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Photon-stimulated desorption from chemically treated Si surfaces
Surface Science 287/288 (1993) 175-177 North-Holland
Photon-stimulated
surface science %A: ./.., ‘.‘. ........... ,,..., ......,,.,.,,, ::::::g$g “‘:‘~‘~‘~‘~~~~~ ,,,,
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desorption from chemically treated Si surfaces
Isao Ochiai a, Taro Ogawa a, Yuji Takakuwa b and Kozo Mochiji a Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan b Research Institute of Electrical Communication,
a
Tohoku University, Sendai 980, Japan
Received 31 August 1992; accepted for publication 24 November 1992
Experimental results are presented on the synchrotron-radiation-induced desorption of oxygen ions from HF-treated Si surfaces measured with a quadrupole mass analyzer operated in the pulse-counting mode. The effect of atomic hydrogen exposure was studied. Desorption of H+, Of and F+ ions was observed from HF-treated Si surfaces. The yield of Of ions was increased more than 30-fold by exposure to atomic hydrogen.
1. Introduction
As the dimensions of large-scale integrated circuits approach the nanometer level, semiconductor processes with low temperature, low damage and low contamination are becoming necessary. Recently, synchrotron-excited reactions have been studied actively as new processing techniques, i.e. deposition [1,2], etching [l], epitaxial growth [3] and surface cleaning [4]. Reactions induced by synchrotron radiation are thought to be due to electronic transitions, thereby offering the opportunity to initiate processes which are thermally inaccessible with the added advantage of improved pattern definition due to the shorter wavelength. Photon-stimulated desorption (PSD) is considered as one of important reactions for these applications, especially surface fabrication, because particles are desorbed from the surface. We investigated how PSD characteristics are affected by the bonding state on Si surfaces using HF treatment and atomic hydrogen irradiation.
Laboratory of High Energy Physics. A schematic diagram of the experimental set-up is shown in fig. 1. BL-8A has a monochromator equipped with three mirrors and one varied-space plane grating [4]. Zeroth-order light (hv < 2 keV) emerging from the monochromator was used as a SR irradiation source. The photon flux per 100 mA is about 10’3-‘4 photons/s . cm*. Mass analy-
(G-z?)
Photon Factors at National Lab. for High Energy Physics
Semispherical
_
Quadrupole mass analyzer
2. Experimental
The experiments were carried out using beam line BLSA at the Photon Factory at the National 0039-6028/93/$06.00
Fig. 1. Schematic diagram of the experimental set-up.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
176
I. Ochiai el al. / PSD from chemically treated Si
3. Results and discussion
Fig. 2. Schematic drawing of the atomic hydrogen source.
sis of photon-stimulated desorbed ions under SR irradiation was performed using a quadrupole mass analyzer (QMA) operated in the pulsecounting mode. A filament in the ionization cell of the QMA was switched off. A potential voltage of 20 V was applied to the specimen to extract ions from the specimen surface. Mass assignment was performed by comparison with the spectrum of residual gases. The mass resolution of the present system (AM) was 0.5. The monochromatic light and a spherical-sector electron energy analyzer were used for X-ray photoelectron spectroscopy (XPS). The cross-sectional view of the atomic hydrogen source is shown in fig. 2. By atomization of molecular hydrogen at the hot inner surface (2000°C) of a tungsten pipe, an intense atomic hydrogen beam was obtained. The pipe was heated by electron bombardment. The acceleration voltage was 1500 V and the current was 100 mA. Exposure to the atomic hydrogen beam was typically performed under the condition of a H, filling pressure of 1.0 X lop6 Torr and an exposure time of 180 s without SR irradiation. The flux of atomic hydrogen was estimated to be 3 x 10-l” atoms/s. cm2 from the measured decrease of H, partial pressure and H, influx. The distance between the outlet of the source and specimens was 10 cm. The temperature rise during the exposure was about 20°C. The specimens used here were Si(lll> and Si(100) wafers dipped in aqueous 0.5%, 5%, and 50% HF solution.
From the Si2p photoelectron spectrum of the SK1111 dipped in 50% HF, a native oxide ‘aye,’ was formed and the thickness was about 5 A estimated by the attenuation of bulk Si photoelectrons. The PSD ion mass spectra before and after atomic hydrogen exposure are shown in fig. 3. Desorption of H+, CH+, O+ and F+ ions was primarily detected during SR irradiation. After atomic hydrogen exposure the yield of O+ ions was increased by 36 times, while the change of the yield of H+ and F+ was within a factor of 2. The yield of CHT was decreased. The yield of OH+ ions was negligibly small in both cases. The results in the cases of Si(ll1) dipped in 5% HF, Si(ll1) in 0.5%, Si(100) in 50%, Si(100) in 5%, and Si(100) in 0.5% showed similar O+ enhancement (from 15 to 50 times). All measurements described above were made at room temperature. Following lh irradiation with SR, the decrease of the oxide peak (- 5%) was observed as shown in fig. 4, where the oxide peak corresponds to photoemission of the Si 2p core-level electron from Si atoms in the oxide layer. The change was smaller than that in ref. [6]. The base pressure of the chamber was 1 x lo-” Torr and that in ref. of [6] was 4 x lo- ” Torr. The recombination
I
I
I
H’
0
I
I
I
I
I
I
1
x10
I
I
15 20 25 30 mass number Fig. 3. Mass spectra of photon-stimulated desorbed ions from 50%HF-treated Si(ll1) surface. Lower trace: before atomic hydrogen exposure. Upper trace: after atomic hydrogen exposure. (H+ peak before hydrogen exposure is higher than that after. Spectra between mass number 11 and 18 are enlarged by a factor 10.) 5
IO
I. Ochiai et al. / PSD from chemically treated Si 10000-
,
I
I
3 .E
I
I
bulk Si
Si4+ a000 -
3 + m c
breaking of the Si-0
bond has oc-
4. Conclusions
Gi
z
subsequent curred.
177
2000 -
i
!z
-2000
I
I
I
I
I
28
30
32
34
36
photoelectron energy (eV)
Fig. 4. Si2p photoelectron spectra (specimens: 50%-HFtreated Si(lll)): (al after atomic hydrogen exposure, (b) after 1 h SR irradiation.
residual gases on the Si surface may be the cause of the small change. After treatment by the hot tungsten pipe without H, gas, the tungsten pipe at room temperature with H, gas, or heating of the specimen to 500°C in H, gas of 1 X 10e6 Torr, the increment of oxygen ion desorption was not observed. These results show that the enhancement is due to the effect of atomic hydrogen exposure. Moreover, the desorbed rate of oxygen ions from a thick, thermally oxidized Si(100) surface (oxide thickness N 50 A> increased by 700 times by the 20 min irradiation of atomic hydrogen at a specimen temperature of 270°C. From the Si2p photoelectron spectrum of the surface without atomic hydrogen exposure, the Si surface dipped in HF seems to have an intermediate oxidation state [5] partly terminated with H and F atoms. The Si-0-Si bonds may be broken by the atomic hydrogen and SI-OH bonds may be formed. The oxygen atom in the Si-OH bond, where the number of bonds attached to Si is one, is thought to be desorbed more easily by SR irradiation than that in the Si-0-Si bond. As mentioned above, OH+ ions were not observed. This suggests that the O-H bond is broken and
The enhancement of O+ PSD ions was observed following atomic hydrogen exposure to a naturally oxidized Si surface which was formed on the HF-treated Si surface. The breaking of the Si-0-Si bond and formation of the Si-OH bond is thought to be a mechanism of the enhancement by the exposure of H atoms. This shows that photon-stimulated desorption is strongly affected by the bonding state at the surface.
Acknowledgements
The authors wish to express their gratitude to Dr. M. Niwano, Dr. N. Miyamoto, Dr. M. Suemitu and Dr. Y. Ishibashi of Tohoku University for valuable discussions and their assistance with the experiment. This work was carried out under the approval of the Photon Factory Program Advisory Committee (Proposal No. 91-005) of the National Laboratory for High Energy Physics. References
ill T. Urisu and H. Kyuragi, J. Vat. Sci. Technol. B 5 (1987) 1436.
l21Y. Akazawa, Y. Utsumi, J. Takahashi and T. Urisu, Appl. Phys. Lett. 57 (1990) 2302; Y. Nara, Y. Sugita, N. Nakayama and T. Itou, J. Vat. Sci. Technol. B 10 (1992) 274. [31 R.A. Rosenberg, F.K. Perkins, D.C. Mancini, G.R. Harp, B.P. Tanner, S. Lee and P.A. Dowben, Appl. Phys. Lett. 58 (1991) 607. [41 M. Itou, T. Harada and T. Kita, Appl. Opt. 28 (1989) 146. El F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi and J.A. Yarmoff, Phys. Rev. B 38 (1988) 6084. l61M. Niwano, H. Katakura, Y. Takakuwa, N. Miyamoto, A. Hiraiwa and K. Yagi, Appl. Phys. Lett. 56 (1990) 1125.
Photon-stimulated
surface science %A: ./.., ‘.‘. ........... ,,..., ......,,.,.,,, ::::::g$g “‘:‘~‘~‘~‘~~~~~ ,,,,
..%. m.. “‘A:.: ........3: ‘,.,‘:.; :.:;,...,.: ‘.“.‘.‘i.‘.‘.‘.‘.‘. .....v.v .............(..._ ..,.. ::.:.:..:..__. ,,, ““.‘.‘.:.::::?::~~::~3l:~~~:~:::~:~.:~~~~ .....,,(,:(,.:...; .....
desorption from chemically treated Si surfaces
Isao Ochiai a, Taro Ogawa a, Yuji Takakuwa b and Kozo Mochiji a Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan b Research Institute of Electrical Communication,
a
Tohoku University, Sendai 980, Japan
Received 31 August 1992; accepted for publication 24 November 1992
Experimental results are presented on the synchrotron-radiation-induced desorption of oxygen ions from HF-treated Si surfaces measured with a quadrupole mass analyzer operated in the pulse-counting mode. The effect of atomic hydrogen exposure was studied. Desorption of H+, Of and F+ ions was observed from HF-treated Si surfaces. The yield of Of ions was increased more than 30-fold by exposure to atomic hydrogen.
1. Introduction
As the dimensions of large-scale integrated circuits approach the nanometer level, semiconductor processes with low temperature, low damage and low contamination are becoming necessary. Recently, synchrotron-excited reactions have been studied actively as new processing techniques, i.e. deposition [1,2], etching [l], epitaxial growth [3] and surface cleaning [4]. Reactions induced by synchrotron radiation are thought to be due to electronic transitions, thereby offering the opportunity to initiate processes which are thermally inaccessible with the added advantage of improved pattern definition due to the shorter wavelength. Photon-stimulated desorption (PSD) is considered as one of important reactions for these applications, especially surface fabrication, because particles are desorbed from the surface. We investigated how PSD characteristics are affected by the bonding state on Si surfaces using HF treatment and atomic hydrogen irradiation.
Laboratory of High Energy Physics. A schematic diagram of the experimental set-up is shown in fig. 1. BL-8A has a monochromator equipped with three mirrors and one varied-space plane grating [4]. Zeroth-order light (hv < 2 keV) emerging from the monochromator was used as a SR irradiation source. The photon flux per 100 mA is about 10’3-‘4 photons/s . cm*. Mass analy-
(G-z?)
Photon Factors at National Lab. for High Energy Physics
Semispherical
_
Quadrupole mass analyzer
2. Experimental
The experiments were carried out using beam line BLSA at the Photon Factory at the National 0039-6028/93/$06.00
Fig. 1. Schematic diagram of the experimental set-up.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
176
I. Ochiai el al. / PSD from chemically treated Si
3. Results and discussion
Fig. 2. Schematic drawing of the atomic hydrogen source.
sis of photon-stimulated desorbed ions under SR irradiation was performed using a quadrupole mass analyzer (QMA) operated in the pulsecounting mode. A filament in the ionization cell of the QMA was switched off. A potential voltage of 20 V was applied to the specimen to extract ions from the specimen surface. Mass assignment was performed by comparison with the spectrum of residual gases. The mass resolution of the present system (AM) was 0.5. The monochromatic light and a spherical-sector electron energy analyzer were used for X-ray photoelectron spectroscopy (XPS). The cross-sectional view of the atomic hydrogen source is shown in fig. 2. By atomization of molecular hydrogen at the hot inner surface (2000°C) of a tungsten pipe, an intense atomic hydrogen beam was obtained. The pipe was heated by electron bombardment. The acceleration voltage was 1500 V and the current was 100 mA. Exposure to the atomic hydrogen beam was typically performed under the condition of a H, filling pressure of 1.0 X lop6 Torr and an exposure time of 180 s without SR irradiation. The flux of atomic hydrogen was estimated to be 3 x 10-l” atoms/s. cm2 from the measured decrease of H, partial pressure and H, influx. The distance between the outlet of the source and specimens was 10 cm. The temperature rise during the exposure was about 20°C. The specimens used here were Si(lll> and Si(100) wafers dipped in aqueous 0.5%, 5%, and 50% HF solution.
From the Si2p photoelectron spectrum of the SK1111 dipped in 50% HF, a native oxide ‘aye,’ was formed and the thickness was about 5 A estimated by the attenuation of bulk Si photoelectrons. The PSD ion mass spectra before and after atomic hydrogen exposure are shown in fig. 3. Desorption of H+, CH+, O+ and F+ ions was primarily detected during SR irradiation. After atomic hydrogen exposure the yield of O+ ions was increased by 36 times, while the change of the yield of H+ and F+ was within a factor of 2. The yield of CHT was decreased. The yield of OH+ ions was negligibly small in both cases. The results in the cases of Si(ll1) dipped in 5% HF, Si(ll1) in 0.5%, Si(100) in 50%, Si(100) in 5%, and Si(100) in 0.5% showed similar O+ enhancement (from 15 to 50 times). All measurements described above were made at room temperature. Following lh irradiation with SR, the decrease of the oxide peak (- 5%) was observed as shown in fig. 4, where the oxide peak corresponds to photoemission of the Si 2p core-level electron from Si atoms in the oxide layer. The change was smaller than that in ref. [6]. The base pressure of the chamber was 1 x lo-” Torr and that in ref. of [6] was 4 x lo- ” Torr. The recombination
I
I
I
H’
0
I
I
I
I
I
I
1
x10
I
I
15 20 25 30 mass number Fig. 3. Mass spectra of photon-stimulated desorbed ions from 50%HF-treated Si(ll1) surface. Lower trace: before atomic hydrogen exposure. Upper trace: after atomic hydrogen exposure. (H+ peak before hydrogen exposure is higher than that after. Spectra between mass number 11 and 18 are enlarged by a factor 10.) 5
IO
I. Ochiai et al. / PSD from chemically treated Si 10000-
,
I
I
3 .E
I
I
bulk Si
Si4+ a000 -
3 + m c
breaking of the Si-0
bond has oc-
4. Conclusions
Gi
z
subsequent curred.
177
2000 -
i
!z
-2000
I
I
I
I
I
28
30
32
34
36
photoelectron energy (eV)
Fig. 4. Si2p photoelectron spectra (specimens: 50%-HFtreated Si(lll)): (al after atomic hydrogen exposure, (b) after 1 h SR irradiation.
residual gases on the Si surface may be the cause of the small change. After treatment by the hot tungsten pipe without H, gas, the tungsten pipe at room temperature with H, gas, or heating of the specimen to 500°C in H, gas of 1 X 10e6 Torr, the increment of oxygen ion desorption was not observed. These results show that the enhancement is due to the effect of atomic hydrogen exposure. Moreover, the desorbed rate of oxygen ions from a thick, thermally oxidized Si(100) surface (oxide thickness N 50 A> increased by 700 times by the 20 min irradiation of atomic hydrogen at a specimen temperature of 270°C. From the Si2p photoelectron spectrum of the surface without atomic hydrogen exposure, the Si surface dipped in HF seems to have an intermediate oxidation state [5] partly terminated with H and F atoms. The Si-0-Si bonds may be broken by the atomic hydrogen and SI-OH bonds may be formed. The oxygen atom in the Si-OH bond, where the number of bonds attached to Si is one, is thought to be desorbed more easily by SR irradiation than that in the Si-0-Si bond. As mentioned above, OH+ ions were not observed. This suggests that the O-H bond is broken and
The enhancement of O+ PSD ions was observed following atomic hydrogen exposure to a naturally oxidized Si surface which was formed on the HF-treated Si surface. The breaking of the Si-0-Si bond and formation of the Si-OH bond is thought to be a mechanism of the enhancement by the exposure of H atoms. This shows that photon-stimulated desorption is strongly affected by the bonding state at the surface.
Acknowledgements
The authors wish to express their gratitude to Dr. M. Niwano, Dr. N. Miyamoto, Dr. M. Suemitu and Dr. Y. Ishibashi of Tohoku University for valuable discussions and their assistance with the experiment. This work was carried out under the approval of the Photon Factory Program Advisory Committee (Proposal No. 91-005) of the National Laboratory for High Energy Physics. References
ill T. Urisu and H. Kyuragi, J. Vat. Sci. Technol. B 5 (1987) 1436.
l21Y. Akazawa, Y. Utsumi, J. Takahashi and T. Urisu, Appl. Phys. Lett. 57 (1990) 2302; Y. Nara, Y. Sugita, N. Nakayama and T. Itou, J. Vat. Sci. Technol. B 10 (1992) 274. [31 R.A. Rosenberg, F.K. Perkins, D.C. Mancini, G.R. Harp, B.P. Tanner, S. Lee and P.A. Dowben, Appl. Phys. Lett. 58 (1991) 607. [41 M. Itou, T. Harada and T. Kita, Appl. Opt. 28 (1989) 146. El F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi and J.A. Yarmoff, Phys. Rev. B 38 (1988) 6084. l61M. Niwano, H. Katakura, Y. Takakuwa, N. Miyamoto, A. Hiraiwa and K. Yagi, Appl. Phys. Lett. 56 (1990) 1125.