Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx

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Nuclear Instruments and Methods in Physics Research B

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Development of a compact high current low emittance RF ion source

0168-583X/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nimb.2013.07.057

⇑ Corresponding author. Tel.: +91 3323183201; fax: +91 3323183287.E-mail address: npy@vecc.gov.in (P.Y. Nabhiraj).

Please cite this article in press as: R. Menon, P.Y. Nabhiraj, Development of a compact high current low emittance RF ion source, Nucl. Instr. Meth. Bhttp://dx.doi.org/10.1016/j.nimb.2013.07.057

Ranjini Menon, P.Y. Nabhiraj ⇑Variable Energy Cyclotron Centre, Sector-1, Block-AF, Bidhan Nagar, Kolkata, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 March 2013Received in revised form 16 July 2013Accepted 17 July 2013Available online xxxx

Keywords:Inductive coupled plasmaRF ion sourceEmittance

A 13.56 MHz inductively coupled plasma based RF ion source is developed for production of high bright-ness focused ion beams of heavy gaseous elements for high speed milling and light ions for high speedimaging. In order to obtain ion beams with low emittance, no magnetic field of any kind is used in theion source. However, to achieve the high plasma density, the plasma chamber volume is reduced to cou-ple RF power as high as 8–12 W/cm3 to the plasma. Measurements show that the normalized rms emit-tance of 0.6 mA Ar1+ beam to be as low as 0.0075 mm-mrad while it is 0.004 mm-mrad for 1.2 mA of ionbeam from hydrogen plasma. With a simple parallel plate extraction system with an aperture of 2 mmdiameter, 80 mA/cm2 of ion beam from hydrogen plasma could be extracted at 3.5 kV extraction potentialand 300 W of RF power. The ion source has been operated with other heavy gases and results show thatmore than 1 mA of xenon and krypton ion beam could easily be extracted at 5 kV extraction potential and200 W of RF power. In this article, the capability of the ion source to produce high current, low emittanceheavy as well as light ion beams is presented.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

High current low emittance ion beams are in demand forseveral applications such as high current injectors in acceleratorslike cyclotrons, linear accelerators, radio frequency quadrupoles,secondary ion mass spectroscopy (SIMS), focused ion beam system(FIB) etc. The RF inductively coupled plasma ion source (ICPIS)which is recently developed in VECC, Kolkata as a part of the plas-ma based FIB system proved to be one such kind, generating highquality ion beam. The ion source was optimized and characterizedfor ion beam parameters such as energy spread, emittance, bright-ness and long term stability. This ion source was coupled to twolens electrostatic focusing column to focus argon ion beam for highspeed micromachining applications. It displayed superior perfor-mance for probe currents larger than 100 nA and showed the sur-face milling rates that are two orders better than the conventionalliquid metal ion source based (LMIS) FIB [1]. Work is in progress toobtain higher current submicron FIB of heavier gases such as kryp-ton and xenon for high speed machining and proton beams for highspeed imaging applications. This ion source is filamentless and canprovide stable beam over much longer period as compared to fila-ment based ion sources. In this article the basic features of the ionsource and its capability to produce high current low emittance ionbeams of both heavy and light elements are presented.

2. Ion source and ion beam extraction characteristics

The ICPIS developed indigenously operates at 13.56 MHz andhas a plasma chamber made of 25 mm dia quartz tube with100 mm length. The ion source has no magnetic field to improvethe plasma density in its design. The emittance of the ion beamis directly proportional to the magnetic field in the extraction re-gion. Since this ion source has no magnetic field, the emittanceof ion beam is supposed to be lower than other plasma based ionsources that use magnetic field. In particular, for singly chargedions the emittance deteriorates at a very low magnetic field [2].This is one of the major advantages of this ion source over otherplasma based ion sources. The extraction system consists of twoelectrodes with 2 mm diameter aperture and a gap of 2 mm. Theplasma electrode is very thin (0.5 mm) to achieve high currentextraction. In our design we have adapted the guidelines presentedby Coupland et al. [3] for designing the extraction system toachieve high current ion beam with small divergence. The operat-ing RF power is limited to a maximum of 300 W to keep the ionsource air-cooled. Faraday shield made by vertically slotted thincopper foil is employed to minimize the coupling of high RF volt-ages to the plasma. For best performance of the ion source theoperating pressure in the plasma chamber is optimized in therange of 0.005–0.06 mbar. The ion energy spread of argon ion beamas low as 4 eV is reported by authors at 0.03 mbar [4]. The originalion source had large plasma volume where the RF power coupledto the plasma was about 4 W/cm3. Since there is no magnetic fieldused in the ion source, the only way to increase the plasma densityand hence extracted current is by coupling more RF power to the

(2013),

Fig. 2. Extraction characteristics of ion beam from hydrogen plasma at 300 W of RFpower.

2 R. Menon, P.Y. Nabhiraj / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx

plasma. This was achieved by changing the original large volumeplasma chamber (50 cm3) to smaller volume (25 cm3) and therebydoubling the power density inside the plasma chamber under a gi-ven RF power. The new plasma chamber has an inner diameter of25 mm and length of 51 mm. With the new dimensions, powerdensity inside the plasma chamber is as high as 12 W/cm3 at300 W. Variation of the extracted current with extraction voltageis compared for the ion source with large and small plasma vol-umes under same operating conditions and RF power. Fig. 1 showsthe comparison where the ion beam extracted from smaller plasmavolume is 1.7 times that of larger one at 100 W of RF power and atextraction voltage of 6 kV. It was seen that the ratio increased tomore than double at 200 W of RF power. Under same operatingconditions and smaller plasma volume, 1450 lA for xenon,1318 lA for krypton, 900 lA for argon and 450 lA for neon wereextracted. The ion beam extracted from hydrogen plasma 2.5 mAamounting to current density of 80 mA/cm3 could be extracted at300 W of RF power and 3.5 kV extraction potential. Our mainaim in all the experiments is to obtain best quality beam at lowerextraction potential and subsequently accelerate to higher energiesto take advantage of the additional magnification due to ratio ofenergies as given by law of Helmholtz and Lagrange besides themagnification due to geometrical arrangement of lenses in focus-ing column [5].

An extraction characteristic of ion beam from hydrogen plasmais shown in the Fig. 2. This current density is significantly high andis very much comparable to magnetized, filament based ionsources and also ECR ion sources. However this ion source, beingvery simple and having lower emittance has clear edge over otherion sources, which will be explained in following section.

3. Emittance measurements

There are several methods to experimentally evaluate theemittance of the ion beam, where devices such as Allison scanner,pepper pot, slit and wire scanner, apertures and wire scanner etc.are widely used. Authors have earlier used Allison scanner foremittance and brightness measurement before modification ofplasma chamber which are reported in Ref. [6]. The emittancemeasurement equipment that we have employed for the work re-ported in this article consists of a 100 lm thick stainless steel platewith 400 lm diameter apertures and a wire scanner. The aperturesare drilled on the aperture plate with a spacing of 2 mm and are

Fig. 1. Comparison of extracted current from ion source with two different volumesat 100 W of RF power.

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distributed across its diameter. The wire scanner is assembled25 mm downstream of the aperture plate and it moves parallelto the direction of line of apertures. The profiles of individualbeamlet passing through the apertures are obtained by the wirescanner. Care is taken to make sure that there is no overlap ofthe profiles of adjacent beamlets. The signal from the scanningwire is recorded as a function of its position to determine the beamcross-section, the density distribution, the total beam divergenceand the divergences of the beamlets. Wire scanning and dataacquisition of the beam profile is automated using an applicationwritten in LabVIEW. This application also evaluates the normalizedrms emittance of the ion beam and emittances at different beamfractions. Unless otherwise mentioned, the reported emittancesin this paper, are all normalized rms values. Rms emittance is thestatistical quantity and is defined by:

erms ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffihx2ihx02i � hxx02i

qð1Þ

where, hx2i, hx02i and hxx0i2 are the second moments of the beam dis-tribution in x–x0 plane, x is the position of beamlet origin deter-mined by the aperture and x0 is the divergence of beam calculatedfrom the beam profile. Rms emittance is multiplied by bc to obtainnormalized rms emittance, where b and c are the usual relativisticparameters. In order to exclude the particles having large x and x0

for obtaining the correct estimate of emittance, the rms emittanceis evaluated for 98% of the total current. Ion source parameters suchas RF power, extraction voltage and pressure are optimized to ob-tain minimum emittance. In plasma ion sources, for a given extrac-tion geometry, these parameters influence very strongly the shapeof the plasma meniscus and the ion temperature which in turninfluence the beam quality.

At a gas pressure of about 0.01 mbar at the plasma chamber andextraction potential of 5 kV, normalized emittance of Ar1+ ionbeam is analyzed and presented in Fig. 3 for wide range of RFpower. Error in the measurement for normalized rms emittanceis ±0.0001 mm mard. It is seen that at 75 W RF power the emit-tance is 0.0097 mm-mrad and at 200 W it is 0.0075 mm-mrad. At75 W of RF power the total extracted current is 200 lA while it is600 lA at 200 W. It is interesting to note that with increase in theRF power the plasma conditions are better optimized to produceion beams of lower emittance even at larger extracted currents.Calculation shows that a normalized emittance of 0.008 mm mradfor Ar ion beam at 5 kV corresponds to an ion temperature of0.58 eV. However, the ion temperature inside the inductively

pact high current low emittance RF ion source, Nucl. Instr. Meth. B (2013),

Fig. 3. Variation of normalized rms emittance of 5 keV Ar1+ ion beam with RF power. Error in the measurement for normalized rms emittance is ±0.0001 mm mard.

R. Menon, P.Y. Nabhiraj / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx 3

coupled plasma is far less compared to the above estimation. Thusit is obvious in the present case that the contribution of iontemperature to the emittance is negligible. The other factors likechanges in the shape of the plasma meniscus and the related aber-rations in the extraction region give rise to increased emittance.With increased RF power, plasma density increases to optimumlevel in such a way as to offer better matching conditions for theextraction and which also results in lower emittance.

Emittance measurements were also carried out on 1.2 mA, 4 keVion beam from hydrogen plasma. Optimum normalized emittancewas found to be at 150 W of RF power and it is 0.004 mm-mrad.Fig. 4 shows the variation of normalized rms emittance of ion beamfrom hydrogen plasma with RF power. Error in the measurement for

Fig. 4. Variation of normalized rms emittance of 4 keV ion beam from hydrogen pla±0.0005 mm mrad.

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normalized rms emittance is ±0.0005 mm mard. It is slightlydifficult to initiate the hydrogen plasma as compared to the plasmaof heavier gases. The operating parameters of hydrogen plasmawere slightly different as compared to argon plasma. Work is inprogress to measure the proton fraction and molecular hydrogenfraction in the total extracted beam.

The emittance measurements have been carried out on ionbeam from other gases like neon, xenon etc. and have been foundto be in the range of 0.004–0.02 mm-mrad. The ion source has beenunder use for FIB applications for over 1000 h and there is no dete-rioration in the performance. However a very thin metal depositionis taking place on the inside of the quartz tube. This is due to thesputtering of plasma electrode by ions in the plasma that are

sma with RF power. Error in the measurement for normalized rms emittance is

pact high current low emittance RF ion source, Nucl. Instr. Meth. B (2013),

4 R. Menon, P.Y. Nabhiraj / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx

influenced by the large voltages existing in across the antenna.Once the gas pressure in the plasma chamber is stabilized, the ex-tracted current remains stable within 2% over 8–10 h. It is expectedto be similarly stable over longer periods.

4. Conclusion

The compact RF ICP ion source is capable of producing high cur-rent, low emittance ion beams of both heavy and light elements.The extracted ion current has just doubled by reducing the plasmachamber volume by a factor of two thereby making it feasible tooperate the ion source at much lower RF power and at the sametime producing the high current ion beam of better quality as com-pared to that of magnetized plasma based ion sources and ECR ionsources. Since there are no magnetic fields utilized in the construc-tion of ion source, the contribution of the magnetic field to theemittance is completely eliminated. The normalized emittancefor argon (0.6 mA) and hydrogen beam (1.2 mA) are measured to

Please cite this article in press as: R. Menon, P.Y. Nabhiraj, Development of a comhttp://dx.doi.org/10.1016/j.nimb.2013.07.057

be 0.0075 and 0.004 mm mrad respectively. Low emittance andhigh current capability, simplicity, compactness and the lowerpower consumption are the very important characteristics of thision source that make it suitable for large number of applicationsincluding the applications needing the ion source to be floated athigh voltages.

References

[1] P.Y. Nabhiraj, R. Menon, R.K. Bhandari, J. Vac. Sci. Tehcnol. B 29 (2011) 051604-1–051604-6.

[2] Ian G. Brown, The Physics and Technology of Ion Sources, second ed., WILEY-VCH Verlag GmgH &Co, Weinheim, 2004.

[3] J.R. Coupland, T.S. Green, D.P. Hammond, A.C. Riviere, Rev. Sci. Instrum. 44(1973) 1258–1270.

[4] P.Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, R.K. Bhandari, Vacuum 85(2010) 344–348.

[5] D.W.O. Heddle, Electrostatic Lens Systems, second ed., Institute of PhysicsPublishing, Bristol, 2000.

[6] P.Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, R.K. Bhandari, Nucl. Instr.Meth. A 621 (2010) 57–61.

pact high current low emittance RF ion source, Nucl. Instr. Meth. B (2013),