INVESTIGATION OF A VERY THIN AND INHOMOGENEOUS FILM OF SiO FROM SURFACE PLASMON OPTICAL EXCITATION

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INVESTIGATION OF A VERY THIN AND

INHOMOGENEOUS FILM OF SiO FROM SURFACE PLASMON OPTICAL EXCITATION

D. Rivière, G. Roger

To cite this version:

D. Rivière, G. Roger. INVESTIGATION OF A VERY THIN AND INHOMOGENEOUS FILM OF

SiO FROM SURFACE PLASMON OPTICAL EXCITATION. Journal de Physique Colloques, 1983,

44 (C10), pp.C10-113-C10-117. �10.1051/jphyscol:19831024�. �jpa-00223481�

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JOURNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome 44, décembre 1983 page C10-113

I N V E S T I G A T I O N OF A V E R Y THIN A N D I N H O M O G E N E O U S FILM OF SiO FROM S U R F A C E P L A S M O N O P T I C A L E X C I T A T I O N

D. Rivière and G.R. Roger

Institut d'Optique Théorique et Appliquée, Laboratoire d'Expériences Fondamentales en Optique, Bâtiment 503, Centre Universitaire d'Orsay, B.P. 43, 91406 Orsay Cedex, Franoe

Résumé - On utilise l'excitation optique d'ondes superficielles de plasma pour caractériser une couche très mince et inhomogène de SiO, en tenant compte de la rugosité et des effets d'anisotropie.

Abstract - The optical excitation of surface plasma waves is used to characterize a very thin and inhomogeneous film of SiO, the roughness and anisotropy effects being taken into account.

The attenuated total reflection (ATR) technique has been widely used to investigate surface plasma waves (SPW) or surface plasmons (SP) on a metal surface {1,2}. In our experiment, we use. SPW optical excitation in the Kretschtnann configuration to characterize a very thin layer of silicon monoxide thermally evaporated under oblique incidence on a cold layer. Such a film gives a good planar alignment for nematic liquid crystals {3} and is used in electro-optic displays very commonly. Its optical properties must be known to understand liquid crystal-surface interactions in order to measure anchoring energies {4} to improve optical characteristics of liquid crystal displays.

It is the aim of this paper to compare experimental and theoretical results relative to this inhomogeneous and very thin film, and to discuss the influence of various parameters such as refractive index, anisotropy and roughness.

I •• THEORY

Let us consider an incident and monochromatic TM plane wave in the Kretschmann ATR configuration. In the absence of surface layer, SPW can propagate along the metal- air interface, decreasing exponentially in amplitude with distance from this interface. Resonant excitation of SP is achieved with prism coupling : when the light is

incident at an angle 6 larger than the critical angle 6c, it can couple with the SPW and the optimum coupling if reached when {1,5} :

where e0 represents the dielectric constant of the air and e = e' +is" the complex dielectric constant of the metal.

In the other part, it is well known that SPW in the Kretschtnann configuration are perturbated by the prism and the dispersion relation is consequently modified. At 9ATR, the total reflection-of the incident light is attenuated and the reflectance versus the incidence angle has a shape which depends on the thickness and the dielectric constants of the three media (prism/gold/air). If we know the thickness of the metal film (dftU ^ 500 A ) , given by an oscillating quartz, carefully calibra- ted, it is possible to determine e' and e" from experimental results with a good

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831024

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JOURNAL DE PHYSIQUE

accuracy.

A f t e r a p p l y i n g K a x w e l l ' s e q u a t i o n s and boundary c o n d i t i o n s t o o u r system, we have used e x a c t e l e c t r o m a g n e t i c f o r m u l a f o r m u l t i l a y e r s I 6 , 7 1 and f i t t e d t h e o r e t i c a l and e x p e r i m e n t a l v a l u e s o f t h e r e f l e c t a n c e v e r s u s t h e i n c i d e n c e a n g l e R ( e ) . T h i s c u r v e - f i t t i n g a l l o w e d us t o d e t e r m i n e t h e r e a l and i m a g i n a r y p a r t s c f t h e d i e l e c t r i c c c n s t a n t o f t h e c o l d l a y e r . It must be p o i n t e d o u t t h a t t h e f i l m t h i c k n e s s has been o p t i m i z e d i n o r d e r t o o b t a i n a sharp r e s o n a n t c u r v e c o r r e s p o n d i n g t o :

Furthermor?, approximate a n a l y t i c a l f o r m u l a s (assuming

I E " ~ <<

^{] E '}

1

and ^E^sE ' ) a r e l e s s v a l i d f o r o o l d f i l m s t h a n f o r s i l v e r ones and e x a c t n u m e r i c a l computations, as d e f i n e d above, a r e necessary t o a f i n e a n a l y s i s o f t h e o p t i c a l p r o p e r t i e s o f t h e l a y e r . N e v e r t h e l e s s , t h e a n g u l a r p o s i t i o n o f t h e minimum o f r e f l e c t a n c e ( ~ A T R ) i s a p p r o x i m a t i v e l y d e t e r m i n e d by ^{E '}v a l u e and t h e a n g u l a r w i d t h o f t h e r e s o n a n t c u r v e

(aeATR, d e f i n e d a t R(e) = 0.5) by E " v a l u e , t h e t h i c k n e s s l a y e r b e i n g f i x e d . T y p i c a l values a r e summarized i n t h e f o l l o w i n g t a b l e , f o r

x

= 6328

4 ,

d ~ ~ = 530

a

^and

N = 1.9078 :

We have a l r e a d y i n d i c a t e d t h a t a t h i n l a y e r can l e a d t o a m o d i f i c a t i o n i n t h e c h a r a c t e r i s t i c s o f SPU p r o p a g a t i o n . Three p r i n c i p a l e f f e c t s a r e observed on t h e r e f l e c t a n c e c u r v e :

a. a s h i f t o f t h e a n p u l a r p o s i t i o n o f t h e minimum o f r e f l e c t i v i t y : 6eATR b. a b r o a d e n i n g i n t h e shape o f t h e r e f l e c t a n c e c u r v e : 6(aeATR)

c. an i n c r e a s i n p i n t h e minimum o f r e f l e c t i v i t y : 6Rmin

I f we suppose a t h i n , i s o t r o p i c homo?eneous and non-absorbing l a y e r , t h e d i e l e c t r i c c o n s t a n t can be determined e a s i l y f r o m e x p e r i m e n t a l d a t a , t h e t h i c k n e s s b e i n g w e l l known : t h e e s s e n t i a l e f f e c t i s t h e a n n u l a r s h i f t , b u t t h e r e i s no i n c r e a s i n p i n t h e minimum o f r e f l e c t i v i t y . On t h e c o n t r a r y , a s l i r h t a b s o r p t i o n produces b o t h a b r o a - dening o f t h e r e f l e c t a n c e c u r v e and an i n c r e a s i n g 6Rmin, b u t no a n g u l a r s h i f t i s observed.

F o r an i s o t r o p i c l a y e r , t h e r e s u l t s a r e r a t h e r c o m p l i c a t e d even i f t h e medium i s an u n i a x i a l one and t h e o p t i c a l a x i s remains i n t h e p l a n e o f i n c i d e n c e . I n t h i s case, we have used n u m e r i c a l computations based on a r i g o r o u s t h e o r e t i c a l t r e a t m e n t o f t h e r e f l e c t i o n o f a s t a c k o f b i r e f r i n g e n t m u l t i l a y e r s (81 : t h e r e s u l t s s t r o n g l y depend on t h e v a l u e s o f t h e o r d i n a r y ( n o ) and e x t r a o r d i n a r y (ne) i n d i c e s o f t h e medium, p a r t i c u l a r l y f o r s t r o n g o p t i c a l a n i s o t r o p i e s . The f o l l o w i n : t a b l e showsthe impor- tance o f t h e phenomenon, g i v e n f o r a s u p e r f i c i a l l a y e r t h e t h i c k n e s s of w h i c h i s df = 380

4 ,

and X = 6328 A ; t h e g o l d l a y e r has t h e o p t i c a l p r o p e r t i e s mentionned above and t h e o p t i c a l a x i s i s p a r a l l e l t o t h e i n t e r f a c e . I n t h e o t h e r p a r t , i t i s w e l l known t h a t v e r y t h i n f i l m a r e inhomogeneous under a c r i t i c a l t h i c k n e s s w h i c h depends on t h e m a t e r i a l d e p o s i t e d on t h e m e t a l l i c l a y e r , t h e c o n d i t i o n s o f evapo- r a t i o n o r a s u p e r f i c i a l o x i d a t i o n ; th e i n f l u e n c e o f s u r f a c e roughness has a l s o been p r o v e d {91. A l l t h e s e f a c t o r s a l t e r t h e p r o p a g a t i o n o f t h e SPW and c o n s e q u e n t l y modify t h e shape and t h e p o s i t i o n o f t h e r e f l e c t a n c e c u r v e . F o r t h i s reason, i t may

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be convenient t o introduce an e f f e c t i v e index of r e f r a c t i o n r e p r e s e n t a t i v e of t h e o p t i c a l p r o p e r t i e s of t h e l a y e r , a s we s h a l l see i t in t h e next paragraph.

I 1

-

EXPERIMENTAL RESULTS AND DISCUSSION

The experimental s e t up used f o r evaporation i s a vacuum chamber i n which gold and oxide films a r e deposited a t about 10-6 Torr. The l a y e r t h i c k n e s s e s a r e monitored by an o s c i l l a t i n p q u a r t z . In t h e c a s e of gold l a y e r s , t h e q u a r t z has been c a l i b r a t e d by X-ray interferometry and t h e r e s u l t i n g accuracy i s about 5

a.

For s i l i c o n monoxide l a y e r s , t h e c a l i b r a t i o n has been done by o p t i c a l microscopic interferometry with an accuracy ,of about 10 on t h e thickness measurement. I t

must

be pointed o u t t h a t t h e material evaporated i s r a t h e r a SiO

-

SiOZ mixture than pure SiO. Furthermore, the evaporation i s achieved under oblique incidence (65') i n o r d e r t o o r i e n t t h e l i q u i d c r y s t a l molecules p a r a l l e l t o t h e s u b s t r a t e [ p l a n a r alignment). I t has been shown t h a t t h i s alignment: came from mechanical e f f e c t s and provided s t r o n s ancho- r i n g enersy {33.

The ATR experimental technique uses a He-Ne l a s e r ( A = 6328 A ) and a precision goniometer (angular accuracy : 1 ' ) ; t h e r e f l e c t i v i t y i s measured by a PIN-photodio- de and photocurrent, amplified and converted i n t o v o l t a g e , i s recorded i n o r d e r t o p l o t experimental r e f l e c t a n c e curves : R = f ( e ) . The corresponding measurement accuracy i s estimated t o : AR = 0.005 on a l l t h e r e f l e c t i v i t y range : 0 - 1 . The r e f r a c t i v e index of the prism i s N = 1.9078 a t t h e f i x e d wavelength 1 = 6328

#.

In our experiment, t h e o p t i c a l parameters of t h e gold l a y e r a r e :

A c a r e f u l c u r v e - f i t t i n o i s necessary t o determine t h e complex d i e l e c t r i c c o n s t a n t :

These values agree q u i t e well with those obtained by o t h e r a u t h o r s , from d i f f e r e n t experiments I l O , l l , l ~ ~ . The t h i c k n e s s of t h e oxide l a y e r , determined a s i n d i c a t e d above, i s : df = 560 A.The experimental r e f l e c t a n c e curve, i s c h a r a c t e r i z e d by t h e following o p t i c a l -parameters : eATR = 36' 10'

+

¹

'

; aeHTR = 1'47

+

2 ' .

If

we

assume a non-absorbing and i s o t r o p i c l a y e r , t h e angular s h i f t 6eATR l e a d s t o a very low value of t h e r e f r a c t i v e index : n = 1.291 k 0.002. This i s primarily due t o t h e f a c t t h a t t h e e x a c t composition of t h e vacuum deposited s i l i c o n monoxide f i l m i s n o t well known, depending on t h e evaporation conditions I13,14,153:

t h e r e f r a c t i v e index can vary between 2.46 (pure Si02) and 2.00 (pure SiO), f o r a bulk material ; furthermore, t h e deposition under grazing incidence angle induces a g r a n u l a r s t r u c t u r e v i s i b l e on e l e c t r o n micrographs 161. Consequently t h e f i l m must be considered a s inhomo~eneous and t h i s can explain t h e discrepancy between t h e values given by o t h e r authors and o u r s .

For these reasons, t h e value of the r e f r a c t i v e index i n d i c a t e d above r e p r e s e n t s an e f f e c t i v e value, considered a s an o p t i c a l parameter, taking t h e inhomogeneous s t r u c t u r e of the t h i n fi%m into,account: In t h e o t h e r p a r t , we have obtained a

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CIO-116 JOURNAL

DE

PHYSIQUE

r e l a t i v e discrepancy between t h e o r e t i c a l and experimental angular width :

Nevertheless, we have f i t t e d t h e o r e t i c a l and experimental r e f l e c t a n c e curves in t h i s two ways :

a . f i r s t , we have supposed a non-absorbing uniaxial a n i s o t r o p i c medium, with i t s o p t i c a l a x i s p a r a l l e l t o t h e i n t e r f a c e ; t h i s l e a d s t o t h e following values, f o r t h e ordinary and e x t r a o r d i n a r y r e f r a c t i v e i n d i c e s of t h e l a y e r

I t must be pointed out t h a t t h e couple in

,

n,) i s only determined by t h e couple ( ~ A T R , n8A-r~) and t h e assumption oP anisotropic would come from t h e a n i s o t r o p i c condition of evaporation.

b. second, we have supposed an absorbing and i s o t r o p i c medium and introduced a s l i g h t damping in t h e d i e l e c t r i c c o n s t a n t of t h e f i l m . This i s formally e q u i v a l e n t t o a complex r e f r a c t i v e index given by :

-

1 This l e a d s t o an absorption c o e f f i c i e n t : a = 0.14 l m

.

To determine which of t h e two hypothesis i s v a l i d , we have performed an ATR e x p e r i - ment w i t h a s p e c i a l prism c o n s t i t u t e d by an o p t i c a l g l a s s pyramid ( s i m i l a r t o an Egyptian pyramid) on which gold and s i l i c o n monoxide ;layers have been deposited ; in such a c o n f i g u r a t i o n t h e plane of incidence may be e i t h e r p a r a l l e l o r perpendicular t o t h e plane of evaporation and f o r an a n i s o t r o p i c l a y e r , numerical computations show t h a t t h e p o s i t i o n of t h e minimum of r e f l e c t i v i t y i s d i f f e r e n t in t h e two c a s e s . Similar r e s u l t s have been obtained in t h e two geometries and in t h e range of t h e

~xperirnental accuracy. For t h i s reason, we may conclude t h a t t h e second hypothesis i s v a l i d only.

R e f l e c t i v i t y R versus i n c i - dence angle

e

: t h e o r e t i c a l curves ( f u l l l i n e s ) and experimental p o i n t s ( r gold,

x

gold + SiO).

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The e x i s t e n c e o f an i m a g i n a r y p a r t i n t h e r e f r a c t i v e i n d e x can be understood as a s l i g h t a b s o r p t i o n ; t h e e x p e r i m e n t a l v a l u e o f a a p r e e s w i t h t h o s e o b t a i n e d f o r f i l m s o f SiO v e r y w e l l C15). Another e x p l a n a t i o n i s t o c o n s i d e r l o s s e s due t h e s u r f a c e roughness. I n f a c t , X-rays measurements have shown t h e f l l o w i n g v a l u e s f o r t h e

W

r.m.s. s u r f a c e roughness o f t h e SiO l a y e r C16) : 6 = 15

.

Such a roughness produces an i n c r e a s i n g o f damping and c o n s e q u e n t l y a b r o a d e n i n g i n t h e c u r v e o f r e f l ' e c t i v i t y , f o r m a l l y e q u i v a l e n t t o a d i e l e c t r i c l o s s , l e a d i n g t o i n t r o d u c e an i m a g i n a r y p a r t i n t h e d i e l e c t r i c c o n s t a n t o f t h e o v e r l a y e r . I t i s l i k e l y t h a t a l l t h e s e phenomenons o c c u r s i m u l t a n e o u s l y and f u r t h e r experiments would be necessary t o conclude, i n p a r t i c u l a r on t h e roughness i n f l u e n c e .

ACKNOWLEDGMENTS

T h i s r e s e a r c h was s u p p o r t e d by DUET c o n t r a c t 81/629. We a r e much i n d e p t e d t o Y. LEVY f o r v a l u a b l e d i s c u s s i o n s and t o M. BOIX f o r SiO e v a p o r a t i o n s .

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