X-ray photoelectron spectroscopy study of adsorption and photodissociation of dimethylaluminum hydride

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Applied Surface Science 79/80 (1994) 444-448

X-ray photoelectron spectroscopy study of adsorption and photodissociation of dimethylaluminum hydride Masahiro Okawa, Hiroo Tsuruta 1, Mitsugu H a n a b u s a * Department of Electrical and Electronic Engineering, Toyohashi Unicersity of Technology, Tenpaku, Toyohashi 44l, Japan (Received 13 October 1993; accepted for publication 28 December 1993)

Abstract

Adsorption of dimethylaluminum hydride on silicon oxide was detected by the appearance of AI and C signals in X-ray photoelectron spectroscopy (XPS) spectra at room temperature and at 200°C. In addition, the effect of UV irradiation by a deuterium lamp was studied. A change of the C/A1 atomic ratio was found. A marked time dependence of the XPS signals was observed for adsorbates, which was attributed to decomposition induced by the irradiation of the A1 Kc~ line (1487 eV) used for the XPS measurements.

I. Introduction

In photochemical surface reactions the adsorption of source-gas molecules on the substrate surface, as well as subsequent photodissociation of adsorbates, plays a key role for film growth. In the present experiment we used X-ray photoelectron spectroscopy (XPS) to study the adsorption of dimethylaluminum hydride ( D M A H ) on silicon oxide surfaces and the photodissociation of the adsorbate thus formed. D M A H is a source gas suitable for chemical vapor deposition (CVD) of aluminum films because of the low level of carbon contamination [1]. In addition, D M A H was found to be suitable for photochemical vapor deposition (photo-CVD) of aluminum films owing to the strong absorption

* Corresponding author. Fax: (+ 81) 532 48 3422. Present address: Toshiba Corporation, 72 Horikawa-cho, Saiwai-ku, Kawasaki-sbi 210, Japan.

of UV light at wavelengths shorter than 275 nm [2]. We have conducted a series of experiments for p h o t o - C V D of aluminum using D M A H and a deuterium lamp or an ArF laser as the U V light source. With the help of UV light, deposition occurred at substrate temperatures lower than 230°C which is the t e m p e r a t u r e required in normal CVD [3]. Also, in CVD aluminum could not be deposited on silicon oxide surfaces [4], while such a restriction was removed in photo-CVD [5]. We mostly carried out the experiments at low D M A H gas pressures so that surface reactions dominated over gas-phase reactions [6]. Surface reactions are recently regarded as of increasing importance in view of applications such as selective-area deposition and layer-by-layer deposition. The experimental results obtained so far were explained by the two-step model consisting of adsorption of D M A H and subsequent photodissociation of adsorbates by UV light [7]. To support this model we conducted some XPS stud-

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M. Okawa et aL /Applied Surface Science 79/80 (1994) 444-448

ies and detected adsorbates on silicon and silicon nitride [5,8]. Also, the effect induced by ArF laser irradiation on the adsorbate formed on silicon at room t e m p e r a t u r e was studied by XPS [8]. Adsorption should be sensitively influenced by surface conditions, and the results obtained previously with other substrates are not necessarily applicable to explain photo-CVD results obtained for silicon oxide. Therefore, in this experiment we chose silicon oxide. The surface was subjected to U V irradiation supplied by a deuterium lamp to study any photo-induced effect. In addition to r o o m - t e m p e r a t u r e observations, the XPS work was carried out at a substrate t e m p e r a t u r e of 200°C, at which photo-CVD work is often carried out. In XPS measurements caution is advised regarding possible changes of signals induced by X-ray irradiation [9]. Such a phenomenon, which is observed as time-dependent spectrum, presents a serious problem in practical measurements. We encountered such a problem in the present study. This finding imposed both positive and negative impacts on the present work. In the first place, because of the time dependence slow scanning of the electron energy or repeated XPS measurements could not be carried out, both of which are required to improve the spectral profile, hidden under the noise, for the weak signals observed from adsorbates. On the other hand, this finding allowed us to study the photodissociation of adsorbates not only by UV light generated by the deuterium lamp in accordance with p h o t o - C V D , but also by monochromatic X-rays at a much shorter wavelength. The X-ray study is related to the recent interest in synchrotron-radiation-induced phenomena. Considering this background we expanded the scope of the present experiment to include some systematic studies on the time-dependent XPS signals.

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pressure of 1 × 10-10 Torr. It was connected to a reaction chamber via a gate valve, which was evacuated by a turbomolecular p u m p to a base pressure of 1 × 10 .8 Torr. The chamber was originally designed for the use in photodeposition, so adsorption and photodissociation could be carried out exactly in the same manner as deposition. The substrate used in the present experiment was a silicon wafer covered with a 200-nm thick silicon oxide layer formed thermally in an oxygen atmosphere at 1200°C. The carbon contamination on the surface was cleaned by a UV-ozone cleaning method, i.e. by exposure to U V light from the deuterium lamp at 200°C in air. The substrate was heated by an electric heater. D M A H was provided to the surface through a nozzle placed nearby. The D M A H gas pressure was measured by an ionization gauge located on the wall of the reaction chamber. Therefore, the actual pressure on the substrate surface was expected to be much higher than indicated by the gauge. The b e a m from the deuterium lamp was focused by a CaF 2 lens to a b e a m spot size of about 5 m m in diameter with a power density of about 140 W / c m 2 for the vacuum ultraviolet below 180 nm. The surface, exposed to D M A H , was then irradiated by the deuterium lamp whereby the sample was displaced in six steps and illuminated for 1 min at each position to cover the whole area of the substrate. This kind of irradiation was good enough to deposit A1 films in photo-CVD. The XPS spectra were taken using a VSW Scientific Instruments C L 1 0 0 M l l spectrometer with an A1 K a source (1487 eV). The slit was adjusted to 4 × 10 mm, and the resolution was 1.3 eV full-width at half-maximum. To enhance the spectra from adsorbates, the take-off or polar angle was set at 30 ° .

3. Results and discussion 2. Experimental The ultrahigh-vacuum system used for the present XPS experiment was p u m p e d by an ion p u m p and a titanium sublimation p u m p to a base

Adsorption of D M A H on silicon oxide at room temperature is demonstrated by the XPS spectrum shown in Fig. la. The D M A H was dosed at a pressure of 2 × 10 -4 Torr for 500 s (105 L). In addition to Si 2s and 2p, and O 2s signals, the

M. Okawa et al. /Applied Surface Science 7 9 / 8 0 (1994) 444*48

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spectrum contains weak A1 2s and 2p, and C ls signals, indicating the presence of adsorbates. We measured the A1 and C signals in more detail by slow energy scanning. The A1 2p signal is shown in Fig. 2a. It was weak, and the profile was obscured by superposed noises. The normal procedure taken to improve the spectra was not followed because of the change of the spectra during measurement, as explained earlier. On the other hand the area under the signals was counted more reliably, and the C / A 1 atomic

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Binding Energy(eV) Fig. 2. AI 2p signals observed after (a) dosing 105 L of D M A H at room temperature, (b) dosing 100 L at 200°C, (e) dosing followed by U V irradiation by a deuterium lamp at room temperature, and (d) at 200°C.

ratio obtained from the area integration, after counting the atomic sensitivity factor of 0.11 for A1 2p, and 0.21 for C ls [8], was roughly 1.5, indicating chemisorption on silicon oxide even at room temperature. The m e a s u r e m e n t was repeated at 200°C, where deposition was often carried out. The spectrum is shown in Fig. 2b. The D M A H gas pressure was 1 × 10 -6 T o r r with a dosing period of 100 s (100 L). The signals observed at 200°C were weaker than those at room temperature, which could not be improved by increasing the amount of dosing by several times. However, it was clear that the peak of the A1 2p signal was shifted from 75.2 eV at room temperature to 76.1 eV at 200°C. Also the C / A 1 ration was reduced to 1.1. Therefore, the adsorbate formed at 200°C is different from that formed at room temperature. Photodissociation of the adsorbate at room t e m p e r a t u r e and 200°C was observed, as shown in Figs. 2c and 2d, respectively. No major change was observed for A 2p signals after UV irradiation at both room t e m p e r a t u r e and 200°C. The C / A I atomic ratio decreased upon irradiation to 1.1 and 0.8 at room t e m p e r a t u r e and 200°C, respectively. The observed reduced ratio indicates that some change of the adsorbate occurred during irradiation. Note that metallic AI has a peak at 72.7 eV, as indicated in Fig. 2 by arrows, which is located outside the observed signals. This is reasonable because, even if D M A H is decomposed completely, AI reacts with the oxygen embedded in the underlying layer to form an A 1 - O bond. Since the signal-to-noise ration for the AI 2p signal observed at room t e m p e r a t u r e after irradiation happened to be good as a lucky artifact, it could be resolved into three peaks, as indicated in Fig. 2c. The peaks are centered at 74.0, 75.2, and 76.4 eV. These peaks could not be matched with those cited for D M A H adsorbates formed on Si(100) [8] and for triisobutylaluminum on Si(100) with and without native oxide [10]. This result is quite natural because the bonds formed by the adsorbate change sensitively, depending on the surface states with respect to the original molecule.

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M. Okawa et al. / Applied Surface Science 79/80 (1994) 444-448

During the measurements we noticed that the XPS spectra changed noticeably while the X-ray source was kept on. Fig. 3 demonstrates such a change which took place in the spectrum shown in Fig. 1 after 290 min. The peak showing the C ls signal was buried under noises. The height of the original C ls peak was reduced to half in about 55 min. Fig. 4 shows the change of the A1 2p signals as a function of time. The peak which was observed after 30 min at 75.1 eV shifted to 75.9 eV after 115 min. In between (after 48 min) the signal showed the two peaks observed at the beginning and at the end. However, the area under the Al 2p signals remained unchanged during this measurement. Therefore, desorption can be ruled out to explain the change of the spectra including the reduction of the C ls peak height. In fact, the C ls signal became weak because it is spread over a wider energy range. From these observations it was concluded that the adsorbate formed on silicon oxide was dissociated by X-ray absorption. Note that the changes occurred only during X-ray irradiation. If the irradiation ceased, the change in spectra stopped during that period, and upon resumption of X-ray irradiation the change of spectra occurred again. The presently observed effect was induced by monochromatic 1487 eV photons in the AI K a line. Uesugi and Nishiyama observed photodeposition of aluminum on silicon oxide using synSi2p

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Binding Energy(eV) Fig. 4. AI 2p signals during different stages of X-ray irradiation after (a) 30 min, (b) 48 rain, and (c) 115 min. chrotron radiation with photon energy below 300 eV [11]. A recent study indicated that D M A H was decomposed by photons with an energy less than 80 eV via excitation of valence electrons [12]. The present result suggests that excitation of core electrons induced by photons with a much higher energy may induce photodissociation of D M A H as well. However, there is another possibility for decomposition of D M A H caused by the secondary electrons created by the X-ray irradiation. To clarify this point, further investigations are under way. Finally, an interesting fact was observed concerning the UV irradiation effect through the time-dependent XPS observation. Namely, the X-ray-induced effect was not so much pronounced for adsorbates irradiated by the deuterium lamp as for those without irradiation. This is another proof that the adsorbate has been changed by UV irradiation from the deuterium lamp.

4. Conclusion

Binding Energy(eV) Fig. 3. Spectrum observed from the same system as in Fig. 1 after X-ray irradiation for 290 min.

Adsorption of D M A H on silicon oxide and subsequent photodissociation of adsorbates were

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M. Okawa et al. /Applied Surface Science 79/80 (1994") 444*48

studied by XPS. The substrate temperature was varied between room temperature and 200°C. Adsorption was clearly observed by the presence of A1 2p and C ls signals in the XPS spectra. The peak of the AI signals shifted appreciably at 200°C. Under irradiation by the deuterium lamp the C/A1 ratio decreased, but the A1 2p signal peak position remained unchanged. We observed time-dependent XPS spectra of adsorbates formed on silicon oxide. This was attributed to an X-ray-induced effect. It was less pronounced for the adsorbate irradiated before by UV light from the deuterium lamp. Because of the time dependence, signals could not be accumulated to improve the signal-to-noise ratio for weak signals originating from the adsorbates. On the other hand, the result suggested the possibility of decomposition of adsorbates on silicon oxide by excitation of core electrons.

Acknowledgements We thank Tetsuya Shimada and Katsunori Maeda for technical assistance. This work was supported by a Grant-in-Aid on Priority-Area Research on "Photo-Excited Process", sponsored

by the Ministry of Education, Science and Culture, Japan.

References [1] R. Bhat, M.A. Koza, C.C. Chang and S.A. Schwarz. J. Cryst. Growth 77 (1986) 7. [2] T. Cacouris, G. Scelsi, P. Shaw, R. Scarmozzino, R.M. Osgood and R.R. Krchnavek, Appl. Phys. Lett. 52 (1988) 1865. [3] M. Hanabusa, A. Oikawa and P.Y. Cai, J. Appl. Phys. 6a (1989) 3268; 67 (199(I) 3208 [erratum]. [4] K. Tsubouchi and K. Masu, J. Vac. Sci. Technol. A 10 (1992) 856. [5] M. Hanabusa, H. Ouchi, K. Ishida, M. Kawasaki and S. Shogen, Mater. Res. Soc. Symp. Proc. 236 (1992) 85. [6] M. Hanabusa and M. lkeda, Appl. Organometal. Chem. 5 (19911 289. [7] T. Kawai and M. Hanabusa, Jpn. J. Appl. Phys. 32 (1993) 4690. [8] M. Ohashi, S. Shogen. M. Kawasaki and M. Hanabusa, J. Appl. Phys. 73 (1993) 3549. [9] D. Briggs and J.C. Rivi~re. in: Practical Surface Analysis, 2nd ed., Vol. 1, Eds. D. Briggs and M.P. Seah (Wiley. Chichester, 1990) pp. 133-134. [10] D.A. Mantell, Appl. Phys. Lett. 53 (19881 1387. [11] F. Uesugi and I. Nishiyama, IEICE Trans. Electron. E76-C (1993) 47. [12] F. Uesugi and I. Nishiyama, Appl. Surf. Sci. 79/80 (1994) 203.