Chiang Mai J. Sci. 2013; 40(4) 789

Chiang Mai J. Sci. 2013; 40(4) : 789-797http://it.science.cmu.ac.th/ejournal/Contributed Paper

A Tapered Glass Microcapillary Processing Systemfor Focusing a MeV H+ Ion BeamSomrit Unai*[a,b], Michael W. Rhodes [b], Chanvit Sriprom [b],Kanda Singkarat [b,c], Nirut Pussadee [a,b] and Somsorn Singkarat [a,b][a] Plasma and Beam Physics Research Facility, Department of Physics and Materials Science,

Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.[b] Thailand Center of Excellence in Physics, Commission on Higher Education, 328 Si Ayutthaya Road,

Bangkok 10400, Thailand.[c] Department of Physics and Materials Science, Faculty of Science, Chiang Mai University,

Chiang Mai 50200, Thailand.*Author for correspondence; e-mail: somritu@fnrf.science.cmu.ac.th

Received: 17 September 2012Accepted: 14 March 2013

ABSTRACTSeveral recent reports have indicated that a tapered glass capillary tube with a tip size

on the order of micrometers has a focusing effect on transmitted ion beams. In relation to ourparticular area of interest, this could be a simple and cheap way to generate a focused ionbeam for ion beam lithography and beam-in-air analysis and irradiation applications.Here, details of the method and the equipment used to produce a tapered glass microcapillarytube will be described. The equipment, which is simply called a “glass microcapillary puller”,is made using an induction heater. This method ensures that the glass tube, which is insertedinside and along the central axis of the solenoid, is heated uniformly in all directions.The heating temperature is adjusted by a power control, with a maximum temperature ofapproximately 2,000C. The pulling tension is varied over a wide range by using differentweights. Application of the tapered glass microcapillary for 2 MeV H+ ion beam lithographyis also demonstrated.

Keywords: focusing effect, glass puller, induction heater, ion beam lithography, tapered glasscapillary

1. INTRODUCTIONIt has been demonstrated since 2002 that

insulating nanocapillaries can focus a low-energy highly charged ion beam which ispassing through them [1]. The focusing factor,defined as the ratio of outlet current densityto inlet current density, was reported to bebetween 10 and 1,000 [2,3]. Later, Nebikiet al. [4] found this focusing effect of a

tapered glass capillary for a high-energy ionbeam as well. The self-organized charge-upby the ion beam at the inner wall surface ofthe glass capillary is believed to be the maincause of the focusing capability in the case ofslow highly charged ions, while the case of ahigh-energy ion beam is not yet clearlyunderstood [1,2].

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Because of several other advantages ofa tapered glass capillary-such as its simplicity,small size and very low cost, in addition to itsfocusing potential-it has been utilized in someinteresting applications, such as beam-in-airanalysis [5] and irradiation [6].

However, with this technique, the onlyway to vary the diameter of the outlet beamis by changing the tip diameter of thecapillary. This requirement is fulfilled by aglass microcapillary puller. Therefore, wehave designed and constructed our own glasspuller by using induction heating technique;the details of its construction and testing arereported herein. In principle, it has shownseveral advantages over a conventional heatingNichrome coil, as is used in a commercialmicropipette puller, such as more even

Figure 1. Schematic illustration of the glass tube pulling method used in this work.

distribution of heat transfer to a glass tube,more stable and uniform power control, moreflexible in coping with glass tubes of differentsizes, much longer service time and saferaccording to electrical hazards. However, theperformance of our glass puller cannot bedirectly compared with those available in themarket. This is because the commercial glasspullers are mostly for producing micropipettewhich use a small starting glass tube of notlarger than 1.2 mm in outside diameter.In our case, the glass tubes used have innerdiameter of not less than 3 mm since it has tocover the whole inlet proton beam that hasan original diameter of 1-2 mm. By utilizingthe tapered glass microcapillary made by ourglass puller, the MeV proton beam lithographyhas been successfully demonstrated.

2. PRINCIPLESIn general, the heating technique in a glass

puller involves the use of a laser, gas flame,or heating filament [7-10]. However, it isaccepted that induction heating has greaterefficiency due to its high power density, rapidheating-up speed, and excellent control ofinduced power [11]. Another importantcriterion when using a glass capillary in guidingand confining the ion beam is the radialsymmetry of the tapered shape of the

capillary. Induction heating allows asimpler method of heating the original glasstube uniformly in all directions, as depictedin the authors’ design shown in Figure 1.The circular cylindrical metal shell (generallycalled a “work piece” by those whofrequently work with induction heatingsystems) is inserted inside the work coil,which is made of copper tubing in the shapeof a solenoid. Actually, in the present case

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in a rapid temperature increase. Theaverage power dissipated over the volumeof the cylindrical shell (Pdis.) is not onlydependent on the intensity of the eddycurrents but also on the properties of thecylindrical metal shell. From Poynting’stheorem, they are related by the followingequation [12]:

Pdis.

where a, l and are the metal cylinder radius,length and shell thickness, respectively, and is the conductivity of the metal cylinder. Here,K is the complex amplitude of the currentdensity circulating on the surface of thecylinder, which is dependent on thecomplex amplitude of the applied magneticfield (H0) and the oscillation frequency ofthe magnetic field (). Their relationship

the metal cylinder is not a work piece, butinstead serves as a uniform mini-oven forthe glass tube inserted in the middle. Whenhigh-frequency alternating current from aresonant LC tank circuit is applied to thework coil, a high-frequency magnetic fieldis created. For a cylindrical shell made offerromagnetic material, the hysteresis effectis the major cause of heat generation at themetal cylinder; but this drastically declinesat temperatures above the Curie point, i.e.770C for iron [11]. In this experiment, thesystem was operated above the Curie pointof the metal cylinder.

Thus, as predicted by Faraday’s and Lenz’slaws, variation of the external magnetic fieldwill induce an alternating eddy current on thesurface of the cylindrical metal shell. Sincethe metal cylinder appears as a lowimpedance load, the induced eddy currentsare dissipated by Joule heating, resulting

Figure 2. (a) A prototype of the glass microcapillary puller using induction heating technique.(b) A schematic diagram of the induction heating system. (c) Top view of the heater andpuller unit, where the red-hot metal cylinder was heated by Psource = 400 W.

KK* (1)al=

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isK -

when m=0a/2, where 0 is thepermeability of the free space.

Applying both equations, it is found thatpower dissipation increases as the square ofthe time rate of change of the applied magneticfield and the magnitude H0, which dependon the performance of its power source.

3. EXPERIMENTAL SETUP3.1 Induction Heating System

The glass microcapillary puller system isshown in Figure 2(a). It is comprised of threeunits: heater and puller unit, frequencyadjustment unit, and power control unit.The heater and puller unit is separated fromthe electronic units so that it can be alignedeither vertically or horizontally. Here, we willreport on the horizontal alignment only.

The drive system is based on invertertechnology, which involves convertingfrom one frequency to another to adjust

the frequency of a power source (Psource)from which power is delivered to acoupling coil or work coil. The schematicof the induction heating system is shownin Figure 2(b). A variac was used for settinga suitable voltage input to the AC-to-DCconverter. The system consists of: a DCrectifier for converting the 50 Hz AC toDC; an inverter section for converting theDC back to a variable frequency centeredaround 70 kHz; and lastly an LC tankcircuit driven by a H-Bridge MOSFETdriver. The output of the driver drives anLC parallel resonant tank through acoupling coil. The work coil (inductor L) ismade of copper tubing of 5.5 mm innerdiameter and 6.5 mm outer diameter.The copper tube is curved into a coil of 5.5turns with inner diameter 3 cm and length6 cm. The work piece is made of a hollowmetal cylinder 10 cm long, with 1 cm and1.2 cm inner and outer diameters, respectively.For a uniform distribution of heat withinthe work piece, the metal cylinder was

Figure 3. Arrangement of the CMU MeV ion beam lithography system utilizing a taperedglass microcapillary (not to scale).

jmH0

1+jm

(2)=

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inserted in the middle of the work coil witha well-aligned centering.

Here, the principle of the step-downtransformer was utilized. The coupling coilwas used as a primary coil and the work coilas the secondary coil, with a higher voltageand lower current on the coupling coil but alower voltage and higher current flow withinthe work coil. The high AC current flow withinthe work coil will induce the high magneticfield around the work coil.

According to equations (1) and (2),increasing or decreasing the Pdis. can beachieved by increasing or reducing the Psource.In this experiment, the control of Psource canbe accomplished in two ways: by shifting thefrequency to get closer or farther from theresonance frequency of the LC circuit, or byvarying the voltage source of the converter.The Psource can be calculated from the currentand voltage of the coupling coil, which aremonitored and displayed on an LCD powermeter. Figure 2(c) shows the heater and pullerunit; the heated metal cylinder was operatingat Psource of 400 W.

Although the system can provide powerup to 1.5 kW at full resonance condition,we found that 400 W was suitable for theglass pulling process since the softeningtemperature of borosilicate glass is 821C.By using a pyrometer, we found that thetemperature of the metal cylinder at 400 Wpower transfer is 900oC. The heat sourceclosely surrounding the glass tube willprovide heat to the glass tube uniformly.

3.2 Microcapillary ProductionIn this experiment, a cylindrical glass tube

made of borosilicate glass was used as astarting material. The inner and outer diametersof the 20-cm-long glass tube were ~1.5 mmand ~3 mm, respectively. As shown inFigures 1 and 2(a), the glass tube wasinserted in the middle of the metal cylinder.

One of its ends was fixed by screws, whilethe other end was tied to a thread and hungwith a weight over a smooth pulley. Threadwas used in order to prevent a perturbationto the symmetry of the stretched partwhen the weight collides with thesupporter. The weight is released as the glasssoftens. The time for heating up the metalcylinder was ~45 s at 400 W, and theproduction time per piece of a tapered glasscapillary was less than 2 min. Two differentways of pulling were studied. Firstly, theglass tubes were pulled with differentweights, ranging from 1 to 9 N at2 N increments, with a fixed stretching lengthof 15 cm which is less than the breaking lengthof the glass tube at 9 N weight. Secondly,the glass tubes were pulled by a constantweight of 3 N, with different stretching lengthsranging from 10 to 18 cm at 2 cm increments.The microcapillaries were cut at the smallestinner diameter after observation by an opticalmicroscope with 10 m resolution calibrationscale.

4. PROTON BEAM LITHOGRAPHYAPPLICATION

A 22 keV H+ ion beam was generatedfrom a Cs sputter ion source and acceleratedup to 2 MeV by a 1.7 MV tandem“Tandetron” accelerator at the Plasma andBeam Physics Research Facility of Chiang MaiUniversity. The proton beam lithographysystem with a tapered glass microcapillary asshown in Figure 3 was installed inside ananalysis chamber with a base pressure of110-5 mbar. Details of this experiment canbe found elsewhere.

In brief, a pattern of four letters, such as“ThEP” in this case, was created by in-housedrawing software. The coordinates of thepattern are exported as an x/y data pair.The x/y data pair is imported to patterngenerator software which is embedded in

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the sample holder controller software.The irradiation time and ion beam size arealso input parameters to be added to thesoftware. Poly(methyl methacrylate)(PMMA) film was attached to a sampleholder that is translated by the steppermotors with 1 m resolution in the x andy axes, as shown in Figure 3. Ion beamcurrent was measured by a Faraday cupwhich was isolated and buried within thesample holder. The Faraday cup wasconnected to an electrometer (Keithley 640)with 10 fA resolution. The ion beamcurrent and irradiated area were used tocalculate the ion beam flux and fluence. For

Figure 4. (a) A photo of tapered glass microcapillaries which were pulled by different weightsof 1, 3, 5, 7 and 9 N (from bottom to top). (b) A cross-section image of the capillary pulledby 9 N weight. (c) Side view of the tip part of a microcapillary pulled by double stretchingmethod.

writing the pattern, the sample holdercontroller software controls the x, ytranslation motors for moving the sampleholder to a position which is defined inthe pattern generator software. With thecombination of a pattern generator andsample holder controller software, anyrequired patterns can be created. Afterirradiation, the PMMA film was developedin a solution of isopropyl alcohol anddeionized water (7:3 by volume) for 4 min,then rinsed with deionized water and driedunder nitrogen gas flow. An opticalmicroscope was used to observe the qualityof the pattern.

Figure 5. Results from separate measurements showing that the outlet diameter of themicrocapillary depends on either (a) the pulling weight or (b) the stretching length (l).The straight lines are from linear regression fitting.

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5. RESULTS AND DISCUSSION5.1 Microcapillary Characteristics

Each microcapillary shown in Figure 4(a)was stretched by different pulling weights,from 1 to 9 N in 2 N increments. As anillustrative example, the tip cross-section ofthe microcapillary with 9 N pulling weight isshown in Figure 4(b).

As one might imagine, for a narrowertip a longer stretching length is required; but itmay not be possible to fit the capillary insidethe limited space of an analysis chamber.Utilizing a method called double stretching, aminute outlet diameter with short tapered partcan be made. Figure 4(c) shows a capillarywith a 17 m diameter tip and a tapered lengthof only a few centimeters, made by the doublestretching technique. Huang et al. [13] reportedthat the double stretching method can be usedto produce a glass electrode a few micronsin size for biological studies of cellelectrophysiology. Practically, the processconsists of two steps. For a 400 W powersource, as in the present case, the first stepwas pulling the glass tube with 2 N weightand 3 cm stretching length. In this state, thetapered part was formed at the heating pointof the glass tube. The second step beganafter the tube was allowed to cool down.Using the same heating power, the glasstube was pulled with 4 N weight andunlimited stretching length.

Figure 5(a) shows a linear dependencyof the normalized outlet diameters withpulling weights. The tip inner diameter wasnormalized with the tube diameter becausethe supplier could not provide us with thesame diameter glass tubes. It is clearly seenthat when the pulling weight is increasedthe inner diameter of capillary tip isdecreased, which is in good agreement withthe experimental results of Nikita et al.[10]. Figure 5(b) shows a linear relationshipbetween the normalized outlet diameter

and stretching length. The range ofstretching length was between 10 cm to 18cm at 2 cm increments. The experimentalresults show that the inner diameter of themicrocapillary tip can be controlled easilyby changing the pulling weight and/orstretching length. This agrees well with thetheoretical work by Purves [14], whichshows that the diameter of the capillary tipdepends on pulling velocity and force.

5.2 MeV H+ Ion MicrobeamLithography

Figure 6(a) is a photo of a microcapillaryused in the microbeam lithographicexperiment. By careful measurement, it wasfound that the outlet current density was2.7 times the inlet current density.According to the definition mentionedearlier, it is clearly evident that the focusingeffect still exists in the case of a MeV lightion beam. The main advantage of theenhancement of the ion beam currentdensity is to reduce irradiation time for ionbeam lithography.

Figure 6(b) shows a pattern designedusing in-house drawing software. PMMAfilm was irradiated with a 2 MeV protonbeam at ion fluence of ~1.3 1014 ions cm-

2 to induce chain scission at the irradiatedarea. Chain scission of PMMA is bestachieved at ion irradiation fluence between3.3 1013 and 3.5 1014 ions cm-2 [15, 16].After the development process, in whichthe irradiated areas of PMMA are washedaway, the engraved pattern on theunirradiated PMMA film is clearly seen,as shown in Figure 6(c). By enhancing theoutlet beam current density this way, theirradiation time was reduced more than 50% when estimates from the focusing factorof 2.7. The basic practical aspects of themicrobeam technology for ion beamlithography have been achieved such that

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the writing time is reasonably short,irradiation spot can be defined precisely andsidewalls of the pattern are completelyvertical.

5. CONCLUSIONSA glass microcapillary puller based on an

induction heating system has been successfullyconstructed and tested. It is fast and easy touse. A 400 W power is enough for pulling a3 mm diameter glass tube. In the process ofmicrocapillary production, the tip diameter canbe controlled by varying the pulling weightand/or the stretching length. A microcapillarytip a few tens of microns in size can bemade by the double stretching method.Economically, dimensions of all parts shownin Figure 2, both electronically and physically,can be shrunk down more than 70 % whenoptimum design is taken into account. A140 m outlet diameter microcapillary wassuccessfully applied in focusing a 2 MeVproton beam for the purpose of writing amicro-pattern onto PMMA film. Thistechnique will also be applied for beam-in-airirradiation and beam-in-air proton inducedx-ray emission (PIXE) analysis.

ACKNOWLEDGEMENTSThe authors gratefully acknowledge

financial support by the International AtomicEnergy Agency (IAEA, Vienna) and theThailand Center of Excellence in Physics(ThEP Center). The scholarship for SU isprovided by the ThEP Center. The authorswould also like to thank Mr. WitoonGinamoon for technical support.

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