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We are investigating the transmission of low energy ions (<10 eV) through ultrathin films of condensed rare gases. Our goal is to address the issue of the depth of origin of secondary ions that desorb from solid surfaces under the impact of ionizing radiation, such as electrons, photons, or through ion sputtering. The secondary ions are produced by electron stimulated desorption (ESD) from a suitable substrate, such as an oxide or an adsórbate on a metal single crystal; the overlayer gas is condensed onto this substrate. The yield, energy and angular distributions of the ions are measured as a function of overlayer thickness. We find that 7 eV oxygen ions can be transmitted through rare gas films (Kr, Xe) several ML thick. In contrast, O+ is completely suppressed by 0.5 ML of H2O. Surprisingly, we find the F− yield to be 4 times higher in the presence of 1 ML of Xe, compared to the clean surface value, accompanied by a dramatic change in the ions’ angular distribution. We discuss a model which considers elastic scattering and charge transfer of the ions with rare gas atoms, as well as the structure of the surface and the electronic properties of the solid-vacuum interface.
We have employed a radical beam assisted deposition technique to prepare single-crystalline niobium nitride thin films on MgO (100) substrates. The radical beam containing excited species of nitrogen was produced by an ECR plasma source and used to irradiate the growing Nb film, which was simultaneously deposited by an electron-gun vapor source. The nitride film was found to grow epitaxially on the substrates heated to 600 – 650°C. It has resulted in the formation of NbN having predominantly Bl structure, resistivity of 44 (μΩcm at 20 K, and almost equiatomic composition.
The use of ion-beam techniques to enhance selected properties of bioactive materials, such as the adhesion of hydroxylapatite (HA) coatings on titanium-based substrates has been investigated. In this study, very thin HA films on titanium substrates were created by pulsed laser deposition techniques. Ion irradiations were carried out using 260-keV argon ions, with fluences of 0.25-50×1015ions/cm2, and at room temperature. Rutherford backscattering spectrometry was used to evaluate sample composition before and after irradiation. The amount of mixing was quantified by the mixing rate (the amount of atomic displacement due to an irradiation fiuence). This pilot data indicates that mixing was evident after sufficient ion irradiation. The ramification of this preliminary study has provided a quantitative measure of ion mixing as a potential prosthetic biomaterial surface modification technique.
In order to study atomic transport in the radiation enhanced diffusion (RED) region, Pd/Co bilayers were intermixed by 80keV Ar+ in the temperature range from 90 K to 700 K. The critical temperature for the onset of RED was found to be ∼ 400 K, and the transported amount of Pd atoms was found to be always larger than that of Co in the RED region. This result cannot be explained by pre-existing models. Thus we have developed a comprehensive model for atomic transport in the RED region including size effect, damage controlled effect, and cohesive energy effect.
High speed steel with a martensitic microstructure is widely used in tool industry, but often the usage is limited by severe abrasive and corrosive wear. Metal ion implantation is a promising method to improve the tribological behaviour of this steel, as was shown in several publications. In this paper we discuss the application V and Cr implanted high speed steel, both of these elements are alloying constituents of the steel matrix. Combined with implantation of carbon is also carried out.
The tribological tests were performed with a pin-on-disc machine under dry sliding conditions. The obtained tribological results are discussed in relation to the microstructures of the nonimplanted and implanted samples.
Samples of AISI 316 Stainless Steel were nitrogen and argon ion implanted with pulsed beams generated with a Plasma Gun operated in the detonation mode. The residual deformations induced by the beams were studied by double exposure (before and after implantation) holographic interferometry.
The results showed residual deformations corresponding to a concave situation, with the total value depending on the number of single pulses accumulated. A saturation in the deformation is observed when the number of pulses is > 20. A model of the process of pulsed irradiation (based on the strong thermal effect due to the short duration of pulses) and the state of stresses induced in the surface layers is presented.
Samples of pure copper were argon ion irradiated using a Plasma Gun operated in the detonation mode. The irradiated copper samples were studied by means of the Mechanical Dynamical Spectroscopy technique.
The results show an anomalous behavior in the Elasticity Shear Modulus, consisting in an abrupt jump in its value at ∼840K, repeated in the running up as well as in the running down part of the temperature cycling. A Damping Peak at that temperature was also observed.
The advantages of energetic deposition are low temperature processing, oriented or single crystal films, high phase purity, high density and good adhesion to substrates. The time and spatial scales over which the atoms arrange themselves on a surface are not easy accessed experimentally. Therefore, these advantages are customarily verified by ex-situ examination of films after deposition is complete, which gives little information on atomic scale processes that lead to the listed advantages. The addition of energy to the deposition flux effects surface processes that are otherwise only controllable by changing the substrate temperature. Thus, understanding the mechanisms by which energetic atoms alter surface processes in analogy with thermal effects is of paramount interest for optimization of the deposition parameters. This review summarizes the state of knowledge on the effects of energetic ions on film formation. For high quality, defect free films, the energy must be controlled in the energy region of 5 eV to 30 eV. Below about 5 eV, the energy is ineffective for changing the physical processes, and above about 30 eV, defects are added to the film by displacement damage that cannot anneal out due to low temperature of the deposition. Models for the composition and chemistry of films energetically deposited are progressing well, while models for prediction of the phases that form are almost nonexistent. Molecular dynamics provide the best information, but only a handful of cases have been simulated.
A study is presented of the geometrical shape of deposition contours that arise when material is evaporated from a point source onto an inclined substrate, an arrangement common in ion-assisted deposition. The shape of the contours, as determined by the inverse square law and the angles of emission and incidence, is described by a fourth-order algebraic equation in polar coordinates on the surface of the substrate. The equation defines a family of distorted ellipses whose form depends on the angle of tilt. An experimental test of these relations by electron-beam deposition of an ion-bombarded oil film on a tilted silicon wafer will be reported.
We have designed and constructed a large area ion beam apparatus to deposit DLC films onto 1000 cm2 surfaces with various geometries. The use of an efficient RF excited ion gun (13.56 MHz, 1 kW power, 50-3000 eV ion energy) with a diameter of 20 cm, enables us to generate various hydrocarbon ions with high ion beam currents, varying ionic species and less maintenance. The use of a four axis (Χ-ϒ-Θϒ-ΘZ) substrate scanner with computer control can produce uniform DLC films on large areas and curved surfaces. The effects of RF power, ion energy, gaseous composition, and total pressure on the properties of DLC have been systematically investigated.
Molybdenum nitrides are active and selective hydrodenitrogenation (HDN) catalysts. The catalytic properties of molybdenum nitrides were found to be dependent on the structural properties. The purpose of research described in this paper was to synthesize molybdenum nitride thin films with well defined structures and stoichiometries using ion beam assisted deposition. The films were deposited by evaporating Mo metal, and simultaneously bombarding the growing film with low energy nitrogen ions. The phase constituents of the films were determined using x-ray diffraction and the film composition was obtained by Rutherford backscattering spectrometry.
The film composition and phase constituents were strong functions of the ion-to-atom arrival rate ratio, ion energy and ion angle of incidence. Differences in the film composition for different arrival rate ratios and ion angles of incidence were interpreted based on reflection and sputtering effects. Our results suggest that phase formation was governed by the effective energy density per deposited atom. Evaluation of the effective energy density per deposited atom and its physical significance in ion beam assisted deposition is discussed.
We deposit Si films on Si(100) substrates at temperatures of 300 - 350 °C using dc magnetron sputtering, and characterize the structure by in-situ spectroscopic ellipsometry. Changes in the ion or electron bombardment, produced by biasing the sample with respect to the floating potential, are found to exert a strong effect on the kinetics of the crystalline (epitaxial) to amorphous transition for films deposited just below the apparent minimum temperature (350 °C) for sustained epitaxy. At 320°C, the best results are found at the floating potential, which is 25 V below the plasma potential and produces an ion flux equal to the depositing Si flux on the substrate. At +14 V above the floating potential, the volume fraction of crystalline Si decreases exponentially with thickness, and the characteristic decay length is a function of substrate temperature. At -14 V below the floating potential, the deposited film is amorphous with a large void content. These observations demonstrate the subtle tradeoff between enhanced surface mobility and defect creation by low energy ion bombardment.
The growth mechanism of ZrNx films produced by reactive ion beam sputtering with or without concurrent low energy ion bombardment of argon or nitrogen has been investigated. The effect of substrate temperature in the range of 300-680K, partial pressure of nitrogen and ion/atom arrival rate on the composition and microstructure of the films have been studied. RBS analysis has confirmed that the nitrogen content varies over wide range 0-60 at. %, depending on the nitrogen/zirconium arrival rate, and the ion assist flux but it is independent of the ion assist energy. TEM analysis shows that the films are non-columnar and polycrystalline with grain sizes l-15nm which depend on the nitrogen content and the deposition temperature.
Plasma and ion beam based techniques have been used to deposit carbon-based films. The ion beam based method, a cathodic arc process, used a magnetically mass analyzed beam and is inherently a line-of-sight process. Two hydrocarbon plasma-based, non-line-of-sight techniques were also used and have the advantage of being capable of coating complicated geometries. The self-bias technique can produce hard carbon films, but is dependent on rf power and the surface area of the target. The pulsed-bias technique can also produce hard carbon films but has the additional advantage of being independent of rf power and target surface area. Tribological results indicated the coefficient of friction is nearly the same for carbon films from each deposition process, but the wear rate of the cathodic arc film was five times less than for the self-bias or pulsed-bias films. Although the cathodic arc film was the hardest, contained the highest fraction of sp3 bonds and exhibited the lowest wear rate, the cathodic arc film also produced the highest wear on the 440C stainless steel counterface during tribological testing. Thus, for tribological applications requiring low wear rates for both counterfaces, coating one surface with a very hard, wear resistant film may detrimentally affect the tribological behavior of the counterface.
We have studied the growth and the properties of CN films prepared by deposition of mass separated 12C+ and 14N+ ions. The film thickness and density were determined as a function of ion energy between 20 eV and 500 eV and for substrate temperatures of 20 °C and 350 °C. Sputtering effects limit the maximum N concentration to about 30 - 40 at.% even for ion energies as low as 20 eV. IR absorption measurements indicate predominantly C-N and C=N bonding and an amorphous or strongly disordered CN-network. For room temperature deposited CN films with N concentrations up to 25 at.% I-V curves of metal-CN-metal devices show Frenkel-Poole behavior due to field-enhanced thermal activation of localized electrons. Films deposited at 350 °C have N concentrations below 15 at.% and graphitic properties like low resistivity and a density close to graphite.
Negative-ion implantation is a promising technique for forthcoming ULSI (more than 256 M bits) fabrication and TFT (for color LCD) fabrication, since the surface charging voltage of insulated electrodes or insulators implanted by negative ions is found to saturate within so few as several volts, no breakdown of insulators would be expected without a charge neutralizer in these fabrication processes. Scatter-less negative-ion implantation into powders is also possible. For this purpose an rf-plasma-sputter type heavy negative-ion source was developed, which can deliver several milliamperes of various kinds of negative ion currents such as boron, phosphor, silicon, carbon, copper, oxygen, etc. A medium current negative-ion implanter with a small version of this type of ion source has been developed.
A general relation between the implanted dose and the processing time for plasma immersion ion implantation (PHI) can be established through the dynamic sheath model. In practice, etching and charging effects have to be taken into account in PIII dose estimation.
Plasma immersion ion implantation (PII) has been tested in fabrication of semiconductor devices with shallow junctions and in hydrogénation of poly-Si thin film transistors (TFT). PIII doping is more suitable than conventional implantation for such applications because of its high dose rate at lower energy. Since the dose rate in PIII does not depend on the area being treated, the effective current will be higher if a larger implanted area is involved. However, the relation between dose and time is not always straightforward. During PIII processing possible etching and charging will affect the total accumulated doses. This paper presents a model for each which allows a proper compensation to be performed.
Buried oxide layers in Si were fabricated using non-mass analyzed plasma immersion ion implantation (PHI). We call this process of making separation by implantation of oxygen (SIMOX) with implantation by PIII as separation by plasma implantation of oxygen (SPIMOX). The implantation was carried out by applying a large negative bias to a Si wafer immersed in an oxygen plasma and a nominal dose of 2 × 1017 cm”2 of oxygen was obtained in less than three minutes. Cross section transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry (RBS) were used to characterize the wafers. Three distinct modes of microstructure development were observed after post implantation annealing. With a low oxygen dose (< 1 × 1017 cm”2 ), isolated silicon dioxide precipitates did not grow large enough to form a continuous oxide layer. With a high oxygen dose ( > 3 × 1017 cm”2 ), however, a single buried oxide layer was observed. By optimizing the concentration ratio of 0+ and 02+ in the plasma and the implant dose, a double oxide layer (Si/oxide/Si/oxide/Si) structure, was produced in a single implantation step.
Use of MeV ion implantation for mass production of CMOS devices at 0.5um design rule and beyond is now being accepted around the world for 16Mb DRAM, 16Mb Flash memory and CMOS logic/microprocessor technologies. Incorporating MeV well formation for twin well and triple well results in a reduction of up to 3 masking layers corresponding to process simplification and manufacturing cost reduction of 10% to 16%. For CMOS logic application, a new structure called BILLI (Buriedjmplanted Layer for Lateral Isolation) is showing great promise for latch-up free CMOS and when combined with hydrogen denuded bulk Czochraliski (CZ) grown silicon wafers, has the potential to replace epitaxial wafers with improved device performance. This paper will review MeV ion implantation use for these various CMOS applications.
Control of charge carrier collection by high-energy boron-implanted layers has been investigated to clarify the validity of buried well structures against soft errors in dynamic random-access memories (DRAMs) by ion-induced-current measurements using high-energy proton microprobes. A finely focused 1.3 MeV proton beam has been used to irradiate normal to n+p diodes with buried layers fabricated by B+ implantation at 160 — 1000 keV and to doses of 1 × 1012 — 1 × 101 ions/cm2, and reverse-biased at 1 to 5 V. The measured current was induced by carriers generated by ion microprobes. The collection of charge carriers induced by microprobe irradiation could be reduced by a buried layer formed by boron implantation. It was found that the rate of charge collection depended not on the depth but on the implantation dose of the buried layer. The carrier collection efficiency of the n+p diode with twin wells (i.e., a retrograde well) was two thirds of that with a conventional well.