Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992 39

Nanometric Surface Spectroscopy: Atom Probe and Time- Correlated Atom Probe

J. H. Block, J. Dirks, and W. Dracl1sel

Fritz- Haber Institut der Max- Planck- Gesellschaft D- 14195 Berlín- Dahlem

Abstraet

An extension of the atorri probe consists of a device for measuring coincident ion formation events.This new technique has beenused to investigate the dynamics of coupled surface processes. .

Theoretically predicted field stabilized He- dimers, field- adsorbed on a W- surface, were studied byusing a mixture of 3He- and 4He- isotopes. Simultaneous emissionof 3He+- and 4He+- ions was observedwithin a time window of less than 100 ps, which is a large deviation from expectations for stochastic events.

Time - correlated ion formation was also observed in the field desorption current of hydrogen from atungsten emitter. The simultaneous emission of pairs of ions, where each pair contains two of the threeusually observed ions H+, H2+, or H3+ was confirmed by auto- and cross- correlation measurements. Theobserved dependence of ion intensities on field strength and hydrogen pressure, as well as supplementaryexperiments with photon induced field desorption, indicate that the time- correlated ion formation is due toelectron- stimulated field desorption. 'Frorn cross- correlation measurements a linear structure of the H3 -species was deduced.

Resumen

Una extensión de la investigación atómica consistente de un aparato para medir eventos de formacióncoincidencial de iones se uso para estudiar la dinámica de procesos de acoplamiento superficial.

El campo estabilizado, teóricamente predicho, de dimeros de He y el campo adsorbido sobre lasuperficie de W, fueron estudiados usando una mezcla de isotopos 3He y 4He. Se observó la emisiónsimultanea de iones 3He+ y 4He+ dentro de un tiempo de ventana menor a 100 ps, 10 cual es una grandesviación con respecto a las expectativas para eventos estocásticos. .

La correlación del tiempo con la formación de iones tambien fue observada en la corriente dedesadsorción del campo desde un emisor de tungsteno. La emisión simultanea de pares de iones, cada parconteniendo dos de los tres iones usualmente observados H+, H2+, o H3+, fue confimiada por medidas deautocorre1a¿~ y correlación cruzada. La dependencia observada en la intensidad de iones sobre el campo deesfuerzos y la presión de hidrógeno, asi como experimentos suplementarios con desorción inducida defotones, indica que la correlación tiempo-formación de iones es debida a la desorpción de campo electro-estimulada. Para medidas de correlación cruzada, fue deducida una estructura lineal de las especies H3.

Introduetion

In general, field ionization (FI) and fielddesorption (FD) currents are due to statisticalionization events. Neutral particles reach theionization zone in front of the field emitter either byimpinging from the gas phase or by surfacediffusion. In either case ionization processes wouldbeindependent statistical events with a stochasticdistribution of ion intensities. There are, however,intermolecular interactions which may influenceionization probabilities. Another reaction channelfor simultaneous ion formation consists in thephenomenon of electron stimulated field desorption(ESFD). In the time-correlated atomprobe thesecoincident processes can be measured .. This will bedemonstrated with two examples, the fielddesorption of helium and the reaction of hydrogenon tungsten field emitters.

The Field Desorptíon of Helium

The phenomenon of field adsorption ofhelium below 100 K has been studied by a 'numberof investigators [1-3] and a reduced binding distancecompared to the fíeld-free physisorbed helium hasbeen found [3]. In the presence of a high electricfield, even ~ field stabilized He-dimer on a kink siteis conceivable. To test this idea, we have used thefact that the di-cation He22+ is a stable configuration.The stability of this gas phase ion was predicted in1932 by L. Pauling [4], who calculated a bindingenergy of 1.4 eV and a bonding length of 0.741 A,shorter than the distance of 0.778 A in theisoelectronic H2-molecule. The- existence of thisspecies was shown experimentally only recently[5].1n order to look for this di-cation by field ionmass spectrometry, a gas mixture of 3He and 4Hewas used. If di-cations were produced, this would

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40 Revista Latinoamericana de Metalurgia y Materiales. Vol 11. N° 1y 2. 1992

provide a detectable mass ar 3.5 amu, which wouldnot be masked by normal 4He+ field ions.

Experimental

The apparatus is an extension of the photon-induced field desorption atom probe described indetail elsewhere [6, 7] and has been modified for thenewly developed time-correlation measurements (seefig. 1). We used an electrochemically etched (110)orientated tungsten tip attached via a heating loop toa cold-finger filled with liquid nitrogen. Theresidual gas pressure was in the 10-10 mbar rangeand all gases used for the experiments were ofhighest commercially available purity. The emittersurface was cleaned by heating, Ne+-sputtering andfield evaporation. An FIM picture was taken tocheck the emitter surface before and after eachexperiment

near the surface, which passed the probe-hole wasregistrated by the detector and started a time-to-amplitude con verter (TAC).

The next incoming ion stopped the TAC.The signals were accumulated in a multi-channel-analyser (MCA) (pulse-height analyzing mode) andstored in a computer. To achieve a goodsignallnoise (SIN) ratio, the spectra were collectedfor at least 350 s. The performance of theelectronics was well calibrated and the overall timeresolution of the electronic equipment was betterthan 1 ns. The minimum time needed for the TAC todistinguish between a start and a stop ion event wasat most a few nanoseconds and was not anexperimentallimitation in this case since the shortestToF-difference expected for correlated ions of mass2 and 3 (H2+ and H3+) or mass 3 and 4 (3He+ and4He+) was calculated to be greater than 100 ns (for atip voltage of 8 kV and the flight path of 47 cm).

tip-temp.controllerIh •. .., (DUple HV-ampl.

Fig. 1: Experimental setup

The FIM micro channel plate (MCP)contained a probe-hole to pass ions to the detectorfrom a tip-area of approximately 5kink sites at theedge of the W(l10) planeo The TOF-MCP tandemdetector consisted of a chevron channel plate and aphosphorus screen, the latter differed frorn usualarrangements by being divided into two halves alongthe diameter and separated along the division by afixed physical distance of 0.1 mm. The lateralseparation between the detecting areas could be

"further enlarged by covering the central area of thedetector entrance with a metal strip 1.5 mm or 8 mmwide. Any ion created by field desorption, by fieldionization at a critica! distance Xc from the surface,or by gas-phase ionization within a probed region

Latinémerican Journal o/ Metallurgy and Materials, Vol 11, N° Jy 2. 1992

In the auto-correlation mode (ac-mode), bothstart and stop ion events originated from the samehalf of the phosphorus screen, and a cable delayprevented the TAC from being stopped by its ownstart ion signal.

For cross-correlation measurements (cc-mode), signals were taken from different halves ofthe detector, so that the cable delay. was not required.lons with identical q/m-ratio as well as neutrals withhigh kinetic energy (created by charge transferreactions in the gas phase) were detected exclusivelyin the cross-correlation mode. The ion pair rate washowever reduced in the cc-mode, since only ionpairs separated in flight vectors by a minimum angle

Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992 41

of 4 x 10-6 degrees (for theO.l mm screenseparation) were registered.

Finally, after carefully adjusting the iondetection sensitivity for signals from both halves ofthe detector, auto - and cross - correlation

flight (ToF) difference At =t""2 - t"1. is proportionaI

to the difference of the square roots of the ionmasses multiplied by the flight time of a hydrogenion ofmass one (At "'"("m2 - "mI). tH+).

ESFO of 3He,'tHe on W (.110)F=49V/rim,80K ..p =p =p =1'10-smbar~lIe 'He HI

(H',m=O,SI?

( H:~He')

a

3He~~He' impadinduced ·FO ?

. 111_expec t'ddouble peak f or He ESFO b~Vj~,~~W~~M~~~

~H21.', ns -1 ~t~

3He' 0Jd 4Het. ~

x 0.1

~--------------------------------/.-IJ-111,4ns ~

LIFO e

Fig, 2: Field ion correlation measurement in the acomode: The first arriving ion starts a time-te-pulse heightanalyzer (ePA) and the next one stops it. (a) spectrum produced by ESFD with bydrogen as auxiliary gas. The given

times are time-of-flight differences accordíng to dt = (~m2 _.[m;). tw- .. (b) only 3He+ and 4He+ gas, the absence

of the expected double peak excludes ESFD of He gas phase ionization. (e) laser induced field desorption (yield reduced bya factor of lO)

Resultsmeasurements were performed simuItaneousIy usingtwo separate TAC and MCA devices.

The He gases (4He; 99,999% and 3He;99,9% purity) were introduced, at same partialpressures, into the atom probe. The atom probecould be used in the laser-impact mode for masscallbration, but was mainIy used in the fieId ioncorrelation mode as described above with a timeresolution of less than 100 ps. In this mode, a fieIdion arriving ar the detector starts the dock, and thenext one arriving stops it. The measured time-of-

LatinAmerican Journal 01 Metallurgy and Materials,

Since the formation of a specíes like He~+

requiresa doubIe ionization, application of ESFDwith hydrogen as anauxiliary gas seemed to beprornising. The hydrogen provides an electronshower of sufficient impact energy on the tipsurface. Introducing 3He, 4He and HJ2at the samépartial pressures revealed the expected ESFD rnass-

, spectrum (Hg. 2a) for the main peaks, There was,

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42 Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992

however, no eorrelation found for H+ and any otherspecies (3He+, 4He+). Varying the field, changes toother relevant parameters, or using Iaser impact, allproduced the same negative resu1t: no doubly orsing1y eharged He dimer ion was observed.

However, at a comparativeIy 10w ToF-difference of 121.4 ns a signal appeared, whichcorresponded to a mass difference of nearIy 0.2929arnu. The exact time-of-flight difference for this ionpair was in faet the difference in flight times for a3He+ and 4He+ ion, (.J4 amu - "3 amu = 0,26795).This means that a 3He+ and a 4He+ ion are emittedsimu1taneousIy from the emitter surface, and arriveat the detector with a time difference of 121.4 ns. Inorder to test whether ESFD is involved, the auxiliarygas, hydrogen, was removed from the system. ThecorreIation spectrum, fig. 2b, was then observedwith a single peak, again at 121.4 ns. Since the fieldstrength was about 50 VInm during thismeasurement, ESFD by space field ionization ofhelium is conceivable: one He ( 3He+ or 4He+ ) isfieId adsorbed and the other one (4He respective1y,3He) de1ivers the impaet electron by fie1d ionizationin front of the tipo If the necessary energy deficit ofthe e1ectron donating species (> 45 eV) is included,the peak wou1d be shifted by an observable ± 35 ns,which should produce a doub1e peak. This,however, is not observed. So, both He speciesdirectly originate from the surface. This is furtherconfirmed by the use of 1aser impact desorption,shown in fig. 2c. The same ToF-difference of121.4 ns is observed for the He-isotropes due tomany-ion emission events from different sitesinitiated by one 1aser pulse.

FI is composed of independent emissionevents, provided that no correlation is superimposedby impact phenomena such as ESFD or LIFD. Theobserved emission of two He ions at the sameinstant (within less than 100 ps) has to be explainedby some other kind of interaction. To understandthe underIying mechanism, the dependence of thiseffect on relevant parameters, such as partial-pressure and ternperature, were investigated. The3He+ - 4He+ ion pair intensity increased nearlylinearly with the gas pressure. With increasingtemperature (78 < T < 100 K) there was a marginaldecrease in the intensities of the ion pairs and anenergy increase of 54 meV.

The coincidence time-of-flight spectrumclearly indicates that the 3He+/4He+ ion pairs are notformed due to ESFD. The simultaneous ionformation must .be due to a surface dimer He2,which.is field adsorbed at a kink site of the W(1lO)-surface. It is known [3] that the binding energy of aHe-atom increases from a field-free value of "" 6meV with increasing fieId strength to a value of ""250 meV (at ""40 V/nm). The slightly physisorbedHe-species is transferred into a field-inducedchemisorbed formo At the same time, the bindingdistance is shortened by about 30% (from ",.3 A to 2A). The binding energy of the second He-atom wasestimated to have a value of 60 meV [8] which is ingood agreement with the measured binding energyof 54 meV. Details of the interatomic interactionwhich Ieads to the joint ionization process are stillobscure.

-11H,', H.'

H, '. ". • CbaMd ridlll·. 1.0lÍ ns

o I o ~ ~~~~~~.wo 100 200 JOO 400 _ 500 600 700 800 900 1000 JOO 100 500 600 100 800 900 1000

AloI' /o-IN. ATcf I Chonneb'.

JOO

H2 / W (110 lerroces)210

1H,·, H,·

180

Auto:..Correlolio.nT : 80 K

u· ¡.6.BVa

,H;,II;

IOO!rr--------------.--------------~·~li H¡ / W (110 terraces) CrQss-Correlolion I1 1 • 8Q.K

7~1I ~f 1"' ~i SOj~ j

10M rilh !lime q/m-rolio-U. 2.6lkVb

110485

ChQnnd l'ldlh • 0.801 "1

Fig.3: Time-of-flight (T-o-F) mass spectra of hydrogenltungsten. Spectra of time-differences as in fig. 2. Thefirst of the ion pairs yields the electron for ESfO, a = acomode, b= cc-mode

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Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992 43

Field Desorption of Hydrogen

In the case of hydrogen field adsorbed on atungsten field emitter, H+, H2+ and H3+ wereobserved in the field ionization and subsequent fieldinduced dissociation, whereas a surface reactionbetween hydrogen atoms and field-adsorbed H2molecules initiates the formation of H3+. Electronstimulated field desorption ofádsorbed hydrogenspecies has already been usetí to characterize theexternal conditions which are necessary to form H3+ions [9]. With auto-correlation (ac-mode) and crosscorrelation (cc-mode) measurement details of thereaction dynamics during the formation of H3+ -ions should be c1arified.

The experiments were performed bycontinuously dosing hydrogen in a pressure rangefrom 5 x 10-6 to 6 X 10-5 mbar and at a ¡surfacetemperature of 80 K to obtain maximum ion signalintensities [9-11]. At this surface temperature and apressure of 10-5 mbar, the field strength was variedby changing the tip voltage. Above a thresholdfieldstrength of 22 VInm time-correlated ion formationwas observed, as is shown in fig. 3 for a fieldstrength of 24 V/nm (U=2.63 kV).

In the ac-mode (fig. 3a), three peaks relatedto three different time-correlated ion pairs wereobserved above a moderate statistical background.The time window was 1067 ns, and the timeresolution of 1.04 ns corresponded to an energyresolution of 5.2 eV per channeL The observedtimes of the peaks match the flight time differencesof the ion pairs (H2+' H3+), (H¡", H2+), and (Hj+,H3+). The fact that the width of the signals wassignificantly less than 1 ns shows that the total timefor the time-correlated ion pair formation wasdefinitely shorter than 1 ns.

In cc-mode (fig. 3b) thesame ion pairs wereobserved in the time correlation spectrum. Thespectrum was now dominated by a signal around theflight time difference of At = O ns (channel 321)caused by ions with the same q/m-ratio impinging on I

the two different detecting areas (e. g. (H+, H+». Apreliminary investigation of the broadening of thissignal was made using a E&B - (Wien) - filter [7](not shown in fig. 1) to separate the hydrogen ionspecies onto different detector halves. This showedthat the broadening was caused by ion-neutral pairs.The highly energetic neutrals were created by acharge-exchange reaction (most probably e--tunneling) between the ions coming from the tip andthe gas hydrogen molecules present in the time-of-flight tube. These neutrals were not deflected by theWien-filter and gave spots on the Tof-detector,

To investigate the mechanism leading to theobserved time-correlated ion formation, thecorrelated ion pair rate was measured as a functionof the applied electric field (fig. 4).

At the onset field strength of 22 VInm fortime-correlated ion formation, (H2+, H3+) ion pairswere observed only in the acomode. The signalincreased slightly, and at 29 V/nm a moderatemaximum was reached. The ratio, T, of the time-correlated ion pair [ate to the field ion rate, wasfound to be 300 ppm and remained near1y constantbelow 29 V/nm. The (H+, H2+)-correlation signalappeared at 24V/nm, increased continuously withfiel strength, passed a maximum of 400 ppm at 33VInm, and disappeared for F > 40 VInm.

Fi.ld,t "n9th FIV/nm - It dSlrtftglh FIVlrvn -10' 20 lO 40

10' 10' 20 lO 40 104

H2/ W( "Oterraces) FI Raf, H,¡Wr' fl R.t,

auto - T= 80 cress-.. corre\ation .. VI correl.ltion .::? tO' o H~.H; 10) t ? 10' H;,Hi 101

~c: e e O" H;.H; " " ::Je O 1> e H;.,H; e" O•. 6 H;.H; IV , A H;.~~ ••~ •.~ .• •ac IX.. IX .. O<

''¡ -,lO' ~

.~ '1 101 .!e, 10 e, 10e

~e ".!! .s

'!lA. •..I

,1 Ia

"

\ b1(j' \ lO' 10' lO'

2 3 4 S l 1, STip Volla t UI kV - Ti VolI t UI kV -

Fig.4: Field dependence of the ion pair formation, tip potentials (right) are giving field strength data (left), the totalion current is indicated as FI rateo a = acomode. b= cc-mode

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44 Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992

The (H+, H3+) ion pairs behaved similarly tothe (H+, H2+) pairs except that r = 700 ppm wasreached for 30 VInm and that the ion pairs vanishedabove 37 VInm. .

The cc-ion yields bore a similarity to the ac-mode results. For (H+, H3+) ion pairs, the onsetbehavior was identical, andO reached a maximumvalue of 220 ppm and 170 ppm, respectively, at thesame fieldstrength. Only the (H2+, H3+) signalsshowed a steeper increase between 24 and 27 VInm,reaching r = 120 ppm.

The different Tmax-values for auto- andcross-correlation ions indicated that most of the ionpairs, especially the (H+, H3+) pairs, were formedand desorbed within a very small angle « 3.7 x 10-6degrees).

To determine thetrue value for G, ion pairsthat arrive at the detector with the same q/m-ratiohave to he taken into account in the data analysis.Accordingly, the sum of all correlated ion pairs thenreached a maximum of r= 8 % at 33 V/m, whichimplies that up to = 16% of the field ions werecreated by a mechanism leading to time-correlatedion formation at a hydrogen pressure of 10-5 mbar.

> 100QI--'0;¡:QI'Q

>.en

10L'<11c:QI

, ,,',' ! ! I I ! ! 11.11 II I I I

20 30field strel'igth I V/nm -

40

Fig. S: Measured energy deficit (C) 'of the freespace field ionized H+ -ion. The extrapolated value(. )for F = 22 V/nm (the thresbold field strength for correlatedion formation) is "" 11 eVo

in the LIFD process and the time-correlated ionformation process, valuable information about theorigin of the time-correlated ions could be obtained.lons from the tip surface are, at x.; starting with ahigher potential than those which are formed severalten or hundred Á away. They will gain higherkinetic energies and have shorter flight times.The origin of ions can be determined by the energydeficit for instance for the H1+-ion: ~E(Hl+) =EUFD - EESFD·

The result of such an energy analysis isshown in fig. 5 for the (H+, H2+) ion" pair. Theenergy deficit DE of the time-correlated ion sincreased from <= 25 eV at F == 25 V/nm to <= 190eVat F = 35 V/nm. The same tendency towardincreasing energy deficit with increasing fiéldstrength was also observed for the (H2+, H3+) and(H+, H3+) pairs. We therefore conclude that the H+ion forming the (H+, H2+) and (H+, H3+) ion pairs,and the H2+ ions forming the (H2+, H3+) pairoriginate from the gas phase so me hundredÁngstrom away from the surface.

From fig 5 the kinetic energy of the createdelectron was determined to be ::= 11 eV at thethreshold field strengthof 22 V/nm. Although theabsolute value is uncertain within some eV (see errorbars in fig. 5), the extrapolated minimum electronenergy is sufficient to stimulate field desorption ofthe field adsorbed hydrogen molecule.

A similar reaction seems to be valid for the(H+, H3+') and (H2+, H3+) pairs, and the occurrenceof these correlations further sustains the view offield stabilized H3 molecules adsorbed on the emittersurfaee [12].. The faet that the (H2+, H3+) signalvanishes at 33 V/nm, whereas the (H+, H3+)correlation is detected up to 36 VI'nm (fig. 5)indicates that with increasing field strength the fielddissociation of space ionized singly charged H2+into H and H+ becomes more likely. Thedisappearance of the (H+, H3+) signal is eventuallybrought about by the depletion of the H3 on thesurface, sine e the increasing field ionizationof gasphase hydrogen moleeules also leads to a decrease offieladsorbed H and H2 (the source for H3formation) on the surface.

Final support for the above proposedmechanism for time-correlated ion formation is givenby the dependence of the signal intensities of the H+and H2+ ions originating from the surface on thehydrogen partial pressure.

We find that increasing the hydrogenpressure from 6 x 10-610 6 X 10-5 mbar leads to anincrease of the time-correlated ion pairs by a factor

.'Eoshed more light on the mechanism leadingto the observed time-correlated ion formation, laser •induced field desorption (LIFD) experiments werealso carried out. With the help of a N2-pumped dyelaser [10], the ternperature of the hydrogen coveredsurface could be raised in a very short time (<= 5 ns).In this case only field adsorbed molecules aredetected. By comparing time-of-flight differences

LatinAmerican Journal of Metallurgv and Materials, Vol 11, N'o 1y 2, 1992

Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° ly 2, 1992

of 10 as expected from the tenfold higher electronimpingement rate on the emitter surface.

Conclusions

Investigations using the time-correlated atomprobe give principally new information on thereaction dynamics on solid surfaces. In the presentcontribution it could be pro ven that He2 - surfacedimers are formed by field induced binding energiesof the order of 50 to 60 me V. During the fielddesorption process dimers dissociate, and so H2+-ions are not detected.

In the field desorption process of hydrogenabove a threshold field strength of 22 V/nm time-correlated ion fonnation is observed. The formationof the ion pairs (H2+' H3+)' (H+, H2+), and (H+,H3+) is due to electron stimulated field desorption(ESFD) of field adsorbed hydrogen species. Theelectrons are created above a field strength of 22.VInm by field ionization of hydrogen gas moleculesfar away from the surface. They possess enoughenergy to stimulate the field desorption of fieldadsorbed hydrogen species. The (H2+, H3+) ionpairs are mostly desorbed within a very small angle,which indicates that the field adsorbed H3 moleculeis adsorbed as a linear structure, onto the surface.

Acknowledgments

Financial support by the DeutscheForschungsgemeinschaft Bonn, Bad Godesberg(SFB 6), and the Auswartiges Amt, Bonn, for oneof us (J. D.) is gratefulIy acknowledged. We alsothank the Prozessrechner group of the FHI for theirexcellent computer and software support.

References

1. T. T. Tsong, Surf. Sci. Rep. 8 (1988) 127, andreferences therein.

2. N. Emst, Surf. Sci. 219 (1989) 1.

3. N. Ernst, W. Drachsel, Y. Li, 1. H.. Block, andH. J. Kreuzer, Phys. Rev. Lett. 57 (1986) 2682.

4. L. Pauling, J. Chem. Phys. 1 (1933) 56.

5. J. Guilhaus, J. Phys. B17 (1984) L805.

6. W. Drachsel, Th. Jentsch, J. H. Block, Int. J.Mass Spectrom. Ion Phys. 46 (1983) 293.

7. W. Drachsel, J. H.-Block, Surf. Sci. 246 (1981)141.

45

8. H. J. Kreuzer, private communication.

9. N. Ernst and J. H. Block, Surf. Sci. 126 (1983)397, ánd references therein.

10. W. Drachsel, Th. Jentsch and J. H. Block, Int.J. Mass Spectrom. Ion Phys. 46 (1983) 293.

11. G. L. Kellogg, 1. Chem. Phys. 74 (1981) 1479.

12. N_ Ernst and J. H. Block, Phys. Rev. B29(1984) 7092.

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