Thermal oxidation of silicon in O2-SiF4 mixtures

Thin SolM Films, 169 (1989) 173-178 GENERALFILM BEHAVIOUR

173

T H E R M A L O X I D A T I O N OF SILICON IN O2-SiF 4 M I X T U R E S G. PEEV, T. RACHEVA~ E. D A C H E V A AND N. NEDEV

Institute of Chemical Technology, Sofia (Bulgaria)

(ReceivedApriI 18, 1988;acceptedAugust 9, 1988)

The kinetics of the thermal oxidation of silicon in 0 2 - S i F 4 mixtures has been investigated over the temperature range 1000-1200 °C. It is found that the addition of SiF 4 to O2 increases the oxidation rate. This effect is explained mainly by an enhanced diffusion of oxidant in the layer of SiO 2.

Thermal oxidation of silicon in dry or wet oxygen is widely used in the production of silicon devices and integrated circuits. Owing to its great importance the kinetics of this process has been studied very extensively. Deal and Grove a have made quite a successful summary of the experimental results. They suggested a model for the mechanism of the process and on this basis derived the following relationship for predicting the thickness of the obtained oxide layer: X 2 + A X = B('c + t)

(1)

Here X is the oxide thickness, t is the oxidation time and A, B and z are experimentally determined coefficients, accounting for the influence of different factors. The coefficient A, given by A = 2Oeff(1/k + 1/h)

(2)

reflects the effect of the oxidant transportation from the bulk of the gas phase to the wafer surface by the mass transfer coefficient h, the influence of the oxidant diffusion through the oxide layer to the silicon surface by the effective diffusion coefficient Deff and the chemical reaction role by the reaction rate constant k. The coefficient B is related by the expression B = 2DeffC*/N 1

(3)

to the equilibrium concentration C* of the oxidant in the oxide, to the number N~ of oxidant molecules incorporated into a unit volume of the layer and to Deff. The coefficient z accounts for the formation of a thin layer with a thickness X i at the beginning of the process by a mechanism different from the general process: ~" •

( X i 2 -+ A X O / B

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It has been recently ascertained that the introduction of small quantities of some halogen elements in the form of different compounds in the oxidizing medium improves the thermal oxidation of silicon. According to Singh and Balk 2, in the presence of HC1, C12 or C2HC13 the characteristics of metal-oxide-semiconductor transistors are improved. It has been found that the oxidizing process is enhanced in the presence of C12 and HC13. A similar effect has been obtained adding N F a to dry oxygen ~. According to Hirabayashi and Iwamura 5 the introduction of HCI results in a layer in which Deft is greater. The chemical reaction between oxygen and silicon is enhanced as well. In our previous work 6 a it was ascertained that the addition of SiF 4 to a dry oxygen atmosphere leads simultaneously to an enhancement of the process and to an improvement in quality of the Si-SiO2 interface. The best results were achieved at an SiF 4 concentration of about 0.01 vol.~o. In this work we aim to investigate more thoroughly the thermal oxidation kinetics of silicon in dry oxygen in the presence of SiF4. Silicon wafers (n-type; (111) oriented; resistivity, 6-9 f~cm) were used. The oxidation was carried out in a tube diffusion furnace (quartz tube; diameter, 125 mm). The wafers of diameter 2 in were placed horizontally in the direction of the gas flow. The temperature was measured by means of a thermocouple and was kept constant during a given experiment within __ 1 °C. The dry oxygen flow rate was measured by a rotameter and was varied so that the total gas mixture flow rate was 41 min - 1 at 20 °C. SiF~ was added diluted by dry oxygen beforehand by a special mixture system. Its concentration at the furnace entrance was kept constant (0.0125 vol.~o) in all runs. The experiments were carried out at temperatures of 1000, 1050, 1100, 1150 and 1200 °C. Oxidation in dry 0 2 was carried out at the same conditions (temperature, volumetric flow rate etc.) in order to estimate the SiF4 efficiency. The oxide thickness was measured on different points of the wafer and the average value was taken as a final result. Three methods of measurement were used: colour scale, with an ellipsometer and with a pycometer. The relationship between the SiO2 layer thickness and the oxidation time at temperatures of 1050 and 1150°C is shown in Fig. 1. The figure shows that the extrapolation of the curves leads to an intercept which is about 200/~ for oxidation in dry 0 2 and somewhat tess than 200 ~ in the O2-SiF 4 mixture. An analogous value of Xi has been obtained for oxidation in dry 0 2 by other researchers 1. X = fit) curves of similar type were obtained at the other three experimental temperatures. The values of z for the various conditions (Table I) were determined by the X axis intercept of each curve. The data at temperatures of 1000, 1100 and 1200°C are shown in Fig. 2 as a dependence of X on (t + r)/X. It can be seen from the figure that eqn. (1) remains applicable in the presence of SiF4 but its existence in the oxidizing medium leads to a change in the slope and in the intercepts of the lines with the coordinate axes. In order to determine the coefficients A and B the linear correlation shown in Fig. 2 and the least-squares method were used. The calculated values are given in Table I, with the corresponding correlation factors R.

THERMAL OXIDATION OF Si IN O 2 - S i F 4

175

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The conclusion that the addition of SiF 4 increases the coefficient B on average by 36% can be drawn from the results obtained. An increase up to 60% has been noted by Hirabayashi and Iwamura 5 after the addition of considerable quantities of HC1. This results in the formation of water in the reaction system. It is well known that oxidation in wet O2 is much quicker than in dry 021. The higher values of B imply larger effective coefficients of molecular oxygen diffusion in the oxide layer growing in the presence of SiF4. A similar explanation has been suggested for the effect of HC1 addition s. Table I shows that the coefficient A is not affected by the presence of SiF4, which is suggestive regarding the increase in both Deff and k. The acceleration of the

G. PEEV et al.

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THERMAL OXIDATION OF Si IN O 2 - S i F 4

177

reaction between silicon and 0 2 is better illustrated by comparing the values of the linear rate constant B/A. It is evident from Table I that the coefficient, decreases when SiF4 is added but its values at the working temperatures are very small and include a large relative error. The observed increase in k is in good agreement with the hypothesis 4 that SiF4 thermally dissociates to SiFz,F 2 and fluorine atoms. Fluorine compounds and fluorine diffuse to the silicon surface and also react with the silicon. According to the Deal and Grove model ~, the coefficients B and B/A are exponential function of l/T: B = B o e x p ( - AEB/RT )

(5)

B/A = (B/A)o exp(-- AEB/A/RT)

(6)

Our data shown in Fig. 3 illustrate the applicability of eqns. (5) and (6) not only for oxidation in dry oxygen but for oxidation in O / - S i F 4 mixtures as well. This allows the effect of SiF 4 on the activation energies and pre-exponential factors in eqns. (5) and (6) to be appreciated. ~2

~B 6

80,

o) /000/7, 08

Fig. 3. Temperature dependence of B and

B/A in the oxidation process.

The data, treated by the least-squares method, for dry oxygen gave the values AE~ = 30.76 kcal m o l - 1 and AEn/a = 42.4 kcal m o l - 1 with correlation factors of 0.99 and 0.985 respectively. These results are in close agreement with the values obtained by Deal and Grove 1. In the case of O2-SiF 4 mixtures the calculations gave AEB = 31.92 kcal mol-1 and AEB/a = 43.26 kcal tool 1 with correlation factors of 0.985 and 0.976 respectively. The similar values obtained for AEB are expected, since in both cases oxygen diffuses through a silicon dioxide layer. The different structure of SiO2 layer with SiF46, however, follows from the increase in B o from 2.87 x 10 9 ~ 2 m i n - 1 for dry oxygen oxidation to 5.76 x 10 9 ~ 2 m i n - 1. The similar values obtained for AEB/A do not reveal an important catalytic role of SiF~ in the oxidation process at the Si-SiO2 interface. The effect of SiF, is indicated again by the increase in the pre-exponential factor (B/A)o in eqn. (6) from 3.13 x 108 Amin -1 to 4.49 x 108 ~ m i n -1. A negligible variation in the activation energies can be expected from the graphical data for the temperature dependence of B and B/A of Hess and Deal 9.

178

G. PEEr e t a l .

These workers, however, did n o t draw any definite conclusions. The data for the kinetic coefficients a n d their temperature dependence o b t a i n e d in this work reveal more clearly the effect of SiF4 a n d permit desirable conditions for the thermal oxidation of silicon in the presence of this additive to be chosen. REFERENCES 1 2 3 4

B.E. Deal and A. S. Grove, J. Appl. Phys., 16 (1965) 3770. B.R. Singh and P. Balk, J. Electrochem. Soc., 125 (3) (1978) 453. B.E. Deal and D. W. Hess, J. Eleetrochem. Soe., 125 (2) (1978) 339. M. Morita, T. Kubo, T. Ishihara and M. Hirose, Appl. Phys. Lett., 45 (12) (1987) 1312.

5 K. HirabayashiandI. Iwamura, J. Eleetroehem. Soc.,120(ll)(1973)1595.

6 G. Peev, T. Racheva, E. Dacheva, N. Jeleva and D. Dachev, Proc. Conf. on Semiconductor Devices, Botevgrad, June 13, 1987.

7 T. Racheva and E. Dacheva, Invention No. 37821, M P K H O 1 221/30, March 27, 1984, Bulgaria. 8 T. Racheva, E. Dacheva, N. Nedev and D. Dachev, Elektropromst. Priborostr., 8 (1987) 26. 9 P.W. Hess and B. E. Deal, J. Electrochem. Soc., 124 (5) (1977) 735.