In situ IR spectral study of the reaction of a-Si:H:F films with dimethylaluminum hydride

VIBRATIONAL SPECTROSCOPY ELSEVIER

Vibrational Spectroscopy 13 (1996) 107 112

In situ IR spectral study of the reaction of a-Si:H:F films with dimethylaluminum hydride Toshimasa Wadayama *, Yoshihisa Maiwa, Aritada Hatta Department of Materials Science, Faculty of Engineering, Tohoku Unit,ersit~, 980-77 Sendai. Miyagi. Japan

Received 26 April 1996; accepted 23 July 1996

Abstract The reaction of dimethylaluminum hydride (DMAH) with a-Si:H:F films prepared by spontaneous chemical vapor deposition (SCVD) has been studied in situ using polarization modulation IR spectroscopy. It is found that SCVD films before DMAH exposure contain SiF2, SiHzF , and Sill 2 species. Upon exposure to DMAH at a temperature ranging from 373 to 593 K, the IR bands due to SiF2 and SiHzF stretch vibrations decrease in intensity, whereas the intensity of the Sill • stretch band increases. These changes are remarkable at high temperature; in particular the Sill 2 stretch band shifts to lower wavenumber with increasing exposure temperature. These results suggest that DMAH reacts with the SiF~ and SiH,F species to form Sill 2 and Sill 3, We also reveal a temperature dependent correlation between the spectral changes and the deposited amounts of A1, as determined by induced coupled plasma analysis. Kevwords: Polarization modulation; MOCVD; Aluminum; Amorphous silicon; Dimethylaluminum hydride: Reflection absorption

1. Introduction Deposition of metallic films on semiconductor surfaces is one of the most important processes in fabricating large-scale integrated circuits. In the m e t a l - o r g a n i c chemical vapor deposition ( M O C V D ) of AI [1], dimethylaluminum hydride ( D M A H ) is often used as a source gas. O f particular interest in this deposition process is the surface selectivity, i.e., AI can be deposited on metals and H- or F-terminated silicon surfaces but no deposition occurs on silica and silicon nitride [2,3]. This deposition selectivity is obviously an advantageous feature of the M O C V D

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author.

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process in comparison to others including the sputtering process. Some specific surface reactions are suggested to account for that selectivity. However, while a number o f investigations have been performed on the deposition process of Al films [4-12], little attempt has been made to characterize, in situ, the chemical reactions that occur during the deposition process. This is indispensable for an understanding of the mechanism of the selective deposition. Besides, this should provide useful information necessary to achieve a reliable contact between aluminum and silicon. Polarization-modulation IR spectroscopy (PM-IR) [13,14] is one of the powerful methods for observing dynamic behavior of surface vibrational species in the presence of absorbing gas. Indeed, this technique

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T. Wadayama et al. / Vibrational Spectroscopy 13 (1996) 107-112

has recently been used to investigate the reaction of DMAH with photochemically deposited a-Si:H films [15]. The results showed that Sill 3 species present on the topmost surface of the film reacted preferentially with DMAH above 323 K under UV illumination though no reaction took place in the dark even at 553 K. In the present paper, the reaction of DMAH with a-Si:H:F films obtained by spontaneous chemical vapor deposition (SCVD) is investigated using PMIR. In particular, our attention is focused on the IR spectral changes of fluorinated and hydrogenated Si species (SiF 2, SiH2F, and Sill 2) upon exposure to DMAH in the absence of UV irradiation. It will be seen that the absorption bands due to SiF 2 and SiHeF species decrease in intensity by DMAH exposure whereas the band due to Sill 2 increases by that exposure. In harmony with these changes, the deposited amount of A1 increased. These facts reveal that DMAH reacted with SiF 2 and SiH2F resulting in the formation of Sill 2 and Sill 3 along with A1 deposition.

(DMAH). DMAH was admitted into the reaction cell without any carrier gas. The pressure of DMAH was 0.04 Torr. After completion of each in situ IR measurement, deposited aluminum was dissolved in an HC1 aqueous solution and quantified by induced coupled plasma (ICP) analysis.

3. Results and discussion Fig. 1 shows the IR spectral change observed when an a-Si:H:F film was exposed to DMAH at 373 K. In this figure, (A) and (B) concern the stretch modes of the S i - H and S i - F bonds, respectively. For either case, spectrum (a) was recorded prior to DMAH exposure, whereas spectrum (b) was recorded after 180 s DMAH exposure. The spectral difference between (b) and (a) is shown at the bottom. Absorption features at 2200, 2100, 940, and 850 cm -] in spectrum (a) reveal that multiple forms of fluorinated and hydrogenated Si species are incorporated in the resulting a-Si:H:F film. It is already known that a-Si:F films exhibit bands due to S i - F stretch modes in the region of 1050-800

2. Experimental The PM-IR spectrophotometer and the reaction cell used in the present study have been described elsewhere [16,17]. The spontaneous chemical vapor deposition (SCVD) method [18-20] was used to deposit a-Si:H:F films. For this deposition, the reactant gases of Sill 4 and F 2 (diluted to 5% by He) were mixed in front of a specially designed nozzle favoring effective reaction. The flow rates of these gases (SiH4//F 2 = 5//5 ccm) were controlled by mass-flow controllers. The a-Si:H:F films were deposited onto Au films which had been formed on Pyrex glass plates (O 75 × 1.2 mm) by vacuum evaporation. The distance between the nozzle and the Au substrate was 3 cm. During the deposition of a-Si:H:F, the IR windows were purged with N 2 to prevent deposition on their surfaces. In the present work, the total gas pressure was monitored using a capacitance manometer and kept constant at 1.3 Torr during the a-Si:H:F deposition. After 5 min deposition, the substrate was heated in vacuum to a desired temperature and then exposed to dimethylaluminum hydride

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Fig. 1. IR spectral changes of the a-Si:H:F film by DMAH exposure at 373 K in the regions of (A) S i - H stretch and (B) Si-F stretch modes. (a) Before DMAH exposure, (b) after 180 s DMAH exposure, and ( b ) - ( a ) difference spectrum. Each spectrum is shifted vertically to avoid overlapping.

T. Wadayama et al. / Vibrational Spectroscopy 13 (1996) 107-112

cm-1 [21-23]. Shimada et al. [21] have investigated the IR spectra of fluorinated amorphous Si (a-Si:F) films with different contents of fluorine and showed that SiF 3 segments incorporated in the films give rise to bands at 1015 and 838 cm -1 due to the asymmetric and symmetric S i - F stretch modes, respectively, whereas the monofluoride S i - F stretch mode appears at 850 cm -1. They also have shown that paring bands at 965 and 920 cm-1 can be assigned to the split antisymmetric stretch modes of the SiF2-SiF 2 segment and additional paring bands at 870 and 820 c m - ~ to the split symmetric stretch modes, and these four bands have a similar order of magnitude in intensity [21]. These data make it possible to assign the band at 940 cm-1 in Fig. 1B to the antisymmetric stretch of isolated SiF 2 segments in the film and the 850 cm-~ band to the corresponding symmetric stretch mode. In our previous work [24] on a-Si:H:F films deposited under SCVD conditions, we observed two intense bands at 960 and 850 cm -1, and the intensities of these bands increased in parallel with deposition. On the basis of this parallel behavior, both bands were ascribed to the stretch modes of SiF 2 species [24]. As can be seen from Fig. 1B, the bands at 940 and 850 cm-~ show a similar intensity reduction upon DMAH exposure. We thus assign the 940 and 850 cm ~ bands to the antisymmetric and symmetric SiF2 stretch modes, respectively. As will be described later, our a-Si:H:F films contain SiH2F species, and the S i - F stretch mode should be located near 850 cm-1 [21]. Undoubtedly, this band is buried in the strong SiF 2 stretch band. In the S i - H stretching region (Fig. 1A), two bands are located at 2200 and 2100 cm -~ in spectrum (a). The former band position is much higher than those of nonfluorinated Sill, (n ~< 3) incorporated in CVD a-Si:H films [25]. The strong intensities of the SiF 2 stretch bands in Fig. IB indicate that a large amount of F atoms are incorporated in the film. Lucovsky et al. [26] have reported that an S i - H stretch band shifts to higher wavenumber in proportion to the sum of the electronegativities of attached atoms or groups. Since the electronegativity of F (x = 4.0) is much higher than that of H (x = 2.1), the bonding of F atoms on hydrogenated Si in the film should result in a blue shift of the S i - H stretch vibration (so-called inductive effect [27]). Thompson [28] investigated IR spectra of various fluorosilane

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and showed that difluorosilane and monofluorosilane reveal their S i - H bands in the 2240-2250 cm-~ and 2190-2210 cm -~ regions, respectively. Thus, the band at 2200 c m - i in spectrum (a) can be ascribed to S i - H stretching in SiH2F species. It should be noted here that, while SiH2F as well as Sill 2 species is expected to give two bands due to the asymmetric and symmetric Sill 2 stretch modes, they are not differentiated from each other due to their close proximity in frequency. The position of the 2100 cm ~ band observed in (a) of Fig. IA is close to that of isolated Sill 2 (2080 cm -1) and of polymeric (Sill2) n (2100 cm -L) reported for CVD a-Si:H films [25], as well as that of monohydride (Si-H) (around 2080 cm - I ) observed on singlecrystalline Si surfaces [29,30]. In addition, the presence of the inductive effect, as described above, makes it difficult to decide definitely whether the 2100 cm -~ band involves a monohydride or a dihydride species or both. However, we tentatively assign the band to the dihydride stretch mode. Further discussion on this assignment will be made later. As is clearly seen in Fig. l, the DMAH exposure at 373 K results in only a slight decrease in intensity of the SiF~ and SiH2F stretch bands. The Sill 2 stretch band at 2100 cm 1 on the other hand, increases in intensity and shifts to higher wavenumber. As a result of such changes in the Sill 2 band, the peak of the difference spectrum is located at 2120 cm -1 in Fig. 1A. One may surmise that the deposition of A1 on the a-Si:H:F film decreased the IR reflectivity on Au and thereby affected the measured absorption intensities. Assuming a bulk density of 2.7 for A1 to apply, the thickness of AI deposited by 180 s DMAH exposure at 373 K was calculated to be 0.5 nm. The effect of such a thickness upon the observed band intensities was indeed not qualified, it should act on all of the species equaIly. Actually, however, the change in intensity of the Sill 2 stretch band and those of the two SiF 2 stretch bands occurred in an opposite way, as mentioned above. Hence, the effect of the deposited A1 appears to be unessential. Fig. 2 shows the IR spectral change of an a-Si:H:F film observed after 180 s DMAH exposure at 593 K. Spectra (a) and (b) shown in both (A) and (B) were measured before and after that exposure, respectively. For each the difference spectrum ( b ) - (a)

T. Wadayama et al. / Vibrational Spectroscopy 13 (1996) 107-112

110

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was shown at the bottom. It is immediately clear that the bands due to the SiF 2 stretch modes at 593 K before exposure (Fig. 2B(a)) are less intense than those shown in Fig. 1B(a). Additionally, the relative intensities of the SiH2F and Sill 2 bands in Fig. 2A(a) are much different from those in Fig. 1A(a). In the present work, each of the a-Si:H:F films was deposited at 373 K followed by heating in vacuum up to a designated temperature at which DMAH exposure was given. Accordingly, the decrease in absorption intensity of the fluorine-containing species at 593 K probably arose from a partial decomposition of these species during the heating process. It is found from Fig. 2B that upon DMAH exposure two bands become clearly visible at about 900 and 860 cm i (indicated by arrows in the figure) while the SiF 2 bands are reduced in intensity. The 900 cm -~ band is ah'eady observed only weakly before DMAH exposure, whereas the 860 cm -~ band was probably buried in the intense band at 850 cm-J before the exposure. We believe that both of the 900 and 860 c m - ~ bands gained intensities at the expense of the SiF2 band. It should be noted here that the difference spectrum shown in Fig. 2 as well as in Fig. 1 shows only a change in reflectivity caused by the exposure to DMAH, but the change can result from a number of reasons which include

changes in the population of the species and in the band positions due to a change in environmental chemical bonds. These changes give rise to a baseline shift that makes it difficult to draw the base-line in the difference spectrum. Deformation modes of the Sill 2 segments in (SiH2) . chains as well as those of Sill 3 are expected to occur in the region of 950-800 cm-1 whereas Si-H (monohydride) has no deformation mode in the region shown in Fig. 2. Accordingly, it is likely that both the 900 and 860 cm -1 bands are due to the deformation modes of Sill 2 a n d / o r Sill 3 species. These bands gained intensities after DMAH exposure and, simultaneously, the band located at 2100 cm-J also gained intensity with a shift to lower wavenumber (Fig. 2A). Only a subtle structure is observed in the 2140-2130 cm -~ region (Fig. 1A and Fig. 2A) which may arise from the Sill 3 stretch modes and suggests a poor population of Sill 3 species, as would be expected. Thus, the two prominent bands at 900 and 860 cm -~ appeared after exposure (Fig. 2B) are probably due to Sill 2 species. Then it follows that the band at 2100 cm-~ is reasonably assigned to the Sill 2 stretch band [25]. As shown in Fig. 2, the intensity reduction of the SiF2 bands after DMAH exposure was more remarkable at 593 K than at 373 K. Also noticeable in Fig. 2 is that the SiH2F band at 2200 cm -J completely disappeared after the exposure. Moreover, the increment in intensity of the Sill 2 stretch band by DMAH exposure at 593 K ( ( b ) - (a) in Fig. 2A) is about twice as large as that occurred at 373 K (Fig. 1A). These differences are apparently attributable to the increased temperature thereby accelerating the reaction of DMAH with the a-Si:H:F film then allowing a more amount of A1 to deposit on the film surface. It was found from ICP analysis that about 30 nm of A1 in thickness was deposited during the 180 s deposition at 593 K. In what follows, we shall describe in more detail the spectral features varying with exposure temperature. Fig. 3 illustrates the intensities of the Sill 2, SiF2, and SiH2F stretch bands as well as the deposited amounts of Al, as derived as a function of exposure temperature. Also shown in the figure are the temperature-dependent changes of peak position for the Sill 2 stretch band. As previously reported [15], in the absence of UV irradiation, no reaction took place

7". Wadayama et al. / Vibrational Spectroscopy 13 (1996) 107-112 "7, '

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between D M A H and hydrogenated species (Sill 2, SiH~) in photochemically deposited a-Si:H films even at 523 K. For a-Si:H:F films, however, the bands due to the fluorinated Si species (SiF 2, Sill 2F) decreased in intensity upon D M A H exposure in the dark, as shown in Fig. 3B. This fact reveals that the fluorinated species in the a-Si:H:F film is more active than the hydrogenated species in the a-Si:H film. As is seen from Fig. 3A, the Sill 2 stretching band increased in intensity and shifted from 2120 to 2080 c m - ~ as the exposure temperature was changed from 373 to 593 K. As is obvious from Fig. 3C, the deposited amount of A1 increases with increasing exposure temperature. Therefore, the red shift of the Sill + band with temperature must be attributed to the decrease of environmental electronegativities by the reaction with DMAH, whereas the associated enhancement of the band intensity arises from the increasing amount of the Sill 2 species in the film. As Fig. 3B shows, both of the SiF 2 and SiH2F bands decreased in intensity with increasing surface temperature, indicating the promotion of D M A H reaction with the fluorinated species to produce Sill 2 and Sill ~ species. The deposition of A1 may lead to the formation of

111

methyl group on the a-Si:H:F film. Information about the IR bands due to CH 3 group would be helpful for the discussion of the deposition mechanisms. In fact we measured the IR spectra in the C - H stretch region during D M A H exposure, but the intensities of the CH 3 stretch vibrations were too weak to discuss. More elaborate experiments are required to obtain information on the behavior of such a decomposed species. To conclude, in situ PM-IRAS measurements were carried out to investigate the reaction of D M A H with a-Si:H:F films produced by spontaneous chemical deposition. The results reveal that D M A H reacts preferentially with the fluorinated Si species in the film, accompanied by increases of the SiH~ and Sill 3 species.

Acknowledgements One of the authors (T.W.) expresses his cordial thanks to Izumi Science and Technology Foundation for a financial support of this work. This work was partly supported by a Grant-in-Aid (No. 06750712) from the Ministry of Education, Science, and Culture of Japan.

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