bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemisorption of high coverages of atomic oxygen on the Pt(111), Pd(111), and Au(111) surfaces
Authors: Parker Deborah Holmes , Koel Bruce E.
Published: July 2002

Studies of the interaction of oxygen with some transition metal surfaces are limited by activation barriers to adsorption and reaction of O2. These barriers can be surmounted by using oxidants more powerful than O2 to chemisorb oxygen on these surfaces under ultrahigh vacuum (UHV) conditions, but at high ‘‘effective’’ pressures of O2. We report here on the use of very reactive molecules, such as nitrogen dioxide (NO2) and ozone (O3) to study the interaction of high concentrations of oxygen atoms on the Pt(111), Pd(111), and Au(111) single crystal surfaces. Chemisorbed oxygen adatom coverages of Θ0≤0.75 monolayer (ML) on Pt(111), Θ0≤1.4 ML on Pd(111), and Θ0≤0.80 ML on Au(111) surfaces have been characterized. On the Pd(111) surface, higher concentrations (Θ0≤3.1 ML) can be formed and the initial stages of oxidation, including migration of oxygen into the subsurface region and formation of Pd oxide can be studied. This method of cleanly forming high coverages of oxygen on metal surfaces allows for new insight into the nature of oxygen–metal interactions. For example, in the case of Pt(111) and Au(111), temperature programmed desorption (TPD) can be used to determine kinetic parameters of O2 adsorption and desorption as a function of oxygen coverage and upper limits for the metal–oxygen bond strengths over a large range of oxygen coverages can be determined from the activation energy of O2 desorption. Also, the kinetics of oxidation reactions can be studied over a much larger range of oxygen coverage than previously carried out under UHV conditions on surfaces that are relatively unreactive to O2 chemisorption, such as Pt(111) and Au(111). We report preliminary results concerning transient kinetic studies of CO oxidation over Pt(111) and Au(111) surfaces.
Intermediate layers for the deposition of polycrystalline diamond films
Authors: Hartnett T. , Miller R. , Montanari D. , Willingham C. , Tustison R.
Published: July 2002

Thin films of BN, hard carbon, a-C:H, and SiC have been deposited onto ZnS or (100)-silicon as precursors to the growth of polycrystalline diamond films. These layers must protect the substrate from chemical attack at the deposition temperatures used in microwave plasma chemical vapor deposition (CVD) and also promote the nucleation of crystalline diamond. The thermal stability, the optical properties of these coatings, and their ability to nucleate diamond growth are investigated. BN thin films appear to be the best interlayer for both protection and nucleation enhancement. Hard carbon and a-C:H interlayers provide no real benefit in themselves. Silicon carbide was identified at the interface between one type of hard carbon film and the silicon substrate using x-ray photoelectron spectroscopy (XPS). Si–C islands may provide nucleation sites for CVD diamond. Silicon carbide can provide both protection and nucleation enhancement if the crystallinity and texture are optimized.
Diamond-like carbon films grown by a large-scale direct current plasma chemical vapor deposition reactor: System design, film characteristics, and applications
Authors: Ham M. , Lou Karen A.
Published: July 2002

Diamond-like carbon (DLC) films have been grown on a small scale by many researchers. While there are many potential applications of DLC films, a method of growing them on an industrial scale has been lacking. We have developed a low-temperature dc plasma process that is larger than typical lab scale, and can easily be scaled up further. With our unique electrode configuration, a stable dc discharge could be maintained, and highly reproducible films are obtained. Films are also grown on substrates of various geometries. Good films with average thickness of 2 μm were grown on semiconductors, metals, insulators, and plastics. An ionic mechanism dominated the deposition process. Solid state 13C cross polarization magnetic angle spinning nuclear magnetic resonance was used to determine carbon sp2/sp3 ratios which were typically 1.6. Combustion and 15N nuclear reaction tests showed 33 at. % H. X-ray diffraction and transmission electron microscopy were used to check for regions of crystallinity. Infrared spectroscopy was used to confirm C–H bonding. Hardness was measured with a light load microhardness tester (10 g); average hardness was close to SiC. Friction and wear were characterized using standard pin-on-disk measurements; no wear was observed on the film-coated disk, and the coefficient of friction was 0.03–0.07 for film-against-film. The films were resistant to concentrated strong acids and stable to 460 °C in air. Some applications for low friction coatings have been explored.
The cryogenic diffusion pump and its implementation in a complete fusion reactor forevacuum system
Authors: Hemmerich J. L. , Küssel E.
Published: July 2002

A cryogenic diffusion pump has been developed and tested. It uses the main constituent of the fusion reactor exhaust gas, i.e., deuterium–tritium (D–T), as a working fluid in a diffusion pump operating at low temperature, to separate and compress the minor constituent 4He. Since a suitable tritium-handling facility was not available, a prototype was tested using mixtures of deuterium with 1%, 2%, and 4% helium. As the deuterium is pumped by cryocondensation at 4 K, the entrained helium is staying in the gas phase, is compressed, and leaves the pump outlet at pressures up to 80% of the total inlet pressure. Compression ratios of up to 80 (for 1% helium) and inlet pumping speeds ranging from 1 to 2 m3 s−1 have been measured for operating pressures between 3 and 10 Pa. Deuterium carry-over into the helium exhaust was below the detection limit of the equipment. A design is presented incorporating the cryogenic diffusion pump in a complete fusion reactor forevacuum system, including the separation of impurities from hydrogen isotope mixtures.
Comparison of TiAlN coatings grown by unbalanced magnetron and arc bond sputtering techniques
Authors: Münz W.-D. , Hurkmans T. , Keiren G. , Trinh T.
Published: July 2002

Basic research has shown that, during the etching stage of the cathodic-arc process, metal atoms of the coating material become embedded in the surface of the substrate to be coated. If 1200-eV Ti ions are used for etching, the penetration depth may be as great as 1500 Å. Paralleling this, more recent results reveal that an unbalanced magnetron can be used to produce dense, droplet-free TiN layers in an exactly reproducible form. Hence, when the arc-mode etching process is combined with unbalanced magnetron-mode coating, the resulting coating can be expected to have excellent properties. The technical implementation of this combined technique is known as ‘‘arc bond sputtering,’’ and is known commercially as ABSTM. This article describes how the process of producing TiAl-nitride layers is used to demonstrate the advantages of metal-ion pretreatment on the adhesive strength of Ti0.5Al0.5N layers applied to high-speed steel and cemented-carbide substrates. The adhesive-strength criteria are the LC and the Rockwell indentation test. The result of this study indicates that the Ti-ion pretreatment renders the measured adhesive-strength values for the subsequent films much less sensitive to uncontrolled variations of the coating-process parameters. It is also shown how the formation of TiAl droplets during the arc-based etching process can be reduced through the use of shutters.
Reflection high-energy electron diffraction patterns of carbide-contaminated silicon surfaces
Authors: Becker James P. , Long Robert G. , Mahan John E.
Published: July 2002

Carbon contamination of silicon surfaces is a longstanding concern for growers of thin films who utilize silicon wafer substrates. This contamination often takes the form of epitaxial β-SiC particles which grow after the decomposition of adsorbed carbon-bearing molecules, and the subsequent reaction of the freed carbon with the silicon substrate. Positive identification of such SiC contamination is possible via reflection high-energy electron diffraction (RHEED). To provide a complete demonstration and analysis of the relevant RHEED patterns, we prepared within a ‘‘silicon molecular beam epitaxy’’ system carbide-contaminated silicon surfaces using procedures intended to foster such contamination. With conventional RHEED instrumentation, we obtained transmission electron diffraction patterns which resulted from the passage of the RHEED electron beam laterally through the SiC particles. Comparison with theoretically predicted patterns positively identifies the β-SiC phase and shows that the particles are epitaxially aligned, with their cubic axes parallel to those of the substrate. {This finding is in agreement with the widely accepted model for the behavior of carbon on silicon surfaces [Henderson et al., J. Appl. Phys. 42, 1208 (1971)]}. More typically during in-situ silicon substrate preparation for thin film growth, RHEED patterns indicating such contamination contain SiC spots which are mere vestiges of the complete transmission diffraction patterns presented in this work.
Pull-test adhesion measurements of diamondlike carbon films on silicon carbide, silicon nitride, aluminum oxide, and zirconium oxide
Authors: Erck R. A. , Nichols F. A. , Dierks J. F.
Published: July 2002

Hydrogenated amorphous carbon or diamondlike carbon (DLC) films were formed by 400 eV methane (CH4) ion bombardment of various smooth and rough ceramics, as well as ceramics coated with a layer of Si or Ti. Adhesion was measured by a bonded-pin method. Excellent adhesion was measured for smooth SiC and Si3N4, but adhesion of DLC to smooth Al2O3 and ZrO2 was negligible. The use of a Si bonding interlayer produced good adhesion to all the substrates, but a Ti layer was ineffective due to poor bonding between the DLC film and Ti. Bulk thermodynamic calculations are not directly applicable to bonding at the interface because the interface is two dimensional and the compositions of interfacial phases are generally not known. If the standard enthalpy ΔH° for the reaction between CH4 and the substrate material is calculated under the assumption that a carbide phase is produced, a relationship is seen between the reaction enthalpy and the relative adhesion. Large positive enthalpies are associated with poor adhesion; negative or small positive enthalpies are associated with good adhesion. This relation between enthalpy and adhesion was also observed for DLC deposited on Si. The lack of adhesion to the Ti was attributed to inadvertent formation of a surface oxide layer that rendered the enthalpy for the reaction with CH4positive.
Microstructural and morphological effects on the tribological properties of electron enhanced magnetron sputtered hard coatings
Authors: Schneider J. M. , Voevodin A. A. , Rebholz C. , Matthews A.
Published: July 2002

X-ray diffraction measurements and scanning electron microscopy studies on close to stoichiometric titanium carbonitride [TiCxN(1−x)] and carbon containing tungsten coatings produced in an electron enhanced unbalanced magnetron system have been undertaken. These coatings are deposited as interlayers to improve adhesion and load support for metal diamondlike carbon (DLC) multilayers. In this study, however, only the interlayer without the top coating (metal DLC) was deposited to study and optimize this layer. The films were deposited on stainless steel substrates with an argon background pressure in the range of 0.1 Pa, introducing acetylene while sputtering tungsten and a mixture of acetylene and nitrogen while sputtering titanium. The chemical composition of the film was analyzed by a glow discharge optical spectrometer. The peak positions and integral breadth and shape parameters were determined by x-ray diffraction. Microhardness, scratch adhesion, and the tribological performance under sliding wear have been determined and correlated with the chemical composition and microstructure of the films. An optimized TiCxN(1−x) load support and adhesive interlayer could be identified.
Surface chemistry at metallic step defect sites
Authors: Yates John T.
Published: July 2002

The role of step sites on metal surfaces in promoting chemical reactions is discussed showing experimental data from the author’s laboratory. The adsorption of CO on two stepped Pt surfaces, Pt (112) and Pt (335), is compared using various methods that are sensitive to adsorption site character. The existence of one-dimensional arrays of chemisorbed CO molecules along the step sites is demonstrated, and CO...CO repulsive steric interactions are observed to cause orthogonal tilting of the step-bound CO species as the coverage is changed on the step sites. For Pt (335), the most active site for the oxidation of CO is found to involve a chemisorbed oxygen atom at the step site, reacting with a CO molecule chemisorbed on a terrace site. It is also shown that a new surface phenomenon, electron stimulated migration (ESM), may be spectroscopically observed for CO on Pt (335). The atomic steps act as trap sites for CO molecules that have been excited by 150 eV electrons. It is likely that this ESM phenomenon is a property of thermally nonaccommodated species in the electronic ground state of the chemisorbed CO that have been produced by quenching from an excited electronic state.
A morphology study of the thermal oxidation of rough silicon surfaces
Authors: Liu Q. , Spanos L. , Zhao C. , Irene E. A.
Published: July 2002

A report on the thermal oxidation of rough silicon surfaces as measured by spectroscopic immersion ellipsometry and atomic force microscopy (AFM) is given. It was found that, as the thickness of the thermally grown SiO2 overlayer increases, the average radius of the crystalline silicon protrusions (roughness) at the interface decreases. A fractal analysis shows that a simpler surface results from oxidation. The frequency spectra of AFM images are concordant with the fractal analysis and shows that small features oxidize faster. This is in agreement with the predictions of the Kelvin equation that small features are more reactive.
Deep levels near ‘‘buried’’ ZnSe/GaAs(100) heterointerfaces
Authors: Raisanen A. , Brillson L. J. , Vanzetti L. , Sorba L. , Franciosi A.
Published: July 2002

We have used low energy cathodoluminescence spectroscopy (CLS) and laser photoluminescence spectroscopy (PLS) to identify the energies and depth variation of deep levels near the buried, molecular beam epitaxy-grown ZnSe/GaAs(100) heterointerface. Using a combination of electron excitation energies and laser wavelengths to obtain CLS/PLS ‘‘depth profiles’’ through the interface regions of 150–500-nm-thick ZnSe epilayer/GaAs substrate structures, we find that deep electronic states are formed near the heterojunction and within the epilayer whose relative emission intensities vary dramatically with beam pressure ratio. For Zn (Se)-rich interface, features at 0.9, 1.0, and 1.14 eV (1.3 eV) are dominant. An additional feature at 1.9 eV appears with high temperature annealing. In general, intensities of the 1.3 and 1.9 eV features appear most intense for excitation depths corresponding to the buried interface regions. These spectral features demonstrate substantial differences in the localized interface states formed under variant epilayer growth conditions. They also provide an effective in situ monitor of heterointerface quality and stability.
Surface chemical state populations in the molecular beam epitaxy deposition of BaF2 on GaAs by x-ray photoelectron spectroscopy and heavy-ion backscattering spectroscopy
Authors: Stumborg M. F. , Chu T. K. , Santiago F. , Price J. L. , Guardala N. A. , Land D. J.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and heavy-ion backscattering spectroscopy (HIBS) were used to investigate the chemical states of barium atoms in submonolayer BaF2 depositions on GaAs (100) substrates. XPS measurements showed barium atoms to have two different chemical states at the BaF2/GaAs interface. The relative abundance of these two states was also determined by XPS. The number of barium atoms in each state was found by normalizing integrated XPS peak intensities to HIBS measurements of the total number of barium atoms on the surface. The results were used to formulate a model of a two-stage deposition mechanism whereby BaF2 first reacts with gallium at the surface and dissociates, releasing gaseous GaF. This reaction is apparently self-limiting, resulting in a barium ‘‘template’’ layer that enables subsequent BaF2 molecules to form an epitaxial (100)-oriented film.
Dust formation and charging in an Ar/SiH4 radio-frequency discharge
Authors: Stoffels E. , Stoffels W. W. , Kroesen G. M. W. , de Hoog F. J.
Published: July 2002

The formation and charging of submicrometer dust particles in a low pressure argon/silane radio-frequency (rf) discharge was studied using laser-induced photodetachment in combination with a microwave resonance technique. This method allows a measurement of the spatially averaged electron density, the spatially resolved negative ion density, and/or the charge on small clusters in the plasma as a function of time during particle formation. The loss frequency of photodetached electrons yields information about the recharging of small clusters. During the first second after plasma ignition dust particles are formed. Simultaneously, the electron density decreases from about 2×1015 m−3 to about 4×1014 m−3. In the first 10 ms after discharge ignition, charged particles are not present in the plasma and the photodetachment experiment gives a negative ion density of 4×1015 m−3. During the first 50 ms after plasma ignition, nanocrystallites are formed, which is reflected by a strong increase of the loss frequency of photodetached electrons. After 50 ms the particles start to coalesce and acquire a negative charge, which results in a strong increase of the photodetachment signal. After 1 s of plasma operation, the charge density on particles is about 8×1016 m−3. The photodetachment signal decreases with the gas flow rate, indicating that the clusters are expelled from the plasma by the gas flow.
Zinc sulfide on GaP(110): Characterization of epitaxial growth and electronic structure
Authors: Wolfframm D. , Bailey P. , Evans D. A. , Neuhold G. , Horn K.
Published: July 2002

Epitaxial layers of ZnS were grown on cleaved GaP(110) surfaces by molecular beam epitaxy in an ultrahigh vacuum photoelectron spectrometer. The growth mode and the structure of the overlayer were studied by means of low-energy electron diffraction (LEED) and core as well as valence level photoemission using synchrotron radiation. The attenuation of substrate core-level intensities with ZnS deposition indicate layerwise growth. LEED demonstrates the growth of the cubic (zinc-blende) phase as expected for substrate-stabilized growth. A minor interface reaction is evident from changes in the appearance of the substrate (Ga 3d) and overlayer (S 2p) core levels with increasing thickness. S–Ga bonding was observed in a thin interfacial layer. The valence band offset for this lattice-matched heterojunction interface system was determined, and found to be of the straddling type (type I); its magnitude is in agreement with predictions based on the dielectric midgap energy model.
Effect of herringbone reconstruction on interference of two-dimensional electron gas on Au(111) surfaces
Authors: Fujita D. , Amemiya K. , Yakabe T. , Nejoh H. , Sato T. , Iwatsuki M.
Published: July 2002

Low temperature scanning tunneling microscopy study at ∼30 K has revealed that the long-range “herringbone” reconstruction has a significant effect on the formation of standing waves in the Fermi-level surface-state electron density on an Au(111)-(23×∛) reconstructed surface. The anisotropic standing-wave formation has been clearly observed on terraces, on a monolayer island, and in a monolayer depression. The amplitudes of the standing waves were found to be enhanced along pairwise soliton walls. The observed anisotropy of the standing-wave formation suggests the important role of the herringbone-reconstruction structure on the propagation and interference of surface-state electron waves.
Kinetic study of low energy argon ion-enhanced plasma etching of polysilicon with atomic/molecular chlorine
Authors: Chang Jane P. , Arnold John C. , Zau Gavin C. H. , Shin Hyung-Shik , Sawin Herbert H.
Published: July 2002

Surface kinetics of ion-enhanced chlorine plasma etching in the low ion energy regime was studied by utilizing Ar+ for ion bombardment and Cl and Cl2 as reactants. The argon ion and chlorine atom (molecular) fluxes were controlled independently over more than an order of magnitude and at flux levels within an order of magnitude of that typically used in high density plasma processes. The ion-enhanced etching yield was characterized as a function of Ar ion energy, ion flux, neutral-to-ion flux ratio, and the ion incident angle. Possible reaction pathways are proposed and reduced into a two-parameter model which is useable in a profile simulator. The etching yield increases with the increase of flux ratio but gradually saturates at higher flux ratios as the ion flux limits the etching yield. The ion energy dependence was found to scale linearly with (Eion1/2−Eth1/2), where the threshold energy Eth is found to be 16 eV. The etching yield of Cl is found to be similar to that of Cl2 at flux ratios below 10, but 4–5 times higher than that of Cl2 at higher flux ratios. The sticking coefficient of Cl2 is similar to that of Cl for smaller neutral-to-ion flux ratios where the ion bombardment produces highly reactive surface silicon sites for both atomic and molecular chlorine. At high neutral-to-ion flux ratios, however, the dissociative adsorption of molecular chlorine is slower than the adsorption of atomic chlorine on the highly chlorinated surfaces. The angular dependence of ion-enhanced etching yield was also measured. The etching yield was reduced by approximately 35% when an ion impingement angle was changed from normal impingement to 60° off-normal.
Growth and characterization of aluminum oxide thin films for evaluation as reference materials
Authors: Gaarenstroom Stephen W.
Published: July 2002

With the aid of aluminum oxide reference films, analytical techniques that previously provided only relative thickness information can now report absolute mass thickness measurements for aluminum oxide films. These new reference films were dense, nonporous, anodic aluminum oxide films grown by the method of Haas with a thickness range from 12 to 135 nm. Dimensional thickness measurements were made by transmission electron microscopy and spectroscopic ellipsometry. Absolute mass thickness measurements were made by electron probe microanalysis and energy-dispersive x-ray microanalysis. Relative thickness measurements were made by infrared spectroscopy, x-ray photoelectron spectroscopy depth profiling, and wavelength-dispersive x-ray fluorescence spectroscopy. Characterization measurements show the growth recipe (1.35 nm/V) predicts the dimensional thickness within 5%–10% and the mass thickness within 10%. Little or no evidence of hydration or phase change is observed after five years storage. Numerous surface-processing procedures involving aluminum material, such as brazing, welding, wetting, bonding, stamping, and rolling, will benefit from improved measurements of oxide-layer thicknesses on aluminum or aluminum alloys.
Experimental investigation of high Si/Al selectivity during anisotropic etching in tetra-methyl ammonium hydroxide
Authors: Pandy A. , Landsberger L. M. , Nikpour B. , Paranjape M. , Kahrizi M.
Published: July 2002

This work contributes to the understanding of anisotropic etching of silicon, for microsystems technology, by studying Si/Al selectivity during anisotropic etching of silicon in tetra-methyl ammonium hydroxide (TMAH). By under-etch experiments using a wagon-wheel mask pattern, Si/Al selectivity is studied in relation to trends in etch anisotropy, etched surface morphology, and variations of under-etch behavior with mask-edge angle. TMAH at 5 wt % is used, with or without the additives: dissolved silicon and ammonium persulfate. High Si/Al selectivity is accompanied by obvious changes in the roughness, flatness, and etch rate of the {100} cavity bottoms, by large changes in anisotropy as seen in under-etch rate curves, by lower ratio of {101} to {100} etch rates, and by more regular 〈101〉-oriented steps on non-{111} cavity sidewalls. The conditions are consistent with a low rate of attack of 〈101〉-directed periodic bond chains in the Si lattice.
Studies of metallic multilayer structures, optical properties, and oxidation using in situ spectroscopic ellipsometry
Authors: Gao Xiang , Hale Jeff , Heckens Scott , Woollam John A.
Published: July 2002

In situ spectroscopic ellipsometry (SE) has been successfully used to accurately measure sputter deposition rates and optical constants of un-oxidized metal layers and to control the growth of magnetic multilayers. The structures include [Co/Cu]n, [Co/Au]n, [Co/Ni]n, [Co/Pd]n and [Co/Pd/Au]n. Layer thickness precision is better than ±0.05 nm for layer thicknesses in the range of 0.2 nm to 10 nm. Closed-loop feedback control of layer thickness is also demonstrated. Good consistency was obtained by comparing the in situ SE results to x-ray diffraction measurements. Dynamic oxidation studies of [Co/Au]n and [Co/Ni]n multilayer structures are also presented.
Ultrathin Ta2O5 film growth by chemical vapor deposition of Ta(N(CH3)2)5 and O2 on bare and SiOxNy-passivated Si(100) for gate dielectric applications
Authors: Son K.-A. , Mao A. Y. , Kim B. Y. , Liu F. , Pylant E. D. , Hess D. A. , White J. M. , Kwong D. L. , Roberts D. A. , Vrtis R. N.
Published: July 2002

We investigated Ta2O5 films grown by chemical vapor deposition of Ta(N(CH3)2)5 and O2 both bare and SiOxNy-passivated Si(100) using x-ray photoelectron spectroscopy, time-of-flight secondary-ion-mass spectroscopy (TOF-SIMS), and electrical measurements. The SiOxNy-passivated layer was formed by nitric oxide exposure to the Si substrate. Chemical composition of the Ta2O5 films is strongly dependent on the oxygen flow rate during film deposition; lower carbon levels and higher O/Ta ratios are observed for the films grown at higher O2 flow rates. A corresponding leakage current decrease is observed for the films grown at a high O2 flow rate. Compared to Ta2O5 films deposited on bare Si(100), the films deposited on SiOxNy-passivated layers show better electrical properties; with smaller equivalent thickness (Δteq∼6 Å), one order of magnitude lower leakage current was measured. TOF-SIMS data indicate that SiOxNy layers (∼9 Å) incorporate some oxygen during Ta2O5 deposition; however, regions where x=2, y=0 were not detected. Postdeposition annealing of Ta2O5/SiOxNy samples results in displacement of N by O in SiOxNy layers and oxidation of the Si substrate, forming SiO2.
Excimer laser cleaning of silicon wafer backside metallic particles
Authors: Beaudoin F. , Meunier M. , Simard-Normandin M. , Landheer D.
Published: July 2002

An excimer laser cleaning system operating at 248 nm was developed to remove micrometer-sized metallic particles from the backside of silicon wafers. Deliberate iron contamination has been performed using iron-oxide particles having a diameter of 0.5–2 μm. The surface photovoltage (SPV) method was used to characterize the cleaning efficiency through the change in diffusion length and iron concentration in the silicon bulk. Following a rapid thermal annealing at 1050 °C for 4 min, the SPV measures diffusion lengths down to 40 μm for the iron-contaminated wafer, corresponding to an iron concentration up to 1.55×1013 cm−3. The minimum diffusion length increases to 130 μm after two steam laser cleanings done at a laser fluence below the silicon threshold damage of 200 mJ/cm2. The iron concentration measured in the bulk of the iron-contaminated wafers is reduced by more than 91%, to a concentration of 1.4×1012 cm−3.
Inductively coupled plasma etching of bulk 6H-SiC and thin-film SiCN in NF3 chemistries
Authors: Wang J. J. , Lambers E. S. , Pearton S. J. , Ostling M. , Zetterling C.-M. , Grow J. M. , Ren F. , Shul R. J.
Published: July 2002

A parametric study of the etching characteristics of 6H p+ and n+ SiC and thin-film SiC0.5N0.5 in inductively coupled plasma (ICP) NF3/O2 and NF3/Ar discharges has been performed. The etch rates in both chemistries increase monotonically with NF3 percentage and rf chuck power. The etch rates go through a maximum with increasing ICP source power, which is explained by a trade-off between the increasing ion flux and the decreasing ion energy. The anisotropy of the etched features is also a function of ion flux, ion energy and atomic fluorine neutral concentration. Indium-tin-oxide masks display relatively good etch selectivity over SiC (maximum of ∼70:1), while photoresist etches more rapidly than SiC. The surface roughness of SiC is essentially independent of plasma composition for NF3/O2 discharges, while extensive surface degradation occurs for SiCN under high NF3:O2 conditions.
Low temperature plasma deposition of silicon nitride from silane and nitrogen plasmas
Authors: Hanyaloglu Bengi F. , Aydil Eray S.
Published: July 2002

The plasma enhanced chemical vapor deposition of silicon nitride films from SiH4 and N2 gases was investigated below 450 K in a helical resonator plasma reactor using in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier transform infrared spectroscopy. Silicon nitride growth proceeds through nitridation of a thin Si-rich region at the film surface and the effects of the externally controlled parameters, such as plasma power and pressure, on the film composition and structure can be understood in terms of this mechanism. The key factor that affects the film composition and structure is the active nitrogen (N2*) flux arriving at the surface which determines the nitridation rate of Si–Si bonds. Silicon nitride films deposited using low plasma power and high pressure, conditions that produce low N2* flux, contains a relatively high concentration of H in the form of Si–H and a-Si inclusions. Increasing the power or decreasing the pressure results in higher N2* flux and nitridation rate. The films deposited under such conditions have less SiH and a-Si incorporation but higher NHx (x=1,2) which disrupts the interconnectedness of the nitride network resulting in films that contain voids. Film properties can be tailored and optimized between these two extremes by manipulating power and pressure.
Characterization of Cl2/Ar high density plasmas for semiconductor etching
Authors: Eddy C. R. , Leonhardt D. , Douglass S. R. , Thoms B. D. , Shamamian V. A. , Butler J. E.
Published: July 2002

Chlorine-based high density plasmas, commonly used in the etching of elemental and compound semiconductors, are characterized using mass spectrometry, optical emission spectroscopy, and electrostatic probes. Plasma fluxes are characterized by three-dimensional Langmuir probe measurements and optical emission spectroscopy. The flux is further characterized at the substrate platen by mass spectrometry to determine its makeup in terms of charged or neutral species and atomic or molecular species. Langmuir probe investigations show variations in electron temperature (2–6 eV), plasma density (1×1010 to 1×1012 cm−3), and plasma potential (5–25 V) as process conditions (microwave power, total pressure, and fraction of Cl2 in Ar) and measurement location are varied. Concurrent optical emission spectroscopy measurements of ionized species are in general agreement with Langmuir probe results. Further, optical emission spectroscopy of neutral and ionized species provides global insight into the variation of atomic/molecular fractions in the plasma as it is transported to the substrate processing region. At the substrate, mass spectrometric characterizations show Cl+ and Ar+ dominating the flux for low pressure and high powers, while Cl2 and Ar dominate at high pressure and low power. For Cl2 fractions greater than 25% molecular chlorine begins to dominate the flux to the substrate. These observations of processing space are discussed with respect to implications on semiconductor etching and regions most suitable to high rate, anisotropic processing conditions are identified.
Deposition of ultrapure hydrogenated amorphous silicon
Authors: Kamei Toshihiro , Matsuda Akihisa
Published: July 2002

We have succeeded in a drastic reduction of impurity contents in hydrogenated amorphous silicon (a-Si:H) films by a newly developed ultrahigh vacuum plasma-enhanced chemical vapor deposition (UHV/PECVD) system. High sensitivity secondary ion mass spectrometry shows that impurity contents in the films are as low as 1015 cm−3 both for oxygen and carbon, and 1014 cm−3 for nitrogen. These values represent the lowest concentrations of atmospheric contaminants for a-Si:H films reported so far. In particular, oxygen content has not been reduced below 1×1018 cm−3 using conventional UHV/PECVD techniques, and not below 5×1017 cm−3 even by high growth rate process of very high frequency plasma. The essential features of the present UHV/PECVD system are an extremely low outgassing rate of 8×10−9 Torr ℓ/s, extremely low partial pressure of contaminant gas species <10−12 Torr, and purification of feed gas SiH4 at “point of use.” The specific origins of impurities in the films are discussed: outgassing of the reactor wall, the purity of feed gas, and the plasma-wall interaction process.
X-ray photoelectron spectroscopy studies of modified surfaces of α-Al2O3, SiO2, and Si3N4 by low energy reactive ion beam irradiation
Authors: Choi Won-Kook , Koh Seok-Keun , Jung Hyung-Jin
Published: July 2002

Reactions of N2+ ion beams with oxide surfaces of α-Al2O3(0001) single crystal and chemical vapor deposition (CVD) SiO2, and reactions of O2+ ion beams with a nitride surface of Si-rich CVD Si3N4 were investigated as a function of ion beam energy (200–1000 V) and dose (1×1015–1×1017/cm2). The thickness modified by the irradiation of a reactive low kinetic energy ion beam was measured using high resolution cross-sectional images of transmission electron microscopy (HR-XTEM), and the formation of new bonding induced by chemical reaction was analyzed by x-ray photoelectron spectroscopy (XPS). New bonding of Al–O–N on α-Al2O3(0001) started to be observed at 600 V N2+ ion energy and a dose of 1×1016/cm2, and Al–N bonding could be found at an ion beam energy of 1 keV. The thickness of the aluminum oxynitride layer after 800 V N2+ bombardment has been determined to be 10–50 Å by HR-XTEM analysis. In the case of CVD SiO2 surface modification, new bonding related to nitrogen was not clearly resolved in the XPS spectra, irrespective of the change of ion beam energy from 200 to 1000 V and ion dose from 1×1015 to 1×1017/cm2. However, widening of the full width at half maximum of Si 2p core-level XPS spectra for the modified SiO2 surface and the peak position of N 1s around 399 eV were evidence of the existence of nitrogen-related bonding like Si–O–N in the modified CVD SiO2 surfaces. Moreover, it was very interesting that the Si 2p peak of elemental Si appeared in the sample irradiated at a dose of 1×1017/cm2. Its occurrence was considered to be due mainly to the preferential sputtering effect, and was found to be largely dependent on the ion beam energy as well as on the ion dose. In the surface modification of low-pressure CVD Si3N4 by direct ken O2+ ion irradiation, Si–O–N bonding could be successfully created at an ion beam energy of 200 V and it evolved significantly at ion beam energies higher than 500 V. From the above results, low energy reactive ion beam irradiation can successfully create new bonding structures on oxide and nitride surfaces due to a surface chemical reaction like nitridation or oxidation, and is expected to be very useful for the formation of new ultrathin functional layers on ceramic surfaces.
Inductively coupled, point-of-use plasma abatement of perfluorinated compounds and hydrofluorinated compounds from etch processes utilizing O2 and H2O as additive gases
Authors: Tonnis Eric J. , Graves David B. , Vartanian Victor H. , Beu Laurie , Lii Tom , Jewett Rusty
Published: July 2002

A current major environmental concern involves the emission of fluorinated compounds (FCs) used in semiconductor manufacturing for dielectric etching and plasma-assisted chamber cleaning processes in dielectric film chemical vapor deposition (CVD) systems. While progress has been made in reducing FC emissions from CVD tools using alternative chemistries and process optimization, the stringent demands of dielectric etch processes have limited emission reduction from this source. A promising strategy for reducing FC emissions from etch processes is point-of-use plasma abatement. In this approach, a high-density plasma is generated by a device installed downstream of the process chamber in the foreline that dissociates the FC effluent species and recombines the resulting fragments with additive gas fragments to create by-products that are either environmentally benign or scrubbable. In this work, the performance of an inductively coupled plasma abatement device is examined for the treatment of exhaust from a commercial, magnetically enhanced reactive ion etcher. Using either O2 or H2O vapor as abatement additives, FC destruction and removal efficiencies (DREs) of greater than 95% were observed for most process conditions tested. Abatement by-product distributions were found to vary significantly with additive gas identity. Oxygen addition abatement by-products consisted primarily of HF, CO2, COF2, and F2, while H2O addition abatement by-products were primarily HF and CO. Abatement input power was found to be the most significant factor affecting DREs, while argon dilution and total gas load showed little impact over the ranges tested. In addition, wafer metrology indicated no negative process impacts due to abatement device operation. Estimation of the Peclet number within the foreline indicates virtually no probability of abatement product backdiffusion at the foreline pressures and flows tested.
Isolating, imaging, and electrically characterizing individual organic molecules on the Si(100) surface with the scanning tunneling microscope
Authors: Hersam M. C. , Guisinger N. P. , Lyding J. W.
Published: July 2002

Molecular electronics shows great potential as an approach for fabricating nanoelectronic devices and circuits. Despite this potential, many fundamental problems remain unsolved. This article outlines a three pronged approach that addresses key molecular electronic issues for molecules supported on ultrahigh vacuum scanning tunneling microscopy (UHVSTM) patterned hydrogen passivated Si(100) surfaces. First, feedback controlled lithography (FCL) has been developed as a reliable technique for making templates of individual dangling bonds on the Si(100)–2×1:H surface. FCL detects individual H desorption events while patterning, thereby compensating for variations in tip structure. When the surface is then exposed to a flux of molecules, they bind individually to the prepatterned sites. With this technique, norbornadiene and copper phthalocyanine (CuPc) molecules have been intentionally isolated into predefined patterns. STM images reveal intramolecular detail and suggest mechanical behavior such as molecular rotation. Second, using STM spectroscopy, molecules’ electronic properties have been revealed. Filled state tunneling conductance maps of CuPc molecules exhibit an enhanced density of electronic states. However, in empty states, a ring of reduced local density of states surrounds each CuPc molecule. Finally, an all-UHV scheme for isolating and, ultimately, electrically contacting STM-patterned nanostructures has been developed that utilizes a predefined p–n junction on a Si(100) substrate. With STM potentiometry, the junction is easily located, allowing for efficient registration of nanostructures after intermediate processing steps. In addition, by STM patterning across the depletion region, the electrical properties of selectively deposited nanostructures can be directly evaluated when the p–n junction is reverse biased.
Epitaxial growth of Ag films on Ge(001)
Authors: Lince Jeffrey R. , Nelson Jeffrey G. , Williams R. Stanley
Published: July 2002

The deposition of thin films of Ag on clean Ge(001) substrates has been investigated using low energy electron diffraction, Auger electron spectroscopy, and scanning electron microscopy. Ag was deposited from an effusion source onto Ge at both 325 and 600 K substrate temperatures. Deposition on high temperature substrates resulted in the formation of Ag islands with no apparent crystalline order, whereas films deposited at temperatures near 325 K onto clean Ge(001) 2×1 surfaces were oriented. The deposition of the equivalent of one Ag(001) monolayer on the Ge surfaces resulted in a Ge(001) 1×1-like LEED pattern. As deposition was continued (up to 500 monolayers), a pattern evolved which is consistent with two (011) 1×1 Ag domains oriented at 90° to one another. This behavior is interpreted in terms of a predominantly Stranski–Krastanov growth mechanism for the initial deposition of Ag onto near room temperature Ge(001) substrates. Annealing of a sample with a thin, ordered film revealed the original Ge(001) 2×1 LEED pattern, which demonstrated that the chemical interaction between Ge and Ag is very weak.
Etch products from the reaction on Cl2 with Al(100) and Cu(100) and XeF2 with W(111) and Nb
Authors: Winters Harold F.
Published: July 2002

A modulated-beam, mass spectrometer system has been used to obtain information about etch products, reaction probabilities, and etching mechanisms for the reactions of Cl2 with Al(100) and Cu(100) and for the reaction of XeF2 with W(111) and Nb. The influence of ion bombardment on the etching reaction has also been investigated.
Impurity scattering limited momentum relaxation time in a quantum well wire
Authors: Brown Jerry W. , Spector Harold N.
Published: July 2002

The ionized impurity limited momentum relaxation time for carrier scattering within a quantum well wire is calculated for both infinite and finite confining wells models. The transverse part of the carrier wave function is taken to be a Bessel function. For a background impurity distribution, the scattering frequency increases with increasing well depth. For remote ionized impurities or a tube of impurities the scattering frequency decreases with increasing well depth. We assume GaAs/Ga1−xAlxAs for the material of the quantum well wire structure.
A model for stress-induced metal notching and voiding in very large-scale-integrated Al–Si (1%) metallization
Authors: McPherson J. W. , Dunn C. F.
Published: July 2002

The kinetics of stress-induced notching and voiding in very large-scale-integrated circuit Al–Si metallization have been investigated. A generalized time-to-failure model has been developed and an activation energy of 0.4 eV has been observed for the mechanism. The stress state of the passivation overcoat has been found to have a major impact on the notching and voiding tendency of the fine-grain metal.
GaAs and AlGaAs crystallographic etching with low-pressure chlorine radicals in an ultrahigh-vacuum system
Authors: Sugata Sumio , Asakawa Kiyoshi
Published: July 2002

GaAs and AlGaAs have been crystallographically etched with low-pressure chlorine radicals in an electron-cyclotron resonance (ECR) plasma shower with a new reactive-ion-beam etching (RIBE) system for obtaining damage- and contamination-free etching. The etching begins abruptly at 190 °C and increases gradually above 200 °C. The typical etching rate of a (001) plane is 1 μm/min at a substrate temperature of 300 °C. Mesa-shaped and reverse mesa-shaped grooves with {111}A side wall planes are obtained for 〈1̄10〉- and 〈1̄1̄0〉-oriented line and space masks. For square masks with [110] edge lines, on the other hand, high aspect-ratio columns with {100} vertical side wall planes are obtained. These results show that the etching rate of {111}A is much lower than that of {100}, {110}, or {111}B. AlGaAs is etched similarly to GaAs. Cl2 gas (not plasma excited) performs similar etching. However, the substrate temperature must be about 100 °C higher for the beginning of etching than for the Cl radical etching. These etching technologies are promising for microfabrication of GaAs/AlGaAs optoelectronic devices, replacing conventional wet chemical etching.
Preliminary experimental study of the multiple ion beam machine
Authors: Ando Masaaki , Muray Julius J.
Published: July 2002

The multiple ion beam machine (MIBM) that we are developing for nanolithography offers important advantages over other nanolithography machines. A focused ion beam machine [R. L. Seliger, R. D. Olney, J. W. Ward, and V. Wang, J. Vac. Sci. Technol. 16, 1610 (1979)] uses a single ion beam to write the pattern. As a result, each chip on a wafer must be written individually thus leading to low throughput for high-volume production. On the other hand, an ion projection lithography machine (G. Stengel, H. Löshner, and J. J. Muray, Solid State Technol. 1986, 119) has a high throughput; however, it is inconvenient in that it requires a special mask to be made for each lithographic pattern. The MIBM uses multiple focused ion beams, which are deflected and blanked in synchronism, each beam being used to write a single chip. Thus, the exposure time for the whole wafer is the same as is required to write a single chip, resulting in a high throughput.
Advanced e-beam lithography
Authors: Takigawa T. , Wada H. , Ogawa Y. , Yoshikawa R. , Mori I. , Abe T.
Published: July 2002

The acceleration voltage dependence of electron-beam (EB) lithography was investigated. A low acceleration voltage is suitable for a mask (reticle) of several magnifications, because the proximity effect correction is not required. A reticle writing system EX-8 with an acceleration voltage of 12.5 to 20 kV has been developed. A high acceleration voltage is preferable for the direct writing of fine patterns. A direct writing system EX-7 with a typical acceleration voltage of 40 kV has been developed. A new shaped beam calibration method and a new astigmatism correction method have enabled the EX-7 to write a pattern with 0.1 μm. High accuracy shaped beam calibration and alignment as required by small patterns were realized for a high acceleration voltage.
Raman scattering study of dry etching of GaAs: A comparison of chemically assisted ion beam etching and reactive ion etching
Authors: Glembocki O. J. , Dobisz E. A.
Published: July 2002

Chemically assisted ion beam etching (CAIBE) and reactive ion etching (RIE) are two popular dry etching techniques routinely used in the fabrication of submicron features. We have used both in situ and ex situ Raman spectroscopy to study the effects of dry etching on the depletion layer in heavily doped GaAs. We find that both CAIBE and RIE produce large numbers of traps that produce an insulating layer on the surface. CAIBE using ion energies greater than 2 keV also produces a damaged layer. For both techniques the use of chlorine based species reduces both traps and damage. Sputtering with chlorine alone, is shown to be able to eliminate both damage and traps. Finally, Raman spectroscopy is shown as a powerful in situ tool for studying etch damage.
Plasma particulate contamination control. I. Transport and process effects
Authors: Selwyn Gary S.
Published: July 2002

The transport and behavior of particulates during plasma processing is imaged in real time using rastered laser light scattering combined with video detection. Results show the distribution of particles is highly ordered and predictable. Two effects have major influence on the distribution and location of particles: feed gas drag and electrostatic traps. Particle traps form from plasma disturbances and design properties of the electrode and the tooling. These results are confirmed in normal plasma conditions used in microelectronics fabrication. A strategy is presented for plasma contamination control. Complete elimination of trapping effects is not feasible. Instead, process techniques are used to minimize or defeat the attractive nature of the traps. Several examples are discussed.