Design of ion energy distributions by a broad beam ion sourceMichael Zeuner, Jürgen Meichsner, Horst Neumann, Frank Scholze, and Frieder Bigl Citation: J. Appl. Phys. 80, 611 (1996); doi: 10.1063/1.362869 View online: http://dx.doi.org/10.1063/1.362869 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v80/i2 Published by the American Institute of Physics. Related ArticlesUltrafast electron diffraction with radio-frequency compressed electron pulses Appl. Phys. Lett. 101, 081901 (2012) Compact solid-state neutral particle analyzer in current mode Rev. Sci. Instrum. 83, 10D304 (2012) Source fabrication and lifetime for Li+ ion beams extracted from alumino-silicate sources Rev. Sci. Instrum. 83, 043303 (2012) Laser ion sources for radioactive beams (invited) Rev. Sci. Instrum. 83, 02A916 (2012) Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation NeutronSource (invited) Rev. Sci. Instrum. 83, 02A732 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Design of ion energy distributions by a broad beam ion sourceMichael Zeuner and Jurgen MeichsnerTechnische Universita¨t Chemnitz-Zwickau, Institut fu¨r Physik, 09107 Chemnitz, Germany

Horst Neumann, Frank Scholze, and Frieder BiglInstitut fur Oberflachenmodifizierung (IOM), Permoser Strasse 15, 04318 Leipzig, Germany

~Received 26 October 1995; accepted for publication 2 April 1996!

We characterize the performance of a built-in hot filament broad beam ion source by massspectrometry, energy analysis, and beam profile measurements. In the ion energy distribution wedetect various peak structures which can be explained by the potential across the ion source anddifferent charge transfer processes. Depending on the typical cross sections for these processes,differences between the energy distributions of the ion species are observed. The total ion currentobtained with the source is determined by the ionization rate in the discharge and the current sharedirected toward the extraction grid system. The performance of the source is strongly dependent onthe process gas used. We observe much broader energy distributions in oxygen and nitrogen than inargon. This broadening is explained by spatial inhomogeneities in the discharge region and can bereduced by a suitable setting of the source parameters. The main contribution to the ion flux iscaused by species generated directly from the process gas, with important impurities from sourcematerials appearing only with additional chemical reactions in oxygen. ©1996 American Instituteof Physics.@S0021-8979~96!08213-8#

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I. INTRODUCTION

Broad beam ion sources are widely used in material pcessing, especially for sputter etching and reactive ion beetching,1 and a lot of work has been done in improving iosource technology through optimization and new sourdesigns.2–4

To obtain satisfactory source and process performancknowledge of the particle energy, beam composition, abeam profile is necessary. However, in spite of the greimportance of broad beam ion sources in material processlittle effort has gone into fundamental investigations of thebeam parameters.

Most beam characterization has been done by electrmethods, analyzing beam current and beam profile by Faday cup measurements,5 with some work on observing theion composition of the beam.6–8

Most surprisingly, there is almost no information avaiable on measurements of ion energy distributions. Sostudies have been carried out on electron cyclotron resona~ECR! downstream plasma sources,9–12 but for broad beamsources only one rough measurement13 has been published todate.

For this reason we investigated the energy distributioproduced by a hot filament broad beam source. We wpresent mass resolved ion energy distributions overwhole energy range reflecting the potential conditions insthe source. An excellent intensity dynamics enables usobserve a various collision processes of the ions.

II. EXPERIMENTAL APPARATUS

An ultrahigh vacuum~UHV! stainless steel chambe400 mm in diameter and in height is used for our investigtions ~Fig. 1!. The chamber is evacuated by a 250l /s tur-bopump to a pressure of 1026 Pa. The process pressure obetween 0.02 and 0.2 Pa is set by means of the gas flowthe ion source. Mass flow controllers are used to select

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flow of the process gases argon, oxygen, nitrogen, andlium with a purity of 99.99%. The pressure is measuredthe process chamber by a hot filament ionization gauge. Tgas flow conductance of the ion source grids is sufficienlarge for the pressure inside the source not to differ signicantly from the value outside.

The experimental setup consists of a built-in hot filameion source ~Kaufman type! mounted at the chamber topflange. Different beam diagnostics setups can be placed adistance of 100 mm in front of the ion source.

A circular, water cooled target, 90 mm in diameter, alows the detection of the ion energy distribution in the beacenter. Ions passing a central 100mm aperture can be ana-lyzed according to their energy and mass. A larger circultarget contains a 16 channel Faraday cup setup on a raline which enables the measurement of the radial curredensity profile. Additional information on the generated iobeam can be achieved by a movable Langmuir probe anquartz glass fiber coupling to a monochromater~SPEX 750!for the analysis of the optical emission~Fig. 1!.

A. Broad beam ion source

The broad beam ion source ISQ 40~Fig. 2! used for thepresent investigations is the smallest source developed by‘‘Institut fu r Oberflachenmodifizierung Leipzig.’’ Thesebroad beam ion sources are produced in a modular concwith hot filament or ECR excitation, respectively, and beadiameters between 3 and 15 cm. Different types of planand spherical grid systems can be fitted at the sources. TECR sources are also available as downstream types withextraction grids.

The ISQ 40 built-in source is directly mounted at the C100 flange which contains all electrical power and gas flufeedthroughs. The source is housed in a cylindrical stainlesteel body 110 mm in length and fitted with permanent manets 95 mm in diameter~Fig. 2!. The tungsten–rhenium alloyfilaments~d5200mm! are mounted on the cup at the rear o

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the source and can easily be changed. The inner discchamber is made from a stainless steel tube 40 mm in deter and 400 mm in length. The ring anode is made fhigh purity graphite with an inner diameter of 30 mm.front of this system two planar grids of high purity graphare responsible for the extraction and formation of thebeam. The screen grid with a thickness of 0.5 mm andaccelerator grid with a thickness of 1 mm are mounted 1apart and contain on a 40 mm diameter, 1 mm holes inmm grid. The source is fitted on the outside withaluminum–nickel–cobalt~ALNICO! magnets which geneate a magnetic field of about 150 G in the anode region

Different types of beam neutralizers can be mountethe source. For some measurements, we used a hot filaneutralizer, but for most of the present experiments no

FIG. 1. Schematic diagram of the experimental apparatus.

FIG. 2. Hot filament broad beam ion source ISQ 40 with power supp

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tralizer was fitted to exclude any beam disturbance. No cooing is used for the source and during continuous operatithe temperature of the source housing can reach up500 °C.

The ion source is driven by an Iontech MPS 3000 powsupply ~Fig. 2!. The so-called beam supply (0 V<Ub

<1200 V, I b<300 mA), generating the potential of thering anode, dominates the energy of the ion beam.

The potential of the filament is lower than the beamvoltage by the discharge voltage (0 V<Ud<150 V,I d<6 A). The source always starts up with the maximumdischarge voltage which is reduced after ignition to the prset value. By choosing the filament current the resulting dicharge current is fixed. The filament itself is heated by anpower supply (I f<15 A).

The potential of the inner screen grid and the inner walof the discharge volume are floated via an 8 kV resistor fromthe beam voltage, an arrangement which eases the ignitprocess. Initially, emitted electrons flow to the neighborhoosource walls. When the discharge starts up and the wall ptential decreases, the electrons move to the anode.

The accelerator power supply generates a negative voage with respect to ground (21200 V<Ua<0 V,I a<100 mA). The potential difference between the beaand the accelerator grid together with the geometrical dimesions define the ion optical parameters of the source. Ttotal ion current emitted by the source can be determinfrom the beam currentI b reduced by the current flowing tothe accelerator gridI a .

B. Beam diagnostics

Incident ions are analyzed with a commercial mass spetrometer system EQP 300~Hiden Analytical!, samplingthrough a 100mm aperture inside the stainless steel targeThe target and mass spectrometer aperture plate are elecally insulated from ground. The total current to the targecan therefore be measured, or the target can be biased.

The mass spectrometer is differentially pumped by240l /s turbopump down to 1028 Pa, and the pressure duringthe process does not exceed 1025 Pa.

Using the computer controlled spectrometer, mass, eergy, and time resolved spectra of neutral particles, positiand negative ions, and radicals from the process mayobtained. Ion optics in front of the probe are used for tranmitting the ions passing through the aperture. The ion opticause an energy-dependent acceptance angle, which weculated by means of a computer program solving Laplaceequation. The acceptance angle decreases with energy,with the voltage settings used it is 2.3° at 10 eV, 1.0° a100 eV, and 0.7° at 1000 eV. So the measured ion enerdistributions represent only those ions which are incidenearly vertically. This means for our setup that the ions prduced in the beam center are much more represented tthose generated at the side of the ion source.

Following the ion optics is an electron impact sourcused for ionization of neutrals and radicals, with the electroenergy adjustable between 5 and 100 eV. An energy analyis performed by an electrostatic sector field analyzer, whiccan be driven up to 1000 eV with a standard resolution of 0.

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FIG. 3. ~a! Ion energy distribution of Ar1 ~Ub5300 V,Ua52400 V,Ud560 V, I d50.5 A, p50.13 Pa!. ~b! Ion energy distribution of Ar21 ~Ub5300 V,Ua52400 V,Ud560 V, I d50.5 A, p50.13 Pa!.

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eV. This analysis takes place at a constant pass enethrough the analyzer, and an energy scan is performedshifting the reference potential of the spectrometer, so thstable ions are accelerated or decelerated to the pass enbefore entering the analyzer. Following this, a triple quadrpole mass filter separates the ions according to their mascharge ratio up to a maximum of 300 amu, and finallychanneltron secondary electron multiplier with a dynamrange of 7 decades is used for ion detection. The countelectronics can be gated externally.

For the detection of the current profiles across the bethe mass spectrometer arrangement can be replaced bmultichannel Faraday cup setup. Within a 200 mm circultarget 16 Faraday cups are arranged on a radial line. TFaraday cups sample through 3.5 mm holes and are arran12 mm away from each other. With the electronics usecurrent densities between 0.01 and 10 mA/cm2 can be mea-sured. The total current determined from the integral of tradial density profile differs by not more than 10% from thtotal current emitted from the source, even at the highestenergies.

III. RESULTS AND DISCUSSION

A. Structures in the ion energy distributions obtainedfrom a hot filament broad beam source

Figures 3~a! and 3~b! show typical ion energy distribu-

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tions for Ar1 and Ar21using argon as the process gas. Thenergy distributions show significant peak structure, mostit with low intensity, which can be explained by the potentiadistribution across the ion source and process chamberdifferent charge exchange collision processes.

The origin of the peaks labeled with capital letters iFigs. 3~a! and 3~b! is explained below, with the corresponding potential structure shown in Fig. 4. The final ion detecteat the target is always marked in bold.

~A! The peak is caused by ions which are generated bcharge exchange collision outside the source in tprocess chamber between a primary ion and a netral. For Ar1 it is a simple transfer: Ar1~300 eV!1Ar→Ar~;300 eV!1Ar1. For the doubly chargedion the most likely process is Ar1~300 eV!1Ar→Ar~;300 eV!1e21Ar 21. The position ofthe peak is determined by the space charge potenUsp and the number of chargesz ~see Sec. III B 1!:EA5zeUsp.

~B! This peak appears only in the Ar21 ion energy dis-tribution and is related to a collision process outsidthe source generating an energetic doubly chargion from a singly charged one: Ar1~300

eV!1M→Ar 21~;300 eV!1e21M. The energy isdetermined by the beam voltageUb : EB5eUb .

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~C! The multiple peak structure at the low energy sidethe main peak D consists of three different peakThe clear peak near the letter C is due to ionizatioor charge transfer at the screen grid. The two othless clear peaks on the low energy side of the mapeak D and of peak C are caused by a radially inhmogeneous discharge structure and are discuslater in Sec. III B 3. The energy of peak C is roughldetermined by beam voltageUb and discharge volt-ageUd : EC5ze„Ub2Ud….

~D! The main peak results from single or multiple ionization in the anode region: Ar1e2

→Ar z11(z11)e2 . The energy is given by the ionchargez and beam voltageUb :ED5zeUb .

~E! This peak appears in the Ar1 distribution only and isdue to charge transfer of a doubly charged ion ouside the source generating a singly charged ioAr21~600 eV!1M→Ar1~;600 eV!1M1. The en-ergy is determined by the beam voltageUb :EE52eUb .

~F! This structure with a long low energy tail overlaywith peak E. It is seen only in the Ar1 energy distri-bution and is caused by a double charge transfer. Tfirst transfer process happens at the accelerator gproducing a fast neutral which is able to escape frothe negative accelerator potential: Ar1~700 eV!1Ar→Ar~;700 eV!1Ar1. Because this processcould happen across the whole accelerator potenwell the low energy tail is created. The second tranfer occurs outside the source and generates a higenergetic ion again: Ar~;700 eV!1Ar→Ar1~;700eV!1e21Ar. The maximum energy is fixed by thetotal potential difference between beamUb and ac-celerator voltageUa :EF5e„Ub1zUaz….

FIG. 4. Potential distribution across the ion source and the vacuum chber.

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~G! The peak appears only in the Ar21 energy distribu-tion. It is caused by a transfer process outside tsource, generating a doubly charged ion from a trply charged one: Ar31~900 eV!1M→Ar 21~;900eV!1M1. The energy amounts to:EG53eUb .

~H! The structure is only significant for the Ar1 energydistribution and likely to be caused by a multiplecharge transfer. The first transfer happens at zepotential within the grids: Ar21~600eV!1M→Ar1~;600 eV!1M1. The Ar1 ion is ac-celerated toward the accelerator grid, and in a seond transfer, generates an energetic neutrAr1~;1000 eV!1M→Ar~;1000 eV!1M1. Thethird transfer takes place outside the sourcAr~;1000 eV!1M→Ar1~1000 eV!1e21M. The resulting energy is:EH5e„2Ub1Ua….

The peak structure D, generated by the primary ions,found exactly at the position expected from the beam voltaUb . By contrast, most of the peaks produced by charge echange, especially peaks B, E, F, G, and H, have a positwhich is about 10–20 eV lower than expected. This fact cbe explained by an energy loss in electronic excitation14,15ora partially elastic character of the collision.

Finally, the influence of the space charge potentialfront of the source on the peak position has to be taken inaccount. For example, a doubly charged ion emitted from tsource (Ub 5 300 V, Usp5 20 V) has an energy of 560 eVoutside the source. If a process E, generating a sincharged ion happens, this ion can only accumulate 20 eVtraveling to the target and has in total only 580 eV, 20 eless than expected without space charge.

It can be seen from the above examples that the mimportant contribution to the ion beam is caused by ionwhich are produced by impact with primary electrons in thanode region. The multiply charged ions can accumulatehigher energy. Various charge exchange processes areserved which cause characteristically low intensity pestructures. Depending on the specific cross sections for dferent charge transfer processes, these low intensity strtures differ from one kind of ion to the other.

Using process gases other than argon, additional effeshifting the position of the peaks and broadening the peaoccur ~see Sec. III C 1!, but the structure and origin of thepeaks are similar.

B. Influence of the source parameters on the energydistribution, current density, and current profilein argon

Ion energy, current density, beam profile, and ion composition are the most important characteristics of an iosource. We discuss the influence of the external sourcerameters on these characteristics by means of the measion energy distributions:

~1! beam voltage in correlation to ion energy and ion curent;

~2! accelerator voltage in correlation to the beam profile;

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~3! discharge voltage, current, and pressure in correlatiothe ionization rate and ion current.

Most of the ion energy distributions were measured ahigher pressure than the values used for the Faradaymeasurements of the beam profile to show more clearlystructures caused by charge exchange in the energy distion. Because the Ar1 ion is the dominant ion in argon~seeSec. III C 2!, only the energy distribution of this ion is usefor discussion.

1. Beam voltage

Figure 5 shows the ion energy distribution of Ar1 ob-tained at different beam voltages. As can be seen, the ption of the main peak D, generated by ionization at theode, is exactly~62 eV! determined by the beam voltagPeak E, produced by charge exchange of Ar21 to Ar1, shiftswith the double beam voltage. Because of the increasingtal voltage drop between the grids, the energetic conditfor the multiple charge transfer processes F and H changwell, these peaks are moving to higher energies as explain Sec. III A.

The position of peak A, generated by charge exchaoutside the source, is determined by the space charge ptial in the beam. Decreasing energy and increasing bcurrent increase the space charge potential and shift this

FIG. 5. Ion energy distributions of Ar1 at different beam voltage~param-eter: beam voltage,Ua52400 V,Ud560 V, I d50.5 A, p50.13 Pa!.

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toward higher energy values. Figure 6 shows the peak potion at a constant low ion energy of 150 eV to be dependeon the beam current which results from different source prameter settings. At constant energy the space charge inbeam is only determined by the local current density, and fthat reason the position of peak A in Fig. 6 should be prportional to the beam current. The expected linearity is nexactly fulfilled because the beam profile and the resultispace charge change with discharge and accelerator voltas well. But nevertheless, the tendency that a higher spcharge shifts peak A toward higher energies is clearly see

Because of the asymmetric shape of the anode peakand collisional processes, the mean ion energy is alwayslow the energy related to the beam voltage. Figure 7 shothe relative ion energy which is the ratio between the meenergy determined from the measured distribution functiand the position of the maximum of the anode peak D. Tmean energy is up to 10% lower than the position of the pemaximum.

The increase of relative energy in the low energy ranin Fig. 7 is caused by a charge transfer cross section decreing with energy.16 The decrease in the upper range could ban effect caused by the acceptance angle of the ion optBecause the acceptance angle decreases at higher enhighly energetic beam ions are less represented than theenergy ions produced by charge exchange. Differencestween Ar1 and Ar21 are caused by differences in collisioncross sections.15,16

Figure 8 shows the total ion current from the IQS 4calculated from the radial profile which was measured withe multichannel Faraday cup. The total current increaswith the beam voltage and approaches a saturation vawhich is not achieved within the observed parameter rang

FIG. 6. Position of the peak A generated by charge exchange dependenthe beam current which results from different settings of discharge aaccelerator voltage.

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and the current is only slightly dependent on the accelervoltage.

An upper limit for the total current is set by the ioniztion rate in the ion source. This ionization rate is a functof source geometry, process gas density, electron curand electron energy. The share of ions extracted fromsource depends on the ratio of the ion current flowing toinner walls of the ion source, including the extraction griand the current passing the grid holes. Under given dischconditions and constant ionization rate, as in Fig. 8,

FIG. 7. Relative energy of Ar1 and Ar21 ~Ua52100 V,Ud550 V, I d50.5A, p50.03 Pa!.

FIG. 8. Total ion current from ISQ 40 measured by multichannel Faracup ~parameter: accelerator voltage,Ud550 V, I d50.5 A, p50.02 Pa!.

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changes in the total ion current can only be explained bchanges in the ratio between the current to the inner sourwalls and to the grid holes, which can be achieved bylarger axial potential gradient inside the discharge region.

In Fig. 9 we show the full width energy at half-maximum of the anode peak D. Because the resulting ioenergy is determined by the potential at the place where thionization occurs, this energy width characterizes the axiapotential gradient in the discharge. The energy width increases with the beam voltage and is less dependent onaccelerator voltage. This means that a larger axial potentigradient is present at a higher beam voltage and for threason the ion current toward the source grids is increascompared to the radial current flowing to the inner sourcwalls. This fact explains the increase in the total current measured by the Faraday cups~Fig. 8!.

An explanation for the effect cannot be derived from thenormal plasma presheath models, which describe the ion curent injected into the plasma sheath. A complete presheawill not be created under our conditions, because the dimesions of the discharge chamber are smaller at this pressuthan the expected presheath thickness.17

In this context it should be pointed out that the explanation of the total current from the Child–Langmuir equation,with the voltage taken as the total voltage between the etraction grids, and the sheath thickness taken as the grdistance,2,3 is incorrect. The equation describes the screeninof the discharge plasma against the screen grid potential acharacterizes the plasma sheath at the screen grid, but noany way an externally applied potential difference betweethe grids.

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FIG. 9. Full energy width at half-maximum of the main peak D in the Ar1

energy distribution~parameter: accelerator voltage,Ud550 V, I d50.5 A,p50.03 Pa!.

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2. Accelerator voltage

Figure 10 shows the influence of the accelerator voltaon the ion energy distribution. Only the positions of peaksand H, produced by multiple charge exchange at the acerator grid, shift with the accelerator voltage. Besides ththe low energy shoulder of peak C, characterizing ion pduction at the screen grid, is extended to lower energies

For comparison with the total current~Fig. 8!, the cur-rent density in the beam center at different beam and acerator voltages is plotted in Fig. 11. Both voltages togetaffect the ion optic characteristics of the combination ofscreen and an accelerator grid hole, producing an ion belet, and the total beam profile results from the superpositof all beamlets. The smaller current values in the beam cter at higher accelerator voltage are caused by a broadeof the beam profile.

For a sharp and homogeneous beam profile the focuthe beamlet should be located at the plasma sheath bounbut the sheath thickness as well as the shape of the sheaa screen grid hole are not known and depend on thecharge parameters.18,19We carry out numerical calculationbased on a self-consistent solution of Poison’s equation,taking space charge effects into account.20

FIG. 10. Ion energy distributions of Ar1 at different accelerator voltage~parameter: accelerator voltage,Ub5300 V,Ud560 V, I d50.5 A, p50.13Pa!.

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3. Discharge voltage, current, and pressure

Figure 12 shows the ion energy distribution of Ar1 atdifferent discharge voltages. Only peak C, generated bcharge transfer or ionization at the screen grid, moves towalower energies when the discharge voltage is increased. Teffect is caused by shifting the electron energy distributionhigher values21 and for that reason there is a higher floatingand a lower absolute potential at the screen grid. The littpeaks in the low energy shoulder of peaks C and D~seearrows in Fig. 12! shift together with the larger neighboringpeak. We believe, for that reason, that the shoulder perepresents radial inhomogeneities in the discharge. The mpeak D is caused by ions generated within the ring anodand because of the small acceptance angle of the ion optof the mass spectrometer, this peak dominates the enedistribution. The small shoulder peak could be produced bions generated near the inner discharge wall in the 15 mgap between the ring anode and screen grid~see Fig. 2!.Carrier density and space charge should be lower at this psition, and because of the acceptance angle, this peak strture is less represented in the energy distribution. Peak Ccreated by charge exchange at the screen grid, and for treason the structure typical for peak D appears as wellpeak C. Because peak C is generated near the accelergrid, this structure can be influenced by the accelerator voage~see Fig. 10!.

We should be able to investigate the effect of radial inhomogeneity in more detail in the future using the smaacceptance angle of the mass spectrometer. By radially shing the spectrometer and ion source with respect to eaother or by tilting the ion source, ions created at the sidregions of the ion source should be more clearly representin the energy distribution.

FIG. 11. Current density measured in the beam center~parameter: accelera-tor voltage,Ud550 V, I d50.5 A, p50.02 Pa!.

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Because of differences in dissociation or ionization crsections, the discharge voltage can be used to changratios between different ion species created from the progas. As can be seen from Fig. 13, doubly charged ionatomic nitrogen ions are generated with a higher probabat higher discharge voltage.

The total current obtained with the source is a functof the discharge voltage. Where the electron-energy dedent ionization cross sections show a maximum, a maximis found in the total current as well. Because of the thermizing effects of the primary electrons,21 this maximum isfound at a higher discharge voltage than expected frommaximum in the cross sections.

Figure 14 shows the discharge current dependent ionergy distributions. The main effect of the discharge currenthat the peak structures narrow, and in particular the peakthe shoulder of peaks C and D tend to disappear withcreasing discharge current. This effect can be explainedthe higher current causing a higher plasma density andthat reason better radial homogeneity of the dischargeditions.

The total current emitted from the source increanearly linearly with the discharge current. A saturation tdency is only seen at very high discharge currents ab1.5 A.

FIG. 12. Ion energy distributions of Ar1 at different discharge voltage~pa-rameter: discharge voltage,Ub5400 V, Ua52200 V, I d50.5 A, p50.13Pa!.

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FIG. 13. Intensity ratio integrated over the energy distribution for differenionic species from argon and nitrogen~Ub5400 V, Ua52200 V, Ud560V, I d50.5 A, p50.13 Pa!.

FIG. 14. Ion energy distributions of Ar1 at different discharge current~pa-rameter: discharge current,Ub5400 V, Ua52200 V, Ud560 V, p50.13Pa!.

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The discharge pressure influences the source conditin the observed range between 0.02 and 0.2 Pa only vlittle, and there is almost no effect on the beam charactetics. This is understandable since each electron is abledeposit the energy accumulated in the cathode region inlisions. There is no excess energy left for ionization ofadditional number of target gas atoms or molecules.

All peaks in the ion energy distribution generatedcollisional processes increase in intensity with respectpeak D, especially peak A, characterizing the simple chaexchange outside the source. Because of the increasinglision probability, the mean ion energy decreases with prsure.

C. Performance of the ion source using differentprocess gases

1. Ion energy distribution

We compared ion energy distributions in argon, oxygenitrogen, and helium–nitrogen mixtures. The three lowcurves in Fig. 15 show the distribution function for the mointense ion in Ar, O2, and N2. There are obvious differencebetween the registered distributions, but nevertheless,peak structures and their origin can be explained as for arunder Sec. III A.

An important difference between the argon beam andtwo other gases is that the main peak D, generated by

FIG. 15. Energy distribution of the main ion in argon, oxygen, and nitrog~parameter: gas and accelerator voltageUb5300 V, Ud560 V, I d50.5 A,p50.13 Pa!.

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onseryris-tool-n

ytorgecol-es-

n,erst

theon

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ization at the anode potential, clearly appears at lower engies than expected from the beam voltage, exactly at 278in N2 and 239 eV in O2, respectively. Additionally the peakstructures show a dramatic broadening compared with argwhich is especially seen in the O2

1 energy distribution. Thefull width energy at half-maximum of the peak is 7.5 eV inAr, 30.4 eV in N2, and 127 eV in O2. On the other hand, thebroadening generates energies in the anode peak D whichmuch higher than the beam voltage, up to 200 eV abobeam voltage in the case of O2.

These observed effects may be explained by inhomogneities in the discharge potential, especially in the radial drection. Such inhomogeneities can be generated or inflenced by inhomogeneities in the magnetic field,22 but morelikely in electron current and electron energy.

With our mass spectrometer we observe mainly the iogenerated at the potential in the center of the ring anodBecause the main peak is shifted to lower energies, the ptential across the ring anode must decrease toward the cenNevertheless, we also detect, with a lower probability, iongenerated at higher potential closer to the ring anode. Fexample, at low accelerator voltage, a small peak is toseen in the high energy shoulder of the main peak of N2

1

exactly at the position of the beam potential~the upper curvein Fig. 15!.

The differences, as compared to the argon discharge,caused by differences in the ionization process, by cross stions, and probably by the presence of negative ions, escially for O2. For that reason, changes in the discharge prameters should affect the spatial discharge inhomogeneitas seen in Fig. 16. Increasing the discharge voltage ordischarge current and at the same time increasing the chacarrier density changes the shape of the resulting N2

1 energydistribution dramatically. The broad and low energy shifteanode peak caused by the inhomogeneous potential strucis resolved with increasing discharge voltage to the samsharp structures seen in the argon energy distribution, whis due to a nearly homogeneous discharge structure withthe anode diameter.

It seems that a minimum ionization rate is necessary fproper operation of the ion source. Because the ionizaticross section for nitrogen is lower at the same electron eergy than for argon, the same ionization rate is realized fN2 at a higher electron energy.

Spatial inhomogeneities of the electron energy distribtion can be derived from the energy distribution of ions witdifferent ionization potentials. In Fig. 17 energy distributionof N2

1 and He1 from a nitrogen–helium mixture are plotted.It can be seen that the main peak of the He1 distribution islocated at the position determined by the beam voltagThese helium ions with an ionization potential of 24.6 eV aronly generated by the most energetic electrons, which accmulate this energy transiting the maximum potential drobetween cathode and anode potential. Consequently, theergy of these ions produced at the place with the highespace charge potential is determined by the anode potentBecause of the small proportion of highly energetic eletrons, the helium peak is about three orders less in intensthan the N2

1 peak.

en

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n

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In contrast to He1, for the N21 ion with an ionization

potential of 15.7 eV the energy distribution shows a brolow energy structure with different peaks. These structuare due to ionization by less energetic electrons or Penionization by metastable helium in other discharge regio

2. Ionic composition of the generated ion beam

Figures 18~a!–18~c! show ion mass spectra obtainewith argon, oxygen, and nitrogen as the process gases.mass spectra were taken at the maximum energy of the bpeak D. Because the main contribution to the ion enedistribution is found in this peak, differences in the enedistribution between different ions should not affect thequalitative results significantly. For the discussion the sptra are not corrected according to the mass dependent stivity of the mass spectrometer, which decreases to abouat a mass of 300 compared to the sensitivity at low mas

It can be seen that the dominating ion is alwayssingly ionized process gas molecule. For argon@Fig. 18~a!#we detect multiply charged ions up to triply charged at 13amu, which is seen as a peak structure between othespecies contributing to the peaks at 13 and 14 amu. Accing to the ionization potentials of 88 eV for Ar31 and 258 eVfor Ar41,23 up to triply charged argon ions are expected toproduced. Multiple impact processes could play an imporrole in the generation of multiply charged ions.

FIG. 16. Influence of the discharge voltage on the N21 energy distribution

~parameter: discharge voltage,Ub5400 V, Ua52200 V, I d50.5 A,p50.13 Pa!.

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For oxygen and nitrogen@Figs. 18~b! and 18~c!# the dou-bly charged molecule is seen. At mass 16 and mass 14 tlargest contribution is caused by the ionized nitrogen anoxygen atoms, respectively, compared with the doublcharged molecule. We observe a small amount of doubcharged nitrogen and oxygen atoms at masses 8 and 7well, and a contribution from ions produced by attachmenprocesses is present, such as Ar2

1 , O31 , and N3

1 .Because of the differences in ionization and dissociatio

cross sections, the ratio between the different species creadirectly from the process gas can be dramatically affected bthe electron energy as shown in Sec. III B 3.

Near the peaks of the main isotopes40Ar, 14N, and16O,contributions of isotope peaks~38Ar, 36Ar, 18O, 17O, 15N! andcombinations of them are observed.

As important pollutants in the beam we find carbon andits oxides~CO2

1 , CO1, C21 , C1!. The oxides can be caused

by small impurities in the process gas, which supply thoxides directly, or oxygen which reacts with the graphiteparts of the source. Especially when using oxygen itself athe process gas, there is a very large contribution of thcarbon oxides, on the order of several tens of percent of thtotal current. The carbon and carbon-oxide content depenon the discharge parameters and on the ion energy.7,24

Other important pollutants in the ion beam are caused breactions with desorbing water. The water desorbs mainfrom the chamber walls and creates compounds directly witthe process gas.~ArH1, NHX

1) and with carbon and carbonoxides (CXHY

1). The pollution caused by water depends onthe pumping time since the system was vented, and espcially on chamber temperature. Because the chamber waheat up to about 60 °C over several hours during ion sourcoperation, this water desorption is difficult to control.

FIG. 17. Normalized ion energy distributions from a nitrogen–helium mix-ture ~Ub5300 V, Ua52400 V, Ud560 V, I d50.5 A, pN250.08 Pa,pHe50.05 Pa!.

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FIG. 18. Ion spectrum in argon~Eion5400 eV,Ub5400 V, Ua52200 V,Ud560 V, I d50.5 A, p50.13 Pa!. ~b! Ion spectrum in oxygen~Eion5339eV, Ub5400 V, Ua52200 V, Ud560 V, I d50.5 A, p50.13 Pa!. ~c! Ionspectrum in nitrogen~Eion5378 eV,Ub5400 V,Ua52200 V,Ud560 V,I d50.5 A, p50.13 Pa!.

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Depending on the ion energy, we find small amountsstainless steel sputtered from the target. The filament marial is only detected in the mass spectrum with oxygen asprocess gas@Fig. 18~b! subplot#, because the filament iseroded by a chemical reaction and not only by sputterinThe filament lifetime is only a quarter of an hour with oxygen, whereas more than 10 h are achieved with argonnitrogen.

IV. CONCLUSIONS

We have measured ion energy distributions and beprofiles obtained from a hot filament, broad beam ion sourThe main component of the ion energy distribution consisof ions generated by electron impact near the ring anodethe ion source. Additionally, multiple peak structures witlow intensity are observed in the energy distribution. Thecan be explained by the potential across the source and vous charge exchange processes. The multiple peak structdiffer between the ion species according to the specific clisional cross sections.

The most important ions detected in the beam are gerated by ionization, dissociation, or attachment in the pcess gas. With the exception of oxygen, contaminants inbeam are of smaller intensity. The main sources for theimpurities are sputter processes at the carbon grids and atarget, as well as reactions with desorbed water. Becaus

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the reactivity of oxygen, a high content of carbon oxides aof etched cathode material is found when using this gas.

The total ion current obtained using the source at a cstant ionization rate depends only on the ratio of the icurrent emitted from the source to the current flowing to tinner source walls. The axial potential gradient inside tsource generated by the grid potentials affects this ratio dmatically. We were able to correlate the axial potential gdient, determined from the width of the main peak in tenergy distribution, with the total current extracted from tsource.

Dramatic differences in the resultant ion energy distribtion were observed depending on the process gas anddischarge parameters used. In comparison to argon, the pin oxygen and nitrogen show a shift toward lower energand a large broadening, which is caused by radial inhomgeneities in the source discharge structure. We showed thcareful setting of the source discharge parameters enableto reduce these inhomogeneities and to achieve the ssharp structures as those obtained using argon as the progas.

Inhomogeneities in the internal potential structure in tsource were detected by using a gas admixture of nitroand helium. Spatial inhomogeneities in the potential cainhomogeneities in the electron energy with the result tions with higher ionization energy are only producedplaces with the highest potential and that, consequen

621Zeuner et al.

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h

gnh

n

n

these ions appear in the ion energy distribution at higvalues.

By utilization of the small acceptance angle of the iooptics of the mass spectrometer, we should be able toinformation on radial discharge inhomogeneities by shiftior tilting the ion source and spectrometer against each otWith these results it should be possible to construct a begeometrical source layout which reduces these inhomogeities.

ACKNOWLEDGMENT

This work was supported by the BMBF under Grant N13N6115.

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