A study of the 150–320 keV proton beam transmission through

tapered glass (borosilicate) capillaries with different diameters of the input and output of the capillary was performed. The focusing effect was observed. The areal

density of the transmitted beam is enhanced by approximately 20 times. It was shown that changing a taper angle from 0.5 deg to 1.7 deg evidences increase of the

transmission coefficient by more than 300 times keeping the initial energy spectrum of ions. The ion transmission through self-ordered nanoporous alumina membranes prepared by anodic oxidation of high-purity aluminium was studied for different energies of ions

Grishin, P. A.

Kamyshan, A. S.

Komarov, F. F.

English

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DOI 10.1393/ncc/i2011-10920-0Colloquia: Channeling 2010IL NUOVO CIMENTO Vol. 34 C, N. 4 Luglio-Agosto 2011Peculiarities of proton transmission through tapered glasscapillariesF. F. Komarov(∗), A. S. Kamyshan and P. A. GrishinInstitute of Applied Physics Problems BSU - Kurchatov Str. 7, 220118 Minsk, Belarus(ricevuto il 22 Dicembre 2010; pubblicato online il 25 Luglio 2011)Summary. — A study of the 150–320 keV proton beam transmission throughtapered glass (borosilicate) capillaries with different diameters of the input andoutput of the capillary was performed. The focusing effect was observed. The arealdensity of the transmitted beam is enhanced by approximately 20 times. It wasshown that changing a taper angle from 0.5 deg to 1.7 deg evidences increase of thetransmission coefficient by more than 300 times keeping the initial energy spectrumof ions. The ion transmission through self-ordered nanoporous alumina membranesprepared by anodic oxidation of high-purity aluminium was studied for differentenergies of ions.PACS 34.35.+a – Interactions of atoms and molecules with surfaces.1. – IntroductionSlightly tapered glass capillaries with micro or submicro outlet diameters attract muchattention as ion beam focusing lens [1-4]. The smallest beam spot size obtained so farwas 0.3μm [1]. Previous studies have demonstrated the focusing effect with respect tomany kinds of ions and energies [1-4]. The focusing factor of a glass capillary, defined asthe ratio of areal current density at the inlet and outlet, depends on the ion species, theincident energies and the shape of glass capillary. This factor of 8 keV Ar8+ is about 10and for 2MeV He+ beam amounts to about 1000 [1-4].The purpose of the present paper is to study the dependence of proton transmissionthrough capillaries on capillary taper angles. An ion energy region was chosen to satisfythe experimental conditions of local implantation, local elemental and structure anal-ysis, micro- and nanolithography, X-ray radiography, and applications in biology andmedicine [3,5]. For example, this technique enables in-air PIXE measurements of varioussamples that are not compatible with the vacuum environment [2] (wet solids, liquidsand gases). Slightly tapered glass capillary optics is applied to work as a differential(∗) E-mail: KomarovF@bsu.byc© Societa` Italiana di Fisica 365366 F. F. KOMAROV, A. S. KAMYSHAN and P. A. GRISHINFig. 1. – Shape and angular distribution of a 200 keV proton beam transmitted through thecapillary.pumping orifice as well as a focusing lens [2]. Recently, the processes of interaction ofaccelerated protons with the surface of cylindrical insulating capillaries as well as timeand angular distributions of ions transmitted through such capillaries have been studiedin our paper [6].2. – Experimental techniqueA schematic of the experimental setup designed for studying the transmission ofaccelerated ions through capillaries has been reported in our paper [6] (fig. 1 in thispaper.) This setup, which enters the implantation complex formed on the basis of 1MeVelectrostatic ion accelerator, consists of four units: 1) a system of ion beam formation, 2)a scattering chamber, 3) a measuring chamber, and 4) a system for detecting scatteredions. The parameters of a setup are as follows: the error in determining angles inmeasurements of angular distributions is not larger than 3.3 · 10−3 deg and the error inthe capillary orientation with respect to the beam axis is not larger than 2.5 · 10−2 deg.The used proton beam energy of 150–320 keV was determined by a magnetic analyzerwith an error of 0.1%.We measured the angular and the time distributions of protons transmitted throughglass capillaries with the inlet diameters of 3.5 and 0.5mm and the outlet diameter of0.1mm and the length of 150mm in the range of taper angles of 0.5 to 2.2 deg. Inpution currents from 10−13 to 10−9 A were used in these experiments. Angular distributionsand energy spectra of transmitted protons were measured directly using mobile siliconsurface barrier detector positioned at a distance of 90 cm from a capillary holder. Thetotal measured energy resolution of the recording system does not exceed 16 keV.3. – Experimental results and discussionThe angular distribution of protons transmitted through the tapered capillary withan inlet diameter larger than an ion beam diameter and an outlet diameter of 100μmis shown in fig. 1. It should be mentioned that a practically uniform distribution of ionPECULIARITIES OF PROTON TRANSMISSION THROUGH TAPERED GLASS CAPILLARIES 367Fig. 2. – Count rate of particles transmitted through the capillary vs. proton current at theinput of the capillary.beam density with sharp edges is registered in this case. Moreover, a spot size amounts to3.8mm that corresponds to a beam divergence of 0.13 deg. An initial beam divergence wasonly 0.015 deg. An integral of an area under the curve in fig. 1 shows that the fraction oftransmitted protons is equal to 80%, i.e. the number of transmitted ions relative to thoseentering into the capillary. Therefore, taking into account that the outlet diameter of thecapillary is 100μm and the initial beam diameter amounts to 500μm, the focusing factorreaches up to 20. An effort to measure angular distributions of transmitted protons athigh currents at the input of the capillary was failed as the measuring system functioningin a regime of individual particle registration was overloaded. Scanning of transmitted ionbeams, at input beam currents of the order of 10−9 A, showed that angular distributionsof beams transmitted through the capillary are independent of the incident current. Incase of this capillary oriented along the beam axis, the count rate versus the beam currentat the input of the capillary is shown in fig. 2. The results presented in fig. 2 demonstratea strong nonlinear behavior of the current at the output of the capillary versus intensityof the input beam up to an input current of 5 · 10−13 A. It is well accepted that suchprotons are guided electrostatically due to the charging up of the inner wall of capillariesmade of insulating material [7].Figure 3 shows the time dependences for swift protons transmitted through cylindricaland tapered capillaries with the diameter of the inlet hole less than the diameter of theion beam. In spite of the practically equal currents at the input of these capillaries,time evolutions of beam currents at the output of the capillaries are strongly differentin shapes and frequencies of the beam intensity oscillation. Current pulse frequencies ofions transmitted through the tapered capillary exceed essentially those for the cylindricalcapillary. On the contrary, shorter pulse durations are typical for the tapered capillary.It should be also noted that a pedestal at the level of 1300 pulse/s is observed in fig. 3b.In the case of the tapered capillary such an interpulse transparency is not revealed.The above-mentioned experimental results confirm our recent assumption [8] on thedominant role of charging up a face part of the capillary in the transformation of con-tinuous ion beams into oscillating ones. This effect is not observed if an input hole ofcapillaries exceeds the beam diameter. A conducting protection of the face part of the368 F. F. KOMAROV, A. S. KAMYSHAN and P. A. GRISHINFig. 3. – Time distributions of protons transmitted through a tapered capillary (a) and cylin-drical capillary (b).tapered capillary (fig. 3a) causes a total suppression of this effect (fig. 4). The angulardistribution of transmitted particles in fig. 4 looks like the distribution presented in fig. 1.Nevertheless, the transparency of this capillary with a taper angle of 0.5 deg is a few hun-dred times less than this parameter for the capillary with a taper angle of 1.7 deg. Ananalogous strong decrease of the transmission coefficient was observed in the capillarywith a taper angle of 2.2 deg (fig. 5). At zero entrance angle (fig. 5) three peaks areobserved in the central part of the distribution. Such a behavior has been revealed for anangular distribution of 240 keV protons transmitted through a cylindrical capillary witha diameter of 0.5mm and length of 178mm (fig. 3 in ref. [6]).The energy spectra of the initial and transmitted protons are shown in fig. 6. Themost of ions transmit the capillary without energy loss, however, the low energy tailcertainly exists (fig. 6, curve 2). It means that those transmitted ions moving with higherFig. 4. – Shape and angular distribution of 200 keV protons transmitted through a capillarywith a taper angle of 0.5 deg.PECULIARITIES OF PROTON TRANSMISSION THROUGH TAPERED GLASS CAPILLARIES 369Fig. 5. – Angular distribution of 320 keV protons transmitted through a capillary with a taperangle of 2.2 deg.transverse energies and suffering the small angle scattering lose their energy interactingwith an inner surface of the capillary. It should be noted that these particles cause onlya very modest widening of the initial energy distribution (less than 5 to 6%).Generally speaking, the key to the understanding of these experimental observationsis the notion of the two distinctly different time constants for formed charge transportalong the surface and within the bulk of capillaries. The interplay between these timeconstants is responsible for providing a dynamic equilibrium keeping up a Coulomb fieldlarge enough to deflect charged particles and small enough not to initiate a Coulombblockade at the entrance of the capillary.Proton transmission through nanoporous alumina was also studied. The samples wereprepared by two-step anodization of 0.6mm thick high-purity (99.999%) aluminium foilsfollowing the procedure of Masuda and Fukuda [9]. The remaining aluminium layer wasFig. 6. – Energy spectrum of transmitted protons with an initial energy of 320 keV with andwithout capillary. The taper angle is 1.7 deg. Curve 1 is for the initial beam and curve 2 is forthe transmitted beam.370 F. F. KOMAROV, A. S. KAMYSHAN and P. A. GRISHINFig. 7. – The SEM picture of the nanoporous alumina sample from a surface.removed by saturated HgCl2 solution. Pore ends were opened in a 5% H3PO4 solution.The samples were finally cleaned in deionised water. Alumina samples with thickness of42μm were used in these experiments (fig. 7). The diameter of pores was in the rangeof 63 to 72 nm and the density of pores amounts to 1.2 · 1010 cm−2. The relative surfacearea covered by pores was about 45 to 50%.Figure 8 shows a cleaved cross-section of this sample imaged by SEM at different mag-nifications. As can be seen, the pores extend through the entire depth. The dominantpart of capillaries displays a regular and parallel behaviour. However, due to unavoidableirregularities in the nanochannel growing process of anodized Al2O3 capillary systems,deviations from the ideal, parallel-arranged structure of the individual capillary, distor-tions as well intersections are revealed in the examined samples.Orienting the capillaries parallel to the proton beam was a complicated process ingeneral. Before each set of measurements, the optimum positions were searched byseeking the minimal deviation and maximal transmission of the proton beam in order tofind the correct zero point of the title angle scale. Figure 9 shows the angular distributionsof protons with the energies of 150 and 320 keV transmitted through untitled capillaries(ϕ = 0◦). It can be seen that each of both the angular distributions consists of a sharpFig. 8. – The SEM image of a side view of the nanoporous alumina sample at different magnifi-cations.PECULIARITIES OF PROTON TRANSMISSION THROUGH TAPERED GLASS CAPILLARIES 371Fig. 9. – Angular distribution of H+ ions transmitted through capillaries in the Al2O3 membranewithout metallic coverage at 0◦ tilt angle: 1–150 keV, I = 17.5 nA; 2–320 keV, I = 20nA.peak and two broad low-intensity peaks disposed symmetrically around the central peak.It should be mentioned that for protons with an energy of 150 keV these low-intensityside peaks are displayed more clearly than for the proton beam with an energy of 320 keV.Moreover, the width of the central transmission peak for low energy protons (fig. 9 (curve1)) is approximately up to 20% exceeding the angular distribution width for protons withan energy of 320 keV. It should be also noted that the angular width of the ion beambehind the Al2O3 membrane is practically equal to the angular width of the initial beam.The tapered glass capillary makes it considerably broader (by a factor of 4 and more). Inour opinion, the side maxima in fig. 9 are caused by the above-mentioned non-parallelismof capillaries and structure imperfections (see fig. 8). The ion beam transmission throughthe Al2O3 membrane, defined by integrating the angular distributions, has amounted toabout 2 · 10−4. In order to improve transparency of such a capillary system the inlet andoutlet surfaces of a membrane should be covered with a thin metal layer (10 to 20 nm inthickness). It enables us to suppress influences of sample surface charging and increasethe transparency coefficient up to a few percents [8]. Non-parallelism of nanocapillariesand other capillary imperfections may also strongly influence the guiding parametersand, therefore, the Al2O3 membrane transparency.The proton beam intensities presented in fig. 9 were measured behind the samplewith a thickness of 42μm that considerably (more than one order of magnitude) exceedsranges of protons in this material. This is an evidence of the anomalous motion of ionslike the hyperchanneling of charged particles along the low-index directions of crystallattices.4. – Conclusion– We have confirmed that a few hundred keV proton beams are successfully focusedby the tapered capillary optics.– The areal density of the transmitted beam is enhanced by approximately 20 times.– Charging up a face part of the capillary causes the transformation of continuousion beams to oscillating ones.372 F. F. KOMAROV, A. S. KAMYSHAN and P. A. GRISHIN– The most of protons (94–95%) in the energy range of 150 to 320 keV transmit thecapillary without energy loss.– Changing a taper angle from 0.5 deg to 1.7 deg evidences increase of the trans-mission coefficient by more than 300 times keeping the initial energy spectrum ofions.– Compared with the conventional micro-beam facilities, the usage of tapered cap-illaries is certainly simple and low-cost, thus providing an interesting techniqueof submicron RBS or PIXE elemental analyses. Moreover, if the ion species areextended to heavier elements, the present method provides highly local versatilemaskless ion implantation technique.– Angular width of the ion beam behind the Al2O3 membrane is practically equalto the angular width of the initial beam. It means that nanochannel alumina is apromising template for pattern transfer by ion lithography.REFERENCES[1] Nebiki T., Yamamoto T., Narusawa T., Breese M. B. H., Teo J. E. and Watt F.,J. Vac. Sci. Technol. A, 21 (2003) 1674.[2] Nebiki T., Robir H. and Narusawa T., Nucl. Instrum. Methods B, 249 (2006) 226.[3] Hasegawa J., Shiba Sh., Fukuda H. and Oguri Y., Nucl. Instrum. Methods B, 266(2008) 2125.[4] Nakayama R., Tona M., Nakamura N., Watanabe H., Yoshiyasu N., Yamada C.,Yamazaki A., Ohtani S. and Sakurai M., Nucl. Instrum. Methods B, 267 (2009) 2381.[5] Razpet A., Possnert G., Johansson A., Hallen A. and Hjort K., Nucl. Instrum.Methods B, 222 (2004) 593.[6] Komarov F. F., Kamyshan A. S. and Karwat Gz., Vacuum, 83 (2009) 51.[7] Stolterfoht N., Hellhammer R., Bundesmann J. and Fink D., Nucl. Instrum. MethodsB, 267 (2009) 226 (and cited in this paper).[8] Kamyshan A. S., Komarov F. F. and Partyka J., Proceedings of 8-th InternationalConference on Interaction of Radiation with Solids, Vol. 32 (Minsk, BSU Publ. House)2009.[9] Masuda H. and Fukuda K., Science, 268 (1995) 1466.

Peculiarities of proton transmission through tapered glass

capillaries

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Peculiarities of proton transmission through tapered glass capillaries

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