Decreasing Ion Energy Research Articles

Heteroepitaxial growth of GaAs on Si substrates using low energy Ga and As ion beams

GaAs thin films were heteroepitaxially grown on (1 0 0) and (1 1 1) Si substrates at a temperature of 500°C using Ga and As ion beams at energies lower than about 50 eV. The growth rates increased with decreasing ion energy. However, the GaAs films were heteroepitaxially grown on the (1 0 0) and (1 1 1) Si substrates by mixed beams containing larger amounts of the neutral Ga and As molecular beams added to the ion beams even when the ion beams were accelerated to energies higher than 60 eV.

Damage, defects and diffusion from ultra-low energy (0–5 keV) ion implantation of silicon

Continued use of ion implantation for doping of silicon integrated circuits will soon require implantation energies below 5 keV to form electrical junctions less than 50 nm deep. At such low energies, dopant diffusion and formation of extended defects is modified by both the proximity of the surface and by the large volume concentrations of point defects and dopant atoms that arise from reduced range straggling. This brief review summarizes our recent experiments which measured defect formation and evolution, as well as enhanced diffusion, in silicon implanted with Si + and B + ions at energies as low as 0.5 keV. The results have demonstrated that {311}-type extended defects are generated from Si + implants even within 3 nm of the surface. However, when these defects eventually dissolve, the surface acts as a perfect sink to efficiently annihilate the released interstitials. As a result, the amount of TED from Si + implantation measured by epitaxially-grown B markers decreases approximately linearly with decreasing ion energy. For sub-keV B + implants typical doses currently used for source-drain doping lead to a boron diffusion enhancement of 3–4× despite the proximity of the surface. Enhanced diffusion is also observed from molecular beam-deposited silicon layers containing a high boron concentration. This newly emerged diffusion enhancement mechanism, boron-enhanced-diffusion (BED), is associated with the formation of a fine-grain polycrystalline silicon boride phase in the implanted layer during activation annealing. These investigations of ultra-low energy (ULE) implantation have thus reinforced and validated our understanding of the role of implantation damage in enhancing dopant diffusion in silicon, while simultaneously revealing some important new materials issues which will impact semiconductor processing in coming device generations.

Enhancement of Reactivity in Au Etching by Pulse-Time-Modulated Cl2 Plasma

In Al, Au and Pt metal etching processes, low etching rate and low etching selectivity are serious problems. To achieve a breakthrough in these problems, metal etching by pulse-time-modulated plasma was investigated. In particular, the Au etching rate was increased significantly in the pulsed plasma even when the ion energy decreases. However, an increase in the etching rate cannot be observed in Al etching. As a result, it is speculated that the increase in the Au etching rate is caused by the increase in the evaporation rate of Au etching products, which results from the injection of negative ions.

Amorphisation and surface morphology development at low-energy ion milling

Amorphisation and surface morphology development of Si and GaAs was studied by means of XTEM after ion bombarding at various low energies in our novel ion milling unit. In this device, the specimen is rotated and the ion energy can be reduced down to 0.12 keV. It was shown that the thickness of the amorphised layer is about 1 nm for Si/0.25 keV, and not observed for GaAs/0.25 keV and Si/0.12 keV. This amorphisation is much thinner than those measured at higher energies, e.g. 5 nm, 2.1 nm for Si/3 keV, GaAs/1.6 keV. Dynamic TRIM simulation could be successfully applied for the description of amorphisation. It will be shown for the first time that the interface roughness also decreases with decreasing ion energy.

Low Bias Dry Etching of Sic and Sicn in ICP NF3 Discharges

ABSTRACTA parametric study of the etching characteristics of 6H p+ and n+ SiC and thin film SiC0.8N0.2 in Inductively Coupled Plasma NF3/O2 and NF3/Ar discharges has been performed. The etch rates in both chemistries increase monotonically with NF3 percentage and rf chuck power reaching 3500Å·min−1 for SiC and 7500 Å·min−1 for SiCN. 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-tinoxide( ITO) 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. The high ion flux available in the ICP tool allows etching even at very low dc self-biases, ≤ −10V, leading to very low damage pattern transfer.

Thin Film Formation by Positive and Negative Ion Beams Simultaneously Irradiated Deposition.

A dual ion beam deposition apparatus was newly developed, which consists of a positive ion beam line, a negative ion beam line and an ultra-high vacuum deposition chamber. The machine can generate mass-analyzed very low energy ion beams with positive and negative charges at the same time with the ion energy range from 10 eV to 20 keV. It is possible to deposit both ions not only simultaneously but also alternatively. The machine has a wide possibility to fabricate ultra-pure materials or non-traditional materials, and is also useful to study fundamental processes of ion beam deposition and/or ion solid surface interactions. Using this new equipment, carbon nitride films were fabricated by simultaneous deposition of C- and N+ ions with the ion energy varying from 50 to 400 eV. The films were analyzed by Rutherford backscattering (RBS), Fourier Transform Infrared Spectroscopy (FTIR) and Raman scattering. It was found that the maximum composition ratio (N/C)c was about 0.9, the film structure was like amorphous carbon and the amount of C-N triple bonds tended to decrease with decreasing nitrogen ion energy.

Sputtering of boron-doped graphite USB15—Investigation of the origin of low chemical erosion

The changes in surface composition of USB15—a boron doped graphite containing 15 wt.% of boron—during bombardment with D ions were determined by in situ Auger electron spectroscopy at temperatures from 300 up to 1000 K. For energies above 100 eV no strong increase of the boron surface concentration could be observed even around 800 K, i.e., at the maximum for chemical erosion of pure graphite. Chemical factor analysis of the carbon Auger peak in this energy-regime results in a much larger carbidic fraction of carbon atoms than suspected from the boron content of 15%. Thus, boron influences much more carbon atoms in their chemical reactivity with deuterium ions than is expected for the stoichiometric B4C precipitates. For ion energies below 100 eV a strong increase of boron surface concentration with decreasing ion energy at room temperature was observed. The chemical erosion of carbon in this energy regime is not suppressed by boron doping and indicates a different, surface related release process of hydrocarbon molecules.

Synthesis of aluminium nitride thin films by ion-vapour deposition method

Aluminium nitride (AlN) thin films have been synthesized by evaporation of aluminium and simultaneous irradiation with nitrogen ions (ion-vapour deposition method). The substrate temperature has been kept at room temperature or 473 K and the kinetic energy of the incident nitrogen ion beam has been varied from 0.5 to 2.0 keV. The microstructure of the synthesized films has been examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the XRD patterns of films synthesized at both room temperature and 473 K the diffraction line indicating the AlN (100) plane can be discerned and the broad peak composed of two lines indicating the AlN(002) and AlN(101) planes is also observed. TEM observations reveal that the films synthesized at 473 K show a more highly crystallized structure rather than the films synthesized at room temperature. The intensity of the AlN(002) and AlN(101) planes is found to increase with decreasing nitrogen ion energy at the substrate temperature of 473 K. The present results suggest that the synthesis of c-axis-oriented AlN films is feasible by controlling the substrate temperature and the incident ion energy.

Preparation of thin gold films by the forward-sputtering method

Au films are deposited on air-cleaved MgO(100) substrate by forward sputtering under various conditions of ion energy and ion species and the differences in their structural features are examined. The films have a mixture of (111) and (100) orientations. When the ion energy decreases or when the ion mass increases, the growth of (100)-oriented deposits is enhanced. Mostly (100)-oriented Au films are obtained below 500 °C. The defect density in Au films deposited by Au3+ ions seems to be lower than that in Au films deposited by other ions. The surface morphology is also changed by the ion species. With lighter ions (C+, O+) a relatively uniform surface is observed, while with heavier ions (Si+, Au3+) a grain structure becomes clear. From these results it is believed that the structural features of Au deposits can be favourably changed by forward-sputtering deposition.

透過電子顕微鏡その場観察用極低エネルギーイオン照射装置（水素イオン入射装置）

Ion irradiation equipment has been developed which allows in-situ observation of microstructural evolution by transmission electron microscope (TEM) during ion irradiation at energies below 100 eV. In conventional ion source TEM interfaces, the flux of extracted ions from the ion source decreases with decreasing ion energy, and the loss of ion current during ion beam transport increases drastically. To overcome these drawbacks, the beam transport distance is shortened and the ion source and the beam transport are insulated from the ground. Ions are first extracted to a maximum of 10 keV, then transported to the specimen chamber of TEM and finally decelerated to a desired energy just before incidence at the specimen in TEM. A hydrogen ion beam current of 1, μA/2 mmφ at 100 eV is obtained at the specimen position allowing in situ microstructural observations.

The sputtering of SiO 2 and its dependence on oxygen partial pressure

The sputtering yield of SiO 2 has been measured for 150–300 keV N +, Ne +, Ar + and Kr + ions by monitoring the secondary electron yield during sputtering through thin SiO 2 films on Si. Measurements were performed both in ultrahigh vacuum and in low pressure O 2, CO and N 2 ambients. The ultrahigh vacuum sputtering yield was found to increase with increasing ion mass and decreasing ion energy in agreement with sputtering theory. Quantitatively, though, considerable deviations were found from a direct proportionality between the sputtering yield and the nuclear energy deposited at an SiO 2 surface. Since there is a correlation between these deviations and the electronic energy deposition, it is suggested that the sputtering yield could be enhanced by electronic processes. The sputtering of SiO 2 films was found to decrease strongly with increasing O 2 partial pressure but no dependence on N 2 or CO partial pressure was observed. The reduction in sputtering at O 2 exposure is larger the smaller the ion mass and can be as much as almost 50% for N + ions. Most probably, the sputtering decrease in O 2 ambients is due to a replacement of sputtered oxygen atoms from the surface by oxygen from the ambient. For light ions, however, it cannot be ruled out that the apparent sputtering reduction, to a small degree, is caused by a simultaneous oxide growth induced by the ion beam. Finally, it is argued that the dependence of the sputtering reduction on ion mass can be due to preferential sputtering of oxygen and/or ion induced incorporation of oxygen in the surface.

Convoy electrons produced at glancing-angle incidence of MeV heavy ions on crystal surfaces

Convoy electrons produced at glancing-angle incidence of MeV H, He, Li and C ions on a SnTe(001) surface are studied. Acceleration of the convoy electrons is observed. The acceleration increases with increasing atomic number of the projectile ion and with decreasing ion energy, in harmony with the surface-wake-acceleration model. The acceleration of convoy electrons by the surface wake is calculated with a classical-trajectory Monte Carlo simulation. The calculated results reproduce the characteristic features of the observed convoy electrons showing the validity of the surface-wake-acceleration model.

The Characteristics of Ion-Beam-Induced Spontaneous Etching of GaAs by Low-Energy Focused Ion Beam Irradiation

We have investigated the characteristics of ion-beam-induced spontaneous etching (IBISE) of GaAs in Cl2 ambient by using a Ga-focused ion beam (FIB) with an energy ranging from 3 to 15 keV. The etched depth of the irradiated region was more than 20 times greater than that of unirradiated region. When the sputtered depth by FIB irradiation amounted to around 8 Å at each ion energy, the etched depth in Cl2 ambient for 1 hour became saturated. The saturated etched depths were 450, 550, 750 and 800 Å at the ion energy of 3, 5, 10 and 15 keV, respectively. The residual damage of the etched surface was also investigated by photoluminescence (PL) measurement. The maximal PL intensity was obtained at around the threshold dose of IBISE and increased with decreasing ion energy. The full recovery of PL intensity was observed at the ion energy of 3 keV after annealing at 400°C.

Determination of the optimal energy range for obtaining diamond-like films by ion implantation

Pure films of carbon were grown by implanting C + ions at energies in the range 0.1–2 keV in various substrates. Analysis of their compositions by various ion beam interactions showed that there was little mixing with the substrate layers and radiation-enhanced diffusion in the films, and the impurity content decreased with decreasing ion energy. On the basis of spectroscopic investigations of optical absorption and soft X-ray emission, we also established that the percentage of diamond-like carbon (DLC) increased monotonically with ion energy. DLC was only obtained during the preliminary stages of implantation in silicon or SiC at energies below 1 keV, because of their catalytic effect on the sp 3 hybridization of carbon atoms. Films formed at 2 keV exhibited similar properties to those grown by laser-plasma deposition and contained about 75% diamond microcrystals. They were also more diamond-like than films grown by sputtering processes.

Reduction of droplet emission in random arc technology

The influence of a static magnetic field (0–10 mT at the emission plane of the cathode) in random arc reactive physical vapour deposition (PVD) (TiN, CrN) and non-reactive PVD (titanium, chromium, copper, nickel) has been investigated using various cathode designs. The dominant effect of an increased magnetic field was a more confined arc trace. In non-reactive processes, detrimental selective cathode erosion was observed. In reactive processes, improved nitriding resulted. A distinct reduction in droplet emission was achieved only with a high melting nitride layer (TiN). Cathode design may assist in droplet reduction by further confinement of the arc trace. In all reactive processes, changes in layer microstructure were detected indicating a decrease in ion energy with increasing magnetic field. Surface roughness values comparable with those obtained in steered arc PVD can be achieved, using strong static magnetic fields combined with modified cathode design, if the process is run at a stabilized steady state nitrogen pressure of about 1 Pa from its very start.

Process control issues for ion implantation using large tilt angles

Using large tilt angles (7°–60°) and wafer rotational repositioning during ion implantation results in several effects which can compromise process control. These effects include: (1) tilt-related nonuniformity, (2) effective dose reduction from decreased ion flux density, (3) effective ion energy decrease, (4) effective dose reduction from dopant loss by surface scattering, and (5) effective surface film thickness increase. In this paper we characterize these effects and explain their physical basis.

Characterization of heavy-ion damage in ruthenium. II. Cascade collapse

Abstract Vacancy loop formation in cascades formed during heavy-ion irradiation of the h.c.p. metal ruthenium at room temperature has been investigated by transmission electron microscopy. The ion energy and the ion mass were varied between 10 and 100 keV and 84 (Kr+) to 184 (W+) respectively to produce a wide variety of cascade morphologies. Information about the cascade collapse process has been obtained from the measurement of defect yield and cascade efficiency values. The former defines the probability that an incident ion forms a visible loop, and the latter the proportion of vacancies per cascade that remain in the form of a dislocation loop. The defect yield increases with increasing ion energy and has a complex dependence on ion mass, first increasing and then decreasing with increasing mass at low and high energies respectively. Cascade efficiency is not as complex and increases with increasing ion mass and decreasing ion energy. These effects have been interpreted in terms of current cascade co...

Photoelectron spectroscopic study of copper surfaces changed by oxygen ion bombardment

The oxidation of polycrystalline copper surfaces under oxygen ion bombardment at 80 and 300 K has been investigated with XPS and UPS. The degree of oxidation has been estimated from XPS data involving the relative intensity of the shake-up peak of CuO and the shift of Cu(L 3VV) Auger line. Copper surfaces were mainly oxidized to Cu 2O by 0.5, 3 and 10 keV ion bombardment at 300 K. The amount of CuO increased with decreasing ion energy. On the other hand, the sample surface was completely oxidized to CuO with some adsorbates under 0.5 keV oxygen ion bombardment at 80 K. These phenomena indicate that the oxidation by oxygen ion bombardment with lower ion energy is similar to that under higher oxygen potential and that the lowering of the substrate temperature, which gives rise to the lowering of oxidation free energy, causes the increasing degree of oxidation.

Radiation damage in vitreous fused silica induced by MeV ion implantation

The nature of E′ 1 defects in vitreous fused silica induced by high energy (1–17 MeV) Cl and F ion implantation in terms of ion fluence, ion energy, saturation behavior and annealing feature, has been studied and compared with results obtained after 2 MeV proton and 0.633 MeV γ-ray irradiation using the electron spin resonance (ESR) method. The ESR spectra of E′ 1 defects created by γ-ray, proton, and MeV ion (at low ion fluence) irradiation are similar, with a mean g factor of 2.0005 ± 0.0004. For MeV heavy ion implantation the density of E′ 1 defects reaches a maximum at a deposited energy density of the order of 10 21 keV cm, and the average yield of E′ 1 defects per unit energy increases with increasing ion mass and decreasing ion energy for a given ion fluence. The influence of ion flux on defect creation as seen in low-energy heavy ion implanted crystal GaAs was not observed. The E′ 1 defects caused by either γ-ray or MeV heavy ion irradiation anneal at a unique stage and bleach at temperatures above 450°C. The annealing feature is dependent on the ion fluence. For 2 MeV proton implantation, the E′ 1 defects decay more slowly and beaach at temperatures above 600°C. The surface cracking produced by MeV heavy-ion implantation has been observed. The crack density increases with ion fluence, reaches a maximum, and then declines. The cracks close up for subsequent higher ion fluences. No surface cracks were seen for 2 MeV proton implantation in the ion fluence range of 1 × 10 14 to 2 × 10 17 protons cm −2.

Interaction of low energy reactive ions with surfaces. III. Scattering of 30–200 eV Ne+, O+, C+, and CO+ from Ni(111)

Time-of-flight (TOF) and energy distributions of 30–200 eV Ne+, O+, C+, and CO+ scattered from Ni(111) have been investigated using pulsed ion beam techniques and classical trajectory simulations. The experiments probe the interaction potentials and the applicability of the binary collision approximation in the low energy range. The experimental scattering energies are in good agreement with the energies predicted by the classical treatment. Scattering of atomic projectiles at primary energies as low as 30 eV (scattered energies ∼15 eV) has been detected and is characterized by sharp scattering peaks. The lower limit of detectability is determined by the sensitivity of the detector to slow neutrals. The reactive ions are completely neutralized in the scattering collision while some of the Ne+ ions survive single scattering events. Scattering of molecular CO+ produces a broad scattered flux distribution due to partial dissociation and scattering of both molecular and atomic species. Trajectory simulations have been performed using a purely repulsive Biersack–Ziegler potential and a Biersack–Ziegler potential combined with a Morse potential to include an attractive component. The importance of the attractive potential in describing the trajectories of the reactive ions increases with decreasing ion energy. This attractive potential is shown to be responsible for preferential neutralization of reactive ions by altering the trajectories so that the distance of closest approach is shorter and the time spent near the surface is longer, thus enhancing electronic interaction between colliding species. Energy level diagrams are used to discuss the neutralization transitions and the differences between the reactive and noble ions.