ON THE ENERGY DISTRIBUTION OF CLUSTERS FROM LIQUID GOLD ION SOURCES

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ON THE ENERGY DISTRIBUTION OF CLUSTERS FROM LIQUID GOLD ION SOURCES

S. Papadopoulos, D. Barr, W. Brown

To cite this version:

S. Papadopoulos, D. Barr, W. Brown. ON THE ENERGY DISTRIBUTION OF CLUSTERS FROM LIQUID GOLD ION SOURCES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-101-C2-106.

�10.1051/jphyscol:1986215�. �jpa-00225646�

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JOURNAL DE PHYSIQUE

Colloque C2, supplement au n03, Tome 47, mars 1986 page c2-101

ON THE ENERGY DISTRIBUTION OF CLUSTERS FROM LIQUID GOLD ION SOURCES

S. PAPADOPOULOS*, D.L. BARR and W.L. BROWN

AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A.

Abstract - W e have studied the changes in t h e energy distribution of the Aa$+ and A U ; species from Au liquid metal ion sources (LMIS) with increasing current. T h e distributions are single peaked a t low currents b u t develop secondary maxima a t higher currents. W e evaluate t h e feasibility of several mechanisms t h a t have been proposed in order t o explain such changes a n d find them unsatisfactory. W e propose two alternative models which may be effective. W e have also studied the angular

~ n t e n s i t y and the energy distributions of t h e above species a s well a s those of t h e AU+' species as a function of angle of emission. These results further emphasize t h e complexity of t h e processes by which these species are created, a n d which, as yet, are not well understood.

I - INTRODUCTION

As it has already been pointed out, /e.g. 1,2/ t h e energy distribution of the various ionic species emitted from liquid metal ion sources (LMIS) is an important indication of t h e mechanisms involved in t h e creation of these species and of the general operation of these sources. T h e energy spread of ions from LMIS's is also of technological importa.nce as it constltutes a major limiting factor in the current density in focused beams from these sources / 3 / A number of energy distribution studles have been reported in the literature, several specifically devoted t o elemental Au LMIS. /4,5,2/ This paper IS concerned with t h e three species, A U + + , AU;+ a n d AU;

I1 - EXPERIMENTAL A P P R O A C H

T o obtain t h e energy distributions we have made use of a double focuslng 60 " magnetic sector spectrometer with a nominal 40 cm radius of curvature. Entrance slits l i m ~ t t h e solid angle accepted by the spectrometer t o 3.2X steradians. T h e mass and energy separated beams are examined with a 100pm wide exit slit a t the spectrometer focus. This yields a n energy resolut~on of

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1.5eV as calibrated with a C s surface ionization source. T h e ions passlng through t h e slit strike a collector plate and are measured as a current with an electrometer Secondary electrons created a t the surface of t h e collector are suppressed by applying a negative voltage of a few hundred volts on a wire loop placed between t h e slit and the collector. T h e mass a n d energy distribution of t h e ions from t h e source are t h e n obtained by scanning t h e m a g n e t ~ c field of the spectrometer. T h e total current, I T ( a ) for a particular ionic specles a t an emission angle a within t h e solid angle subtended by the entrance slits was measured by opening u p t h e exit slit so t h a t the whole of t h e energy distribution of t h e species was allowed through. Alternatively, I T ( a ) was calculated from t h e area under t h e energy distribution curves.

'permanent address : Department of Engineering Science, Wolfson College, University of Oxford, GB-Oxford, OX2 6UD

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986215

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JOURNAL DE PHYSIQUE

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E N E R G Y DISTRIBUTIONS F O R AXIAL EMISSION

Figs. l a and b show t h e axial energy distributions ( a = 0 ) of AU?+ a n d A U ~ respectively a t four different values of t h e source current I,. T h e relative abundance of these species with respect t o t h e prima,ry species ( A U + ) varies from 3.2% a t I,= 2 p A t o 7.6% a t 1 8 = 4 0 p A for ,4u3++ and from 7% a t 2 p A t o 7.6% a t 40pA for A U ; , SO they are substantial components of the beam. i t is evident in both figures t h a t t h e energy spectra are single peaked a t low source currents ( 2 and 4 p A ) b u t develop a second peak a t a lower energy a s I, is increased.

T h e well resolved low energy peak in t h e A U ; spectrum a t 40pA is

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135 eV below t h e higher energy peak. T h e shoulder in t h e A U ; + spectrum a t 40pA is lower t h a n t h e main peak by

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65 eV. Similar spectra have been reported by Waugh/5/ using a time of flight mass spectrometer. Sudraud e t al /4/ have also reported double peaks b u t have n o t given any results at low source currents.

nu;. a . o- 4 -

5

D:

3 0 0

4 P

Z

I

~ ~ [ e v ] 31 evldiv AE [ev]

Figs. 1 a,b: Energy d i s t r i b u t ~ o n s of AU;' a n d A U ; a s a function of source current. All peak heights are normalized t o 1.

We believe t h e higher energy peak a t high currents a n d t h e single peak a t low currents in t h e spectra of Fig. 1 have t h e same origin. This seems likely t o be direct emission from the liquid metal tip a s A U ; and A U ; t h e latter of which is then post-ionized t o A = : + . W e have considered several models t h a t have been suggested t o explain t h e lower energy peak of t h e distribution:

1) Ionization of a neutral cluster, Au,, by secondary electrons falling towards t h e liquid metal tip a s proposed by Sudraud e t al. /4/ T h e flux of these secondary electrons near t h e tip is n o t known, nor ^IStheir origin. However, secondary electrons will doubtless arise from collisions of the primary ionic species of t h e beam A U + a n d A u + + with larger, more slowly moving clusters or droplets according t o the process:

A U +

+

A U ~ +

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^{A U +}

⁺

A U ( , ~ + ' ) +

+

e -

These secondary electrons could then impact ionize neutral aggregates emerging from t h e liquid metal t i p . However, we note t h a t t h e secondary electron emission in a n ion-cluster collision will be small for ion energies less t h a n

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¹keV so t h a t such secondaries will be created a t a potential a t least 1 keV below t h e tip potential.. These electrons will then have

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1 k e V of kinetic energy a s they interact with neutrals n e a r , t h e tip. T h e interaction cannot take place with 100 eV electrons as proposed by Sudraud e t al, t h e energy a t which ionization becomes most probable. There is also n o reason for t h e interaction t o occur a t 3 2 or 135 volts away from t h e tip as would be required t o produce t h e low energy peaks in t h e AU?+ a n d A U ; ion energy distributions respectively. I t should also be noted t h a t this mechanism is a high order process: Ions from t h e source have t o make electrons by striking other emitted species from the source and these electrons have t o make ions by striking other emitted

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species from the source. T h e efficiency of such a high order process IS extremely small and it could certainly n o t give rise t o a lower energy peak t h a t may be more t h a n 1% of the primary specles.

2) Nonresonant charge transfer of t h e type:

as proposed by Sudraud e t al. /4/ During this process A U + ions interact with A u 2 neutrals whereby the charge of t h e ^{A U +} is transferred t o t h e A u 2 . Using even the extraordinary neutral flux proposed by Sudraud e t al ( 6 ~ neutrals/cm2sec), and t h e extreme assumptions t h a t all of these neutrals are emitted a s A u 2 a n d t h a t they possess thermal velocity only - which sets an upper limit t o their volume density

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i t can be shown t h a t with a typical non-resonant charge transfer cross section of l ~ - ' ~ c m ~ only 1 in lo5 A U + ions will undergo charge transfer wlthin a radial interval whose equivalent in energy corresponds t o t h e width of t h e low energy peak of Fig. l b . T h u s t h e low energy AU; current produced in this way cannot be larger t h a n of the A U + current. However, o u r measurements yield an AU; (low energy)/ A U + current ratio which is larger t h a n Producing t h e A%$+ low energy peak by charge exchange requires

This is even less l ~ k e l y because of the repulsive Coulomb interaction which further reduces the cross section.

3) A t t a c h m e n t of a n A U + ion t o an Au neutral as:

also proposed by S u d r a u d e t al. /4/ W e note t h a t energy conservation d e m a n d s t h a t any lowering of t h e kinetic energy of the final product ion below t h e energy t h a t ^{A U +} would have if it had proceeded without coll~sion m u s t be made up by energy stored a s internal energy in t h a t final ionic species. I t is impossible t o imagine having 135 eV of internal energy stored in an AU; ion. T h e same argument applies t o t h e formation of AU;+ according t o

4) Sputtering of droplets or large clusters by energetic A U + or AU'+ ions in order t o produce AU; or A U $ + ions corresponding t o the lower energy peak. Sputtering of droplets has been suggested by Wagner /6/ as giving rise t o the ejection of both neutral a n d ionized molecular clusters. In examining whether such a mechanism can give rise t o t h e low energy peaks of t h e above species we note t h a t the idea is t o create a new ion a t a potential

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¹³⁵

volts (AU;) a n d

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35 volts (AU:+) below t h e potential a t which t h e ions corresponding t o t h e higher energy peak are formed, so t h a t t h e final kinetic energy of t h a t species will match the low energy pea.k of Fig. l a a n d l b . Therefore, a n A u + ion doing t h e sputtering can only have an energy of 65 eV or 135 eV a t t h e sputtering point in order t o produce ^AU; or A U $ + respectively. If a n A U + + ion is responsible for sputtering its energy a t t h e point of sputtering will be twice t h a t of an A U + . Total sputtering cross sections are very small a t such low energies and t o have t h e sputtering occur in a narrow radial interval (as discussed in connection with ( 2 ) above) requires a large cross section a n d one t h a t falls off with energy above

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100 eV rather t h a n increasing a s sputtering cross sections d o in this energy range

/?I.

T h e cross section for sputtering t o produce an ion is even smaller a n d t o produce a doubly charged lor, a s in A U $ + is smaller skill.

We have considered two additional mechanisms which might give rise t o t h e lower energy ~ e a k s in Figs l a and b . They are discussed briefly below.

5) Spontaneous fragmentation of a cluster (which could be charged t o its R a y l e ~ g h limit 181). According t o this idea t h e parent cluster comes off t h e liquid tip in a highly vibrationally excited s t a t e a s a result of t h e strong polarization force of t h e t i p electric field.

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c2-104 JOURNAL DE PHYSIQUE

Fragmentation is most likely t o occur in a time of t h e o r d e r p f the vibrational period of t h e cluster,

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10-l3 sec, in whic) time it will have moved

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^{25A or}

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135 volts away from t h e tip a t a tip field of 5.3 volt/A ( t h e field evaporation field for solid Au 191).

6)-Collision induced fragmentation. In this mechanism the probability of fragmentation of a cluster is enhanced by Coulomb scattering of a passing 50 t o 100 eV A s + or 100 t o 200 eV A s + + primary Ion. Mechanisms 5) and 6) seem t o be likely candidates a s sources for t h e low energy peaks in the A S $ + and t h e A U ; energy spectra. T h e increase of these peaks with increasing current is qualitatively consistent with t h e idea t h a t because of the increasing turbulence in t h e surface of t h e liquid tip which must be present when h ~ g h currents are b e ~ n g drawn from it, t h e probability t h a t ionized clusters are extracted is increased together with an increase in the vibrational excitation and hence instability of these clusters.

IV - SPECTRAL D E P E N D E N C E O N EMISSION ANGLE A . Energy Distributions

Figs. 2a, b and c show the emission angle (a) dependence of t h e shape of t h e energy distributions of t h e A U + + , A s : + and A U ; species respectively. T h e A U + + species shows n o significant change in peak shape. T h e peak height simply decreases with angle. However, the behaviour of A%:+ and A U ; is different: In both cases as a increases t h e lower energy peak (B) decreases in intensity b u t t h e higher energy peak (A) increases. This observation 1s an indication t h a t , whatever t h e mechanism of formation of the ions corresponding to peak B, they are preferentially created close t o t h e axis of t h e source.

Figs. Ba, 6 , c: Energy distributions of A U + + , A U ~ + + and A u z a s a function of angle of e m i s s ~ o n , a

B. Differential current

Figs. 3a, b and c show t h e variation of t h e total c u r r e n t per u n i t solid angle IT ( a ) a s a function of a a t different values of t h e source current. IT(&) in the case of the A S + + a n d A U ; species decreases with increasing a a t all source currents. A&+ behaves differently:

I T ( a ) decreases with increasing a only a t I,=2pA. A t larger I,, IT(a) develops a local minimum a t a = O and maxlma nearly symmetrically placed on either side of a = 0 " . This is most completely seen in the curve corresponding t o I,= 4 p A . As I, IS further increased, these maxima have moved t o angles larger t h a n those accessible in t h e experimental arrangement.

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Figs. $ a , b , c : Differential c u r r e n t per u n i t s o l ~ d angle of A ~ + + , A ~ $ + a n d A U ~a s a function of angle of emission a t different source c u r r e n t s .

T h e IT(@) dependence of t h i s species resembles closely t h a t of t h e p r i m a r y A U + species. /2/

C . Energy W i d t h s

Figs. 4a, b a n d c display t h e variation of t h e F W H M of t h e energy distribution of t h e t h r e e species a s a function of a a t different source c u r r e n t s . I t is obvious from F i g . 4a t h a t unlike t h e A U + species / 2 / , t h e F W H M of t h e A U + + species is angle i n d e p e n d e n t a t all values of I, In t h e case of AU;+ (Fig. 4 b ) a t I,= 2 a n d 4 p A t h e energy s p e c t r a are single peaked (see Fig.

l a ) a n d t h e F W H M is almost angle i n d e p e n d e n t . A t I,= 1OpA t h e presence of t h e lower energy peak becomes perceptible a n d t h e F W H M displays a s o m e w h a t stronger angular dependence because t h e presence of t h e lower energy p e a k broadens t h e energy distribution a t small angles. A s cu increases t h e lower energy p e a k diminishes a n d t h e energy distribution gets narrower. A t I,= 20 a n d 40pA t h e t w o p e a k s a r e clearly distinguishable from each o t h e r . In order t o o b t a i n t h e F W H M of both p e a k s a non-linear least squares c o m p u t e r fit w a s used with t h e a s s u m p t i o n t h a t t h e peaks a r e gaussian in s h a p e . A n y a n g u l a r variation t h a t may exist in t h e F W H M of peak B a t 20 a n d 40pA is obscured b y t h e s c a t t e r of t h e d a t a points.

However, t h e F W H M of t h e higher energy p e a k A a t these c u r r e n t s is angle I n d e p e n d e n t AU; displays a similar behaviour t o t h a t of AU;+ (Fig 4c). S u c h angular P e p e n d e n c e s of t h e F W H M suggest t h a t longitudinal space charge effects c a n n o t b e t h e d o m i n a n t source of t h e energy s p r e a d , neither in t h e case of A U + + which is a significant c o m p o n e n t of t h e beam (-40%), nor In t h e case of t h e much less a b u n d a n t AU;+ a n d AU; species w h ~ c h move a t different velocities t h a n t h e primary species a n d therefore electrostatic interactions wlth t h e m c a n n o t be c o h e r e n t .

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JOURNAL DE PHYSIQUE

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Figs. 4 a , b,c: Energy spread of

70 -

the same specles as a functlon of angle of emlsslon a t different source currents B refers t o t h e lower energy peak and A t o t h e h ~ g h e r energy one

-20 -15 -10 -5 0 5 10 15 20 a ['I

V - CONCLUSIONS

I t is evident t h a t our understanding of the mechanisms which account for t h e above d a t a is incomplete. T h e angular dependence of IT and t h e energy d i s t r i b u t ~ o n of different s p e c ~ e s e m p h a s ~ z e t h e complexity of t h e processes t h a t m u s t be active. T h e b e h a v ~ o u r of gold LMIS a t high currents leads one once again /2/ t o consider the highly chaotic a n d d y n a m ~ c character of the surface of the tip which makes it far from a smooth Taylor cone from which ions are being quietly field evaporated. A t higher currents these effects lead t o greatly enhanced emission of ionized (and possibly neutral) atomic clusters a n d droplets. From this viewpoint t h e w ~ d t h s of the energy peaks in t h e species considered here are associated with the perturbation of t h e e l e c t r ~ c field which takes place during the emission of these clusters and droplets /2/ rather t h a n with longitudinal space charge effects which have been generally believed t o be the dominant source of the energy spread in ions from LMIS /e g. 101

R E F E R E N C E S

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/2/ S . Papadopoulos, D . L. Barr, W . L . Brown a n d A. Wagner, Journal d e Physique Colloque C9, supplement au Tome 45, (1984) 217.

/3/ R . L. Seliger, J. W . Ward, W. Wang a n d R. L. Kubena, Appl. Phys. L e t t ,

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(1979) 310.

,141 P . Sudraud, C . Colliex and J. van de Walle, Journal de Physique, 40, (1979) 207.

/5/ A. R. Waygh, J. P h y s . D, 19, (1980) 203.

/6/ A . Wagner, 30th Intl. Field Emission Symposium, Philadelphia, P e n n . (1983).

/7/ G . Carter a n d J . S . Colligon, Ion Bombardment of Solids, Elsevier 1968.

/a/

Lord Rayleigh, Proc. Roy. Soc., 14 (1882) 184.

/9/ T . T. Tsong, Suf. Sci., 70 (1978) 211.

/ l o / W . K n a u e r , O p t i k , 59 ('1981) 335.