Study of static electricity in wafer cleaning process M. Wada 1a, T. Sueto 1b, H. Takahashi 1c, N. Hayashi 1d, and A. Eitoku 1e

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1 Solid State Phenomena Vol. 134 (28) pp Online available since 27/Nov/2 at (28) Trans Tech Publications, Switzerland doi:1.428/ Study of static electricity in wafer cleaning process M. Wada 1a, T. Sueto 1b, H. Takahashi 1c, N. Hayashi 1d, and A. Eitoku 1e 1 Dainippon Screen Mfg., Co.Ltd.,24261, Mikami, Yasu, Shiga , Japan a m.wada@prp.screen.co.jp, b sueto@prp.screen.co.jp, c hi.takahashi@prp.screen.co.jp, d n.hayashi@prp.screen.co.jp, e eitoku@prp.screen.co.jp Keywords: Static Electricity, Wafer Cleaning, Plastic material, Silicon wafer Introduction Static electricity has always received attention as a major factor in yield loss for the semiconductor manufacturing process. The problems related to static electricity are particle attachment to the wafer surface by electrostatic attraction [1] and pattern (device) damage by electrostatic discharge. Moreover, there are also concerns for the ignition of organic solvents and the related fire or explosion hazards. So, controlling static electricity is very important for improving yields and safety. In wet cleaning and etching processes, highly corrosive chemicals, such as H 2 SO 4, HF and HCl, have been used. Therefore, even materials that surround the wafer need to have resistance against these chemicals. Although materials consisting of fluorinecontaining plastic, such as PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyl polymer), PVDF (polyvinylidene fluoride), and PCTFE (polychlorotrifluoroethylene), have high resistance to highly corrosive chemicals and compatibility in high quality environmentals, at the same time those plastic materials can easily have electrostatic charge due to the triboelectric phenomena (~15kV). Hence, it is necessary to investigate the charging effect on plastic materials to circumvent. To solve the problem of electrostatic charging, many studies have been reported. For example, the noncontact method of ionizer irradiation [2], using soft Xray and corona discharge with high frequency, is commercially available. It has also been reported that IPA vapor solution could reduce this charging phenomena. [3] In this paper, we will report the effect of charged plastic materials on the wafer and chemicals in wet cleaning equipments. Experimental 1. Influence of charged plastic materials to the wafer. In order to investigate the wafer charging from the surrounding plastic materials, the surface voltage on wafer was measured by the static field meter (Ion System: Model 775) inside and outside of the chamber. The surface voltage was compared when the wafer was grounded or not grounded, both inside and outside of the chamber and with or without chemical processing. For chemical processing, dhcl (RT mixture of HCl : DIW) was used followed by DI water rinse and spin dry. To remove electrostatic charge on wafer by contacting conductive materials, PCTFE (1 16 Ω/square), conductive ethylenetetrafluoroethylene (ETFE)type A (1 1 Ω/square) and conductive ETFEtype B (1 4 Ω/square) were used as grounding materials. The contact point was wafer edge as indicated in Figure 1. The wafer was first placed on the charged plastic materials, and held by those materials, then the surface voltage was measured. 2. Influence of charged plastic materials to the chemical. As shown in Figure 2, the surface voltage of dhcl (.2%) filled in PTFE beaker was measured under various conditions. In this case, the effect of charged PTFE plate under the beaker and grounding of chemical were evaluated with the static field meter. 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 TTP, (ID: , Pennsylvania State University, University Park, United States of America14/6/14,18:43:39)

2 264 Ultra Clean Processing of Semiconductor Surfaces VIII Results and Discussions 1. Influence of charged plastic materials to the wafer. Table 1 shows the change of surface voltage on Siwafer at each position (in and out of the chamber). Due to negatively charged materials around the wafer in the process chamber, the surface voltage was negative (4.5kV). When the wafer went back to outside of chamber (transfer area), the voltage returned to V. In case of grounding inside chamber, surface voltage was V, but after transferring to out of the chamber, the surface showed 4kV. Also if dhcl was dispensed to wafer in chamber, the voltage became V in chamber and 4kV when the wafer was transferred out of chamber. It is understood that the charged materials around the wafer create either a repulsive or attractive force to the electrons in the wafer, and then repulsed electrons escape through a grounding point or dispensed chemical. Inside the chamber, the surface voltage was able to keep neutral by grounding. But on charged plastic materials, the surface voltage became positive due to lack of electrons when those wafer left charged materials. When the wafer was grounded both inside and outside of the chamber, the surface voltage was kept neutral at all positions. Therefore to avoid wafer charging, grounding of the wafer is important. In Figure 1, the electrostatic charge removal efficiency by using conductive materials is indicated. In the case of PCTFE, the surface voltage was 2.5kV. On the other hand, when conductive ETFE typea and B were used, the surface voltage was reduced about ~.5V. It is suggested that PCTFE has no ability to remove electrons, and conductive ETFE typeb is one of the more effective materials for electrostatic charge removal and chemical resistance. Table 1: Surface voltage on Silicon wafer. Wafer position Sequence Transfer area Transfer area Chamber (Before) (After) I : Transfer only (without grounding) Wafer V 4.5 kv V Ⅱ Grounding : Transfer only (with grounding) V.1 kv 4 kv Ⅲ dhcl Process : dhcl process inside chamber V.1 kv 4 kv

3 Solid State Phenomena Vol Surface Volt age (kv) Probe Contact materials (PCTFE, ETFEA, ETFEB) PCTFE ETFE typea ETFE typeb Wafer Materials Figure 1: Charging removal efficiency depended on each contact material. 2. Influence of charged plastic materials to the chemical. Figure 2 illustrates results of surface charge of chemical. When a discharged PTFE beaker was filled with dhcl (.2%), the surface voltage of the dhcl was V. When the PTFE beaker was placed on a charged PTFE plate with 18kV (Fig. 2III), the surface voltage of dhcl became negative (1kV). And when a grounding cable was inserted in the chemical, the surface voltage became V (Fig. 2IV). If the PTFE plate was removed from the beaker, the surface voltage changed to 7kV (Fig. 2V). If the grounding cable was reinserted, the surface voltages returned to V. From these results, it is thought that electrons in the dhcl were induced by the PTFE plate, as well as silicon wafer, and then able to escape from the chemical by grounding of the dhcl. Due to the lack of electron, surface voltage became positive outside of PTFE plate. By regrounding of the dhcl, lost electrons were recovered through the grounding. The chemical can have induction charging like the wafer. Addition to above when dhcl charged by PTFE plate was dispensed on a wafer, a voltage meter connected to the wafer showed a spike of 15mV at the moment of chemical touch as shown in Figure 3I. On the other hand when dhcl contained uncharged beaker was dispensed to the wafer no spiking was observed (Fig.3II). kv 1 kv Ⅰ. Discharged Ⅱ. Filled with dhcl Ⅲ. Put on charged Teflon plate(13kv) kv +7 kv kv + Ⅳ. Grounding Ⅴ. Removed plate Ⅵ. Regrounding Figure 2: Experimental procedure and results for surface voltage measurement in chemical.

4 266 Ultra Clean Processing of Semiconductor Surfaces VIII Time (sec) Time (sec) Voltage (mv) Chemical dispense Voltage (mv) Chemical dispense (I) dhcl dispensed from charged PTFE beaker. (II) dhcl dispensed from neutral PTFE beaker. Figure 3: Voltage variations on wafer surface after chemical dispensing. Summary Fluorine plastic materials generally used in cleaning equipment easily produce electrostatic charge, and it causes charging to the wafer surface. In case of chemical, it has also the same affect as the wafer. The electrostatic charge on both surfaces may affect process performance, such as particle contamination by staticattracted force. Controlling static electricity on plastic materials is essential factor. However, to avoid unaware wafer charging, to account for grounding inside and outside of chamber to the wafer is important. References [1] Larry B. Lerit, Tom M. Haniey, Frank. Curran: Solid State Technology, June 2 edition [2] A. Steinman: The basics of air ionization for hightechnology manufacturing applications, Compliance engineering, 26 Annual reference guide [3] T. Ohmi, S. Sudoh and H. Mishima: IEEE Transactions on semiconductor manufacturing. VOL. 7, No. 4, November 1994

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