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Thin vapour-deposited niobium pentoxide films
Thin SolM Films, 201 ( 1991 ) 137 145
137
PREPARATION AND CHARACTERIZATION
T H I N V A P O U R - D E P O S I T E D NIOBIUM P E N T O X I D E FILMS J. J. VAN GLABBEEK AND R. E. VAN DE LEEST
Philips Research Laboratories, Eindhoven ( The Netherlands) (Received September 20, 1990; accepted November 22, 1990)
Thin vapour-deposited niobium pentoxide films exhibit optical absorption which limits their application as a high refractive index layer for optical interference filters. Water adsorption in the pores of the film is not the cause of the poor optical quality, as is generally assumed for this type of film. Decomposition of Nb205, a thermodynamically defined process, leads to non-stoichiometric films of poor quality in terms of structure and morphology. Decomposition of the electron-beambombarded Nb205 melt aggravates the situation. Stoichiometry can be restored by heating in an oxygen ambient.
1. INTRODUCTION
Thin niobium pentoxide films are used as the high refractive index layers in optical interference filters which consist of a multilayer stack of high index and low index materials. When deposited by electron beam evaporation, the niobium pentoxide layers gradually exhibit more optical absorption, which limits the application of this material. A generally accepted explanation for the deterioration of these oxides is that the porous morphology of evaporated films 1 causes water to adsorb in the pores of the film. Water has a different refractive index and is able to react with oxides. As a consequence the optical properties of these films, and hence the interference filter, change 2, 3. As well as the water content of the films there may be other quality-determining factors such as morphology, microstructure and chemical composition (stoichiometry). As the deterioration of niobium pentoxide films is rather severe in this otherwise desirable material a detailed study of the layer and the evaporation process was undertaken. 2. EXPERIMENTAL DETAILS
Niobium pentoxide was evaporated in a Balzers BAK 550 electron beam Elsevier Sequoia/Printed in The Netherlands
138
J. J. V A N G L A B B E E K ,
R. E. V A N D E L E E S T
system. The rest gas composition was monitored by a quadrupole mass spectrometer. Film thickness was measured in situ with an optical method and by a quartz crystal oscillator. The substrates were glass (B-270), for optical characterization, and silicon. IR measurements were made with double-polished p-type silicon wafers as substrates. The film structure was characterized by X-ray diffraction (XRD). The film composition has been determined by X-ray fluorescence, Rutherford backscattering and energy-dispersive analysis of X-rays (EDAX). Film morphology was observed by scanning electron microscopy (SEM). Water in the film was determined by IR spectrophotometry. The refractive index was determined from spectrophotometric measurements. Internal stress was measured with the cantilevcr method. The process parameters which have been varied are the growth rate, the partial oxygen pressure and the electron beam sweep. Before starting the evaporation process, the vacuum chamber was pumped down to 1 x 10 4 Pa. 3. RESULTS 3.1. As-depositedlilms Table l gives an overview of the various N b 2 0 5 evaporation experiments and the properties of the resulting films. Three sets of experiments have been carried out: (A) evaporation without oxygen and without sweeping the electron beam over the melt; (B) evaporation under partial oxygen pressure and with electron beam sweeping; (C) evaporation without oxygen but with electron beam sweeping. The refractive index of various N b 2 0 5 films could not be determined because the films exhibited too much optical absorption. The films display a compressive stress which varies with changing process conditions. The evaporation rate has a TABLE
I
SURVEY OF EVAIR')RATION EXPERIMENTS
Charge
Process eharaeteristics
Film properties
~ll,l D l b e r
Growth rate (nms
1 2
0.3 0.7
3
1.3
4
2.2
1)
Po2
Sweep
(Pa)
t
n
(pm)
---
--
cr (GPa)
0.64 0.56
--
0.49
--
--
0.47
__
0.67
Yes Yes
0.79 0.65 0.74
-2.09
Yes
0.55
1.97
0.09 0.14 0.11 0.22
B 5
0.45
1.9 × 10
6 7 8
0.5 0.6 0.7
4.9 × 10 -' 2.9 × 10 2 5 . 8 × 10 - 2
2
9
1.4
0.07 0.04 0.01
C
t, t h i c k n e s s : n, r e f r a c t i v e i n d e x ; a , i n t e r n a l stress.
0.02
VAPOUR-DEPOSITED
139
N b 2 0 5 FILMS
strong influence: the faster the e v a p o r a t i o n rate, the greater the compressive stress. I n t r o d u c i n g oxygen in the c h a m b e r (reactive e v a p o r a t i o n ) a n d sweeping the electron b e a m over the melt yield low compressive stress films. W h e n the electron b e a m is swept over the N b 2 0 S melt, then films with lower compressive stress are deposited. The i n t r o d u c t i o n of oxygen into the e v a p o r a t i o n c h a m b e r yields films with a higher refractive index.
3.2. Annealed NbeO sfilms The N b 2 0 5 films have been heated in air at T = 370 °C. The properties of asdeposited films a n d heated films are presented in Table II. TABLE II PROPERTIES OF VAPOUR-DEPOSITED N b 2 0 5
Charge number
FILMS
As deposited l
Annealed ?1
(lam)
O-
t
(GPa)
(~tm)
r/
O"
(GPa)
1 2 3 4
0.64 0.56 0.49 0.47
----
0.09 0.14 0.1 ! 0.22
0.56 0.54 0.48 0.36
2.10 2.14 2.17 2.17
-0.15 -0.09 --0.08
5 6 7 8
0.67 0.79 0.65 0.74
--
0.07 0.04
2.09 --
0.01
0.62 0.70 0.64 0.68
2.04 2.08 2.11 2.09
- 0.09 -- 0.22
9
0.55
1.97
0.02
0.54
2.13
t, thickness;n, refractiveindex; a, internal stress. The films become t r a n s p a r e n t o n a n n e a l i n g in air a n d the refractive indices can be determined. The film thickness decreases, which m e a n s that the N b 2 0 5 films densify on heating a n d hence a n increase in refractive index is observed. The compressive stress of as-deposited films changes to a tensile stress on annealing. The i n t e r n a l stress could n o t always be d e t e r m i n e d because in some cases the N b 2 0 5 films behave as antireflecting coatings for the wavelength used to m e a s u r e the b e n d i n g of the N b 2 O s - c o a t e d silicon strip. The N b 2 0 5 films with the highest refractive indices are those which are e v a p o r a t e d in the absence of oxygen a n d w i t h o u t electron beam sweeping. These films also exhibit low i n t e r n a l stress.
3.3. Water adsorption As-deposited films c o n t a i n physically a d s o r b e d water, as is indicated by the b r o a d IR a b s o r p t i o n b a n d between 3200 cm t a n d 3500 c m - 1 (stretching) a n d the
140
J. J. V A N G L A B B E E K ,
R . E. V A N D E L E E S T
small peak at 1640 cm 1 (bending)4. O n heating at T = 370 :'C the absorption bands disappear (Fig. 1), Water is not re-adsorbed into the fihns when the heated films are cooled to r o o m temperature.
8O TRANSMISSIO~
60
t 40
\ '/x /
20
400~
3000
2000
1000
WAVENUMBER ('cm "1 )
Fig. 1. IR spectra of evaporated Nb205 films:
, as-deposited films; - , heated at T = 370'C.
3.4. X-rayfluorescence As-deposited N b 2 0 s films have a lower N b : O ratio than heated films. The water content of as-deposited films, which is m u c h higher, offers an explanation for this result. X-ray fluorescence is therefore not a quantitative m e t h o d for determining the chemical composition of water-containing oxides. A striking result is found for the N b 2 0 s melt. The melt is nearly metallic after electron beam melting and evaporation.
3.5. Morphology and structure As-deposited films are a m o r p h o u s , as shown by X R D measurements. Heating at T = 370°C does not induce the change from a m o r p h o u s to crystalline states. S E M examinations of the surfaces and cross-sections reveal m a r k e d changes. Asdeposited films exhibit a s m o o t h surface and have no distinctive features. Heated films exhibit a c o l u m n a r m o r p h o l o g y and cracks can be seen at the surface. The film characteristics, as observed with SEM, are summarized in Tables I I I and IV. The sample n u m b e r s correspond to the sample numbers in Tables I and II, where some properties of the N b 2 0 5 films are summarized. There is a wide variation a m o n g as-deposited films. Figure 2 shows the cross-sections of films 1 and 2.
VAPOUR-DEPOSITEDN b 2 0 5 FILMS
141
T A B L E 1II MORPHOLOGICAL
FILM
CHARACTERISTICS:
EVAPORATION
WITHOUT
OXYGEN
AND
NO ELECTRON
BEAM
SWEEPING
Charge
1
2
3
4
Rough No Yes
Smooth No No
Smooth No Yes
Smooth No Yes
Cracks -blisters Yes Yes
Cracks blisters Yes Yes
Cracks blisters Yes Yes
A s deposited
Surface Columnar morphology Voids H e a t e d at T = 370°C
Surface C o l u m n a r morphology Voids
T A B L E IV MORPHOLOGICAL
FILM C H A R A C T E R I S T I C S : E V A P O R A T I O N U N D E R P A R T I A L O X Y G E N P R E S S U R E A N D W I T H
E L E C T R O N BEAM S W E E P I N G
Charge
5
6
7
8
Cracks No Yes
Cracks No No
Cracks No Yes
Cracks No Yes
Smooth No No
Smooth Yes Yes
A s deposited
Surface C o l u m n a r morphology Voids H e a t e d at T = 370°C
Surface C o l u m n a r morphology Voids
Cracks Yes Yes
(a) (b) Fig. 2. SEM photographs of cross-sections of evaporated Nb205 films: (a) charge 1; (b) charge 2.
On heating at T = 370 °C morphological changes do occur, as can be seen in Fig. 3, where the cross-sections of the as-deposited and heated N b 2 0 5 film 3 are shown.
142
(a)
J. J. VAN GLABBEEK, R. E. VAN DE LEEST
(b)
Fig. 3. SEM photographs of cross-sections of evaporated N b 2 0 s films (charge 31: (a) as deposited; (bl heated at T = 370' C.
There are only small morphological variations among the annealed films. Asdeposited films have cracks at the surface. The cross-sections reveal a grainy texture with a large void area. On heating to T = 370 ~'C morphological changes do occur as can be seen from Fig. 4. The cracks at the surface become narrower or disappear and the morphology seems to evolve towards a columnar texture.
(a) (b) Fig. 4. SEM photographs of cross-sections of evaporated N b 2 0 ~ films (charge 5): (a) as deposited; (b) heated at T = 370'C.
4. DISCUSSION Figure 1 shows that physically adsorbed water desorbs completely when the Nb20~ films are heated at T = 370 ~C. Re-adsorption does not take place when the films are cooled down to room temperature. SEM photographs (Figs. 3 and 4) show that on heating a columnar morphology develops. One would therefore expect to see an increase in water adsorption in the pores, created by heating the films. Quite the reverse happens. A columnar morphology alone is not enough to trigger water adsorption in the films. The N b ~ 0 s films, as deposited as well as heated, are amorphous according to XRD measurements. The microstructure therefore does not account for the differences in optical and mechanical properties. The morphological characteristics of as-deposited films and heated films are
VAPOUR-DEPOSITED
Nb,O,
FILMS
143
different and so are their properties. In general, as-deposited films are smooth and exhibit no distinct features. On heating, the morphology changes to a columnar texture with a large void area. The internal stress of the films changes from compressive to tensile on heating. It is therefore tempting to correlate the morphological changes with changes in properties. On closer observation the correlation between morphology and properties is not so straightforward. The heated films from experiment A look exactly the same (Table III). Nevertheless, their properties such as internal stress and refractive index are different. As-deposited films, deposited with a different evaporation rate (Table I), have different internal stress. The morphology of these films does not vary much (Table II). The presence of oxygen during the evaporation process and the electron beam sweeping lead to Nb,O, films with a lower internal stress. There are cracks at the surface and the void area is larger than for films deposited without oxygen and without electron beam sweeping. The two sets of experiments (A and B) lead to different morphologies and different properties. The morphology of the films offers an explanation for the difference in properties. However, again, when looking more closely the correlation is not as straightforward. Within the two sets of experiments the morphological variations are very small, while the difference in properties is rather large (see Table II). Morphological differences of evaporated Nb,O, films cannot entirely account for the observed differences in properties. EDAX and X-ray fluorescence measurements of Nb,O, films deposited on silicon substrates suggest that the films might be non-stoichiometric. A non-stoichiometric film means non-congruent evaporation or decomposition of the Nb,O, melt. According to thermodynamic calculations5, Nb,O, is completely decomposed on evaporation to NbO,, NbO and oxygen (Fig. 5). Partial pressure measurement with a mass spectrometer indicates a sudden rise in the partial oxygen pressure at the onset of evaporation. This phenomenon is in agreement with the thermodynamic calculations. Chemical analysis of the Nb,O, residue after evaporation reveals a large deficiency of oxygen. Nb,O, is stable in the molten phase according to thermodynamic calculations. The decomposition of the Nb,O, melt must therefore be caused by the electron bombardment from the electron beam. When the Nb,O, melt becomes gradually more deficient in oxygen the evaporated films will also become more absorbing. It can therefore be concluded that the deviation from stoichiometry in the films is the principal cause of the poor optical properties. Heating the films in oxygen restores stoichiometry and changes the properties. The films become more transparent (Fig. 6) up to a point where they can be used for optical interference filters. Heating a single film presents no difficulty, but heating a multilayer stack composed of NbO, films and alternating low index films is not practical, especially when optical-grade glass is used as substrate (low melting temperature). 5.
CONCLUSIONS
Water is physically adsorbed in evaporated Nb,O, films and irreversibly on heating. Water adsorption does not offer an explanation observed differences in optical and mechanical properties of the film.
desorbs for the
_I. J. VAN GLABBEEK,
:L’,.\._ 1 !
Fig. 5. Decomposition of Nb,O, 02; ---, NbO: -‘-, NbO,.
*.ae
on heating
I
according
R. E. VAN DE LEEST
I
to thermodynamic
calculations:
p,
0,; _....,
rr
Fig. 6. Optical
transmission
of evaporated
Nb,Os
films: y3
as deposited:
---,
heated at 7‘ = 370 c.
VAPOUR-DEPOSITED
Nb,O,
145
FILMS
The Nb,O, films, as deposited as well as heated at T= 370 “C, are amorphous. Microstructure also fails to offer an explanation. Morphology partly provides an explanation for the optical and mechanical properties of the Nb,O, films. The chemical composition of the films or the deviation from stoichiometry offers a good explanation for the observed differences in optical and mechanical properties. The decomposition of Nb,O, to other niobium oxides and oxygen causes the deposition of non-stoichiometric niobium oxide films. Heating in oxygen ambient restores stoichiometry and changes the properties. REFERENCES
I 2 3 4 5
A. G. Dirks and H. J. Leamy, Thin SolidFilms, 47(1977) 219. P. J. Martin, Vide, Couches Minces. 246 (1989) 115. H. K. Pulker, Proc. Sm. Photo-Opt. Instrum. Eng., 952 (1988) 788. K. Nakamoto, Infrared Spectra qflnorganic and Coordination Compounds, E. Schnedler. Philips J. Re.s., 38 (1983) 247.
Wiley. New York.
1963.
137
PREPARATION AND CHARACTERIZATION
T H I N V A P O U R - D E P O S I T E D NIOBIUM P E N T O X I D E FILMS J. J. VAN GLABBEEK AND R. E. VAN DE LEEST
Philips Research Laboratories, Eindhoven ( The Netherlands) (Received September 20, 1990; accepted November 22, 1990)
Thin vapour-deposited niobium pentoxide films exhibit optical absorption which limits their application as a high refractive index layer for optical interference filters. Water adsorption in the pores of the film is not the cause of the poor optical quality, as is generally assumed for this type of film. Decomposition of Nb205, a thermodynamically defined process, leads to non-stoichiometric films of poor quality in terms of structure and morphology. Decomposition of the electron-beambombarded Nb205 melt aggravates the situation. Stoichiometry can be restored by heating in an oxygen ambient.
1. INTRODUCTION
Thin niobium pentoxide films are used as the high refractive index layers in optical interference filters which consist of a multilayer stack of high index and low index materials. When deposited by electron beam evaporation, the niobium pentoxide layers gradually exhibit more optical absorption, which limits the application of this material. A generally accepted explanation for the deterioration of these oxides is that the porous morphology of evaporated films 1 causes water to adsorb in the pores of the film. Water has a different refractive index and is able to react with oxides. As a consequence the optical properties of these films, and hence the interference filter, change 2, 3. As well as the water content of the films there may be other quality-determining factors such as morphology, microstructure and chemical composition (stoichiometry). As the deterioration of niobium pentoxide films is rather severe in this otherwise desirable material a detailed study of the layer and the evaporation process was undertaken. 2. EXPERIMENTAL DETAILS
Niobium pentoxide was evaporated in a Balzers BAK 550 electron beam Elsevier Sequoia/Printed in The Netherlands
138
J. J. V A N G L A B B E E K ,
R. E. V A N D E L E E S T
system. The rest gas composition was monitored by a quadrupole mass spectrometer. Film thickness was measured in situ with an optical method and by a quartz crystal oscillator. The substrates were glass (B-270), for optical characterization, and silicon. IR measurements were made with double-polished p-type silicon wafers as substrates. The film structure was characterized by X-ray diffraction (XRD). The film composition has been determined by X-ray fluorescence, Rutherford backscattering and energy-dispersive analysis of X-rays (EDAX). Film morphology was observed by scanning electron microscopy (SEM). Water in the film was determined by IR spectrophotometry. The refractive index was determined from spectrophotometric measurements. Internal stress was measured with the cantilevcr method. The process parameters which have been varied are the growth rate, the partial oxygen pressure and the electron beam sweep. Before starting the evaporation process, the vacuum chamber was pumped down to 1 x 10 4 Pa. 3. RESULTS 3.1. As-depositedlilms Table l gives an overview of the various N b 2 0 5 evaporation experiments and the properties of the resulting films. Three sets of experiments have been carried out: (A) evaporation without oxygen and without sweeping the electron beam over the melt; (B) evaporation under partial oxygen pressure and with electron beam sweeping; (C) evaporation without oxygen but with electron beam sweeping. The refractive index of various N b 2 0 5 films could not be determined because the films exhibited too much optical absorption. The films display a compressive stress which varies with changing process conditions. The evaporation rate has a TABLE
I
SURVEY OF EVAIR')RATION EXPERIMENTS
Charge
Process eharaeteristics
Film properties
~ll,l D l b e r
Growth rate (nms
1 2
0.3 0.7
3
1.3
4
2.2
1)
Po2
Sweep
(Pa)
t
n
(pm)
---
--
cr (GPa)
0.64 0.56
--
0.49
--
--
0.47
__
0.67
Yes Yes
0.79 0.65 0.74
-2.09
Yes
0.55
1.97
0.09 0.14 0.11 0.22
B 5
0.45
1.9 × 10
6 7 8
0.5 0.6 0.7
4.9 × 10 -' 2.9 × 10 2 5 . 8 × 10 - 2
2
9
1.4
0.07 0.04 0.01
C
t, t h i c k n e s s : n, r e f r a c t i v e i n d e x ; a , i n t e r n a l stress.
0.02
VAPOUR-DEPOSITED
139
N b 2 0 5 FILMS
strong influence: the faster the e v a p o r a t i o n rate, the greater the compressive stress. I n t r o d u c i n g oxygen in the c h a m b e r (reactive e v a p o r a t i o n ) a n d sweeping the electron b e a m over the melt yield low compressive stress films. W h e n the electron b e a m is swept over the N b 2 0 S melt, then films with lower compressive stress are deposited. The i n t r o d u c t i o n of oxygen into the e v a p o r a t i o n c h a m b e r yields films with a higher refractive index.
3.2. Annealed NbeO sfilms The N b 2 0 5 films have been heated in air at T = 370 °C. The properties of asdeposited films a n d heated films are presented in Table II. TABLE II PROPERTIES OF VAPOUR-DEPOSITED N b 2 0 5
Charge number
FILMS
As deposited l
Annealed ?1
(lam)
O-
t
(GPa)
(~tm)
r/
O"
(GPa)
1 2 3 4
0.64 0.56 0.49 0.47
----
0.09 0.14 0.1 ! 0.22
0.56 0.54 0.48 0.36
2.10 2.14 2.17 2.17
-0.15 -0.09 --0.08
5 6 7 8
0.67 0.79 0.65 0.74
--
0.07 0.04
2.09 --
0.01
0.62 0.70 0.64 0.68
2.04 2.08 2.11 2.09
- 0.09 -- 0.22
9
0.55
1.97
0.02
0.54
2.13
t, thickness;n, refractiveindex; a, internal stress. The films become t r a n s p a r e n t o n a n n e a l i n g in air a n d the refractive indices can be determined. The film thickness decreases, which m e a n s that the N b 2 0 5 films densify on heating a n d hence a n increase in refractive index is observed. The compressive stress of as-deposited films changes to a tensile stress on annealing. The i n t e r n a l stress could n o t always be d e t e r m i n e d because in some cases the N b 2 0 5 films behave as antireflecting coatings for the wavelength used to m e a s u r e the b e n d i n g of the N b 2 O s - c o a t e d silicon strip. The N b 2 0 5 films with the highest refractive indices are those which are e v a p o r a t e d in the absence of oxygen a n d w i t h o u t electron beam sweeping. These films also exhibit low i n t e r n a l stress.
3.3. Water adsorption As-deposited films c o n t a i n physically a d s o r b e d water, as is indicated by the b r o a d IR a b s o r p t i o n b a n d between 3200 cm t a n d 3500 c m - 1 (stretching) a n d the
140
J. J. V A N G L A B B E E K ,
R . E. V A N D E L E E S T
small peak at 1640 cm 1 (bending)4. O n heating at T = 370 :'C the absorption bands disappear (Fig. 1), Water is not re-adsorbed into the fihns when the heated films are cooled to r o o m temperature.
8O TRANSMISSIO~
60
t 40
\ '/x /
20
400~
3000
2000
1000
WAVENUMBER ('cm "1 )
Fig. 1. IR spectra of evaporated Nb205 films:
, as-deposited films; - , heated at T = 370'C.
3.4. X-rayfluorescence As-deposited N b 2 0 s films have a lower N b : O ratio than heated films. The water content of as-deposited films, which is m u c h higher, offers an explanation for this result. X-ray fluorescence is therefore not a quantitative m e t h o d for determining the chemical composition of water-containing oxides. A striking result is found for the N b 2 0 s melt. The melt is nearly metallic after electron beam melting and evaporation.
3.5. Morphology and structure As-deposited films are a m o r p h o u s , as shown by X R D measurements. Heating at T = 370°C does not induce the change from a m o r p h o u s to crystalline states. S E M examinations of the surfaces and cross-sections reveal m a r k e d changes. Asdeposited films exhibit a s m o o t h surface and have no distinctive features. Heated films exhibit a c o l u m n a r m o r p h o l o g y and cracks can be seen at the surface. The film characteristics, as observed with SEM, are summarized in Tables I I I and IV. The sample n u m b e r s correspond to the sample numbers in Tables I and II, where some properties of the N b 2 0 5 films are summarized. There is a wide variation a m o n g as-deposited films. Figure 2 shows the cross-sections of films 1 and 2.
VAPOUR-DEPOSITEDN b 2 0 5 FILMS
141
T A B L E 1II MORPHOLOGICAL
FILM
CHARACTERISTICS:
EVAPORATION
WITHOUT
OXYGEN
AND
NO ELECTRON
BEAM
SWEEPING
Charge
1
2
3
4
Rough No Yes
Smooth No No
Smooth No Yes
Smooth No Yes
Cracks -blisters Yes Yes
Cracks blisters Yes Yes
Cracks blisters Yes Yes
A s deposited
Surface Columnar morphology Voids H e a t e d at T = 370°C
Surface C o l u m n a r morphology Voids
T A B L E IV MORPHOLOGICAL
FILM C H A R A C T E R I S T I C S : E V A P O R A T I O N U N D E R P A R T I A L O X Y G E N P R E S S U R E A N D W I T H
E L E C T R O N BEAM S W E E P I N G
Charge
5
6
7
8
Cracks No Yes
Cracks No No
Cracks No Yes
Cracks No Yes
Smooth No No
Smooth Yes Yes
A s deposited
Surface C o l u m n a r morphology Voids H e a t e d at T = 370°C
Surface C o l u m n a r morphology Voids
Cracks Yes Yes
(a) (b) Fig. 2. SEM photographs of cross-sections of evaporated Nb205 films: (a) charge 1; (b) charge 2.
On heating at T = 370 °C morphological changes do occur, as can be seen in Fig. 3, where the cross-sections of the as-deposited and heated N b 2 0 5 film 3 are shown.
142
(a)
J. J. VAN GLABBEEK, R. E. VAN DE LEEST
(b)
Fig. 3. SEM photographs of cross-sections of evaporated N b 2 0 s films (charge 31: (a) as deposited; (bl heated at T = 370' C.
There are only small morphological variations among the annealed films. Asdeposited films have cracks at the surface. The cross-sections reveal a grainy texture with a large void area. On heating to T = 370 ~'C morphological changes do occur as can be seen from Fig. 4. The cracks at the surface become narrower or disappear and the morphology seems to evolve towards a columnar texture.
(a) (b) Fig. 4. SEM photographs of cross-sections of evaporated N b 2 0 ~ films (charge 5): (a) as deposited; (b) heated at T = 370'C.
4. DISCUSSION Figure 1 shows that physically adsorbed water desorbs completely when the Nb20~ films are heated at T = 370 ~C. Re-adsorption does not take place when the films are cooled down to room temperature. SEM photographs (Figs. 3 and 4) show that on heating a columnar morphology develops. One would therefore expect to see an increase in water adsorption in the pores, created by heating the films. Quite the reverse happens. A columnar morphology alone is not enough to trigger water adsorption in the films. The N b ~ 0 s films, as deposited as well as heated, are amorphous according to XRD measurements. The microstructure therefore does not account for the differences in optical and mechanical properties. The morphological characteristics of as-deposited films and heated films are
VAPOUR-DEPOSITED
Nb,O,
FILMS
143
different and so are their properties. In general, as-deposited films are smooth and exhibit no distinct features. On heating, the morphology changes to a columnar texture with a large void area. The internal stress of the films changes from compressive to tensile on heating. It is therefore tempting to correlate the morphological changes with changes in properties. On closer observation the correlation between morphology and properties is not so straightforward. The heated films from experiment A look exactly the same (Table III). Nevertheless, their properties such as internal stress and refractive index are different. As-deposited films, deposited with a different evaporation rate (Table I), have different internal stress. The morphology of these films does not vary much (Table II). The presence of oxygen during the evaporation process and the electron beam sweeping lead to Nb,O, films with a lower internal stress. There are cracks at the surface and the void area is larger than for films deposited without oxygen and without electron beam sweeping. The two sets of experiments (A and B) lead to different morphologies and different properties. The morphology of the films offers an explanation for the difference in properties. However, again, when looking more closely the correlation is not as straightforward. Within the two sets of experiments the morphological variations are very small, while the difference in properties is rather large (see Table II). Morphological differences of evaporated Nb,O, films cannot entirely account for the observed differences in properties. EDAX and X-ray fluorescence measurements of Nb,O, films deposited on silicon substrates suggest that the films might be non-stoichiometric. A non-stoichiometric film means non-congruent evaporation or decomposition of the Nb,O, melt. According to thermodynamic calculations5, Nb,O, is completely decomposed on evaporation to NbO,, NbO and oxygen (Fig. 5). Partial pressure measurement with a mass spectrometer indicates a sudden rise in the partial oxygen pressure at the onset of evaporation. This phenomenon is in agreement with the thermodynamic calculations. Chemical analysis of the Nb,O, residue after evaporation reveals a large deficiency of oxygen. Nb,O, is stable in the molten phase according to thermodynamic calculations. The decomposition of the Nb,O, melt must therefore be caused by the electron bombardment from the electron beam. When the Nb,O, melt becomes gradually more deficient in oxygen the evaporated films will also become more absorbing. It can therefore be concluded that the deviation from stoichiometry in the films is the principal cause of the poor optical properties. Heating the films in oxygen restores stoichiometry and changes the properties. The films become more transparent (Fig. 6) up to a point where they can be used for optical interference filters. Heating a single film presents no difficulty, but heating a multilayer stack composed of NbO, films and alternating low index films is not practical, especially when optical-grade glass is used as substrate (low melting temperature). 5.
CONCLUSIONS
Water is physically adsorbed in evaporated Nb,O, films and irreversibly on heating. Water adsorption does not offer an explanation observed differences in optical and mechanical properties of the film.
desorbs for the
_I. J. VAN GLABBEEK,
:L’,.\._ 1 !
Fig. 5. Decomposition of Nb,O, 02; ---, NbO: -‘-, NbO,.
*.ae
on heating
I
according
R. E. VAN DE LEEST
I
to thermodynamic
calculations:
p,
0,; _....,
rr
Fig. 6. Optical
transmission
of evaporated
Nb,Os
films: y3
as deposited:
---,
heated at 7‘ = 370 c.
VAPOUR-DEPOSITED
Nb,O,
145
FILMS
The Nb,O, films, as deposited as well as heated at T= 370 “C, are amorphous. Microstructure also fails to offer an explanation. Morphology partly provides an explanation for the optical and mechanical properties of the Nb,O, films. The chemical composition of the films or the deviation from stoichiometry offers a good explanation for the observed differences in optical and mechanical properties. The decomposition of Nb,O, to other niobium oxides and oxygen causes the deposition of non-stoichiometric niobium oxide films. Heating in oxygen ambient restores stoichiometry and changes the properties. REFERENCES
I 2 3 4 5
A. G. Dirks and H. J. Leamy, Thin SolidFilms, 47(1977) 219. P. J. Martin, Vide, Couches Minces. 246 (1989) 115. H. K. Pulker, Proc. Sm. Photo-Opt. Instrum. Eng., 952 (1988) 788. K. Nakamoto, Infrared Spectra qflnorganic and Coordination Compounds, E. Schnedler. Philips J. Re.s., 38 (1983) 247.
Wiley. New York.
1963.