Rapid three-dimensional imaging of defect distributions using a high-intensitypositron microbeamN. Oshima, R. Suzuki, T. Ohdaira, A. Kinomura, T. Narumi, A. Uedono, and M. Fujinami Citation: Applied Physics Letters 94, 194104 (2009); doi: 10.1063/1.3137188 View online: http://dx.doi.org/10.1063/1.3137188 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characterization of Vacancy Defects in Carbon Ion Irradiated Graphite Using Positrons AIP Conf. Proc. 1349, 1281 (2011); 10.1063/1.3606335 Evolution of Defects in Pu During Isochronal Annealing and SelfIrradiation AIP Conf. Proc. 673, 121 (2003); 10.1063/1.1594573 Low energy positrons at semiconductor surfaces AIP Conf. Proc. 576, 753 (2001); 10.1063/1.1395416 Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique usingmonoenergetic positron beams J. Appl. Phys. 90, 181 (2001); 10.1063/1.1372163 Defects in p + -gate metal–oxide–semiconductor structures probed by monoenergetic positron beams J. Appl. Phys. 86, 5385 (1999); 10.1063/1.371535

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Rapid three-dimensional imaging of defect distributions usinga high-intensity positron microbeam

N. Oshima,1,a� R. Suzuki,1 T. Ohdaira,1 A. Kinomura,1 T. Narumi,2 A. Uedono,2 andM. Fujinami31National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba,Ibaraki 305-8568, Japan2Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan3Department of Applied Chemistry, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8552, Japan

�Received 30 March 2009; accepted 24 April 2009; published online 15 May 2009�

An intense positron microbeam generated by an electron accelerator has been developed forobtaining three-dimensional positron lifetime mappings in a sample to permit visual evaluation ofdefect distributions. The beam diameter at the sample was 80–100 �m. The counting rate of thepositron annihilation � rays used to measure positron lifetime was as large as 3�103 s−1.Three-dimensional imaging was demonstrated of positron lifetimes in a SiO2 sample, which wasirradiated with ion beams through a mesh mask. The time to obtain a single image �3500 pixels foran area of 2.5�3.5 mm2� was 0.5–1 h. © 2009 American Institute of Physics.�DOI: 10.1063/1.3137188�

When a positron, which is the antiparticle of an electron,encounters an electron in a material it annihilates producing� rays. Both the positron lifetimes and the Doppler broaden-ing of the annihilation � rays are strongly dependent onatomic-scale defects and nanometer and subnanometer orderpores at the positron annihilation site. Positron annihilationspectroscopy �PAS�, which includes positron annihilationlifetime spectroscopy �PALS� and Doppler broadening of an-nihilation radiation, is an effective technique for nondestruc-tive evaluation of such defects in a material.1,2

Slow positron beams are used to investigate defects nearthe surface of a material at any depth between �0.01 and�1 �m.2 However, PAS with a conventional positron beamsuffers from a poor lateral spatial resolution of several mil-limeters. In order to improve the lateral spatial resolution ofPAS, several kinds of positron microbeams/microprobes�PMs� with beam spot sizes of �1–100 �m have beendeveloped.2–6 By scanning the lateral injection position �xyplane� and the implantation depth �z� of the beams, thesemicrobeams are capable of producing two- or three-dimensional PAS mappings �i.e., two-dimensional images atan arbitrary depth�, which enables visual evaluation of defectdistributions in a sample.3–5

The PMs that have been developed so far use radioiso-topes as their positron sources. Consequently, they do nothave very high beam intensities and so a considerable time�e.g., of the order of a day or even a week� is required toobtain a single PAS image. In order to reduce the time re-quired to obtain an image, intense positron microbeams thatuse electron linear accelerators �LINACs� or nuclear reactorsas positron sources have been studied.7–9 We have developeda high-efficiency �10%� brightness enhancement system forthe intense positron beam produced by a LINAC.8 Using thissystem, the diameter of the positron beam was reduced by afactor of �1 /300 at the sample relative to that at the positronsource.10 The beam intensity produced by a LINAC is typi-cally two orders of magnitude greater than that obtained in

PMs that use radioisotopes.2 Consequently, our PM can ob-tain PAS images in relatively short times. In addition, thedeveloped system can detect scattered particles �positronsand secondary electrons� emitted from the beam injectionpoint. Scattered particle images are useful for confirming thelocation of positron beam injection in a sample.

In this letter, we briefly describe the PM system and useit to obtain three-dimensional mapping of positron lifetime ina SiO2 sample that was partially ion-irradiated through amesh mask. Scattered particle images obtained by the PMare also reported.

The developed PM consists of a positron source, a mag-netic beam transport system, a brightness enhancement sys-tem, and a measurement system. A pulsed slow positronbeam with an intensity of more than 107 e+ /s is generatedby a LINAC.11,12 A typical pulse width is �1 �s and thepulse rate is 100 pulses/s. The magnetic beam transport sys-tem guides the slow positrons from the positron source to theexperimental room. This system includes a linear storagesection where the positron pulse width is stretched from�1 �s to �5 ms to prevent detector pileup for PAS mea-surements. The brightness enhancement system consists of afocusing lens and a transmission remoderator �a Ni thin filmwith a thickness of 200 nm�. The positron beam extractedfrom the solenoid is focused by the lens at the remoderatorwhere the beam divergence is reduced so that the beambrightness can be improved. The beam is chopped and com-pressed by using two pulsing schemes to know when posi-trons inject into a sample.12,13 The measurement system con-sists of a xy-translation system, microchannel plates �MCPs�,a �-ray detector �BaF2 scintillation detector�, and an objec-tive lens �see Fig. 1�. The MCPs detect scattered particles�positrons and/or electrons� emitted from the beam injectionspot. The positron lifetime can be determined by the timedifference between the pulse trigger signal and the detectionsignal of the annihilation � ray. The lateral position of thebeam injection point can be moved by the xy-translation sys-tem with an accuracy of better than 1 �m. The positronbeam can be focused to a diameter of �30–100 �m on thea�Electronic mail: nagayasu-oshima@aist.go.jp.

APPLIED PHYSICS LETTERS 94, 194104 �2009�

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sample by adjusting the power of the objective lens.8,10 Theimplantation energy of the positron beam is variable from1–30 keV. The time resolution for the positron lifetime mea-surement is typically �0.28 ns, which is sufficient forPALS. The maximum �-ray count rate is about 3�103 s−1,which is one and a half orders of magnitude greater thanthose obtained by PMs developed so far.

We used a SiO2 sample to obtain positron lifetime im-ages using the developed system. The sample was preparedaccording to the following steps. The SiO2 sample was irra-diated by a 50 keV H+ beam through a mesh mask �wirediameter: 250 �m, mesh spacing: 30 wires/in.�. The meshmask was rotated 45° on the sample after H+ irradiation andthe sample was then irradiated by a 150 keV Ar+ beam. Thetotal doses of H+ and Ar+ were 1016 and 1015 cm−2, respec-tively. Projected ranges of H+ and Ar+ in SiO2 were, respec-tively, estimated to be Rp�H+�=600 nm and Rp�Ar+�=200 nm using the TRIM code, where point defects would begenerated between the surface and those ranges.14 It isknown that some point defects induced by ion beam irradia-tion in SiO2 prevent the formation of positronium, resultingin a shortening of the positron mean lifetime.15 Ta ribbonswere partially laid on the SiO2 sample �see Fig. 2�a�� andthey were used as positional marks in images. A 2.5�3.5 mm2 area on the sample was scanned by the PM in50 �m intervals in the x and y directions. The positron beamsize at the sample was 80–100 �m. The surface potential ofthe MCPs was �100 V relative to the sample so that second-ary electrons with energies less than 100 eV could not bedetected by the MCPs.

Figures 2�b�–2�d� show positron lifetime images ob-tained for positron implantation energies of Epos=4.7, 6.4,and 7.8 keV, respectively. Positron mean implantation depths�zpos� in SiO2 are approximately 200, 350, and 500 nm forEpos=4.7, 6.4, and 7.8 keV, respectively.2 The measurementtime for each pixel was 1 s. The contrast �i.e., the positronmean lifetime of each pixel� of these images is defined as�=�initi /�ini, where i is the channel number of the positronannihilation spectrum recorded by a microchannel analyzer,ni is the number of � rays recorded in channel i, ti is the timecorresponding to channel i and is taken to be zero at thepositron annihilation peak channel. The time range used forcalculating � was from �0.59 to 4.56 ns. The statistical errorof � was typically �0.03 ns in this experiment, which ismuch shorter than the full scales of the images �0–0.75 ns�.

Portions of the Ta ribbons could be clearly observed in life-time images that had relatively low values of �. Only the H+

irradiation pattern was clearly observed at Epos=7.8 keV�see Fig. 2�d��, whereas both H+ and Ar+ irradiation patternscould be observed at Epos=4.7 keV �see Figs. 2�b� and 2�e��.These results appear to be reasonable because almost all pos-itrons stopped around Rp�Ar+� for Epos=4.7 keV�zpos�Rp�Ar+��Rp�H+�� and almost all positrons arestopped between Rp�Ar+� and Rp�H+� for Epos=7.8 keV�Rp�Ar+��zpos�Rp�H+��. Even though the measurementtime was only 0.5 s/pixel, the lifetime image clearly revealedion beam irradiation patterns �see Figs. 2�f� and 2�g��. There-fore, one positron lifetime image of 3500 pixels can be mea-sured in 30 min.

Figures 2�h� and 2�i� show images obtained simulta-neously with the image of Fig. 2�b�. Their contrasts are equalto the counting rates of the signals obtained by the �-raydetector �I�� and by the MCPs �IMCP�, respectively. Theyclearly show that I� is much lower in Ta than in SiO2, whileIMCP is much higher in Ta than in SiO2. These images appearto be reasonable because the backscattering probability ofinjected positrons is expected to be much higher in Ta re-gions than in SiO2 regions.2 In a different measurement, we

BaF2scintillation

detector

Intense e+

microbeam

Microchannel

plates

γ

Objective

lens

Sample

Vacuum

chamber

Bellowsxy-translation

system

e+ / e-

FIG. 1. Experimental setup of the measurement system.

(b) τ, 4.7 keV (c) τ, 6.4 keV (d) τ, 7.8 keV

(e) τ, 4.7 keV

(h) Iγ, 4.7 keV (i) IMCP

, 4.7 keV

(a)

Ta

SiO2

400 µm

Ta

Image contrast scales

τ (ns) : 0 0.75

Iγ (s-1) : 0 2800

IMCP

(s-1) : 0 200

Unit : Min. Max.

(f) τ, 4.7 keV (g) τ, 7.8 keV

(j) IMCP

, 7.8 keV

Step

FIG. 2. �Color� �a� Schematic view of the sample with a lateral scale of400 �m. Two Ta ribbons with thickness of 10 �m were partially laid onthe SiO2 sample. ��b�–�j�� images obtained using the PM. The kind of con-trast ��, I�, and IMCP� and positron energies Epos during measurement of theimages are noted above the images. The three color bars contrast scales ��,I�, and IMCP�. ��b�–�g�� are positron lifetime images. �e� is the same as �b�, inwhich the black solid lines express the mesh mask pattern for H+ and Ar+

beams. Measurement time was 1 s/pixel for ��b�–�d��, while it was 0.5s/pixel for ��f� and �g��. ��h� and �i�� are �-ray count rate and scatteredparticle images measured simultaneously with �b�. Mesh pattern during Ar+

beam irradiation is visible in the scattered particle image �j�.

194104-2 Oshima et al. Appl. Phys. Lett. 94, 194104 �2009�

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obtained the mesh pattern in the scattered particle image dur-ing Ar+ beam irradiation �see Fig. 2�j��. Measurements by asurface analyzer revealed that about 10 nm was etched bysputtering of the Ar+ beam.

In summary, an intense PM using a LINAC was devel-oped and annihilation �-ray count rates of �3�103 s−1 inpositron lifetime measurements were achieved. The devel-oped system can simultaneously acquire three kinds of im-ages �positron lifetime image, �-ray count-rate image, andscattered particle image� very rapidly �0.5–1 s/pixel�. Theseperformances ensure that this is a practical technique forevaluating defect distributions in various materials.

The authors are grateful to T. Akahane, M. Doyama, K.Ito, S. Jinno, Y. Kobayashi, T. Kurihara, M. Matsuya, T. Oka,A. Sakai, and T. Suzuki for helpful discussions. This studywas partially financially supported by the Budget for NuclearResearch of the Ministry of Education, Culture, Sports, Sci-ence and Technology, based on screening and counseling bythe Atomic Energy Commission, and the “Development ofSystem and Technology for Advanced Measurement andAnalysis” Program at the Japan Science and TechnologyAgency �JST�.

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