eV Ion Beam Research Articles

Deposition of mass-selected ions in neon matrices: CS2+ and C6F6+

Infrared and visible absorption spectra and laser-induced fluorescence (LIF) and excitation spectra are obtained for several simple cations deposited from a mass-selected ion beam. In the present preliminary study we demonstrate successful and clean mass selection by presenting spectra of samples obtained by depositing the isotopic S1234C32S+ ion in natural isotopic abundance, and analyzing its spectrum. Spectra of C6F6+ deposited from a 20 eV ion beam exhibit quite different inhomogeneous line profiles, suggesting that even the relatively low kinetic energy results in considerable damage to the solid. Analysis of the spectra indicates that the Jahn–Teller-distorted vibrational structure in the doubly degenerate ground state of C6F6+ is strongly perturbed in the newly formed sites, which are presumably of lower symmetry. A 33–46 cm−1 splitting of the origin and other totally symmetric bands in emission is tentatively attributed to the spin–orbit splitting in the E1g2 ground state.

Thiosulfoxides (X 2SS) and disulfanes (XSSX): : first observation of organic thiosulfoxides

Using a combination of tandem mass spectrometry methodologies, collisional activation of fast (8 keV) or slow (20–30 eV) ion beams, neutralization–reionization and ion–molecule reactions, it is shown that thiosulfoxides X 2SS (X = H, CH 3, C 2H 5) are stable in the gas phase as radical cations as well as neutral molecules. The absence of isomerization into the more conventional disulfane structure, XSSX, is firmly demonstrated by ion-molecule reactions of the isomeric ions with methyl isocyanide and by collisional activation of “survivor” ions generated in neutralization–reionization experiments. All the mass spectral data have been recorded with a single hybrid mass spectrometer of sectors-quadrupole-sectors configuration. Molecular orbital calculations demonstrate that ionized thiosulfoxides and disulfanes are local minima on the MP2/6-31G∗ potential energy surface. The disulfane structures are found to be the most stable species by 91, 65 and 59 kJ/mol for X = H, CH 3 and C 2H 5, respectively. Heat of formation values of 1000, 810 and 760 kJ/mol are estimated for X 2SS ·+ ions (X = H, CH 3 and C 2H 5, respectively).

Soft landing of ions onto self-assembled hydrocarbon and fluorocarbon monolayer surfaces

Low energy (≤ 10 eV) ion beams can be soft landed onto a C 12-hydrocarbon self-assembled monolayer (H-SAM) surface. The H-SAM surface causes much less collateral fragmentation of the deposited ions under the same conditions than the corresponding fluorinated self-assembled monolayer (F-SAM) surface. The energy dependence for release of ions deposited onto an F-SAM monolayer surface shows that smaller ions are ejected more readily than sterically bulky species. The threshold energy for release of the soft-landed species by Xe +· coincides with that of the typical chemical sputtering product CF 3 +, implying that the deposited species are strongly held inside the SAM matrix and that C–C bond cleavage assists in efficient release of the trapped ions. In cases where fragment ions of the deposited projectile are released from the surface, it is shown, by varying the energy and nature of the releasing projectile ion, that dissociation primarily occurs upon impact of the soft-landed ion and not upon its release. Examination of various odd- and even-electron ions confirms the earlier conclusion that the former are efficiently neutralized and only the latter can be soft landed as ions onto F-SAM surfaces.

Patterning amorphous fluoropolymer films by reactive ion milling

Amorphous fluoropolymer films have low dielectric constants and high chemical resistance and, so, have potential to be used as the insulator for high speed interconnects and as protection layers. Many applications would require high resolution patterning of the fluoropolymer film. We have found that these films are easily etched by reactive ion beam etching using an O2/Ar gas mixture. High etching rates of 600 nm/min with a 0.2 mA/cm-2,500 eV ion beam were obtained. This technique allows good selectivity with typical underlayers such as Si, Au, and photoresist. We have also found that a short Arion milling of the fluoropolymer surface allows good wettability of the film by photoresist.

Chemically Clean Planar Interface Synthesis: Substrate Surface and Interface Cross Section Microscopy

An ultra high vacuum (UHV) planar interface unit has been constructed to study the effect of interface/boundary structure and chemistry on properties. We report here initial observations of substrate morphology and chemistry prior to bonding and resulting interface morphology obtained using austenitic stainless steel.To synthesize chemically clean planar interfaces by diffusion bonding, the substrate must be macroscopically and microscopically flat and chemically clean. Macro-flatness, necessary for bonding to occur over large areas, was ensured by conventional mechanical polishing and lapping. Substrate surfaces were cleaned by a broad (3cm) 500 eV ion beam (Ar or Xe) at 15° incidence. The resulting changes in substrate near-atomic-scale roughness and chemistry were analyzed using Auger spectroscopy (AES) and Atomic Force Microscopy (AFM). Before ion beam cleaning, the sub-strates exhibited high oxygen and carbon contamination (Fig la). Both Xe and Ar ion cleaning reduced these values; the result for 5 minutes Ar cleaning is shown in Fig lb.

Low-Energy Focused Ion Beam Doping during Molecular Beam Epitaxial Growth for the Fabrication of Three-Dimensional Devices: The Effect of Dopant Surface Segregation

An extreme example of surface segregation is found in Sn-doped GaAs grown by molecular beam epitaxy (MBE). Abrupt changes in the doping profile are not possible, instead the dopant concentration decreases exponentially towards the wafer surface from the point at which doping was terminated. In this work it is shown that segregation can be suppressed by implanting the Sn from a very-low-energy (50 to 300 eV) ion beam during growth. The effect of ion implantation energy is studied using secondary ion mass spectroscopy (SIMS) to measure the depth profile of the implanted Sn. It is found that the level of incorporation can be increased by up to a factor of eight using a 300 eV ion energy.

Compositional and structural modifications of amorphous SiO 2 by low-energy ion and neutral beam irradiation

Compositional and structural modifications induced by irradiating amorphous SiO 2 with low-energy (300–500 eV) ion and neutral beams of inert gases have been studied. The ion beams were extracted from microwave excited plasmas of inert gases (Ne, Ar and Kr), and the neutral beams were generated by subsequent charge-exchange reactions. The SiO 2 layers modified with these beams have been characterized by Auger electron spectroscopy, Rutherford backscattering spectroscopy, reflection high-energy electron diffraction and transmission electron microscopy. It is shown that neutral beam irradiation does not cause preferential sputtering of oxygen from SiO 2, whereas ion beam with the same energy causes significant preferential sputtering. For neutral beam irradiation, densification and crystallization of SiO 2 were observed: nanometer sized α-cristobalite or α-quartz crystals were formed when amorphous SiO 2 was irradiated with more than 10 17 neutrals/cm 2 and the densified SiO 2 layer extended several nanometers beneath these crystals. The reasons for these densification and crystallization phenomena have been discussed in terms of the high temperatures and pressures in local spots formed as a result of bombardment by energetic neutral atoms. For ion beam irradiations, these densification and crystallization phenomena have not been observed.

Modification of the Wettability of Plexiglas and Carbon by Means of Ion Beam Implantation: Application to Fluid Management and Materials Processing in Microgravity.

AbstractIn the absence of gravity, the behaviour of a liquid within a container will depend on its ability to wet the container. For instance, we evidenced that wetting property is involved in the propagation of perturbation, at the liquid-air interface. Because of perturbation in space, there is a strong interest in the modification of wetting properties for fluid management and materials processing in microgravity. The use of low energy (-500 eV) ion beams is attractive. Combining their low mean range (a few nm) and erosion rate, they are well suited for the modification at will of surface composition. Nitrogen, oxygen and neon ions of low energy were used to bombard the surface of plexiglas (polymethylmethacrylate or PMMA) and carbon samples at different fluences. Advancing and receding contact angles were measured before and after the implantation. A decrease of the contact angle as a function of the fluence of implanted ions was measured. A partial recovery of the contact angles as a function of time after the implantation was also observed. There was evidence that the long term value of the contact angles was influenced by the composition of the gas surrounding the samples. These results are discussed in terms of the development of a new technology for the modification at will of wetting properties of materials.

Development of broad beam ion sources at CSSAR

High-energy and intense beam current broad beam ion sources have been developed for ion implantation and dynamic recoil mixing at CSSAR. The sources can be operated over beam energy and current ranges of 3–120 keV and 5–70 mA, respectively. For sputter coating of thin films, a series of focusing beam ion sources with different structures has also been developed. The energy and current range from 1–10 keV and 100–350 mA for different applications. For some applications, low-energy (below 100 eV) ion beams are required. CSSAR has developed a 6-cm-diam broad beam ion source. The source can be operated at beam energy 10–70 eV, and the beam current 15–80 mA has been extracted. Typical structures and operational data are given for the sources mentioned above. Recently a new type of broad beam metal ion source (Electron Beam Evaporation Metal Ion Source EBE) is being studied. Ion beams of several kinds of materials such as C, W, Ta, Mo, Cr, Ti, B, Cu, etc. have been extracted from the source. Typical operation conditions and ion yields are given in this paper.

The deposition mechanism of diamond-like a-C and a-C: H

Diamond-like carbon (DLC) is a dense, partially sp 3 bonded metastable phase of non-crystalline carbon or hydrogenated amorphous carbon formed by the deposition from medium energy (50–500 eV) ion beams. The sp 3 bonding arises from C ions entering subsurface atomic sites and producing a quenched-in density increase. The process has an optimum ion energy of about 100 eV because the C ions must have sufficient energy to penetrate the surface, but any excess energy could anneal out the density increment. The process is an ion-induced compression of sp 2 a-C into the denser sp 3 phase. Deposition of a-C: H is more complex, involving a growth step, ion-induced dehydrogenation and ion-induced compression of the C-C skeleton. In the growth step, ions incorporate directly into the films whereas slow neutral species first form an adsorbed layer which then may incorporate into the bulk. Ion bombardment causes dehydrogenation by the preferential displacement of hydrogen and compresses sp 2 sites into sp 3 sites, forming extra bonds in the C-C skeleton, as in a-C.

Strain Relief by Ion Beam Mixing. Molecular Dynamics Simulations Applied to Metallic Hetero-Structures

ABSTRACTThis work presents molecular dynamics simulations of low-energy (40-80 eV) ionbeam mixing of thin metallic hetero-structures. The results indicate that the propagation of the cascade may maintain or even restore crystallinity in disordered interfacial regions.

Silicon nitride formation by low energy N+ and N+ 2 ion beams

Reactions of N+ and N+2 ions with Si(100) surface are examined as a function of both ion kinetic energy and dose using a low energy ion beam instrument. The Si surface is exposed to low energy (1–300 eV) ion beams in an ultrahigh vacuum environment and the resulting surface species are characterized by Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The absolute reaction probability Pr is measured for nitridation processes. Pr(N+) has a value of ∼0.25 and stays constant in the energy range of 1–25 eV. Pr(N+2) increases from zero to ∼0.25 in the same range. Continued exposure of the ion beams to a dose ≳5×1015 ions/cm2 leads to a saturation and formation of a dense and stable silicon nitride layer. Variation of Pr with energy and dose is explained in terms of elementary reaction steps such as charge neutralization of the projectile ion, collisional dissociation of N+2, nitridation reaction, and chemically induced desorption of surface nitrogen species. A mechanism is proposed to explain enhancement in the stability and ordering of the nitride layer during the low energy ion–surface interaction. At saturation the processes of both nitridation and chemically induced desorption simultaneously occur, which, under hyperthermal environments, effectively remove defects and interstitial nitrogen atoms from the layer.

Densification and Crystallization of Amorphous SiO2 by Neutral Beam Bombardment

ABSTRACTAmorphous SiO2 films formed by thermal oxidation of silicon have been bombarded by low-energy (350 — 400 eV) ion beam and neutral beam of inert atoms. The modified SiO2 layers have been characterized by Auger electron spectroscopy (AES), Rutherford backscattering spectroscopy (RBS) and reflection high energy electron diffraction (RHEED). It is shown that neutral beam bombardment does not cause preferential sputtering of oxygenfrom SiO2, whereas ion beam of the same energy causes significant preferential sputtering. For neutral bombardment, densification and crystallization of SiO2 have been observed. The formation of α-cristobalite and α-quartz from amorphous SiO2 has been observed for high dose bombardments (>1017neutrals/cm2). These densification and crystallization phenomena can be attributed to high temperature and high pressure local spot formation upon the incidence of energetic neutral atoms. For ion beam bombardments, these densification and crystallization phenomena have not been observed.

Structural and electrical properties of Co grown on Si(111) by low energy ion beam deposition

Co/Si interface formation was monitored by a high-depth-resolution medium-energy ion scattering (MEIS) spectroscopy. High-purity 59Co thin films were deposited either at room temperature (RT) or at 400°C onto a clean Si(111) substrate using a very-low-energy (50 eV) ion beam deposition (IBD). In situ Auger electron spectroscopy (AES) was used to ascertain the atomic cleanliness of the silicon surface before growth. A post furnace anneal under N 2 was performed in the range 300–700°C after Co deposition for the silicide formation. The structure and the effect of annealing on the electrical properties of these epitaxial film were investigated by MEIS and four-point probe measurements, respectively. Finally barrier heights were measured in this range of annealing temperature using the C V technique.

Monitor for displaying real-time images of low energy ion beams

A monitor that provides real-time images of low energy (<50 eV) ion beams has been designed, constructed, and tested. The cross-sectional image of the beam at the entrance aperture of the monitor is magnified by a factor of 6.5 and displayed on a CRT, following current amplification by using a dual microchannel plate assembly. The monitor provides unambiguous information regarding the cross section of any low energy ion beam. Application in the design and testing of quadrupole-based mass spectrometers is emphasized.

Ultra low energy (100–2000 eV) boron implantation into crystalline and silicon-preamorphized silicon

Very low energy (<1 keV) B + ion implantation has been carried out on both crystalline and silicon-preamorphized silicon. The amorphous layer depth was determined using Rutherford backscattering analysis (RBS). The penetration depth and channelling tail of B + ions into silicon was studied as a function of ion energy (100–2000 eV) and low temperature regrowth/annealing (T < 600° C), using secondary ion mass spectroscopy (SIMS). These studies also involved low energy (40–60 eV) ion beam deposition (IBD) of 10–20 nm isotopic 28Si + cap layer at room temperature to enable accurate SIMS depth profile measurements to be carried out. The results show the extent of the channelling tails at these low energies and indicate that sub 40 nm p +-n junctions with estimated carrier concentrations as high as 10 20 cm −3 can be obtained.

Studies of hydrogen ion beam cleaning of silicon dioxide from silicon using in situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy

The removal of a thin oxide layer from a silicon substrate without significant damage has been achieved at temperatures as low as 500 °C using a low-energy hydrogen ion beam produced by a high-intensity and low-energy ion source in a high-vacuum system. In situ spectroscopic ellipsometry was found to be a sufficiently sensitive and nondestructive method for simultaneously monitoring silicon surface cleaning and ion-induced substrate damage. This letter reports the optimum cleaning parameters for silicon (i.e., minimum ion-induced damage with maximum etch rate of SiO2) to be 300 eV ion beam energy, 60° beam incidence, and 500 °C substrate temperature.

A-Si:H prepared by low-energy ion beam assisted reactive evaporation

a-Si:H has been deposited using low-energy (~55 eV) ion beam assisted reactive evaporation. An ion beam formed in an ion source using silane as the feed-through gas is directed at the substrate during film deposition. For this a-Si:H deposited on substrates held at temperatures above 490 K, the resulting films have only the silicon monohydride bonding configuration observable by IR absorption. Small amounts of the silicon dihydride or polyhydride configurations may be present in samples made on substrates held near 395 K. Electrical conductivity, optical band gap and the total hydrogen concentration all exhibit a reduced dependence on the substrate temperature relative to that observed in samples made without the use of the ion beam. Dark to light (AM1) conductivity ratios typically exceed 103.

Effect of growth temperature on photoluminescence from ion-beam-doped molecular beam epitaxial Si:As

Abstract Low energy (500 eV) ion beam doping with As + ions during silicon molecular beam epitaxial growth has been exploited to obtain thick (about 5 microm) homo-epitaxial 3×10 16 cm −3 Si:As layers on n + (5×10 18 Sb doped) silicon substrates. These samples grown over a range of growth temperatures, 500–800 °C, were studied using low temperature (4.2 K) photoluminescence (PL) spectroscopy and deep-level transient spectroscopy (DLTS). The PL results for two growths, at 500 and 800°C, are presented here. Present in both spectra, although less intense for the 500°C sample, are three sharp lines due to the decay of arsenic-bound excitons: the no-phonon, transverse acoustic-phonon-assisted and transverse optic-phonon-assisted (TO) lines. A much broader and weaker Sb-BE(TO) peak from the nearly degenerate substrate was also observed. Only the higher temperature sample was free of the additional peaks (generally with lower energy than the arsenic-bound exciton peaks, due to residual ion-induced lattice defects) and the emission background rising below 900 meV for 500°C growth. This broad background signal appeared to be related to the presence of electron traps in the 500°C samples at a concentration of about 2×10 14 cm −3 as determined by DLTS. However, the 800°C sample was of much better electronic quality since it exhibited no electron traps in DLTS (sensitivity, approximately 10 13 traps cm −3 ) and had a PL spectrum containing only sharp, relatively intense, bulk-like bound exciton peaks.

Ion-assisted processing of GaAs for optical devices

Gas etching techniques, such as reactive ion etching, ion beam milling and chemically assisted ion beam etching, have been used to produce a range of optical and electronic components in GaAs. Large structures, typically 10–100μm deep, such as spherical microlenses, mesas, columns and fibre-location-channels, may be fabricated in GaAs by a fast reactive ion etching process using Freon-12 (CCl 2F 2). Three-dimensional ir microlenses of spherical geometry have been fabricated in single crystal (111) and (100) GaAs. Microlens structures of this form were transferred from a hemispherical photoresist mask cap into the GaAs substrate by ion beam milling and by CCl 2F 2 assisted ion beam etching. Etching rates for crystalline GaAs by RIE with CCl 2F 2 were fast; at a power density of 6 W cm −2 and gas pressure of 18 Pa (∼150 mtorr) an etch rate of 3.5 μm min −1 was recorded. However, the selectivity between photoresist mask and GaAs using RIE was more difficult to control. By inert argon ion beam milling at a beam energy of 500 eV and current density 0.55 mA cm −2, etch rates for crystalline GaAs of circa 700 Å min −1 were obtained. Improving surface finish is also important if highly optically efficient lenses are to be produced with reduced losses (i.e. scattering, aberrations). Low damage etching of GaAs has been demonstrated with ⩽ 100 eV ion beams, resulting in improved surface finish.