Sputtering of silicon carbide coatings by low-energy hydrogen ions

Various types of silicon carbide coatings made by reactive ion-plating have been bombarded with a 3.0 keV H⁺₃ ion beam at temperatures around 500°C. The sputtering yield in stoichiometric samples (i.e. Si : C = 1 : 1) at 500°C was $\text{1.15 × 10}{}^{\mathrm{?2}}$ atoms/H⁺. As the stoichiometry deviates from this point, the sputtering yield has larger values. The temperature dependence of the sputtering yield in stoichiometric samples was negligible below 600°C. No surface topography changes occurred in stoichiometric samples even at a high fluence of $\text{2 × 10}{}^{20}\text{H}{}^{\mathrm{+}}\text{/cm}{}^2$, while severe erosion took place in non-stoichiometric samples. By Auger electron spectroscopy (AES), carbon exists on the surface in the form of carbide in stoichiometric SiC before and after bombardment, while it exists in the form of graphite in carbon rich samples, which suggests that the bound state of carbon in the form of carbide should correspond to the low sputtering yield in stoichiometric SiC coatings. The surface stoichiometry changes due to hydrogen bombardment were observed by AES, where the carbon population increases in stoichiometric SiC, while it decreases in carbon rich samples, which was supported as well by the results from electron probe X-ray microanalysis.     

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10^?2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH₄ during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.     

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Surface modifications and erosion yields of silicon and titanium doped graphites due to low energy D bombardment

Influences of low energy D⁺ ion bombardment and target temperature on surface topography, surface concentration and erosion yield of carbon based binary compounds were investigated. The samples contained 10 at.% Si and 10 at.% Ti, respectively. The surface concentration was determined in situ by Auger electron spectroscopy and the topography ex situ by scanning electron microscopy. During low energy D⁺ bombardment a pronounced conelike surface developed with silicon respective titanium rich ‘caps’ protecting the underlaying carbon rich shafts from erosion. The average dopant surface concentration was up to 7 × the bulk concentration. The erosion mechanism was determined by surface concentration and chemical state of the surface: At high temperatures carbidic bindings dominated, while at room temperature a mixture of graphite and carbide covered the surface.     

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Sputtering of two-layered films composed of nickel (~5000 Å) and nickel carbide (~1500 Å) at 600° C by 5 keV Ar⁺ bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H⁺ ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.     

Raman scattering and SEM studies of graphite and silicon carbide surfaces bombarded with energetic protons,deuterons and helium ions

Experimental investigations on the chemical and physical effects of 10–15 keV H₁⁺, D₁⁺ and He⁺ ion bombardments to fluences up to 10¹⁹ ions/cm² on graphite and SiC have been conducted using the techniques of Raman scattering and scanning electron microscopy (SEM). Raman scattering data for ion bombarded graphite reveal the formation of an amorphous surface layer as indicated by the appearance of a broad band in the spectrum centered at 1525 cm which replaces the bands due to microcrystalline carbon at 1585 cm^?1 and 1360 cm^?1. The microcrystalline structure could be partially restored upon vacuum annealing at 1040°C for several hours. A weak, broad band centered at 2150 cm also appears after bombardment which is indicative of the formation of ?C = C? bonds. Surfaces of “KT” SiC were also amorphized on ion bombardment as indicated by changes in the Raman spectra. Chemical trapping of the incident h₁⁺ and D₁⁺ ions to form bulk C-H, C-D and Si-H species was observed. Preferential sputtering of Si leaving a carbon rich surface region also occurred. Blister formation was observed in the SEM studies.     

The structure of graphite bombarded with light,gaseous ions

Natural and highly-oriented pyrolytic graphite crystals have been injected with helium, deuterium and carbon ions at energies up to 100 keV and doses up to 10¹⁸ ions/cm². This results in twinning in thick samples, the density of twinning increasing with energy and dose, and varying with target temperature. Twinning is shown to provide an important plastic deformation mode and a model of twinning related to the depth variation of atomic displacements is presented: a method of calculating the strains involved is discussed. It is suggested that surface distortions observed in other graphites originate in the same way. Measurements show that some gas is retained in the samples, but its direct role is uncertain. Very thin (thickness ? 0.1 μm) crystals blister under gas-ion bombardment and are demonstrated to retain sufficient gas to produce the blisters observed. It is shown that the blisters can grow laterally by cleavage in graphite, rather than by increasing their curvature and bursting as in metals. The origin of these blisters is obscure.     

Ion mass effects on bombardment-induced disordering of γ'-Ni3Si

The kinetics of disordering of the ordered compound γ'-Ni₃Si during ion bombardment were investigated at a temperature of 270°C. The disordering followed first order kinetics and the rate constant was measured as a function of incident ion flux, ion energy, and ion mass. The rate constant varied approximately in direct proportion with the ion flux during bombardment with 1 MeV ⁸⁴Kr⁺-ions: from ~ 0.28 min ^?1 at ~ 70 nA/cm² to ~ 50 min ^?1 at ~ 4.4 μA/cm ². During bombardment with a flux of $\text{~ 1 μA}\text{cm}{}^2$ of ¹³²Xe⁺-ions, the rate constant was directly proportional to the calculated near-surface damage ra te. It varied from ~ 50 min ^?1 for 200 KeV ions with calculated damage rates of ~ 2.3 × 10^?1 dpa to ~ 1.8 min ^?1 for 3 MeV ions with damage rates of 2 × 10^?3dpa/s. The disordering rate was also observed to be directly proportional to the near surface damage rate for heavier mass ions (⁴⁰Ar⁺, ⁸⁴Kr⁺ and ¹³²Xe⁺); the ratio of disordering rate to damage rate was ~ 17. This ratio dropped to ~ 3 for ²⁰Ne⁺-ion bombardment, and to ~ 0.4 for bombardment with ⁴He⁺-ions. The decrease in the ratio with decreasing mass is discussed in terms of cascade size, replacement-to-displacement ratio, and random recombination.     

Oxidative erosion of graphite in air between 600 and 1000 K

The oxidative erosion of seven types of graphite has been investigated by heating in air at temperatures between 600 and 1000 K. The specimens include pyrolytic graphite, fine-grain graphites, carbon-fibre composites (CFC), and graphites doped with Si and Ti. The weight loss was measured using a microbalance, the surface morphology by scanning electron microscopy, and the composition of the surface layer by MeV ion beam techniques. Pyrolytic graphite is least affected by erosion, while pure and Si-doped CFCs erode particularly fast. Typical erosion rates for specimens with a surface area of ?4 cm² are below 0.2 μg/m² s at 600 K for all graphite types, and at 900 K range from 0.34 mg/m² s for pyrolytic graphite to about 9 mg/m² s for the strongest eroding types. The temperature dependence of the erosion rate of all types of graphite studied is well described by an activation energy of 1.7 eV. The erosion rates of these graphites are by far lower than the removal rates for deposited amorphous hydrocarbon layers. In contrast to all other types, the Ti-doped graphite absorbs a significant amount of oxygen reaching up to ?5% of its original mass. Once the oxygen uptake is saturated, it erodes with rates similar to those of the strongest eroding types.     

Reliability,detection limit and depth resolution of the elastic recoil measurement of hydrogen

Reliability, detection limit and depth resolution were studied in the elastic recoil measurement of hydrogen mainly in silicon compounds by bombardment with argon ions accelerated up to 50 MeV. For the quantitative determination of hydrogen, recoil silicon atoms proved to serve satisfactorily as an internal monitor. The detection limit was shown to be about 1 to 2×10¹² (atoms/cm² for hydrogen on surface and about 1 wt. ppm for hydrogen in bulk. The depth resolution was found to be about 50 nm in most silicon compounds.     

Measurements of the erosion of stainless steel, carbon, and SiC by hydrogen bombardment in the energy range of 0.5 – 7.5 keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H₂⁺ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10⁻³ to 2.6 × 10⁻² atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 10¹⁸ particles/cm², but disappear at doses of 5 × 10 particles/cm² . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.     

X-ray reflectivity investigation of near-surface density changes induced in Al–Au multilayers by high-current ion beam bombardment

Alternating Al–Au multilayers (typical thickness of each layer 150 nm) were deposited on polished glassy carbon substrates by evaporation under high-vacuum conditions at 278 K and subsequently interdiffused with high-current 2.0 MeV ⁴He⁺ ions. After ion beam bombardment, non-destructive X-ray reflectometry measurements reveal a significant decrease of the density in the near-surface region from 19 to 12 g/cm³. This change in density is caused by the Al–Au interdiffusion during ion beam bombardment, as measured with RBS and X-ray diffraction. Based on the advantage of X-ray reflectometry of no specific sample preparation, detailed integral information of the surface roughness is achieved, additionally. For example, the surface roughness of the interdiffused Al–Au layers increases from 3.1 to 4.1 nm accompanied by the appearance of a gradient layer at the surface that even increases in thickness after irradiation. In addition, the density of this gradient layer decreased from the as-deposited to the irradiated state.     

Erosion yield of Ti from TiC-deposited graphite by hydrogen ion bombardment at high temperatures

Erosion yields of Ti atoms from a TiC-deposited graphite by bombardment with 1 keV hydrogen ion beam of various current densities at 900°C have been investigated by means of the Rutherford backscattering (RBS) technique. It has been observed that the sputtering yields for Ti atoms at 900°C are almost zero below a critical ion flux of 1 × 10¹⁵/cm²·s, compared with the sputtering yield of Ti atoms at room temperature which has been measured to be 1 × 10⁻²atoms/ion. No sputtering of Ti atoms observed at 900°C is explained in terms of self-sustaining coating of the TiC surface with segregated carbon layer. The condition for the self-sustaining coating is discussed.     

Two types of MeV ion beam enhanced adhesion for Au films in SiO2

The ion beam-enhanced adhesion of thin Au films on vitreous silica sus studied for a wide range of Cl ion beam doses for beam energies between 6.5 MeV and 21.0 MeV. Since the residual adhesion of Au on SiO₂ is low, the improved adhesion can be easily seen using the Scotch Tape Test. The threshold in the enhanced adhesion corresponding to passing the tape test occurs at two different dose ranges for a given energy; one at very low dose centered around 1 × 10¹³ /cm², the other at higher doses with a threshold of around 1.5 × 10¹⁴ /cm² (depending upon the beam energy). At low doses (2 × 10¹² to 5 × 10¹³ /cm²) surface cracks occur on the SiO₂ substrates, these cracks close up at doses higher than 5 × 10¹³ /cm². A possible explanation of enhanced adhesion in the low dose range is associated with the surface crazing of the SiO₂ substrate. To make the adhesion test more quantitative, a scratch test was also used on the samples.     

Sputtering process of a silicon carbide surface with energetic ions by means of an AES-SIMS-FDS combined system

Surface phenomena on silicon carbide following interaction with energetic hydrogen ions and argon ions have been studied by means of simultaneous, in situ measurements with a combined system of AES-SIMS-FDS (Flash Desorption Spectroscopy). Bombardment by 0.7 and 1.5 keV argon ions was observed to sputter the surface atoms, both silicon and carbon, with the same sputtering yields. In the case of bombardment by hydrogen ions, on the other hand, silicon atoms were sputtered out preferentially through chemical sputtering to form silicon hydrides at room temperature. In-depth composition profiles of silicon carbide irradiated by 100-keV D⁺ ions were also examined by the combined system.     

Charge state distributions and resulting isotopic fractionation effects of carbon and chlorine in the 1–7 MeV energy range

Equilibrium charge state distributions have been measured for 2–7 MeV ³⁵Cl ions stripped in ca. 10 μg/cm² carbon foils and for 2–7 MeV ¹²C ions stripped in ca. 880 ng/cm² oxygen gas. In addition, for 4.95 MeV gas stripped ¹²C ions, the non-equilibrium distribution has been recorded for stripper thicknesses of 50–2500 ng/cm². The results are compared with previous measurements. Isotopic fractionation versus energy is shown for some charge states of carbon, chlorine and beryllium.     

The effect of fission fragment irradiation upon the passive oxidation of silicon-bonded silicon carbide by oxygen at 950°C

The effect of fission fragment irradiation on the passive oxidation of a silicon-bonded silicon carbide by oxygen has been examined at 950°C. The fission fragment flux ranged between 6.3 – 12.2 × 10¹⁰ ff/cm² · s, and the exposure period varied up to 4768 h. The extent of oxidation was increased slightly by the fission recoil bombardment. The enhancement was more significant during the initial exposure (? 500 h), when it was increased by a factor of between 2.5 – 3.5. Subsequently, the factor became progressively decreased to 1.6 – 1.9 after 4768 h. Comparison between the extents of oxidation of the silicon carbide and free silicon constituents suggest that it was the oxidation of free silicon which was increased, particularly over the initial exposure period. Thereafter, silica film growth on both components was enhanced to comparable extents. The most probable mechanism for the continuing radiation induced attack was that the silica film properties controlling oxidation were influenced by chemical contamination, with fission products and uranium oxide, ejected from the sources with the emerging fission fragments.     

Effect of radiation damage on the erosion of graphite by atomic hydrogen

The reactivity of graphite with atomic hydrogen is strongly enhanced after irradiation with 2 MeV ⁴He⁺ at doses of about 10¹⁷?10¹⁸ cm^?2. The observed effect is explained in terms of crystal lattice stress induced by the radiation damage in the irradiated area.     

Behavior of graphite under heat load and in contact with a hydrogen plasma

Graphite is extensively used in large tokamaks today. In these machines the material is exposed to vacuum, to intense heat loads, and to the edge plasma. The use of graphite in such machines, therefore, depends on the outgassing behavior, the heat shock resistance, and thermochemical properties in a hydrogen plasma. Investigations of these properties made at different laboratories are described here.Experiments conducted at Sandia National Laboratories (SNL), Livermore, and the Max-Planck-Institut für Plasmaphysik (IPP) in Garching showed that the outgassing behavior of fine-grain reactor-grade graphite and carbon fiber composites depends on the pretreatment (manufacturing and/or storage). However, after proper outgassing the samples tested behave similarly in the case of fine-grain graphite, but the outgassing remains high for the carbon fiber composites.Heat shock tests have been made with the Electron Beam Test System (EBTS) at SNL, Albuquerque. Directly cooled graphite samples (FE 159 graphite brazed onto Mo tubes) showed no failure at a heat load of 700 W/cm², 20 s; or 10 kW, 1 s. Thermal erosion due to sublimination and particle emission from the graphite surface was observed. This effect is related to the surface temperature and becomes significant at temperatures above 2500°K. Fourteen different types of graphite were tested; the main differences among these samples were the different surface temperatures obtained under the same heating conditions. Cracking due to heat shocks was observed in some of the samples, but none of the carbon fiber composites failed.Thermochemical properties have been tested in the PISCES plasma generator at UCLA for ion energies of around 100 eV. The formation of C-H compounds was observed spectroscopically at sample temperatures of around 600°C. However, this chemical reaction did not lead to erosion as observed in beam experiments but to a drastic change of the surface structure due to redeposition. Carbon-hydrogen lines were still observed at sample temperatures of around 100°C. Under these conditions the erosion yield is high and in agreement with those measured in beam experiments.     

Passage of fission fragments through thin film plastic scintillators

The slowing down of fission fragments from the spontaneous fission of ²⁵²Cf has been studied in the self-supporting thin films of plastic scintillator NE 102A, using a surface barrier detector. The measured residual energies and hence the energy losses of the mean light and heavy fragments after passing through the thin films of thickness ranging from ～-100 to 1200 μg/cm² of NE 102A are reported. These measurements were carried down to 20.0 MeV and ～- 14.0 MeV for light and heavy fragments respectively. The shape spectrum parameters from the slowed spectra are determined. The measured energy straggling parameters obtained from these spectra show a maximum around 300 μg/cm² of NE 102A and the energy bunching effect due to fission fragments is observed in thicker plastic scintillator films (>300 μg/cm²). The stopping power obtained from these measurements is compared with the theoretical predictions.