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Top 20 Most Read Articles
February 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Application of porcelain enamel as an ultra-high-vacuum-compatible electrical insulator J. Vac. Sci. Technol. A 18, 1751 (2000); http://dx.doi.org/10.1116/1.582418 (4 pages)
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Many accelerator vacuum system components require electrical insulation internal to the vacuum system. Some accelerator components at Brookhaven National Laboratory are installed in ultra-high-vacuum systems which require the insulation to have excellent vacuum characteristics, be radiation resistant, and be able to withstand high temperatures when used on baked systems. Porcelain enamel satisfies all these requirements. This article describes the process and application of coating metal parts with porcelain enamel to provide electrical insulation. The mechanical and vacuum testing of Marman flanges coated with porcelain and using metal Helicoflex seals to form a zero-length electrical break are detailed. The use of porcelain enameled parts is attractive since it can be done quickly, is inexpensive and environmentally safe, and most of all satisfies stringent vacuum system requirements. © 2000 American Vacuum Society. |
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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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Microfabrication of cantilever styli for the atomic force microscope J. Vac. Sci. Technol. A 8, 3386 (1990); http://dx.doi.org/10.1116/1.576520 (11 pages)
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Atomic force microscopy (AFM) is a newly developed high resolution microscopy technique which is capable of mapping forces near surfaces or, by means of these forces, the topography of the surface itself. In one mode of operation, AFM can resolve individual atoms on both conducting and insulating surfaces. A crucial component for the AFM is a flexible force‐sensing cantilever stylus, whose properties should include, among other things: a sharp tip, a low force constant, and a high mechanical resonance frequency. These requirements can be met by reducing the size of the cantilever stylus through microfabrication techniques and employing novel methods to construct a sharp tip. Presented here are a number of microfabrication processes for constructing cantilever styli with properties ideal for the AFM. These fabrication processes include (1) a method for producing thin film SiO2 or Si3N4 cantilevers without tips, (2) a method for producing Si3N4 cantilevers with integrated pyramidal tips formed by using an etch pit on the (100) surface of Si as a mold, (3) a method for producing SiO2 cantilevers with conical tips formed by a combination of isotropic and anisotropic plasma etching of a small Si post, and (4) a method for producing SiO2 cantilevers with integrated tetrahedral tips formed by anisotropically etching a corner of a small Si post to a sharp point. Each of these processes uses a (100) Si wafer as a substrate and relies on conventional batch fabrication techniques. The quality (i.e., sharpness) of the tips produced by the above methods matches or exceeds that of conventional tips used in the AFM or scanning tunneling microscope (STM). Alternative methods for producing tips by evaporation of material through an orifice or by selective chemical vapor deposition of W metal into a pyramidal etch pit in Si have been demonstrated, but these methods have not yet been successfully used in cantilever assemblies. |
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Metal–organic interface and charge injection in organic electronic devices J. Vac. Sci. Technol. A 21, 521 (2003); http://dx.doi.org/10.1116/1.1559919 (11 pages) Online Publication Date: 18 March 2003
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Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend most directly on the interface offset. The charge injection process is described as thermally assisted tunneling from the delocalized states of the metal into the localized states of the semiconductor, whose energy includes contributions from the mean barrier height, the image potential, the energetic disorder, and the applied electric field. There is no completely satisfactory analytic theory for the field and temperature dependence of the injection current, which, for well characterized interfaces, exhibits behavior relating to both thermionic emission and field-induced tunneling. © 2003 American Vacuum Society. |
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Fabrication of layered self-standing diamond film by dc arc plasma jet chemical vapor deposition J. Vac. Sci. Technol. A 25, L1 (2007); http://dx.doi.org/10.1116/1.2409940 (3 pages) Online Publication Date: 4 January 2007
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Layered self-standing diamond films, consisting of an upper layer, buffer layer, and a lower layer, were fabricated by fluctuating the ratio of methane to hydrogen in high power dc arc plasma jet chemical vapor deposition. There were micrometer-sized columnar diamond crystalline grains in both upper layer and lower layer. The size of the columnar diamond crystalline grains was bigger in the upper layer than that in the lower layer. The orientation of the upper layer was (110), while it was (111) for the lower layer. Raman results showed that no sp3 peak shift was found in the upper layer, but it was found and blueshifted in the lower layer. This indicated that the internal stress within the film body could be tailored by this layered structure. The buffer layer with nanometer-sized diamond grains formed by secondary nucleation was necessary in order to form the layered film. Growth rate was over 10 μm/h in layered self-standing diamond film fabrication.
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Module to guide the expert use of x-ray photoelectron spectroscopy by corrosion scientists J. Vac. Sci. Technol. A 25, 1 (2007); http://dx.doi.org/10.1116/1.2406058 (27 pages) Online Publication Date: 29 December 2006
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This contribution, to the potential development of data systems having some degree of “expert” character for use in x-ray photoelectron spectroscopy (XPS), illustrates the manner in which models of “Rules” might be developed by the user community. The field of corrosion science is taken as an example of one community of researchers who make regular use of XPS for well defined needs. These “needs” are redefined as a series of Goals that have to be reached in order to characterize the surface in terms of layer sequences and the enrichment of given elements within them. Rules are written to allow a structured approach to achieve each Goal. A feature of this set of Rules is that they are designed expressly to allow automated interpretation of the survey scan. This approach is facilitated by the use of a recommendation that the survey spectrum be acquired as a series of accumulated scans instead of the usual approach of making a single scan through the spectrum. Repeat scans enable the information extracted by the operation of the Rules to be processed and displayed for information during the period that is normally used for the survey scan. It is intended that this information will inform the setting up of any subsequent high resolution scans and their interactive interpretation. It will also inform any future operations such as ion etching or angle-resolved measurements. In some cases, the information made available may be all that is required by the user and in this case the “expert module” approach becomes particularly cost effective. The operation of the rules is illustrated throughout by an examination of data obtained for passivated stainless steel, giving a data set of measurements, typical of those made by corrosion scientists, that can be compared with the literature values obtained by more conventional XPS interpretation.
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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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Phase-change characteristics of chalcogenide Ge1Se1Te2 thin films for use in nonvolatile memories J. Vac. Sci. Technol. A 25, 48 (2007); http://dx.doi.org/10.1116/1.2388956 (6 pages) Online Publication Date: 29 December 2006
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In the present work, the authors show that Ge1Se1Te2 thin films provide a promising alternative for phase-change random access memory (PRAM) applications to overcome the problems of conventional Ge2Sb2Te5 PRAM devices. 100‐nm-thick chalcogenide Ge1Se1Te2 thin films were prepared by evaporating a stoichiometric bulk target, and Ge1Se1Te2 thin-film PRAM devices with a 20‐μm-sized memory cell have been fabricated. The devices exhibited a successful switching between an amorphous and a crystalline phase by applying a 50 ns, 7.3 V set pulse and a 30 ns, 7.4 V reset pulse with a switching dynamic range (the ratio of Rhigh to Rlow) as high as 103. For a static-mode switching operation, two different resistance states in Ge1Se1Te2 thin films have been observed at low voltages, depending on the two different crystalline states of the film. The first phase-transition temperature of Ge1Se1Te2 thin film is found to be 110 °C, which is clearly lower than that of Ge2Sb2Te5 films from the temperature-dependent conductivity measurements. From field emission scanning electron microscope and x-ray diffraction analyses, the authors confirmed that phase-change properties of Ge1Se1Te2 materials are closely related to the structure of the amorphous state and crystalline state.
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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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Effect of poly (3-hexylthiophene) film thickness on organic thin film transistor properties J. Vac. Sci. Technol. A 24, 1228 (2006); http://dx.doi.org/10.1116/1.2202858 (5 pages) Online Publication Date: 21 June 2006
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We present the effect of poly (3-hexylthiophene) (P3HT) thickness on the performance of organic thin film transistors (OTFTs). The P3HT film thickness varies from 11 to 186 nm. The devices have channel lengths of 5, 10, 20, 40, and 80 μm and a channel width of 500 μm. The mobility and on/off ratio are up to 0.08 cm2/V s and 7×103, respectively. The drain current and the mobility increase with thickness. At the same P3HT thickness, the drain current and mobility become higher when the channel length is reduced. The on/off ratio decreases quickly and then saturates for thickness >64 nm. Short channel devices have higher on/off ratio than long channel devices. For short channel devices (5 μm), the on/off ratio does not change significantly with thickness. The devices with shorter channel length and thicker P3HT films tend to have smaller threshold voltages. The threshold voltage saturates for long channel (20–80 μm) devices, for films thicker than 110 nm. The gate leakage (ID offset) is higher for thicker film devices. The performance dependence as a function of P3HT film thickness can be explained by the bulk conductance model and the SiO2 surface potential change. Our results suggest that the performance of P3HT OTFTs should be optimized based on the specific application.
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Crystalline alumina coatings by reactive ac magnetron sputtering J. Vac. Sci. Technol. A 25, L5 (2007); http://dx.doi.org/10.1116/1.2431353 (4 pages) Online Publication Date: 29 January 2007
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Alumina coatings were deposited on silicon (111 orientation) substrates by reactive ac magnetron sputtering. Film deposition was done using Al targets and three O2/Ar gas flow rate ratios at 5 kW power. X-ray diffraction studies showed that films were crystalline and contained several phases of alumina. Secondary ion mass spectroscopy analyses were used to measure O/Al atomic ratio and Ar and H concentrations in the films. Hydrogen content in the coatings depended on the O2 partial pressure used during sputtering and also on the arrival rate of Al and O species on the substrates and seemed to influence the crystallinity of the coatings.
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Epitaxial (SrTiO3/NiO)n/MgO multiferroic heterostructure J. Vac. Sci. Technol. A 25, 37 (2007); http://dx.doi.org/10.1116/1.2388952 (5 pages) Online Publication Date: 29 December 2006
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High quality epitaxial (SrTiO3/NiO)n/MgO thin film multiferroic heterostructures have been fabricated using reactive off-axis sputtering. Crystal quality is verified using x-ray diffraction and ion channeling. These heterostructures comprise of dielectric and antiferromagnetic layers that exhibit dielectric resonance and antiferromagnetic resonance, respectively, at terahertz frequencies. In order to achieve epitaxy despite the large lattice mismatch between SrTiO3 and NiO or MgO, high processing temperature is required. The high temperature has led to a small amount of interdiffusion between any SrTiO3/MgO interfaces. Less interdiffusion is evident for the NiO/MgO interface as seen through Rutherford backscattering. NiO and SrTiO3 have been shown to be compatible and nonreacting at temperatures as high as 1500 °C through the use of bulk ceramic techniques.
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J. Vac. Sci. Technol. A 19, 1800 (2001); http://dx.doi.org/10.1116/1.1380721 (6 pages)
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We have demonstrated large-scale production (10 g/day) of high-purity carbon single-walled nanotubes (SWNTs) using a gas-phase chemical-vapor-deposition process we call the HiPco process. SWNTs grow in high-pressure (30–50 atm), high-temperature (900–1100 °C) flowing CO on catalytic clusters of iron. The clusters are formed in situ: Fe is added to the gas flow in the form of Fe(CO)5. Upon heating, the Fe(CO)5 decomposes and the iron atoms condense into clusters. These clusters serve as catalytic particles upon which SWNT nucleate and grow (in the gas phase) via CO disproportionation: CO+CO⇒CO2+C(SWNT). SWNT material of up to 97 mol % purity has been produced at rates of up to 450 mg/h. The HiPco process has been studied and optimized with respect to a number of process parameters including temperature, pressure, and catalyst concentration. The behavior of the SWNT yield with respect to various parameters sheds light on the processes that currently limit SWNT production, and suggests ways that the production rate can be increased still further. © 2001 American Vacuum Society. |
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J. Vac. Sci. Technol. A 3, 1027 (1985); http://dx.doi.org/10.1116/1.573115 (8 pages)
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The ultraviolet (UV)/ozone surface cleaning method is reviewed. The UV/ozone cleaning procedure is an effective method of removing a variety of contaminants from surfaces. It is a simple‐to‐use dry process which is inexpensive to set up and operate. It can rapidly produce clean surfaces, in air or in a vacuum system, at ambient temperatures. By placing properly precleaned surfaces within a few millimeters of an ozone producing UV source, the process can produce clean surfaces in less than 1 min. The technique is capable of producing near‐atomically clean surfaces, as evidenced by Auger electron spectroscopy, ESCA, and ISS/SIMS studies. Topics discussed include: the variables of the process,the types of surfaces which have been successfully cleaned, the contaminants which can be removed, the construction of a UV/ozone cleaning facility, the mechanism of the process, UV/ozone cleaning in vacuum systems, rate enhancement techniques, safety considerations, effects of UV/ozone other than cleaning, and applications. |
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High-power pulsed sputtering using a magnetron with enhanced plasma confinement J. Vac. Sci. Technol. A 25, 42 (2007); http://dx.doi.org/10.1116/1.2388954 (6 pages) Online Publication Date: 29 December 2006
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High-power pulsed dc magnetron discharges for ionized high-rate sputtering of metallic films were systematically investigated. The depositions were performed using two unbalanced circular magnetrons of different types with a directly water-cooled planar copper target of 100 mm in diameter. The repetition frequency was 1 kHz at a fixed 20% duty cycle and an argon pressure of 0.5 Pa. Time evolutions of the discharge characteristics were measured to provide information on absorption of energy in the discharge plasma and on transfer of arising ions to the substrate at a target power density in a pulse up to 950 W/cm2. Time-averaged mass spectroscopy was performed at the substrate position to characterize ion energy distributions and composition of total ion fluxes onto the substrate. The deposition rate of the copper films formed on a floating substrate at the distance of 100 mm from the target was 2.2 μm/min at an average target power density over a pulse period of 96 W/cm2. Very effective ionization of sputtered copper atoms resulted in a strong predominance of copper ions (up to 92%) in total ion fluxes onto the substrate. Trends in measured values of the deposition rate per average target power density and the ionized fraction of sputtered copper atoms in the flux onto the substrate (up to 56%) were explained on the basis of model predictions.
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Nucleation theory and the early stages of thin film growth J. Vac. Sci. Technol. A 21, S96 (2003); http://dx.doi.org/10.1116/1.1600454 (14 pages) Online Publication Date: 2 September 2003
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A review is given of nucleation and growth models as applied to the earliest stages of thin film growth. Rate equations, kinetic Monte Carlo, and level set simulations are described in some detail, with discussion of remaining uncertainties, in particular the functional form of the so-called capture numbers in rate equations. Recent examples are given of sub-monolayer nucleation at surface defects, attachment-limited capture, and Ostwald ripening. The experimental literature is cited, and experiment–theory comparisons are made where possible. Emphasis is given to fast computational models that can span a large range of length and time scales, which might be further developed in the direction of on-line process control. © 2003 American Vacuum Society. |
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Growth and structure of epitaxial CeO2 by oxygen-plasma-assisted molecular beam epitaxy J. Vac. Sci. Technol. A 17, 926 (1999); http://dx.doi.org/10.1116/1.581666 (10 pages)
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The epitaxial growth of CeO2 films on SrTiO3(001) has been investigated over a wide range of growth parameters using oxygen-plasma-assisted molecular beam epitaxy. The lattice mismatch for CeO2 on SrTiO3(001) is 2.0% (compressive) if the film nucleates with a 45° rotation about [001] relative to the substrate (i.e., CeO2(001)‖SrTiO3(001) and CeO2[110]‖SrTiO3[100]). Pure-phase, single-crystalline epitaxial films of CeO2(001) with the above epitaxial relationship readily grew on SrTiO3(001) for substrate temperatures ranging from 550 to 700 °C. However, small amounts of (111) and (220) minority orientations also nucleated at the higher substrate temperatures. In addition, the film surface was observed to become progressively smoother with increasing substrate temperature due to more extensive island agglomeration. The highest-quality film surface grown at 700 °C is unreconstructed and oxygen terminated. © 1999 American Vacuum Society. |
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Quantum-dot nanodevices with carbon nanotubes J. Vac. Sci. Technol. A 24, 1349 (2006); http://dx.doi.org/10.1116/1.2201054 (7 pages) Online Publication Date: 22 June 2006
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We review our recent work on quantum-dot devices with carbon nanotubes. We conclude that the single-wall carbon nanotube quantum dot is an artificial atom with two- or four-electron shell structures. Zeeman splitting of single particle levels was observed, which is advantageous for the spin based quantum computing device (spin qubit) because the single spin is generated by putting one electron in the shell. Single-electron devices such as single-electron inverter and single-electron exclusive-OR gates have been fabricated, and their performance has been demonstrated at liquid-helium temperature. Despite the expected room-temperature operation from the single-electron charging energy, the operation temperature of our devices was limited to ∼ 10 K because of the low height of the tunnel barrier.
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Aspect ratio dependent etching lag reduction in deep silicon etch processes J. Vac. Sci. Technol. A 24, 1283 (2006); http://dx.doi.org/10.1116/1.2172944 (6 pages) Online Publication Date: 22 June 2006
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Microelectromechanical system (MEMS) device fabrication often involves three dimensional structures with high aspect ratios. Moreover, MEMS designs require structures with different dimensions and aspect ratios to coexist on a single microchip. There is a well-documented aspect ratio dependent etching (ARDE) effect in deep silicon etching processes. For features with different dimensions etched simultaneously, the ARDE effect causes bigger features to be etched at faster rates. In practice, ARDE effect has many undesired complications to MEMS device fabrication. This article presents a physical model to describe the time division multiplex (TDM) plasma etch processes and thereafter the experimental results on ARDE lag reduction. The model breaks individual plasma etch cycles in the TDM plasma etch processes into polymer deposition, polymer removal, and spontaneous silicon etching stages. With the insights gained from the model and control over the passivation and etch steps, it has been demonstrated that ARDE lag can be controlled effectively. Experiments have shown that a normal ARDE lag can be changed to an inverse ARDE lag. Under optimized conditions, the ARDE lag is reduced to below 2%–3% for trenches with widths ranging from 2.5 to 100 μm, while maintaining good etch profile in trenches with different dimensions. Such results are achieved at etch rates exceeding 2 μm/min.
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Metal-gate-induced reduction of the interfacial layer in Hf oxide gate stacks J. Vac. Sci. Technol. A 25, 261 (2007); http://dx.doi.org/10.1116/1.2435376 (8 pages) Online Publication Date: 13 February 2007
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The properties of high-κ metal oxide gate stacks are often determined in the final processing steps following dielectric deposition. We report here results from medium energy ion scattering and x-ray photoelectron spectroscopy studies of oxygen and silicon diffusion and interfacial layer reactions in multilayer gate stacks. Our results show that Ti metallization of HfO2/SiO2/Si stacks reduces the SiO2 interlayer and (to a more limited extent) the HfO2 layer. We find that Si atoms initially present in the interfacial SiO2 layer incorporate into the bottom of the high-κ layer. Some evidence for Ti–Si interdiffusion through the high-κ film in the presence of a Ti gate in the crystalline HfO2 films is also reported. This diffusion is likely to be related to defects in crystalline HfO2 films, such as grain boundaries. High-resolution transmission electron microscopy and corresponding electron energy loss spectroscopy scans show aggressive Ti–Si intermixing and oxygen diffusion to the outermost Ti layer, given high enough annealing temperature. Thermodynamic calculations show that the driving forces exist for some of the observed diffusion processes.
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