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AUTOCAR LAMPS COATED WITH A YELLOW MULTILAYER FILTER BY LPCVD
C. Mulder, W.H.M.M. van de Spijker, J. Jensma, G. Verspui, G. Heijnen
To cite this version:
C. Mulder, W.H.M.M. van de Spijker, J. Jensma, G. Verspui, G. Heijnen. AUTOCAR LAMPS
COATED WITH A YELLOW MULTILAYER FILTER BY LPCVD. Journal de Physique Colloques,
1989, 50 (C5), pp.C5-241-C5-248. �10.1051/jphyscol:1989530�. �jpa-00229553�
JOURNAL DE PHYSIQUE
Colloque C5, supplement au n05, Tome 50, mai 1989
AUTOCA* LAMPS COATED WITH A YELLOW MULTILAYER FILTER BY LPCVD
C.A.M. MULDER, W.H.M.M. VAN DE SPIJKER, J. JENSMA*, G. VERSPUI and G.H.C. HEIJNEN*"
Philips Lighting B.V.. NL-5600 JM Eindhoven, The Netherlands
'philips Centre for manufacturing Technology, NL-5600 MD Eindhoven, The Nether1 ands
"philips Gluhlampwerk, 0-5100 Aachen, F.R.G.
~dsumd
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A fin d'amdliorer le flux lumineux de phares jaunes, un filtre bleu d'interfdrence rdfldchissant multicouches a dtd appliquddirectement sur les ampoules. Un tel filtre a dtd produit par le ddp6t altern6 de couches Si3N4 et SiOz en systgme LPCVD standard opdr6 d seulement un profil de tempdrature. Par une bonne sdlection de la pression, des caract6ristiques de ddbit gazeux et de la gdomdtrie de charge dans le tube en quartz du four pour chaque couche, on a pu optimiser les deux processus. La croissance du filtre a 6td contr8lde par un appareil optique de contrale d'dpaisseur in situ.
Actuellement plusieurs centaines de lampes aux halog6nes (H4) ont 6td revstues d'un filtre bleu d'interfdrence, rdfl6chissant composd de 1 1 couches de Si3N4/Si02. Cette operation a eu lieu dans un rdacteur LPCVD, le temps de cycle dtant de moins de 3 heures. L'efficacitd optique des lampes jaunes multicouches H Qtd amdliorde de plus de 20%
relativement aux lampes conventionnelles dquipdes d'une enveloppe ext6rieure en verre absorbant s6lectivement la lumisre bleu.
Abstract
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To improve the light output of yellow autocar headliqhts a blue reflecting multilayer interference filter was deposited directly on the bulbs. Such a filter was produced by the alternate deposition of Si3N4 and SiOa layers in a standard LPCVD system operated at one temperature profile only. By properly selectinq the pressure, qas flow characteristics and load qeometry in the quartz furnace tube for each layer material, both processes could be optimized. The growth of the filter was monitored by an in-situ optical thickness controller.Presently, several hundreds (H4) halogen lamps were coated
simultaneously with an 11-layer Si3N4/Si02 blue reflecting interference filter in a LPCVD reactor with a cycle time of less than 3 hours. The layer thickness uniformity of the deposited filter on each lamp was better than
+
2%. The optical efficiency of the yellow multilayer lamps was improved by more than 20% with respect to conventional onesequipped with an outer glass jacket that selectively absorbs blue light.
1
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INTRODUCTIONYellow autocar headlights are regarded as safer since contrasts, and
therefore obstacles, are easier to see and that driver fatique is somewhat less due to a kinder light emanating from the headliqhts of oncominq traffic. Also there is evidence that yellow light penetrates foq, mist and rain more efficiently than white light / l / althouqh more recent discussions disclaim the advantages of yellow light for road traffic purposes /2/.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989530
JOURNAL DE PHYSIQUE
r-
visible light7
Fig. 1
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Transmittance of the cadmium-doped jacket glass (measured) that selectively absorbs blue light ("selectiva") and transmittance of a blue reflecting multilayer interference filter at normal incidence (calculated).Conventionally, the yellow is produced by filtering white light throuqh an outer glass jacket that selectively absorbs blue liqht. This is optically inefficient (see fig. 1 ) due to the absorbinq nature of the filter and
reflection losses as a consequence of the extra outer glass jacket. An ideal filter should have a low transmittance for visible light in the reqion below approx. 530 nm (high blue reflecting filter) followed by a sharp transition to a high transmittance for the rest of the visible spectrum.
An alternative manner to achieve such a colour filter is to use a multilayer interference filter which can be deposited directly on the bulb itself. Such a filter consists of a number of temperature resistant layers with
alternating high and low refractive indices which reflects a specified wavelength band with rather sharp edges (see fig. 1). An improvement of at least 20% in optical efficiency can be reached as compared with conventional halogen lamps equipped with a yellow glass jacket. In addition, the
manufacturing process no longer involves the use of the cadmium containinq glass.
A very suitable method to deposit a number of homogeneous layers on
complex-shaped materials is that of low pressure chemical vapour deposition (LPCVD). In the present study yellow multilayer filters were produced by the alternate deposition of Si3N4 and Si02 layers in a hot-wall LPCVD furnace.
The aim was to be able to perform both deposition processes at one temperature profile in the reactor and to uniformly deposit an 11-layer Si3N4/Si02 blue reflecting interference filter on several hundreds (H,) halogen lamps simultaneously. A preliminary account to the present results was given by Verheijen et al. / 3 / .
2
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KINETICALLY CONTROLLED LPCVDIn chemical vapour deposition a chemical reaction of gases or vapours near a surface is used to deposit material onto the surface. The chemical reaction can be activated by means of temperature, but also plasma or liqht radiation can be used.'Normally a thin layer is deposited on a substrate surface. By varying the experimental conditions
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substrate temperature, composition of the reaction gas mixture, total pressure, gas flows, etc.-
materials with different properties can be deposited. CVD processes can be classified into two categories /4/: diffusion controlled processes when the mass transport of reactants to the surface is the growth determininq step in the process, and kinetically controlled processes when the chemical reaction at the surface is rate determining. Surface kinetics control is favourable for obtaininqcoherent coatings of uniform thickness and of low porosity on more complicatedly shaped substrates. Whether a CVD process is diffusion or kinetically controlled can be determined from the temperature dependence of the overall reaction rate Rdep which follows an Arrhenius equation:
Rdep = A exp (-Ea/RT)
where E, is the activation energy. Typical values of E, for kinetically controlled reactions range from 100 to 400 kJ/mol. For diffusion controlled reactions Ea is well below 100 kJ/mol as the diffusion rate D is less dependent on temperature (D T ~ . ~ - ~ . ~ 1.
3
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LPCVD OF Si3114 AND Si0,In the present study two known processes from the semiconductor industry
/ 5 - 9 / were combined, i.e. Si3N4 from dichlorosilane and ammonia, and SiO,
from tetraethoxysilane (TEOS) accordinq to the following reaction equations:
Si(OC2H5 14
-
Si02,/, +
byproducts.In the case of interference filters for lamps, a flat thickness profile for both processes at one deposition temperature profile had to be established.
In the temperature range from 730 to 780°C normally a positive slope of axial thickness profile for the Si3N4 process and a negative slope for the SiO2 process were observed. To obtain a reasonable growth rate for both processes the average temperature in the deposition zone was selected around 750°C.
Then the most important process parameters for adjusting the growth rate in the reactor tube were vapour and gas flow-rate, concentration of reactants and total pressure. Typical growth rates were 4 nm/min for Si3N4 at a
C5-244 JOURNAL DE PHYSIQUE
p r e s s u r e o f 67 P a ( 0 . 5 t o r r ) and 25 nm/min f o r S i 0 2 a t 27 P a ( 0 . 2 t o r r ) . Under t h e s e c o n d i t i o n s t h e A r r h e n i u s a c t i v a t i o n e n e r q i e s E, f o r S i 3 N 4 and S i 0 2 amounted a p p r o x . 170 kJ/mol and 190 k J / m o l , r e s p e c t i v e l y , and w e r e w e l l i n t h e k i n e t i c a l l y c o n t r o l l e d r e q i o n . The r e s u l t i n g d e p o s i t e d S i 3 N 4 and S i 0 2 l a y e r s were f u l l y t r a n s p a r e n t f o r v i s i b l e l i g h t and had i n t h i s w a v e l e n q t h r a n g e r e f r a c t i v e i n d i c e s o f 1 . 9 9 and 1 . 4 4 , r e s p e c t i v e l y . The a r o w t h r a t e and r e f r a c t i v e i n d e x o f e a c h l a y e r m a t e r i a l w e r e d e t e r m i n e d i n s e p a r a t e p r o c e s s c a l i b r a t i o n e x p e r i m e n t s . L a y e r s were d e p o s i t e d o n s m a l l s i l i c o n s a m p l e s p l a c e d o v e r t h e l e n g t h o f t h e s u b s t r a t e h o l d e r and t h e t h i c k n e s s and r e f r a c t i v e i n d e x w e r e m e a s u r e d b y means o f a n e l l i p s o m e t e r .
4
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EQUIPMENTThe e x p e r i m e n t s a r e p e r f o r m e d i n a c o n v e n t i o n a l 5 z o n e r e s i s t a n c e - h e a t e d LPCVD r e a c t o r e q u i p p e d w i t h a l a r g e d i a m e t e r , f l a n g e l e s s q u a r t z t u b e . H e a t s c r e e n s a r e p l a c e d o n b o t h s i d e s o f t h e s u b s t r a t e h o l d e r . A d i g i t a l
t e m p e r a t u r e c o n t r o l l e r s t a b i l i z e s and r e p r o d u c e s t h e t e m p e r a t u r e i n s i d e t h e r e a c t o r t u b e t o w i t h i n 0 . p C . A l l t h e q a s e s a r e s u p p l i e d a u t o m a t i c a l l y t o t h e r e a c t o r d i r e c t l y a t t h e h e a t e d f r o n t f l a n g e and c o n t r o l l e d w i t h t h e r m a l m a s s f l o w c o n t r o l l e r s . The TEOS is v a p o u r i z e d i n a p y r e x v e s s e l which i s immersed i n a t h e r m o s t a t e d o i l b a t h . The TEOS s u p p l y l i n e t o t h e f r o n t f l a n q e i s e q u i p p e d w i t h a p r e s s u r e - b a s e d m a s s f l o w c o n t r o l l e r p l a c e d i n a n i s o l a t e d and t e m p e r a t u r e c o n t r o l l e d h o u s i n g . T h i s s u p p l y l i n e is h e a t e d t o a
t e m p e r a t u r e w h i c h is a t l e a s t 10°C h i g h e r t h a n t h a t o f t h e v a p o u r i z e r . The p r e s s u r e i n t h e r e a c t o r is r e g u l a t e d w i t h a t h r o t t l e v a l v e .
5
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OPTICAL THIN-FILM FILTERSTo o b t a i n a n o p t i c a l f i l t e r w h i c h s p e c i f i c a l l y r e f l e c t s t h e s h o r t w a v e l e n q t h p a r t o f t h e v i s i b l e l i g h t , a r e l a t i v e l y s i m p l e f i l t e r d e s i g n c o u l d b e a p p l i e d / 1 0 , 1 1 / . The r e f l e c t i o n o f a n i n t e r f e r e n c e f i l t e r is d e t e r m i n e d by ( t h e d i f f e r e n c e b e t w e e n ) t h e r e f r a c t i v e i n d i c e s o f t h e u s e d m a t e r i a l s and t h e number o f l a y e r p a i r s .
L e t u s c o n s i d e r a t h i n - f i l m s t r u c t u r e c o n s i s t i n g o f a h i q h r e f r a c t i v e i n d e x m a t e r i a l o n t h e s u b s t r a t e f o l l o w e d b y a l o w - i n d e x m a t e r i a l and a g a i n t h e h i g h - i n d e x m a t e r i a l ( f i g . 2 ) . When a beam o f l i g h t p a s s e s t h r o u g h s u c h a s t a c k o f l a y e r s and t h r o u g h t h e s u b s t r a t e , p a r t o f t h e i n c i d e n t l i g h t w i l l b e r e f l e c t e d a t t h e t o p and a t t h e b o t t o m s u r f a c e s o f e a c h t h i n f i l m . The beams r e f l e c t e d a t t h e f o u r b o u n d a r i e s c a n i n t e r f e r e c o n s t r u c t i v e l y o r
d e s t r u c t i v e l y d e p e n d i n g on t h e i n d i v i d u a l p a t h l e n g t h d i f f e r e n c e s and o n t h e p h a s e s h i f t s ( l a p ) t h a t o c c u r a t low t o h i g h - i n d e x b o u n d a r i e s . I f t h e o p t i c a l t h i c k n e s s ( = g e o m e t r i c a l t h i c k n e s s X r e f r a c t i v e i n d e x ) o f e a c h t h i n f i l m i s o n e q u a r t e r w a v e l e n g t h t h i c k , i t i s e a s y t o show t h a t t h e r e f l e c t e d beams w i l l r e a p p e a r a t t h e f r o n t s u r f a c e a l l i n p h a s e a n d , h e n c e , r e c o m b i n e c o n s t r u c t i v e l y . By a d d i n g more h i g h / l o w - i n d e x p a i r s t h e e f f e c t i v e r e f l e c t a n c e
thin film
\/
F i g . 2
-
A t h i n - f i l m s t r u c t u r e c o n s i s t i n g o f a s t a c k o f a l t e r n a t e h i g h and low r e f r a c t i v e i n d e x f i l m s on a s u b s t r a t e .o f t h e a s s e m b l y c a n be made v e r y h i g h ( s e e f i g . 3 ) . A s t h e p a t h l e n g t h d i f f e r e n c e s a r e w a v e l e n g t h d e p e n d e n t , i t i s found t h a t t h e r e f l e c t a n c e r e m a i n s h i g h o v e r o n l y a l i m i t e d r a n g e o f w a v e l e n g t h s d e p e n d i n g on t h e r a t i o o f t h e h i g h and low r e f r a c t i v e i n d i c e s . O u t s i d e s u c h a zone t h e r e f l e c t a n c e d r o p s t o a low v a l u e .
For a b l u e r e f l e c t i n g f i l t e r t h e w a v e l e n g t h o f maximum r e f l e c t i o n is 455 nm, which i m p l i e s a g e o m e t r i c a l t h i c k n e s s f o r t h e q u a r t e r w a v e l e n g t h l a y e r s o f S i 3 N 4 and S i 0 2 o f a p p r o x . 58 nm and 7 9 nm, r e s p e c t i v e l y . A s t a c k o f e l e v e n l a y e r s s t a r t i n g w i t h t h e h i g h - i n d e x m a t e r i a l is s u f f i c i e n t t o r e n d e r t h e same d e g r e e o f c o l o u r s a t u r a t i o n a s f o r t h e j a c k e t o f a b s o r b i n g g l a s s . The
t r a n s m i t t a n c e o f t h e c a l c u l a t e d f i l t e r i s shown i n f i g . 1. B y r e d u c i n q t h e
r
r
visible light100 [
_ I
I
F i g . 3
-
R e f l e c t a n c e a t normal i n c i d e n c e o f m u l t i l a y e r s t a c k s of 3 , 7 and 11 a l t e r n a t i n g q u a r t e r w a v e l e n g t h l a y e r s o f h i g h nH = 1.99 and low n~ = 1.44 r e f r a c t i v e i n d e x on a g l a s s s u b s t r a t e ( n s = 1 . 5 0 ) .C5-246 JOURNAL DE PHYSIQUE
thickness of the first and last (high-index) layer, a hiqh transmittance independent of wavelength in the rest of the visible spec'trum is obtained.
6
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IN-SITU OPTICAL THICKNESS CONTROLThe growth of an optical filter on the substrates in the LPCVD reactor can be monitored as follows. One end of a quartz optical fibre is positioned in the reactor tube in the deposition zone. Chopped light of a monochromatic source is coupled into the cold end of the fibre. Reflected light is fed to a
Si-detector connected with a lock-in amplifier. During deposition, chanqes in light reflection occur as a consequence of the qrowth of high or low
refractive index layers. As the hot end of the fibre is close to the
substrates and if the growth rate is homogeneous over the batch, changes in reflection of a layer during deposition are a direct measure of the film thickness on the substrates. This enables a control of the optical thickness of the growing layer and, hence, the determination of the exact switching point for every individual layer of a prescribed filter design. A typical example of the monitored reflection signal during a deposition process is given in fig. 4. The shape of this reflection curve is strongly dependent on the chosen monitor wavelength as can be inferred from fiq. 3. Durinq the deposition of the filter strong oscillations occur at the wavelenqth of (and close to) the maximum.
0 20 40 60 80 100 120
-
time ( m i dFig. 4
-
Typical recorder curve of the reflection signal during a deposition run of an 11-layer interference filter at a monitor wavelenqth of 540 nm(Si3N4
-
drawn lines, Si02-
dashed lines). The horizontal lines indicate the switching time between two process steps.r -
visible light-1
300 400 500 600 700 800
A wavelength (nm)
Fig. 5
-
Measured transmittance and reflectance of an 11-layer Si3N4/Si02 interference filter on a quartz (H4) haloqen lamp.The in-situ optical thickness control is more accurate than deposition determined from growth rate calibration experiments. Generally speaking optical effects should preferably be measured by an optical method.
7
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RESULTS AND DISCUSSIONFor both Si3N4 and Si02 processes a flat growth rate over a relatively lonq deposition zone in the LPCVD reactor was obtained at a temperature profile at approx. 750°C. The choice of the temperature profile was mainly determined by the Si02 process. The Si02 growth rate profile was then adjusted with the TEOS vapour flow and a relatively low pressure for this process. The thickness profile of the second process (Si3N4) was flattened with the NH3 flow and a relatively high value for the total pressure. The multilayer colour filters were produced by the alternate deposition of Si3N4 and Si02. A switch-over time between the two deposition processes of at least 30 S was necessary in order to stabilize the new required deposition pressure. During the switch-over argon was introduced to purqe the reactor.
An eleven-layer blue reflecting interference filter was be deposited
simultaneously on several hundreds, densely packed (H4) haloqen lamps. Total cycle time including time for loading/unloading and temperature stabilization was less than 3 hours per batch. The filters were absorption free and had an excellent colour uniformity which is linearly related to layer thickness uniformity (batch-to-batch reproducibility better than
+
2%). A typical example of a transmission and reflection curve of a coated lamp is given in fig. 5. A good adherence of the filter on the quartz substrates and noC5-248 JOURNAL DE PHYSIQUE
deterioration of optical properties of the filter were observed from
scotch-tape tests before and after temperature treatment (up to 600°C in air for 24 h) and tropical chamber tests.
8
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CONCLUSIONMultilayer blue reflecting interference filters can be produced on an industrial scale by the alternate deposition of Si3N4 and Si02 on complex-shaped substrates in a standard hot-wall LPCVD system at one
temperature profile only. The growth of the filter is monitored by an in-situ optical thickness controller. On (H4) halogen lamps this results in
absorption-free filters with excellent colour uniformity (better than f 2%).
ACKNOWLEDGEMENT
Thanks are due to A. Boekkooi, D. Guilmant, J. de Ridder and E. van der Stelt for support and stimulatinq discussions.
REFERENCES
/l/ Monnier, A. and Mouton, M., Lux 10 (1934) 144.
/2/ Schreuder, D.A. : White or ~ e l l o w ~ i q h t s for Vehicle Head-Lamps?
,
ed.E. Asmussen (Institute for Road Safety Research, The Netherlands, 1976)
G,
pp. 9-54./3/ Verheijen, J., Bongaerts, P. and Verspui, G., Proc. 10th Int. Conf. on Chemical Vapor Deposition (Hawai, USA, 1987) pp. 977-981.
/4/ Bryant, W.A., J. Mater. Sci. 12 (1977) 1285.
/5/ Rosler, R., Solid State ~echn- (1977) 63.
/6/ Kern, W. and Rosler, R.S., J. Vac. Sci. Techn. 14 (1977) 1082.
/7/ Makino, T., J. Electrochem. Soc.
130
(1983) 40:5/8/ Huppertz, H. and Engl, W.L., IEEE Trans. Electr. Dev. ED 26 (1979) 658.
/9/ Becker, F.S., Pawlik, D., Schafer, H. and Staudigl, G., J. Vac. Sci.
Techn. B 4 (1986) 732.
/10/ ~acleod,.~. : Thin-film Optical Filters (Hilger, U.K., 1986).
/11/ Pulker, H.K. : Coatings on Glass (Qlsevier, The Netherlands, 1984).
