Secondary Ion Mass Spectrometry: A Multidimensional Technique

Oct 17, 1985 - Richard J. Colton1, David A. Kidwell1, George O. Ramseyer2, and Mark M. Ross1. 1 Chemistry Division, Naval Research Laboratory, ...
0 downloads 0 Views 2MB Size
Download PDF
10

Secondary Ion Mass Spectrometry: A Multidimensional Technique Downloaded via YORK UNIV on December 15, 2018 at 07:00:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

1

1

2

1

RichardJ.Colton ,DavidA.Kidwell ,GeorgeO.Ramseyer ,andMarkM.Ross 1

Chemistry Division, Naval Research Laboratory, Washington,DC20375-5000 General Electric Company, Syracuse,NY13221

2

This paper discusses SIMS as a multi-dimensional technique for the analysis of inorganic and organic materials. The paper is divided into two parts: inorganic SIMS and organic (molecular) SIMS. The inorganic SIMS part focuses on methods of quantitative analysis and depth profiling applications. In particular, the parameters that makes SIMS difficult to quantify -secondary ion yield, matrix effects, and instrumental effects - are reviewed as well as the various physical models and empirical methods used to quantify SIMS data. The instrumental and experimental parameters that affect SIMS depth profiling are also reviewed. The organic SIMS part discusses the method of ionization and the various sample preparation and matrix-assisted procedures used for analysis. The matrices include various solid-state and liquid matrices such as ammonium chloride, charcoal, glycerol, and gallium. A neutral beam source was developed to analyze thick, insulating films. Various chemical derivatization procedures have been developed to enhance the sensitivity of molecular SIMS and to selectively detect components in mixtures.

Bombarding a s o l i d s u r f a c e w i t h low energy (keV) i o n s o r n e u t r a l s r e s u l t s i n t h e e m i s s i o n o f secondary p a r t i c l e s : p o s i t i v e and n e g a t i v e i o n s , n e u t r a l s , e l e c t r o n s , and photons. T h i s phenomenon, known as s p u t t e r i n g , i s dependent on s e v e r a l important parameters such as t h e energy, mass, and a n g l e o f t h e i n c i d e n t beam and t h e mass, s t r u c t u r e , and b i n d i n g energy o f t h e atoms which form t h e s u r f a c e o f t h e t a r g e t [ J j . Mass a n a l y s i s o f t h e s p u t t e r e d secondary i o n s forms t h e b a s i s o f secondary i o n mass s p e c t r o m e t r y (SIMS) [ 2 ] . 0097-6156/85/0291 -0160509.50/0 © 1985 American Chemical Society

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10.

COLTON ET AL.

161

SIMS

As a s u r f a c e a n a l y t i c a l t o o l , SIMS has s e v e r a l advantages over X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) and Auger e l e c t r o n s p e c t r o s c o p y (AES). SIMS i s s e n s i t i v e t o a l l elements and i s o t o p e s i n t h e p e r i o d i c t a b l e , whereas XPS and AES cannot d e t e c t H and He. SIMS a l s o has a lower d e t e c t i o n l i m i t o f «ν 10-5 atomic p e r c e n t (at.JS) compared t o 0.1 at.5S and 1.0 a t . % f o r AES and XPS, r e s p e c t i v e l y . However, SIMS has s e v e r a l d i s a d v a n t a g e s . I t s e l e m e n t a l s e n s i t i v i t y v a r i e s over f i v e o r d e r s o f magnitude and d i f f e r s f o r a g i v e n element i n d i f f e r e n t sample m a t r i c e s , i . e . , SIMS shows a s t r o n g m a t r i x e f f e c t . This matrix e f f e c t makes SIMS measurements d i f f i c u l t t o q u a n t i f y . Recent p r o g r e s s , however, has been made e s p e c i a l l y i n t h e development o f q u a n t i ­ t a t i v e models f o r t h e a n a l y s i s o f s e m i c o n d u c t o r s [ 3 - 5 ] . SIMS methodology has e v o l v e d a l o n g two d i s t i n c t l i n e s . The f i r s t and o r i g i n a l method showed SIMS as an a n a l y t i c a l t o o l f o r depth p r o f i l i n g and m i c r o a n a l y s i s . S p e c i a l i z e d i n s t r u m e n t s w i t h microscope o r microprobe c a p a b i l i t i e s were developed f o r depth p r o f i l i n g , i o n imaging and m i c r o - a r e a a n a l y s i s [ 2 , 6 ] . T h i s SIMS method, commonly r e f e r r e d t o as "dynamic SIMS", uses a r e l a ­ t i v e l y h i g h primary i o n beam f l u x (> 1x10"^ A/cm ) t o g e n e r a t e specimen s p u t t e r i n g r a t e s o f > 5 0 A / m i n . The h i g h s p u t t e r i n g r a t e s enhance t h e s e n s i t i v i t y o f t h e method. The dynamic SIMS method has been a p p l i e d p r i m a r i l y t o s t u d i e s i n e l e c t r o n i c t e c h n o l o g y and m a t e r i a l s c i e n c e [ 7 - 9 ] . 2

c

The second SIMS method was p i o n e e r e d by A. Benninghoven ( U n i v . o f Munster, West Germany) i n t h e l a t e I 9 6 0 s and i s c a p a b l e o f a n a l y z i n g s u r f a c e monolayers [ 1 0 ] , To a c h i e v e monolayer s e n s i t i v i t y , i t i s f i r s t n e c e s s a r y t o reduce t h e sample s p u t t e r i n g r a t e by l o w e r i n g t h e primary i o n beam f l u x ( t y p i c a l l y < 1 X 1 0 " A/cm ) and second, i n o r d e r t o compensate f o r t h e c o r r e s p o n d i n g l o s s i n s i g n a l i n t e n s i t y (due t o t h e lower f l u x ) , t h e a n a l y s i s a r e a i s i n c r e a s e d by broadening o r r a s t e r i n g the p r i m a r y i o n beam. T h i s SIMS method known as " s t a t i c " [12,13] o r low damage [ 1 4 ] SIMS has been a p p l i e d t o t h e study o f g a s - s u r f a c e i n t e r a c t i o n s [7-9,15-17]. (The p i o n e e r i n g work o f M a c f a r l a n e (Texas A&M U n i v e r s i t y ) d e a l i n g w i t h C a l i f o r n i u m - 2 5 2 plasma d e s o r p t i o n mass s p e c t r o m e t r y was i m p o r t a n t i n e s t a b l i s h i n g t h a t l a r g e o r g a n i c m o l e c u l e s c o u l d be desorbed as i n t a c t m o l e c u l a r and m o l e c u l a r - l i k e i o n s [ 1 1 ] ) . 1

9

2

As a sub-element o f t h e s t a t i c SIMS methodology, SIMS has become (most r e c e n t l y ) a new i o n i z a t i o n source f o r t h e a n a l y s i s of n o n v o l a t i l e and t h e r m a l l y l a b i l e m o l e c u l e s i n c l u d i n g polymers and l a r g e b i o m o l e c u l e s such as p r o t e i n s . S i n c e most o f t h e s e l a t t e r s t u d i e s deal with the emission of polyatomic or molecular i o n s , t h e name " m o l e c u l a r SIMS" has been a p p l i e d [ 1 8 - 2 1 ] . The a p p l i c a t i o n o f m o l e c u l a r SIMS a s a s e n s i t i v e i o n i z a t i o n source f o r n o n v o l a t i l e and t h e r m a l l y l a b i l e m o l e c u l e s compares f a v o r a b l y w i t h o t h e r new i o n i z a t i o n methods i n mass s p e c t r o m e t r y such as f i e l d d e s o r p t i o n ( F D ) , C a l i f o r n i u m - 2 5 2 plasma d e s o r p t i o n (PD), f a s t heavy i o n induced d e s o r p t i o n ( F H I I D ) , l a s e r desorp-

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

162

DESORPTION MASS

SPECTROMETRY

t i o n (LD) o r l a s e r m i c r o p r o b e mass a n a l y s i s (LAMMA), and f a s t - a t o m bombardment (FAB) o r l i q u i d SIMS [21-23], In each o f t h e s e t e c h n i q u e s , t h e m o l e c u l e s a r e desorbed and i o n i z e d d i r e c t l y from t h e s o l i d s t a t e and appear as m o l e c u l a r and/or m o l e c u l a r - l i k e (protonatçd, d e p r o t o n a t e d , and/or c a t i o n i z e d ) i o n s , e.g., Μ±·, [M ± H ] " and [M + c a t i o n ] * . T h i s paper d i s c u s s e s SIMS as a m u l t i - d i m e n s i o n a l technique f o r t h e a n a l y s i s o f i n o r g a n i c and o r g a n i c m a t e r i a l s . The paper i s d i v i d e d i n t o two p a r t s : i n o r g a n i c and o r g a n i c ( o r m o l e c u l a r ) SIMS. The i n o r g a n i c SIMS p a r t f o c u s e s on t h e methods o f q u a n t i t a t i v e a n a l y s i s and depth p r o f i l i n g a p p l i c a t i o n s . I n p a r t i c u l a r , SIMS m a t r i x e f f e c t s a r e d e f i n e d and t h e p h y s i c a l models and e m p i r i c a l methods used t o q u a n t i f y SIMS r e s u l t s a r e reviewed. The e m i s s i o n o f m o l e c u l a r i o n s i n o r g a n i c SIMS i s d i s c u s s e d w i t h r e s p e c t t o t h e method o f i o n i z a t i o n and t h e v a r i o u s sample p r e p a r a t i o n and m a t r i x - a s s i s t e d p r o c e d u r e s used. The m a t r i c e s i n c l u d e v a r i o u s s o l i d - s t a t e and l i q u i d m a t r i c e s such as ammonium c h l o r i d e , c h a r c o a l , g l y c e r o l , and g a l l i u m . A n e u t r a l beam source i s described t o analyze t h i c k i n s u l a t i n g f i l m s . Various c h e m i c a l d e r i v a t i z a t i o n p r o c e d u r e s have been developed t o enhance t h e s e n s i t i v i t y o f m o l e c u l a r SIMS and t o s e l e c t i v e l y d e t e c t components i n m i x t u r e s . I n o r g a n i c SIMS The r e s u l t s d i s c u s s e d i n t h i s s e c t i o n d e a l p r i m a r i l y w i t h t h e methods used t o q u a n t i f y dynamic SIMS r e s u l t s o b t a i n e d from depth p r o f i l i n g s t u d i e s o f i n o r g a n i c m a t e r i a l s such as semi­ conductors. Q u a n t i t a t i v e A n a l y s i s . SIMS has many unique f e a t u r e s (compared t o o t h e r s u r f a c e a n a l y t i c a l t e c h n i q u e s ) such as hydrogen and i s o t o p e d e t e c t i o n , a d e t e c t i o n l i m i t as low as 10"^g, s u r f a c e / monolayer s e n s i t i v i t y , compound s p e c i f i c i t y , and h i g h s p a t i a l r e s o l u t i o n ( 1 X 10"^ A/cm ).

of

2

F i g u r e s 4 and 5 show t h e m o l e c u l a r SIMS s p e c t r a o f phenan­ t h r e n e and 9-aminophenanthrene t a k e n from f o u r d i f f e r e n t m a t r i c e s : Ag f o i l , NH4CI, c a r b o n , and a l i q u i d m e t a l [ 1 0 3 ] . We found t h a t t h e m o l e c u l a r i o n y i e l d s and i o n i z a t i o n e f f i c i e n c e s f o r phenanthrene ( a t m/z 178) were s i m i l a r from the Ag, NH4CI and c a r b o n m a t r i c e s , but t h a t the s i g n a l - t o - b a c k g r o u n d r a t i o was much b e t t e r from the c a r b o n m a t r i x . (The mass peak a t m/z 191 i s a t t r i b u t e d t o a contaminant i n e i t h e r t h e vacuum chamber o r sample. S i n c e many d i f f e r e n t p o l y c y c l i c a r o m a t i c hydrocarbon compounds had been s t u d i e d over many months, t h e vacuum chamber and sample c a r r o u s e l had t o be c l e a n e d p e r i o d i c a l l y t o reduce a c o n t a m i n a t i o n problem.) For t h e 9-aminophenanthrene, on t h e o t h e r hand, i t s secondary i o n y i e l d and i o n i z a t i o n e f f i c i e n c y were enhanced by the NH4CI m a t r i x as e v i d e n t by t h e h i g h e r c o u n t i n g r a t e i n F i g u r e 5. T h i s o b s e r v a t i o n demonstrates how t h e c h e m i c a l p r o p e r t i e s o f NH4CI can i n f l u e n c e t h e secondary i o n e m i s s i o n o f m o l e c u l e s c o n t a i n i n g c e r t a i n f u n c t i o n a l groups. F u r t h e r comparison o f t h e m a t r i c e s showed t h a t t h e Ag m a t r i x was t h e e a s i e s t t o prepare but r e q u i r e d a s t a t i c p r i m a r y beam t o e n s u r e low background and good s i g n a l - t o - n o i s e . The NH4CI m a t r i x enhanced the i o n e m i s s i o n f o r most s u b s t i t u t e d compounds and worked b e s t ( i . e . , h i g h s i g n a l - t o - n o i s e ) w i t h a s t a t i c i o n beam. The u n s u b s t i t u t e d PACs were most e a s i l y a n a l y z e d from c a r b o n where use o f a dynamic primary beam was p o s s i b l e w i t h o u t i n c r e a s i n g the background i o n i n t e n s i t y .

L i q u i d M e t a l S u b s t r a t e . I n i t i a l e x p e r i m e n t s u s i n g a CAMECA i o n m i c r o s c o p e (5.5 keV Ar+ o r 0 + i o n beam at >1 Χ 1 0 " A/cm ) demonstrated t h a t t h e l i q u i d m e t a l ( a g a l l i u m / i n d i u m a l l o y ) p r o v i d e d a s u i t a b l e s u b s t r a t e from which l o n g - l i v e d Μ+· i o n e m i s s i o n o f o r g a n i c m o l e c u l e s o c c u r r e d w h i l e u s i n g a dynamic p r i m a r y i o n beam [100, 102], The e x p e r i m e n t a l s e t - u p f o r the l i q u i d metal s u b s t r a t e i s shown s c h e m a t i c a l l y i n F i g u r e 6. We found t h a t i o n bombardment o f the l i q u i d metal s u r f a c e , upon which sample p a r t i c l e s were d e p o s i t e d , r e s u l t e d i n movement o f t h e sample s p e c i e s towards the primary i o n beam where they a r e desorbed and f i n a l l y d e t e c t e d by t h e mass a n a l y z e r . T h i s l i q u i d metal s u b s t r a t e o f f e r s s e v e r a l advantages over c o n v e n t i o n a l l i q u i d or s o l i d m a t r i c e s (see Table I ) . For example, t h e g a l l i u m / i n d i u m a l l o y has a smooth s u r f a c e onto which s o l i d s can be d i s p e r s e d . The l i q u i d metal i s a l s o c o n d u c t i n g and has a low vapor p r e s s u r e . 6

2

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985. JUAJLJWVA*^^

"•ίif"' •.184

1

4

(M-fOe)" "

m/z»-»*

jJluJUL^

"Ί "Ίι

In?

250cp>

Ζ

j

Ί

, Js^^x

(M-Hn) 293 ι

2500cps

sample

5 k « V A r 1 X1 0 - 7 A / c m 2 30μς

F i g u r e 4. SIMS s p e c t r a o f phenanthrene t a k e n from s i l v e r f o i l , ammonium c h l r o i d e , c a r b o n , and l i q u i d g a l l i u m / i n d i u m a l l o y . Reproduced w i t h p e r m i s s i o n from R e f . 103. C o p y r i g h t 1983, E l s e v i e r S c i e n c e P u b l i s h i n g Co.

liquid Ga/ln

DILUTION

C/Ag

1:100

178

DESORPTION MASS SPECTROMETRY

SPECTRUM

6 k e V A r luu.aJ >

?

METHYLATED METHAMPHETAMINE

3

(b) Derivatized with CH I

JUU

3 ng

(METHAMPHET A MINE+H)

>L i,Ul,i,i...L.

136

llilhtJuuillhl

109

(a) Underivatized

DRUG S T A N D A R D S E T : Group II - Basic Drugs in Human Urine

Ο

q

m

Ό

ζ/5 C/S C/Î

>

Ο Ζ

JO

Ο

s

10.

COLTON ET AL.

SIMS

185

(a) Cortisone

(Μ+ΗΓ

1000 ng

1

(KhAgJ* 469

467

ω υ c

361

2800 ope

L

(M+Ag-60)

(Ο

•σ c

ZS

100 ng

1(1

2500 cpe

m/z

( b ) Derivatized Cortisone

10 ng

T3 C 3