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Top 20 Most Read Articles
February 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Prevention of overload in high‐vacuum systems J. Vac. Sci. Technol. A 10, 2629 (1992); http://dx.doi.org/10.1116/1.577949 (4 pages)
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High‐vacuum pumps, like other compressors, have basic limitations in regard to the maximum pressure difference (and pressure ratio) and maximum mass (and volume) flow rates that they can produce. Because high‐vacuum pumps are usually made to discharge into another pump, their tolerable discharge pressure must be associated with the characteristics of the backing pump. However, it is also necessary to coordinate the performance of the high‐vacuum pump with the performance of the pumping system used for pre‐evacuation of the vacuum chamber. The traditional concept of a single absolute value for the crossover pressure used for the initiation of high‐vacuum pumping is fundamentally incorrect because it is not based on a clear mass flow limitation. To prevent overloading high‐vacuum pumps during and immediately after switching from pre‐evacuation to high‐vacuum pumping, a simple rule must be observed: the crossover must be performed when the gas mass flow from the vacuum chamber is less than the maximum throughput capacity of the high‐vacuum pump. Typically, at the end of the pre‐evacuation period, there are two somewhat distinct gas quantities associated with the vacuum chamber, the gas in the space of the chamber and the quasisteady outgassing rate. There are distinct pressure decays associated with those two gas quantities. The overloading of the high vacuum pump due to the space gas can be prevented by opening the high‐vacuum valve slowly or by using a parallel low‐conductance bypass. However, the overload due to the outgassing rate can only be prevented by following the golden rule of mass flow limitation. An immediate corollary of matching mass flows is that the larger the roughing pump, the lower the crossover pressure must be. In capture pumps, the maximum throughput value for the crossover condition must be correlated with the period of regeneration (for cryopumps) or cathode replacement (for ion pumps). |
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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 26, 224 (2008); http://dx.doi.org/10.1116/1.2835090 (4 pages) Online Publication Date: 23 January 2008
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ZnO films have been grown on Si(111) substrates by metal-organic chemical vapor deposition using diethylzinc and CO2 as precursors. The effects of growth temperature on growth rate, structure, and optical properties of the ZnO films were investigated in a temperature range between 450 and 700 °C. As growth temperature increased, the growth mode changed from kinetics-limited to mass-transfer-limited, and finally, to desorption-limited mode. Using the Arrhenius equation, the activation energy for the kinetics-limited growth mode was estimated to be 105 meV. The crystalline quality was also strongly dependent on growth temperature. With increasing growth temperature, the width of x-ray diffraction peaks decreases and the photoluminescence intensity enhances. Using CO2 as the oxygen source, we found that the optimal growth temperature was near 600 °C.
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Conventional triode ionization gauge with carbon nanotube cold electron emitter J. Vac. Sci. Technol. A 26, 1 (2008); http://dx.doi.org/10.1116/1.2803713 (4 pages) Online Publication Date: 14 December 2007
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The authors presented a conventional triode ionization gauge with a linear-type carbon nanotube cold electron emitter, which was made by painting technology on a nickel wire. The gauge used the ratio of the ion current to the electron current to indicate the vacuum. Although there was fluctuation in the cathode’s emission current, the ratio of the ion current to the electron current kept stable with a variation of about ±10% in each pressure decade from 10−7 to 10−3 torr. The gauge showed good measurement linearity in the vacuum range from 10−6 to 10−3 torr.
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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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Process integration for through-silicon vias J. Vac. Sci. Technol. A 23, 824 (2005); http://dx.doi.org/10.1116/1.1864012 (6 pages) Online Publication Date: 24 June 2005
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The formation of a through-silicon via (TSV) enables three-dimensional (3D) interconnects for chip-stacking applications that will be especially important for integrating heterogeneous devices. Many processing steps are involved with the major areas including: via formation; deposition of via insulation, barrier, and Cu seed films; Cu electroplating for via-fill; wafer thinning; and backside processing. The via diameter is 4–8 μm, via depth is 15–20 μm, and a 20 μm pitch is used in this study. Each step will be described in the process flow with the considerations discussed for successful process integration.
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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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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 26, 151 (2008); http://dx.doi.org/10.1116/1.2821747 (10 pages) Online Publication Date: 2 January 2008
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The surface roughness evolutions of single crystal silicon, thermal silicon dioxide (SiO2), and low dielectric constant film coral in argon plasma have been measured by atomic force microscopy as a function of ion bombardment energy, ion impingement angle, and etching time in an inductively coupled plasma beam chamber, in which the plasma chemistry, ion energy, ion flux, and ion incident angle can be adjusted independently. The sputtering yield (or etching rate) scales linearly with the square root of ion energy at normal impingement angle; additionally, the angular dependence of the etching yield of all films in argon plasma followed the typical sputtering yield curve, with a maximum around 60°–70° off-normal angle. All films stayed smooth after etching at normal angle but typically became rougher at grazing angles. In particular, at grazing angles the rms roughness level of all films increased if more material was removed; additionally, the striation structure formed at grazing angles can be either parallel or transverse to the beam impingement direction, which depends on the off-normal angle. More interestingly, the sputtering caused roughness evolution at different off-normal angles can be qualitatively explained by the corresponding angular dependent etching yield curve. In addition, the roughening at grazing angles is a strong function of the type of surface; specifically, coral suffers greater roughening compared to thermal silicon dioxide.
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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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J. Vac. Sci. Technol. A 26, 185 (2008); http://dx.doi.org/10.1116/1.2827492 (8 pages) Online Publication Date: 17 January 2008
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Nanocrystalline zirconia thin films have been deposited at ambient temperature by dc magnetron sputtering on glass and quartz substrates. The crystallite size as calculated from the x-ray diffraction patterns in the films varies between 10 and 25 nm and is dependent on oxygen percentage in the sputtering gas. Interestingly, the presence of monoclinic and cubic phase is observed for the films deposited on glass at 40%, 60%, and 80% of oxygen in the sputtering gas, while those deposited on quartz showed only the monoclinic phase. Refractive index decreased with increase in percentage of oxygen in the sputter gas. Significantly, even at 100% oxygen in the sputtering gas, films of thickness of the order of 500 nm have been grown starting from the metallic Zr target. The dielectric constants were measured using the extended cavity perturbation technique at X-band frequency (8–12 GHz). The dielectric constant and loss tangent showed a very small decrease with increase in frequency but exhibited a stronger dependence on processing parameters. The dielectric constants of the films at microwave frequencies ranged between 12.16 and 22.3.
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Effects of interfacial layer structures on crystal structural properties of ZnO films J. Vac. Sci. Technol. A 26, 90 (2008); http://dx.doi.org/10.1116/1.2821741 (7 pages) Online Publication Date: 14 December 2007
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Single crystalline ZnO films were grown on Cr compound buffer layers on (0001) Al2O3 substrates by plasma assisted molecular beam epitaxy. In terms of lattice misfit reduction between ZnO and substrate, the CrN and Cr2O3/CrN buffers are investigated. The structural and optical qualities of ZnO films suggest the feasibility of Cr compound buffers for high-quality ZnO films growth on (0001) Al2O3 substrates. Moreover, the effects of interfacial structures on selective growth of different polar ZnO films are investigated. Zn-polar ZnO films are grown on the rocksalt CrN buffer and the formation of rhombohedral Cr2O3 results in the growth of O-polar films. The possible mechanism of polarity conversion is proposed. By employing the simple patterning and regrowth procedures, a periodical polarity converted structure in lateral is fabricated. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezo response microscope images and the opposite hysteretic characteristic of the piezo response curves, which are strict evidences for the validity of the polarity controlling method as well as the successful fabrication of the periodical polarity controlled ZnO structure.
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Rarefied gas flow through short tubes into vacuum J. Vac. Sci. Technol. A 26, 228 (2008); http://dx.doi.org/10.1116/1.2830639 (11 pages) Online Publication Date: 24 January 2008
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A rarefied gas flow into vacuum through a tube of finite length is investigated over the whole range of gas rarefaction by the direct simulation Monte Carlo method. The nonequilibrium effects at the inlet and outlet of the tube have been considered by including in the computational domain large volumes of the upstream and downstream reservoirs. Results for the dimensionless flow rate and for the flow field are presented for a wide range of the gas rarefaction and for various values of the length to radius ratio in the range from 0 to 10. The influence of the gas-surface interaction model, as well as the effect of the intermolecular potential model on the gas flow, is examined. A good agreement has been obtained between the present numerical results and the corresponding experimental ones available in the literature.
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Phosphor coatings to enhance Si photovoltaic cell performance J. Vac. Sci. Technol. A 25, 61 (2007); http://dx.doi.org/10.1116/1.2393298 (6 pages) Online Publication Date: 2 January 2007
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Polycrystalline Si photovoltaic cells were coated with photoluminescent phosphors to increase the power conversion efficiency by down-conversion of high energy ultraviolet photons to less energetic visible photons, for which the Si solar cell conversion efficiency is greater. Phosphor coatings consisting of Y2O3:Eu3+, Y2O2S:Eu3+, 3,9-perylenedicarboxylic acid bis(2-methylpropyl) ester, and tetraphenylporphyrin dispersed in either polyvinyl alcohol or polymethylmethacrylate were tested. Cells coated with the Y2O3:Eu3+ or Y2O2S:Eu3+ showed an increased power conversion efficiency by up to a factor of 14 under UV illumination. The increased power conversion efficiency was shown to depend on the quantum efficiency of the down-conversion phosphor, the overlap between the photoluminescence excitation and the illumination spectra, and the overlap of the emission spectrum and the solar cell spectral response function.
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Angular etching yields of polysilicon and dielectric materials in Cl2/Ar and fluorocarbon plasmas J. Vac. Sci. Technol. A 26, 161 (2008); http://dx.doi.org/10.1116/1.2821750 (13 pages) Online Publication Date: 2 January 2008
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The angular etching yields of polysilicon in Cl2/Ar plasmas, and dielectric materials (thermal silicon dioxide and low-k dielectric coral) in fluorocarbon plasmas, have been characterized in an inductively coupled plasma beam apparatus. The effects of ion energy, feed gas composition, and plasma source pressure are studied. The experimental results showed that these etching parameters had a significant impact on the resulting angular etching yield curve. In particular, the angular etching yield curve was more sputteringlike at low plasma source pressure and/or low effective gas percentage (Cl2 and C4F8), with a peak around 60°–70° off-normal ion incident angle. In contrast, ion-enhanced-etching-like angular curves, which dropped gradually with off-normal angle, were formed at high plasma source pressure and/or high effective gas percentage. Further analysis indicated that the effective neutral-to-ion flux ratio reaching the surface was the primary factor influencing the angular etching yield curve. More specifically, the angular etching yield curve had physical sputtering characteristics at low neutral-to-ion flux ratios; while etching process was really dominated by ion-enhanced etching at high ratios and the angular curve was ion-enhanced-etching-like. The polymer deposition effects are also discussed in this article.
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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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J. Vac. Sci. Technol. A 18, 2646 (2000); http://dx.doi.org/10.1116/1.1290371 (15 pages)
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The bulk of developmental work on transparent conducting oxides (TCOs) has been somewhat empirical. This statement applies both to more familiar materials such as indium tin oxide (ITO) and to less-well-known materials that have emerged in recent years. In this article, we place a greater emphasis on more fundamental research. Our eventual goal is to gain a thorough understanding of these materials, their potential for further improvement, whether or not they suggest new and potentially superior materials, and the way their properties are influenced by structural and other issues. We also hope to provide guidelines to other researchers working in this area. We have investigated films of cadmium oxide (CdO), cadmium stannate (Cd2SnO4 or CTO), and zinc stannate [Zn2SnO4 (ZTO)]. The CdO was prepared by chemical-vapor deposition, whereas the stannates were prepared by rf sputtering. In both cases, Corning 7059 glass substrates were used. However, some depositions were also made onto tin oxide, which had a profound effect on the nucleation of CdO, in particular. It is well known that a high free-carrier mobility is essential for a TCO with near-ideal electro-optical properties. Increasing the free-carrier concentration also increases the free-carrier absorbance but a higher mobility reduces it. We have achieved free-electron mobilities in CdO (Eg∼2.4 eV) of greater than 200 cm2 V−1 s−1, of almost 80 cm2 V−1 s−1 in CTO (Eg∼3.1 eV), but of only 10–15 cm2 V−1 s−1 in ZTO (Eg∼3.6 eV). We have characterized these materials, and will show key data, using techniques as diverse as the Nernst–Ettingshausen effect; Mössbauer, Raman, optical, and near-infrared spectroscopies; atomic-force and high-resolution electron microscopy; and x-ray diffraction. These measurements have enabled us to determine the effective mass of the free carriers and their relaxation time, the probable distributions of cations between octahedral and tetrahedral sites, the role of the deposition parameters on the carrier concentrations, and the nature of the dominant scattering mechanisms. We also consider issues relating to toxicity of cadmium and to reserves of indium. Both are of great significance to prospective large-volume manufacturers of TCO films and must be taken into account by researchers. © 2000 American Vacuum Society. |
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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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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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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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