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Mechanism of polishing of SiO2 films by CeO2 particles
Journal of Non-Crystalline Solids 283 (2001) 129±136
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Mechanism of polishing of SiO2 ®lms by CeO2 particles Tetsuya Hoshino *, Yasushi Kurata, Yuuki Terasaki, Kenzo Susa Hitachi Chemical Co. Ltd., Wadai 48, Tsukuba-shi, Ibaraki 300-4247, Japan Received 17 April 2000; received in revised form 19 December 2000
Abstract To examine the polishing mechanism in chemical mechanical polishing of a thermally grown SiO2 ®lm by CeO2 particles, the surface structure of the ®lm and the polishing waste were investigated by various analytical means. Fourier-transformed-infrared-attenuated-totra indicated that the ®lm surface structure was strained as a result of a reaction with CeO2 . A small amount of Si was found by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) in the waste supernatant and it was detected as particles by optical interference measurement. The scanning electronic microscopy (SEM) image of the particles showed a not-well-de®ned shape like cotton scraps, but their IR transmission spectrum resembled that of the SiO2 ®lm. From these results we concluded that the SiO2 ®lm surface is ®rst reacted with CeO2 particles and a multiple number of chemical bondings of Si±O±Ce are formed on the surface. Then mechanical tearing of Si±O±Si bonds leads to the SiO2 removal as a lump instead of Si OH4 monomer, and the lump is released from the CeO2 particles downstream. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 68.60.Bs; 81.05.Kf; 81.65.Ps
1. Introduction Ceria has been commonly used as an abrasive for glass polishing. It has the fastest polishing rate among those of SnO2 , TiO2 , ZrO2 , Cr2 O3 , Al2 O3 , Y2 O3 , La2 O3 and so on [1]. Since CeO2 is softer than SiO2 [2], it in¯icts less damage such as scratches on the SiO2 ®lm than the other particles mentioned. Recently, ceria slurry has also been used in the ®eld of semiconductor CMP (chemical mechanical polishing) process for planarization of intermetal dielectrics [3±5], and good polishing * Corresponding author. Tel.: +81-298 64 4000; fax: +81-298 64 4008. E-mail addresses: [email protected] (T. Hoshino), [email protected] (Y. Kurata), [email protected] (Y. Terasaki), [email protected] (K. Susa).
performance has been demonstrated. To improve the polishing performance it is important to understand how the polishing of the SiO2 ®lm with ceria slurry takes place. So the purpose of this paper is to study the polishing mechanism in SiO2 ®lms by CeO2 particles. The CeO2 particles feature softness and high polishing rate. If mechanical polishing is dominant, soft particles should give a slower polishing rate, but the fact is reversed. So, there should be a large contribution from chemical reactivity in the polishing mechanism. Polishing mechanisms have been studied in the ®eld of optical glass [1,6,7] for more than 40 years, as reviewed by Izumitani [6] and Holland [7]. And also in the ®eld of semiconductor the mechanisms have been studied [8,9]. However, the chemical contribution has not been identi®ed in these studies [10].
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 3 6 4 - 7
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T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
Lee [1] reviewed the polishing mechanism of SiO2 with CeO2 and proposed the most probable mechanism in the sense of CMP. He thought that SiO2 ®lm and CeO2 particles would make a Ce±O± Si bonding and Ce±O±Si OH3 must have been removed from the surface. Then the dissolution of Si OH4 follows. To con®rm this, we paid attention to the surface structure of the SiO2 ®lm and the dissolved species of Si(OH4 in the polishing waste. If Ce±O±Si bonding is formed during the polishing, the surface structure of SiO2 must be distorted, which may be detected by IR spectroscopy. And if Si OH4 has dissolved in the waste, it must be found by chemical analysis.
2. Experiment As Fig. 1 shows, CMP was carried out on a wafer polisher with a polyurethane pad. During the polishing experiment, the wafer was mounted on a template assembly of 6 in. diameter. The polishing pressure was controlled by a dead weight scheme. The down force pressure was kept at 16 kPa and the polisher was rotated at 30 rpm. A thermally grown SiO2 ®lm on silicon substrate was used as the sample. The SiO2 ®lm was made from Si which was oxidized by hot water vapor at 1000°C. The abrasive slurry was made of CeO2 particles dispersed in water with a surfactant. The particles used had a mean diameter of
Fig. 1. The scheme of polishing of SiO2 ®lm by the polyurethane pad with abrasive salary.
about 80 nm with an impurity level of less than 10 ppm. An organic polymer-type surfactant (0.0091 wt%) was added to get a stable suspension or a slurry of proper pH. The concentration of CeO2 was about 1 wt%. The pH value of the slurry was ®xed at 8 to get the most stable suspension. We con®rmed through optical microscope observations combined with a particle counter that the polishing caused few scratches larger than 0.2 lm on the surface of SiO2 ®lms. At ®rst, we checked how the surface of SiO2 ®lm had changed after the polishing. The ®lm thickness was measured by non-contact spectrore¯ectometry. The surface structure was measured by the Fourier-transformed-infrared-attenuated total re¯ectance (FT-IR-ATR). The experimental setup was made of a 60° germanium ATR plate 90 15 5 mm). The plate geometry provided nine re¯ections at an incident angle of 60°. Two specimens cut from the thermally oxidized SiO2 ®lm were attached to both sides of the ATR plate. The thickness of the ®lm was 1500 and 400 nm before and after polishing, respectively. If needed, the polished ®lm was treated in acid for 15 min with sonication. Then the ®lm was washed in water for 15 min with sonication. Next, we checked the composition of the waste. After the polishing, the waste was centrifuged at 4 104 Gal for 1 h. To eliminate the majority of CeO2 particles in the waste, the supernatant was analyzed for composition and particle size distribution. The concentration of Ce and Si was measured by the induced coupled plasma atomic emission spectroscopy (ICP-AES). The particle size distribution was measured by optical interference measurement using a Coulter counter. When we calculated the size distribution, we assumed the particle to be spherical. The supernatant of the waste was further checked for SiO2 solid component. The SiO2 component was extracted by the following concentration process. The supernatant was ®rst placed in a plastic vessel and was evacuated through a cold trap and was rotated at 20°C until the reconcentration ratio reaches as much as 100 times. The vessel was kept in the ambient atmosphere for one week to get a white precipitate deposited on the wall of the vessel. The precipitate
T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
was subjected to analysis by IR, SEM and XMA (X-ray micro-analysis). IR spectra were recorded with a spectral resolution of 4 cm 1 and using 64 scans. For transmission IR, the samples were dispersed in powdered KBr. SEM pictures were taken and XMA spectra were measured. The samples were platinum-coated for SEM and XMA to get clear SEM images at high resolution.
3. Results 3.1. The change in surface structure of SiO2 ®lm To see whether or not polishing causes structural change on surface of SiO2 ®lms, we used FT-IR-ATR. The results are shown in Fig. 2. The spectrum had changed after polishing as in
131
Fig. 2(b) compared with 2(a), but the change was restored by washing the surface in an acidic solution (conc: HNO3 /30%H2 O2 1/1 by volume) as in Fig. 2(c). There are two peaks indicated by arrows near 1100 cm 1 , which belong to asymmetric stretching modes. These two peaks result from one peak, which splits into two peaks due to the polarized light. The peak of the upper wavenumber is the LO mode and the lower is the TO mode [11±14]. This splitting width was changed by polishing, with a blue shift of 57 cm 1 for the LO mode and a red shift of 5 cm 1 for the TO mode. However, when we washed the polished SiO2 ®lm with acid, the peaks returned to the original positions, indicating that the surface was once modi®ed by CeO2 but restored by the post-acid treatment. 3.2. Analysis of polishing waste To seek out the SiO2 component removed from the ®lm surface, we investigated the contents of the polishing waste. The pH of the waste is 7.4, which is not far from the pH of the slurry. After the centrifugation of the waste, the supernatant was found to have Ce 3:7 0:7 ppm and Si 2:9 0:1 ppm by ICP-AES. The particle size distribution of the supernatant is shown in Fig. 3. The peak of 80 nm is assigned to CeO2 and that of 600 nm to SiO2 . The CeO2 in the original polishing slurry has a mean particle diameter of 80 nm, which does not change even after polishing. Besides, if CeO2 par-
Fig. 2. Changes in FT-IR-ATR spectrum of SiO2 ®lm: (a) before polishing; (b) after polishing and; (c) washed with HNO3 =H2 O2 after polishing.
Fig. 3. Particle size distribution of waste.
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ticles had aggregated to have a mean diameter of 600 nm, they should have been carried down during the centrifugation, faster than the individual CeO2 particles. So the new peak at the diameter of 600 nm must be assigned to SiO2 particles. If such large particles were scraped, it should have caused much damage on the SiO2 ®lm. But no actual damage was observed. So the particles must have grown larger due to aggregation. It is noted that total weight of Ce particles estimated from the peak area is as much as that of SiO2 , which agrees with the results of ICP-AES. By the way, why did such large particles of SiO2 stay in the waste? The image of SiO2 was observed in the next section.
isolated. The peak at 1100 cm 1 is rather weak [17]. On the other hand, the SiO2 gel made from Si OCH2 CH3 4 has a broader peak of the SiO stretching mode at 1070 cm 1 and a Si±O±Si bending mode at 450 cm 1 [18]. Thus, the spectral similarity teaches us that the SiO2 particles in the waste and the SiO2 ®lm have a similar structure. While in Fig. 5(a), the SiOH peak seems to appear in addition to the SiO2 peaks. Moreover, the surfactant peak appears in the range 1400± 1800 cm 1 . Thus, aggregation may be caused by SiOH and/or the surfactant.
3.3. Characterization of particles in waste
4. Discussion
For a more detailed study we conducted further analysis on the solid component in the supernatant of the waste by SEM with XMA and FT-IR. The sample for SEM observation was a white lump of more than 2 mg, prepared from the waste volume of 500 ml in agreement with the results from ICPAES. The SEM observation results are shown in Fig. 4. As in Fig. 4(a) SiO2 particles show a notwell-de®ned shape like `cotton scraps'. On the other hand, CeO2 particles, as in Fig. 4(b), were round and as large as 100 nm. The XMA results are shown in Fig. 4(c) and (d). The corresponding parts analyzed are designated by the cross-hatch in Fig. 4(a) and (b). It is clear that the cotton scrap is Si-rich and the round particle is Ce-rich. The shape of round particles is another good identi®cation of the ceria particles because the shape of the ceria particles used in the polishing slurry has been round. The FT-IR absorption spectroscopy of the white precipitate is shown in Fig. 5(a). The peak of CeO2 is not seen here. The spectrum of the SiO2 ®lm is also shown in Fig. 5(b). The results are also shown in Table 1. It is already known that the spectrum of SiO2 ®lms diers in accordance with the kind of ®lms [15,16]. When SiO2 particles are made from a solution, for example, prepared from the hydrolysis and condensation reaction of Si OCH2 CH3 4 , its spectrum has a broad and intense peak for SiOH at 1400 cm 1 , indicating that the hydroxyl group is
4.1. Interpretation of analytical results The data from FT-IR-ATR show that the SiO2 peak shifts after polishing and restores after washing with HNO3 =H2 O2 aqueous solution. The similar peak shift is also observed when the SiO2 ®lm on Si wafer becomes thinner [11,12,19]. They calculated the shift theoretically [13,14], and concluded that the distortion of the SiO2 network structure of thin layer neighboring Si layer caused the shift. Moreover, Naoki et al. observed the similar distortion, experimentally [15]. Some other possible reasons for the shift by distortion in the glass network could be due to the densi®cation of silica or the formation of dangling bond in Si atom, chemical adsorption of surfactant on the ®lm, or the formation of Ce±O±Si bonding. Judging from the experimental results on the retention of SiO2 IR peak shift after washing with acid, the most plausible cause for the shift should be the formation of Ce±O±Si bonding in this particular case. This is because H2 O2 =HNO3 may dissolve CeO2 , but cannot destroy Si±O±Si bonding or dissolve organic molecules of the surfactant. If the peak shift is due to the formation of Ce±O± Si bonding on the surface, these IR data will support the proposal that Ce±O±Si bonding is formed during the polishing process. On the other hand, the ratio of SiO2 to CeO2 in the polishing waste supernatant is much the same as the concentration ratio of Si to Ce as con®rmed
T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
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Fig. 4. SEM and XMA of particles in waste: (a) SEM of SiO2 particles; (b) SEM of CeO2 particles. The cross-hatched area is analyzed by XMA; (c) XMA of SiO2 particles; (d) XMA of CeO2 particles.
by the particle size distribution analysis and ICPAES. If a SiO2 ®lm is removed in the form of Si(OH4 as Lee proposed [1], the analysis ®gure for Si should have been much bigger than Ce. This
means that the amount of Si OH4 is much less than that of the SiO2 particle. So, Si should have existed not as a molecule of Si OH4 , but as a lump of SiO2 .
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Fig. 5. FT-IR absorption spectroscopy of a SiO2 ®lm and cotton scraps: (a) polished SiO2 ®lm and (b) cotton scraps.
Moreover, the SiO2 particle had almost the same chemical structure as the SiO2 ®lm. This means that SiO2 particles did not change the structure and were much bigger than Si OH4 or its oligomer. From the results of SEM inspection, the SiO2 rich paste had a not-well-de®ned shape. But as the paste looked like a root, SiO2 particles were possibly scrubbed and taken from the CeO2 particle. The size of SiO2 was as large as 600 nm, which was much larger than the scratch in the polished surface of the SiO2 ®lm. So the particles should have
aggregated. An experiment to measure surface charges and the SiO2 particle size was performed [17]. Generally, the SiO2 particles are dispersed as the SiO2 surface is negatively charged [9]. However, the SiO2 particles of 10 nm diameter were not charged very much [17]. So, they easily aggregated, probably because their surface charges were low. One might argue that SiO2 is made from Si OH4 in the waste by a condensation reaction. There is an equilibrium between SiO2 ®lm and Si OH4 . In fact, the saturated concentration of SiO2 in water of pH 8 is as much as 100 ppm [1]. This means that if the concentration of Si is less than 100 ppm, solid SiO2 cannot exist in water. While the results of ICP gave 3 ppm for the Si component and yet solid SiO2 was observed in the waste. One of the reasons why Si did not saturate in the waste is that it takes a long time for Si to reach equilibrium [20]. There is little possibility that SiO2 particles were made from Si(OH)4 by chemical mechanical interaction between CeO2 and SiO2 , because the area where CeO2 particles contacted with SiO2 ®lms was small and Si OH4 was of very dilute species. Thermodynamically, Si OH4 is more stable than SiO2 particles in the concentration. So as a conclusion, the ®lm did not dissolve but was scraped in the shape of a lump. Furthermore, the obser-
Fig. 6. The polishing mechanism by Lee (after [1]).
Table 1 FT-IR peak position (cm 1 ) and assignment Assignment
SiO2 ®lms
Silica in the waste
Si±O±Si wagging Defect between Si and SiO2 Si±O±Si bending Si±OH Si±O±Si assymmetric stretching OH stretching of H2 O
462 610 812 No peak 1094 No peak
472 No peak 803 970 1111 3446
T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
135
Fig. 7. The polishing mechanism used in the present work.
vation of few scratches on the wafer after polishing suggested that the white lump of SiO2 was not made by scratching but by wearing.
®lm surface, followed by the release of a SiO2 particle from the CeO2 particles downstream.
4.2. Polishing mechanism
Acknowledgements
Lee's model tells us that Si OH4 is released from the reacted surface of CeO2 particle which is removed, in advance, from the SiO2 ®lm as shown in Fig. 6. By contrast, through the discussion of the previous section, a new model for polishing may be proposed as shown in Fig. 7. At ®rst, multiple number of bondings between a CeO2 particle and a SiO2 ®lm are formed. Then, the CeO2 particle with SiO2 lump is removed from the SiO2 ®lm, and then the SiO2 lump instead of Si OH4 monomer is released from the surface of the CeO2 particle. We believe that SiO2 is removed as a lump, which is scraped from the surface. It is then dispersed into the waste. So, the polishing rate is affected by the formation rate of Ce±O±Si bonding and the cleaving rate of Si±O±Si. The cleaving of Si±O±Si is controlled not only by chemical depolymerization but also by mechanical tearing.
We thank Dr Munsok Kim for his intensive discussion on the IR spectrum. Thanks are also to Dr Masato Yoshida, Mr Yo-ichi Machii, Mr Toranosuke Ashizawa, Mr Jun Matsuzawa, Mr Hiroto Otsuki, Mr Kiyohito Tan-no and Mr Tsuyoshi Sakurada for their help in preparing CeO2 slurry and polishing performance evaluations.
5. Conclusions Analysis of the waste and the surface of polished wafer, using SEM, XMA, FT-IR-ATR and transmission IR, leads to the following conclusions. First, the SiO2 ®lm reacts with CeO2 particles, and then the cleaving of Si±O±Si takes place to remove SiO2 lumps with CeO2 particles from the
References [1] L.M. Cook, J. Non-Cryst. Solids 120 (1990) 152. [2] G.V. Samsonov, The Oxide Handbook, 2nd Ed., IPI/ Plenum Data, New York, 1982, p. 192. [3] H. Nojo, M. Kodera, R. Nakata, IEEE 96 (1996) 349. [4] K. Susa, in: A Workshop on CMP, Lake Placid, NY, August, 1997. [5] T. Ashizawa, in: A Workshop on CMP, Lake Placid, NY, August, 1999. [6] T. Izumitani, in: M. Tomozawa, R. Doremus (Eds.), Treatise on Material Science and Technology, vol. 17, Academic Press, New York, 1979, p. 115. [7] L. Holland, in: The Properties of Glass Surfaces, Chapman and Hall, London, 1964. [8] W. Bensch, W. Bergholz, Semicond. Sci. Technol. 5 (1990) 421. [9] Y. Hayashi, M. Sakurai, T. Nakajima, K. Hayashi, S. Sasaki, S. Chikaki, T. Kunio, Jpn. J. Appl. Phys. 34 (1995) 1037. [10] F. Sugimoto, H. Horie, Y. Arimoto, T. Ito, Jpn. J. Appl. Phys. 34 (1995) 30. [11] J.A. Trogolo, K. Rajan, J. Mater. Sci. 29 (19) (1994) 4554. [12] J.E. Olsen, F. Shimura, J. Appl. Phys. 53 (20) (1988) 1934. [13] C.H. Bjorkman, T. Yamazaki, S. Miyazaki, M. Hirose, J. Appl. Phys. 77 (1) (1995) 313.
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[14] J.A. Moreno, B. Garido, J. Samitier, J.R. Morante, J. Appl. Phys. 81 (4) (1997) 1933. [15] N. Yasuda, S. Takagi, A. Toriumi, Appl. Surf. Sci. 117 (1997) 216. [16] P. Lange, J. Appl. Phys. 66 (1) (1989) 201. [17] M. Mitsui, A. Ichiki, M. Horio, H. Kamiya, Character of dispersion and aggregation and surface state of a silica ®ne
particle in solution ± A seminar of Jouhou-GijutsuKyoukai on ultra-®ne particles of silica, 1994, p. 54. [18] M. Yamane, S. Okano, Yogyo-Kyokai-Shi 8 (1979) 87. [19] G.J. Pietsch, Y.J. Chabal, G.S. Higashi, J. Appl. Phys. 78 (3) (1995) 1650. [20] A. Helebrant, I. Pekarkova, Ber. Bunsenges. Phys. Chem. 100 (9) (1996) 1519.
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Mechanism of polishing of SiO2 ®lms by CeO2 particles Tetsuya Hoshino *, Yasushi Kurata, Yuuki Terasaki, Kenzo Susa Hitachi Chemical Co. Ltd., Wadai 48, Tsukuba-shi, Ibaraki 300-4247, Japan Received 17 April 2000; received in revised form 19 December 2000
Abstract To examine the polishing mechanism in chemical mechanical polishing of a thermally grown SiO2 ®lm by CeO2 particles, the surface structure of the ®lm and the polishing waste were investigated by various analytical means. Fourier-transformed-infrared-attenuated-totra indicated that the ®lm surface structure was strained as a result of a reaction with CeO2 . A small amount of Si was found by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) in the waste supernatant and it was detected as particles by optical interference measurement. The scanning electronic microscopy (SEM) image of the particles showed a not-well-de®ned shape like cotton scraps, but their IR transmission spectrum resembled that of the SiO2 ®lm. From these results we concluded that the SiO2 ®lm surface is ®rst reacted with CeO2 particles and a multiple number of chemical bondings of Si±O±Ce are formed on the surface. Then mechanical tearing of Si±O±Si bonds leads to the SiO2 removal as a lump instead of Si OH4 monomer, and the lump is released from the CeO2 particles downstream. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 68.60.Bs; 81.05.Kf; 81.65.Ps
1. Introduction Ceria has been commonly used as an abrasive for glass polishing. It has the fastest polishing rate among those of SnO2 , TiO2 , ZrO2 , Cr2 O3 , Al2 O3 , Y2 O3 , La2 O3 and so on [1]. Since CeO2 is softer than SiO2 [2], it in¯icts less damage such as scratches on the SiO2 ®lm than the other particles mentioned. Recently, ceria slurry has also been used in the ®eld of semiconductor CMP (chemical mechanical polishing) process for planarization of intermetal dielectrics [3±5], and good polishing * Corresponding author. Tel.: +81-298 64 4000; fax: +81-298 64 4008. E-mail addresses: [email protected] (T. Hoshino), [email protected] (Y. Kurata), [email protected] (Y. Terasaki), [email protected] (K. Susa).
performance has been demonstrated. To improve the polishing performance it is important to understand how the polishing of the SiO2 ®lm with ceria slurry takes place. So the purpose of this paper is to study the polishing mechanism in SiO2 ®lms by CeO2 particles. The CeO2 particles feature softness and high polishing rate. If mechanical polishing is dominant, soft particles should give a slower polishing rate, but the fact is reversed. So, there should be a large contribution from chemical reactivity in the polishing mechanism. Polishing mechanisms have been studied in the ®eld of optical glass [1,6,7] for more than 40 years, as reviewed by Izumitani [6] and Holland [7]. And also in the ®eld of semiconductor the mechanisms have been studied [8,9]. However, the chemical contribution has not been identi®ed in these studies [10].
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 3 6 4 - 7
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T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
Lee [1] reviewed the polishing mechanism of SiO2 with CeO2 and proposed the most probable mechanism in the sense of CMP. He thought that SiO2 ®lm and CeO2 particles would make a Ce±O± Si bonding and Ce±O±Si OH3 must have been removed from the surface. Then the dissolution of Si OH4 follows. To con®rm this, we paid attention to the surface structure of the SiO2 ®lm and the dissolved species of Si(OH4 in the polishing waste. If Ce±O±Si bonding is formed during the polishing, the surface structure of SiO2 must be distorted, which may be detected by IR spectroscopy. And if Si OH4 has dissolved in the waste, it must be found by chemical analysis.
2. Experiment As Fig. 1 shows, CMP was carried out on a wafer polisher with a polyurethane pad. During the polishing experiment, the wafer was mounted on a template assembly of 6 in. diameter. The polishing pressure was controlled by a dead weight scheme. The down force pressure was kept at 16 kPa and the polisher was rotated at 30 rpm. A thermally grown SiO2 ®lm on silicon substrate was used as the sample. The SiO2 ®lm was made from Si which was oxidized by hot water vapor at 1000°C. The abrasive slurry was made of CeO2 particles dispersed in water with a surfactant. The particles used had a mean diameter of
Fig. 1. The scheme of polishing of SiO2 ®lm by the polyurethane pad with abrasive salary.
about 80 nm with an impurity level of less than 10 ppm. An organic polymer-type surfactant (0.0091 wt%) was added to get a stable suspension or a slurry of proper pH. The concentration of CeO2 was about 1 wt%. The pH value of the slurry was ®xed at 8 to get the most stable suspension. We con®rmed through optical microscope observations combined with a particle counter that the polishing caused few scratches larger than 0.2 lm on the surface of SiO2 ®lms. At ®rst, we checked how the surface of SiO2 ®lm had changed after the polishing. The ®lm thickness was measured by non-contact spectrore¯ectometry. The surface structure was measured by the Fourier-transformed-infrared-attenuated total re¯ectance (FT-IR-ATR). The experimental setup was made of a 60° germanium ATR plate 90 15 5 mm). The plate geometry provided nine re¯ections at an incident angle of 60°. Two specimens cut from the thermally oxidized SiO2 ®lm were attached to both sides of the ATR plate. The thickness of the ®lm was 1500 and 400 nm before and after polishing, respectively. If needed, the polished ®lm was treated in acid for 15 min with sonication. Then the ®lm was washed in water for 15 min with sonication. Next, we checked the composition of the waste. After the polishing, the waste was centrifuged at 4 104 Gal for 1 h. To eliminate the majority of CeO2 particles in the waste, the supernatant was analyzed for composition and particle size distribution. The concentration of Ce and Si was measured by the induced coupled plasma atomic emission spectroscopy (ICP-AES). The particle size distribution was measured by optical interference measurement using a Coulter counter. When we calculated the size distribution, we assumed the particle to be spherical. The supernatant of the waste was further checked for SiO2 solid component. The SiO2 component was extracted by the following concentration process. The supernatant was ®rst placed in a plastic vessel and was evacuated through a cold trap and was rotated at 20°C until the reconcentration ratio reaches as much as 100 times. The vessel was kept in the ambient atmosphere for one week to get a white precipitate deposited on the wall of the vessel. The precipitate
T. Hoshino et al. / Journal of Non-Crystalline Solids 283 (2001) 129±136
was subjected to analysis by IR, SEM and XMA (X-ray micro-analysis). IR spectra were recorded with a spectral resolution of 4 cm 1 and using 64 scans. For transmission IR, the samples were dispersed in powdered KBr. SEM pictures were taken and XMA spectra were measured. The samples were platinum-coated for SEM and XMA to get clear SEM images at high resolution.
3. Results 3.1. The change in surface structure of SiO2 ®lm To see whether or not polishing causes structural change on surface of SiO2 ®lms, we used FT-IR-ATR. The results are shown in Fig. 2. The spectrum had changed after polishing as in
131
Fig. 2(b) compared with 2(a), but the change was restored by washing the surface in an acidic solution (conc: HNO3 /30%H2 O2 1/1 by volume) as in Fig. 2(c). There are two peaks indicated by arrows near 1100 cm 1 , which belong to asymmetric stretching modes. These two peaks result from one peak, which splits into two peaks due to the polarized light. The peak of the upper wavenumber is the LO mode and the lower is the TO mode [11±14]. This splitting width was changed by polishing, with a blue shift of 57 cm 1 for the LO mode and a red shift of 5 cm 1 for the TO mode. However, when we washed the polished SiO2 ®lm with acid, the peaks returned to the original positions, indicating that the surface was once modi®ed by CeO2 but restored by the post-acid treatment. 3.2. Analysis of polishing waste To seek out the SiO2 component removed from the ®lm surface, we investigated the contents of the polishing waste. The pH of the waste is 7.4, which is not far from the pH of the slurry. After the centrifugation of the waste, the supernatant was found to have Ce 3:7 0:7 ppm and Si 2:9 0:1 ppm by ICP-AES. The particle size distribution of the supernatant is shown in Fig. 3. The peak of 80 nm is assigned to CeO2 and that of 600 nm to SiO2 . The CeO2 in the original polishing slurry has a mean particle diameter of 80 nm, which does not change even after polishing. Besides, if CeO2 par-
Fig. 2. Changes in FT-IR-ATR spectrum of SiO2 ®lm: (a) before polishing; (b) after polishing and; (c) washed with HNO3 =H2 O2 after polishing.
Fig. 3. Particle size distribution of waste.
132
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ticles had aggregated to have a mean diameter of 600 nm, they should have been carried down during the centrifugation, faster than the individual CeO2 particles. So the new peak at the diameter of 600 nm must be assigned to SiO2 particles. If such large particles were scraped, it should have caused much damage on the SiO2 ®lm. But no actual damage was observed. So the particles must have grown larger due to aggregation. It is noted that total weight of Ce particles estimated from the peak area is as much as that of SiO2 , which agrees with the results of ICP-AES. By the way, why did such large particles of SiO2 stay in the waste? The image of SiO2 was observed in the next section.
isolated. The peak at 1100 cm 1 is rather weak [17]. On the other hand, the SiO2 gel made from Si OCH2 CH3 4 has a broader peak of the SiO stretching mode at 1070 cm 1 and a Si±O±Si bending mode at 450 cm 1 [18]. Thus, the spectral similarity teaches us that the SiO2 particles in the waste and the SiO2 ®lm have a similar structure. While in Fig. 5(a), the SiOH peak seems to appear in addition to the SiO2 peaks. Moreover, the surfactant peak appears in the range 1400± 1800 cm 1 . Thus, aggregation may be caused by SiOH and/or the surfactant.
3.3. Characterization of particles in waste
4. Discussion
For a more detailed study we conducted further analysis on the solid component in the supernatant of the waste by SEM with XMA and FT-IR. The sample for SEM observation was a white lump of more than 2 mg, prepared from the waste volume of 500 ml in agreement with the results from ICPAES. The SEM observation results are shown in Fig. 4. As in Fig. 4(a) SiO2 particles show a notwell-de®ned shape like `cotton scraps'. On the other hand, CeO2 particles, as in Fig. 4(b), were round and as large as 100 nm. The XMA results are shown in Fig. 4(c) and (d). The corresponding parts analyzed are designated by the cross-hatch in Fig. 4(a) and (b). It is clear that the cotton scrap is Si-rich and the round particle is Ce-rich. The shape of round particles is another good identi®cation of the ceria particles because the shape of the ceria particles used in the polishing slurry has been round. The FT-IR absorption spectroscopy of the white precipitate is shown in Fig. 5(a). The peak of CeO2 is not seen here. The spectrum of the SiO2 ®lm is also shown in Fig. 5(b). The results are also shown in Table 1. It is already known that the spectrum of SiO2 ®lms diers in accordance with the kind of ®lms [15,16]. When SiO2 particles are made from a solution, for example, prepared from the hydrolysis and condensation reaction of Si OCH2 CH3 4 , its spectrum has a broad and intense peak for SiOH at 1400 cm 1 , indicating that the hydroxyl group is
4.1. Interpretation of analytical results The data from FT-IR-ATR show that the SiO2 peak shifts after polishing and restores after washing with HNO3 =H2 O2 aqueous solution. The similar peak shift is also observed when the SiO2 ®lm on Si wafer becomes thinner [11,12,19]. They calculated the shift theoretically [13,14], and concluded that the distortion of the SiO2 network structure of thin layer neighboring Si layer caused the shift. Moreover, Naoki et al. observed the similar distortion, experimentally [15]. Some other possible reasons for the shift by distortion in the glass network could be due to the densi®cation of silica or the formation of dangling bond in Si atom, chemical adsorption of surfactant on the ®lm, or the formation of Ce±O±Si bonding. Judging from the experimental results on the retention of SiO2 IR peak shift after washing with acid, the most plausible cause for the shift should be the formation of Ce±O±Si bonding in this particular case. This is because H2 O2 =HNO3 may dissolve CeO2 , but cannot destroy Si±O±Si bonding or dissolve organic molecules of the surfactant. If the peak shift is due to the formation of Ce±O± Si bonding on the surface, these IR data will support the proposal that Ce±O±Si bonding is formed during the polishing process. On the other hand, the ratio of SiO2 to CeO2 in the polishing waste supernatant is much the same as the concentration ratio of Si to Ce as con®rmed
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Fig. 4. SEM and XMA of particles in waste: (a) SEM of SiO2 particles; (b) SEM of CeO2 particles. The cross-hatched area is analyzed by XMA; (c) XMA of SiO2 particles; (d) XMA of CeO2 particles.
by the particle size distribution analysis and ICPAES. If a SiO2 ®lm is removed in the form of Si(OH4 as Lee proposed [1], the analysis ®gure for Si should have been much bigger than Ce. This
means that the amount of Si OH4 is much less than that of the SiO2 particle. So, Si should have existed not as a molecule of Si OH4 , but as a lump of SiO2 .
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Fig. 5. FT-IR absorption spectroscopy of a SiO2 ®lm and cotton scraps: (a) polished SiO2 ®lm and (b) cotton scraps.
Moreover, the SiO2 particle had almost the same chemical structure as the SiO2 ®lm. This means that SiO2 particles did not change the structure and were much bigger than Si OH4 or its oligomer. From the results of SEM inspection, the SiO2 rich paste had a not-well-de®ned shape. But as the paste looked like a root, SiO2 particles were possibly scrubbed and taken from the CeO2 particle. The size of SiO2 was as large as 600 nm, which was much larger than the scratch in the polished surface of the SiO2 ®lm. So the particles should have
aggregated. An experiment to measure surface charges and the SiO2 particle size was performed [17]. Generally, the SiO2 particles are dispersed as the SiO2 surface is negatively charged [9]. However, the SiO2 particles of 10 nm diameter were not charged very much [17]. So, they easily aggregated, probably because their surface charges were low. One might argue that SiO2 is made from Si OH4 in the waste by a condensation reaction. There is an equilibrium between SiO2 ®lm and Si OH4 . In fact, the saturated concentration of SiO2 in water of pH 8 is as much as 100 ppm [1]. This means that if the concentration of Si is less than 100 ppm, solid SiO2 cannot exist in water. While the results of ICP gave 3 ppm for the Si component and yet solid SiO2 was observed in the waste. One of the reasons why Si did not saturate in the waste is that it takes a long time for Si to reach equilibrium [20]. There is little possibility that SiO2 particles were made from Si(OH)4 by chemical mechanical interaction between CeO2 and SiO2 , because the area where CeO2 particles contacted with SiO2 ®lms was small and Si OH4 was of very dilute species. Thermodynamically, Si OH4 is more stable than SiO2 particles in the concentration. So as a conclusion, the ®lm did not dissolve but was scraped in the shape of a lump. Furthermore, the obser-
Fig. 6. The polishing mechanism by Lee (after [1]).
Table 1 FT-IR peak position (cm 1 ) and assignment Assignment
SiO2 ®lms
Silica in the waste
Si±O±Si wagging Defect between Si and SiO2 Si±O±Si bending Si±OH Si±O±Si assymmetric stretching OH stretching of H2 O
462 610 812 No peak 1094 No peak
472 No peak 803 970 1111 3446
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135
Fig. 7. The polishing mechanism used in the present work.
vation of few scratches on the wafer after polishing suggested that the white lump of SiO2 was not made by scratching but by wearing.
®lm surface, followed by the release of a SiO2 particle from the CeO2 particles downstream.
4.2. Polishing mechanism
Acknowledgements
Lee's model tells us that Si OH4 is released from the reacted surface of CeO2 particle which is removed, in advance, from the SiO2 ®lm as shown in Fig. 6. By contrast, through the discussion of the previous section, a new model for polishing may be proposed as shown in Fig. 7. At ®rst, multiple number of bondings between a CeO2 particle and a SiO2 ®lm are formed. Then, the CeO2 particle with SiO2 lump is removed from the SiO2 ®lm, and then the SiO2 lump instead of Si OH4 monomer is released from the surface of the CeO2 particle. We believe that SiO2 is removed as a lump, which is scraped from the surface. It is then dispersed into the waste. So, the polishing rate is affected by the formation rate of Ce±O±Si bonding and the cleaving rate of Si±O±Si. The cleaving of Si±O±Si is controlled not only by chemical depolymerization but also by mechanical tearing.
We thank Dr Munsok Kim for his intensive discussion on the IR spectrum. Thanks are also to Dr Masato Yoshida, Mr Yo-ichi Machii, Mr Toranosuke Ashizawa, Mr Jun Matsuzawa, Mr Hiroto Otsuki, Mr Kiyohito Tan-no and Mr Tsuyoshi Sakurada for their help in preparing CeO2 slurry and polishing performance evaluations.
5. Conclusions Analysis of the waste and the surface of polished wafer, using SEM, XMA, FT-IR-ATR and transmission IR, leads to the following conclusions. First, the SiO2 ®lm reacts with CeO2 particles, and then the cleaving of Si±O±Si takes place to remove SiO2 lumps with CeO2 particles from the
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