Nuclear Instruments and Methods in Physics Research B 268 (2010) 2808–2814

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

Ge and Ti post-ion acceleration from laser ion source

L. Torrisi a,b,*, L. Giuffrida a,b, M. Rosinski c, C. Schallhorn d

a INFN-LNS di Catania, Via S. Sofia 62, 95123 Catania, Italyb Dipartimento di Fisica, Università di Messina, Ctr. Papardo 31, 98166 S. Agata, Messina, Italyc Institute of Plasma Physics and Laser Microfusion, 23 Hery Str. 01-497 Warsaw, Polandd Department of Physics, University of California, Portola Plaza 430, 90095 Los Angeles, CA, United States

a r t i c l e i n f o

Article history:Received 18 March 2010Received in revised form 8 June 2010Available online 19 June 2010

Keywords:Laser–plasmaIon post-accelerationIon energy distributionsTOF technique

0168-583X/$ - see front matter � 2010 Elsevier B.V.doi:10.1016/j.nimb.2010.06.020

* Corresponding author at: INFN-LNS di Catania, VItaly. Tel.: +39 090 6765052; fax: +39 090 395004.

E-mail addresses: Lorenzo.Torrisi@unime.it, torrisi

a b s t r a c t

Laser ion sources (LIS) are employed with success to generate, in vacuum, Ge and Ti ion beams with highcurrent, ion energy, charge states and directivity.

Nanoseconds infrared laser pulses, with intensities of the order of 1010 W/cm2, induce high ablation inGe and Ti targets. Ions are produced in vacuum with energy distribution following the Coulomb–Boltz-mann-shifted distribution and they are ejected mainly along the normal to the target surface. The freeion expansion process occurs in a constant-potential chamber placed at 30 kV positive voltage. An electricfield of 5 kV/cm was used to accelerate the ions emitted from the plasma at INFN-LNS laser facility. Time-of-flight technique is employed to measure the mean ion energies of the post-accelerated particles. Ioncharge states and energy distributions were measured through an ion energy spectrometer.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction At laser intensities on the order of 1010 W/cm2, the ns laser

Nanosecond laser irradiating solid targets and operating withone joule per pulse may accelerate ions at energies on the orderof 0.1 keV/amu [1]. An inexpensive laser can be proposed to gener-ate ions from plasma with high current, energy and charge state.Moreover, the ion energy can be increased using a suitable post-ion acceleration system applied to the ions emitted from the laserirradiated target. In order to couple the laser ion source (LIS) withthe post-acceleration system a special ion extractor system mustbe used.

The post-accelerated ions are characterized by multi-energeticenergy distributions that depend on the applied high voltage andon the number of intrinsic charge states. One advantage of themultiplicity of energy states in the beam concerns its applicabilityin the field of ion implantation. Ions can be implanted at variousdepths, modifying the chemical and physical properties of the sur-face layers of different materials [2]. Other advantages of post-ionacceleration method reside in the production of a large number ofions per laser shot. Particularly, it is possible to use high repetitionrate lasers to generate any type of ion and to obtain greater chargestates with high ion directivity along the normal to the target sur-face, with the goal of becoming available in an economical andcompact accelerator system. The main disadvantages concern thelimitation of the maximum ion energy, generally below 1 MeV,and the multi-energetic nature of the extracted ion beams.

All rights reserved.

ia S. Sofia 62, 95123 Catania,

@lns.infn.it (L. Torrisi).

pulse–matter interaction produces, in vacuum, a very fast sublima-tion process and ionization of the vapor with an adiabatic gasexpansion at supersonic velocity. Many phenomena are responsi-ble for the plasma generation and time–space plasma evolution,which depends strongly on the laser parameters as well as targetproperties and irradiation geometry. At these intensities, the plas-ma core temperature is generally higher than 10 eV and the plasmadensity, at the end of the laser-matter interaction, is of the order of1017/cm3. This density value is measurable at about 1 mm distancefrom the target surface and at 1 ls delay from the laser shot [3].Temperature and density of the plasma decrease exponentiallywith the distance from the target and time after the laser shot,reaching negligible values at distances of several centimeters andafter times of a few hundred ns.

The non-equilibrium processes developed in the plasma areresponsible of the not-uniform spatial charge distribution, duemainly to the relatively high electron mobility and to the slowerion emission, which produces a large electric field in front of thetarget surface. Indirect measurements of this electric field featurevalues higher than MeV/cm and high directivity along the normalto the target surface [4]. In order to increase the energy of the ex-tracted ions, a positive voltage may be applied to the laser irradi-ated target. To avoid arcs when the extraction voltage is applied,it is convenient to produce a plasma expansion in a constant-potential chamber and to extract the ions through a ‘‘nose” byapplying an electric field to a suitable geometry of electrodes,which permits the ion extraction and acceleration.

The laser repetition rate, generally of the order of tens Hz, per-mits to extract a near constant ion beam from the plasma, but

L. Torrisi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2808–2814 2809

really it is a pulsed beam and it is not comparable with the con-stant current extractable from conventional radiofrequency ionsources.

In this work, some results on Ge and Ti ion plasma production,extraction and acceleration by use of the INFN-LNS LIS ion sourceare presented. The ion beam diagnostics are based on time-of-flight (TOF) analysis techniques.

2. Experimental

A Nd:Yag laser operating at 1064 nm fundamental wavelength,9 ns pulse duration and 0.9 J maximum pulse energy, is employedto irradiate Ge and Ti targets in an expansion chamber placed in-side a large vacuum chamber at 10�6 mbar pressure. The incidenceangle of the laser beam is 30� and the ion diagnostics are placedalong the normal to the target surface.

Fig. 1 shows some designs and photos of the experimental set-up. The laser ablated ions are generated inside a plasma expansionchamber, with a parallelepiped shape, 26 cm long and 11 cm �11 cm square base, and with the longitudinal axes orthogonal tothe target surface (Fig. 1a).

A lateral input hole permits the laser to enter and hit the targetplaced at the center of the parallelepiped chamber. Expansionchamber and target are insulated from the ground, are electricallytogether connected and both can be placed at an electrical poten-tial ranging between 0 kV and 30 kV though a high voltage powersupply (Heinzinger, 30 kV–200 mA).

The ion extraction occurs along the longitudinal chamber axisthough 12 mm circular aperture in front of the target at 15 cm dis-

Fig. 1. Scheme and dimensions of the extraction chamber (a), of the accelerating discs ansetup apparatus (d).

tance from it. After the extraction exit, 12 metallic discs (2 mmthickness, 10 cm diameter and 8 mm central hole) are aligned par-allel to the central axes and held to a relative distance of 5 mm onefrom the other, as in the photo of Fig. 1b. The discs are connected tothe high voltage (HV) and to the ground through a resistive circuitby use of different resistors at 1 MX each, as reported in thescheme of Fig. 1c.

The 12th disc, the last one, is connected directly to the ground.The maximum extraction–acceleration voltage is +30 kV; it is ap-plied to the extraction system (target and extraction chamber)and the total distance chamber extraction hole–ground was60 mm. The corresponding maximum electric field is 5 kV/cmand the field applied between two proximal discs is 2.5 kV over5 mm. Fig. 1d shows a photo of the whole setup used in this exper-iment with the opened vacuum chamber.

The discs permit the accelerating electric field to be maintainedat an uniform value in the acceleration region, in agreement withthe simulations carried out with the Opera 3D code [5].

The large and intense ion emission is largely stopped inside theextraction chamber, due to the interaction with the chamber wallsand due to the strong limitation of the extraction aperture (about50 mm2). Simulation code has demonstrated that only about 10%of the total emitted ions from the target go through the extractioncollimator hole and reach the final grounded disc [6].

The target holder can be moved externally both in vertical andin rotation in order to adjust the spot position and the incidenceangle. The Ge and Ti targets consist in pure sheets, 2 cm � 2 cmsurface and 2 mm thickness.

Generally the laser pulse energy is settled to low values, be-tween 50 and 500 mJ, corresponding to a laser fluence ranging be-

d their electric connections (c) and photo of the extraction chamber (b) and of the all

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tween 5 and 50 J/cm2 and laser intensity between 5.6 � 108 and5.6 � 109 W/cm2. Such low intensities are used in order to reducethe number of ions emitted from the plasma, so that the arcs canbe minimized and the HV power supply can efficiently work witha low flow current. The arcs, in fact, short-circuit the target-expan-sion chamber to ground and the accelerating voltage value may lit-tle decrease, thus provoking a power supply extra current.

At the incidence angle of 30�, the laser spot on the target is1 mm2, and the maximum pulse intensity is generally maintainedbelow 5.6 � 109 W/cm2.

A ring ion collector (ICR), an annular cup with an inner holediameter of 15 mm and an external diameter of 25 mm, with a sur-face of about 314 mm2, is placed along the normal to the targetsurface (the direction of maximum ion acceleration) at a target dis-tance of 100 cm. TOF technique is employed by using as a start sig-nal a trigger photodiode induced by the laser shot, and as a stopsignal a signal from the ICR ions detectors. Signals are recordedon a fast Tektronics storage oscilloscope with 500 MHz frequency,2 Gs/s sampling velocity and 50 X input resistance.

An electrostatic ion energy analyzer (IEA) is used to measurethe energy-to-charge ratio of the detected particles emitted fromthe plasma along the normal to the target surface. IEA spectrome-ter electro-statically deflects the ions by 90�, so that they aredetected by a windowless electron multiplier (WEM). Thetarget-first WEM dynode is 160 cm from the target, and TOF tech-nique is employed for the ion energy measurements. Ion chargestates and ion energy distributions were investigated with IEA,which permits to select the energy-to-charge ratio, E/z, by

Fig. 2. Optical microscope image of a typical Ge crater (a), corresponding crater profile

fixing the deflection bias of the deflecting plates, as reported inRef. [7].

Because the E/z ratio is proportional to the ion deflection biaspotential, ±U, by changing the deflection bias voltage, different en-ergy-to-charge ratios can be selected. Thus IEA is an ion filter anddetects the ions in terms of ion energy and charge states. The plotof the ion yield vs. energy and vs. charge state, permits to evincethat ions follow a Boltzmann energy distribution, which is shiftedtowards high energy increasing their charge state.

A surface profiler (Tencor P-10), with 1 nm and 1 lm depth andlateral resolution, respectively, is employed in order to evaluatethe crater volume produced by a known amount of laser pulsesas a function of the laser pulse fluence. The measurement of thecrater volume permits a precise calculation of the ablation yieldin terms of removed atoms per single laser shot.

3. Results

Starting from the crater volume measurements performed withthe high precision surface profiler, the ablation yields of Ge and Titargets were measured, in terms of removed atoms per pulse, as afunction of the laser pulse energy. The acquired data are shown inthe plot of Fig. 2c. The experimental ablation thresholds, belowwhich no ablation occurs, were 1.05 J/cm2 for Ge and 0.65 J/cm2

for Ti, in good agreement with the theoretical previsions of0.86 J/cm2 and 0.5 J/cm2, respectively. These theoretical valueswere obtained taking into account the Ti and Ge atomization latent

generated by 10 laser shots (b) and ablation yield of Ge and Ti vs. laser fluence (c).

L. Torrisi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2808–2814 2811

heat, the thermal conductivity, the density and the specific heat, asreported in [8].

At the fluence of 34 J/cm2, the measured ablation yield was1.6 � 1015 ats/pulse for Ge and 3.4 � 1015 ats/pulse for Ti. Thesedata were collected from investigations performed with opticalmicroscopy and surface profiling of the craters produced duringthe laser ablation. As an example Fig. 2a shows a photo of a craterin Ge, due to 10 laser shots at 340 mJ, and Fig. 2b the relative depthprofile which permits to calculate the ablated volume. Measure-ments of angular distributions indicated that the ion emission oc-curs mainly along the normal to the target surface and its apertureis about ±30� both for Ge and Ti targets, in agreement with previ-ous works [6].

Fig. 3 shows the ICR-TOF spectra for Ge and Ti ion detectionwithout post-acceleration voltage (HV = 0 kV) (a and b) and withthe post-acceleration voltage application (HV = 30 kV) (c and d).When the extraction–acceleration voltage increases, the ICR ionpeaks shift towards lower TOF times, indicating that ions are suc-cessfully accelerated by the extraction electric field. In particular,Fig. 3a and b represents the detected integral ions coming directlyfrom the plasma producing a TOF peak time at about 22 ls and20 ls for Ge and Ti ion species, respectively. The maximum TOFspectrum peak corresponds to an ion velocity of about 4.5 �104 m/s for Ge and 5 � 104 m/s for Ti ions. Such velocities indicatea mean kinetic energy of about 765 eV for Ge and 623 eV for Tiions. Assuming the target-first acceleration disc to be 16 cm, dur-ing which the ions are not substantially accelerated by the externalelectric field, at these energies the Ge and Ti ions spend about3.6 ls (drift time) and 3.2 ls, respectively, to reach the effectiveacceleration zone from the target surface.

For comparison, Fig. 3c and d reports the spectrum obtained byusing a voltage acceleration of 30 kV. In this case, the TOF peak isdetected at about 7.5 ls and 6 ls for Ge and Ti, respectively. Byconsidering the maximum peak position and by subtracting thedrift time, i.e. by considering the real extraction–ICR distance of

Fig. 3. TOF-ICR spectra comparison, at 100 cm distance from the target, for Ge and Ti ionand d).

about 84 cm, it is possible to calculate the new ion velocity dueto the post-ion acceleration system. The corresponding averageion energy of about 18 keV and 22 keV is calculable for Ge and Tiions, respectively. The distributions show a long tail indicatingthe presence of many ions at lower velocity and a sharp front indi-cating the presence of faster ions.

The evaluation of the initial position of the ion spectra is notsimple because of the intense background signals due to photonsand electrons hitting the collector. The ion energy correspondingto the maximum half height of the front TOF ion peak is about90 keV both for Ge and Ti ions, a value corresponding to the30 kV ion acceleration applied to three times ionized atomicspecies.

This last result, indicating the possible presence of three chargestate of atomic ionization, finds confirmation in the IEA measure-ments at the incident laser fluence of 34 J/cm2 hitting Ge and Titargets. A fourth charge state is also present both for Ge and Tiatoms but its contribution is negligible with respect to the othercharge states.

Fig. 4 shows two typical Ge-IEA spectra acquired for ±30 V (a)and ±45 V (b) deflection bias corresponding to an E/z ratio filterof 600 eV and 900 eV, respectively.

The ion energy distributions, obtained by varying the IEAdeflection bias, indicate that the ions follow a Boltzmann distribu-tion but it is different for each charge state three charge states andit follows a ‘‘Coulomb–Boltzmann-shifted” distribution, as re-ported in the literature [9]. The mean ion energy is about 400 eV,800 eV and 1200 eV for Ge1+, Ge2+ and Ge3+, respectively, as re-ported in the energy distributions of Fig. 5a. Similar ion energy dis-tributions are measured for Ti ions. In this case the mean energy isabout 360 eV, 720 eV and 1080 eV for Ti1+, Ti2+ and Ti3+, respec-tively, as reported in the energy distributions of Fig. 5b.

Here the points represent the measurements obtained by vary-ing the ion deflection bias, i.e. the E/z ratio. The lines refer to thefits made by using the Coulomb–Boltzmann-shifted function:

s without acceleration voltage (a and b) and with 30 kV post-acceleration voltage (c

Fig. 4. TOF-IEA spectra at 160 cm distance from the Ge target, at 34 J/cm2 laserfluence, by using a E/z ratio of 600 eV (a) and 900 eV (b).

Fig. 5. Experimental ion energy distributions of the three Ge and Ti ion species(points) without post-acceleration and data fits with the Coulomb–Boltzmann-shifted function (lines).

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f ðEÞ ¼ Affiffiffiffiffiffiffi2mp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

ðpkTÞ3

sEexp �1

T

ffiffiffiEp�

ffiffiffiffiffiEk

p�

ffiffiffiffiffiEC

p� �2� �

ð1Þ

Here m is the Ge or the Ti atomic mass, k the Boltzmann constant, Tthe temperature, E the total kinetic ion energy, Ek the adiabatic ionenergy and EC the Coulomb ion energy.

The Ge Boltzmann distributions are in agreement with anequivalent plasma temperature of about 40 eV and a plasma intrin-sic ion acceleration of about 400 V, which is responsible of the en-ergy shift towards high energy increasing the charge state. At thistemperature, obtained by the experimental data fit, as presented inprevious articles [10], the mean electron energy should be:

Ee ¼32

kT ¼ 60 eV ð2Þ

This value is in agreement with the possible number of generatedcharge states, on the base of the ionization potentials of Ge atoms,which is 46 eV for the fourth ionization and 94 eV for the fifth ion-ization. Moreover, the Ge2+/Ge1+ ion yield ratio is about 0.75 whilethe Ge3+/Ge1+ is about 0.35. These values are in good agreementwith the Ge electron ionization cross-section ratios calculable fromliterature by using the Lotz approach [10].

The Ti Boltzmann distributions are in agreement with an equiv-alent plasma temperature of about 33 eV and a plasma intrinsic ionacceleration of about 360 V, which is responsible of the energyshift towards high energy increasing the charge state. At this tem-perature the mean electron energy should be:

Ee ¼32

kT ¼ 50 eV ð3Þ

This value is in agreement with the possible number of generatedcharge states, on the base of the ionization potentials of Ti atoms,which is 43 eV for the fourth ionization and 99 eV for the fifth ion-ization. Moreover, the Ti2+/Ti1+ ion yield ratio is about 0.66 while

Fig. 6. Typical TOF-IEA spectra obtained at three different E/z ratios for Ge+ (a), Ge2+

(b) and Ge3+(c).

L. Torrisi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2808–2814 2813

the Ti3+/Ti1+ is about 0.4. These values are in good agreement withthe Ti electron ionization cross-section ratios calculable from liter-ature by using the Lotz approach [11].

By increasing the laser fluence a simultaneous increase of theablation yield, of the maximum charge state, of the mean ion en-ergy and of the plasma ion temperature occurs.

The analysis of the ICR spectra without extraction voltage(Fig. 3a and b) gives a signal of about 40 mV � 40 ls duration forGe and 40 mV � 20 ls duration for Ti, corresponding to the emis-sion of a charge state of about 32 nC/pulse for Ge and 16 nC/pulsefor Ti. By using 30 kV acceleration extraction, these values decreaseto about 10 nC/pulse and 6 nC/pulse for Ge and Ti accelerated ions,respectively (see Fig. 3c and d).

By using the 30 kV acceleration voltage, the IEA ion energy dis-tributions of the post-accelerated ions have been acquired bychanging the E/z ratios around the mean energy of 30 keV,60 keV and 90 keV for the three charge states of the two ion spe-cies. Three typical IEA spectra, obtained analyzing the post-acceler-ated Ge ions, are reported in Fig. 6.

The comparison of these last spectra with those reported inFig. 4 demonstrates that a peak shift towards low TOF values withincreasing the charge state occurs. However, in order to calculatecorrectly the ion energy from the IEA spectra, a special attentionshould be given to the ion path. The ion energy of the IEA peaksmust be measured taking in account the drift time of the ion inthe iso-potential extraction chamber (16 cm length) and by con-

Fig. 7. Ion energy distributions of the Ge and Ti ions post-accelerated by 30 kVvoltage.

sidering the real flight length relative to the species extractedfrom the chamber and arriving to the WEM (144 cm netdistance).

With the drift time correction, the IEA-TOF spectra indicate thatthe detected ions have energies very near to 30 keV, 60 keV and90 keV. The peaks of Fig. 6, for example, are in good agreementwith the energies of 29 keV (a), 59 keV (b) and 88 keV for the ionsGe1+, Ge2+ and Ge3+, respectively. No ions at different energies weredetected.

Fig. 7 shows the experimental data obtained for 30 kV post-acceleration of Ge (a) and Ti (b) ions. As expected each charge statehas a different energy distribution which is shifted towards higherenergy proportionally to the charge state and with a yield propor-tional to the ionization cross-section.

The experimental post-accelerated ion energy distributions ap-pear very narrow with a full width half maximum (FWHM) ofabout 0.8 keV, 1.7 keV and 2.3 keV, for Ge1+, Ge2+ and Ge3+, corre-sponding to an energy resolution DE/E of 2.7%, 2.8% and 2.6%,respectively. In the case of Ti post-acceleration the distributionFWHM is 4.5 keV, 3 keV and 2.5 keV, for Ti1+, Ti2+ and Ti3+, corre-sponding to an energy resolution of 15%, 5% and 2.8%, respectively.

These values of the energy straggling are due mainly to thepower supply instability induced by the high current spikes duringplasma pulse formation and/or to the laser pulse energyreproducibility.

Preliminary measurements of post-ion acceleration and ionimplantation of silicon and polyethylene substrates have been per-formed using 10 Hz repetition rate pulses. The extraction hole of8 mm in diameter, and the high beam directivity produces an ionbeam spot on the substrate of about 70 mm2 at 1 m distance fromthe target, as measured from the marked area on the polyethylenesubstrate. In these conditions, 2 h laser irradiation of Ge or Ti targetproduces a multi-energetic ion implantation dose of about1015 ions/cm2.

4. Discussion and conclusions

The post-ion acceleration from laser generated plasma repre-sents an innovative and useful approach in the acceleration tech-nologies to build new compact ion accelerators with highcurrents and energies. Such simple, table-top and economical lowenergy ion accelerators are demonstrating high applicationpotentiality, especially in the field of the ion implantationprocesses.

Measurements on the ion energy distributions after 30 kVacceleration show that single charge states are mono-energeticwith an energy spread of about 3–5%. However, the total emittedbeam is multi-energetic due the number of charge statescontained. A magnetic separation should be employed after thepost-acceleration stage in order to select a mono-energetic ionbeam.

Although the ion beam is composed of ions at different chargestates and energies, these multi-energetic ion beams can be usefulin many different applications. The multi-energy ion beams of Geand Ti ions here described, for example, can be implanted to mod-ify the physical and chemical surface properties of many sub-strates. The projected range of 90 keV Ge and Ti ions inpolyethylene is 111 nm and 138 nm, respectively. Because ionshave different energy in the range 0–90 keV, the multi-energeticion implantation will modify the surface layers about uniformlyin all the projected range. The ion implantation substrate modifica-tions concern the polymer carbonization, the electrical conduc-tance, the surface hardness, the chemical passivation, the surfaceroughness and wetting ability, etc., as reported in the literature[12,13].

2814 L. Torrisi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2808–2814

The interest for such compact ion implanter accelerator and forthe possibility to use multi-energy ion implantation to modify thefirst superficial layers comes from different scientific areas,such as microelectronics, chemistry, physics, engineering andbio-medicine.

In this investigation an important role is played by the extractorHV power supply geometry, which must support high electron andion currents.

In comparison with ultra-intense laser systems, using intensi-ties of the order of 1016–20 W/cm2, that can permit the ion acceler-ation up to energies of the order of tens MeV/charge-state [14], theadvantage of the proposed method is that the laser source coupledto the post-acceleration system is very economical and that highercurrents can be obtained. Ultra-intense lasers, in fact, use complexlaser apparatuses irradiating very thin films, with ion streams ofthe order of 1010 particles/laser-pulse. The main disadvantage con-sists in the low ion energy.

A work is in progress in order to minimize the current on thepower supply by using larger collimators between the positiveion extraction chamber and the final grounded collimator. Forth-coming investigations will concern the possibility to increase theion acceleration voltage and the ion current up to about 100 keV/charge-state and 1015 particles/laser-pulse, respectively. Moreoverelectro-magnetic systems will be employed to filter the ion beamand to extract a single ion type with a quasi-mono-chromatic en-ergy. Finally, the future trend will be focused on the increasing ofthe pulse repetition rate (frequency) to obtain high current ofmono-chromatic ion beams.

Acknowledgements

The authors thank the 5th National Scientific Committee ofINFN for the financial support given to the PLEIADI project, inframe of which this research has been developed.

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