X-ray Photoelectron Spectroscopy Applied to Microelectronic Materials

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8 X-ray Photoelectron Spectroscopy Applied to Microelectronic Materials William F. Stickle and Kenneth D. Bomben Physical Electronics Division, Perkin-Elmer Corporation, Mountain View, CA 94043

This review describes some of the recent studies that have used X-ray Photoelectron Spectroscopy to investigate materials commonly used in the microelectronics industry. It is divided into two sections: first, an introduction that is intended to give a general overview of the technique and second, a review, divided into five categories, of some of the work done over the last five years characterizing material systems of interest to the semiconductor industry.

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 has i t s o r i g i n s i n a p h y s i c a l p r i n c i p l e t h a t has been u n d e r s t o o d f o r n e a r l y e i g h t y y e a r s — a p r i n c i p l e t h a t was p a r t i a l l y r e s p o n s i b l e f o r a r e v o l u t i o n i n c h e m i s t r y and p h y s i c s . The p r i n c i p l e i s the p h o t o e l e c t r i c e f f e c t ; the r e v o l u t i o n a r o s e because o f the concept o f quanta o f energy. E i n s t e i n s account (1) o f the p h o t o e l e c t r i c e f f e c t can be expressed as: 1

E = hv - w

[1]

where E i s the k i n e t i c energy o f the p h o t o e l e c t r o n , hv i s the i n c i d e n t photon energy and w i s the minimum energy r e q u i r e d t o remove an e l e c t r o n from the sample and i s c h a r a c t e r i s t i c o f the m a t e r i a l b e i n g s t u d i e d . A l t h o u g h the e a r l y work on t h e p h o t o e l e c t r i c e f f e c t d e a l t m a i n l y w i t h the c o n d u c t i o n band e l e c t r o n s o f m e t a l s , the above e q u a t i o n can be a p p l i e d t o the d e s c r i p t i o n o f the p h o t o e j e c t i o n o f core e l e c t r o n s . E l e c t r o n s p e c t r o s c o p y can be d i v i d e d i n t o s e v e r a l c a t e g o r i e s . These would i n c l u d e X - r a y 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 ( X P S ) , a l s o known a s E l e c t r o n S p e c t r o s c o p y f o r C h e m i c a l A n a l y s i s (ESCA), U l t r a v i o l e t 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 (UPS) and Auger E l e c t r o n S p e c t r o s c o p y (AES). Other e l e c t r o n s p e c t r o s c o p i e s i n c l u d e Penning i o n i z a t i o n and i o n n e u t r a l i z a t i o n . XPS uses s o f t X r a y s a s the 0097-6156/86/0295-0144$06.00/0 © 1986 American Chemical Society In Microelectronics Processing: Inorganic Materials Characterization; Casper, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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i o n i z i n g source of radiation for the ejection of core electrons. UPS makes use of u l t r a v i o l e t l i g h t and i s generally used to study the valence electrons of atoms and molecules. AES i s a secondary e l e c tron process where electron emission occurs because of the coulombic rearrangement induced by a core hole created by a photon or a highenergy electron beam. H i s t o r i c a l l y , electron spectroscopy has matured i n two separate but related areas. One has been the use of electron spectroscopy as applied to a n a l y t i c a l problems, e s p e c i a l l y those that r e l a t e to surfaces, such as f a i l u r e analysis, corrosion, c a t a l y s i s , or tribology. In such studies, the technique i s often used i n conjunction with other techniques such as low energy electron d i f f r a c t i o n (LEED), secondary ion mass spectrometry (SIMS), or ion scattering spectroscopy (ISS). Another related area i s the use of electron spectroscopy to examine the e l e c t r o n i c structure of materials or chemical species. In the past, the study of matter and i t s i n t e r a c t i o n with radiation was largely confined to the measurement of electromagnetic radiation. The e x c i t a t i o n and de-excitation of atoms and molecules by photons emitted or absorbed when electrons made t r a n s i t i o n s between d i f f e r e n t discrete states has been well studied. Scanning electron microscopy (SEM), for example, makes routine use of energy dispersive X-ray analysis (EDX). In t h i s case the sample emits X rays as a by-product of the technique. High i n t e r e s t i n the applications of photoelectron spectroscopy was stimulated by Kai Siegbahn and his group at Uppsala (2-4) i n the 1950 s. Their major i n i t i a l contribution was to increase the k i n e t i c energy resolution of t h e i r spectrometer over that of e x i s t i n g i n s t r u ments. The photoelectron spectra were characterized by d i s t i n c t l y narrow features which could be energetically located with great accuracy as i l l u s t r a t e d i n Figure 1. Assuming an o r b i t a l description of the e l e c t r o n i c structure of matter, each feature corresponded to photoionization of successive o r b i t a l s or structure associated with the photoionization process. In t h i s example, the 2s, 2p, 3s and 3p levels of titanium are ionized by Mg X rays and appear i n the binding energy spectrum. Additional t r a n s i t i o n s , attributable to relaxation processes and discussed below, also occur. Over the past several years electron spectroscopy has e s t a b l i s h ed i t s e l f as a powerful technique i n the e l e c t r o n i c studies of matter and has gained wide acceptance as an unparalleled t o o l for surface analysis. The escape depth of the electrons i s material and energy dependent but, i n general, the information depth i s about 50 A thus making electron spectroscopy a uniquely surface-sensitive a n a l y t i c a l technique. This i s because electrons that t r a v e l through a material for distances greater than t h i s have a r e l a t i v e l y high p r o b a b i l i t y of suffering energy losses due to i n e l a s t i c c o l l i s i o n s with bound e l e c trons. Hence, electrons emitted from deep within the sample (caused by X rays which can penetrate several micrometers) loose energy before leaving the surface and thereby contribute only to the background. The purpose of t h i s chapter i s to discuss the p r i n c i p l e s of photoelectron spectroscopy and i t s applications i n the semiconductor and microelectronics industries. Other recent reviews (5-10) have dealt with the use of XPS, AES, SIMS and ISS for f a i l u r e analysis and materials characterization for these and related i n d u s t r i e s . f

In Microelectronics Processing: Inorganic Materials Characterization; Casper, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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X-Rav P h o t o e l e c t r o n

Spectroscopy

XPS o f f e r s a d i r e c t method f o r measuring t h e b i n d i n g e n e r g i e s o f c o r e o r b i t a l s . The e j e c t e d e l e c t r o n l e a v e s t h e atom w i t h a w e l l - d e f i n e d k i n e t i c energy unique f o r t h a t o r b i t a l and t h a t atom. S i n c e t h e d i p o l e s e l e c t i o n r u l e s a l l o w f o r e l e c t r o n e m i s s i o n from any o r b i t a l , p h o t o e j e c t i o n o c c u r s as l o n g as t h e photon energy i s g r e a t e r than t h e b i n d i n g energy o f a p a r t i c u l a r l e v e l . Thus XPS a l l o w s t h e s t u d y o f a l l a c c e s s i b l e e l e c t r o n i c l e v e l s r e g a r d l e s s o f symmetry ( 1 1 ) . XPS i s a s u r f a c e s e n s i t i v e t e c h n i q u e b u t i t s g r e a t e s t s t r e n g t h l i e s i n i t s a b i l i t y t o p r o v i d e i n f o r m a t i o n on t h e s u r f a c e c h e m i s t r y o f a sample. The b i n d i n g energy can be measured t o h i g h r e s o l u t i o n w i t h t o d a y ' s s p e c t r o m e t e r s and i s s e n s i t i v e t o changes i n t h e c h e m i c a l environment. Thus, i n many c a s e s t h e p o s i t i o n o f t h e XPS peak, a f t e r c o r r e c t i o n f o r s t a t i c c h a r g i n g , can a s s i g n t h e c h e m i c a l environment. A l a r g e f r a c t i o n o f t h e XPS s t u d i e s t o d a t e have i n v o l v e d t h e p r e c i s e measurement o f c o r e - e l e c t r o n b i n d i n g e n e r g i e s and t h e measurement o f t h e c h e m i c a l s h i f t s o f t h e s e e n e r g i e s . The term " c h e m i c a l shift r e f e r s t o the f a c t that c o r e - o r b i t a l i o n i z a t i o n s o f t e n vary measurably due t o changes i n t h e chemi- c a l environment. F o r example, t h e p r e s e n c e o f a n a t i v e o x i d e on s i l i c o n can be e a s i l y demonstrated by XPS a s i s shown i n F i g u r e 2. The s i l i c o n 2p p h o t o e l e c t r o n spectrum shows two d i s t i n c t peaks, c o r r e s p o n d i n g t o t h e d i f f e r e n t environments o f t h e s i l i c o n atoms, one, a t h i g h e r b i n d i n g e n e r g y , o x i d i z e d and t h e o t h e r , a t l o w e r b i n d i n g e n e r g y , e l e m e n t a l . The asymmetry o f t h e lower b i n d i n g energy peak a r i s e s because o f s p i n - o r b i t s p l i t t i n g i n t h e 2p o r b i t a l . F i n a l - s t a t e e f f e c t s , d i s c u s s e d below, obscure t h e s p i n - o r b i t e f f e c t i n t h e o x i d e peak. XPS i s w e l l s u i t e d f o r t h e s e t y p e s o f s t u d i e s because t h e common e x c i t a t i o n s o u r c e s (Mg Ka = 1253.6 eV and A l Ka = 1486.6 eV) can e a s i l y cause p h o t o e j e c t i o n . The more common u l t r a v i o l e t s o u r c e s a r e l i m i t e d t o t h e v a l e n c e r e g i o n and weakly-bound c o r e l e v e l s . X-ray monochrometers c a n be used t o narrow t h e e x c i t a t i o n spectrum and remove s a t e l l i t e and b r e m s s t r a h l u n g c o n t r i b u t i o n s , however, t h e y s u f f e r a concomitant l o s s i n i n t e n s i t y t h a t i n c r e a s e s t h e d a t a - g a t h e r i n g t i m e . S y n c h r o t r o n r a d i a t i o n can be used t o g e t around t h e s e l i m i t a t i o n s , a l t h o u g h such l i g h t s o u r c e s a r e n o t common and a r e not i n w i d e s p r e a d use among c h e m i s t s . S t u d i e s t h a t use t u n a b l e l i g h t sources o f t e n use t h e term S o f t X-ray Photoelectron Spectroscopy (SXPS). Core l e v e l s may be c o n s i d e r e d as r e p r e s e n t a t i v e o f t h e f i l l e d s u b s h e l l s o f an atom and a r e o f t e n found by XPS t o be r e l a t i v e l y sharp i n energy. The w i d t h o f t h e c o r e p h o t o e l e c t r o n l i n e depends on s e v e r a l f a c t o r s b o t h i n h e r e n t and i n s t r u m e n t a l . I n h e r e n t s o u r c e s i n c l u d e t h e l i f e t i m e o f t h e s u b s h e l l c o r e h o l e c r e a t e d by p h o t o i o n i z a t i o n and t h e v a r i o u s p o s s i b l e v a l u e s o f t h e f i n a l - s t a t e energy. The f i n a l - s t a t e energy w i l l be i n f l u e n c e d by m u l t i p l e t s p l i t t i n g s , m u l t i e l e c t r o n e f f e c t s o r v i b r a t i o n a l b r o a d e n i n g . Another s o u r c e o f b r o a d e n i n g i s t h e presence o f u n r e s o l v a b l e c h e m i c a l l y - s h i f t e d peaks. Instrumental sources o f broadening i n c l u d e t h e width o f the X-ray beam, t h e f i n i t e r e s o l v i n g power o f t h e s p e c t r o m e t e r , and non-uniform sample c h a r g i n g . M u l t i - e l e c t r o n e f f e c t s a r e o f t e n termed " s a t e l l i t e s t r u c t u r e " . Due t o t h e sudden change i n t h e p o t e n t i a l o f an atom, a s e x p e r i e n c e d 1 1


8. STICKLE AND BOMBEN


Ti

147


Auger

Ti Ti

1000

3p

3s

bOO

Binding Energy (eV)

F i g u r e 1. P h o t o e l e c t r o n spectrum o f c l e a n T i m e t a l .

F i g u r e 2. S i l i c o n 2p p h o t o e l e c t r o n r e g i o n showing o x i d e and m e t a l ( r i g h t ) .

(left)

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 In Microelectronics Processing: Inorganic Materials Characterization; Casper, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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i n p h o t o i o n i z a t i o n , an e l e c t r o n i n a v a l e n c e o r b i t a l may go i n t o an unoccupied bound o r continuum s t a t e . These phenomena have been termed " e l e c t r o n shake-up" and " e l e c t r o n s h a k e - o f f " and appear as s a t e l l i t e s on t h e h i g h b i n d i n g energy s i d e o f t h e p r i m a r y i o n i z a t i o n . Shake-up s t r u c t u r e appears as d i s c r e t e s a t e l l i t e l i n e s i n t h e photoe l e c t r o n spectrum w h i l e s h a k e - o f f has no s t r u c t u r e and appears as a c o n t i n o u s l y r i s i n g background. B o t h t y p e s o f s t r u c t u r e o c c u r a t l o w e r k i n e t i c energy t h a n t h e main p h o t o e l e c t r o n peak. The f i n a l - s t a t e energy o f a p h o t o i o n i z e d s p e c i e s w i l l a l s o be i n f l u e n c e d by m u l t i p l e t s p l i t t i n g . M u l t i p l e t s p l i t t i n g w i l l o c c u r i f t h e r e a r e one o r more u n p a i r e d e l e c t r o n s i n t h e v a l e n c e s h e l l w i t h u n p a i r e d s p i n s . P h o t o i o n i z a t i o n i n a n o t h e r s h e l l can l e a d t o more t h a n one f i n a l s t a t e depending on how t h e u n f i l l e d s h e l l s c o u p l e . I n XPS, bound e l e c t r o n s a r e e j e c t e d t o f r e e s t a t e s o u t s i d e t h e atoms. The k i n e t i c energy o f t h e s e p h o t o e l e c t r o n s i s w e l l d e f i n e d and i s a measure o f t h e e l e c t r o n ' s b i n d i n g energy. From c o n s e r v a t i o n o f energy: E. = E - E + E». [2] hv kin f i where E and E a r e t h e t o t a l e n e r g i e s o f t h e f i n a l and i n i t i a l s t a t e s and E' i s t h e k i n e t i c energy o f t h e p h o t o e j e c t e d e l e c t r o n , f i E - E can be d e f i n e d as t h e b i n d i n g energy, E , o f t h e p h o t o e l e c t r o n . F o r gaseous samples t h i s r e l a t i o n s h i p i s unambiguous because i t i s r e f e r e n c e d t o t h e vacuum l e v e l , but f o r s o l i d s t h i s i s t r u e o n l y when r e f e r e n c e d t o t h e vacuum l e v e l o f t h e s p e c t r o m e t e r . C o n s i d e r , f o r example, p h o t o e j e c t i o n from t h e K s h e l l i n s o l i d s . From e q u a t i o n 2 i t i s seen t h a t : E. = w' + E» [3] hv kin where w» i s t h e sum o f t h e work f u n c t i o n and t h e b i n d i n g energy o f the sample. E n t e r i n g t h e s p e c t r o m e t e r s l i t s , t h e k i n e t i c energy o f the e l e c t r o n i s s l i g h t l y a l t e r e d due t o an e l e c t r o s t a t i c f i e l d between t h e sample and s p e c t r o m e t e r . T h i s f i e l d i s from t h e d i f f e r ences i n t h e work f u n c t i o n o f t h e sample and t h e m a t e r i a l from which the s p e c t r o m e t e r i s c o n s t r u c t e d . Common g r o u n d i n g m e r e l y r e q u i r e s the F e r m i l e v e l s be e q u a l and so any d i f f e r e n c e i n work f u n c t i o n w i l l r e s u l t i n such a f i e l d . Upon e n t e r i n g t h e s p e c t r o m e t e r t h e e l e c t r o n a c q u i r e s a new k i n e t i c e n e r g y , and i t i s t h i s energy which i s measured and conseq u e n t l y r e l a t e d t o t h e b i n d i n g energy. C h o o s i n g t h e F e r m i l e v e l as a r e f e r e n c e , c o n s e r v a t i o n o f energy r e q u i r e s : f

1

fe

E^ r E. - E. . - b hv kin sp Y

[4]

F where E i s t h e b i n d i n g energy r e f e r e n c e d t o t h e F e r m i l e v e l and i s t h e work f u n c t i o n o f t h e s p e c t r o m e t e r and i s t h u s independent o f the e x c i t a t i o n s o u r c e . T h e r e f o r e , t h e same work f u n c t i o n may be a p p l i e d t o a l l measurements made on t h e same i n s t r u m e n t . fc

Auger E l e c t r o n E m i s s i o n . I n a d d i t i o n t o t h e p r i m a r y p h o t o - p r o c e s s , secondary p r o c e s s e s can o c c u r . F o l l o w i n g c o r e e x c i t a t i o n , t h e atom can r e l a x by e m i t t i n g e i t h e r a photon o r an e l e c t r o n . The e l e c t r o n -


sp


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e j e c t i o n phenomenon i s c a l l e d t h e Auger p r o c e s s and t h e e l e c t r o n i s an Auger e l e c t r o n . T h i s p r o c e s s was d i s c o v e r e d by P i e r r e Auger (12) w h i l e u s i n g a W i l s o n c l o u d chamber. The presence o f a double t r a c k a l o n g the X - r a y beam was i n d i c a t i v e o f a t w o - e l e c t r o n p r o c e s s . The f i r s t t r a c k was t h e e l e c t r o n due t o p h o t o i o n i z a t i o n , w h i l e t h e second t r a c k was due t o what i s now c a l l e d the Auger e l e c t r o n . E n e r g e t i c a l l y , the c o r e h o l e i s f i l l e d by an e l e c t r o n c a s c a d i n g from a h i g h e r energy o r b i t a l a l o n g w i t h the s i m u l t a n e o u s e j e c t i o n o f y e t a n o t h e r e l e c t r o n . The p r o c e s s i s a s i m u l t a n e o u s t w o - e l e c t r o n coulombic r e a d j u s t m e n t by the r e m a i n i n g e l e c t r o n s t o the c o r e h o l e . As s u c h , i t competes d i r e c t l y w i t h X - r a y f l u o r e s c e n c e (XRF) but i t i s not l i m i t e d by t h e d i p o l e o p e r a t o r s e l e c t i o n r u l e s . A l l e n e r g e t i c a l l y a l l o w e d t r a n s i t i o n s a r e observed i n an Auger e l e c t r o n spectrum. I n a d d i t i o n , the e l e c t r o n escape depth i s a l s o a few t e n s o f angstroms, u n l i k e XRF where t y p i c a l escape depths a r e on t h e o r d e r o f t e n s o f thousands o f angstroms. D i f f e r e n t a p p r o x i m a t i o n s can be used t o e s t i m a t e t h e energy o f the Auger e l e c t r o n s (13>14). I n g e n e r a l , the energy r e g i o n f o r a p a r t i c u l a r t r a n s i t i o n can be e a s i l y c a l c u l a t e d . The energy o f a K L j L j j t r a n s i t i o n , f o r example, i s g i v e n by: E

A

=E

f

- E

1

= E(K) - E ( L ) - E ( L I

I ; [

)

[5]

p l u s a ( n e g a t i v e ) term o f magnitude 10-20 eV which accounts f o r the i n t e r a c t i o n s o f t h e f i n a l h o l e s . E^ i s t h e k i n e t i c energy o f t h e Auger e l e c t r o n and E ( K ) , E ( L j ) and E f L j j ) a r e t h e b i n d i n g e n e r g i e s o f the K, L j and L ^ s h e l l e l e c t r o n s , r e s p e c t i v e l y . By p r e c i s e l y measuring t h e k i n e t i c energy o f the Auger e l e c t r o n s , c h e m i c a l i n f o r m a t i o n may be deduced. I n f a c t , t h e c h e m i c a l s h i f t o f Auger e l e c t r o n s i s , i n some c a s e s , l a r g e r than t h a t o f the a s s o c i a t e d p h o t o e l e c t r o n l i n e s (13,15-17). Auger Parameter. The e n e r g i e s o f the X - r a y i n d u c e d Auger e l e c t r o n s , i n c o n j u n c t i o n w i t h p h o t o e l e c t r o n e n e r g i e s , have been used by Wagner t o develop the concept o f t h e Auger parameter ( 1 8 ) , which i s d e f i n e d as t h e k i n e t i c energy o f the Auger e l e c t r o n minus t h a t o f t h e photoe l e c t r o n . T h i s parameter, measurable w i t h o u t the n e c e s s i t y o f s t a t i c charge c o r r e c t i o n , i s a q u a n t i t y c h a r a c t e r i s t i c o f a m o l e c u l a r o r s o l i d s t a t e . Chemical s h i f t s i n t h i s q u a n t i t y r e p r e s e n t , approximat e l y , d i f f e r e n c e s i n t h e p o l a r i z a t i o n energy o f t h e f i n a l s t a t e ( 1 9 ) . When a n i o n i s c r e a t e d w i t h a c o r e vacancy, r e l a x a t i o n can o c c u r by t h e e m i s s i o n o f an X - r a y photon, by t h e e m i s s i o n o f an Auger e l e c t r o n , o r by the e m i s s i o n o f a n e l e c t r o n i n a C o s t e r - K r o n i g p r o c e s s (19). The p r o b a b i l i t y o f each o f these p r o c e s s e s depends on s e v e r a l f a c t o r s and t h e r e s u l t i n g p a r t i c l e e m i s s i o n may be d e t e c t e d by w e l l known methods. The i n i t i a l s t a t e f o r each o f these p r o c e s s e s c o n t a i n s a c o r e h o l e on a n atom. I n AES, t h e vacancy i s u s u a l l y c r e a t e d e i t h e r by a high-energy e l e c t r o n (5-10 keV) o r by a photon. Commonly used photon s o u r c e s i n c l u d e Mg and A l , a s noted above, Z r (2042 e V ) , Au (2123 eV) and Ag (2984 e V ) . The h i g h e r energy s o u r c e s a r e u s e f u l f o r c r e a t i n g deeper l y i n g c o r e h o l e s t h a n a r e c o n v e n i e n t l y a c c e s s i b l e w i t h Mg o r A l , but a r e not t o o u s e f u l f o r r o u t i n e p h o t o e l e c t r o n s t u d i e s because o f t h e l a r g e l i n e w i d t h ( 2 0 ) . Once t h e h o l e i s c r e a t e d , an e l e c t r o n



150


k i n e t i c energy spectrum can be measured which can y i e l d c h e m i c a l , a n a l y t i c a l , and e l e c t r o n i c s t r u c t u r e i n f o r m a t i o n . I n f a c t , i t i s i m p o s s i b l e t o a v o i d t h e appearance o f s p e c t r a l peaks due t o Auger e l e c t r o n s d u r i n g t h e c o u r s e o f a p h o t o e l e c t r o n experiment a s seen i n F i g u r e 1. Some workers i n XPS have viewed t h e s e l i n e s a s a n u i s a n c e , b u t t h e presence o f t h e Auger l i n e s i n c r e a s e s t h e amount o f i n f o r m a t i o n a v a i l a b l e i n a photon-induced e l e c t r o n k i n e t i c energy spectrum. S e v e r a l workers have made u s e o f the b r e m s s t r a h l u n g r a d i a t i o n from A l and Mg s o u r c e s t o l o o k a t Auger e l e c t r o n e m i s s i o n due t o c o r e h o l e s o f energy h i g h e r t h a n t h e c h a r a c t e r i s t i c l i n e o f t h e X-ray s o u r c e (13»20-23). Wagner has a l s o developed a t w o - d i m e n s i o n a l c h e m i c a l s t a t e p l o t (24-25) t h a t makes use o f t h e Auger parameter t o d i s t i n g u i s h between compounds o f an element t h a t have t h e same, o r n e a r l y t h e same, b i n d i n g energy. I n cases where t h e XPS c h e m i c a l s h i f t a l o n e i s n o t enough t o determine t h e c h e m i c a l s t a t e , t h e s e p l o t s o r t h e Auger parameter by i t s e l f can be used t o h e l p i d e n t i f y t h e t y p e o f compounds p r e s e n t i n t h e s u r f a c e . Depth P r o f i l e s . By i o n m i l l i n g a s u r f a c e w i t h i n e r t gas i o n s , t h e c h e m i c a l c o m p o s i t i o n c a n be f o l l o w e d as a f u n c t i o n o f d e p t h , y i e l d i n g a "Chemical Depth P r o f i l e " . F o r example, t h e change from o x i d e t o m e t a l n e a r an i n t e r f a c e can be c h a r t e d unambiguously by examining t h e changes i n peak shape and p o s i t i o n f o r each element a t a s e r i e s o f d e p t h s . Care must be t a k e n , however, i n t h e i n t e r p e r t a t i o n o f such d a t a because i o n - i n d u c e d c h e m i c a l changes, such as r e d u c t i o n , may o c c u r . The c h e m i c a l i n t e r a c t i o n s a t i n t e r f a c e s govern t h e w i d t h o f t h e i n t e r f a c e , t h e r a t e o f d i f f u s i o n a c r o s s i t and i n f l u e n c e t h e e l e c t r i c a l and p h y s i c a l p r o p e r t i e s o f i n t e r f a c e s . F i g u r e 3 shows a depth p r o f i l e o f t a n t a l u m s i l i c i d e on s i l i c o n . The s p u t t e r r a t e i s a p p r o x i m a t e l y 200A/min. I n t h i s example, t h e i n t e r f a c e can be e a s i l y i d e n t i f i e d a t a s p u t t e r time o f t e n m i n u t e s . F i g u r e s 4 i s t h e Ta 4 f p h o t o e l e c t r o n r e g i o n p r i o r t o t h e s t a r t o f s p u t t e r i n g (spectrum a) and a f t e r f i v e minutes (spectrum b ) . Spectrum a shows t h a t Ta i s p r e d o m i n a t e l y i n t h e form o f a s i l i c i d e w i t h o n l y a s a m l l amount o f o x i d e e v i d e n t . I n spectrum b, o n l y t h e s i l i c i d e i s p r e s e n t i n d i c a t i n g t h a t Ta i s o n l y o x i d i z e d a t t h e sample s u r f a c e . The S i 2p r e g i o n ( F i g u r e 5) shows t h e S i t o be h e a v i l y o x i d i z e d p r i o r t o s p u t t e r i n g (spectrum a) as c a n be seen i n t h e h e i g h t o f t h e o x i d e peak. However, a f t e r f i v e minutes o f s p u t t e r i n g , spectrum b shows o n l y t h e s i l i c i d e . I n s t r u m e n t a t i o n . The b a s i c i n s t r u m e n t a t i o n needed t o do 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 i s shown i n F i g u r e 6. The components i n c l u d e an X-ray s o u r c e , sample o r s o u r c e chamber, e l e c t r o n energy a n a l y z e r , electron detector o r m u l t i p l i e r , counting e l e c t r o n i c s , the a l l - e n c o m p a s s i n g vacuum system and a d a t a o u t p u t d e v i c e . Samples a r e i n t r o d u c e d t h r o u g h a l o a d l o c k mechanism i n t o t h e u l t r a - h i g h vacuum r e g i o n o f t h e s p e c t r o m e t e r . Some commercial s p e c t r o m e t e r s a r e now equipped w i t h X-ray s o u r c e s w i t h m u l t i p l e anode m a t e r i a l s , a l l o w i n g s i m p l e s w i t c h i n g o f e x c i t a t i o n l i n e s t o "move" i n t e r f e r e n c e from Auger t r a n s i t i o n s o r t o i n d u c e more e f f i c i e n t i o n i z a t i o n o f d e e p - l y i n g c o r e l e v e l s t o observe Auger l i n e s f o r Auger parameter experiments. R e c e n t l y , equipment m a n u f a c t u r e r s have begun p r o d u c i n g " s m a l l



8.

151


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0

20 Sputter Time (min.)

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Depth p r o f i l e o f t a n t a l u m s i l i c i d e on s i l i c o n .

BINDING ENERGY (eV)

F i g u r e 4. Ta 4 f p h o t o e l e c t r o n r e g i o n a t time=0 (a) and time=5 (b) minutes.



152


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F i g u r e 5. minutes.

BINDING ENERGY (eV)

r 95

S i 2p p h o t o e l e c t r o n r e g i o n a t time=0 ( a ) and time=5 ( b )

MAGNETIC

X-RAY SOURCE

COMPUTER CONTROL AND DATA HANDLING SYSTEM F i g u r e 6. Schematic o f a t y p i c a l X-ray spectrometer.

photoelectron


8.

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153


s p o t " XPS systems w i t h t h e a b i l i t y t o examine, r o u g h l y , a 150 m i c r o meter d i a m e t e r s p o t on a sample s u r f a c e (26-30). These instruments have two advantages; f i r s t t h e y b r i n g t h e major s t r e n g t h o f XPS, the a b i l i t y t o d e t e r m i n e c h e m i c a l i n f o r m a t i o n , t o bear on s m a l l e r a r e a s and, second, i t means t h a t XPS depth p r o f i l e s can be done a t s p u t t e r r a t e s t h a t approach those commonly used i n AES, a s shown i n F i g u r e 3t above. Some r e c e n t i n s t r u m e n t s a l s o a l l o w t h e o p p o r t u n i t y t o do c o n t i n u o u s s p u t t e r p r o f i l e s which i s a s i g n i f i c a n t time s a v e r . S o f t X-Rav 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 . T y p i c a l l y , SXPS uses s y n c h r o t r o n r a d i a t i o n t o tune the X-ray e n e r g y f o r maximum a b s o r p t i o n by one element. The c h e m i c a l s h i f t i n f o r m a t i o n a v a i l a b l e v i a t h i s t e c h n i q u e i s e x a c t l y analogous t o the c h e m i c a l s h i f t s d i s c u s s e d above. Examples u s i n g SXPS a r e i n c l u d e d i n the examples below and a r e meant t o i n d i c a t e f u r t h e r how XPS can be used t o s o l v e problems i n the m i c r o e l e c t r o n i c s i n d u s t r y . Because o f the a b i l i t y t o tune t h e e x c i t a t i o n s o u r c e t o a p a r t i c u l a r element, d a t a can be g a t h e r e d r a p i d l y . SXPS i s o f t e n used t o i n v e s t i g a t e the i n t e r a c t i o n s o f m a t e r i a l s being deposited at an i n t e r f a c e . The one drawback t o the t e c h n i q u e i s t h a t i t r e q u i r e s time on a s y n c h r o t r o n . Applications The a p p l i c a t i o n o f XPS t o m i c r o e l e c t r o n i c m a t e r i a l s t y p i c a l l y f o c u s e s on two a r e a s . F i r s t , a s a s u r f a c e i n v e s t a g a t i v e t e c h n i q u e , XPS can be used t o e s t a b l i s h the c h e m i c a l i n t e r a c t i o n s between two m a t e r i a l s o r between a m a t e r i a l and the ambient atmosphere. Second, i n c o n j u n c t i o n w i t h depth p r o f i l i n g , the changes i n c h e m i c a l composit i o n w i t h depth can be f o l l o w e d . The f o l l o w i n g s e c t i o n s g i v e examples o f the k i n d s o f i n f o r m a t i o n t h a t can be g a i n e d by u s i n g XPS t o i n v e s t i g a t e a sample s u r f a c e . The f i r s t t h r e e s e c t i o n s c o v e r m i c r o e l e c t r o n i c m a t e r i a l s and d e s c r i b e r e c e n t work i n the c h a r a c t e r i z a t i o n o f t h e s e m a t e r i a l s . The n e x t s e c t i o n c o v e r s i n t e r a c t i o n s a t i n t e r f a c e s and t h e l a s t s e c t i o n i s a p o t p o u r r i o f t o p i c s i n c l u d i n g p a c k a g i n g and c l e a n i n g . W h i l e an attempt was made t o keep t h e s e c a t e g o r i e s from o v e r l a p p i n g , some o f t h e c i t e d work c o n t a i n s m a t e r i a l t h a t i s a p p l i c a b l e t o more t h a n one section. S i l i c o n and i t s O x i d e s . I n the r e c e n t p a s t s i l i c o n and s i l i c o n d i o x i d e have become v e r y i m p o r t a n t because o f t h e i r widespread use i n the semiconductor i n d u s t r y . Most d e v i c e s c u r r e n t l y b e i n g manufactured use s i l i c o n a s a p r i m a r y m a t e r i a l , making s i l i c o n and i t s o x i d e s the s u b j e c t o f many s t u d i e s t h a t c h a r a c t e r i z e t h e s e m a t e r i a l s and t h e i r interfaces. Hofmann and Thomas ( 3 D have d i s c u s s e d t h e e f f e c t o f i o n bombardment on t h e r m a l l y grown s i l i c o n d i o x i d e . XPS showed t h a t s m a l l changes i n the s u r f a c e c h e m i s t r y were o b s e r v a b l e . The S i 2p and 0 1s peaks were seen t o broaden due t o i o n damage which presumably i n c r e a s e d bond-angle d i s o r d e r r e s u l t i n g i n charge r e d i s t r i b u t i o n . 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 has a l s o been a p p l i e d t o the s t u d y o f t h i n S i 0 l a y e r s on S i (32,33). A n g u l a r r e s o l v e d XPS (34-36) can be used t o produce n o n - d e s t r u c t i v e depth p r o f i l e s and t h e r e b y a v o i d the i o n - i n d u c e d damage d e s c r i b e d by Thomas ( 5 ) . Moulder and Hammond (37) 2



154


found t h a t by c h a n g i n g the a n g l e between t h e sample s u r f a c e and t h e e n t r a n c e s l i t o f the a n a l y z e r ( F i g u r e 7) an e f f e c t i v e depth p r o f i l e c o u l d be g e n e r a t e d . M u l t i p l e o x i d a t i o n s t a t e s o f s i l i c o n were i d e n t i f i e d i n t h e i n t e r f a c i a l r e g i o n ( F i g u r e 8) and f o l l o w e d as a f u n c t i o n o f escape depth ( F i g u r e 9 ) . B e r t r a n d and F l e i s c h e r (38) have s t u d i e d the c h e m i c a l d e p o s i t i o n o f s i l i c o n d i o x i d e on i n d i u m phosphide. They found t h a t on u n o x i d i z e d , e t c h e d s u r f a c e s , o x i d e coverage was "always p a t c h y " . I f the I n P had a monolayer o f chemisorbed oxygen, an a p p r o x i m a t e l y 6 0 A - t h i c k o x i d e f i l m c o u l d be formed a t room temperature t h r o u g h t h e f o r m a t i o n o f Si-O-P bonds a t t h e i n t e r f a c e . I n t e r d i f f u s i o n o f m e t a l s i n s i l i c o n h a s a l s o been s t u d i e d by SXPS (39) where i t was r e v e a l e d t h a t Au and A l i n t e r a c t i n v e r y d i f f e r e n t ways w i t h s i l i c o n . The A u - S i i n t e r f a c e e x h i b i t s a s t r o n g c h e m i c a l i n t e r a c t i o n w h i l e the A l - S i i n t e r f a c e shows much weaker i n t e r a c t i o n s . A f u l l y r e a c t e d S i - C r i n t e r f a c e was found t o be an e f f e c t i v e b a r r i e r f o r Au-Si i n t e r d i f f u s i o n (40). T h i n f i l m s on S i o r SiO can a l s o be i n v e s t i g a t e d by XPS. The C u - S i i n t e r f a c e (41) was found t o be s i m i l a r t o the d i f f u s e A u - S i i n t e r f a c e s t u d y done by SXPS. Comparison o f C u - S i w i t h Cu-InP shows s t r o n g s i m i l a r i t i e s between t h e two systems ( 4 2 ) . Nefedov e t a l . (43) examined the F e N i / S i O ^ i n t e r f a c e and found t h a t t h e uppermost l a y e r o f the f i l m was e n r i c h e d w i t h i r o n o x i d e s w h i l e the next l a y e r was d e p l e t e d o f i r o n atoms. T o r r i s i e t a l . (44) used XPS t o s t u d y the a d h e s i o n o f v a r i o u s m e t a l s t o p o l y s i l o x a n e i n an a t t e m p t t o u n d e r s t a n d a d h e s i o n o f f i l m s t o common s e m i c o n d u c t o r m a t e r i a l s . S i l i c i d e s . The most commonly used g a t e m a t e r i a l i n the t e c h n o l o g y o f MOS i n t e g r a t e d c i r c u i t s i s doped p o l y c r y s t a l l i n e s i l i c o n . As d e v i c e d i m e n s i o n s become v a n i s h i n g l y s m a l l the h i g h s h e e t r e s i s t a n c e o f p o l y s i l i c o n becomes a l i m i t i n g f a c t o r i n d e v i c e performance. The w e l l - e s t a b l i s h e d technology o f p o l y s i l i c o n gates has l e d t o the i n c o r - p o r a t i o n o f m e t a l s i l i c i d e s i n t o the d e v i c e s . The s i l i c i d e s o f t a n t a l u m , molybdenum, t i t a n i u m and t u n g s t e n (45) a r e o f t e n used. U s i n g XPS t o examine s i l i c i d e s a l l o w s f o r t h e e x a m i n a t i o n o f t h e c h e m i s t r y o f the s u r f a c e e i t h e r a f t e r t r e a t m e n t o r upon r e a c t i o n . Tungsten s i l i c i d e , f o r example, has been i n v e s t i g a t e d by XPS ( 4 6 ) . The e x p e r i m e n t a l r e s u l t s suggest bonding s i m i l a r t o t h a t found i n m e t a l c a r b o n y l s i n terms o f a d o n a t i o n and TT back bond i n g . Dubois and Nuzzo (47) have observed the r e a c t i o n o f s i l a n e w i t h c l e a n N i , Rh, P t , Mo, T a , W, Co and Au. S u r f a c e compound f o r m a t i o n i s s t r o n g l y suggested by the o b s e r v a t i o n o f s h i f t s t o h i g h e r b i n d i n g energy o f the s i l i c o n and m e t a l c o r e l e v e l s . These s h i f t s a r e i n d i c a t i v e o f s i l i c i d e f o r m a t i o n (48,49). I n another r e p o r t (50), r u t h e n i u m , rhodium and p a l l a d i u m s i l i c i d e s were s t u d i e d by XPS. B i n d i n g e n e r g i e s f o r t h e v a r i o u s s i l i c i d e s a r e t a b u l a t e d , showing these d i f f e r e n c e s i n b i n d i n g energy between t h e s i l i c i d e s and t h e m e t a l s a r e v e r y s m a l l . T h e r e f o r e , f o r some m e t a l s , the t r a n s i t i o n from m e t a l t o s i l i c i d e can be f o l l o w e d by XPS b u t f o r o t h e r m e t a l s i t i s not so s t r a i g h t f o r w a r d . Rare e a r t h s i l i c i d e s have been s t u d i e d f o r s e v e r a l r e a s o n s . Of fundamental i n t e r e s t i s the f a c t t h a t the r a r e e a r t h s i l i c i d e s w i l l form s o l i d s o l u t i o n s showing b o t h b i v a l e n t and t r i v a l e n t r a r e e a r t h atoms a s w e l l as compounds showing mixed v a l e n c e b e h a v i o r (51-53). More p r a c t i c a l r e a s o n s f o r such s t u d i e s i n c l u d e the f a c t t h a t r a r e




STICKLE AND BOM BEN

F i g u r e 7. E x p e r i m e n t a l p r i n c i p l e s o f A n g u l a r R e s o l v e d 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 (ARXPS). Reproduced w i t h p e r m i s s i o n from Ref. 37. C o p y r i g h t 1985 Research and Development.



156

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS

107

102

Binding

energy

CHARACTERIZATION

97

(eV)

F i g u r e 8. ARXPS s p e c t r a o f s i l i c o n 2p r e g i o n o f s i l i c o n w i t h a n a t i v e o x i d e showing m u l t i p l e o x i d a t i o n s t a t e s . A ) 0 = 10; B) 6 = 20; C) 9 =90. Reproduced w i t h p e r m i s s i o n from Ref. 37. C o p y r i g h t 1985 Research and Development.

100

ANALYSIS DEPTH (x) F i g u r e 9. R e l a t i v e i n t e n s i t i e s o f the s i l i c o n s p e c i e s v e r s u s a n a l y s i s d e p t h . Reproduced w i t h p e r m i s s i o n from Ref. 3 J . C o p y r i g h t 1985 Research and Development.


8.

STICKLE AND BOMBEN

157


e a r t h c o n t a c t s e x h i b i t a s m a l l S c h o t t k y b a r r i e r . One such system s t u d i e d was t h e Y b - S i i n t e r f a c e ( 5 4 ) . Mixed v a l e n c e compounds were found where two s t a b l e and u n i f o r m c o m p o s i t i o n s were o b s e r v e d . P i r r i e t a l . (55) s t u d i e d c o b a l t d i s i l i c i d e e p i t a x i a l growth on the s i l i c o n (111) s u r f a c e . A t room t e m p e r a t u r e , a s t r o n g r e a c t i o n forms C o S i d u r i n g t h e d e p o s i t i o n o f up t o f o u r monolayers o f Co. However, more t h a n f o u r monolayers o f Co r e s u l t s i n m e t a l l i c c o b a l t being deposited. Annealing t h i s surface r e s u l t s i n the formation o f the d i s i l i c i d e . XPS g i v e s c o n f i r m a t i o n o f t h e s p ^ c o v a l e n t o f t h e C o - S i bond i n C o S i .

nature


2

N o n - S i l i c o n Semiconductor M a t e r i a l s . I I I - V compounds a r e f i n d i n g w i d e r uses i n t h e semiconductor i n d u s t r y . One a r e a o f p r i m a r y i n t e r e s t i s t h e c o n c e r n o v e r t h e c l e a n l i n e s s o f s u b s t r a t e s used i n M o l e c u l a r Beam E p i t a x y (MBE) s t u d i e s . S u b s t r a t e p r e p a r a t i o n i s i m p o r t a n t because t h e s u b s t r a t e may p l a y a r o l e i n f a i l u r e t h a t o c c u r a f t e r a d d i t i o n a l p r o c e s s i n g s t e p s . The s u r f a c e s e n s i t i v i t y o f XPS makes i t i d e a l l y s u i t e d f o r such i n v e s t i g a t i o n s . V a r i o u s c l e a n i n g p r o c e d u r e s have been s t u d i e d by s e v e r a l groups u s i n g XPS. I t was observed (56) t h a t l o w energy i o n e t c h i n g combined w i t h s i m u l t a n e o u s a n n e a l i n g was an e f f i c i e n t c l e a n i n g p r o c e d u r e . Vasquez e t a l . (57) examined d i f f e r e n t c l e a n i n g p r o c e d u r e s f o r GaAs (100) s u b s t r a t e s . The GaAs o x i d e d e c o m p o s i t i o n was observed t o p r o ceed by t h e t h e r m a l r e d u c t i o n o f A s 0 _ which l e d t o t h e f o r m a t i o n o f o

G a 2

°3

a

n

d

As m e t a l .

Minimum carbon c o n t a m i n a t i o n

was observed by

combining t h e g e n e r a l l y a c c e p t e d o x i d e p a s s i v a t i o n growth sequence w i t h an HCl/EtOH s t r i p . S t o i c h i o m e t r i c GaAs c o u l d be o b t a i n e d a t t e m p e r a t u r e s a s low as 350 C. Woodall e t a l . (58) have a l s o d e s c r i b e d a wet c h e m i c a l technique f o r p a s s i v a t i n g a i r - e x p o s e d GaAs s u r f a c e s . XPS showed t h a t t h e r e was l i t t l e o r no Ga m e t a l o r Ga and As compounds i n t h e As f i l m c r e a t e d d u r i n g p a s s i v a t i o n o r a t t h e As/GaAs i n t e r f a c e . I n t e r d i f f u s i o n o f m e t a l s i n t o GaAs has a l s o been s t u d i e d w i t h m e t a l s such a s Pd (59) and Au ( 6 0 ) . The e f f e c t o f oxygen on t h e i n t e r m i x i n g o f t h e GaAs/Au i n t e r f a c e has a l s o been s t u d i e d (61) where oxygen was found t o i n h i b i t i n t e r d i f f u s i o n . K e n d e l w i c z e t a l . (62) have found e v i d e n c e f o r t h e f o r m a t i o n o f p a l l a d i u m phosphide a t t h e Pd/InP (110) i n t e r f a c e . They found t h a t , between 2.5 and 15 monolayers o f Pd coverage on I n P , d e p o s i t i o n l e a d s t o t h e f o r m a t i o n o f a s t a b l e Pd^P compound. They add t h a t " f o r t h e f i r s t time f o r a n o n - e l e m e n t a l semiconductor i n t e r f a c e t h e d e t a i l e d n a t u r e o f t h e c h e m i c a l bond has been s t u d i e d by f o l l o w i n g t h e e v o l u t i o n o f t h e . . . . c o r e l i n e s " and t h e y n o t e s i m i l a r r e s u l t s f o r N i on InP. T h i s l e a d s them t o deduce t h a t t r a n s i t i o n m e t a l phosphide f o r m a t i o n may be a g e n e r a l phenomenon on I I I - V m a t e r i a l s . XPS has a l s o been used i n an attempt t o i n v e s t i g a t e S c h o t t k y b a r r i e r h e i g h t s (SBH) and t h e e f f e c t s o f h e a t i n g on semiconductor m a t e r i a l s . P e t r o e t a l . (63) found t h a t t h e SBH i n c r e a s e s d u r i n g t h e d e p o s i t i o n o f up t o 25 monolayers o f Au on GaAs and a t t r i b u t e t h i s t o i n t e r m i x i n g o f t h e Au w i t h t h e GaAs. XPS was used t o demonstrate t h a t the Au i s n o t c h e m i c a l l y bound o r a l l o y e d w i t h t h e GaAs. I t was a l s o found t h a t h e a t i n g i n h i b i t s t h e SBH i n c r e a s e by removing d e f e c t s t a t e s from t h e s u r f a c e r e g i o n .



158


Pan e t a l , (64) used XPS t o demonstrate the p r e f e r e n t i a l s u r f a c e s e g r e g a t i o n o f As d u r i n g t h e i n t e r m i x i n g o f n o b l e m e t a l s on the GaAs (110) s u r f a c e . P e t r o e t a l . (64) used SXPS t o f i n d s i g n i f i c a n t Au i n t e r m i x i n g i n the Au-GaAs (110) i n t e r f a c e . F u r t h e r m o r e , As moves t o the s u r f a c e a t t h e h i g h e s t Au coverages and h e a t i n g t h e sample causes the Ga t o become m e t a l l i c . I n an SXPS e x a m i n a t i o n o f Au on InP ( 1 1 0 ) , B a b a l o l a e t a l . (66) found t h a t , between 1 and 37 monolayers, InP + Au -> Au + I n and P. At l e s s t h a n one monolayer o f Au, t h e y saw no measurable c h e m i c a l changes. S k e a t h a t a l . (67) were a b l e t o use SXPS t o examine Sb on GaAs (110) w h i l e K e n d e l e w i c z e t a l . (68) examined room temperature exchange r e a c t i o n s a t t h e A l - I n P (110) i n t e r f a c e by SXPS. I n t e r f a c e s . I n t e r f a c e s p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g t h e c h a r a c t e r i s t i c s and r e s p o n s e s o f s e m i c o n d u c t o r s . T y p i c a l l y , a l a y e r o r l a y e r s o f a m a t e r i a l i s l a i d down on the s u b s t r a t e as a m e t a l f i l m , as an i n t e r m e t a l l i c o r as a compound i n an a t t e m p t t o i m p a r t p a r t i c u l a r e l e c t r i c a l p r o p e r t i e s t o the d e v i c e . I n d o i n g so, c l o s e a t t e n t i o n must be p a i d t o the i n t e r f a c e , because the s u r f a c e o f a m a t e r i a l i s not l i k e the b u l k . F o r t u n a t e l y , f i l m s on semiconductor s u r f a c e s produce s t r o n g a t o m i c and charge rearrangements a t the m i c r o s c o p i c i n t e r f a c e t h a t changes can be c h a r a c t e r i z e d by XPS. As B r i l l s o n (69) n o t e s , XPS r e v e a l s t h a t the "magnitude and s t o i c h i o m e t r y o f ( s u b s t r a t e d i s s o c i a t i o n and d i f f u s i o n o f c a t i o n and a n i o n i n t o the m e t a l ) depends s y s t e m a t i c a l l y on the s t r e n g t h and n a t u r e o f the i n t e r f a c e c h e m i c a l bonding and t h a t m e t a l i n d i f f u s i o n a l s o takes place." As an example o f the i n f o r m a t i o n t h a t can be a c q u i r e d by u s i n g XPS t o s t u d y i n t e r f a c e s , c o n s i d e r the r e s u l t o f some r e c e n t work by Hirokawa e t a l . ( 7 0 ) . I n l o o k i n g a t Cu o r Fe on S i 0 they found t h a t 2

Cu, as a m e t a l , d i f f u s e d s l i g h t l y i n t o S i 0 f u r t h e r h e a t i n g , CuO

formed on S i 0

2

and

2

a t 500-800 C.

Upon

changed t o C u 0 o r Cu^O 2

plus

Cu. Fe began r e a c t i n g w i t h SiO a t about 600 C and m i g r a t e d i n t o the +2 SiO as Fe . F r a n c i o s i e t a l . (71) demonstrated t h a t S i - C r i n t e r f a c e f o r m a t i o n a t room temperature r e s u l t s i n r e a c t e d phases t h a t d i f f e r from b o t h b u l k chromium s i l i c i d e and a S i - r i c h chromium silicide. M i s c e l l a n e o u s A p p l i c a t i o n s . The packages t h a t i n c o r p o r a t e semicond u c t o r d e v i c e s a r e o f t e n the s o u r c e o f f a i l u r e s . Alumina i s commonly used i n p a c k a g i n g s u b s t r a t e s u s i n g Cr/Au o r Cr/Cu m e t a l l i z a t i o n s . As s m a l l e r g e o m e t r i e s a r e employed the s u b s t r a t e roughness becomes a c o n s i d e r a b l e f a c t o r i n terms o f r e l i a b i l i t y . Changes i n s u r f a c e topography can cause r e d u c t i o n i n a d h e s i o n , b u t , i n an XPS s t u d y by Orent and Wagner ( 7 2 ) , they found t h a t s p u t t e r e d Cr/Au and Cr/Cu t h i n f i l m s show c h e m i c a l bonding t o the s u b s t r a t e which may c o n t r i b u t e t o a d h e s i o n . They a l s o found s i l i c o n n i t r i d e t o be an a c c e p t a b l e r e p l a cement f o r a l u m i n a . F u r t h e r , t i t a n i u m m e t a l l i z a t i o n s on s i l i c o n n i t r i d e were s t u d i e d and t h e y observed t h a t t i t a n i u m n i t r i d e had been formed d u r i n g t h e t i t a n i u m d e p o s i t i o n . I t i s a l s o p o s s i b l e t o use XPS t o a n a l y z e the e f f e c t s o f m e c h a n i c a l h a n d l i n g and c l e a n i n g on s u r f a c e s . Wasche e t a l . (73) i n v e s t i g a t e d A l samples and f i l m s i n an attempt t o determine i f


8.

STICKLE AND BOMBEN


159

m e c h a n i c a l t r e a t m e n t o f A l i n . vacuo r e s u l t e d i n c l e a n m e t a l s u r f a c e s . They were a b l e t o observe the d i s a p p e a r a n c e o f the o x i d e peak and concluded t h a t m e c h a n i c a l p r e t r e a t m e n t o f A l r e s u l t s i n s u r f a c e s t h a t compare f a v o r a b l y w i t h s u r f a c e s p r e p a r e d by o t h e r t e c h n i q u e s . S p u t t e r - d e p o s i t e d o r s p u t t e r - e t c h e d s u r f a c e s can a l s o be e x a m i n e d . R e a c t i v e l y s p u t t e r - i o n p l a t e d T i N f i l m s showed the s u r f a c e t o be o x i d i z e d t o T i 0 ( 7 4 ) . S u r f a c e f i l m s d e p o s i t e d d u r i n g plasma e t c h i n g 2


of S i 0

on S i by CHF^ were s t u d i e d by a c o m b i n a t i o n o f XPS and AES

2

( 7 5 ) . I t was found t h a t two type o f f i l m s were d e p o s i t e d . A " n o n - p e r s i s t e n t " f i l m c o u l d be removed by an oxygen plasma and c o n s i s t e d o f a f l u o r o c a r b o n p o l y m e r . The " p e r s i s t e n t " f i l m formed oxygen and f l u o r i n e compounds o f s i l i c o n and c o u l d not be removed by an oxygen p l a s m a . The e f f e c t s o f i o n bombardment were i n v e s t i g a t e d by C h r i s t i e a l . (76) on a range o f Groups I I and I V compounds. They were a b l e t o observe s t o i c h i o m e t r i c changes i n d u c e d by the i o n bombardment o f s u r faces. T h i s has i m p o r t a n t consequences f o r i n e r t gas i o n bombardment and plasma e t c h i n g p r o c e s s e s i n the f a b r i c a t i o n o f s e m i c o n d u c t o r devices. Summary T h i s r e v i e w has attempted t o demonstrate how the c h e m i c a l i n f o r m a t i o n t h a t i s i n h e r e n t i n XPS can be used t o i n v e s t i g a t e problems o f i n t e r e s t i n the semiconductor and m i c r o e l e c t r o n i c s f i e l d s . A number o f a r e a s have been d i s c u s s e d where XPS was found t o p l a y an i m p o r t a n t r o l e i n a t t e m p t i n g t o understand the c h e m i c a l and p h y s i c a l p r o c e s s e s t h a t o c c u r i n semiconductor m a t e r i a l s and d e v i c e s , however, t h i s r e v i e w i s not a l l - e n c o m p a s s i n g . F u r t h e r m o r e , the a r e a s t h a t can be i n v e s t i g a t e d by XPS a r e n o t l i m i t e d t o t h e s e examples. XPS o f f e r s u n i q u e advantages t h a t complement o r s u r p a s s o t h e r a n a l y t i c a l techniques. The a b i l i t y t o a n a l y z e i n s u l a t o r s as w e l l as c o n d u c t o r s makes t h i s t e c h n i q u e p a r t i c u l a r i l y u s e f u l . I n a d d i t i o n , t h e r e i s u s u a l l y l i t t l e damage t o the sample. With the s p a t i a l r e s o l u t i o n s t h a t a r e now a v a i l a b l e on s m a l l s p o t i n s t r u m e n t s , the t e c h n i q u e i s no l o n g e r l i m i t e d t o l a r g e a r e a a n a l y s i s .

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