Ultrahigh Vacuum Ion Research Articles

Change of preferred orientation in TiN thin films grown by ultrahigh vacuum reactive ion beam assisted deposition

Changes of the preferred orientation in TiN thin films grown on Si(0 0 1) by ultrahigh vacuum ion beam assisted deposition has been studied by using XRD, RBS and AFM. The preferred orientation of the TiN thin films changed systematically from (1 1 1) to (2 0 0) as the assist energy of reactive N 2 + ion increased. In the case without reactive ion irradiation, the TiN thin film grew with the (1 1 1) orientation having the lowest strain energy. For an assist ion energy over about 100 eV, the TiN thin film grew with the (2 0 0) orientation having the lowest surface energy. The average grain size of TiN thin films increased from 10 to 100 nm in diameter as the reactive ion assist energy increased.

Ultra-thin oxides on silicon fabricated using ultra-slow multicharged ion beams

We have used ultra-slow multicharged ions in ultra-high vacuum with a low partial pressure of O 2 to grow ultra-thin layers (0.3 to 2.0 nm) of SiO 2 on silicon. The advantage of using ultra-slow ions is that they interact with the surface only through their potential energy, not their kinetic energy, so they do not penetrate below the surface, avoiding implanted-induced damages to the substrate. This paper presents several analysis we have performed to qualify these ultra-thin SiO 2 layers, such as Fourier transform infrared spectroscopy–attenuated total reflection, Auger electron spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry and surface charge analysis. The oxidation process was monitored in situ and in real time using visible light emitted during the irradiation, and the thickness of the SiO 2 layers could be controlled. We plan to used these ultra-thin SiO 2 layers in next generation MOS gate dielectric stack, as a buffer layer between the channel in silicon and a high- k dielectric.

Large-area surface treatment by ion beam technique

A large-area ion beam deposition system has been used extensively for depositing diamond-like carbon films by a direct ion-beam process. An ultra high vacuum ion beam system, consisting of a 20-cm diameter RF excited (13.56 MHz) ion gun and a four-axis substrate scanner, has been used to modify large surfaces (up to 1000 cm 2) of various materials, including 304 and 316 stainless steel, 440C and M50 steels, aluminum, aluminum alloys, Ti-6Al-4V, silicon carbide, silicon nitride, polycarbonates, infrared windows and polycrystalline diamond, by depositing varying chemical compositions of diamond-like carbon films. The influence of ion energy, RF power, and gas composition (H 2/CH 4, Ar/CH 4, O 2/H 2/CH 4 and N 2/H 2/CH 4), on the diamond-like carbon characteristics has been investigated. Particular attention was focused on the adhesion, environmental effects, coefficient of friction and wear factors of the diamond-like carbon films on the various substrates under space-like environments. A quadrupole mass spectrometer and a total ion-current measuring device have been utilized to monitor the ion compositions of the gas mixtures of CH 4/H 2, CH 4/Ar, CH 4/H 2/O 2 and CH 4/H 2/N 2 during the deposition process for quality control and process optimization.

Oxide nanodots and ultrathin layers fabricated on silicon using nonfocused multicharged ion beams

We have used slow multicharged argon ions in ultrahigh vacuum with a partial pressure of oxygen to form ultrathin (subnanometer) oxide dots of a few tens of nanometer diameter on a silicon surface. The main characteristic of this technique is that the incident multicharged ion does not penetrate below the surface, so there is no implantation-induced modifications of the substrate. Thus, we utilized a unique possibility provided by the multicharged ions interaction with surfaces of solids, the noncontact trampoline effect, to open the bonds of hydrogenated silicon, and replace it with oxygen. This article presents the fundamentals of slow multicharged ion interaction with a surface, the experimental multicharged ion beam line build at X-ion laboratory, and obtained results of oxide dot formation on silicon, using them as a mask in a reactive ion etching process to grow three-dimensional crystalline silicon structures. Potential applications are in nanoflash–multidot type nonvolatile memories.

Ion beam deposition of 107Ag(111) films on Ni(100)

Monolayer thick silver films were grown on an Ni(100) surface using direct ion beam deposition of 107Ag + ions with 20 eV kinetic energy and the substrate at 25°C. Deposition was performed by means of a mass-selected, low-energy, ultra-high vacuum ion beam system with a well-defined ion energy. The films were analyzed in situ by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and by low energy electron diffraction (LEED). Classical ion trajectory simulations were used to model the TOF-SARS spectra. Results of the TOF-SARS spectra and azimuthal scans are consistent with the growth of a monolayer Ag film with a fcc (111) structure on the Ni(100) surface. The deposited films exhibited a faint hexagonal c(2×8) LEED pattern. Molecular dynamic simulations of the deposition process are in agreement with the experimental observations. The growth of such isotopically pure metal monolayers at room temperature with controlled orientations provides the opportunity for exploring a variety of unique physical properties.

Quantitative compositional depth profiling of Si 1− x− yGe xC y thin films by simultaneous elastic recoil detection and Rutherford backscattering spectrometry

Elastic recoil detection in conjunction with time-of-flight spectroscopy (ERD-TOF) has been used to simultaneously depth profile all elements in thin SiGeC films on Si (0 0 1). Ge-rich Si 1− x− y Ge x C y (0.025 < (1 − x − y) < 0.08 and 0.026 < y < 0.216) films were grown by ultra-high vacuum ion beam sputter deposition. X-ray diffraction and transmission electron microscopy (TEM) showed that samples with low carbon concentration (typically, y ≤ 0.07) were single crystals with stacking faults and twins while films with 0.07 < y < 0.15 were polycrystalline. Samples with y > 0.15 were amorphous. There was no evidence of SiC precipitates. Ge depth profiles were obtained from the recoil spectra as well as from the Rutherford scattered Cl spectra in the same ERD-geometry. The H concentration was determined simultaneously by selective-absorber ERD. Films with C concentrations approaching a fraction of an at.%, and Si concentrations as low as ∼ 1 at.%, could be routinely depth profiled in a single experiment even in the presence of relatively strong surface contamination.

Radiative Lifetime Measurement of theBa+MetastableD3/2State

The extremely long lifetimes of the metastable $D$ states of the ${\mathrm{Ba}}^{+}$ ion make it difficult to measure them accurately due to their high sensitivity to collisional effects. Thus a discrepancy exists between theories and experiments for the ${\mathrm{Ba}}^{+}$ ${D}_{3/2}$ state. We have measured the ${D}_{3/2}$ lifetime under a nearly ideal condition, i.e., single trapped ${\mathrm{Ba}}^{+}$ ions in ultrahigh vacuum. The obtained value of $79.8\ifmmode\pm\else\textpm\fi{}4.6\mathrm{s}$ resolves the existing discrepancy. The collisional quenching is found insignificant in our experiment, but there are indications of a non-negligible fine-structure mixing effect.

Resistance measurements of platinum-titania thin film gas detectors in ultra-high vacuum (UHV) and reactive ion etcher (RIE) systems

The application of a conductometric Pt-TiO x thin film gas detector for monitoring oxygen and hydrogen impurities in vacuum and in semiconductor process gases is described. The gas detector exhibits reversible resistance responses to gases such as O 2 and H 2 in the 10 −5–10 −3 Pa range, but does not register a response to inert gases such as CF 4. The gas detector utilizes an evaporated 65 Å Ti-35 Å Pt film deposited on a micromachined dielectric window of the silicon based structure. Additional features of the gas detector structure include Ti-Ir electrodes arranged on the sensing film in a four point probe configuration and a boron-doped silicon heater beneath the dielectric window which is capable of varying the temperature of the sensing film from ambient to 750 °C. The gas detector is activated for sensing with oxidation in 1.3 × 10 −4 Pa O 2 at 750 °C followed by reduction in 1.3 × 10 −4 Pa H 2 at 400 °C. This series of activation steps converts the 65 Å Ti-35 Å Pt film into a discontinuous network structure of discrete Pt-TiO x islands. Gas detector responses to oxygen and hydrogen in vacuum, both under isothermal and temperature-programmed operation, are reported and mechanistically discussed. The utility of the Pt-TiO x gas detector to detect small leaks and to track in-situ the effluent dynamics during operation of an Applied Materials hexode reactive ion etcher (RIE) is demonstrated.

Composition and morphology of InP{100} surfaces as a function of low energy Ar + bombardment and annealing

The surface composition and morphology of the InP{001} surface have been studied by in situ Auger electron spectroscopy (AES) and ex situ atomic force microscopy (AFM) as a function of Ar + ion bombardment energy over the range 5 eV to 500 eV and annealing temperatures up to 600°C. Ion bombardment has been performed by means of a low energy, mass-selected, ultrahigh vacuum ion beam system. The surface contamination can be removed by Ar + ion bombardment with ≥ 30 eV ion energies for carbon and ≥ 60 eV ion energies for oxygen species. However, Indium-rich clusters and islanding are observed for Ar + ion energies above ∼ 40 eV. The results show that a combination of 30 eV Ar + ion bombardment and annealing to ∼ 300°C can produce an InP{100} surface which is devoid of observable impurities in AES and devoid of defect features in AFM.

Large Area Surface Treatment by Ion Beam Technology

AbstractAn ultra high vacuum ion beam system, consisting of a 20 cm diameter Rf excilted (13.56 MHz) ion gun and a four-axis substrate scanner, has been used to modify large surfaces (up to 1000 cm2) of various materials, including; infrared windows, silicon nitride, polycrystalline diamond, 304 and 316 stainless steels, 440C and M50 steels, aluminum alloys, and polycarbonates; by depositing different chemical compositions of diamond-like carbon films. The influences of ion energy, Rf power, gas composition (H2/CH4 , Ar/CH4 and O2/CH4/H2), on the diamond-like carbon characteristics has been studied. Particular attention was focused on adhesion, environmental effects, IR(3–12 μm) transmission, coefficient of friction, and wear factors under spacelike environments of diamond-like carbon films on various substrates. A quadrupole mass spectrometer was utilized to monitor the ion beam composition for quality control and process optimization.

Low-energy (5&amp;lt;E i&amp;lt;100 eV), high-brightness, ultrahigh vacuum ion source for primary ion beam deposition: Applications for Al and Ge

The design, operation, and use of a double-grid, electron-impact, ultrahigh-vacuum compatible, low-energy ion source capable of operating with relatively low-vapor-pressure solid source materials such as Al and Ge are described. The source can run at cell temperatures up to 1450 °C and deliver ion current densities Ji≳30 mA cm−2 (corresponding to 1.9×1017 cm−2 s−1) at source-to-substrate separations d of 2 cm, and Ji≳100 μA cm−2 (6.3×1014 cm−2 s−1) at d=40 cm with beam energies which can be varied from 5 to 100 eV. The uniformity in the ion current density from a 2.6-cm-diam source over 6-cm-diam substrates is better than 97%. A postextraction magnetic filter was used to separate the ion and neutral components and provide a pure ion beam. Initial nucleation and growth experiments on amorphous carbon substrates at 30 °C using thermal and accelerated (EAl+=35 and 75 eV) Al+ beams show that the average island size in layers with thicknesses between 2 and 6 nm increases by factors ranging from 3 to 14 with increasing ion energy. Layers deposited with thermal Al have an island size distribution which is near-Gaussian while Al islands formed from ion beams exhibit a bimodal size distribution.

(100) Epitaxial and (111) Polycrystalline Spin Valve Heterostructures on si (100): Magnetotransport and the Importance of Interface Mixing in Ion Beam Sputtering

ABSTRACTWe have investigated magnetoresistance properties of (100) epitaxial, (11) textured and polycrystalline spin valve heterostructures of the form Ni80Fe20/Cu/NiNi80Fe20/Fe50Mn50 on (100) Si substrates by ultra high vacuum (UHV) ion beam sputtering at room temperature. Magnetoresistance was measured as a function of Cu interlayer thickness (ti) with 10 Å ≤ti ≤ 100 Å and the maximum was found at 20 Å in the case of (100) epitaxial spin valves. Highly (11) textured spin valves with heterostructure configurations similar to the (100) spin valves were found to have a slightly lower magnetoresistance than the (100) heterostructures, but the functional dependence of the magnetoresistance on ti was very similar.Interface mixing during the sputtering process by energetic neutral bombardment was found to significantly affect the magnetoresistance. Samples were made under various sputtering conditions (gas pressure, ion beam energy, target and substrate configuration) that could enhance or suppress high energy neutral bombardment of the growing film surface. Samples made under the conditions that suppressed neutral bombardment showed higher magnetoresistance and more abrupt interfaces as confirmed by small angle X-ray diffraction analysis of interface mixing by energetic neutral bombardment during sputter deposition.

Epitaxial growth of Cu (001) on Si (001): Mechanisms of orientation development and defect morphology

We describe the evolution of microstructure during ultrahigh vacuum ion beam sputter deposition of Cu (001) at room temperature on hydrogen-terminated Si (001). In situ reflection high energy electron diffraction indicates growth of an epitaxial Cu (001) film on Si (001) with the intensity of the Bragg rods sharpening during 5–20 nm of Cu film growth. Post-growth x-ray diffraction indicates the Cu film has a mosaic spread of (001) textures of about ±2° and that a small fraction (0.001–0.01) is of (111) textures. High-resolution transmission electron microscopy shows an abrupt Cu/Si interface with no interfacial silicide, and reveals an evolution in texture with Cu thickness so as to reduce the mosaic spread about (001). Moiré contrast suggests a nearly periodic elastic strain field extending into the Cu and Si at the interface. Other aspects of film growth which are critical to epitaxy are also discussed.

Growth of homoepitaxial Ge(001)2 × 1 by ultrahigh vacuum ion beam sputter deposition

High quality epitaxial Ge(001)2 × 1 films have been grown on Ge(001) substrates by ultrahigh vacuum (UHV) ion beam sputter deposition. The load-locked multichamber growth system is equipped with in situ reflection high-energy electron diffraction. Sputter deposition was carried out using a 1 keV Kr + ion beam generated by a modified UHV Kaufman-type ion source with post-extraction electrostatic ion optics. All films were 1 microm thick and deposited at a rate of 0.5 microm h −1. Results of plan-view, cross-sectional, and convergent-beam transmission electron microscopy analyses showed that films deposited at temperatures T s between 300 and 650°C had a high degree of perfection with no observable defects. Temperature-dependent carrier mobilities were found to be equal to or to exceed the best reported values for bulk Ge. A1-doped p-type films grown at T s = 400–600° C exhibited 77 K hole mobilities ranging from 3890 to 1200 cm 2 V −1 s −1 for corresponding hole concentrations between 2.8 × 10 16 and 1.5 × 10 17 cm −3.

Formation of low reverse current ion-implanted n+p junctions by low-temperature annealing

By employing stress compensation technology in conjunction with an ultrahigh vacuum ion implanter, we have obtained two orders of magnitude smaller reverse-bias current levels in n+p junctions annealed at 550 °C as compared to the previous data. When samples are made with an optimum stress compensation condition, the magnitude of reverse-bias currents in n+p junctions as well as the carrier deactivation during post-implantation anneal becomes smallest. In order to further reduce the reverse-bias current level, it is shown that the elimination of contamination involvement during the implantation process is most essential.

Accelerator-based chemiluminescence from steady-state gas-surface reactions

Some solid elemental targets have been bombarded with 80 keV argon ions in ultrahigh vacuum as well as in an oxygen atmosphere at pressures of up to 10 −3 Pa. Bombardment-induced photon emission has been studied (170–900 nm). With oxygen present, the spectra consist of atomic line radiation related to sputtering of excited atoms, superimposed on a broad continuum. When the targets are in ultrahigh vacuum, the continuum is absent, only the atomic line radiation is seen. The continua are attributed to chemiluminescence due to oxygen reacting with the target surfaces. The ion bombardment causes a continuous cleaning of the target surface because of sputtering. Thus, by keeping the projectile flux and the oxygen gas pressure constant, it is possible to study chemiluminescence from gas-surface reactions under steady-state conditions. The active surface area increases with increasing angle of incidence of the projectiles, implying that this chemiluminescence source is very bright at large angles of incidence.

Epitaxial growth of yttria-stabilized zirconia films on silicon by ultrahigh vacuum ion beam sputter deposition

Yttria-stabilized zirconia (YSZ) films have been grown on Si (100) substrates by ion sputter deposition using ultrahigh vacuum system with in situ diagnostic equipment. Epitaxial conditions of YSZ (100) films were achieved between 700 and 800 °C by using an oxygen partial pressure around 3×10−4 Pa during the deposition. The film stoichiometry was measured by Rutherford backscattering spectrometry and nuclear reaction analysis. Crystalline quality of the YSZ layers was determined by x-ray diffraction, channeling of 4He+ ion beam, and in situ reflection high-energy electron diffraction pattern.

On the intrinsic secondary ion emission from light metals

Two ionization mechanisms are known to contribute to the intrinsic secondary ion emission from light metals: surface excitation by electron tunneling during the separation of an outgoing particle from the target and collisional process by Auger de-excitation of excited atoms ejected outside the target with an inner electronic vacancy. Bombardment by noble gas ions in ultrahigh vacuum of those alloys where only symmetric collisions between light atoms are effective for the subsequent ionization by collisional process, allows one to distinguish between both processes and to evaluate their respective contributions at given initial energy from the concentration dependence of the ion intensities. This method and its theoretical assumptions are presented here in detail.

Summary Abstract: Some observations on the calibration of two types of ultrahigh vacuum ion gauges

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A UHV-compatible ΔE− E gas telescope for depth profiling and surface analysis of light elements

We have developed a compact ΔE− E gas telescope that can easily be incorporated into a standard ultrahigh vacuum ion scattering chamber. If energetic, heavy ions are available as a primary beam, the gas telescope can be used to detect light elastic recoils scattered from the surface region of a sample. The detector consists of a gas ionization chamber in front of a surface-barrier detector. The energy loss signal in the gas ( ΔE) is taken in coincidence with the energy signal in the surface-barrier detector ( E) to identify both the atomic number and the total energy of the elastic recoil. The design parameters of the detector have been chosen to allow detection of elements from H to O with good depth resolution ( < 100 Å) and submonolayer sensitivity. The mass resolution is adequate to easily separate all elements from H to O.