SUPPRESSION OF MOLECULAR IONS IN SECONDARY ION MASS SPECTRA

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SUPPRESSION OF MOLECULAR IONS IN SECONDARY ION MASS SPECTRA

N. Mcintyre, W. Chauvin, J. Metson, G. Bancroft

To cite this version:

N. Mcintyre, W. Chauvin, J. Metson, G. Bancroft. SUPPRESSION OF MOLECULAR IONS IN

SECONDARY ION MASS SPECTRA. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-143-C2-

146. �10.1051/jphyscol:1984232�. �jpa-00223945�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°2, Tome 45, février 1984 page C2-H3

SUPPRESSION OF MOLECULAR IONS IN SECONDARY ION MASS SPECTRA

N.S. Mclntyre, W.J. Chauvin, J.B. Metson and G.M. Bancroft

Surface Science Western and Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A SB7

Résumé - Des perfectionnements dans l'élimination des ions moléculaires dans les spectres SIMS ont été obtenus à l'aide d'un analyseur ionique : Cameca IMS-3F. Cette étude montre une diminution du nombre d'ions moléculaires d'un facteur 10 à 100 par rapport aux techniques classiques utilisant un décalage d'énergie.

Abstract - Improved suppression of molecular ions in SIMS spectra has been ob- tained using a Cameca IMS-3F ion microscope. The approach described results

in the suppression of molecular ions by factors of 10-100 fold more than by classical "energy offset".

Introduction

One of the most attractive characteristics of Secondary Ion Mass Spectrometry (SIMS) for surface analysis is its capability for detecting many elements at a trace con- centration. Unfortunately, in practice, the detection of trace elements is often masked by interference from molecular ions of the same nominal mass. In the past, one method of reducing such interference has been to use higher mass resolving power

in the spectrometer to distinguish molecular and atomic ions from their mass defects.

An approach more generally applicable to all SIMS systems has been the use of kine- tic energy analysis of the secondary ions to suppress molecular species. The kine- tic energy distributions of atomic and molecular ions usually differ significantly within the lowest 100 eV of energy distribution. The work described here outlines some improvements to the use of kinetic energy analysis which result in some signi- ficant improvements of the trace analytical capabilities of SIMS.

During our recent SIMS studies of insulating geologic materials (1), it was dis- covered that evidence of molecular ions in the spectrum was reduced dramatically when an electrically insulated specimen surface was bombarded with a diffuse beam of 0

ions. In this note we describe new experiments to eluciate the molecular ion suppre- ssion effect, as well as applications of the effect to the analysis of silicon, metal alloy and geologic surfaces.

Experimental

A Cameca IMS-3F ion microscope was used for all experiments. The instrument uses a duoplasmatron ion source from which either 0

or 0 primary ion beams can be ex- tracted. Certain of the experiments were done with a primary ion beam mass filter in place. Generally, a primary beam current of 200-400 nA was focussed into a 50 urn spot and could, if desired, be rastered over a 250 X 250 urn area.

The only modifications to the normal instrumentation occurred in the specimen holder (see Figure 1). The specimen, normally mounted in contact with the stage, was instead isolated electrically and offset behind the stage with a 1 mm teflon spacer. A tan- talum mask with a 5 mm diameter hole was secured above the specimen and attached electrically to the stage. During certain of the experiments the specimen potential was allowed to float freely, controlled by the primary and secondary ion currents in equilibrium. In other experiments, the specimen or specimen and mask potentials were controlled by a Sorensen high voltage power supply.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984232

JOURNAL DE PHYSIQUE

0-

TO ANALYZERS BEAM

-\- -IMMERSION LENS f

FIGURE 1 - E x p e r i m e n t a l Arrangement f o r Secondary

I o n Mass S p e c t r o m e t r y by t h e Specimen l s o l a t i o n Technique i n t h e Cameca

IMS-3F.

\ /ILLUMINATION HOLE

c,\<Lzy~~~~~2 ^(+4500V)

INSULATOR (-1 mm)

'FLOATING" SAMPLE STAGE (<+4500V)

R e s u l t s

( i ) S i l i c o n - ^A specimen o f p o l i s h e d s i n g l e c r y s t a l s i l i c o n was mounted i n t h e s p e c t r o m e t e r , so t h a t i t was e l e c t r i c a l l y i s o l a t e d f r o m t h e mask. A mass f i l t e r e d 0 p r i m a r y beam produced t h e p o s i t i v e secondary i o n spectrum shown i n F i g u r e 2.

I MASS NUMBER - ^I

lo6-

T

>-

5 ^1oL-

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W I-

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FIGURE 2 - Secondary I o n Mass S p e c t r o m e t r y o f S i n g l e C r y s t a l S i l i c o n o b t a i n e d by t h e Specimen I s o l a t i o n Technique.

A n o r m a l l y mounted spe$imen+of s i l i c o n would c o n t a i n l a r g e c o n t r i b u t i o n s f r o m +

m o l e c u l a r s such as S i O , Si,, Si,O , e t c . w i t h t y p i c a l s ~ o + / s ~ + i n t e n s i t y r a t i o s o f .- 5 X 10 '. I n t h i s case, u s i n g t h e specim n i s o l 2 t i o n ( S l ) + t e c h n i q y e , t h e o n l y m o l e c u l a r ions v i s i b l e , w i t h i n a f a c t o r o f 1 0 : o f S i , a r e SiO and Si,. Thus, con- t r i b u t i o n s f r o m s u r f a c e i m p u r i t i e s such as Fe , i.li+, Cu, Zn and Sb become more read- i l y d e t e c t a b l e . O f p a r t i c u l a r i n t e r e s t , i s t h e v e r y low c o n t r i b u t i o n f r o m ~ O S ~ H + a t mass 31, which n o r m a l l y p a r t i a l l y masks t h e d e t e c t i o n o f phosphorhs.

10 20 30 40 50 60 70 80 90 100

Si+ Single Crystal Silicon

( i i ) S t e e l A l l o y Reference M a t e r i a l - A s e r i e s o f NBS 1260 s e r i e s i r o n a l l c y s were analyzed, b o t h under e l e c t r i c a l l y i s o l a t e d ( S I ) c o n d i t i o n s and w i t h an e x t e r n a l v o l t a g e c o n t r o l l i n g t h e specimen p o t e n t i a l .

S I Conditions

Primary Beam Fiitered

SiO+

Si;

I I I I ^I I .

o+

F+

I

Si*

C+

0"

I

The m o l e c u l a r i o n s u p p r e s s i o n i n c r e a s e d i n v e r s e l y w i t h t h e p o t e n t i a l d i f f e r e n c z appl i e d between t h e specimen s u r f a c e and t h e normal mask v o l t a g e ( + 4 5 0 0 ~ ) . TotaT secondary i o n c u r r e n r decreased w i t h t h e p o t e n t i a l d i f f e r e n c e . The spectrum o b t a i n - ed w i t h a specimen p o t e n t i a l o f -300 v o l t s i s shown i n F i g u r e 3a.

S?ECIMEN

-

F 1 GURE 3. A Comparison o f S lMS S p e c t r a o f NBS r e f e r e n c e mater i a l 1263 t a k e n under M o l e c u l a r I o n Suppression C o n d i t i o n s . ( a ) S l + c o n d i t i o n s w i t h an 0 p r i m a r y beam.

(b) Normal Cameca o f f s e t c o n d i t i o n s w i t h an 0, p r i m a r y beam.

M o l e c u l a r i o n s such as F ~ O + and C ~ O + a r e about 3-4 o r d e r s o f magnitude lower i n l n - t e n s i t y t h a n t h e y would be i n an e q u i v a l e n t spectrum taken under n o n - i s o l a t e d con- d i t i o n s . As a r e s u l t , m i n o r and t r a c e elements, p a r t i c u l a r l y t h o s e w i t h tl/e>60, a r e more r e a d i l y d e t e c t a b l e .

L i n e a r c a l i b r a t i o n p l o t s f o r t h e a l l o y s e r i e s y i e l d t h e f o l l o w i n g ( 3 6 d e t e c t i o n l i m i t s i n an i r o n m a t r i x : Ti-0.6 ug/g; Cr-2 ug/g;Cu-15ug/g and As-70 ug/g.

P r e v i o u s l y used methods o f k i n e t i c energy a n a l y s i s i n SlMS ( 2 ) have a l s o produced a p p r e c i a b l e s u p p r e s s i o n o f t h e m o l e c u l a r i o n component, b u t n o t t o t h e same e x t e n t as i n t h e s e p r e s e n t e x p e r i m e n t s . I n t h e Cameca IMS-3F t h e normal method o f k i n e t i c energy a n a l y s i s i s t o o f f s e t t h e normal specimen s t a g e b i a s by up t o 125 v o l t s and t o reduce t h e k i n e t i c energy spread passed t h r o u g h t h e e l e c t r o s t a t i c a n a l y z e r . The use o f such a p r o c e d u r e w i t h t h e same a1 l o y s t a n d a r d i s shown i n F i g u r e 3b. Both o f f s e t v o l t a g e and s l i t w i d t h s were s e t t o o b t a i n a minimum =+ r a t i o . The

Fe

C2-146 JOURNAL DE PHYSIQUE

s u p p r e s s i o n f a c t o r o b t c i i i e d was a b o u t two o r d e r s o f magnitude l e s s t h a n o b t s i n e d by specimen i s o l a t i o n and was a c h i e v e d w i t h an o v e r a l l l o s s i n i o n i n t e n s i t y .

( i i i ) G e o l o g i c Specir~iens - The a n a l y s i s o f ceramic and m i n e r a l s u r f a c e s has been v e r y d e s i r a b l e f o r t h a s t u d y o f i s o t o p e r a t i o s , s u r f a c e t r a n s f o r m a t i o n s and t r a c e element m i c r o d i s t r i b u t i o n . However, t h e s p e c t r a a r e u s u a l l y r a t h e r complex due t o m o l e c u l a r i o n c o n t r i b u t i o n s and c h a r g i n g o f t h e s u r f a c e o f t e n reduces o r c o m p l e t e l y e l i m i n a t e s t h e secondary i o n c u r r e n t . The use o f specimen i s o l a t i o n c o n d i t i o n s f o r t h e a n a l y s i s o f m i n e r a l s has t h u s been p a r t i c u l a r l y v a l u a b l e . I n s u l a t i n g specimens a r e sometimes mounted w i t h a s p u t t e r e d g o l d o v e r l a y e r , b u t most specimens have been s u c c e s s f u l l y a n a l y z e d w i t h n o o v e r l a y e r , s i n c e t h e g e o m e t r i c arrangements appears t o c r e a t e c o n d i t i o n s l e c d i h ~ ; t o a s t a b l e c h a r g e e q u i l i b r i u m , n o t u s u a l l y e n c o u n t e r e d w i t h i n s u l a t o r s i n SlMS e x p e r i m e n t s .

A SlMS spectrum o f a n a t u r a l sphene (CaSiTiO ) m i n e r a l , wi'th an uncoated s u r f a c e i s shown i n F i g u r e 4. Of p a r t i c u l a r n o t e i s t h z a l m o s t t o t a l absence o f v i s i b l e o x i d e peaks and t h e c l e a r i n d i c a t i o n s o f t h e t r a c e l a n t h a n i d e and a c t i n i d e elements.

FIGURE b - SlMS Spectrum o f a N a t u r a l Sphene ancr croft, O n t a r i o ) o b t a j n e d under S I c o n d i t i o n s u s i n g an 0 p r i m a r y beam.

10E4

10E2

lOE0

100 120 UO 163 180 200 220 240

MASS

D i s c u s s i o n

A complete u n d e r s t a n d i n g o c t h e m o l e c u l a r i o n s u p p r e s s i o n e f f e c t i s s t i l l n o t p o s s i b l e . However, i t i s c l e a r t h a t some o f t h e e f f e c t s r e s u l t s f r o m an extreme d i s c r i m i n a t i o n o f t h e k i n e t i c e n e r g i e s o f a t o m i c and m o l e c u l a r i o n s . I t i s a l s o c l e a r t h a t t h e e f f e c t i s n o t c h i e v e d s i m p l y by a l a r g e r v o l t a g e o f f s e t t h a n i s n o r m a l l y used i n t h e Cameca IMS-3F. A t t e m p t s t o use a l a r g e h o l e i n t h e specimen mask r e s u l t s i n extreme

l o s s o f i o n i n t e n s i t y t h r o u g h o u t t h e spectrum. A l s o , a n a l y s i s o f t h e k i n e t i c e n e r c y d i s t r i b u t i o n s o b t a i n e d under normal sample m o u n t i n g c o n d i t i o n s shows t h a t , a t an o f f , s e t v o l t a g e o f 300-400 eV, a v e r y l o w t o t a l secondary i o n c u r r e n t would be o b t a i n e d . Thus, t h e p r e s e n t specimen i s o l a t i o n mount appear t o c r e a t e an e f f e c t above t h e specimen s u r f a c e which may cause some i n i t i a l s e p a r a t i o n o f k i n e t i c e n e r g i e s . References

( 1 ) METSON, J.B., BANCROFT, G.M., MclNTYRE, N.S. and CHAUVIN, W.J.,"Suppression oi' M o l e c u l a r Ions i n t h e Secondary I o n Mass S p e c t r a o f M i n e r a l , S u r f a c e a n d I n t e r f a c i a l A n a l y s i s " ( a c c e p t e d ) .

(2) SHIMIZU, N., N a t u r e , 299, (1961) 575.