Time-dependent ellipsometric measurements were made of the water absoprtion in SiO2 films grown by chemical vapour deposition at low temperatures. These films were exposed to water vapour at different pressurures. We determined in the ellipsometric volume percentage of water in the oxide film from the changes in the ellipsometric parameters due to water absorption. We also obtained a diffusion coefficient of water of the order of 1.3 x 10-13 cm2 s-1. From the reversible changes in the ellipsometric parameters due to dehydration we conclude that the water is physically adsorbed

Rooij, N.F. de

Sieverdink, R.S.J.

Tromp, R.M.

English

de Rooij, N.F.

University of Twente Research Information

• n t t t  o v l l u  • ~ l t t o ~  n,  i ~ 1 • t t ! ~ 1 1 - - ~  • u © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands 21 ! AN I N V E S T I G A T I O N  OF T H E  H Y D R A T I O N  P R O P E R T I E S  OF C H E M I C A L L Y  V A P O U R - D E P O S I T E D  SILICON D I O X I D E  FILMS BY MEANS OF E L L I P S O M E T R Y  N. F. DE ROOIJ, R. J. S. SIEVERDINK AND R. M. TROMP Department of Applied Physics, Twente University of Technology, Enschede (The Netherlands) (Received June 9, 1977; accepted July 1, 1977) Time-dependent ellipsometrfc measurements were made of  the water absor- ption in SiO 2 films grown by chemical vapour  deposition at low temperatures. These films were exposed to water vapour  at different pressures. We determined a volume percentage of  water in the oxide film from the changes in the ellipsometric parameters due to water absorption. We also obtained a diffusion coefficient of  water of  the order of  1.3 x 10 - 1 3  cm 2 s -1. From the reversible changes in the ellipsometric parameters due to dehydration we conclude that the water is physically adsorbed. 1. INTRODUCTION In the literature on glass electrodes it has been reported that an important  factor which determines the electrode function is the extent to which the glass surface can undergo hydration. The water uptake for optimal operation is 1 of  the order of  60-120 mg cm-3.  In order to improve the operation of  the ion-sensitive field effect transistor (ISFET) 2 with respect to the required electrode function it is necessary to investigate the hydration properties of  several types of  oxide films. SiO 2 grown by chemical vapour  deposition (CVD) at low temperatures has proved to be quite responsive to water absorption 3. Arai and Terunuma 3 have estimated the diffusion coefficient of  water in CVD SiO 2 to be of  the order of  10-15-10 -14 cm 2 s -  1 at room temperature. The purpose of  the present work was to investigate the influence of  water vapour  pressure on the refractive index and thickness of  CVD S i O  2. This was done by means of time-dependent ellipsometric measurements, which yielded data on the water content of  the glass and on the mechanism of  water penetration into the glass. 2. EXPERIMENTAL 2.1. Preparation of  chemically vapour-deposited silicon dioxide films The CVD SiO 2 films were grown on p-type 10 D cm silicon (111) wafers provided by the Philips Company.  First the wafers were cleaned in nitric acid (98 ~ ) ;  this was followed by rinsing in a mixture of  one part  of  50 ~ HF  solution to six parts of  50 % NH4F. Next the wafers were put into boiling nitric acid (65 %). After each treatment the wafers were rinsed in deionized water (10 MD cm). The 212 N.F.  DE ROOIJ, R. J. S. SIEVERDINK, R. M. TROMP oxides were grown by introducing Sill 4 and oxygen (with argon as the carrier gas) to the wafers, which were kept at a temperature of  about 325 °C. Typical flow rates of  Sill 4, oxygen and argon were 0.0071 min - t, 0.041 min - 1 and 71 min - 1 respectively. The corresponding deposition rate for SiO 2 was 0.1 lam min-1. The experiments were carried out on samples which had oxide films 500, 1000 and 1900/~ thick. 2.2. Hydration and dehydration experiments The hydration and dehydration experiments were carried out in a stainless steel vacuum chamber at water vapour pressures of between 0.2 and 2.2 Torr. Starting from a pressure of  about 10-3 Torr, the desired water vapour pressure was reached within 25 s. The system pressure could be reduced to below 10-3 Torr by pumping with a sorption pump. The pressure was measured by means of  a Pirani gauge (Thermovac TM 203, Leybold-Heraeus). The water vapour pressure was con- trolled by heating a glass tube filled with hydrated silica gel, separated from the system by a leak valve. The sample was kept at a temperature of  25 °C. The ellipsometer used was an automatically controlled nulling ellipsometer built in our laboratory according to the details in ref. 4. The ellipsometer was provided with Glan-Thompson prisms mounted in rotatable discs. These were supplied with stepping motor drives and anti.-backlash gears. One motor step corresponded to a rotation of 0.005 ° of  the optical element. The polarizer and analyser null positions could be located in about 8 s. A quarter-wave plate was placed between the polarizer and the input window of the hydration chamber, with the fast axis at + 45 ° or - 45 ° with respect to the plane of incidence. The light source was a He-Ne laser of wavelength 632.8 nm. The alignment of the ellipsometer and static four-zone measurements were made by the method of McCrackin et al. 5 The time-dependent hydration and dehydration experiments were conducted in zones 1, 2 and 4 successively. An angle of incidence of  between 69.0 and 71.0 ° was used. This angle was measured optically to an accuracy of  0.1 °. 3. EXPERIMENTAL RESULTS Table I gives the results computed from the ellipsometric data A and ff before hydration and also at different water vapour pressures when the changes with time of  A and of  ~k due to hydration become zero. These results comprise the initial refractive index n o and the thickness d o of  the dry oxide films, together with the change ~Sn in refractive index and the change 8din thickness of the films. A one-layer model was assumed with homogeneous absorption of  water throughout the layer. The calculations were made by means of  a computer program 6; the refractive index used for silicon was 7 ti = 3.85 -i0.02.  In nearly all cases the refractive index and the thickness increased with water vapour pressure. The volume fraction of  water was calculated by using the Lorentz-Lorenz formula (no +8n)2-- 1 d o no2-1  nw2-1 - d o + 6 d n o 2 ~ + q e n w 2 + 2 -  (1) (no +~5n)2 + 2 where qe is the volume fraction of  water in the SiO 2 and n~ is the refractive index of  water (n~ = 1.3318 from ref. 8). The calculated values for the volume fraction as a HYDRATION PROPERTIES OF CVD SiO 2 FILMS 213 TABLE I RESULTS OBTAINED FROM ELLIPSOMETRIC FOUR-ZONE MEASUREMENTS BEFORE HYDRATION AND AT DIFFERENT WATER VAPOUR PRESSURES Water  vapour pressure n o d o ~n ~d Volume percentage water (Torr) (A) (A) < 10 -3 1.428-+_0.004 500_+4 0.2 0.006 0.6 1.7 0.5 0.006 2.1 2.1 1.2 0.010 1.6 2.9 2.2 0.012 3.2 3.8 <10 -3 1.446+0.001 1013+2 0.2 0.006 1.3 1.5 0.5 0.007 1.4 1.9 1.2 0.009 1.6 2.5 2.2 0.012 3.5 3.4 <10 -3 1.452+0.001 1930+4 0.2 0.005 1.6 1.4 0.5 0.006 2.0 1.6 1.2 0.009 2.7 2.4 2.2 0.013 6.4 3.6 volume percentage are given in Table  I. The absolute  values ISA[ and  [St# I of  the changes with time of A and  ~b caused by the hydra t ion  of SiO 2 films of  different thicknesses at different water vapour  pressures are shown in Figs. 1-6 (full 1.O E i i i i 0 0 5 10 1,5 20 25 t ime (rain) Fig. 1. The change 8A in A with time due to hydration at different water vapour pressures for CVD SiO 2 500 A thick : O, computed change in A. curves). F r o m  these figures it can be seen that  the time needed to reach equi l ibr ium depends on  the film thickness and  the water vapour  pressure. The film thickness in part icular  strongly affects this time. Thus  it can be concluded that a diffusion mechanism plays the major  role. We shall assume that the diffusion coefficient is 214 N . F .  D E  R O O I J ,  R .  J .  S. S I E V E R D I N K ,  R .  M .  T R O M P  .3 PH.gO -- 2.2 Tort o ° o o .2 o o o --  o 1.2 v 0 0 5 10 15 2O 25  t ime (rain) Fig. 2. The change 60 in 0 with time due to hydration at different water vapour pressures for CVD SiO 2 500 • thick : ©, computed change in ~. 1.0 .8 .6 z .2 0 .2 o o o o .5 o . 2  o o 5 10 15 20  '25 t ime  (rain) Fig. 3. The change 6A in A with time due to hydration at different water vapour pressures for CVD SiO 2 1013 A thick : ©, computed change in A. independent  o f  the concentrat ion o f  water in the film. The  diffusion can be treated as a one -d imens iona l  problem with a coordinate  x perpendicular to the interface planes.  The  diffusion equat ion  is given by 8c 82c ~- = O U x  ~ (2) and the initial and boundary  condi t ions  are c = 0  O < ~ x < ~ d  a t t = 0  c = c  e a t x = d  t > O  ~c - - = 0  a t x = O  t~>O 8x (3) HYDRATION PROPERTIES OF CVD S i O  2 FILMS 215 1.0 [ o P 0=22 Torr .8 F o 6 o o t2 o 4 5 ~2 "0 I I I I 0 5 10 15 20 25 time (mini Fig. 4. The change 6~0 in 0 with time due to hydration at different water valour pressures for CVD SiO 2 1013 A thick: C), computed change in 0. - -  . 2  " i i I i O0 40 50 10 2O 3O time (mini Fig. 5. The change 6J in A with time due to hydration at different water vapour pressures for CVD SiO 2 1930 A thick: C), computed change in zt. 1.6 ~ 1.2 ~ o .4 131 I I 0 10 20 o pH:~O. 2.2 Tor r o o _ _  time (mini I r 30 40 50 Fig. 6. The change 5~, in ~b with time due to hydration at different water vapour pressures for CVD SiO 2 1930 A thick: ©, computed change in ~b. 216 N. F. DE ROOIJ, R. J. S. SIEVERDINK, R. M. TROMP where c is the concentra t ion o f  water, c e the equilibrium concentra t ion o f  water, D the diffusion coefficient and d the film thickness. The x = 0 position corresponds to the Si-SiO 2 interface. The solution for the diffusion equat ion for this case is 9 c(x,t) q(x,t) Ce qe 4 ~L (-1)' f O(21+l)Zz2t)  f(21+l)rtx'~ = 1 - r t  1=2-o 21+]-exp~ 4-d' ; c ° s  l 2 d  J (4) Here, q is again the volume fraction o f  water at a certain time t during the hydrat ion process at a position x within the film. By means o f  eqn. (4) the water distribution inside the S i O  2 film can be determined as a function o f  the dimensionless parameters  Dt/d 2 and x/d. For  the purpose o f  calculation the film layer was subdivided into a number  o f  imaginary sublayers, and the volume percentage o f  water belonging to each sublayer was calculated for a given value of  qe. The refractive index for each sublayer can be found by using the following equat ion:  n 2 -- 1 d o no 2 - 1 nw 2 - 1 (5) n 2 + 2  - d o +~q  n0 2 + 2  +-qnw 2 + 2  Here it is assumed that the thickness increases linearly with the volume fraction with a coefficient ~. F rom Fig. 7 R is clear that  this linear relation is rather poor.  The ellipsometric parameters  d and ~ were calculated from the refractive indices o f  the sublayers in this multilayer model,  together with the optical constants  o f  the substrate, the angle o f  incidence and the wavelength o f  the light used (for details o f  this calculation see ref. 6). D 6 5 i I • i I I  i 3 1 / . ( /  / / /  o 0 I I I I 1 2 3 4 5 q x l O  2 Fig. 7. ThethicknessincreaseSdofdifferentCVDfilmsasa function ofthe volume fraction q ofwater in the oxide: O, 500 A; O, 1013 A; []. 1930 A. H Y D R A T I O N  PROPERTIES OF CVD S i O  2 FILMS 217 The open circles along the full curves in Figs. 1-6 indicate the calculated values for the changes 8,4 and 8~k in A and ~k as a result of  a fit of  the theoretical 8A amd 8~b versus D t / d  2 curves and the experimental 8/1 and 8~, versus t curves. F rom this fit a value for the diffusion coefficient can be obtained. The diffusion coefficient for different thicknesses and different water vapour  pressures is given in Table II. After T A B L E  II  THE DIFFUSION COEFFICIENT OF WATER IN CVD SiO 2 Oxide thickness (A) Diffusion coefficient (cm 2 s -  1) Water vapour pressure of  2.2 Torr Water vapour pressure of l.2 Torr 500 1.2 x 10 -13 6.7 x 10 -1`* 1013 1.7 × 10 -13 7.8 x 10 -14 1930 2 . 0 x  10 -13 1 .6x  10 -13 pumping off the water vapour,  A and ~k returned to their initial values. The time needed to return to the initial situation was about  four times longer than that needed to reach the equilibrium situation during the inward diffusion. 4. DISCUSSION From the volume percentages of  water that we found in CVD SiO2, it can be concluded that the oxide absorbs a considerable amount  of  water. The volume fraction of water at the dew-point at room temperature is about  30 % according to a rough linear extrapolation, which is too high for the desired electrode function to be achieved 1. On heat treatment of  the oxide prior to exposure to water vapour  the amount  of  water which can be taken up by the film decreases a so that the opt imum water uptake for a good electrode function can be expected. This needs further investigation. In contrast, the water uptake of thermally grown oxides is very low 1°. Good  results with this oxide as an ion-sensitive layer are unlikely. The diffusion coefficient data obtained from this investigation are of  the same order of  magnitude as those of  Arai and Terunuma a. Ellipsometry in this respect promises to be an interesting tool for measuring the diffusion coefficients of  species in thin films with an accuracy to within a factor 10. The dependence of  the diffusion coefficient on the water vapour  pressure is at present under investigation. From the fact that after hydration the A and ~ values regain their initial values on pumping off the water vapour, it can be concluded that the water is physically adsorbed (see refs. 3, 11). The difference in duration between the hydration and the dehydration processes can be explained by the difference in activation energy for the inward diffusion and the outward diffusion processes. Rupture of  hydrogen bonds in the latter case may be of importance here. 5. CONCLUSIONS It appears that in situ ellipsometry is a useful tool for obtaining data about  the hydration of  SiO 2 films and the diffusion coefficients of  species (e.g. water) in these films. 218 N.F. DE ROOIJ, R. J. S. SIEVERDINK, R. M. TROMP CVD SiO 2 adsorbs a reasonable amount  of water, which makes it an interesting sensitive layer for application in ISFETs. Moreover, it is possible to prepare special mixtures of oxides by means of  CVD technology (e.g. SiO 2 with A1203 (ref. 12)) and it has already been established that they have good electrode properties13. ACKNOWLEDGMENTS The authors wish to express their gratitude to Professor J. Th. G. Overbeek and Professor M. J. Sparnaay for their critical reading of  the manuscript, to Ing. K. O. van der Werf for designing the ellipsometer and to J. Holleman for preparing the samples. This work was supported by the Netherlands Organization for the Advance- ment of  Pure Research (Z.W.O.). REFERENCES 1 D. Hubbard, S. Black, S. Holley and G. Rynders, J. Res. Natl. Bur. Stand., 46 (1961) 168. 2 P. Bergveld, IEEE Trans. Biomed. Eng., 19 (1972) 342. 3 E. Arai and Y. Terunuma, J. Electrochem. Soc., 121 (1974) 676. 4 J.L. Ord and B. Wills, Appl. Opt., 6 (1967) 1673. 5 F.L. McCrackin, E. Passaglia, R. R. Stromberg and H. L. Steinberg, J. Res. Natl. Bur. Stand., Sect. A, 67(1963) 363. 6 F.L. McCrackin, FORTRAN Program for Analysis of Ellipsometer Measurements, Natl. Bur. Std. Tech. Note 479, 1969. 7 W.C.  DashandR. Newman, Phys. Rev.,99(1955) l151. 8 Smithsonian Physical Tables, Smithsonian Institution Press, Washington, D.C., 1969. 9 J. Crank, The Mathematics of  Diffusion, Oxford University Press, London, 1956. 10 G.L. Holmberg, A. B. Kuper and F. D. Miraldi, J. Electrochem. Soc., 117 (1970) 677. 11 M. L. Hair, lnfrared Spectroscopy in Surface Chemistry, Marcel Dekker, New York, 1967. 12 W. Kern and R. C. Helm, J. Electrochem. Soc., 117 (1970) 562. 13 G. Eisenman (ed.), Glass Electrodes for Hydrogen and Other Cations, Marcel Dekker, New York, 1967. 

An investigation of the hydration properties of chemically vapour-deposited silicon dioxide films by means of ellipsometry

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