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Atomic-hydrogen-induced desorption of fluorine from silicon surfaces
applied surface science Applied
ELSEVIER
Surface
Science
81 (1994) 223-227
Atomic-hydrogen-induced desorption of fluorine from silicon surfaces Yoji Saito * Department of Electrical Engineering and Electronics, Faculty of Engineering, Seikei University, Musashino 3, Tokyo 180, Japan Received
3 March
1994; accepted
for publication
1 June 1994
Abstract Extraction of fluorine adsorbates on silicon surfaces is induced by exposure to atomic hydrogen. The decay process of fluorine has been investigated with Auger electron microscopy and is discussed in terms of chemical kinetics. The desorption rate depends on the hydrogen exposure time, the substrate temperature, and the supply of atomic hydrogen. In the initial stage, the reaction proceeds with second-order kinetics with an activation energy of about 0.4 eV. Moreover, two hydrogen atoms seem to take part in the reaction at the same time, according to the rate dependence on the supply of atomic hydrogen. With the decrease of the fluorine coverage, a first-order process dominates the reaction, and its activation energy is about 0.56 eV. The fluorine adsorbates on silicon are completely removed by annealing above 200°C for 30 min, using the atomic hydrogen.
1. Introduction
With the down-scaling of device dimensions in semiconductor integrated circuits, the precise control of surfaces and interfaces of the substrates becomes more important. Halogen removal from the silicon surface is required to obtain high-quality films at low substrate temperature, before the epitaxial growth of silicon [l] or during the film growth using halogenated source gases [21. The substrates should be annealed to above 700°C to remove the residual fluorine in UHV (ultra-high vacuum) [2]. Ultraviolet light excited gas mixtures of H, and F, can reduce the residual fluorine, but the mechanism of the fluorine extraction is not clear [3].
* Tel: 0422-37-3725;
Fax: 0422-37-3871.
0169-4332/94/$07.00 0 1994 Elsevier SSDI0169-4332(94)00153-R
Science
Recently, it has been reported [4,5] that chlorine and bromine adsorbates on semiconductor surfaces can be extracted by exposure to atomic hydrogen, but an atomic hydrogen-fluorine reaction has not been examined. In this study, we have investigated the fluorine extraction phenomena on silicon surfaces induced by atomic hydrogen. The atomic hydrogen was supplied onto the fluorine-covered silicon surface at llO-200°C. The decay process of fluorine has been investigated with Auger electron spectroscopy and is discussed in terms of chemical kinetics.
2. Experimental
procedure
We used (lOO)-oriented p-type single-crystalline silicon substrates with a resistivity of 2-4 B.V. All rights reserved
224
Y. Saito /Applied Surface Science 81 (1994) 223-227
s1. cm. The substrate was degreased in boiling methanol and rinsed in de-ionized water. Then, the substrate was boiled in an alkali solution of NH,OH, H,O,, and H,O (1: 1:6) for 10 min, rinsed with de-ionized water, and dipped into a diluted HF solution (1%). Finally, the substrate was rinsed again in de-ionized water and blown dry with nitrogen. The etching, hydrogen exposure, and analysis were carried out in a UHV system with four chambers made of stainless steel, which can be spatially separated from each other with gate valves. The sample can be transferred between the chambers in UHV. The base pressures in the preparation, etching, hydrogen process, and analysis chambers were below lo-‘, lo-‘, 10m9, and 10e9 Torr, respectively. The schematic diagram of the experimental apparatus except the hot filament has been shown elsewhere [6]. After the chemical pretreatments, the sample was mounted on the sample holder, introduced into the preparation chamber, and transferred into the etching chamber. The substrate was exposed to diluted fluorine gas (5% in He) at 5 Torr for 3 min at room temperature. In this treatment, the silicon surface was slightly etched (a few monolayers [7]), but the fluorine atoms were adsorbed onto the surface. Then, the substrate was transferred onto the resistive heater in the hydrogen process chamber, and was heated up to a temperature of 110-200°C in UHV. The deviation from the target temperature was within 5°C. Desorption of the fluorine adsorbates can be ignored during the heating. The atomic hydrogen, which was thermally generated by a hot tungsten filament from a pure hydrogen gas, was supplied onto the substrate at 2 x 1O-5 Torr. The tungsten filament was in a BN tube with 4 mm diameter to decrease the thermal loss, as described in Ref. [Sl. The filament temperature was measured with a pyrometer having a built-in reference filament, where the absolute error in the measurement would be within a few tens of degrees. Before and after the hydrogen exposure, the decay process of the residual fluorine on the surface was investigated with AES (Auger electron spectroscopy) in the analysis chamber. The
acceleration energy, the current, and the diameter of the primary electron beam were 2 keV, 5 PA, and about 0.1 mm, respectively. The differential AES signal was fed to the computer system through a analog-to-digital converter. Only the fluorine peak measurement was repeated several times, and the data were averaged to decrease the noise component. ESD (electron-beam-stimulated desorption) of fluorine was observed during the AES measurements. Without special operation, the decay time constant for ESD was a few seconds. To minimize the influence of electron-stimulated desorption, the observed point on the sample was changed at each measurement and scanned by moving the sample slowly to horizontal directions of the surface during the observation. With this operation, the decay time constant for ESD has increased up to several tens of seconds. The relative error was estimated to be less than 20%, taking account of the averaging operation and the measurement time of about 25 s for each observation.
3. Experimental
results and discussion
AES measurements were performed on the silicon surface after the F2 etching treatment. AES signals near 650 eV from fluorine atoms are observed except for the primary signals near 92 and 1610 eV which stem from silicon atoms. The average fluorine coverage on the substrate was estimated to be 0.7-0.8 monolayers. Fig. la shows the dependence of the normalized density of the fluorine, [F]/[F],, on substrate temperature, as a function of hydrogen exposure time, where [F] and [F], are the fluorine signal intensity at an arbitrary time and that for time zero (initial), respectively. The filament temperature was 1360°C. The normalized density of the fluorine decreases rapidly at the initial stage, and then, exponentiahy with the exposure time. Therefore, a first-order kinetic reaction dominates the desorption process, decreasing the fluorine coverage. In analogy to the reaction between atomic hydrogen and other halogen species on silicon [41, the latter reaction should be Si-F( ads) + H + Si( s) + HF( g) .
(1)
225
Y. Saito /Applied Surface Science 81 (1994) 223-227
The reaction rate constant k, was obtained from the slope of the linear region in Fig. la for each substrate temperature. The dependence of k, on the inverse temperature is shown in Fig. lb. The activation energy of the first-order reaction is obtained to be 0.56 k 0.04 eV from the slope of the fitted line in Fig. lb. This value is larger than 0.3 eV for Cl desorption [5], because the bond dissociation energy of Si-F (about 6 eV) is larger than that of Si-Cl (4 eV) [9]. On the other hand, however, an exponential behavior cannot be observed at the initial stage in Fig. la. The early stage of the data in Fig. la is replotted in Fig. 2a according to a second-order kinetic reaction, where the vertical axis, [F],/[F] - 1, corresponds to the desorbed fluorine quantity. In Fig. 2a, a rather good linear dependence on the exposure time is obtained at the initial stage. Thus, it is concluded that the fluorine adsorbates desorb with a second-order reaction when the fluorine coverage exceeds 0.4-0.5 monolayers at least. The reaction rate constant k, was also obtained from the slope of the linear region in Fig. 2a for each substrate temperature. k, as a function of the inverse temperature is shown in Fig. 2b. The activation energy of the second-order reaction is obtained to be 0.40 + 0.04 eV from the slope of the fitted line in Fig. 2b, and is less than that of the first-order reaction in Fig. lb.
0
100 Time (min) (a)
200
2.1 2.2 2.3 2.4 25
T-1 (XlO-3
2.6 2.7
K-')
@I
Fig. 1. (a) Dependence of the normalized density of fluorine, [F]/[F]u, on substrate temperature, as a function of hydrogen exposure time, where [F] and [F], are the fluorine signal intensity at arbitrary time and that for time zero (initial), respectively. The filament temperature was 1360°C. (b) Reaction rate constant, k,, as a function of inverse substrate temperature.
22
Time (min) (a)
23
24
25
26
2.7
T-1 (X10-3K-') (b)
Fig. 2. (a) Replotted data of the early stage in Fig. la according to a second-order kinetic reaction, where the vertical axis, [F],/[F]1, corresponds to the desorped fluorine quantity. (b) Reaction rate constant, k,, as a function of inverse substrate temperature.
The large activation energy causes an abrupt decrease of residual fluorine with a slight increase of the substrate temperature. The required temperature is dominated by the firstorder reaction for the complete removal of fluorine. In this case, the fluorine adsorbates can be completely removed by exposure to atomic hydrogen for 30 min above 200°C. We have investigated the dependence of the reaction rate on the supply of atomic hydrogen to confirm the above suppositions and to estimate the reaction mechanisms. The supply of atomic hydrogen was controlled by varying the filament temperature, and was assumed to be proportional to the decomposition probability of hydrogen by the hot filament. The decomposition probability of hydrogen is estimated from the filament temperature, comparing with the data previously reported [lo]. Fig. 3a shows the dependence of the normalized density of the fluorine, [F]/[F],, on the filament temperature, as a function of hydrogen exposure time, where the substrate temperature was 150°C. In this figure, the normalized density of the fluorine decreases exponentially with the exposure time for each filament temperature, decreasing the fluorine coverage. The correlation of the reaction rate of first-order kinetics, R,, obtained from Fig. 3a to the supply of atomic hydrogen is shown in Fig. 3b. R, is roughly proportional to the supply of the atomic hydrogen. This
226
Y Saito /Applied Surface Science 81 (1994) 223-227
netic reaction should be related to both two fluorine atoms and two hydrogen atoms. Moreover, according to Ref. [ll], a fluorine etched surface consists of SiF, (X = 1-3) layers. The favorable second-order reactions are likely,
IO’0
20
40
60
60
000’ 00
Atomic
Time (min)
02
04
06
hydrogen
08
supply (a u
’
10
)
Fig. 3. (a) Dependence of the normalized density of fluorine, [Fl/[Fl,, on the filament temperature, as a function of hydrogen exposure time, where the substrate temperature was 150°C. (b) Correlation between the reaction rate of first-order kinetics, R,, and the supply of atomic hydrogen.
(a)
Square
of atomic
hydrogen
SuPply @.u
(b)
Fig. 4. (a) Replotted data of the early stage in Fig. 3a according to a second-order kinetic reaction. (b) Correlation between the reaction rate of second-order kinetics, R,, and the square of the supply of atomic hydrogen.
Si-F,(ads)
(2b)
+ 2 H + SiH,F,(g).
2 Si-F(ads)
+ 2 H + Si( s) + 2 HF( g) ,
(2c)
but the probability for this four-molecular related reaction to occur is less than those for the reactions of Eqs. (2a) and (2b). F, may also be possible as a reaction product. However, such a reaction should be unfavorable, taking into account the larger free energy of F, compared to that of HF, where the Gibbs free energies of F, and HF are 0 and - 273 kJ mol-‘, respectively [12]. Therefore, at present, the author considers Eqs. (2a) and (2b) as the secondorder reaction. Further photoelectron- and thermal desorption spectroscopy experiments are required to determine the reaction.
behavior is consistent with the first-order reaction between F and H, and agrees with Eq. (1). The early stage of the data in Fig. 3a is replotted in Fig. 4a according to a second-order kinetic reaction. In Fig. 4a, a good linear dependence on the exposure time is obtained in the initial stage. The reaction rate for the second-order reaction, R,, is obtained from the slope of the line for each filament temperature. However, R, is not proportional to the supply of atomic hydrogen. Thus, R, as a function of the square of the supply of atomic hydrogen is shown in Fig. 4b. A rather good linear relationship is obtained in Fig. 4b. The result indicates that two hydrogen atoms are required for the second-order reaction. On the basis of the above results, the second-order ki-
Time (min)
(2a)
Considering a threshold of the fluorine coverage for the second-order kinetics, the following reaction is also possible:
W
(4
Si-F,( ads) + 2 H + Si(s) + 2HF(g),
4. Conclusions
)
An accelerated desorption of fluorine adsorbates on silicon surfaces is induced by atomic hydrogen exposure. The desorption rate increases with increasing hydrogen exposure time, substrate temperature, and supply of atomic hydrogen. In the initial stage, the reaction proceeds with second-order kinetics with an activation energy of 0.40 k 0.04 eV. Moreover, two hydrogen atoms seem to take part in the reaction at the same time, according to the rate dependence on the supply of atomic hydrogen. With the decrease of the fluorine coverage, a first-order kinetic process dominates the reaction, and its activation energy is 0.56 _t 0.04 eV. The fluorine adsorbates on silicon are completely removed by annealing above 200°C for 30 min, using the atomic hydrogen.
I’. Saito /Applied Surface Science 81 (1994) 223-227
Acknowledgments
The author thanks Prof. A. Yoshida and Dr. A. Namiki of Toyohashi University of Technology for encouragement and helpful discussions. The author is also grateful to M. Okada and T. Momma for technical assistance. This work was partially supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan.
References [l] T. Aoyama, T. Yamazaki and T. Ito, J. Electrochem. Sot. 140 (1993) 366. [2] Y.T. Kim, J.S. Hong and S.-K. Min, Appl. Phys. Lett. 59 (1991) 3136.
227
[3] T. Aoyama, T. Yamazaki and T. Ito, J. Electrochem. Sot. 140 (1993) 1704. [4] SM. Cohen, T.I. Hukka, Y.L. Yany and M.P.D.‘Evelyn, Thin Solid Films 225 (1993) 155. [5] J.T. Yates, Jr., C.C. Cheng, Q. Gao, M.L. Colaianni and W.J. Choyke, Thin Solid Films 225 (1993) 150. [6] Y. Saito and A. Yoshida, J. Electrochem. Sot. 139 (1992) L115. [7] M. Cheng, V.T. Minkiewicz and K. Lee, J. Electrochem. Sot. 126 (1979) 1946. [81 T. Sugaya and M. Kawabe, Jpn. J. Appl. Phys. 30 (1991) L402. 191 R. Walsh, Act. Chem. Res. 14 (1981) 246. I101 T. Sakurai, M.J. Carodillo and H.D. Hagstrum, J. Vat. Sci. Technol. 14 (1977) 397. Ill] F.R. McFeely, J.F. Morar and F.J. Himpsel, Surf. Sci. 165 (1986) 277. [12] D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Bailey and R.H. Schumm, NBS Tech. Notes 270-3, Selected Values of Chemical Thermodynamic Properties, Tables for the First Thirty-Four Elements in the Standard Order of Arrangement (US Government Printing Office, Washington, DC, 1968).
ELSEVIER
Surface
Science
81 (1994) 223-227
Atomic-hydrogen-induced desorption of fluorine from silicon surfaces Yoji Saito * Department of Electrical Engineering and Electronics, Faculty of Engineering, Seikei University, Musashino 3, Tokyo 180, Japan Received
3 March
1994; accepted
for publication
1 June 1994
Abstract Extraction of fluorine adsorbates on silicon surfaces is induced by exposure to atomic hydrogen. The decay process of fluorine has been investigated with Auger electron microscopy and is discussed in terms of chemical kinetics. The desorption rate depends on the hydrogen exposure time, the substrate temperature, and the supply of atomic hydrogen. In the initial stage, the reaction proceeds with second-order kinetics with an activation energy of about 0.4 eV. Moreover, two hydrogen atoms seem to take part in the reaction at the same time, according to the rate dependence on the supply of atomic hydrogen. With the decrease of the fluorine coverage, a first-order process dominates the reaction, and its activation energy is about 0.56 eV. The fluorine adsorbates on silicon are completely removed by annealing above 200°C for 30 min, using the atomic hydrogen.
1. Introduction
With the down-scaling of device dimensions in semiconductor integrated circuits, the precise control of surfaces and interfaces of the substrates becomes more important. Halogen removal from the silicon surface is required to obtain high-quality films at low substrate temperature, before the epitaxial growth of silicon [l] or during the film growth using halogenated source gases [21. The substrates should be annealed to above 700°C to remove the residual fluorine in UHV (ultra-high vacuum) [2]. Ultraviolet light excited gas mixtures of H, and F, can reduce the residual fluorine, but the mechanism of the fluorine extraction is not clear [3].
* Tel: 0422-37-3725;
Fax: 0422-37-3871.
0169-4332/94/$07.00 0 1994 Elsevier SSDI0169-4332(94)00153-R
Science
Recently, it has been reported [4,5] that chlorine and bromine adsorbates on semiconductor surfaces can be extracted by exposure to atomic hydrogen, but an atomic hydrogen-fluorine reaction has not been examined. In this study, we have investigated the fluorine extraction phenomena on silicon surfaces induced by atomic hydrogen. The atomic hydrogen was supplied onto the fluorine-covered silicon surface at llO-200°C. The decay process of fluorine has been investigated with Auger electron spectroscopy and is discussed in terms of chemical kinetics.
2. Experimental
procedure
We used (lOO)-oriented p-type single-crystalline silicon substrates with a resistivity of 2-4 B.V. All rights reserved
224
Y. Saito /Applied Surface Science 81 (1994) 223-227
s1. cm. The substrate was degreased in boiling methanol and rinsed in de-ionized water. Then, the substrate was boiled in an alkali solution of NH,OH, H,O,, and H,O (1: 1:6) for 10 min, rinsed with de-ionized water, and dipped into a diluted HF solution (1%). Finally, the substrate was rinsed again in de-ionized water and blown dry with nitrogen. The etching, hydrogen exposure, and analysis were carried out in a UHV system with four chambers made of stainless steel, which can be spatially separated from each other with gate valves. The sample can be transferred between the chambers in UHV. The base pressures in the preparation, etching, hydrogen process, and analysis chambers were below lo-‘, lo-‘, 10m9, and 10e9 Torr, respectively. The schematic diagram of the experimental apparatus except the hot filament has been shown elsewhere [6]. After the chemical pretreatments, the sample was mounted on the sample holder, introduced into the preparation chamber, and transferred into the etching chamber. The substrate was exposed to diluted fluorine gas (5% in He) at 5 Torr for 3 min at room temperature. In this treatment, the silicon surface was slightly etched (a few monolayers [7]), but the fluorine atoms were adsorbed onto the surface. Then, the substrate was transferred onto the resistive heater in the hydrogen process chamber, and was heated up to a temperature of 110-200°C in UHV. The deviation from the target temperature was within 5°C. Desorption of the fluorine adsorbates can be ignored during the heating. The atomic hydrogen, which was thermally generated by a hot tungsten filament from a pure hydrogen gas, was supplied onto the substrate at 2 x 1O-5 Torr. The tungsten filament was in a BN tube with 4 mm diameter to decrease the thermal loss, as described in Ref. [Sl. The filament temperature was measured with a pyrometer having a built-in reference filament, where the absolute error in the measurement would be within a few tens of degrees. Before and after the hydrogen exposure, the decay process of the residual fluorine on the surface was investigated with AES (Auger electron spectroscopy) in the analysis chamber. The
acceleration energy, the current, and the diameter of the primary electron beam were 2 keV, 5 PA, and about 0.1 mm, respectively. The differential AES signal was fed to the computer system through a analog-to-digital converter. Only the fluorine peak measurement was repeated several times, and the data were averaged to decrease the noise component. ESD (electron-beam-stimulated desorption) of fluorine was observed during the AES measurements. Without special operation, the decay time constant for ESD was a few seconds. To minimize the influence of electron-stimulated desorption, the observed point on the sample was changed at each measurement and scanned by moving the sample slowly to horizontal directions of the surface during the observation. With this operation, the decay time constant for ESD has increased up to several tens of seconds. The relative error was estimated to be less than 20%, taking account of the averaging operation and the measurement time of about 25 s for each observation.
3. Experimental
results and discussion
AES measurements were performed on the silicon surface after the F2 etching treatment. AES signals near 650 eV from fluorine atoms are observed except for the primary signals near 92 and 1610 eV which stem from silicon atoms. The average fluorine coverage on the substrate was estimated to be 0.7-0.8 monolayers. Fig. la shows the dependence of the normalized density of the fluorine, [F]/[F],, on substrate temperature, as a function of hydrogen exposure time, where [F] and [F], are the fluorine signal intensity at an arbitrary time and that for time zero (initial), respectively. The filament temperature was 1360°C. The normalized density of the fluorine decreases rapidly at the initial stage, and then, exponentiahy with the exposure time. Therefore, a first-order kinetic reaction dominates the desorption process, decreasing the fluorine coverage. In analogy to the reaction between atomic hydrogen and other halogen species on silicon [41, the latter reaction should be Si-F( ads) + H + Si( s) + HF( g) .
(1)
225
Y. Saito /Applied Surface Science 81 (1994) 223-227
The reaction rate constant k, was obtained from the slope of the linear region in Fig. la for each substrate temperature. The dependence of k, on the inverse temperature is shown in Fig. lb. The activation energy of the first-order reaction is obtained to be 0.56 k 0.04 eV from the slope of the fitted line in Fig. lb. This value is larger than 0.3 eV for Cl desorption [5], because the bond dissociation energy of Si-F (about 6 eV) is larger than that of Si-Cl (4 eV) [9]. On the other hand, however, an exponential behavior cannot be observed at the initial stage in Fig. la. The early stage of the data in Fig. la is replotted in Fig. 2a according to a second-order kinetic reaction, where the vertical axis, [F],/[F] - 1, corresponds to the desorbed fluorine quantity. In Fig. 2a, a rather good linear dependence on the exposure time is obtained at the initial stage. Thus, it is concluded that the fluorine adsorbates desorb with a second-order reaction when the fluorine coverage exceeds 0.4-0.5 monolayers at least. The reaction rate constant k, was also obtained from the slope of the linear region in Fig. 2a for each substrate temperature. k, as a function of the inverse temperature is shown in Fig. 2b. The activation energy of the second-order reaction is obtained to be 0.40 + 0.04 eV from the slope of the fitted line in Fig. 2b, and is less than that of the first-order reaction in Fig. lb.
0
100 Time (min) (a)
200
2.1 2.2 2.3 2.4 25
T-1 (XlO-3
2.6 2.7
K-')
@I
Fig. 1. (a) Dependence of the normalized density of fluorine, [F]/[F]u, on substrate temperature, as a function of hydrogen exposure time, where [F] and [F], are the fluorine signal intensity at arbitrary time and that for time zero (initial), respectively. The filament temperature was 1360°C. (b) Reaction rate constant, k,, as a function of inverse substrate temperature.
22
Time (min) (a)
23
24
25
26
2.7
T-1 (X10-3K-') (b)
Fig. 2. (a) Replotted data of the early stage in Fig. la according to a second-order kinetic reaction, where the vertical axis, [F],/[F]1, corresponds to the desorped fluorine quantity. (b) Reaction rate constant, k,, as a function of inverse substrate temperature.
The large activation energy causes an abrupt decrease of residual fluorine with a slight increase of the substrate temperature. The required temperature is dominated by the firstorder reaction for the complete removal of fluorine. In this case, the fluorine adsorbates can be completely removed by exposure to atomic hydrogen for 30 min above 200°C. We have investigated the dependence of the reaction rate on the supply of atomic hydrogen to confirm the above suppositions and to estimate the reaction mechanisms. The supply of atomic hydrogen was controlled by varying the filament temperature, and was assumed to be proportional to the decomposition probability of hydrogen by the hot filament. The decomposition probability of hydrogen is estimated from the filament temperature, comparing with the data previously reported [lo]. Fig. 3a shows the dependence of the normalized density of the fluorine, [F]/[F],, on the filament temperature, as a function of hydrogen exposure time, where the substrate temperature was 150°C. In this figure, the normalized density of the fluorine decreases exponentially with the exposure time for each filament temperature, decreasing the fluorine coverage. The correlation of the reaction rate of first-order kinetics, R,, obtained from Fig. 3a to the supply of atomic hydrogen is shown in Fig. 3b. R, is roughly proportional to the supply of the atomic hydrogen. This
226
Y Saito /Applied Surface Science 81 (1994) 223-227
netic reaction should be related to both two fluorine atoms and two hydrogen atoms. Moreover, according to Ref. [ll], a fluorine etched surface consists of SiF, (X = 1-3) layers. The favorable second-order reactions are likely,
IO’0
20
40
60
60
000’ 00
Atomic
Time (min)
02
04
06
hydrogen
08
supply (a u
’
10
)
Fig. 3. (a) Dependence of the normalized density of fluorine, [Fl/[Fl,, on the filament temperature, as a function of hydrogen exposure time, where the substrate temperature was 150°C. (b) Correlation between the reaction rate of first-order kinetics, R,, and the supply of atomic hydrogen.
(a)
Square
of atomic
hydrogen
SuPply @.u
(b)
Fig. 4. (a) Replotted data of the early stage in Fig. 3a according to a second-order kinetic reaction. (b) Correlation between the reaction rate of second-order kinetics, R,, and the square of the supply of atomic hydrogen.
Si-F,(ads)
(2b)
+ 2 H + SiH,F,(g).
2 Si-F(ads)
+ 2 H + Si( s) + 2 HF( g) ,
(2c)
but the probability for this four-molecular related reaction to occur is less than those for the reactions of Eqs. (2a) and (2b). F, may also be possible as a reaction product. However, such a reaction should be unfavorable, taking into account the larger free energy of F, compared to that of HF, where the Gibbs free energies of F, and HF are 0 and - 273 kJ mol-‘, respectively [12]. Therefore, at present, the author considers Eqs. (2a) and (2b) as the secondorder reaction. Further photoelectron- and thermal desorption spectroscopy experiments are required to determine the reaction.
behavior is consistent with the first-order reaction between F and H, and agrees with Eq. (1). The early stage of the data in Fig. 3a is replotted in Fig. 4a according to a second-order kinetic reaction. In Fig. 4a, a good linear dependence on the exposure time is obtained in the initial stage. The reaction rate for the second-order reaction, R,, is obtained from the slope of the line for each filament temperature. However, R, is not proportional to the supply of atomic hydrogen. Thus, R, as a function of the square of the supply of atomic hydrogen is shown in Fig. 4b. A rather good linear relationship is obtained in Fig. 4b. The result indicates that two hydrogen atoms are required for the second-order reaction. On the basis of the above results, the second-order ki-
Time (min)
(2a)
Considering a threshold of the fluorine coverage for the second-order kinetics, the following reaction is also possible:
W
(4
Si-F,( ads) + 2 H + Si(s) + 2HF(g),
4. Conclusions
)
An accelerated desorption of fluorine adsorbates on silicon surfaces is induced by atomic hydrogen exposure. The desorption rate increases with increasing hydrogen exposure time, substrate temperature, and supply of atomic hydrogen. In the initial stage, the reaction proceeds with second-order kinetics with an activation energy of 0.40 k 0.04 eV. Moreover, two hydrogen atoms seem to take part in the reaction at the same time, according to the rate dependence on the supply of atomic hydrogen. With the decrease of the fluorine coverage, a first-order kinetic process dominates the reaction, and its activation energy is 0.56 _t 0.04 eV. The fluorine adsorbates on silicon are completely removed by annealing above 200°C for 30 min, using the atomic hydrogen.
I’. Saito /Applied Surface Science 81 (1994) 223-227
Acknowledgments
The author thanks Prof. A. Yoshida and Dr. A. Namiki of Toyohashi University of Technology for encouragement and helpful discussions. The author is also grateful to M. Okada and T. Momma for technical assistance. This work was partially supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan.
References [l] T. Aoyama, T. Yamazaki and T. Ito, J. Electrochem. Sot. 140 (1993) 366. [2] Y.T. Kim, J.S. Hong and S.-K. Min, Appl. Phys. Lett. 59 (1991) 3136.
227
[3] T. Aoyama, T. Yamazaki and T. Ito, J. Electrochem. Sot. 140 (1993) 1704. [4] SM. Cohen, T.I. Hukka, Y.L. Yany and M.P.D.‘Evelyn, Thin Solid Films 225 (1993) 155. [5] J.T. Yates, Jr., C.C. Cheng, Q. Gao, M.L. Colaianni and W.J. Choyke, Thin Solid Films 225 (1993) 150. [6] Y. Saito and A. Yoshida, J. Electrochem. Sot. 139 (1992) L115. [7] M. Cheng, V.T. Minkiewicz and K. Lee, J. Electrochem. Sot. 126 (1979) 1946. [81 T. Sugaya and M. Kawabe, Jpn. J. Appl. Phys. 30 (1991) L402. 191 R. Walsh, Act. Chem. Res. 14 (1981) 246. I101 T. Sakurai, M.J. Carodillo and H.D. Hagstrum, J. Vat. Sci. Technol. 14 (1977) 397. Ill] F.R. McFeely, J.F. Morar and F.J. Himpsel, Surf. Sci. 165 (1986) 277. [12] D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Bailey and R.H. Schumm, NBS Tech. Notes 270-3, Selected Values of Chemical Thermodynamic Properties, Tables for the First Thirty-Four Elements in the Standard Order of Arrangement (US Government Printing Office, Washington, DC, 1968).