Materials Science in Semiconductor Processing 5 (2002) 5–10 Suppression of oxidation-induced stacking fault generation in argon ambient annealing with controlled oxygen and the effect upon bulk defects Toshiharu Suzuki* New Display Device Division, Core Technology Development Center, Sony Corporation Core Technology and Network Company, 4-16-1 Okada, Atsugi-shi, Kanagawa 243-0021, Japan Abstract The effect of oxygen partial pressure during annealing in argon on the suppression of the generation and growth of oxidation-induced stacking faults (OSFs) was investigated by precisely and widely controlling the oxygen partial pressure. Similarly to the case of OSF in the surface region, the generation of OSFs in the deep region of substrate was effectively suppressed by an annealing with the oxygen partial pressure below the critical value of 6 10 3 atm. The formation of precipitation was also decreased throughout the substrate. Mechanism by which the generation of OSFs was suppressed has been attributed to the oxidation mechanism in these oxygen partial pressures. It is suggested that an electric ﬁeld is built across the oxide layer when the oxygen partial pressure is below the critical value. The electric ﬁeld causes silicon interstitials and impurities, such as metals, to drift from the substrate to the outer surface of the oxide. During the annealing the origins of OSFs, such as Si–O clusters and metallic impurities, are eliminated and any mechanical damage is annealed out before climbing to the nuclei of the OSFs. It is suggested that Si interstitials and metallic impurities deep in the substrate are also eliminated by the diffusion, when their concentration at the surface region is decreased. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Oxidation-induced stacking fault; Bulk defect; Suppression of defect; Preliminary annealing in argon; Oxygen concentration; Oxidation mechanism 1. Introduction Stacking faults generated during the oxidation of silicon often decorate metallic impurities at the partial dislocation, and this causes deterioration of the electrical characteristics of silicon devices, such as an increase in the leakage current of metal-oxide–semiconductor (MOS) transistors and the image characteristics of charge-coupled devices (CCDs). In recent MOS LSIs miniaturized beyond 0.18 mm, the thickness of the gate oxide must be scaled down to o3 nm, the thickness at which direct tunneling makes a non-negligible contribution to the leakage current through the gate oxide. The leakage current through defects or impurities in the thin *Fax: +81-46-227-2135. E-mail address: [email protected] (T. Suzuki). oxide, however, can be greater than the pure tunneling current at that regime . The number of defects and impurities that occur in the tunnel oxide of the coming generation of MOS transistors must be minimized. Because these defects and impurities mainly originate from the crystalline defects and impurities of the substrate [2,3], it is essential that the formation of crystalline defects during the oxide growth will be suppressed in the coming generation of MOS technology. The mechanisms responsible for the generation and growth of oxidation-induced stacking faults (OSFs) have been investigated for several decades. The sites of metallic impurities , oxygen precipitates or Si–O clusters [5,6], and mechanical damage [7,8] have been identiﬁed as the origins of OSFs. OSFs grow through the coalescence of silicon interstitials [5,7], which are 1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 4 4 - 6 6 T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10 generated at the oxide–substrate interface during oxidation . Many researchers have attempted to suppress the generation of OSFs by performing a preliminary annealing of the substrate wafers in an inert ambient, such as nitrogen or argon, before oxidizing them [10–12]. This author annealed wafers in an argon ambient that included oxygen and found that the suppression of OSF generation in the surface region was effective only when the oxygen partial pressure was lower than the critical partial pressure of 6 10 3 atm, and that the difference in ability to suppress OSFs between below and above the critical oxygen partial pressure can be attributed to the difference in oxidation mechanisms . The oxidation-related species, such as silicon interstitials, are transported from the oxide– substrate interface to the oxide surface by the electric ﬁeld when oxygen partial pressures are lower than the critical value. This decreases the interstitial concentration at the interface and results in the suppression of the generation and growth of OSFs. When the surface concentration of interstitials is decreased, the concentration of interstitials in the depth of substrate must be lowered by a diffusion mechanism. Thus, the generation of stacking faults in the deep region of a substrate can be suppressed. This article reports the results of an investigation in which the suppression effect of argon annealing with reduced oxygen partial pressure on the generation of OSFs in the depths of the substrate was studied. 2. Experiment A Czochralski-grown silicon crystal doped with boron to a concentration of about 1015 cm 3 was used in the experiment. The crystal was sliced into a (1 0 0) surface orientation and polished to a mirror surface. Final thickness of the wafers was 300 mm. The OSF density on the surface of the wafers was initially examined by oxidizing the wafers at 11001C and was found to be on the orders of 102–105 cm 2. After the wafers were cleaned with a warm mixture of sulfuric acid and nitric acid, they were dipped into diluted hydroﬂuoric acid (5% HF) to remove the chemical oxide that had grown during the acid-mixture treatment. Finally, the wafers were rinsed in deionized water using ultrasonic agitation and then dried. Argon gas was chosen as the annealing ambient in order to avoid the chemical reaction reported to occur when silicon wafers were annealed in nitrogen . A resistance-heating furnace was used for the annealing and oxidation, and the heat-treatment temperature was 11001C. The oxygen partial pressure of the 1-atm annealing ambient was controlled to a value between 10 6 and 10 1 atm. The air tightness of the gasdistribution system was tested by ﬁlling the system with nitrogen to a pressure of 3 kg cm 2; after 48 h there was no reduction in the pressure. Some of the annealed wafers were oxidized in the same furnace by changing the ambient to dry oxygen in order to grow the OSFs to a size visible under an optical microscope. After annealing and oxidation, the oxide was removed using a buffered HF solution. OSFs were delineated using a Secco etch solution  for 1 min. The surface density of the OSFs on the samples was evaluated using an optical microscope. The depth distribution of the stacking faults generated during annealing and oxidation was obtained by repeating the etching and measurement of OSFs. Then, the number of OSFs was differentiated by depth to count the newly delineated OSFs. Defects and precipitates in the whole region of the substrate were observed by X-ray section topography using AgKa1 with asymmetric /4 4 4S reﬂection. 3. Results Annealing at an oxygen partial pressure below 3 10 5 atm resulted in surface roughening in the form of anomalous ﬁlm growth and pitting . The inﬂuence of oxygen partial pressure on OSF generation in the surface region and the deep region of the substrate was investigated by annealing wafers for 30 min under oxygen partial pressures from 3 10 5 to 1 10 1 atm and then oxidizing them for 120 min. Fig. 1 shows the oxygen partial pressure dependence of the OSF density in the surface region observed after the subsequent oxidation. For oxygen partial pressures higher than 6 10 3 atm, the OSF density was high: from 10 to 105 cm 2. In contrast, the OSF density was o20 cm 2 for the wafer annealed with oxygen partial pressure below a critical value of 6 10 3 atm. This shows that annealing in argon with an oxygen partial pressure lower than the critical value, suppresses the generation of OSFs in the surface region. Surface roughening that occurred in the oxygen partial pressure lower than 3 10 5 atm is comprised of SO2 ﬁlm deposition and the formation of square pits with inverted pyramidal shapes and sometimes accompanying the deposition of a ﬁber-like material. This surface roughening has been attributed to the evaporation of SiO before the formation of stable SiO2 in the extremely low oxygen partial pressures. This mechanism was veriﬁed by considering the stagnant region around the substrate . The OSF density in the deep region of the substrate was evaluated by repeating the step etching and measurement of the OSF number. Fig. 2 shows the depth variation of the OSF density newly delineated by every 1.5-mm etching. The measured results for the substrates oxidized for 120 min with and without annealing are also shown in the ﬁgure. For the substrate T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10 7 Fig. 1. Oxygen partial pressure dependence of OSF density observed after annealing wafers for 30 min at 11001C under various oxygen partial pressures and then oxidizing them for 120 min in dry oxygen at the same temperature. Fig. 2. Depth distribution of OSF after oxidation for 120 min with and without preliminary annealing in argon with oxygen of 2 10 3 atm at 11001C for 30 min. OSF densities were measured after every etching by 1.5 mm and differentiated with the depth. annealed with the oxygen partial pressure of 2 10 3 atm was suppressed effectively to a density o40 cm 2 to the depth of 30 mm. This value is almost the same level with the surface OSF density of the annealed substrate in low oxygen partial pressure as shown in Fig. 1. The effect of the annealing on the defect generation in the whole region of a substrate was analyzed by X-ray section topography. In Fig. 3(a)–(c) images of the section topography of an annealed substrate, an annealed and oxidized substrate, and an only oxidized substrate are shown, respectively. The oxidation time after annealing was for 4 h. Topographic image of the annealed substrate, Fig. 3(a) shows striped patterns without any speckles. The topographic image in Fig. 3(c) of the substrate oxidized without preliminary annealing shows many speckles in the region deeper than the socalled denuded zone of about 10 mm. On the contrary, the image in Fig. 3(b), which is the image of annealed and oxidized substrate, shows very few speckles. These speckles are considered to be oxide precipitates [17,18]. Annealing in argon with the oxygen partial pressure lower than the critical value suppresses both the OSF generation and formation of the precipitates in the depth of substrate, effectively, even for an annealing time this short. 4. Discussion without preliminary annealing, the density of OSFs was about 1 103 cm 2, almost the same density as that obtained from the preliminary substrate examination. The generation of the bulk OSFs of the substrate Many researchers have reported that annealing in an inert ambient can suppress the generation of OSFs in one case [11,12], and that surface roughening and the increase in OSF density were induced by a pre-oxidation 8 T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10 Fig. 3. Section topography of silicon wafers: (a) annealed in argon with 2 10 4 atm oxygen for 30 min at 11001C, (b) annealed in argon with 2 10 4 atm for 30 min and oxidized for 4 h at 11001C, and (c) oxidized for 4 h at 11001C. annealing in nitrogen in another case [10,14]. The results of this experiment clariﬁed that the generation of OSFs at the surface region of the substrate is suppressed when the substrates are annealed in argon with an oxygen partial pressure that is lower than the critical value, as shown in Fig. 1. Furthermore, the scarcity of OSFs after subsequent oxidation suggests that argon annealing under oxygen partial pressures lower than the critical value can eliminate the nuclei and embryos of OSFs . The contradictory results reported in previous reports [10–12,14] may have been brought about by the nocontrolled oxygen partial pressure and the nitridation reaction on the surface of the substrates. When the oxygen partial pressure is higher than the critical value, the generation and growth of OSFs are considered to obey the mechanism reported previously [19–21], in which the interstitials generated through the reaction of oxygen and silicon climb to be OSFs. The results of the previous experiments done by this author  shown that the oxidation modes above and below the critical oxygen partial pressure are different. Analysis based on the Cabrera–Mott model  suggested that the drift of the ions of the oxidationrelated species through the electric ﬁeld in the oxide is dominant in the low oxygen partial pressures. This mechanism has also been suggested by Kamigaki et al.  for the oxygen partial pressure of 1 10 3 atm. Furthermore, the oxidation-related species in this region are considered to be the ionized excess silicon in the oxide rather than oxygen. As stated by Cabrera and Mott in Ref. , the oxidation rate is independent of oxygen partial pressure when the oxide grows as a result of the outward movement of the oxidation-related species. This must be the case for the results in the reports [13,24] because of the very weak oxygen partial pressure dependence of oxide thickness. A model, which explains both the oxide growth and the suppression of OSF generation in the surface region at the oxygen partial pressures lower than the critical value, has also been proposed in the previous paper . A fraction of the oxygen adsorbed on the oxide surface is ionized at elevated temperatures. At the same time, the oxidation-related species, such as silicon interstitials, are ionized at the substrate–oxide interface and they drift through the oxide. Then the species are oxidized at the surface region of the oxide resulting in oxide growth. Along with this process, metallic impurities, when there are any in the surface region of substrate, are thought to drift to the oxide surface in the form of ions. The mostly contained metallic impurities in the substrate, such as Fe, Ni, and Cu, are considered to be ionized easily because their ionization energies are smaller than that of silicon . Consequently, the nuclei or the embryos of OSFs are eliminated from the surface region of the substrates. It is supposed that under the conditions in which the silicon interstitials and metallic impurities are sucked from the substrate, the other origins of OSFs, i.e., oxygen precipitates or Si–O clusters and mechanical damage, are annealed out during this annealing before climbing to be OSFs or growing into nuclei of OSFs. Once the nuclei or the embryos are annihilated from the surface region of the substrates, OSFs will hardly be generated during the subsequent oxidation even if the oxidation is for a long time. In this situation, concentrations of the interstitials and metallic impurities in the bulk of the substrate are decreased due to diffusion to the surface region. For the suppression effect of the OSF generation in the bulk region and the decrease of precipitates in the whole wafer, annealing for 30 min is sufﬁcient in this experiment. As the effect reaches a long range, diffusion of some species, which have large diffusion coefﬁcient, may be the mechanism responsible for the suppression of the generation of OSFs and the reduction of precipitates. Diffusion coefﬁcients of the elements related to this system are summarized in Table 1. Diffusion coefﬁcients of the metallic impurities, such as Fe, Ni, and Cu, are very large , and the diffusion length during an annealing for 30 min at 11001C was calculated to be more than 900 mm. This diffusion length T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10 9 Table 1 Diffusion coefﬁcients of relevant species in the oxidation of Si at 11001C Species Diffusion coefﬁcients at 11001C (cm2/s) Diffusion length for 30 min (cm) Ref. Fe Ni Cu Si interstitial O Si interstitial O Si interstitial D5 10 D4 10 5 D1.2 10 5 D3.02 10 7 8 10 11 3.91 10 5 1.5 10 10 2.3 10 9 D9.5 10 D2.7 10 D1.5 10 D2.3 10 3.8 10 27 10 5.2 10 2.0 10 . U. Gosele  . U. Gosele  . U. Gosele  . U. Gosele  . U. Gosele  T. Abe  T. Abe  T. Okino  6 is sufﬁciently large compared to the 300 mm of the substrate thickness. When the metallic impurities at the surface region are removed, those in the depth of the bulk can easily be eliminated. The diffusion length of excess Si during the annealing was calculated to be more than tens of a micrometer [25–27]. Elimination of bulk stacking faults to the depth of 30 mm is probable. The diffusion coefﬁcient of oxygen is very small [25,26] and its diffusion length for 30 min at 11001C was calculated to be about several micrometers. In the X-ray topography image of the annealed and oxidized substrate, the speckles had been decreased in the central region of the substrate, as shown in Fig. 3(b). Thermal diffusion of oxygen contributed little to the decrease in the precipitate density. Since the concentration of the oxygen that was dissolved at the crystal growth step was about 1.7 1018 cm 3, it cannot be decreased through diffusion by this annealing condition to the level at 1.9 1017 cm 3, which is the equilibrium solubility at 11001C . The diffusion of species other than oxygen atoms or some complex of atoms is speculated to be the mechanism responsible for the decrease in precipitates. 5. Conclusion Oxidation-induced stacking faults in silicon substrates, that adversely affect the gate-oxide quality in scaled MOS transistors, were suppressed effectively by preliminary annealing of the wafer before oxidation in argon with oxygen partial pressure less than the critical value at 6 10 3 atm. The suppression of the generation of OSFs was found to be effective deep into the bulk of the substrates. At the same time, the precipitates generated during the long-time oxidation in dry oxygen were also decreased through the preliminary annealing in argon with low oxygen partial pressure. The mechanism of the OSF generation suppression in the low oxygen partial pressure was attributed to the difference of the transport mechanisms of oxidationrelated species from the transport mechanism at higher 2 1 1 2 4 1 4 3 oxygen partial pressures than the critical value. The decrease in OSF in the bulk was also attributed to the decrease in Si interstitials and metallic impurities in the surface region, and diffusion from the bulk to the surface. The decrease in precipitates in the whole region of the substrates could not be interpreted by diffusion of oxygen in the substrate. Acknowledgements The author extends his immense thanks to Dr. Y. Hayashi for his encouragement to this work. The authors also thank Mr. H. Mori for his support to this work. 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