HIGH SPATIAL RESOLUTION SIMS WITH THE UC-HRL SCANNING ION MICROPROBE

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HIGH SPATIAL RESOLUTION SIMS WITH THE UC-HRL SCANNING ION MICROPROBE

R. Levi-Setti, Y. Wang, G. Crow

To cite this version:

R. Levi-Setti, Y. Wang, G. Crow. HIGH SPATIAL RESOLUTION SIMS WITH THE UC-HRL SCANNING ION MICROPROBE. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-197-C9-205.

�10.1051/jphyscol:1984933�. �jpa-00224413�

HIGH S P A T I A L R E S O L U T I O N SIMS W I T H T H E U C - H R L S C A N N I N G ION M I C R O P R O B E R. Levi-Setti, Y.L. Wang and G. Crow

The Enrico Fermi Institute and Department of Physios, The University of Chicago, Chicago, Illinois 60637, U.S.A.

Résumé - Une nouvelle sonde ionique (Ga+) à balayage nous permet d'obtenir des images qui montrent une resolution latérale près de 40 nm, en

utilisant les ions secondaires analysés par un filtre quadrupolaire RF de masse.

Abstract - A new Ga+ scanning ion microprobe yields images at lateral resolution approaching 40 nm, making use of the secondary ions analyzed by an RFquadrupole mass filter.

The prospects for high resolution imaging microanalysis using focused ion beams from liquid metal ion sources (LMIS) have been previously examined in detail[l], and an optical column design proposed for a 55 KeV probe to reach the spatial reso- lution level of ^ 10 nm. Considerations of the size of the collisional cascade leading to the sputtering process compe![2] to regard the 10 nm level as a limit

intrinsic to the method for either imaging microanalyzers (IMMA)[3] or scanning ion probe microanalyzers (SIPM)[4,5]. Over the past four years, the high resolution scanning ion microprobe (SIM) conceived in ref. [1] has been developed and con- structed in a collaboration between the University of Chicago (UC) and Hughes Research Laboratories (HRL).

While our development was taking place, reports of several efforts to apply LMIS-based probes to secondary ion mass spectrometry (SIMS) microanalysis and mapping appeared in the literature. Much progress has already been made since the first submicron maps obtained with a Ga+ probe[6], notably by the VG Scientific, Ltd. group[7, 8, 9 ] . Also, quantitative SIMS data for an In+ probe have been collected and discussed by the Vienna group[10,ll].

The UC-HRL SIM became operational at UC in November 1983 with a Ga+ probe and early results on its focusing and imaging performance have been recently re-

p o r t e d ^ ] . We present here the first results obtained with a high transmission SIMS system, which we have operated since May 1984, with emphasis on elemental maps at high spatial resolution, well in the sub-100 nm region.

I - The UC-HRL SIM

The microprobe optical column and related instrumentation are shown schema- tically in Fig. 1. The column comprises, in order, a Ga+ LMIS, an extraction aper- ture, a beam defining aperture at the entrance of an asymmetrical triode gun Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984933

C9-198 JOURNAL DE PHYSIQUE

LlOUlD METAL ION SOURCE EXTRACTION ELECTRODE BEAM DEFINING APERTURE

CTUPOLE DEFLECTOR IFFERENTIAL PUMPING APERTURE

RF OUADRUPOLE MASS FILTER

DISRAY ELECTRONICS

F i g . 1. Schematics of t h e UC-HRL h i g h - r e s o l u t i o n SIM/SIMS.

( o p e r a t i n g i n t h e d e c e l e r a t i n g mode [13]), an o c t u p o l e d e f l e c t o r t o scan a c r o s s - o v e r / d i f f e r e n t i a l pumping a p e r t u r e , a d u a l o c t u p o l e d o u b l e - d e f l e c t i o n system and a n e i n z e l l e n s f o r t h e f i n a l f o c u s i n g o f t h e beam on t h e t a r g e t . We o p e r a t e t h e c o l - umn a t a probe v o l t a g e Vp i n t h e 4 0 - 50 kV range, w i t h a source e x t r a c t i o n v o l t a g e Vs o f 7

-

9 kV.

A p l o t o f t h e c a l c u l a t e d [l 21 probe d i a m e t e r dp vs source acceptance a n g l e a.

i s shown i n F i g . 2 f o r probe v o l t a g e s 30 and 50 kV, and f o r two h y p o t h e t i c a l

c h o i c e s o f t h e v i r t u a l s i z e o f t h e source, 1 0 a n d 50nm r e s p e c t i v e l y . We have t h u s f a r

e x p l o r e d t h e probe shape and s i z e f o r d~

Probe Current for

2

= 20 l S r two s e t t i n g s o f t h e beam d e f i n i n g a p e r -

,

^{10-~4 10-~3} , 0 - ~ 2

1 d ' O

t u r e , 25 and 12.5 pm i n diameter, 0 UC-HRL HIGH RE OLUTION SIM which determine a n g l e s a. o f 0.78 and Go' probe slze vs a,

for A € = 10 eV F W H M ,

0.39 mr r e s p e c t i v e l y . A FWHM o f ~rnoqe d~stance = 3 cm Vs = l 0 kV 3 0 keV-

90 and 43 nm has been observed f o r t h e 102

,/O

/ 50 keV

gaussian-shaped p r o f i l e o f t h e probe i n 5 0 n m Vtrtuol

t h e s e two cases, a t 40 kV, by s p u t t e r - e t c h i n g grooves i n Au-coated S i wafers.

Measured F W H M probe

The c o r r e s p o n d i n g probe c u r r e n t s f o r s u e ot 40 keV

i d e n t i c a l source c u r r e n t Is o f .L 2 UA ^a

were 32 and 8 pA. These r e s u l t s i n d i - Source Size

c a t e a s t r i c t c h r o m a t i c - a b e r r a t i o n - l i m i t e d regime, w i t h probe d i a m e t e r

1

- I O - ~ 1 0 ‘ ~ I O - ~ I O - ~

Accepted Holf-Angle ot Source a, (rod1

p r o p o r t i o n a l t o a,, and probe c u r r e n t F i g . 2. C a l c u l a t e d probe d i a m e t e r ( d p ) t o a,'. kle w i l l , i n due course, versus beam acceptance h a l f - a n g l e a t

t h e source ( a o ) .

t r a n s f e r r i n g Rayleigh's c r i t e r i o n f o r A i r y ' s d i s k s t o Gaussian d i s k s , should c o r r e - spond approximately t o t h e FWHM of t h e beam s p o t . Indeed, i t has been p o s s i b l e t o resolve s t r u c t u r e s 40 nm a p a r t i n micrographs obtained a t t h e s e t t i n g s which yielded a s p o t FWHM of 43 nm.

11. DETECTION, IMAGING AND SIMS INSTRUMENTATION

The d e t e c t i o n of t h e ion-induced secondary e l e c t r o n (ISE) signal o r ion-induced secondary

ion

( I S I ) s i g n a l , f o r imaging of t h e s u r f a c e topography and/or of material c o n t r a s t , i s accomplished i n t h e same manner a s described [14,15] f o r our extensive i n v e s t i g a t i o n s of SIM imaging with t h e prototype UC-SIM. In t h e UC-HRL SIM, two channel e l e c t r o n mu1 t i p 1 i e r d e t e c t o r s (CEM) overlook t h e t a r g e t region, a s shown i n Fig. 1 . A 280 based microprocessor c o n t r o l s a l l t h e beam handling o p e r a t i o n s , scan f u n c t i o n s and video image processing, f o r s i g n a l s processed through an analog a m p l i f i e r . All micrographs shown here were obtained i n s t e a d by pulse-mode imaging, i n which each detected pulse i s amplified, shaped and displayed with v a r i a b l e width and amplitude on t h e CRT. The d i g i t a l r a s t e r s i z e and dwell time per pixel can be varied over a wide range t o provide f o r rapid scans f o r visual focusing, astigmatism c o r r e c t i o n and specimen searching, a s well a s s i n g l e pass high r e s o l u t i o n scans f o r image recording. The CRT used has a r e s o l u t i o n of 1024 l i n e s and an image s p o t s i z e of s 0.1 m m on an 8 X 1 0 cm2 ( 8 X 8 cm2 used) s c r e e n . Images, 7 X 7 cm2 i n s i z e , a r e recorded on Polaroid f i l m .

The c o l l e c t i o n , energy a n a l y s i s and t r a n s p o r t of t h e secondary ions t o t h e RF quadrupol e mass f i l t e r (Extranuclear 300-1 0 ) follows a scheme conceptually s i m i l a r t o t h a t developed by Wittmaack.[l6] The secondary ions emerging from t h e t a r g e t a r e f i r s t a c c e l e r a t e d and energy analyzed by a 90" c y l i n d r i c a l e l e c t r o s t a t i c prism. A high transmission t r a n s p o r t system subsequently focuses t h e secondary i o n s , a f t e r d e c e l e r a t i o n , so a s t o match t h e acceptance requirements of t h e RF quadrupole. In i t s present c o n f i g u r a t i o n , t h e a n a l y s i s - t r a n s p o r t system (ATS) t r a n s m i t s ions within a 10 eV energy window. The e n t i r e ATS i s contained within a depth of 2 cm down- stream of t h e o b j e c t i v e l e n s so a s not t o unduly i n c r e a s e t h e working d i s t a n c e of t h e l a t t e r , with consequent s p o t s i z e degradation. An o f f s e t voltage of 5 - 1 0 V i s u s u a l l y maintained between t h e t a r g e t and t h e quadrupole. We have thus f a r operated with p o s i t i v e I S I ' s . Minor high v01 tage feedthrough modifications a r e being implemented f o r an e f f i c i e n t SIMS d e t e c t i o n of negative ions. The SIMS CEM signal i s handled i n t h e same manner a s t h e ISE and IS1 s i g n a l s a r e used f o r pulse- mode imaging. Mass s p e c t r a a r e accumulated with a mu1 tichannel s c a l e r (MCS) and p r i n t e d on a c h a r t recorder.

I11

-

CONDITIONS FOR HIGH SPATIAL RESOLUTION SIMS IMAGING

To f u l l y e x p l o i t t h e a v a i l a b l e 106 pixels/frame i n our SIM o r SIMS d i s p l a y sys- tem, c e r t a i n conditions have t o be met. The f i r s t regards t h e useful magnification.

C9-200 JOURNAL

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PHYSIQUE

O p t i m a l l y , one wishes i n f a c t t o nap a sample a r e a o f 1000 dp X 1000 dp o n t o a lOOOx 1000 p i x e l s on t h e CRT image, 8 X 8 cm2 i n s i z e . T h i s i m p l i e s a maximum m a g n i f i - c a t i o n o f "-' 2000, f o r a s p o t s i z e of a 40 nm, w h i c h a l l o w s t o v i e w a n a r e a 40 X 40 um2 on t h e specimen. No f u r t h e r improvement i n o b j e c t r e s o l u t i o n i s g a i n e d a t h i g h e r m a g n i f i c a t i o n , b u t r a t h e r a r e d u c t i o n i n t h e e f f e c t i v e number o f p i x e l s p e r frame w i l l t a k e p l a c e . For t h i s reason, t h e SIMS maps t o be shown h e r e w i l l be m o s t l y a t t h e m a g n i f i c a t i o n s c l o s e s t t o t h e optimum ( a c t u a l l y x2000 o r ~ 4 0 0 0 ) .

Another c o n d i t i o n i s imposed b y t h e t i m e o f f l i g h t o f t h e secondary i o n s . I t t a k e s an i o n w i t h mass M and average k i n e t i c energy E, a t i m e tf = R ( ~ E / M ) - $ t o t r a v e l t h r o u g h t h e ATS

+

RF quadrupole, a n o v e r a l l d i s t a n c e R. I n o r d e r t o m a i n t a i n on t h e CRT a s i g n a l synchronous w i t h t h e r a s t e r scan, t h e d w e l l t i m e p e r p i x e l , t d , must be l o n g e r t h a n tf. For o u r system, t h i s c o n d i t i o n i s s a t i s f i e d f o r

t d > I O - ~ ( M ) ~ ( s e c ) , f o r M i n amu. T h i s i s shown i n t h e p l o t o f t d vs M o f F i g . 3.

A s i d e f r o m t h e above g e o m e t r i c a l and k i n e m a t i c a l c o n d i t i o n s , one wishes o f c o u r s e t o m a i n t a i n t h e s t a t i s t i c s o f c o u n t s / p i x e l a t a l e v e l e n s u r i n g a m e a n i n g f u l s i g n a l / n o i s e r a t i o . T h i s i m p l i e s t h e c h o i c e o f a l a r g e r l o w e r l i m i t on td, as d i c - t a t e d by t h e i o n y i e l d s f o r a p a r t i c u l a r sample. An upper bound on t d however e x i s t s , imposed by t h e r e q u i r e m e n t t o m a i n t a i n adequate oxygen coverage o f t h e sam- p l e , needed t o m a x i m a l l y enhance I S 1 e m i s s i o n . [ l 7 ] Two cases must be d i s t i n g u i s h e d here:

a ) . No oxygen replacement i s available. Then one must r e l y on t h e o r i g i n a l O2 coverage. Assuming a monolayer coverage such t h a t t h e number o f oxygen atoms p e r u n i t area e q u a l s t h e s u r f a c e d e n s i t y No o f sample atoms, and equal s p u t t e r - i n g y i e l d Y f o r both, t h e l i m i t on td i s t d < No/JpY, where JP i s t h e probe c u r r e n t d e n s i t y . Such l i m i t f o r Y = 2, JP = 0.5 A/cm2 i s ^.L 160 vs, as shown i n F i g . 3. I n p r a c t i c e , we have found i n many cases t h a t d w e l l t i m e s p e r p i x e l up t o "-' 500 u s can be t o l e r a t e d b e f o r e t h e I S 1 y i e l d s a r e s e r i o u s l y a f f e c t e d b y oxygen d e p l e t i o n . Furthermore, when t h e a r e a scanned i s l a r g e r t h a n t h e op- t i m a l 40 X 40 urn2 ( u n i f o r m l y s p u t t e r e d w i t h 1 0 6 p i x e l s ) , gaps untouched by t h e beam w i l l e x i s t between p i x e l s d e p l e t e d i n a f i r s t scan. A s l i g h t s h i f t i n o b j e c t p o s i t i o n w i l l t h e n a l l o w ad-

d i t i o n a l maps t o be r e c o r d e d even a t maximum d w e l l t i m e p e r p i x e l .

b )

.

Oxygen replacement i s availa3le. ,,j3 Here t h e r e e x i s t s a we1 l known

2

c o n d i t i o n [ l 7 1 on t h e r a t e o f oxygen m o l e c u l e i m p a c t on t h e sample, u l t i m a t e l y on t h e O2 am-

2

b i e n t p r e s s u r e P(O2), t o m a i n t a i n

s a t u r a t e d Oz coverage d u r i n g t h e Secondary Ion Mass Number (amul s p u t t e r i n g process. E x t e n d i n g t h e

F i g . 3. Dwell t i m e c o n d i t i o n s f o r h i g h d e r i v a t i o n b y B l a i s e and s p a t i a l r e s o l u t i o n SIMS imaging.

frame, we want t o m a i n t a i n t h e f r a c t i o n a l oxygen coverage 8 equal t o 1, i . e . ,

where S(0) i s t h e O2 s t i c k i n g c o e f f i c i e n t , m t h e mass o f t h e 07. molecule, k t h e Boltzmann c o n s t a n t , T t h e a b s o l u t e temperature, Y ( 8 ) t h e s p u t t e r i n g y i e l d o f t h e sample, no t h e number d e n s i t y o f t h e e q u i v a l e n t a d s o r p t i o n s i t e s and o t h e e j e c t i o n c r o s s s e c t i o n f o r t h e adsorbed 02. Since S ( @ ) 5 1 and Y(8) i s t o remain c o n s t a n t i n t d , a c o n d i t i o n on P(02) can be d e r i v e d from ( 1 ), t o m a i n t a i n s a t u r a t e d oxygen coverage :

I f we assume [ l 8 1 no = No = s u r f a c e atomic d e n s i t y o f t h e s o l i d , and o i s regarded as a geometric c r o s s s e c t i o n (no@ = l ) , we o b t a i n , f o r JP =0.5A/cm2,Y($)=2, T = 300°K, n = 106, as r e l e v a n t i n o u r case, P ( 0 2 )

2

1.8 X 10-E t o r r .

I V

-

ELEMENTAL MAPS WITH THE UC-HRL SIM/SIMS: FIRST RESULTS

We have a p p l i e d o u r new i n s t r u m e n t t o o b t a i n p o s i t i v e I S 1 SIMS s p e c t r a and elemental maps o f a v a r i e t y o f samples, i n a p i l o t program o f r e s e a r c h a p p l i c a t i o n s . As much as f e a s i b l e , we have approached t h e c o n d i t i o n s discussed above f o r h i g h spa- t i a l r e s o l u t i o n SIElS imaging, a l t h o u g h we have r e l i e d on t h e p r i s t i n e O2 coverage f o r I S 1 y i e l d enhancement ( an O2 j e t w i l l be i n s t a l l e d s h o r t l y ) . We have o p e r a t e d w i t h t h e t a r g e t chamber i n t h e l o w 10-E t o r r range. T h i s may have c o n t r i b u t e d some O2 replacement and enabled us t o use e f f e c t i v e l y p i x e l d w e l l t i m e s up t o 500 us.

From t h e p r e v i o u s d i s c u s s i o n , i t appears t h a t we have o p e r a t e d o u r SIM/SIMS system i n c o n d i t i o n s i n t e r m e d i a t e between " s t a t i c " and "dynamic" SIMS [18]. I n f a c t , f o r a sample surface area o f 40 X 40 um2, t h e s p u t t e r e r o s i o n r a t e i s 6 ^X 10-3 atomic l a y e r s / s e c , f o r Y = 2 and 8 pA o f probe c u r r e n t .

We have examined a number o f p a s s i v a t e d i n t e g r a t e d c i r c u i t s i n S i and GaAs, s i l i c a t e m i n e r a l s i n m e t e o r i t e s , glasses, m i n e r a l s o f o r g a n i c o r i g i n , m e t a l s and

a l l o y s . A l i g h t Pd-Au c o a t i n g I I 1 I

o f i n s u l a t i n g samples has p r o - ^- UC-HRL SIM/ S I ~ S

(Gd

probe')

ven adequate i n p r e v e n t i n g Chondr~te ) Pos~t~ve Secondary -

c h a r g i n g e f f e c t s . A1 k a l i- _- r i c h m i n e r a l s have y i e l d e d

'$'

0

elemental c o u n t i n g r a t e s as

h i g h as 2 X TO5 cps o r 2 X 1 0 " ~ ' ~ -

0

cps/pA, Ca i n o x a l a t e s and a p a t i t e , and Mg i n o l i v i n e and spine1 r a t e s o f c 1 0 3

cps/pA, A1

,

S i , T i and a1 so I I I I Channel Nurnber IOPO]

Cu, r a t e s i n t h e 1-2 X 10' 0 20 40 M/e 60 80 100

F i g . 4. Example o f SIllS spectrum.

cps/pA range. F i g . 4 shows

C9-202 J O U R N A L DE PHYSIQUE

an example o f o u r SIMS mass s p e c t r a , o b t a i n e d i n t h e constant-aM mode o f o p e r a t i o n o f t h e RF quadrupole, accumulated i n a 200 X 200 pm2 scan o v e r 42 m i n u t e s . The o b j e c t i s a p o l i s h e d s e c t i o n o f a s t o n y m e t e o r i t e (Mezo-Madaras, t y p e 3 c h o n d r i t e ) , composed p r i m a r i l y o f s p i n e l , o l i v i n e and o t h e r s i l i c a t e m i n e r a l s and g l a s s e s . Elemental maps o f ~ a ~ ~ and Mg2' f o r one area, and Fes6 f o r a n o t h e r a r e a o f t h e s e c t i o n a r e shown i n F i g . 5. Continuous t o n e images a r e o b t a i n e d i n s e v e r a l a r e a s o f t h e maps, a t s p a t i a l r e s o l u t i o n which approaches t h e probe r e s o l u t i o n . The extreme d i f f e r e n t i a t i o n we observe, on a w i d e r range of elemental sampling, a c t u a l l y p e r m i t s i d e n t i f i c a t i o n o f i n d i v i d u a l m i n e r a l g r a i n s .

F i g s . 6 a,c, a r e L i 7 maps o b t a i n e d f r o m an Au-coated S i wafer, a t X 1800 and x 3 6 0 0 r e s p e c t i v e l y . S i m i l a r maps have been observed f o r K3', which suggests we a r e d e t e c t i n g t h e r e s i d u e o f a d e t e r g e n t o r e t c h a n t smear. I s l a n d s v a r y i n g i n s i z e f r o m a few pm t o t h e l i m i t s o f image r e s o l u t i o n (2.40 nm i n t h e o r i g i n a l m i c r o g r a p h o f F i g . 6 c ) a r e c l e a r l y o u t l i n e d i n b o t h maps, b u t many o f t h e s m a l l e r g r a i n s d i d n o t s u r v i v e t h e i n c r e a s e d e r o s i o n r a t e o f t h e x3600 scan. F i g . 6 b , d , a r e maps o f a sec- t i o n o f F e r m i l a b s u p e r c o n d u c t i n g w i r e (Ti-Nb a l l o y w i r e s embedded i n a Cu m a t r i x ) , f o r C Uand T i k 8 r e s p e c t i v e l y . ~ ~ A Nbg3 map, s i m i l a r t o t h a t o f T i 4 8 , has a l s o been o b t a i n e d . A l t h o u g h t h e s e c t i o n was e t c h e d i n n i t a l ( 1 0 % HNO, i n e t h a n o l ) t o e l i m - i n a t e t h e smearing o f t h e s t r u c t u r e due t o s e c t i o n i n g and p o l i s h i n g , some Cu f r o m t h e m a t r i x i s s t i l l p r e s e n t i n t h e Ti-Nb w i r e s e c t i o n s , b u t n o t t h e r e v e r s e .

Elemental maps o f a p a s s i v a t e d i n t e g r a t e d c i r c u i t a r e shown i n F i g . 7 f o r AI2', S i 2 8 and Ca40, t o g e t h e r w i t h an I S 1 image d e s c r i p t i v e o f t h e s u r f a c e topography.

Only t h e A1 map c o u l d be o b t a i n e d p r i o r t o c o a t i n g t h e sample w i t h Pd-Au, due t o c h a r g i n g o f t h e i n s u l a t i n g areas. The maps shown h e r e and o t h e r s , t a k e n a f t e r c o a t - ing, r e v e a l S i areas presumably f r o m SiO, and t h e presence o f a g l a s s l a y e r c o v e r i n g t h e e n t i r e sample.

V

-

CONCLUSION

T h i s i n t r o d u c t o r y p r e s e n t a t i o n o f SIMS r e s u l t s o b t a i n e d w i t h t h e UC-HRL SIM f u r t h e r s t h e r e a l i z a t i o n , a l r e a d y p e r c e i v e d i n view o f p r e v i o u s r e s u l t s [6-111, t h a t a d e c i s i v e advance i n SIMS imaging m i c r o a n a l y s i s has m a t e r i a l i z e d w i t h t h e use o f LMIS i n f o c u s i n g i o n probes. Elemental maps o f unusual d e f i n i t i o n and l a t e r a l r e s o - l u t i o n c l o s e t o t h e 40 nm l e v e l have been shown t o be p r a c t i c a l w i t h a 40 kV Gaf probe, even p r i o r t o r e c o u r s e t o O2 I S 1 y i e l d s enhancement. We e x p e c t an i n c r e a s e i n l a t e r a l r e s o l u t i o n t o t h e 1 0 nm l e v e l , where t h e probe c u r r e n t w i l l be %l pA, t o be w i t h i n r e a c h and s t i l l u s e f u l , i n view o f t h e elemental I S 1 y i e l d s observed w i t h o u r SIllS system. A j u d i c i o u s c h o i c e o f t h e m a g n i f i c a t i o n and p i x e l d w e l l t i m e can y i e l d o p t i m a l i n f o r m a t i o n i n t h e mapping o f s e v e r a l i m p o r t a n t elements a t t h e p r e s e n t l e v e l o f r e s o l u t i o n , i n v o l v i n g a sample consumption l i m i t e d t o .L 1 atomic monol ayer.

a. ~ a ' ~ , 256 sec., 4.3 ^X 1 0 6 counts. c . Mg2", same area as a., 5 1 2 s e c . , 1 . 5 x 1 0 6 counts.

b. A I z 7 , 512 sec., 3 . 2 ~ 1 0 5 c o u n t s . d. Fe56, 512 sec., 4 . 5 ~ 1 0 5 c o u n t s . Ac know1 edgements

T h i s work was s u p p o r t e d b y t h e A i r Force O f f i c e o f S c i e n t i f i c Research (Con- t r a c t F 49620-83-C-Oll O), t h e N a t i o n a l Science F o u n d a t i o n under Grant No. DMR-8007978, and p a r t i a l l y by t h e NSF M a t e r i a l s Research L a b o r a t o r y a t t h e U n i v e r s i t y o f Chicago.

We a r e i n d e b t e d t o P.H. LaMarche f o r h i s p a r t i c i p a t i o n i n t h e most c r i t i ~ a ! f i n a l phase of t h e c o n s t r u c t i o n and t u n i n g o f o u r SIM/SIMS system. We w i s h t o t h a n k o u r c o l l a b o r a t o r s N. W. Parker, W. P. Robinson, R. L. S e l i g e r and J. W. Ward f o r t h e i r most v a l u a b l e c o n t r i b u t i o n s t o t h e r e a l i z a t i o n o f t h e new microprobes. We a r e a l s o i n - debted t o R. L. Sel i g e r f o r t h e l o a n o f s e v e r a l Ga LMIS, and t o E. Olson o f F i e l d Museum of N a t u r a l H i s t o r y f o r t h e l o a n o f t h e m e t e o r i t e s e c t i o n .

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