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A MICROPROCESSOR CONTROLLED SPECTROELLIPSOMETER FOR STUDY OF
ELECTROCHEMICAL SYSTEMS
S. Pihlajamäki, J. Kankare
To cite this version:
S. Pihlajamäki, J. Kankare. A MICROPROCESSOR CONTROLLED SPECTROELLIPSOMETER
FOR STUDY OF ELECTROCHEMICAL SYSTEMS. Journal de Physique Colloques, 1983, 44 (C10),
pp.C10-53-C10-56. �10.1051/jphyscol:19831010�. �jpa-00223459�
JOURNAL DE PHYSIQUE
Colloque CIO, s u p p l e m e n t a u n012, T o m e 44, d k e m b r e 1983 page C 1 0-53
A MICROPROCESSOR CONTROLLED SPECTROELLIPSOMETER FOR STUDY OF ELECTROCHEMICAL SYSTEMS
S.V. P i h l a j a r d i k i a n d J.J. Kankare
Department of Chemistry, University of W k u , SF-20500 hcrku 50, Finland
RBsum6
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Un ellipsom&tre a u t o m a t i q u e B modulation d e phase e s t ddcrit.L ' i n s t r u m e n t p e u t simultandment f a i r e des m e s u r e s ellipsomdtriques e t d l e c t r o -
chimiques, contrbl6esparunmicroprocesseur. Leslongueursd'ondeentre250nmet
550 nm peuvent S t r e utilisCes. L ' i n s t r u m e n t a d t d utilisd pour d e t e r m i n e r l a composition d ' u n film adsorb4 sur s u l f u r e d e cuivre(I) d a n s une solution deoxygdnde d e x a n t h a t e d ' d t h y l e ri un p o t e n t i e l c o n t r b l i . .
Abst t a c t m e n t c a n controlled
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An a u t o m a t i c phase-modulation ellipsometer is described. T h e instru- p e r f o r m ellipsometric a n d v o l t a m m e t r i c m e a s u r e m e n t s simultaneously,by a microprocessor. T h e wavelength range t h a t c a n be used is 250
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550 nm. T h e instrument has b e e n used t o d e t e r m i n e t h e composition of a film adsorbed o n copper([) sulfide in deoxygenated e t h y l x a n t h a t e solution a t controlled potential.
I n t r o d u c t i o n
In many e l e c t r o c h e m i c a l processes a thin film is f o r m e d o r removed, o r t h e topo- graphy o f a b a r e s u r f a c e changes. Such r e a c t i o n s c a n b e studied by ellipsometry in s i t u simultaneously with conventional e l e c t r o c h e m i c a l measurements.
When t h e c o u r s e of a reaction is followed, many m e a s u r e m e n t s must b e m a d e during t h e process. T o o b t a i n information o n t h e composition of a film f o r m e d a s t h e re- sult of t h e reaction, m e a s u r e m e n t s must b e m a d e a t several wavelengths. Such m e a s u r e - m e n t s a r e impossible o r a t l e a s t very tedious with a manual instrument.
T h e a i m of this work w a s t o c o n s t r u c t a n a u t o m a t i c s p e c t r o e l i i p s o m e t e r f o r study of e l e c t r o c h e m i c a l systems. A microprocessor was used b o t h for c o n t r o l of t h e instru- m e n t and f o r d a t a acquisition, in o r d e r t o m a k e it e a s y t o a d a p t t h e instrument for dif- f e r e n t m e a s u r e m e n t schemes.
T h e e l e c t r o c h e m i c a l reaction s t u d i e d as a n application of t h e new i n s t r u m e n t w a s adsorption of e t h y l x a n t h a t e o n copper(1) sulfide f r o m deoxygenated solution of potassium e t h y l x a n t h a t e . T h i s r e a c t i o n is very i m p o r t a n t , as alkyl x a n t h a t e s a r e widely used in s e p a r a t i o n of sulfide minerals from o r e s by flotation.
S t r u c t u r e o f t h e i n s t r u m e n t
A block diagram of t h e ellipsometer is shown in fig. 1. T h e instrument is a phase-modulation e l l i p s o m e t e r , in which t h e polarization s t a t e of light is varied by a p h o t o e l a s t i c modulator /I/. Information o n t h e o p t i c a l p r o p e r t i e s of t h e s a m p l e is retriev- a b l e from harmonic analysis of t h e r a t i o of t h e a c t o d c c o m p o n e n t s o f t h e photomulti- plier o u t p u t c u r r e n t .
T h e a c signals a r e measured by lock-in a m p l i f i e r s , t h e output of which is f e d ' t o a microprocessor via analog-to-digital converters. T h e microprocessor also c h a n g e s t h e wavelength, t h e p o t e n t i a l of t h e working e l e c t rode, a n d t h e s t rain-inducing voltage of t h e piezoelectric unit of t h e photoelastic modulator.
T h e wavelength range of t h e i n s t r u m e n t is 250 - 550 nm.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831010
JOURNAL DE PHYSIQUE
Fig. 1. Block diagram of t h e a u t o m a t i c ellipsometer. Dashed lines represent light, solid lines e l e c t r i c connections. Xe-lamp: ( O s r a m ) , Oriel power supply 8540; Monochromator:
H.20 (Jobin Yvon); Polarizer a n d analyzer: Glan-Taylor prisms, schlieren-free c a l c i t e , ex- tinction r a t i o 0.00001 ( K a r l L a m b r e c h t ) ; Photoelastic modulator: Morvue PEM-3; Photo- multiplier 1P28A.
T h e inner p a r t of t h e s a m p l e cell is m a d e of carbon-filled PTFE, which is b o t h black and inert. T h e o u t e r p a r t is of aluminum t o provide sufficient mechanical strength.
T h e windows of t h e cell a r e made of fused silica microscope slides, a n d s e a l e d with rings of silicone rubber using as l i t t l e pressure as possible t o avoid s t r a i n birefringence.
T h e s a m p l e holder is a t t a c h e d t o t h e lid of t h e s a m p l e c e l l s o t h a t t h e s a m p l e s u r f a c e is vertical. T h e a n g l e of incidence is 45'.
S t r u c t u r e - o f t h e c o n t r o l p r o g r a m
T h e control p r o g r a m is w r i t t e n in assembly language. T h e program is s t o r e d on a floppy disk of Apple 11+ c o m p u t e r a n d loaded by a d a t a transmission link. A flowchart o f t h e p r o g r a m is shown in fig. 2. T h e p r o g r a m consists of t w o main branches: t h e wave- l e n g t h s c a n and t h e potential scan. Input p r o c e d u r e s and subroutines a r e c o m m o n t o b o t h branches.
T h e input p a r a m e t e r s t h a t d e t e r m i n e t h e d e t a i l s of t h e m e a s u r e m e n t process a r e wavelength limits for t h e modulator control voltage s e t t i n g s , wavelength l i m i t s f o r m e a - s u r e m e n t , wavelength i n c r e m e n t , p o t e n t i a l limits, p o t e n t i a l i n c r e m e n t , n u m b e r o f readings t a k e n f o r e a c h A/D-conversion, and number of duplicate measurements. F o r t h e p o t e n t i a l
I N P U T
CONTROL W A V f L I G T W
AND l N l T l M
S E T
1
w . v t L f w r *1 I I
S E T ELECTRODE r n n w i I A L
1
O P T I C A L S I G N A L S
1-p
V A L U E S6 - 4
Fig. 2. Flowchart of t h e control program.
s c a n additional control p a r a m e t e r s a r e t h e value of t h e initial p o t e n t i a l a n d i t s hold t i m e , s w e e p r a t e , and s w e e p t y p e ( r a m p o r triangle).
T h e smallest wavelength increment t h a t c a n be used is 1 nm. T h e lowest s c a n r a t e is 1 mV/s. T h e AID-converter uses a r e f e r e n c e voltage of -10 V, which gives a res- olution of 5 mV. T h e integrating AID-conversion process f u r t h e r improves this resolution.
T h e D/A-converter for t h e potentiostat uses a r e f e r e n c e voltage of -5 V, which gives a resolution of 1 0 mV.
D a t a p r o c e s s i n g
T h e d a t a processing is done by a n Apple II+ c o m p u t e r with a program w r i t t e n in CP/M-FORTRAN.
F o r a n ambient-film-substrate s y s t e m t h e film p a r a m e t e r s a r e c a l c u l a t e d by iter- a t i o n using t h e e x a c t Drude equations a n d Fresnel reflection c o e f f i c i e n t s /2/. T h e itera- tion is done by a s t e p method 131.
T h e wavelength range, in which t h e extinction coefficient of t h e film, k , is zero, is given as input. T h e program c a l c u l a t e s t h e refractive index, n , a n d t h e thickness of t h e film, d , in t h i s range. An average value of thicknesses is t h e n used t o c a l c u l a t e n a n d k a t o t h e r wavelengths.
CIO-56 JOURNAL DE PHYSIQUE
T h e e r r o r function minimized in t h e i t e r a t i o n process is
where R, a n d R , a r e t h e measured signals:
A d s o r p t i o n o f x a n t h a t e o n c o p p e r ( 1 ) s u l f i d e
X a n t h a t e s a d s o r b on different m e t a l sulfides forming e i t h e r dixanthogen o r m e t a l x a n t h a t e s /4/. T h e reaction is of e l e c t r o c h e m i c a l n a t u r e requiring e i t h e r t h e presence of oxygen in t h e solution o r oxidation products o n t h e s u r f a c e of t h e sulfide, o r suitable potential.
T o d e t e r m i n e t h e composition of t h e film t h a t is formed, when e t h y l x a n t h a t e r e a c t s with t h e s u r f a c e of s y n t h e t i c d j u r l e i t e a t controlled p o t e n t i a l in deoxygenated solution, t h e s p e c t r u m of t h e optical c o n s t a n t s of t h e film was measured by ellipsometry b e t w e e n wavelengths 350 and 550 nm. T h e lower limit was s e t by t h e strong absorption of light by x a n t h a t e in t h e solution.
T h e s p e c t r u m of t h e extinction c o e f f i c i e n t of t h e film a n d t h e absorption spec- t r u m of a colloidal w a t e r solution of copper(I) x a n t h a t e a r e shown in fig. 3. T h e similar- i t y of t h e s e s p e c t r a c o n f i r m s t h e p r e s e n c e of copper(1) x a n t h a t e on t h e s u r f a c e of t h e sulfide.
Fig. 3. S p e c t r a o f copper(1) x a n t h a t e a n d t h e x a n t h a t e adsorption film on copper(1) sul- fide. Conditions: Colloidal w a t e r solution: C(CuEX) = 0.000047 M; Adsorbed film:
C(KEX) = 0.0004 M, c(NaCI) = 0.07 M , adsorption potential -140 mV vs. SCE, c a l c u l a t e d film thickness 58.4 nm (wavelength range 510 - 550 nm).
R e f e r e n c e s
1. JASPERSON, S. N., SCHNATTERLY, S. E., Rev. Sci. Instrum. 40 (1969) 761.
2. AZZAM, R. M. A., BASHARA, N. M., Ellipsometry and PolarizTd Light, North-Holland Publishing Co., A m s t e r d a m , 1977, s. 286, 288.
3. CHANDLER, J. P., Program 66.1, Quantum Chemistry Program Exchange, Indiana Uni- versity, Bloomington, Indiana, 1966.
4. ALLISON, S. A., GOOLD, L. A., NICOL, M. J., GRANVILLE, A., Metall. Trans.
3 (1972) 2613.
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