HAL Id: jpa-00226902
https://hal.archives-ouvertes.fr/jpa-00226902
Submitted on 1 Jan 1987
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
A TIME-OF-FLIGHT MASS SPECTROMETER FOR SIMS AND FIELD IONISED NEUTRAL ANALYSIS
USING A PULSED LMIS
A. Waugh, D. Kingham, C. Richardson, M. Goff
To cite this version:
A. Waugh, D. Kingham, C. Richardson, M. Goff. A TIME-OF-FLIGHT MASS SPECTROMETER
FOR SIMS AND FIELD IONISED NEUTRAL ANALYSIS USING A PULSED LMIS. Journal de
Physique Colloques, 1987, 48 (C6), pp.C6-577-C6-582. �10.1051/jphyscol:1987694�. �jpa-00226902�
A TIME-OF-FLIGHT MASS SPECTROMETER FOR SIMS AND FIELD IONISED NEUTRAL ANALYSIS USING A PULSED LMIS
A.R. Waugh, D.R. Kingham, C.H. Richardson and M. Goff VG IONEX Ltd., Charles Avenue, Maltings Park, Burgess Hfll West Sussex, RH15 9TQ, U.K.
ABSTRACT
A new surface analysis instrument has been developed u s i n g an energy- compensated time-of-flight mass spectrometer. Samples are ionised f o r analysis either b y microfocussed laser irradiation or b y sputtering w i t h a microfocussed (<0.25~lml mass-filtered beam o f Ga' ions from a liquid metal ion source. A framestore-based data system allows the simultaneous capture o f both SIMS mass spectra and mass-resolved ion images. Data are presented i l l u s t r a t i n g b o t h SIMS and Laser Microprobe performance. Data a r e also presented showing enhanced ion yield from t h e sputtering o f a gold tip, due t o field ionisation of sputtered neutrals.
I. INTRODUCTION
I n order t o make best possible use o f the relatively small number of secondary ions sputtered from a t h i n surface layer b y a microfocussed ion beam it i s necessary to use a mass spectrometer with parallel detection o f all masses, rather than a tuned spectrometer such as a quadrupole or magnetic spectrometer. A n energy compensated time-of-flight mass spectrometer was devised b y Poschenreider 111 and i s well suited t o t h i s purpose 121. We have combined such a spectrometer w i t h a microfocussed i o n gun u s i n g a l i q u i d metal ion-source 131 which has been developed to g i v e a pulsed beam o f nanoseconds duration. The instrumentation i s also f i t t e d with a Q-switched Nd:YAG laser using second o r f o u r t h harmonic radiation (532 o r 266nm) which can be focussed t o a few microns diameter, as an alternative way o f ionising sample materials.
I I. INSTRUMENTATION
The instrument (Figure 1 ) i s constructed as a f u l l y bakeable UHV system i n a high-stability chamber w i t h a sample stage capable of <50nm resolution when the liquid-metal ion g u n i s operated as a continuous beam. The 'Poschenreider' spectrometer is the same as we have used previously f o r t h e 'FIM100' atom- probe field ion microscope 141 w i t h t h e addition of a variable i r i s to control t h e spectrometer acceptance angle.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987694
C6-578 JOURNAL DE PHYSIQUE
T h e liquid-metal i o n gun i s c u r r e n t l y f i t t e d w i t h a gallium l i q u i d metal i o n source o p e r a t i n g a t u p t o 30kV. T h e g u n has condenser a n d focus asymmetric einzel lenses, a Wien mass f i l t e r w i t h s u f f i c i e n t r e s o l u t i o n t o separate 6 9 ~ a + f r o m 71Ca+, octupole stigmators a n d scan r o d s . A u x i l i a r y i n t e r n a l d e f l e c t o r p l a t e s can b e u s e d t o c h o p t h e beam ( 5 t o 50ns pulses c a n b e selected b y computer c o n t r o l ) b y s c a n n i n g i t across a n a p e r t u r e . A spatial r e s o l u t i o n o f
<50nm has been achieved u s i n g a continuous beam a n d <0.25 p m u s i n g a p u l s e d beam. T h e i o n beam can b e p u l s e d a t u p t o 2OkHz.
Time-of-Flight SlMS spectra a r e collected u s i n g a time-to-digital c o n v e r t e r w i t h I n s r e s o l u t i o n ( L e C r o y 4208). D i g i t a l delay g e n e r a t o r s can b e used t o v e t o signals f r o m u n w a n t e d i n t e n s e peaks ( e g a l k a l i metals) t o a v o i d s a t u r a t i n g t h e acquisition system, if necessary. A f a s t CAMAC processor i s u s e d t o s o r t t h e d a t a a n d t r a n s f e r i t p e r i o d i c a l l y t o a PDP 11/73 i n c o r p o r a t i n g a d i g i t a l frame- store. T h e datasystem c a n e i t h e r - j u s t a c q u i r e mass spectra w i t h a c q u i s i t i o n times o f t y p i c a l l y 10-100 seconds, o r can simultaneously a c q u i r e b o t h a mass s p e c t r u m a n d m u l t i p l e mass-resolved secondary i o n images o f 642, 1282 o r 25fj2 p i x e l resolution, w i t h a c q u i s i t i o n time t y p i c a l l y a r o u n d 120 seconds. Due t o t h e p a r a l l e l detection o f t h e time-of-flight spectrometer, images c a n i n c l u d e ions a t d i f f e r e n t masses ( e g all t h e isotopes a n d all t h e cluster-ions c o n t a i n i n g a g i v e n element). While SlMS s p e c t r a a r e a c q u i r e d b y accumulating secondary i o n c o u n t s o v e r many p r i m a r y i o n pulses, Laser M i c r o p r o b e data a r e generated u s i n g a single-shot laser p u l s e w i t h analogue a c q u i s i t i o n u s i n g a 200MHz,8 b i t 32,000 channel t r a n s i e n t r e c o r d e r . T h e laser beam i s focussed u s i n g a m i r r o r microscope o b j e c t i v e w h i c h can also b e used for v i e w i n g t h e sample v i a a n eyepiece o r a T V camera: t h i s i s also connected t o t h e d i g i t a l framestore, allowing optical m i c r o g r a p h s t o b e r e c o r d e d o n d i s k if required.
I II PERFORMANCE OF T H E INSTRUMENT 1 1 1. 1. SPECTRAL AND IMAGING PERFORMANCE
Examples showing spectral a n d image i n f o r m a t i o n obtained w i t h t h e new i n s t r u m e n t a r e shown in f i g u r e s 2, 3 and 4. F i g u r e 2 shows t r a c e amounts o f molybdenum (as MO+, MOH+ a n d MOOH+) a n d cobalt, as well as hydrocarbons, o n t h e s u r f a c e o f a n alumina c a t a l y s t p a r t i c l e removed f r o m a r e a c t i o n vessel; t h e 2 7 ~ 1 + has been t r u n c a t e d severely (19000 c o u n t s full scale) i n t h e f i g u r e .
F i g u r e 3 shows simultaneously-acquired time-of-flight SlMS images o f ~ e + , ~ i + ,
~ r + a n d
sii
f r o m a metallographic section o f an o x i d i s e d nuclear fuel-can.( c o u r t e s y C e n t r a l E l e c t r i c i t y Research Laboratories)
F i g u r e 4 shows an example o f a laser-induced mass s p e c t r u m f r o m a polymer - a molecular-weight s t a n d a r d o f p o l y s t y r e n e w i t h a d i s t r i b u t i o n o f ( C 8 H 8 ) n chains c e n t r e d o n n=6. A l t h o u g h fragmentation o f t h e chains t o g i v e C x H ~ + ions o f masses
<
200amu was observed, t h e p a r e n t molecular ions w e r e v e r y c l e a r l y observed w i t h an i n t e n s i t y d i s t r i b u t i o n r e f l e c t i n g t h a t i n t h e sample material.111. 2 , F I E L D IONISATION O F SPUTTERED N E U T R A L ATOMS
I n general o n l y a small percentage o f atoms s p u t t e r e d in SlMS experiments a r e ionised, a n d s t r o n g d i f f e r e n c e s a r e o b s e r v e d between d i f f e r e n t elements. T o achieve h i g h e r a n d more u n i f o r m s e n s i t i v i t y it i s necessary t o post-ionise t h e
F~~.I+cHEHATIc DIAGRAH OF IXZ3LS TIHE-OF-PLIGHT SMS AND LASER HICROPROBE INSTRUMENT
VG IONEX
Positive Time-Of-Flight SIMS Channel width 0.050 emu
KEXPAD
Vertical scale Xi00
Atomic Mass Units
Fig. 2 - ~ l u m i n a catalyst particle a f t e r exposure t o reactants,
C6-580 JOURNAL DE PHYSIQUE
~ i3 ~
- &,
. F ~ + , ~ i * and S? f k m a section of an oxidised fhl canToF L a s e r M i c r o p r o b e P o s i t i v e I o n s C h a n n e l w i d t h 0.125 amu
0
200 300 400 500 600 700 800 900 100C
Atomic Mass U n i t s
Fig.4- P o l y s t y r e n e w i t h n o m i n a l M o l . W t . 600.
r e p o r t h e r e an alternative technique u s i n g field-ionisation o f s p u t t e r e d g o l d atoms from a b l u n t g o l d tip. It i s o f course well established in t h e atom-probe t h a t field-ionisation p r o v i d e s an excellent way o f completely ionising atoms near a surface. A gold. t i p was electropolished in .dilute HCL: glycerol a n d mounted a t room temperature i n t h e instrument. A t 5 k V a weak continuous signal d u e t o field-ionisation o f residual gases a n d water v a p o u r was observed, ( b a c k g r o u n d p r e s s u r e was mbar d u r i n g t h i s experiment). When t h e pulsed ~ a + ion- beam was focussed o n t h e t i p a n d a time-of-flight 'SIMS' spectrum obtained, peaks d u e t o AU++, Au+, Au2+ a n d Au3+ were observed. ( U n d e r normal SIMS conditions t h e Au+ i o n y i e l d from s p u t t e r e d g o l d i s extremely low, a n d t h e Au++, Au2+ etc would be negligible. Sinall peaks a t mass 16 (0+), 213 (AUO') a n d 229(Au0,+) were i n i t i a l l y observed: these decreased p r o p o r t i o n a t e l y w i t h time, as d i d - t h e b a c k g r o u n d d u e t o continuous ionisation; b p t h were r e s t o r e d each time t h e t i p voltage was raised. T h e spectra were acquired i n 60 seconds w i t h lOKHz pulse r e p e t i t i o n rate.
I V DISCUSSION AND SUMMARY
T h e time-of-flight SIMS instrument described h e r e has demonstrated t h e acquisition o f sub-micron resolution mass-resolved images w i t h sensitivities a t least an o r d e r o f magnitude b e t t e r t h a n a n y p r e v i o u s i n s t r u m e n t known t o t h e authors. A laser microprobe attachment gives v e r y r a p i d analytical information, w i t h h i g h s e n s i t i v i t y o v e r t h e whole mass r a n g e o f elements.
T h e demonstration o f t h e field-ionisation o f s p u t t e r e d neutrals described h e r e raises t h e possibility o f a number o f new experimental techniques:-
1) Analysis o f t i p s too b l u n t o r too delicate t o b e analysed b y conventional APFlM o r Laser Atom-Probe
2) Quantitative o r semi-quantitative measurements o f t h e composition and d i s t r i b u t i o n o f fragments s p u t t e r e d b y i o n impact from samples t h a t can be prepared as t i p s .
3) Quantitative o r semi-quantitative analysis o f materials deposited o r adsorbed on a s u p p o r t t i p .
4) B y v a r y i n g t h e ionisation field, t o f i n d t h e threshold value f o r ionisation o f d i f f e r e n t species, i t should b e possible t o p r o b e t h e population o f excited states o f s p u t t e r e d n e u t r a l s a n d n e u t r a l clusters.
REFERENCES
(1) W.P. Poschenreider, Int J Mass Spectrom I o n P h y s
9
(1972) 357(2) P. Steffens, E. Niehuis, T. Friese, D. G r e i f e n d o r f and A. Benninghoven J Vac Sci Technol A
3
(1985) 1322(3) A.R. Waugh, A.R. Bayly, M. Walls, P. V o h r a l i k and D. Fathers J. d e Physique Coll C2,
47
(1986) C2(4) A. Cerezo, G.D.W. Smith and A.R. Waugh. J de Physique Coll C9
45
(1984) C9
( 5 ) D. L i p i n s k y , R. Jede, 0. Ganschow and A. Benninghoven. J Vac Sci Technol A3(5) (1985) 2007
(6) J. Morellei, D. Normand, G. Petite. Adv. A t . k l . Phys.
2
(1982) 97 (7) G.S. Hunt, M.G. Payne, S.D. Kramer and J.P. Young. Rev Mod P h y s51
(1979) 767
JOURNAL
DE
PHYSIQUE) SPT: GOLD10
200 m u
C J 0 100
0
0 20 40 60 80 100 120 140 160 180 200 220 240 Atomic Mass u n i t s
Atomic Mass U n i t s
) SPT: GOLD20 VG IONEX
P o s i t i v e T i m e - o f - F l i g h t SIMS
Channel width 0.125 amu
60
50
40 m .iJ
;
300 U
20
10
0
0 20 40 60 80 100 120 140 160 180 200 220 240 Atomic Mass u n i t s
Fig.5- old i o n s and c l u s t e r s f r o m t i p s p u t t e r e d by 20 k V Ga*.
