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Top 20 Most Read Articles
September 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2 J. Vac. Sci. Technol. A 28, 1060 (2010); http://dx.doi.org/10.1116/1.3456123 (13 pages) Online Publication Date: 2 September 2010
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There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objective of this work was to determine relative sputter rates at a fixed angle, the measurements also examined (i) the angle dependence of the relative sputter rates, (ii) the energy dependence of the relative sputter rates, and (iii) the extent of ion beam induced reduction for some oxides. Oxide films examined include SiO2, Al2O3, CeO2, Cr2O3, Fe2O3, HfO2, In–Sn oxide, Ta2O5, TiO2 (anatase, rutile, and amorphous), and ZnO. The authors found that the sputter rates for the oxides can vary up to a factor of 2 (usually lower) from that observed for SiO2. The ratios of sputter rates relative to those of SiO2 appear to be relatively independent of ion beam energy in the range of 1–4 kV and for incident angles <50°. As expected, the extent of ion beam induced reduction of the oxides varies with the sputter angle.
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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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Vacuum-calibration apparatus with pressure down to 10−10 Pa J. Vac. Sci. Technol. A 28, 1099 (2010); http://dx.doi.org/10.1116/1.3457934 (6 pages) Online Publication Date: 2 September 2010
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The vacuum-calibration apparatus is mainly composed of the extreme high-vacuum (XHV) system, ultrahigh vacuum system, and separated-flow system. The ultimate pressure of 7.9×10−10 Pa was obtained in the XHV calibration chamber by combining the magnetically levitated turbomolecular pump and nonevaporable getter pump (NEGP). The separated-flow method was used to extend the lower limit of vacuum-gauge calibration to 10−10 Pa. The uncertainty at lower limit was reduced when taking inert gases as test gases because NEGP has no pumping speed for inert gases. For this apparatus, the combined standard uncertainties were estimated to range from 1.5% at 10−4 Pa to 3.5% at 10−10 Pa.
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On the frequency characteristic of inductor in the filter of Hall thrusters J. Vac. Sci. Technol. A 28, L9 (2010); http://dx.doi.org/10.1116/1.3457152 (5 pages) Online Publication Date: 17 August 2010
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It was found that low frequency discharge current oscillation of two inductors with the same nominal value is extremely different. To decide the general characteristic of thruster inductors, relationship between the frequency characteristic of inductors and low frequency oscillation is studied in experiments at the same operation conditions. Experimental results indicate that the differences of low frequency discharge current oscillation were caused by the different frequency characteristic of inductor. As the corner frequency of inductors increases, the discharge current low frequency oscillation declines at first and then slightly turns up after a certain frequency. Through analysis from the perspective of control system, advantageous frequency characteristics of thruster inductors in filter are suggested.
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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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200-mm-diameter neutral beam source based on inductively coupled plasma etcher and silicon etching J. Vac. Sci. Technol. A 28, 1169 (2010); http://dx.doi.org/10.1116/1.3474977 (6 pages) Online Publication Date: 3 September 2010
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The authors developed a neutral beam source consisting of a 200-mm-diameter inductively coupled plasma etcher and a graphite neutralization aperture plate based on the design of a neutral beam source that
Samukawa et al. [Jpn. J. Appl. Phys., Part 2 40, L779 (2001)]
developed. They measured flux and energy of neutral particles, ions, and photons using a silicon wafer with a thermocouple and a Faraday cup and calculated the neutralization efficiency. An Ar neutral beam flux of more than 1 mA/cm2 in equivalent current density and a neutralization efficiency of more than 99% were obtained. The spatial uniformity of the neutral beam flux was within ±6% within a 100 mm diameter. Silicon etching using a F2-based neutral beam was done at an etch rate of about 47 nm/min, while Cl2-based neutral beam realized completely no undercut. The uniformity of etch rate was less than ±5% within the area. The etch rate increased by applying bias power to the neutralization aperture plate, which shows that accelerated neutral beam was successfully obtained. These results indicate that the neutral beam source is scalable, making it possible to obtain a large-diameter and uniform neutral beam, which is inevitable for application to mass production.
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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 28, 774 (2010); http://dx.doi.org/10.1116/1.3305963 (5 pages) Online Publication Date: 29 June 2010
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This study investigated the microstructure and properties of nanocrystalline TiZrN films on AISI 304 stainless steel substrate. TiZrN films were prepared by reactive magnetron sputtering based on the previous optimum coating conditions (substrate temperature, system pressure, nitrogen flow, etc.) for TiN and ZrN thin films. The composition ratio of TiZrN coatings were adjusted by changing the Zr target power, while keeping the Ti target power constant. Experiments were conduced to find the optimum composition with desired properties. The ratio of TiZrN composition was analyzed by x-ray photoelectron spectroscopy and Rutherford backscattering spectrometer. In terms of phase formation, there were two types of coatings that were considered: single-phase solid solutions of TiZrN and interlacing nuclei of TiZr in the matrix of TiZrN. The thickness of all TiZrN films as measured by the secondary ion mass spectroscopy was about 500 nm, and the composition depth profiles indicated that the compositions in the TiZrN films were uniform from the film surface to the 304 stainless steel substrate. The crystal structure of the TiZrN films was determined by x-ray diffraction using a M18XHF-SRA diffractometer with Cu Kα radiation. A diffraction peak of TiZrN (002) was observed between that of TiN (002) and ZrN (002); similarly, a diffraction peak of TiZrN (111) was observed between that of TiN(111) and ZrN(111), respectively. The corrosion resistance of the TiZrN film deposited on the 304 stainless steel has been investigated by electrochemical measurement. The electrolyte, 0.5M H2SO4 containing 0.05M KSCN, was used for the potentiodynamic polarization. The potentiodynamic scan was conducted from −800 to 800 mV standard calomel electrode (SCE).
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Low energy Ar+ bombardment of GaN surfaces: A statistical study of ion reflection and sputtering J. Vac. Sci. Technol. A 28, 1263 (2010); http://dx.doi.org/10.1116/1.3480344 (6 pages) Online Publication Date: 3 September 2010
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Statistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar+ ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence θi. As θi becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for θi = 45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statistical results differ from the predictions of continuous ion bombardment simulations since the surfaces are not allowed to evolve self-consistently during the gathering of impact statistics.
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J. Vac. Sci. Technol. A 28, 1105 (2010); http://dx.doi.org/10.1116/1.3460904 (6 pages) Online Publication Date: 2 September 2010
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Results from molecular dynamics simulations of continuous 50–200 eV Ar+ bombardment on wurtzite and zinc blende GaN surfaces are reported. A new analytical bond-order potential, originally developed for growth process studies, is used to investigate the low-energy physical sputtering of GaN compounds. Preferential sputtering of N atoms is initially observed up to 3.5×1015 ions/cm2 fluence, after which the layers reach steady state sputtering. The crystalline structure of the GaN sample does not have a major influence on the sputtering yield due to the rapid amorphization of the top surface after a few hundred impacts. Concentration depth profiles indicate a surface enrichment in gallium with a N/Ga concentration ratio equal to 0.59±0.1 for 100 eV bombardment, in agreement with published experimental studies. For the same conditions, Ga, N, and GaN species represent 25, 60, and 7% of the sputtered products. A significant fraction of those products leave the surface with kinetic energies sufficiently high to damage the passivation layers on sidewalls during etching processes dominated by physical bombardment.
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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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Electron stimulated desorption from bare and nonevaporable getter coated stainless steels J. Vac. Sci. Technol. A 28, 1215 (2010); http://dx.doi.org/10.1116/1.3478672 (11 pages) Online Publication Date: 3 September 2010
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An installation for investigation of the electron stimulated desorption (ESD) from both pumping and nonpumping tubular samples was designed and built. This installation allows studying ESD and sample sticking probability as a function of electron dose up to about 1023 e−/m2, electron energy in the range 10 eV–6.5 keV, and sampling temperature in the range 0–80 °C. Two samples were investigated: bare and Ti–Zr–V coated stainless steels. The ESD yields were measured as a function of electron accumulated dose, electron energy, and different NEG coating activation temperatures. The effect of electron stimulated pumping of CO saturated NEG coating was demonstrated for the first time and is in a good agreement with the effect of photon stimulated NEG activation measured earlier.
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Microstructural evolution during film growth J. Vac. Sci. Technol. A 21, S117 (2003); http://dx.doi.org/10.1116/1.1601610 (12 pages) Online Publication Date: 2 September 2003
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Atomic-scale control and manipulation of the microstructure of polycrystalline thin films during kinetically limited low-temperature deposition, crucial for a broad range of industrial applications, has been a leading goal of materials science during the past decades. Here, we review the present understanding of film growth processes—nucleation, coalescence, competitive grain growth, and recrystallization—and their role in microstructural evolution as a function of deposition variables including temperature, the presence of reactive species, and the use of low-energy ion irradiation during growth. © 2003 American Vacuum Society. |
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Effect of process conditions on the microstructural formation of dc reactively sputter deposited AlN J. Vac. Sci. Technol. A 28, 1210 (2010); http://dx.doi.org/10.1116/1.3478670 (5 pages) Online Publication Date: 3 September 2010
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Thin film aluminum nitride (AlN), because of its attractive properties, is a material with many applications. Its microstructure and hence properties are greatly influenced by the deposition process conditions. In this work, AlN was reactively deposited in a dc magnetron sputtering system at different proportions of nitrogen in the process gas mixture and at different process conditions. The microstructure and composition of the films were analyzed using x-ray diffraction data, energy dispersive spectroscopy, and scanning electron microscopy. Results show that for a process gas pressure of 0.67 Pa, a magnetron power of 100 W, and a substrate-target distance of 10 cm, a near stoichiometeric AlN can be prepared at nitrogen proportions as low as 20%. At these process conditions, (002) was the preferred crystal orientation. Dense fibrous structures were obtained, especially at low deposition rates with high proportions of nitrogen. Increase in magnetron power and decrease in distance result in a more porous structure. High kinetic energies (average) of the sputtered Al particles and high deposition rates tend to favor AlN(101) formation, while low kinetic energies of the Al particles and low deposition rates generally favor more of the AlN(100) formation.
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J. Vac. Sci. Technol. A 28, 1269 (2010); http://dx.doi.org/10.1116/1.3480919 (6 pages) Online Publication Date: 3 September 2010
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Nanocrystalline TiO2 films were deposited on SnO2:F (FTO) coated glass substrate by the facing-target sputtering technique. Ruthenium complex-based dye was used to sensitize TiO2 films and carbon paste coated on FTO glass was used as a counterelectrode. The dye-sensitized solar cell (DSC) performance was investigated with various sputtering Ar/O2 gas-flow ratios of 8:2, 7:3, and 6:4. The structure of TiO2 films has shown a penniform columnar structure and the morphology has the clear dependence on the gas-flow ratio. The DSC with TiO2 electrode deposited at 7:3 gas-flow ratio exhibits the highest photocurrent (4.74 mA/cm2), maximum photoelectric-conversion efficiency (1.38%), and incident photon-to-current efficiency (58.9%) at 520 nm. Moreover, this DSC shows a longer lifetime which implies low recombination rate.
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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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Effective atomic layer deposition procedure for Al-dopant distribution in ZnO thin films J. Vac. Sci. Technol. A 28, 1111 (2010); http://dx.doi.org/10.1116/1.3460905 (4 pages) Online Publication Date: 2 September 2010
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A zinc-metal dopant-oxygen precursor exposure cycle is demonstrated as a modified deposition procedure for better distribution of Al-dopants in ZnO films by atomic layer deposition with the aim to reduce the formation of nanolaminate thin films that might form with the typically used alternating ZnO and metal oxide deposition procedure. The distribution and chemical bonding states of Al-dopants were studied with various dopant deposition intervals of Zn–Al–O precursor and Zn–O cycles at 1::4, 1::9, 1::14, and 1::19 ratios. The smallest resistivity of Al-doped ZnO film without degradation of transparency was obtained at 250 °C with 5.37×10−4 Ω cm.
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Plasma deposition of optical films and coatings: A review J. Vac. Sci. Technol. A 18, 2619 (2000); http://dx.doi.org/10.1116/1.1314395 (27 pages)
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Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others. © 2000 American Vacuum Society. |
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Control of surface roughness during high-speed chemical dry thinning of silicon wafer J. Vac. Sci. Technol. A 28, 1073 (2010); http://dx.doi.org/10.1116/1.3456124 (5 pages) Online Publication Date: 2 September 2010
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In this study, the evolution and reduction of the surface roughness during the high-speed chemical dry thinning process of Si wafers were investigated. The direct injection of NO gas into the reactor during the supply of F radicals from NF3 remote plasmas was very effective in increasing the Si thinning rate, due to the NO-induced enhancement of the surface reaction, but resulted in the significant roughening of the thinned Si surface. However, the direct addition of Ar gas, together with NO gas, decreased the root mean square surface roughness of the thinned Si wafer significantly. The process regime for the enhancement of the thinning rate and concomitant reduction of the surface roughness was extended at higher Ar gas flow rates. In this way, Si wafer thinning rates as high as 22.8 μm/min and root-mean-squared surface roughnesses as small as 0.75 nm could be obtained. The results indicate that the high-speed chemical dry thinning process using F radicals and directly injected NO/Ar gases can be applied to ultrathin Si wafer thinning with controlled surface roughness.
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Magnetic tunnel junctions with Co-based perpendicular magnetic anisotropy multilayers J. Vac. Sci. Technol. A 28, 973 (2010); http://dx.doi.org/10.1116/1.3430549 (6 pages) Online Publication Date: 29 June 2010
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Magnetic CoFeB/MgO/CoFeB-based tunnel junctions with perpendicular magnetic anisotropy Co/M multilayers (M = Ni, Pd, Pt) have been investigated as a function of structural and magnetic properties. Magnetometry, ferromagnetic resonance, x-ray diffraction, stress tests, and local electrode atom probe tomography were carried out primarily on Co/Ni multilayers. A statistical design of experiments was conducted to optimize the perpendicular magnetic anisotropy and damping parameter α of these multilayers. Seed layers, thickness, and thickness ratios are all critical to achieve perpendicular behavior. Perpendicular MgO-based magnetic tunnel junctions with Co/Ni and Co/Pd reference and free layers were fabricated and tested. Sharp MR-H switching characteristics were observed for the Co/Pd multilayers, and a somewhat softer transition was observed for the Co/Ni multilayer with a Cu seed, which did not have as high a perpendicular anisotropy. Tunneling magnetoresistance (TMR) values were limited to about 10%, primarily because the fcc-bcc-fcc transition does not promote the “MgO giant TMR” symmetry filtering effect.
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