C.J. Sofield

Ion beam modification of polymers

Abstract Recent studies of the electrical conductivity of heavy ion irradiated poly-ether-ether-ketone (PEEK) have shown that the conductivity of the highly damaged polymer is greater for ions of the greater electronic stopping power. The conductivity of the modified material appears to fall into two different regimes depending on the ion energy deposition (dose dE/dx product) into the polymer. As a result of this observation an examination of the “carbonaceous” irradiation modified polymer samples was made with a Raman microprobe. The highest energy heavy ions used had sufficient range so that the damaged layer could be sectioned at a shallow angle and Raman spectra obtained at varying depths along the ion implant range. These observations show two kinds of “carbonaceous” material, diamondlike and graphitic carbon, are formed depending on the dE/dx of the damaging ion. This observation is discussed with reference to a track formation model, and an energy threshold for graphitization is derived.

Ion implantation of polymers for electrical conductivity enhancement

Abstract The polymers PET, PAN, PES and PEEK were implanted with ions of He, B, C, N, Ar and As at an energy of 50 keV. PEEK was implanted with I at 22.5 MeV, Xe at 24 MeV, and Ni at 47 MeV. Surface resistivity for electrical conduction indicated a plateau effect in the dose range 1016–1017 ions/cm2. The more aliphatic polymer, PET, indicated the lowest resistance of the low energy implants. The high energy ions produced much lower resistivities at much lower doses. The temperature dependence of the resistivities indicate a quasi-one-dimensional variable range hopping mechanism for electrical conduction.

Sputtering by MeV ions

Abstract The ability of fast heavy ions to produce efficient erosion of dielectric surfaces has recieved only limited attention from the experimental and theoretical communities. The effect was first observed in association with studies of track formation processes in dielectrics, and the sputtering yields of a limited number of combinations of ion and substrate combinations quantified. The process is poorly understood, with significant variation of sputter yields in similar materials with the same ion. The observation that the process appears to “switch off” for dielectric layers below a critical thickness (approximately 2 nm) is also interesting and unexplained. We present results of MeV sputtering using beams from the Harwell 7 MV Tandem accelerator, using a novel technique to simultaneously sputter and assess film thickness. For thermally grown SiO2 films on silicon substrates we have observed the variation of sputter yield with ion species, charge state and incident angle. A dramatic decrease in sputter yield is observed for films less than 2 nm thick. The sputter yield is found to be roughly consistent with a (sin τ)−1 variaton in incident angle, and increases dramatically with incident charge state, indicating the process to be dominated by interactions within the near surface region. The use of isotopically labelled thin oxide layers has been used to gain an insight into the depth of origin of the sputtered species, and evidence that the process is not purely a surface phenomenon is presented.

In-situ low temperature cleaning of silicon surfaces using hydrogen atoms

Abstract In this work we discuss in-situ cleaning of Si (100) and Si (111) using a hydrogen atom source, with particular reference to carbon and oxygen removal and the quality of the substrate surface after treatment. It is shown that hydrogen atoms are effective at surface carbon removal at temperatures around 350 °C and may contribute to lowering the thermal desorption temperature of thin oxides. Exposure of the substrate to hydrogen atoms at high temperature (~ 750 °C), though successful at removing oxide, caused significant damage to the silicon surface.

MeV ion enhanced adhesion in the gold on tantalum system

Abstract The improvement in adhesion of a thin (50 nm) gold film on samples of tantalum sheet with high energy ion irradiation has been studied. Adhesion enhancement was assessed using the scotch tape and scratch testing methods. The dependence of the effect as a function of ion stopping power has been measured using beams of 1H, 4He, 12C, 16O, 19F, 35Cl and 81Br with energies between 1 and 40 MeV. The threshold dose (Dth) to pass the scotch tape test has previously been measured by two groups for this system and found to have a power law dependence on stopping power ( d E d x ), but a large discrepancy in the exponent has been reported (−1.6 and −3.0). The value obtained in the present work is −1.42 ± 0.11 (scotch tape) and −1.48 ± 0.12 (scratch test). The role of sample preparation is discussed.

Time-of-flight medium energy ion scattering study of epitaxial SiSi1 − xGex superlattice structures

Abstract The ability to determine structural and compositional information from the sub-surface region of a semiconductor material has been demonstrated using a new time-of-flight medium energy ion scattering (ToF-MEIS) system. A series of silicon-silicon/germanium ( Si Si 1 − x Ge x ) heterostructure and multilayer samples, grown using both solid source molecular beam epitaxy (MBE) and gas source chemical vapour deposition (CVD) on Si(100) substrates, have been investigated. These data indicate that each individual layer of Si1 − xGex (x ∼ 0.22) in both 2 and 3 period samples, can be uniquely identified with a resolution of approximately 3 nm. A comparison of MBE and CVD grown samples has also been made using layers with similar structures and composition. ToF-MEIS showed that the MBE grown samples exhibit a much higher Ge concentration per layer than CVD grown samples. Surface segregation of Ge was observed for the MBE grown samples, but appears to be suppressed in the case of CVD grown samples. These data indicate the ability of ToF-MEIS to probe the differences in the kinetics associated with Ge incorporation and segregation between the two techniques. The total Ge content of each sample was confirmed independently using conventional Rutherford backscattering spectrometry (RBS).

Radiation enhanced adhesion by MeV ions

Abstract The ability of fast heavy ions to induce adhesion between a thin film and substrate has been observed by several groups. As yet, no thorough and rigorous explanation of the phenomenon has emerged, although the observation of similar adhesion enhancements with electron beam and UV photon irradiation suggests a common root cause in direct electronic excitation. At this conference, we have presented the results of a rigorous study of the adhesiion of gold films to tantalum substrates, with precise control of interface composition in terms of native oxide, adsorbate and hydrocarbon layers, using a unique UHV (ultrahigh vacuum) sample preparation facility, known by the acronym SURF, and vacuum transfer to a UHV irradiation facility. The as deposited, and radiation enhanced adhesion was assessed by the scotch tape and scratch testing methods. The results obtained are compared with the published results of other groups where a large discrepancy in the observed dose threshold for adhesion enhancement has been reported. The results of the present study provide strong evidence for the mechanism to the direct radiolysis involving interfacial contaminant species, since irradiation produced no beneficial effects for samples with atomically clean interfaces.

Ion implantation of new polymers containing imide and amide units

Abstract Six new polymers containing imide and amide units, have been implanted by 50 keV Xe and 180 keV As ions. The measurements of surface resistance indicated that the conductivities of all polymers increased more than five orders of magnitude from their pristine values after implantation. The temperature-dependent conductivities showed that these implanted new polymers conduct by the charging energy limited tunnelling mechanism. Raman spectrometry showed that diamond-like structure was present in some of the polymer films implanted by 180 keV As ions. Infrared spectroscopy indicated that the molecular structure of pristine films was damaged by incident ions, and some cross-link structure was also created at the high dose of implantation.

Adhesive failure of gold films on tantalum pentoxide produced by MeV ion irradiation

Abstract Here we report work on the MeV irradiation adhesion modification of gold films on deposited tantalum pentoxide. For this system irradiation produces bubbling of the film at comparatively low doses, which in some cases leads directly to failure of the gold film. This is in contrast to the results of irradiation of gold on native or thermally grown tantalum oxide samples where enhanced adhesion is observed. Irradiation by 5 × 10 13 ions/cm 2 of 20 MeV 35 Cl and 1 × 10 13 ions/cm 2 of 40 MeV 81 Br were observed to release 100 and 5000 H atoms per ion, respectively. The nature of the bonding of the hydrogen and the details of the ion-induced release are unknown, although electronic stopping is thought to be the driving force rather than bulk heating effects.