Figure 1: Increased small-size defects for wafers treated in DHF and dried with and without IPA.

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1 Solid State Phenomena Online: ISSN: , Vol. 195, pp doi: / Trans Tech Publications, Switzerland Influence of ammonia gas ambient in IPA drying process of the single wafer cleaning system Yoshiya Hagimoto 1, a, Hayato Iwamoto 1, b, Yosuke Kawabuchi 2,c, and Teruomi Minami 2,d 1 Sony Corporation, Asahi-cho, Atsugi-shi, Kanagawa, Japan 2 Tokyo Electron Kyushu Limited, 1-1 Fukuhara, Koshi City, Kumamoto Japan a Yoshiya.Hagimoto@jp.sony.com, b Hayato.Iwamoto@jp.sony.com, c Yosuke.Kawabuchi@tel.com, d Teruomi.Minami@tel.com Keywords: IPA (isopropyl alcohol), dry, NH 3, ammonia gas, single wafer cleaning system, defect, chamber ambient Introduction High-performance drying techniques using IPA (isopropyl alcohol) are widely used in the silicon wafer cleaning process. IPA-based drying techniques help prevent the formation of watermarks because they effectively displace any water remaining on a wafer surface. They are thus frequently used in the single wafer cleaning system for advanced devices in which ultra-clean process performance is required. However, as devices are becoming physically smaller, the formation of extremely small defects during cleaning has become a serious problem. It is therefore important to elucidate the mechanism of the defect formation and to take measures to prevent it for future device technologies in which small-size defects can be killer defects during production. In this paper, we performed experiments focused on the process chamber atmosphere in IPA drying of the single wafer cleaning system and describe the mechanism of the defect formation. Experimental Figure 1 shows the number of defects on 300-mm silicon wafers treated in dilute hydrofluoric acid solutions (DHF) to completely remove native oxides and dried under different conditions using the single wafer cleaning system. One sample was dried with IPA and the other without IPA. As shown in Fig. 1, the number of small-size defects on the wafer dried with IPA is larger than that without IPA (spin dry). Previous studies [1, 2] have shown that the atmosphere in the cleaning process chamber has a significant effect on defect formation on wafer surfaces, so we evaluated the relationship between defect formation and process chamber atmosphere during IPA drying of the single wafer cleaning system. Figure 1: Increased small-size defects for wafers treated in DHF and dried with and without IPA. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,04:30:15)

2 232 Ultra Clean Processing of Semiconductor Surfaces XI Experimental procedure. 300-mm silicon wafers were treated in DHF solutions to remove native oxides and dried using IPA in the single wafer cleaning system. The number of defects on samples after the treatment was measured using a wafer surface inspection system. The size of the defects inspected was larger than 60-nm. The atmosphere in the process chamber was collected using an impinger system and quantitatively analyzed by ion chromatography. Ionic components quantitatively analyzed were F, Cl, NO 2, Br, NO 3, SO 4, PO 4, NH 3, Li, Na, K, Mg, and Ca. Figure 2: Relationship between defect formation and ionic concentration in the process chamber. Results We investigated the relationship between defect formation and ionic concentration in the process chamber. Figure 2 shows the experimental result. The left graph shows the result for the anionic ions (F, Cl, NO 2, Br, NO 3, SO 4, and PO 4 ), and the right graph for the cationic ions (NH 3, Li, Na, K, Mg, and Ca). We found that there was no relationship between the defect formation and the ionic concentrations (except for the NH 3 ion) in the process chamber when the concentrations were low enough. However, the number of defects increased when the NH 3 ion concentration became higher, indicating that the concentration has a great influence on defect formation on wafer surfaces even when it is only a few ppb. It is noteworthy that an extremely small amount of NH 3 ion in the process chamber affects the formation of defects on a wafer surface. Figure 3 shows the relationship between the defect formation and NH 3 and F ion concentration in the process chamber. Figure 3: Relationship between defect formation and NH 3 and F ion concentration in the process chamber.

3 Solid State Phenomena Vol Discussion In order to elucidate the mechanism of defect formation under NH 3 ambient in the IPA drying process of the single wafer cleaning system, we need to observe the defects themselves. We characterized the defects on wafer surfaces using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses. Figure 4 shows SEM images and the results of EDX analyses of defects for IPA drying (left) and spin drying without IPA (right). We detected not only silicon and oxygen but also fluorine and nitrogen in the defects for IPA drying; these were not detected in the defects for spin drying without IPA. This suggests that the defect formation for IPA drying is caused by some type of chemical reaction. Figure 4: SEM images and results of EDX analyses of defects for IPA drying (left) and spin drying without IPA (right). Next, we discuss the mechanism of defect formation under NH 3 ambient in the IPA drying of the single wafer cleaning system. In this system, the process is as follows: DHF treatment (60 sec) DIW rinse IPA dry With these process steps is mind, we describe how defects are formed on a wafer surface. First, a silicon wafer surface becomes hydrophobic once the native oxide layer is completely removed by a DHF solution. Next, deionized water is dispensed to the wafer surface for eliminating residual chemicals on the surface. In this step, however, if there is any ammonia or fluorine in the process chamber or on the wafer surface, ammonium fluorides are formed and dissolved into the water on the wafer surface. In the final step, liquid IPA is dispensed to the wafer to displace water remaining on the wafer surface and ammonia fluorides dissolved into water are precipitated since they are not dissolved into IPA. This is how defects, which are mainly composed of ammonia fluorides, are formed on the wafer surface under NH 3 ambient in the IPA drying of the single wafer cleaning system. A schematic of how these defects are formed is shown in Fig. 5. NH 3 F- H 2 O NH 4 F IPA F- NH 4 F Defect DIW rinse IPA dry Figure 5: Schematic of how defects are formed.

4 234 Ultra Clean Processing of Semiconductor Surfaces XI We performed a simple experiment to verify the mechanism. The experimental setup is shown in Fig. 6. A silicon wafer, ammonia water, and a hydrofluoric acid solution were placed inside a closed box. DIW/IPA mixed solutions were dropped onto the wafer surface with volatile gas of ammonia, and hydrofluoric acid filled the box. After the droplet was dried, we observed the location where it had existed using SEM and EDX (shown in Fig. 7). We found that these defects resembled those observed in the previous experiment (Fig. 4, left). This result supports the mechanism of defect formation mentioned above. Figure 6: Experimental setup. Figure 7: SEM images and results of EDX analyses of defects. Summary We performed experiments focused on the process chamber atmosphere in the IPA drying of the single wafer cleaning system and described the mechanism of defect formation. Defects are formed on wafer surfaces that are primarily composed of ammonia fluorides precipitated from water into IPA. It is interesting that an extremely small amount of NH 3 ion in the process chamber affects the formation of defects on a wafer surface. The results demonstrate the increasing importance of strictly controlling the atmosphere in the process chamber of the single wafer cleaning system for future device technologies. References [1] H. Namba, T. Orii, H. Ohno, and G. W. Gale, Solid State Phenomena., , p 83 (2005). [2] N. Kurumoto, A. Eitoku, and K. Miya, Solid State Phenomena., , p 91 (2009).

5 Ultra Clean Processing of Semiconductor Surfaces XI / Influence of Ammonia Gas Ambient in IPA Drying Process of the ngle Wafer Cleaning System /