Sputtering of Glass Dust Grain by Ions and Electrons

Transcription

1 WDS'14 Proceedings of Contributed Papers Physics, , ISBN MATFYZPRESS Sputtering of Glass Dust Grain by Ions and Electrons M. Vyšinka, J. Vaverka, J. Pavlů, Z. Němeček, and J. Šafránková Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Glass (SiO 2 ) is sputtered not only by ion impact, but the sputtering yield can be enhanced by electron impact as well. In the case of small silicate dust grains in the space environment, such effects can influence the lifetime of the grains. Moreover, there is a suggestion that electric field could further enhance the sputtering yield from charged non-conducting grains. To reveal these effects, we performed experiments with a single dust grain levitated in an electrodynamic quadrupole trap. As a representative of silicate-type space dust, we used spherical SiO 2 grains with diameters around 1 micron. The grain in the trap was bombarded by 2-keV Ar + ions and 1-keV electrons. The sputtering yield of the ion-only bombardment (i.e., at a high surface field) was 1.4 SiO 2 /ion, in the case of a combined ion and electron bombardment, the yield was 1.5 SiO 2 /ion, i.e., slightly higher. Introduction Sputtering by a beam of energetic particles is an important process of destructing dust grains in space. A typical source of such particles is the solar wind that consists of ions with energies in the kev range and electrons in the ev range. Other source can be shock waves, supernova explosions, and others. A dust grain is a small spatially limited object, often with an irregular shape. It means that there is no preferred impact angle for a particular grain. Sputtering is strongly angular-dependent with the maximum sputtering yield at around and can be up to 7 times higher compared to the normal incidence. As a results, the grain is sputtered by different rates at different areas of the surface. Papers dealing with the sputtering and the lifetime of the grain [e.g., Flower et al., 1996] usually assume a spherical grain and calculate the sputtering yield over the sphere using values for a given incident angle and an angular dependence of the yield. Such approach neglects that the real grain need not necessarily preserve a spherical shape during the sputtering leading to change in sputtering yield due to angular dependency and some experimental confirmations of the sputtering rate will be appreciated. Dust grain sputtering experiments are usually based on sputtering of dust grain beds using a mass spectroscopy for detection of sputtered particles [Meyer et al., 2011]. Such experimental arrangement is intricate due to a possible recaption of sputtered atoms by other grains and a precise estimation of the sputtering yield is quite difficult. The first study of spherical object sputtering was performed by Wehner, [1959]. Pavlů et al. [2007] studied sputtering of a single gold grain with a diameter of 1 micron. The authors sputtered the grain at a high surface potential and found that the sputtering yield was enhanced by a factor of 4 in comparison to the normal incidence on a flat sample. They analyzed that a factor of 2 is given by the geometry and the rest was attributed to a field enhancement caused by the high surface electric field during the sputtering. They speculated that for an insulating grain, the field factor should be much larger. To test check their deductions, we performed a similar experiment as Pavlů et al. [2007] but with a glass grain as a more space-related material instead of the originally investigated gold. Our experiment was based on sputtering of a single glass grain by Ar + ions at a high surface potential ( 1 kev, the potential was limited by field ion emission) and at a low surface potential ( 10 ev, the potential was kept by an 1-keV electron beam). The mass of the grain was measured every 3 hours of bombardment. The experiment was similar to that published 371

2 Figure 1. Mass measurement. Secular frequency (and Q/m) jumps occur due to the capture or the release of a single or a few electrons (left). The corresponding charge of the grain in numbers of electrons is shown in the right side. The data was measured after 31 hours of bombardment and the obtained grain mass was m = (8.45 ± 0.15) kg. earlier [Vyšinka et al., 2013] but we developed a new technique for determination of the amount of impinging ions that improved the sputtering yield determination. Experimental techniques The experimental set-up is similar to that used by Pavlů et al. [2007]. It is based on an electrodynamic quadrupole (Paul trap) placed in a UHV vacuum chamber. A single dust grain levitates in the trap and its charge-to-mass ratio (Q/m) can be calculated from the secular frequency of grain oscillations. To obtain this frequency, light of a red laser diode is scattered off the oscillating grain. The scattered light is focused by a lens system to a position-sensitive detector that provides coordinate signals for determining the secular frequency. The grain can be charged or discharged by electron (KIMABLL PHYSICS EMG-14) and ion (COLUTRON G-2-D) guns with energies in the range of kev. More details about the experimental set-up can be found in the previous papers [Čermák et al., 1995, Žilavý et al., 1998, Pavlů et al., 2004, Němeček et al., 2011]. Mass measurement The mass of the grain can be determined from changes of the charge-to-mass ratio caused by a change of the grain charge by a single or a few electrons (the method was firstly used for dust grains by Žilavý et al., 1999). The mass can be calculated according to the following Equation: m = e, (1) where e is electron charge, Q m is the change of the charge-to-mass ratio of the grain corresponding to the elementary charge change. The grain was discharged to about hundred electrons to obtain a higher resolution, thus better precision. Such an example of measurement is illustrated in Figure 1. Estimation of number of impinging ions When the grain undergoes a short shot from the ion gun we can measure a change of the Q/m ratio ( Q/m). If the grain mass is known (from mass measurement) and under the assumption that the mass change can be neglected, the grain charge change can be estimated ( Q). If we assume that all impinging ions are captured on the grain surface we can calculate the number of impinging ions during the shot. The total ion current from the ion gun is measured by a Faraday cup, the signal is amplified by an electronic system and read in volts. If we take a Q m 372

3 Figure 2. Measurements of impinging ions. Left changes of the charge-to-mass ratio (top, black dots) caused by the ion shots (blue line). One particular ion shot is magnified (bottom) to show the ion signal area used to determination of the number of impinging ions. Right the charge-to-mass ratio changes as a function of the time integral of the ion signal showing a relationship between the ion signal and the number of impinging ions. time integral of the signal (the area under the chart in Figure 2, left) we can plot the dependence of Q/m on this ion signal area (Figure 2, right). The dependence is linear and we can obtain the coefficient from a linear regression. Such procedure allows us to estimate the total number of impinging ions from the ion signal measured by the Faraday cup. This type of measurements was done prior to each sputtering session, thus we could account for the changes of both the ion-gun beam intensity and the grain cross-section. The total number of impinging ions (N) during each sputtering session is given by the Equation 2, N = B I a m (2) e where B is the slope of the dependence of Q m vs. the integrated ion signal that converts the Faraday cup signal to incoming charge (see Figure 2), I a is the time integral of the ion signal for the sputtering session (the total ion signal area), m is the grain mass and e is the elementary charge. When the grain is charged to high surface potentials, the number of impinging ions (N i ) is reduced according to Equation 3: ( N i = N 0 1 e φ ) max (3) E 0 where N 0 is the number of impinging ions according to Equation 2, e is the elementary charge, φ max is the equilibrium surface potential of the grain, and E 0 is the energy of impinging ions. 373

4 Sputtering at low surface potentials To study the sputtering, we used the SiO 2 grain with an initial mass m = kg and with diameter D = 1.01 µm. The grain mass during the sputtering by 2-keV Ar + ions and 1-keV electrons is depicted in Figure 3 as a temporal evolution (a) and as a dependence on the number of impinged ions (b). The surface potential was 10 V during the sputtering. The temporal evolution of the grain mass is not linear because: (1) a reduction of the grain cross-section (the grain becomes smaller due to the sputtering thus less ions reach the surface), and (2) a reduction of the ion flux from the ion gun (given probably by degradation of the ion gun cathode). Measuring the number of impinging ions as described above, we get a linear dependence of the grain mass on the ion dose as it is demonstrated in Figure 3b. During 101 hours of sputtering, kg of the grain material (70 % of the initial mass) was sputtered. The total ion dose was ions and it corresponds to the dose of ions/cm 2. The sputtering yield of a SiO 2 dust grain as a function of the ion dose (i.e., the number of the ions that hit the surface) is depicted in Figure 4 left. No dependence of the sputtering yield on the ion dose (i.e., on the grain diameter) was observed. A mean value of the sputtering yield was: Y SiO2 = (90.2 ± 0.3) amu ion = (1.501 ± 0.004)SiO 2 ion, where amu = kg is the atomic mass unit. The value was obtained as a slope of a linear regression of the mass vs. ion dose dependence (Figure 3b). Sputtering at high surface potentials In this case, the initial grain mass was m = kg (diameter D = 1.04 µm). The average charge-to-mass ratio during the sputtering was approx 61 C/kg and it corresponds to a surface potential of 1.3 kv. The energy of the ion beam was 3 kev, but due to the grain potential, the ions impinge with a resulting energy of 1.7 kev. The number of impinging ions was reduced according to Equation 3. Due to some experimental complications (leading to a higher probability of the grain loss from the trap during high charge-to-mass ratio changes), there are less data points available than in the experiment at low surface potentials. During 15.5 hours of sputtering, kg of the grain material (5 % of the initial mass) was sputtered. The total ion dose was ions that corresponds to a dose of ions/cm 2. The temporal evolution of the grain mass and its dependence on the number of impinging ions is depicted in Figures 3c and 3d). The sputtering yield of a SiO 2 dust grain as a function of the ion dose (i.e., the number of ions that hit the surface) is depicted in Figure 4 right. The grain was sputtered by Ar + ions with an effective energy of 1.7 kev and the grain surface potential was 1.3 kv. The mean value of the sputtering yield was: Y SiO2 = (82 ± 38) amu ion = (1.36 ± 0.64)SiO 2 ion, The value was obtained again as a slope of a linear regression of the mass vs. ion dose dependence (Figure 3d). Discussion Glass (SiO 2 ) is a rather complicated material from the sputtering point of view. It exhibits preferential sputtering of oxygen by both electron [Fujita et al., 2006] and ion impacts (with a comparison to neutral beam bombardment) [Mizutani, 1991]. Such preferential sputtering can lead to changes of the surface composition and to an increase of the surface conductivity as it was observed by Gonzales de Vicente et al. [2007] after an He + -ion bombardment of 374

5 (a) (b) (c) (d) Figure 3. Temporal evolution of the grain mass during sputtering by 2-keV Ar + and 1-keV electrons (a) and the dependence of the grain mass on the ion dose (b). The sputtering by 3-keV Ar + beam (surface potential 1.3 kv, hence impinging energy 1.7 kev) as a temporal evolution of the grain mass (c) and its dependence on the ion dose (d). The lines in the panels (a) and (c) are intended to guide the eyes, the lines in the panels (b) and (d) are linear fits. Error bars are given by the errors of the mass measurement data. Figure 4. The sputtering yield as a function of the ion dose measured on a single SiO 2 grain. Left sputtering at low surface potentials by 2-keV Ar + and 1-keV electron beams. Right sputtering at high surface potentials (1.3 kv) by 3-keV Ar + (impinging energy was 1.7 kev). a glass surface. The increase of the surface conductivity was observed after reaching some threshold dose ( ions/cm 2 ). Such circumstances may explain a broad range of reported sputtering yields for SiO 2 ( atoms/ion for 2 kev Ar + ions [Seah et al., 2010]). In the case of a spherical grain, the sputtering yield should be enhanced by a factor of due to its angular dependence [Pavlů et al., 2007]. Our results are consistent with these findings. 375

6 The charge of the grain influences the number of impinging ions and their effective energy. When the charge is compensated by beam electrons, these effects can be neglected as the grain surface potential is around 10 V. When the charge is not compensated by the electron beam, the surface potential is stabilized by field ion emission (equilibrium between the primary current, the field ion emission current, and the background current of electrons generated on the electrodes) and some correction is needed. The problem of the grain sputtering at high surface potentials is a necessity of discharging the grain in a broad charge range for the mass measurements. This step increases a risk of losing the grain from the trap and it makes the experiment time consuming. Long-time experiment and further data on the sputtering at high surface fields are needed for a stronger conclusion. It is not clear why the sputtering yield at the 2 nd data point in Figure 4 right is so low. Generally, the ion dose at the sputtering at high surface fields was low and the data point scatter can be just a result of this. Pavlů et al. [2007] analyzed a similar experiment on a gold grain at high surface potentials (without the ion beam compensation by the electron bombardment) and found a sputtering yield about 2-times larger than expected. They ascribed it to the influence of the surface field and suggested that the enhancement should be much more pronounced for an insulating grain but our results do not confirm this hypothesis. The possible reasons can be that the electron beam used for the ion compensation has a larger effect on the sputtering yield than the electric field or the preferential sputtering of oxygen leads to a more conductive surface and the grain loses its insulating properties. Our experimental set-up does not allow us to measure mass spectra of sputtered particles to confirm the preferential sputtering. Conclusion The sputtering yield of an 1-µm SiO 2 grain bombarded simultaneously by 2-keV Ar + ions and 1-keV electrons that keep the grain surface potential low is Y SiO2 = (90.2 ± 0.3) amu ion = (1.501±0.004) SiO 2 ion. It is consistent with the data published in the literature for SiO 2 sputtering [Seah et al., 2010]. The grain mass evolution shows a linear dose dependence during a prolonged time of the bombardment. Totally, kg (70 %) of the glass grain was sputtered during 101 hours of bombardment. No dependence of the sputtering yield on the ion dose was observed during the ions bombarded the grain surface ( ions/cm 2 ). At a high surface potential (1.3 kv), the sputtering yield of the SiO 2 grain bombarded by 1.7-keV Ar + ions is Y SiO2 = (82 ± 38) amu ion = (1.36 ± 0.64) SiO 2 ion. Totally, only kg (5 %) of the glass grain was sputtered during 15.5 hours of bombardment. The total ion dose was ions and it corresponds to the dose of ions/cm 2, an order of magnitude lower than at the low-surface-potential experiment. Reaching a higher ion dose for the highsurface-potential experiment will be needed for a better comparison of the sputtering yield. A hypothesis about the yield enhancement due to a high surface field of non-conducting materials according to Pavlů et al. [2007] could not be confirmed. At least, the field enhancement (if any) for the glass grain is smaller than the influence of the electrons. Acknowledgments. The authors would like to express their appreciation to the Charles University Grant Agency for a support of this work under contract No Prof. DelVítek and his group is greatly acknowledged for never-ending encouragement and endless discussions during the experiment. References Čermák, I., E. Gruen, and J. Švestka: New results in studies of electric charging of dust particles, Adv. Space Res. 15, (1995). Flower, D.R., G. Pineau des Forêts, D. Field, and P.W. May: The structure of MHD shocks in molecular outflows: grain sputtering and SiO formation, Mon. Not. R. Astron. Soc. 280, (1996). 376

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