 28, 1307 (2010); http://dx.doi.org/10.1116/1.3484138 (5 pages) Online Publication Date: 23 September 2010
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The structural, electrical, and optical properties of ZnO films fabricated by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) under various gas flow ratios of [H2O]/[DEZn] (VI/II ratio) ranging from 0.55 to 2.74 were systematically examined. Hall effect measurements exhibited an evident effect of the VI/II ratio on the conduction type of the intrinsic films. An n-type film was fabricated at the VI/II ratio = 0.55; however, p-type ZnO films with the hole concentration of the order of 1017 cm−3 could be achieved at VI/II ratios higher than 1.0. In particular, the highest mobility of 91.6 cm2/V s and the lowest resistivity of 0.369 Ω cm have been achieved for the specimen fabricated at the VI/II ratio = 1.10. Moreover, room-temperature photoluminescence (PL) measurements demonstrated an interstitial Zn (Zni) donor defect related emission at 2.9 eV for the n-type film, while a Zn vacancy (VZn) acceptor defect related one at 3.09 eV for the p-type films. The existence of material intrinsic defects was further confirmed by low temperature PL measurements conducted at 10 K. Conclusively, the conduction type of undoped ZnO films deposited by AP-MOCVD is resolved by the VI/II ratio used, which causes the formation of various kinds of intrinsic defects, Zni otherwise VZn. p-type ZnO films with the hole concentration in the range of (1.5–3.3)×1017 cm−3 can be achieved with good reproducibility by modulating a VI/II ratio the range 1.0–2.2 for the AP-MOCVD process.
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