Sputtering of bismuth thin films under MeV Cu heavy ion irradiation: Experimental data and inelastic thermal spike model interpretation.

Abstract

The sputtering of bismuth (Bi/Si) thin films deposited onto silicon substrates and irradiated by swift Cu q+ heavy ions (q = +4 to +7) was investigated by varying both the ion energy over the 10 to 26-MeV range and the ion fluence ϕ from 5.1 × 10 13 cm −2 to 3.4 × 10 15 cm −2 . The sputtering yields were determined experimentally via the Rutherford backscattering spectrometry technique using a 2-MeV He + ion beam. The measured sputtering yields versus Cu 7+ ion fluence for a fixed incident energy of 26 MeV exhibit a significant depression at very low ϕ-values flowed by a steady-state regime above~1.6 × 10 14 cm −2 , similarly to those previously pointed out for Bi thin films irradiated by MeV heavy ions. By fixing the incident ion fluence to a mean value of~2.6 × 10 15 cm −2 in the upper part of the yield saturation regime, the measured sputtering yield data versus ion energy were found to increase with increasing the electronic stopping power in the Bi target material. Their comparison to theoretical predicted models is discussed. A good agreement is observed between the measured sputtering yields and the predicted ones when considering the contribution of 2 competitive processes of nuclear and electronic energy losses via, respectively, the SRIM simulation code and the inelastic thermal spike model using refined parameters of the ion slowing down with reduced thermophysical proprieties of the Bi thin films. KEYWORDS electronic sputtering yield, nuclear and electronic stopping power, Rutherford backscattering spectrometry (RBS), inelastic thermal spike model, sss

Experimental study and thermal spike modeling of sputtering in SiO2 thin films under MeV Auq+ heavy ion irradiation

“…Finally, the experimental sputtering yield for each irradiated target sample was determined, as previously proceeded, 9,12 by considering the induced change in its areal density before and after Au ion irradiation to be proportional to the amount of the removed Si and O atoms: where ρ at . ∆ x | 0 and ρ at .…”

“…For the comparison of our measured data with the theoretical predictions, we first applied the i‐TS model 29,30 in order to determinate numerically the induced sputtering yields versus the target electronic stopping power. The numerical procedure used here has been described in detail in our previous published paper 12 on the sputtering of Bi thin films under MeV Cu q + heavy ion irradiation. Thus, we only give a brief review of the main numerical aspects by including the changes made on the different input‐parameters related to the ion slowing down, the thermophysical properties, and the composite nature of the SiO 2 target material.…”

“…An arbitrary radial function proposed by Fageeha et al 32 was also included in this work to correct the K‐W model by better improving the fit to experimental and Monte Carlo data of the radial dose distribution F ( r ) for small distances around 0.1–10 nm 31,32 . Besides, the contribution to the sputtering yield due to elastic (nonlinear) collision spikes when the projectile or the target is heavy was also taken into account, by including the contribution of the nuclear energy losses into the heat diffusion Equation of the atomic system as an additional energy source term 4,12 of the following form: where B 0 is a normalization factor related to the integration of B n ( r , t ) in space and time that gives the deposited nuclear energy in the SiO 2 composite target material, τ a ~ 10 −13 s is the mean deposition time equivalent to the time it takes the ion or the recoil atoms to come to rest at the end of its range, and R denotes the spike radius corresponding to the mean range of scattered recoils forming the elastic collision cascades 33–35 . Notice that the target nuclear stopping power S n ( E ) in Equation was evaluated using the ZBL nuclear interaction formalism of Ziegler et al 28 combined to Bragg's rule in case of SiO 2 composite target material.…”

“…Then, we solved numerically the two nonlinear Equations and by using an explicit finite difference method, as previously proceeded, 11,12 with the condition of convergence, , where the time and radial steps (Δ t , Δ r ) are assumed appropriately to amount to ~10 −16 s and ~0.2 nm, respectively. Also, the boundary conditions (ambient temperatures) for the electronic and atomic subsystems in regions far from the ion path (i.e., for r → ∞) are assumed 42 to be T e = T a = 300 K. On the other hand, the specific heat and thermal conductivity are taken in the present work from the fit to their corresponding experimental and numerical values in case of SiO 2 thin films 43–50 with thicknesses close to that used in the present experiment.…”

“…However, for thin film materials with reduced sizes, the magnitude of the induced sputtering yields should be different due to the reduction of thermophysical properties of the target material 2,4 . The present work is still again a contribution to the continuous investigation of our research group on ion beam–induced surface sputtering that started more than 10 years ago 7–12 . Here, we are interested to determinate experimentally the sputtering yields induced in (SiO 2 /Si) thin films deposited onto silicon substrates and irradiated by swift Au q + heavy ions ( q = +4 to +9) of different kinetic energies ranging from 10 to 40 MeV.…”