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Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films

Issue 6 • Date Nov 2005

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Displaying Results 1 - 25 of 41
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • When seeing is not believing: Oxygen on Ag(111), a simple adsorption system?

    Page(s): 1487 - 1497
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    A number of recent studies indicate that, under the oxygen rich conditions of oxidation catalysis, some transition metal catalysts may be covered by thin oxide overlayers. Moreover, it has been suggested that such “surface-oxide” layers are catalytically active, possibly more active than the pure metal surfaces as was traditionally assumed. This contemporary picture can be traced back to Ag catalysis, where over 30 years ago it was suggested that the top layer of Ag(111) reconstructed to an epitaxial Ag2O like overlayer upon exposure to oxygen [Rovida etal, Surf. Sci. 43, 230 (1974)]. Extensive experimental work, including scanning tunneling microscopy studies in which the oxide was apparently imaged with atomic resolution, as well as density-functional theory calculations, largely confirmed this interpretation. However, a review of published experimental data and new density-functional theory results presented here indicate that previous conclusions are significantly incomplete and that the structure of this original surface oxide must be reconsidered. View full abstract»

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  • Quest for high brightness, monochromatic noble gas ion sources

    Page(s): 1498 - 1508
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    Focused ion beam (FIB) machines are key tools for state-of-the art sample preparation in electron microscopy, for characterization and repair in material sciences, for the semiconductor industry and for nanotechnology in general. Liquid-metal ion sources (LMIS) are widely used in FIB machines because they meet the minimum ion source requirements such as source brightness and reliability. However, in FIB machines, noble gas ion sources are favorable for sputtering, beam-induced etching and deposition, because the implanted ions do not change the electrical behavior of the substrate significantly. There are several efforts by various researchers to develop noble gas ion sources that can be used in FIB machines instead of LMIS. The gas ion sources could not meet the minimum ion source requirements. Therefore, LMIS are still a popular choice among FIB machine users. This review article takes a critical look at the reported efforts in the literature to develop noble gas ion sources for FIB machines. View full abstract»

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  • Prediction of ultraviolet-induced damage during plasma processes in dielectric films using on-wafer monitoring techniques

    Page(s): 1509 - 1512
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    We measured electron-hole pairs generated in dielectric film using our developed on-wafer monitoring technique to detect electrical currents in the film during the plasma etching processes. The electron-hole pairs were generated by plasma induced ultraviolet (UV) photons, and the number of electron-hole pairs depends on the UV wavelength. In SiO2 film, UV light, which has a wavelength of less than 140 nm, generates electron-hole pairs, because the band gap energy of the film is 8.8 eV. On the other hand, in Si3N4 film, which has a band gap energy level of 5.0 eV, UV light below 250 nm induces the electron-hole pairs. Additionally, we evaluated the fluorocarbon gas plasma process that induces UV radiation damage using multilayer sensors that consisted of both SiO2 and Si3N4 stacked films. In these cases, electron-hole pair generation depended on the dielectric film structure. There were more electron-hole pairs generated in the SiO2 deposited on the Si3N4 film than in the Si3N4 deposited on the SiO2 film. As a result, our developed on-wafer monitoring sensor was able to predict electron-hole pair generation and the device characteristics. View full abstract»

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  • Reactive magnetron sputtering of hard Si–B–C–N films with a high-temperature oxidation resistance

    Page(s): 1513 - 1522
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    Based on the results obtained for C–N and Si–C–N films, a systematic investigation of reactive magnetron sputtering of hard quaternary Si–B–C–N materials has been carried out. The Si–B–C–N films were deposited on p-type Si(100) substrates by dc magnetron co-sputtering using a single C–Si–B target (at a fixed 20% boron fraction in the target erosion area) in nitrogen-argon gas mixtures. Elemental compositions of the films, their surface bonding structure and mechanical properties, together with their oxidation resistance in air, were controlled by the Si fraction (5–75%) in the magnetron target erosion area, the Ar fraction (0–75%) in the gas mixture, the rf induced negative substrate bias voltage (from a floating potential to -500 V) and the substrate temperature (180–350 °C). The total pressure and the discharge current on the magnetron target were held constant at 0.5 Pa and 1 A, respectively. The energy and flux of ions bombarding the growing films were determined on the basis of the discharge characteristics measured for the rf discharge dominating in the deposition zone. Mass spectroscopy was used to show composition of the total ion fluxes onto the substrate and to explain differences between sputtering of carbon, silicon and boron from a composed target in nitrogen-argon discharges. The films, typically 1.0–2.4 μm thick, possessing a density around 2.4 g cm-3, were found to be amorphous in nanostructure with a very smooth surface (Ra≤0.8 nm) and good adhesion to substra- - tes at a low compressive stress (1.0–1.6 GPa). They exhibited high hardness (up to 47 GPa) and elastic recovery (up to 88%), and extremely high oxidation resistance in air at elevated temperatures (up to a 1350 °C substrate limit). View full abstract»

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  • Physical properties of high pressure reactively sputtered TiO2

    Page(s): 1523 - 1530
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    We present a study of the physical properties of TiO2 thin films deposited at 200 °C on Si by high pressure reactive sputtering, a nonconventional deposition method. Just after deposition, the TiO2 films were in situ annealed in the deposition chamber at temperatures between 600 and 900 °C in O2 atmosphere. Morphological, compositional, structural and electrical characterization of the samples was performed by means of several techniques, including transmission electron microscopy, heavy-ion elastic recoil detection analysis, infrared spectroscopy, x-ray and electron diffraction and capacitance-voltage measurements. Microscopy images show that the TiO2 films are polycrystalline, and that a SiO2 film spontaneously grows at the TiO2/Si interface. The unannealed TiO2 films are oxygen rich, as shown by compositional measurements. By annealing this oxygen excess is released. For temperatures above 600 °C the TiO2 films are stoichiometric. Infrared spectroscopy and diffraction measurements show that as-deposited films are a mixture of anatase and rutile grains. During annealing there is a phase transformation, and at 900 °C the anatase phase disappears and only the rutile phase is found. The relative dielectric permittivity of the TiO2 film is calculated from capacitance-voltage measurements, and very high values in the 88–102 ran- - ge are obtained. View full abstract»

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  • Experimental and numerical study of the optimal operation pressure within Crookes radiometer

    Page(s): 1531 - 1534
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    The optimal operation pressure within Crookes radiometer is studied using experimental and direct simulation Monte Carlo (DSMC) method. The rotational rates of the rotor in the Crookes radiometer versus pressure are measured using a freely rotating rotor with two different dimensions, one is called a 20 mm rotor, which the side length of rotor blade is 20 mm, and the other is called a 10 mm one. The experiment results indicate that the optimal operation pressure varies with the side length of rotor blade and appears at 0.1 Pa and 3 Pa in the cases of 20 mm and 10 mm rotors, respectively. The pressure difference distributions of gas on the back and front surfaces of the rotor blades are simulated by DSMC method. It is indicated that the maximum pressure differences appear at 0.1–0.5 Pa in the case of the 20 mm rotor, and at 2–5 Pa in the case of the 10 mm rotor, respectively. The results are consistent with the experiment results. The velocity vector fields of gas within Crookes radiometer with the 20 mm rotor under four different pressures are also presented. View full abstract»

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  • Preparation of bead metal single crystals by electron beam heating

    Page(s): 1535 - 1537
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    For the fabrication of small metal bead crystals a gas flame is used to melt a wire forming a liquid droplet which solidifies upon cooling into a single crystal metal bead. Due to oxidation under ambient conditions bead crystals can be formed only from noble metals using this method. Here we describe a method how to fabricate bead crystals from a wide variety of metals and metal alloys (Cu, Mo, Ru, Rh, Pd, Ag, Ta, W, Re, Ir, Pt, Au, PtPd, Pd80Pt20, PtRh, AuAg, and PtIr) by electron beam heating under vacuum conditions. Narrow x-ray diffraction peaks confirm a high crystal quality of the bead crystals. View full abstract»

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  • Effect of blade-surface-roughness on the pumping performance of a turbomolecular pump

    Page(s): 1538 - 1544
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    Turbomolecular pumps (TMPs) are widely used in the semiconductor and other thin film industries. Some semiconductor processes form corrosive gases such as HCl or HF as byproducts. The elements of a TMP are sometimes coated with ceramic (SiO2) film for the purpose of preventing corrosion of the TMP. The blades coated with SiO2 have relatively rough surfaces. The effect of the surface roughness of the blades on the pumping performance has been studied experimentally and theoretically. Experimental results for TMPs with two rotor disks and one stator disk show that the TMP coated with SiO2 film gives about 11% to 13% higher maximum-compression ratio than the noncoated TMP when the blade speed ratio is 0.47. The theory based on the conic peak/dimple-surface-roughness model that has been proposed by the authors explains the change in the compression ratio with the surface roughness shown in the experiment. View full abstract»

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  • Low-energy sputtering yields of tungsten and tantalum

    Page(s): 1545 - 1547
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    The sputtering yields of tungsten and tantalum due to low-energy (30–125 eV) xenon ion bombardment in a plasma environment are measured both using weight loss and spectroscopic techniques. The yields of both tungsten and tantalum are practically identical and show a reduction of 3 when compared to the molybdenum yield at 125 eV and a reduction of a factor of 10 when compared to the molybdenum yield at 50 eV. View full abstract»

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  • High current pulsed electron beam treatment of AZ31 Mg alloy

    Page(s): 1548 - 1553
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    This paper reports, for the first time, an analysis of the effect of High Current Pulsed Electron Beam (HCPEB) on a Mg alloy. The AZ31 alloy was HCPEB treated in order to see the potential of this fairly recent technique in modifying its wear resistance. For the 2.5 J/cm2 beam energy density used in the present work, the evaporation mode was operative and led to the formation of a “wavy” surface and the absence of eruptive microcraters. The selective evaporation of Mg over Al led to an Al-rich melted surface layer and precipitation hardening from the over saturated solid solution. Due to the increase in hardness of the top surface layer, the friction coefficient values were lowered by more than 20% after the HCPEB treatments, and the wear resistance was drastically (by a factor of 6) improved. The microhardness of the HCPEB samples was also increased significantly down to a depth of about 500 μm, far exceeding the heat-affected zone (about 40 μm). This is due to the effect of the propagation of the shockwave associated with this HCPEB treatment. View full abstract»

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  • Crystallization and chemical structures with annealing in ZrO2 gate insulators studied by photoemission spectroscopy and x-ray absorption spectroscopy

    Page(s): 1554 - 1557
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    We have performed photoemission spectroscopy and x-ray absorption spectroscopy (XAS) to investigate the chemical states and the crystallization from the amorphous structure by annealing of the ZrO2 gate insulators on Si. Angular-dependent core-level photoemission spectra revealed the chemical states including the interfacial layers. Annealing-temperature dependence in valence-band spectra and XAS revealed the relationship between crystallization and the changes in spectral line shapes although core-level photoemission spectra are not sensitive to the crystallization. Valence-band spectra are split into double peak structures and the line shapes of O K-edge x-ray absorption spectra become sharp by the annealing at 800 °C corresponding to the crystallization temperature of amorphous ZrO2 films. It suggests that the valence-band spectra and XAS can be utilized to characterize the crystallization features in the gate insulators. View full abstract»

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  • Modeling complex vapor-transport systems using Monte-Carlo techniques: Radioactive ion beam applications

    Page(s): 1558 - 1567
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    A Monte-Carlo code has been developed that can be used to optimally design vapor transport systems for isotope-separator-on-line-based radioactive ion beam facilities in lieu of costly iterative trial and error design methods. The code provides a powerful means for delineating diffusion-release and effusive-flow (molecular-flow) processes, in combination, the delay times of which are principal intensity limiters of short-lived radioactive species at such facilities. The code provides time dependent particle evacuation, average distance traveled per particle, and particle/wall interaction information during particle transit through a given vapor-transport system under molecular-flow conditions, independent of the chemistry between particles of interest and the materials of which the transport system are constructed. In addition, the code provides powerful graphical insight via particle trajectories that serve as strong assets in arriving at a final design by identifying regions within the transport system where hold-up times are problematical. In this article, we compare simulation and experimental measurement results for transport of noble gases through selected vapor-transport systems using both cosine and isotropic particle re-emission distributions about the normal to the surface following adsorption (isotropic re-emission distributions are found to be in close agreement with experimental measurements) and describe a concept vapor-transport system that reduces transport times over those of conventional systems by two orders of magnitude. View full abstract»

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  • Crystallization and segregation in vitreous rutile films annealed at high temperature

    Page(s): 1568 - 1574
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    Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m-TiO2 (monoclinic P21/c space group) and α-TiO2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation shows that small m-TiO2 crystallites are able to form directly from vitreous TiO2 at 1273 K. However, the preferred mechanism for forming α-TiO2 is likely to be by epitaxial growth at crystalline rutile twin boundaries (secondary crystallization). Both phases are denser than crystalline rutile and reduce the average thermal stress in the films. View full abstract»

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  • Diamond microstructures fabricated using silicon molds

    Page(s): 1575 - 1578
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    We have fabricated diamond microstructures using silicon molds into which microcrystalline diamond was deposited by plasma assisted chemical vapor deposition. Prior to diamond deposition the silicon molds were seeded with the diamond powder of two different grain sizes, 14 and 1 μm. Scanning electron microscopy and atomic force microscopy were used to view and characterize the resultant diamond microstructures. Analysis of the diamond replicas indicates better reproduction fidelity for the 14 μm diamond powder, and a surface roughness of 20 nm. View full abstract»

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  • Morphology evolution on diamond surfaces during ion sputtering

    Page(s): 1579 - 1587
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    We have conducted an extensive study of the evolution of surface morphology of single crystal diamond surfaces during sputtering by 20 keV Ga+ and Ga++H2O. We observe the formation of well-ordered ripples on the surface for angles of incidence between 40 and 70°. We have also measured sputter yields as a function of angle of incidence, and ripple wavelength and amplitude dependence on angle of incidence and ion fluence. Smooth surface morphology is observed for ≪40°, and a transition to a step-and-terrace structure is observed for ≫70°. The formation and evolution of well-ordered surface ripples is well characterized by the model of Bradley and Harper, where sputter-induced roughening is balanced by surface transport smoothing. Smoothing is consistent with an ion-induced viscous relaxation mechanism. Ripple amplitude saturates at high ion fluence, confirming the effect of nonlinear processes. Differences between Ga+ and Ga++H2O in ripple wavelength, amplitude, and time to saturation of amplitude are consistent with the increased sputter yield observed for Ga++H2O. For angle of incidence ≪40°, an ion bombardment-induced “atomic drift” mechanism for surface smoothing may be responsible for suppression of ripple formation. For Ga++H2O, we observe anomalous formation of very large amplitude and wavelength, poorly ordered surface ridges for angle of incidence near 40°. Finally, w- - e observe that ripple initiation on smooth surfaces can take place by initial stochastic roughening followed by evolution of increasingly well-ordered ripples. View full abstract»

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  • On the origin of a-type threading dislocations in GaN layers

    Page(s): 1588 - 1591
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    The origin of threading dislocations (TDs) in GaN epitaxial layers grown on sapphire (0001) substrate is investigated using moiré fringes from plan-view transmission electron microscopy. The studied samples are nucleation layers deposited at 560 °C for times ranging from 20 s to 180 s. This initial stage growth gives rise to islands which are randomly rotated and relaxed with misfit dislocations. The islands that start to coalesce from 60 s growth time keep this random orientation and this leads to the bending of 60° misfit dislocations in the interface plane to form a-type TDs inside low angle boundaries. View full abstract»

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  • Etching of high aspect ratio features in Si using SF6/O2/HBr and SF6/O2/Cl2 plasma

    Page(s): 1592 - 1597
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    We have investigated the etching of high aspect ratio holes (∼4 μm deep, ∼0.2 μm diameter) in silicon using plasmas maintained in mixtures of SF6, O2, and HBr or Cl2 gases. The etching experiments were conducted in a low pressure (25 mTorr), high density, inductively coupled plasma etching reactor with a planar coil. Visualization of the profiles with scanning electron microscopy is used in conjunction with plasma diagnostics such as optical emission and mass spectroscopies to understand the key factors that control the feature profile shape and etch rate. HBr addition to SF6/O2 mixture reduces the F-to-O ratio, increases sidewall passivation and reduces mask undercut. Addition of Cl2 to SF6/O2 discharge also decreases the F-to-O ratio, but Cl-enhanced F chemical etching of silicon significantly increases the mask undercut and lateral etching. In both SF6/O2/HBr and SF6/O2/Cl2 mixtures, reduction of O2 flow rate and subsequent increase of the halogen-to-O ratio eventually results in significant lateral etching because of the lack of oxygen required to form a siliconoxyhalide passivating film on the sidewalls. View full abstract»

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  • Silicon etch by fluorocarbon and argon plasmas in the presence of fluorocarbon films

    Page(s): 1598 - 1604
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    Molecular dynamics simulations have been conducted to study the mechanisms of silicon etch in the presence of fluorocarbon species (CF and C4F4), F atoms and Ar+ ions. The specific goal of the study was to find conditions in which steady Si etching occurs in the presence of a fluorocarbon (FC) film. Results indicate that if incident species are not properly chosen for the simulation, either steady etching is observed with no FC film present, or a FC film is present (often continuously growing in thickness) with no steady etching of the underlying film. With the proper set of incident species, C/F ratio, neutral/ion flux ratio, and ion energy, we observed steady Si etching in the presence of a steady FC film. We also observed that the thicker the FC film, the lower the etch yield. A sufficiently thick film results in no etching and a continuous deposition. Simulation results are in qualitative agreement with analogous experimental measurements. The key is to find FC species that will stick with a high probability, forming a relatively open and porous film. In addition, our results suggest that this film will fluctuate in thickness from impact to impact, resulting in better transport of incident F and SiFx species, to and from the underlying Si, respectively. Steady state etching appears unlikely if the overlying FC film has the hard, dense, cross-linked character of films deposited from energetic fluorocarbon species. The basic mechanisms of etching, and the composition and depth of the underlying layers appear to be largely unaffected by the presence of the FC film. We have found ion energy deposition at a range of depths to be crucially important in the creation and transport of etch products. Ion energy deposition, ion-induced mixing, and reac- - tion promotion are the key processes in all of the ion-assisted processes simulated, including the present case of etching in the presence of FC film. The FC film retards etching by slowing the rate at which etchants (e.g., F) reach the underlying film and the rate at which etch products leave. If the film is sufficiently thick or dense, etching will cease. View full abstract»

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  • Ammonium chloride complex formation during downstream microwave ammonia plasma treatment of parylene-C

    Page(s): 1605 - 1609
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    In this work, parylene-C is exposed to the effluent from a microwave ammonia plasma with a goal of producing primary amine groups on the parylene-C. These amine groups are desired as sites for the attachment of various biomolecules that will influence the biocompatibility of the parylene-C. Ammonia plasma treatment is an effective approach for creating amine species on polymers. In this work, attenuated total reflectance infrared spectroscopy studies showed that no primary amine groups resulted from this treatment of parylene-C. Instead, reactive nitrogen-bearing radicals from the plasma appear to have been complexed by chlorine in the polymer. The formation of these complexes scavenged nitrogen-bearing radicals from the plasma and prevented the formation of nitrogenous species, such as the desired primary amines, on the parylene-C. These results are consistent with results of ammonia plasma treatment of other chlorinated polymers and suggest that alternative approaches are required to create nitrogen-bearing species on parylene-C. View full abstract»

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  • Electron conditioning of technical aluminium surfaces: Effect on the secondary electron yield

    Page(s): 1610 - 1618
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    The effect of electron conditioning on commercially available aluminium alloys 1100 and 6063 was investigated. Contrary to the assumption that electron conditioning, if performed long enough, can reduce and stabilize the secondary electron yield (SEY) to low values (≤1.3, the value for many pure elements), the SEY of aluminium did not go lower than 1.8. In fact, it reincreased with continued electron exposure dose. The SEY was monitored as a function of electron dose and the surface chemistry was measured with x-ray photoelectron spectroscopy (XPS). The XPS carbon and aluminium core levels showed that the late increase in SEY is due to electron desorption of adsorbed gas, thereby exposing high-SEY Al2O3 View full abstract»

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  • Hafnium diboride thin films by chemical vapor deposition from a single source precursor

    Page(s): 1619 - 1625
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    High quality, stoichiometric thin films of hafnium diboride are deposited by chemical vapor deposition from the precursor Hf[BH4]4 at deposition temperatures as low as 200 °C. An activation energy of 0.43 eV(41 kJ/mol) is obtained for the overall process as monitored by temperature programmed reaction studies. Films deposited at low temperatures (≪500 °C) are structurally amorphous to x-ray diffraction; a 12 nm thick film is sufficient to prevent copper diffusion into silicon during a 600 °C anneal for 30 min. Films deposited above 500 °C are crystalline, but have a columnar microstructure with low density. All the films are metallic, but the low temperature amorphous films have the lowest resistivity ∼440 μΩ cm. The process is also highly conformal, e.g., a 65 nm wide trench with a 19:1 depth-width aspect ratio was coated uniformly. View full abstract»

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  • Dry-etch of As2S3 thin films for optical waveguide fabrication

    Page(s): 1626 - 1632
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    Plasma etching to As2S3 thin films for optical waveguide fabrication has been studied using a helicon plasma etcher. The etching effects using the processing gases or gas mixtures of O2, Ar, and CF4 were compared. It was found that the O2 plasma had no chemical etching effect to the As2S3, but it could oxidize the surface of the As2S3. The Ar plasma provided a strong ion sputtering effect to the films. The CF4 plasma exhibited a too strong chemical etch to the As2S3, leading to serious undercutting and very rough sidewalls of the waveguides. Ar and O2 gases were compared as the additives to dilute the CF4 processing gas. The etch rate of the As2S3 was reduced dramatically from over 2000 nm/min to a few hundred nm/min when the pure CF4 gas was heavily diluted with 70% Ar or O2 gas. The undercutting and sidewall roughness of the etched waveguides were also decreased greatly when above dilution was made, which was associated with an enormous weakening of the isotropic chemical etch induced by neutral reactants in the plasma. In addition, the O2 showed a better dilution effect than the Ar in reducing the etch - - rate of the As2S3; and the O2/CF4 plasma also enabled a much lower erosion rate to Al mask layers than the Ar/CF4 plasma at similar plasma conditions. The As2S3 waveguides with near vertical and very smooth sidewalls were obtained using an optimized O2/CF4 plasma. Moreover, the etching behaviors and mechanisms were explained base on the etching results, and on the characteristics of the applied plasma diagnosed using Langmuir probe and optical spectroscopy techniques. View full abstract»

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  • Low-temperature synthesis of gallium nitride thin films using electron cyclotron resonance plasma assisted pulsed laser deposition from a GaAs target

    Page(s): 1633 - 1637
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    Using reactive pulsed laser deposition assisted by electron cyclotron resonance (ECR) plasma, we have synthesized GaN thin films from a polycrystalline GaAs target at low temperatures. This was achieved by ablating the GaAs target in the reactive environment of a nitrogen plasma generated from ECR microwave discharge in pure nitrogen gas and depositing the films with concurrent bombardment by the low-energy nitrogen plasma stream. High-energy ion backscattering spectroscopy analysis shows that the synthesized films are gallium rich. Characterizations by x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy confirm the presence of GaN bonds in the films. The recorded absorption spectrum also reveals GaN stretching mode characteristic of the hexagonal GaN phase. The synthesized GaN films are transparent in the visible region and have a band gap of 3.38 eV. Optical emission from the plume during film deposition reveals that the plume created by pulsed laser ablation of the GaAs target consists mainly of monoatomic atoms and ions of gallium and arsenic. Mechanisms responsible for the formation of GaN molecules and the growth of GaN films are also discussed. View full abstract»

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Aims & Scope

The Journal of Vacuum Science and Technology A is devoted to reports of original research, review articles, and Critical Review articles.

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Editor
G. Lucovsky
North Carolina State University