L.B. Bridwell

Ion beam modification of polymers

Abstract The implantation of polymers has received considerable attention in recent years, primarily to examine doping of conducting polymers and to increase the surface conductivity (by many orders of magnitude) of highly insulating polymers. The interest in these studies was partly motivated by possible applications to microelectronic device fabrication. More recently it has been observed that ion implantation can under some conditions lead to the formation of a hard (e.g. as hard as steel, ca. 3 MPa) and conducting surface layer. This paper will review the ion beam modification of polymers resulting from ion implantation with reference to fundamental ion-solid interactions. This leads us to examine whether or not implantation of polymers is a contradiction in terms.

Ion induced structures and electrical conduction in implanted polymer films

Abstract A conducting-grain picture is presented, based on the idea that the ion-polymer interaction process can be simply described as a rapid dissipation of the thermal energy converted from the kinetic energy in the ion track region. Thermal dissipation and efficiency through electronic ionization and nuclear collisions are quite different. A critical transient temperature, determined by the critical energy deposition density, is necessary to activate a thermodynamic relaxation process along the ion track creating conducting graphitic grains. Our high resolution temperature-dependent dc conductivity data reveal a two component conductivity that depends on both one-dimensional variable-range hopping (VRH) and three-dimensional VRH. The relative importance of 3-D VRH conductivity over the entire conductivity seems to depend on the relative rate of the ion energy loss through electronic ionization processes.

Charge state dependence of dEdx for ions in very thin targets

Abstract Relative stopping powers of 3 MeV/u He, Li and C ions have been measured in very thin carbon targets (2–26 μg/cm2). Energy losses were measured using a high resolution position sensitive detector (PSD) in the focal plane of a magnetic spectrometer, so that only one charge state (the incident charge state) was detected. As expected for fully stripped light projectiles in C targets, the stopping power data for He and Li ions scale as Z12 to within our ~1% experimental precision. However, for C ions, charge-changing effects are found to influence the stopping considerably. The evolution of the charge-state distribution as a function of foil thickness for 3 MeV/u C ions in C foils has been determined in prior experiments. A two-component charge state model is used to predict the corresponding evolution of stopping power for both C6+ and C5+ ions, using these charge state data. Comparison of the model with our observations of d E d x show that charge-changing events themselves contribute significantly to stopping in thicker foils, even in the present high velocity regime. From our data we extract the stopping cross sections S00 and S11 for the fully stripped and one-electron ions in the absence of charge-changing events, and the energy loss U = (2.3 ±0.2) keV undergone in the charge-change cycle C6+ C5+. A higher order Z1 effect of ∼ −2.8% is observed in the total stopping cross-section So for the fully stripped ion. This is comparable with the usual Bloch term plus Lindhards polarisation term, which together yield a −2.0% effect. However, the presence of charge-changing energy losses means that the polarisation theory gives only a very sketchy estimate of the appropriate higher order correction for heavy ions.

Effects of dose rate on the electrical conductivity of ion implanted polymers

Abstract Several kinds of polymers were implanted by 50 keV nitrogen (N + and N + 2 ) with different dose rates (0.16–2.03 μA/cm 2 ). The resistivity was measured using a constant voltage method. The dose rate was found to significantly influence the resistivities of implanted polymers. The temperature of the targets at the end of the implant was more dependent upon the total fluence than the dose rate. While some variation of resistances of the conducting layer was observed for different implant dose rates, no blistering or bubble formation occurred under these conditions. The dose rate was found to also affect the current-voltage characteristic and temperature-dependent resistivity of implanted polymers.

Measurement of polarization, bloch and charge exchange contributions to the stopping power of C and O ions in carbon

Abstract In the high velocity regime stopping power is usually described in terms of the Bethe (Z12L0), polarization (Z13L1), and Bloch (Z14L2) contributions. In recent dE/dx transport theory due to Winterbon charge-exchange contributions were set to zero value in the absence of any theoretical or experimental guidance to their magnitude. We have recently carried out precision measurements of the stopping power of C and O ions relative to He for carbon foils ranging in thickness from values sufficiently thin to suppress charge-exchange up to values thick enough to achieve near charge-state equilibrium. These measurements have provided estimates for L0, L1, L2 and charge-exchange energy loss for non-fully stripped heavy ions. The charge-exchange contribution was found to be significant when compared to L1, and L2 and shows a strong variation with projectile Z.

Electrical conductivity enhancement of polyethersulfone (PES) by ion implantation

Abstract Amorphous polyethersulfone (PES) films have been implanted with a variety of ions (He, B, C, N and As) at a bombarding energy of 50 keV in the dose range 1016−1017 ions/cm2. Surface resistance as a function of dose indicates a saturation effect with a significant difference between He and the other ions used. ESR line shapes in the He implanted samples changed from a mixed Gaussian/Lorentzian to a pure Lorentzian and narrowed with increasing dose. Temperature dependent resistivity indicates an electron hopping mechanism for conduction. Infrared results indicate cross-linking or self-cyclization occurred for all implanted ions with further destruction in the case of As.

Excited state populations and charge-exchange of fast ions in solids

Abstract Excited state populations and charge state fractions of 36 MeV C and 445 MeV Cl ions have been measured for a range of thicknesses of gaseous and solid C targets. Cross-sections for electron capture, loss, excitation and excited state quenching have been determined and these data are found to give a quantitative account of the Bohr-Lindhard density effect model. Implications to stopping power theory are considered.

Stopping powers for energetic ions in carbon targets

Abstract Stopping powers of carbon for 3 MeV/u He, Li, C and O ions and 7 MeV/u He and O ions are presented and compared to the predictions of Ziegler. Although the data is consistent with prediction to within the limited accuracy of the latter it exhibits a trend which supports the occurrence of higher order Z 1 corrections and charge changing energy loss.

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.

Ion implantation of polymers for electrical conductivity enhancement

Ions in the mass range from H to Xe have been used at energies from a few keV to more than a GeV to cause atomic rearrangement and consequent formation of conducting grains of carbonized material in several polymers. Generally, the conductivity increases dramatically-- by several orders of magnitude--until finally demonstrating a saturation effect with further ion dose. These results indicate that the ion specie, the rate of energy deposition due to electronic processes, and the original structure of the polymer influence the final conductivity of the implanted material. The temperature dependence of the conductivity, in most cases, and frequency dependence in our case indicate a hopping mechanism for the conductivity in a one dimensional mode although precise details of the conducting mechanism are still in question.