- Email: [email protected]
Dielectric relaxation of water molecules in oxide films on silicon
Solid State Communications, Vol. 5, pp. 791-793, 1967. Pergamon Press Ltd. Printed in Great Britain
DIELECTRIC RELAXATION OF WATER MOLECULES IN OXIDE FILMS ON SILICON G. Dorda4 and M. Kaderka Institute of Solid State Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia (Received 11 August 1967 by J. Tauc)
Dielectric losses from 102 to 1 o~c/s for both etched and thermally grown oxide films on silicon under different relative humidity of ambient air are investigated. It is shown that absorption of water molecules in the oxide gives rise to relaxation maxima (usually at 0. 8, 8 and 60 kc/s). The magnitude of it is strongly influenced by sodium contamthation of the oxide.
AS IT IS known1 4 useful information about the structure and the dielectric properties of solids can be obtained by measurement of dielectric relaxations. The magnitude and the frequency dependence of the dissipation factor tan 6 in the range from 100 c/s to 100 kc/s on etched samples as well as on samples with thermally grown oxide layers were studied.
-
Here c is the conductivity, n the concentration of dipoles, ~i the dipole moment, c~the radio frequency dielectric constant, r the relaxation time, w the angular frequency of the applied field. The form of the curves at low humidity follows predominantly the conductivity component, i. e. the conductivity of the oxide prevails. This conductivity is usually highest immediately after etching and decreases with time. The value of the conductivity calculated from the gradient of the line in Fig. 1 is c~= 3 x 10~ohnf’ cmt.
Plane parallel discs of silicon single crystals of both types, n and p, were used. The obtamed results did not depend on the resistivity and type of silicon nor on the form of the sample. The sample was placed between two gold circular electrodes, which were pressed to the sample by a spring,
Further it is seen that relaxation maxima around 8 and around 60 kc/s increase with increasing humidity. rhis observed increase of losses with humidity is related to the ability of the etched surface of silicon to adsorb water molecules. ~
In Fig. 1 the frequency dependence of tan 8 for an etched sample at different humidities is shown. This sample was etched in the mixture HF + HNO 3 (ratio 1:2), then boiled in nitric acid and finally washed in the hot vapours of distilled water. According to the Debye theory the overall value of dielectric losses is given by the conductivity component and the relaxation component caused by dipoles 2 ~ tan 6 = 4~ic~+ 38r~ni.i kT ~+ ~ (1) *present address: W. Physikalisches Institut der Universlt~tGöttingen, G~tttngen, Germany.
Further dielectric losses on samples with thick thermally grown oxide layers were studied. The thermal oxidation was carried out at 1100°C In a flowing oxygen atmosphere enriched by water. Typical frequency dependences of dielectric losses found on such samples are shown in Fig. 2(a). It which is seenare that (1) thesituated curve consists two maxima usually around 1ofand 10 kc/s, (2) the influence of humidity on the magnitude of the relaxation maxima was very small in contrast to the change of the conductivity cornponent which markedly increased with humidity.
.
_____________
791
RELAXATION OF WATER MOLECULES
792
100
Il~~l
Vol. 5, No. 10
~
I
a 100
tan
6
-
-
~
-
tan
-
dry air wit afr(85%r~h.)
...
6
...
~
°
~
o~-~
:~
a/tar
~‘
~.-.-,--—----...-~
-
~ \
\
—0%15%
35%_
100
/ (cps)
Dissipation factor tan 6 vs. frequency f for a silicon sample boiled In HNO3 etching and vapours underofdifferent distilled relative water after humidity of ambient air. T = 20°C. The dotted line demonstrates the conductivity component. After desorption of water in dry air the conductivity remained higher and disappeared only after a short heat treatment (about 1 mm) of the sampie in dry air at 500°C. The conductivity was mostly ohmical in character or showed only a very small dependence on the applied fjeld. The nature of this water induced conductivity is not clear, it may be a similar process as reported by Hof ~
Skanavi7 calculated relations for tan 8 for a system of several dielectric layers. It follows that in our case the measured relaxation losses indicate dipoles located in the bulk of the oxide layer. Therefore we conclude that water dipoles are not absorbed by the grown oxide even at high humidity. The effect of heat treatment in a sodium containing environment on the behaviour of the grown silicon dioxide in a wet atmosphere was investigated. The sample with a groWn oxide layer was heated several minutes at 500°C in a vessel with room air and with inclosed NaCl at the neighbourhood of the sample. The drastic influence of sodium on the magnitude of hydration of this oxide is demonstrated in Fig. 2 (b). Why a series of relaxation maxima (located Usually at 0.8, 8 and 60 kc/a) is found is not
b 4
I
10102
10’
I4
V
FIG. 2
V’ ~‘
(cps) -
Dissipation factor tan 6 vs. frequency f for a thermally grown oxide film (~-..0. 12 .i thick) on silicon under different relative humidity of ambient air. T = 20°C. ~d ith t i t tional odi a c t oii n en a um ~ m na OIl. (b) Oxide with high sodium contamination.
‘WI
entirely clear. We assume that water exists either in different surroundings or is bound in different ways. From the form of the curves it may be surmised that with increasing humidity an increase not only of the relaxation maxima but also of the oxide conductivity occurs. Using the data of the relaxation maxima of Fig. 2 (b) equation (1) gives a concentration of water molecules of the order of 10~to 1 0~ crn~. After washing this sodium contaminated oxide several hours in hot vapours of distilled water we obtain again an oxide with low relaxation losses showing a little neglecting dependence on humidity as shown in Fig. 2 (a). The effect of the contaminating and washing process is reverstble. The contamination of a “washed” hydro-
Vol. 5, No. 10
RELAXATION OF WATER MOLECULES
Finally, it was found that carefully prepared thick oxide films showed no measurable losses even at high humidity when sodium contaminatton during the oxidation process was carefully avoided.
phobic sample by immersion in a boiling sodium chloride solution or in a NaHCO~solution for 10 mm did not lead to a markedly change in the hydrophobic character. The temperature was in this case probably too low for a diffusion of sodium into the oxide or the electrolysed water at the surface of the oxide blocked it.6
Acknowledgement - The authors acknowledge valuable discussions with M. Lébi.
References 1.
HIROSE H. and WADA Y.,
Japan J. Appl. Phys.
2.
BURKHARDT P.J.,
3.
PETERS F. G., Am. Ceram. Soc. Bull. 45, 1017 (1966).
4.
DORDA G. and KADERKA M., Verhandl DPG (VI) 2, 41 (1967).
5.
CLAUSSEN B. H.,
6.
HOFSTEIN S.R., IEEE Trans.
7.
SKANAVI G. I.,
IEEE Trans.
ED-13,
793
3, 179 (1964).
268 (1966).
J. Electrochem. Soc. 111, 646 (1964). ED-13, 222 (1966).
Physics of Dielectrics p. 475 G. I. T. T. L., Moscow, Leningrad (1949)-in
Russian. Verlustfaktormessungen von 102 bis 1 0~Hz wurden bet verschiedenen Luftfeuchtigkeiten sowohl am Oberflächenoxyd ge~tzterSiliziumproben ala auch an thermisch aufgewachsener Siliziumdioxyd Schichten durchgefiihrt. Es wurden Relaxationsmaxima bet 0,8, 8, mid 60 kHz gefunden, die durch absorbierte Wassermoleldile hervorgerugen werden. Die Menge der absorbierten Wassermolekille ist von der Natrium-Verunremnigung des Oxydes stark ab1~ngtg. -
DIELECTRIC RELAXATION OF WATER MOLECULES IN OXIDE FILMS ON SILICON G. Dorda4 and M. Kaderka Institute of Solid State Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia (Received 11 August 1967 by J. Tauc)
Dielectric losses from 102 to 1 o~c/s for both etched and thermally grown oxide films on silicon under different relative humidity of ambient air are investigated. It is shown that absorption of water molecules in the oxide gives rise to relaxation maxima (usually at 0. 8, 8 and 60 kc/s). The magnitude of it is strongly influenced by sodium contamthation of the oxide.
AS IT IS known1 4 useful information about the structure and the dielectric properties of solids can be obtained by measurement of dielectric relaxations. The magnitude and the frequency dependence of the dissipation factor tan 6 in the range from 100 c/s to 100 kc/s on etched samples as well as on samples with thermally grown oxide layers were studied.
-
Here c is the conductivity, n the concentration of dipoles, ~i the dipole moment, c~the radio frequency dielectric constant, r the relaxation time, w the angular frequency of the applied field. The form of the curves at low humidity follows predominantly the conductivity component, i. e. the conductivity of the oxide prevails. This conductivity is usually highest immediately after etching and decreases with time. The value of the conductivity calculated from the gradient of the line in Fig. 1 is c~= 3 x 10~ohnf’ cmt.
Plane parallel discs of silicon single crystals of both types, n and p, were used. The obtamed results did not depend on the resistivity and type of silicon nor on the form of the sample. The sample was placed between two gold circular electrodes, which were pressed to the sample by a spring,
Further it is seen that relaxation maxima around 8 and around 60 kc/s increase with increasing humidity. rhis observed increase of losses with humidity is related to the ability of the etched surface of silicon to adsorb water molecules. ~
In Fig. 1 the frequency dependence of tan 8 for an etched sample at different humidities is shown. This sample was etched in the mixture HF + HNO 3 (ratio 1:2), then boiled in nitric acid and finally washed in the hot vapours of distilled water. According to the Debye theory the overall value of dielectric losses is given by the conductivity component and the relaxation component caused by dipoles 2 ~ tan 6 = 4~ic~+ 38r~ni.i kT ~+ ~ (1) *present address: W. Physikalisches Institut der Universlt~tGöttingen, G~tttngen, Germany.
Further dielectric losses on samples with thick thermally grown oxide layers were studied. The thermal oxidation was carried out at 1100°C In a flowing oxygen atmosphere enriched by water. Typical frequency dependences of dielectric losses found on such samples are shown in Fig. 2(a). It which is seenare that (1) thesituated curve consists two maxima usually around 1ofand 10 kc/s, (2) the influence of humidity on the magnitude of the relaxation maxima was very small in contrast to the change of the conductivity cornponent which markedly increased with humidity.
.
_____________
791
RELAXATION OF WATER MOLECULES
792
100
Il~~l
Vol. 5, No. 10
~
I
a 100
tan
6
-
-
~
-
tan
-
dry air wit afr(85%r~h.)
...
6
...
~
°
~
o~-~
:~
a/tar
~‘
~.-.-,--—----...-~
-
~ \
\
—0%15%
35%_
100
/ (cps)
Dissipation factor tan 6 vs. frequency f for a silicon sample boiled In HNO3 etching and vapours underofdifferent distilled relative water after humidity of ambient air. T = 20°C. The dotted line demonstrates the conductivity component. After desorption of water in dry air the conductivity remained higher and disappeared only after a short heat treatment (about 1 mm) of the sampie in dry air at 500°C. The conductivity was mostly ohmical in character or showed only a very small dependence on the applied fjeld. The nature of this water induced conductivity is not clear, it may be a similar process as reported by Hof ~
Skanavi7 calculated relations for tan 8 for a system of several dielectric layers. It follows that in our case the measured relaxation losses indicate dipoles located in the bulk of the oxide layer. Therefore we conclude that water dipoles are not absorbed by the grown oxide even at high humidity. The effect of heat treatment in a sodium containing environment on the behaviour of the grown silicon dioxide in a wet atmosphere was investigated. The sample with a groWn oxide layer was heated several minutes at 500°C in a vessel with room air and with inclosed NaCl at the neighbourhood of the sample. The drastic influence of sodium on the magnitude of hydration of this oxide is demonstrated in Fig. 2 (b). Why a series of relaxation maxima (located Usually at 0.8, 8 and 60 kc/a) is found is not
b 4
I
10102
10’
I4
V
FIG. 2
V’ ~‘
(cps) -
Dissipation factor tan 6 vs. frequency f for a thermally grown oxide film (~-..0. 12 .i thick) on silicon under different relative humidity of ambient air. T = 20°C. ~d ith t i t tional odi a c t oii n en a um ~ m na OIl. (b) Oxide with high sodium contamination.
‘WI
entirely clear. We assume that water exists either in different surroundings or is bound in different ways. From the form of the curves it may be surmised that with increasing humidity an increase not only of the relaxation maxima but also of the oxide conductivity occurs. Using the data of the relaxation maxima of Fig. 2 (b) equation (1) gives a concentration of water molecules of the order of 10~to 1 0~ crn~. After washing this sodium contaminated oxide several hours in hot vapours of distilled water we obtain again an oxide with low relaxation losses showing a little neglecting dependence on humidity as shown in Fig. 2 (a). The effect of the contaminating and washing process is reverstble. The contamination of a “washed” hydro-
Vol. 5, No. 10
RELAXATION OF WATER MOLECULES
Finally, it was found that carefully prepared thick oxide films showed no measurable losses even at high humidity when sodium contaminatton during the oxidation process was carefully avoided.
phobic sample by immersion in a boiling sodium chloride solution or in a NaHCO~solution for 10 mm did not lead to a markedly change in the hydrophobic character. The temperature was in this case probably too low for a diffusion of sodium into the oxide or the electrolysed water at the surface of the oxide blocked it.6
Acknowledgement - The authors acknowledge valuable discussions with M. Lébi.
References 1.
HIROSE H. and WADA Y.,
Japan J. Appl. Phys.
2.
BURKHARDT P.J.,
3.
PETERS F. G., Am. Ceram. Soc. Bull. 45, 1017 (1966).
4.
DORDA G. and KADERKA M., Verhandl DPG (VI) 2, 41 (1967).
5.
CLAUSSEN B. H.,
6.
HOFSTEIN S.R., IEEE Trans.
7.
SKANAVI G. I.,
IEEE Trans.
ED-13,
793
3, 179 (1964).
268 (1966).
J. Electrochem. Soc. 111, 646 (1964). ED-13, 222 (1966).
Physics of Dielectrics p. 475 G. I. T. T. L., Moscow, Leningrad (1949)-in
Russian. Verlustfaktormessungen von 102 bis 1 0~Hz wurden bet verschiedenen Luftfeuchtigkeiten sowohl am Oberflächenoxyd ge~tzterSiliziumproben ala auch an thermisch aufgewachsener Siliziumdioxyd Schichten durchgefiihrt. Es wurden Relaxationsmaxima bet 0,8, 8, mid 60 kHz gefunden, die durch absorbierte Wassermoleldile hervorgerugen werden. Die Menge der absorbierten Wassermolekille ist von der Natrium-Verunremnigung des Oxydes stark ab1~ngtg. -