Impact of Rapid Thermal Oxidation at Ultrahigh-Temperatures on Oxygen Precipitation Behavior in Czochralski-Silicon Crystals II

Oxygen precipitates (OPs) in a Czochralski silicon (Cz-Si) wafer can degrade the performance of semiconductor devices if they are in the active region of the Si wafer, i.e., the surface layer. On the other hand, they can improve device fabrication yield through their ability to enhance impurity gettering and through mechanical strengthening, both of which are important characteristics for semiconductor device fabrication. 1,2 Thus, the OPs must be controlled in an appropriate manner depending on the structure of the intended semiconductor devices and the production process employed. More precise and uniform control of OPs in Si wafers will be a significant factor for future advanced semiconductor devices. To understand this very important issue, we previously investigated the re-formation effect of OP nuclei using ultrahigh-temperature rapid thermal oxidation (RTO), at over 1300 ◦ C, and achieved wide and precise controllability of new OP nuclei. 3 This technique also demonstrated a remarkable ability to eliminate heterogeneity effects such as OP nuclei or related defects in the grown crystal. Ultrahigh-temperature annealing using rapid thermal processing (RTP) has significant advantages in terms of excellent temperature uniformity in the radial direction of the Si wafer as compared to the Cz-Si crystal growth process. As mentioned above, ultrahigh-temperature RTO showed a remarkable ability to control the effects of the OP nuclei. The behavior of OP nuclei in Cz-Si crystals is strongly related to point defects such as vacancies and Si interstitials. 4 Vacancies promote the formation of OP nuclei because these nuclei are formed as a complex between oxygen atoms and vacancies. It is generally known that annealing in an oxygen atmosphere is not a practical way to increase vacancy concentration because Si interstitials are dominant in the Si wafer due to the injection of Si interstitials from the oxidized surface. However, as we previously reported, 3 the behavior of OP nuclei can be controlled by changing the dominant point defects from Si interstitials to vacancies depending on the difference of each thermal equilibrium concentration at ultrahigh temperatures, even though the oxidation atmosphere is used for RTP. Furthermore, in the case of ultrahigh-temperature RTO, it is expected that Si interstitials also exist at high concentration due to the Si surface oxidation even though the dominant point defects are vacancies. Interstitial Si point defects are expected to annihilate vacancy-related defects such as void defects when enough interstitial Si atoms exist. 5 The void defects also influence the semiconductor device performance; therefore, in addition to OP control, annihilating void defects is a very important goal. It is well known that RTP in a nitrogen or argon atmosphere does not effectively annihilate void defects under the surface of the Si wafer. 6‐8 Ultrahigh-temperature RTO would gain a further advantage over RTP in a nitrogen or argon atmosphere if annihilation of void defects is confirmed. In this study, the annihilation behaviors of void defects in the case of the ultrahigh-temperature RTO were evaluated in detail.

Oxygen precipitates in Czochralski Silicon (Cz-Si) wafers effectively act as getter sites for heavy metal impurities in semiconductor devices, 1,2 and they also increase the mechanical strength of the wafer by precipitation hardening. 3 Both these roles are important for stable device manufacturing, but oxygen precipitates are also responsible for decreasing the mechanical strength of the wafers if their size and density are not appropriately controlled. 4 In addition, when oxygen precipitates remain in the device formation region, they can result in failure due to current leakage. 5 For this reason, oxygen precipitation must sometimes be suppressed depending on the kind, structure, and process conditions of a semiconductor device. In particular, because of current advances in device structures such as scaling and threedimensional chip integration, precise control of oxygen precipitation will become more significant as the stress induced in Si wafers during device fabrication becomes increasingly crucial. [6][7][8] Therefore, an effective method that can be used for either promoting or suppressing oxygen precipitation in Cz-Si crystals is needed.
Many studies have been performed on methods for achieving such precise control of oxygen precipitation. However, maintaining the stability of the growing crystal both in the pulling and radial directions still remains difficult. Thus, both local and widespread nonuniformity often exists in the grown crystal. 9,10 To address this very important problem, we have proposed rapid thermal oxidation (RTO) at ultrahigh temperature (ultrahigh-temperature RTO). 11 Our results obviously demonstrated that the oxygen precipitates generated during the crystal growth were dissipated entirely, and dense oxygen precipitate nuclei were formed uniformly in the radial direction during RTO at temperatures over 1350 • C. We believed that the newly formed oxygen precipitates in the wafers that had been subjected to ultrahigh-temperature RTO were closely related to preserved vacancies. In particular, each oxygen precipitate nucleus is thought to consist of an oxygen-vacancy complex (for example, O 2 V). 12 Ultrahigh-temperature annealing using rapid thermal processing (RTP) has great advantages in terms of uniformity of the temperature in the radial direction as compared to the Cz-Si crystal growth process. In the near future, in order to establish more accurate control of the oxygen precipitation, further investigation on ultrahigh-temperature RTO will be necessary. We have already reported that new, very dense and uniformly distributed oxygen precipitates are nucleated when Si wafers are cooled down relatively quickly, for example, at over 25 • C/s, in the RTO process. 11 Depending on the kind of semiconductor device, however, the nucleation should sometimes be properly suppressed so that perfect crystallinity is obtained in the entire bulk Cz-Si wafer, especially for discrete devices, since the oxygen precipitates deteriorate device performance, as mentioned before. When the cooling rate z E-mail: Koji_Araki@sas-globalwafers.co.jp in used the RTO process is lower, oxygen precipitate nucleation can be suppressed even for Cz-Si wafers with high oxygen concentrations (which make the formation of oxygen precipitates easier), since the vacancy concentration (C v ) is lower because of vacancy out-diffusion during the slow cooling. This means that it will be possible to fabricate a Si wafer with no oxygen precipitates even if it has a higher oxygen concentration and is thus mechanically well strengthened because it has sufficient interstitial oxygen atoms. In summary, oxygen precipitation could be either promoted or suppressed by ultrahigh-temperature RTO on wafers made from the same Cz-Si single crystal. In this paper, we demonstrate in detail such suppression of oxygen precipitation in Cz-Si wafers subjected to ultrahigh-temperature RTO.
Several 300-mm-diameter nitrogen-doped Cz-Si (100) wafers were cut from single Cz-Si crystals. The oxygen concentration (according to the old American Society for Testing and Materials (ASTM) standards), nitrogen concentration, and resistivity of the boron-doped wafers were 1.24-1.30 × 10 18 cm −3 , 1.9-2.1 × 10 14 cm −3 , and 26.4-27.1 · cm, respectively. The reason we chose high oxygen concentration range Cz-Si wafers in recent device fabrication is for better understanding of oxygen precipitation behavior. A commercially available RTP unit was used at 1375 • C for 15 s to anneal the wafers in a pure O 2 atmosphere in order to introduce point defects. The cooling rates to 600 • C were 5 • C/s and 120 • C/s. After RTP, these wafers were annealed at 780 • C for 3 h followed by 1000 • C for 16 h (two-step annealing) in a pure O 2 atmosphere using a commercially available vertical batch furnace to grow the oxygen precipitate.
The density of oxygen precipitates at wafer depths of 7-380 μm was evaluated by IR tomography (Raytex MO-441). The detection limits for size and density were 25 nm and 1 × 10 6 cm −3 , respectively.
The relation between the RTO conditions and the radial distribution of oxygen precipitates in the wafers is shown in Fig. 1. Typical IR tomography images of these samples are also shown in Fig. 2. The oxygen precipitate density in the sample produced without RTO treatment decreased at approximately 110-145 mm from the wafer center, as shown in Fig. 1. This corresponds to OSF normally stands for Oxide-induced Stacking Fault ring (so called OSF ring) that originated from large silicon oxide precipitate nucleation during the crystal pulling. Generally, a vacancy-rich region exists inside the OSF ring. 13 This nonuniformity of the oxygen precipitate size is assumed to be caused by the OSF ring, because the vacancy concentration in the OSF ring area is lower than that in other vacancy-rich areas.
It should be noted that no oxygen precipitation was observed along the radial direction of the wafer subjected to ultrahigh-temperature RTO with a cooling rate of 5 • C/s, as shown in Fig. 1. The density was obviously below the detection limit at all measurement points. This is again clearly demonstrated in Fig. 2. Therefore, the oxygen precipitates that already existed in the wafer before RTO (formed during crystal growth) must have been completely dissolved during the ultrahigh-  temperature RTO treatment, and almost all new nucleation during the ultrahigh-temperature RTO process was suppressed. This suppression of oxygen precipitation during ultrahigh-temperature RTO was extremely strong compared to that observed for the RTO at below 1300 • C reported previously (for example, Fig. 2(a) in Ref. 11). On the other hand, the oxygen precipitates were uniformly distributed in the radial direction after ultrahigh-temperature RTO with a cooling rate of 120 • C/s. As in our previous experiment, 11 very dense and very fine oxygen precipitate nuclei appear in the bulk at each wafer position, as shown in Fig. 2. The advantage of this ultrahigh-temperature RTO is not only to dissolve the oxygen precipitates generated in the crystal growth process but also to form new precipitates with excellent uniformity in the radial direction of the Si wafer.
As we reported in the former paper, 11 C v − C i (where C i is the concentration of Si interstitials) is the key parameter determining whether oxygen precipitation proceeds during successive thermal treatment after RTO. 11,14 Therefore, to obtain a good understanding of the oxygen precipitate behavior in this RTO experiment, a C v − C i depth profile was estimated by numerical simulation using the same technique as in references 11, 15, and 16 for the conditions used in this study. The calculated C v − C i depth profiles are shown in Fig. 3. When the cooling rate in the ultrahigh-temperature RTO process is 120 • C/s, the calculated C v − C i in the bulk is over 9 × 10 12 cm −3 which was assumed as the critical value for the formation of oxygen precipitates in our previous study. 11 Oxygen precipitate nuclei are considered to have formed effectively because the C v was high for a cooling rate 120 • C/s. Furthermore, it was confirmed that oxygen precipitates can form with cooling rates as low as 25 • C/s in our previous study. 11 In contrast, ultrahigh-temperature RTO with a cooling rate 5 • C/s led to a calculated C v − C i value lower than the critical value throughout the bulk, as shown in Fig. 3. Although vacancies are slightly more abundant than interstitial Si (C v − C i > 0) in the Si wafer, they are thought to be insufficient to form O 2 V during the ultrahigh-temperature RTO. Decreasing the cooling rate from 120 • C/s to 5 • C/s clearly decreases C v − C i greatly, which qualitatively agrees with the experimental observation. Thus, the formation of oxygen precipitate nuclei was effectively suppressed by lowering the vacancy concentration by reducing the cooling rate in ultrahigh-temperature RTO.
We demonstrated that reducing the cooling rate in ultrahightemperature RTO suppresses oxygen precipitation in Cz-Si wafers with diameters of 300 mm. Ultrahigh-temperature RTO with a cooling rate 5 • C/s provided a wafer with no oxygen precipitates even though it had a relatively high oxygen concentration. This is thought to be due to the greatly reduced vacancy concentration in the wafer. Thus, our results clearly demonstrate that ultrahigh-temperature RTO can effectively control the oxygen precipitation, not only leading to the formation of uniform oxygen precipitates at high cooling rates but also suppressing oxygen precipitation completely in vacancy-rich Cz-Si wafer at low cooling rates, so that the fundamental heterogeneity of crystals caused by intrinsic point defects and the related growth of defects are removed.