CMP characteristics of silicon wafer with a micro-fiber pad, and padconditioningwithhighpressuremicrojet(hpmj)

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1 The 5th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium), Nov , 2008, Kona, Hawaii, USA CMP characteristics of silicon wafer with a micro-fiber pad, and padconditioningwithhighpressuremicrojet(hpmj) Keiji Miyachi 1*, 2,TaroAkama 2, Syuhei Kurokawa 2, Toshiro Doi 2, Kimio Nakayama 3, Yoshiyuki Seike 1,Yoji Matsukawa 2, Yoji Umezaki 2 1 New component division, Asahi Sunac Corporation.5050 Asahimae-cho, Owariasahi, Aichi , Japan 2 Dept. of Mechanical Engineering, Faculty of Engineering, Kyushu University. 744 Motooka, Nishi-ku, Fukuoka , Japan 3 Functional Products Business Promotion Department, Kuraray co., Ltd Tamashimaotoshima, Kurashiki, Okayama , Japan miyachi@sunac.co.jp Abstract In this study, Chemical Mechanical Polishing (CMP) characteristics of silicon wafer and conditioning characteristics with high pressure micro jet (HPMJ) have been examined, taking account of use of micro-fiber pad for the next generation which seems to give better removal rate and improved planarity than unwoven fabric pad used at present. As the results, it is found that CMP process on silicon wafers using the micro-fiber pad can get higher removal rate at initial stage, but the removal rate becomes lowered as the process time goes on. But, when the pad is conditioned by HPMJ using ultra pure water, it recovers normal state without dusts fill-up in relation to time length and pressure. Furthermore, it becomes clear that, if the pad is conditioned after CMP of every one silicon wafer, stabilized polishing characteristic can be kept for long time. The conditioning by HPMJ is a non-destructive method, even the pad for new generation can also have longer life in CMP of silicon wafers and it is expected as conditioning method in future. Key words: Pad Conditioning, High Pressure Micro Jet (HPMJ), Micro-fiver Pad, Silicon Wafer, Chemical Mechanical Polishing (CMP) 1. Introduction For manufacturing of highly precise integrated circuits, it is required in CMP process for silicon wafers to secure stably high removal rate without defects on wafers such as micro-scratches, hazes or orange peels to get high accuracy of their shapes. Especially it is important for getting high surface quality of silicon wafers stably to perform non-destructive conditioning of pads. In the past series of this study, we have proposed, as a part of studying about non-destructive conditioning methods for pads used in CMP process of silicon wafers, a method with high pressure micro jet(hpmj), examining relation of dusts fill-up on pads to polishing characteristic and effect of HPMJ with super pure water for recovery of the polishing characteristic. [2] Now in this study, we have taken account of using micro-fiber pad, which will be a next generation type being expected to give better removal rate and planarity than conventional unwoven fabric pad, and examined CMP characteristics of silicon wafer as well as conditioning characteristics by HPMJ. 2. Features of micro-fiber made polishing pad Fig.1 shows cross section image of micro-fiber polishing pad which was used in our experiments in this study. This pad is made of high density micro-fiber with hard polyurethane impregnated and mainly featured by the microfiber covering its surface. To explain differences from conventional unwoven pad, Table 1 shows characteristics of various pads and Fig.2 shows their cross sectional images. From Table 1 it is found that D hardness of micro-fiber polishing pad, 50 to 58, is higher than the same of unwoven pad and porous pad. This means its smaller elastic deformation can help, when it is used as CMP pad, accuracy improvement for wafers with reduced edge sags, etc. In comparison between three images on Fig.2, it is found that the micro-fiber pad(a), being covered by fibers has the finest structure, so that its superior slurry holding is effective for polishing rate improvement, surface roughness refining and defects reduction. Thus the micro-fiber polishing pad is expected as next generation s pad for wider range of applications aiming at up-graded high precision work, improved productivity and quality. On the contrary, it has problem of Fig. 1 Cross sectional image of microfiber polishing pad

2 easier filling of piled work dust and reacted chemical matters, etc. if CMP works continuously, that resulted in lowered polishing rate. To widen its application range, it will be urgent need to develop effective conditioning technology for it. Table 1 Characteristics of various polishing pads Structural classification Unwoven type Polyurethane resin type Micro-fiber series Usual unwoven fabric Porous type Apparent specific gravity g/cm D hardness Textile diameter μm (a) Micro-fiber series (b) Unwoven fabric pad from other source (c) Porous type from other source Fig. 2 Cross-sectional images of surfaces area of various types of pad 3. Principles of High Pressure Micro Jet(HPMJ) and its application to pad conditioning Working principle of HPMJ is to generate fine drops of pure water and to make them impinge on object surface with high speed to clean it. As described in preceding clause, generally when CMP works continuously, work dusts and reacted chemical matters are increasingly piled on pad surface which clog in fine holes there to make polishing characteristics unstable. To clean off those clogging, fine particles putout from HPMJ are impinged on them, i.e., a non destructive conditioning. Fig.3 shows schematic of system applying HPMJ and Fig.4 shows image of outside view of HPMJ pump unit. [1] Purewaterispressurizedinthepumpupto3to20MPaandsprayed out through a cat- eyed special nozzle having hundreds micrometer orifice to go onto pad surface. The sprayed out water drops are further atomized by air resistance to several tens micrometer and hit the pad surface with velocity of several tens m/s. By means of energy to hit the slurry residue directly, shearing stress waves generated at water drops hitting pad surface and fluid effect by water volume output from nozzle, the dusts and reacted chemical matters filled in small holes on pad surface are dug out to condition the pad physically. In our past studies, it has been confirmed that, when HPMJ pad conditioning system was applied for CMP process with unwoven pad and foamed urethane resin made pad, it could clean not only their surfaces but also their under layers and groove insides. Accumulator High pressure filter valve Nozzle D.I.W. Super high-pressure micro jet pump unit Fig. 3 Schematic of the High Pressure Micro Jet system (HPMJ) Fig. 4Aspect of HPMJ Pump Unit 4. Apparatus and method of experiments Fig.5 shows image of apparatus used for experiments of silicon CMP and Table 2 show conditions for experiments. In addition, the following 4 items were taken as the basic steps for polishing experiments. 1) When to use virgin pad, to perform 60 minutes of break-in by diamond dresser 2) To use a same silicon wafer 3 times in each process experiment, to see removal rate from its weight before and after the experiment

3 3) To confirm surface temperature of pad under 25 at beginning of the experiment 4) To measure surface temperature of pad during the polishing Table 2 Experimental conditions (a) Polishing conditions Polishing object Si wafer, 3 inches Pad used TM-1 KURARE Slurry used Nalco2350 Pressure 0.05 MPa Platen turning speed 60 min -1 Head rotating speed 60 min -1 Slurry flow rate 15 ml/min (b) HPMJ conditioning conditions Pressure 12 MPa Stand off distance 50 mm Nozzle fan angle 25 type Nozzle flow rate 1150 ml/min (at 10 MPa) Time of conditioning 120 s Fig. 5 Polishing machine for silicon CMP experiments 5. Results of experiments and observations 5.1 Changes in removal rate by continuous polishing and change in removal rate by HPMJ pad conditioning Fig.6 shows changes in removal rate in case of 50 times of continuous polishing. Among 50 times of polishing, conditioning was not applied up to 30 times. After then, the pad was conditioned by HPMJ. Fig.7 shows images of outside view of pad surface and Fig.8 shows Images of SEM on pad surface. On both Fig.s, (a) shows image before polishing, (b) shows image after 30 times of polishing, (c) shows image after 50 times of polishing where the pad was being conditioned by HPMJ from 31st polishing. From Fig.6, it is observed that CMP of silicon wafer with micro-fiber pad gives high removal rate around 0.5m/min at initial stage, but it gets down rapidly after about 10 times of polishing and, after 20 times, reaches 50% of initial removal rate. Being compared with the case of same experiment using unwoven fabric pad, micro- fiber pad showed approximately 20% higher removal rate at the initial stage, but its reduction rate is 75% higher than unwoven Without conditioning With HPMJ conditioning Fig. 6 Relationship between the removal rates and the number of experiment

4 pad. [2] It seems typical feature of micro-fiber pad, while such rapid reduction of removal rate should be anyhow prevented. As shown in Fig.6, conditioning of pad by HPMJ after 30 times of polishing could recover removal rate to some extent, but it was far lower than its initial rate. On the other hand, image(b) on Fig.7 shows work dusts accumulated during 30 times of CMP, while image ( c ) shows dusts reduced by HPMJ but not removed completely. This tendency can be more clearly observed on SEM images on Fig.8. As stated above, it has been confirmed that there is possibility to keep removal rate at a certain level by cleaning pad with HPMJ conditioning, but, if residues inside pad increases too much, its conditioning can not be effective. (b) After 30 times ofpolishing (c) After 50 timesof polishing Fig. 7 Images showing outside view of pad surfaces (b) After 30 times ofpolishing Fig. 8 SEM images of pad surfaces (c) After 50 timesof polishing 5.2 Changes in removal rate in the case that pad is conditioned at each time of polishing Fig.9 shows changes in removal rate when 30 times of polishing were performed where, but, pad was conditioned at each time of the polishing. Also Fig.10 shows images of outside view of pad surface and Fig.11 shows SEM images of pad, both in that case. On both Fig.s, (a) shows state before polishing, (b) shows the same after 10 times of polishing with conditioning at each time, (c) for after 20 times and (d) for after 30 times of polishing. From Fig.9 it is observed that, when the pad is conditioned at each time of polishing, the removal rate goes on higher level of 0.55m/min in initial stage that is kept until about 17th polishing, then goes down slowly but kept at 0.43m/min at 30th polishing, approx. 78% of the same in initial stage, which is almost same as the value in initial stage in case the same experiment was conducted on unwoven fabric pad. [2] conditioning Without conditioning Fig. 9 Relationship between the removal rates and the number of experiment

5 Furthermore, as seen on images on Fig. 10 and Fig.11, even when pad is conditioned at each time of polishing, work dusts, etc., are piled up slightly as the polishing goes on, but their increase is retained at minimum. As mentioned above, it has been confirmed that, when combination of micro-fiber polishing pad and its conditioning by HPMJ is used for CMP of silicon wafers, favorable feature of micro-fiber pad can be encouraged showing possibility to obtain higher productivity. It remains as future task to set up conditions for pad conditioning as well as for polishing that keep removal rate at further high level for longer time, to confirm expected effect of surface cleaning by HPMJ for reduction of defects such as micro scratches, etc., and to confirm, as the result of the above, effect to extend pad life for stable CMP polishing. (a) Before polishing (b) After 10 times of polishing (c) After 20 times of polishing (d) After 30 times of polishing Fig. 10 Images of outside view of pad surfaces (a) Before polishing (b) After 10 times of polishing (c) After 20 times of polishing (d) After 30 times of polishing Fig. 11 SEM images of pad surfaces Conclusion Above stated reports of this study are summarized as follows. 1) Micro-fiber polishing pad is greatly expected as the polishing pad in the next generation having characteristics of higher productivity and higher precision work because of its features being covered by micro-fiber and having less elastic deformation value. 2) When it is used for CMP of silicon wafers, micro-fiber pad can get higher removal rate at initial stage, but the rate goes down gradually as its working time passes. 3) If the pad is conditioned by HPMJ at each time of CMP processing, dusts fill-up on pad surface is restrained by HPMJ effect to prevent lowering of removal rate. 4) It is confirmed that the combination of Micro-fiber polishing pad and conditioning by HPMJ can encourage favorable features of the pad to get higher productivity. REFERENCES Yoshiyuki Seike, Sayuri Kawashima, Keiji Miyachi, Toshiro Doi, Ara Philipossian: Proposal and Apparatus of a High Pressure Micro Jet (HPMJ) Cleaning Technique Adaptation to Pad Conditioning -, Int. J. Jpn. Soc. Precis. Eng., Vol. 71, 1 (2005) pp Keiji Miyachi, Yoshiyuki Seike, Shinichi Haba, Syuhei Kurokawa, Toshiro Doi: Impact of a High Pressure Micro Jet (HPMJ) on the conditioning and cleaning of unwoven fabric polyester pads in silicon polishing, Proceedings of The 4 th International Conference on Planarization/CMP Technology 07 (2007) pp