Low temperature formation of nc-si by ICP-CVD with internal antenna. A. Tomyo, H. Kaki, E. Takahashi, T. Hayashi, K. Ogata

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1 Low temperature formation of nc-si by ICP-CVD with internal antenna A. Tomyo, H. Kaki, E. Takahashi, T. Hayashi, K. Ogata Process Research Center, R & D Laboratories, Nissin Electric Co., Ltd., Umezu, Takase, Ukyo-ku, Kyoto -, Japan Tel/Fax: +---/+---, tomyo@rd.nissin.co.jp Keywords: nc-si, SiO, ICP-CVD Abstract. We have investigated structures of nanocrystalline silicon (nc-si) prepared by low-temperature inductively coupled plasma chemical vapor deposition (ICP-CVD) methods. Plan-view transmission electron microscopy (TEM) images showed that spatially isolated nc-si was synthesized and that the diameter and the standard deviation of nc-si could be well controlled with substrate temperature, gas pressure, RF power, synthesis time and pre/post plasma treatments. Under conditions of the substrate temperature of o C and. Pa with oxygen plasma pretreatment and hydrogen plasma posttreatment, the mean diameter and surface density of nc-si were. ±. nm and. cm -, respectively. This result is suitable for quantum effect device applications. Introduction Nanocrystalline silicon has attracted much attention because it exhibits unique features, such as coulomb blockade, light emission and absorption [,]. For example, quantum-dot floating gate memory devices have advantage of the high reliability with respect to the breakdown of tunnel oxide []. In the practical applications of nc-si in quantum effect device, it is important to control the diameter and surface density. In case of floating gate memory devices with nc-si embedded in SiO, the diameter is theoretically expected to be less than nm for room-temperature operation and the surface density should be as high as cm - to gain a sufficient voltage shift for memory operation. At the same time, low temperature synthesis of nc-si is desirable for high throughput in mass production and use of substrates which consist of low melting point materials. In this study, nc-si synthesis and SiO deposition were performed by ICP-CVD with low inductance antenna (LIA) units which were installed inside of a vacuum container so that the induced electric field from an antenna could be used effectively. By reducing the antenna inductance and fully covering antenna conductors with insulator, the plasma potential would decrease and thus the plasma damage could be suppressed [,]. These advantages of high density and low potential plasma are expected to be quite useful for lowering process temperatures and suppressing plasma damage to the underlayer. Here it is worthy to mention that this plasma source could be applicable for the large-area deposition by increasing antenna units.

2 Experimental After removing the natural oxide on the top surface of () oriented n-type.-ωcm CZ-silicon by buffered %-HF (BHF) solution, the substrate was transferred into the SiO deposition chamber. A schematic diagram of the experimental setup is shown in Fig.. While the substrate was heated up to the certain deposition temperature, which was fixed at ºC in the present work, the pressure in the chamber was reduced to approximately - Pa. A mixture of SiH and O gases was introduced into the chamber and then a silicon oxide layer was deposited on the top surface of the substrate by ICP-CVD. Radio frequency (. MHz) power was supplied through internal antenna units. To maintain the plasma stability, optical emission spectrometer (OES) was used. In some cases, oxygen or hydrogen plasma treatments, hereafter pretreatments, were inserted after SiO deposition, and besides, before nc-si synthesis. Deposition conditions are summarized in Table. Subsequently the substrate was transferred to the nc-si synthesis chamber which had the same structure as the SiO deposition chamber, keeping a vacuum state. Nanocrystalline silicon was also synthesized by ICP-CVD under conditions shown in Table. Substrate temperature, gas pressure, RF power and synthesis time were varied in order to investigate optimum conditions for nc-si synthesis. In some cases, hydrogen plasma treatment, hereafter posttreatment, was inserted after nc-si synthesis. Structures of nc-si were observed by a high-resolution transmission electron microscopy (HR-TEM). The diameter and surface density were determined by the plan-view TEM image. In addition, electronic properties of a single SiO film were examined with metal oxide semiconductor (MOS) capacitors formed by vacuum evaporation of aluminum electrodes. Table : Conditions of SiO deposition. SiH /O gas flow rate ratio. RF power (mw/cm ) Deposition temperature ( C) Total gas pressure (Pa). Deposition rate (nm/sec). Table : Conditions of nc-si synthesis. Fig. : The experimental setup of the ICP-CVD apparatus for synthesis of nc-si and deposition of SiO. SiH /H gas flow rate ratio RF power (mw/cm ) ~ Synthesis temperature ( C) ~ Total gas pressure (Pa).~. Process time (sec) ~ Results and discussion Figure shows the typical cross-sectional TEM image of nc-si. This proves that, even though nc-si is synthesized at a very low temperature of o C, the structure is crystalline. In order to determine the diameter and surface density of nc-si, plan-view TEM observation was carried out as shown in Fig. (a)-(r). Each figure shows the variation of the diameter and surface density of nc-si depending on substrate temperature, gas pressure, RF power, synthesis

3 time and pre/post plasma treatments. Si nanocrystals were clearly observed with a diameter of sub- nm. It was found that oxygen plasma pretreatment and hydrogen posttreatment were effective to decrease the diameter of nc-si, while hydrogen pretreatment could increase the diameter and surface density. These results suggest that plasma treatments vary the number of Si-OH bonds on the SiO surface which play a role for nucleation sites of nc-si []. Figures (a)-(d) show the average diameter and surface density of nc-si as functions of substrate temperature, gas pressure, RF power density and synthesis time. Uniformity of diameter of nc-si would be developed at C as shown in Fig. (a). At this temperature, it is expected that the diffusion of radical precursors are suppressed and thus the growth of nc-si would not proceed. We found that the diameter of nc-si varied with gas pressure and small and uniform nanocrystals could be synthesized at a high pressure as shown in Fig. (b). We considered that radical precursors would be decreased by the collision with source gases and thus the growth and coalescence of nc-si would be suppressed. The higher power density led to the small diameter and the high surface density as sown in Fig. (c). It is possible that oversupplied RF power would dissociate radical precursors. A long synthesis time (t sec) increased the standard deviation of the nc-si diameter distribution as shown in Fig. (d). This result also suggests that combination of contiguous nc-si has occurred. Figure shows I-V characteristic of a MOS capacitor using an ICP-CVD SiO film deposited at C. Breakdown voltage was. MV/cm at. - A/cm and leakage current was. - A/cm at. MV/cm for a SiO film with a thickness of nm. Fig. : Cross-sectional TEM image of nc-si synthesized at o C and at. Pa. (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k)

4 (l) (m) (n) (o) (p) (q) (r) Fig. : Plan-view TEM images of nc-si (a), (b), (c), (d), (e) o C at. Pa; (f)., (g)., (h). Pa at o C; (i), (j), (k) mw/cm at o C; (l), (m), (n) sec at o C; (o)without pre/post treatments, (p)with oxygen plasma pretreatment, (q)with oxygen plasma pretreatment and hydrogen plasma posttreatment, (r)with hydrogen plasma pretreatment at o C. (a) Substrate Temperature ( ).... Number Density ( /cm ) (b) Pressure (Pa).... Number Density ( /cm ) Power Density (mw/cm ) (c).... Number Density ( /cm ) Fig. : The dependency of the average diameter and surface density of nc-si on (a)substrate temperature, (b)gas pressure, (c)power density and (d)synthesis time. (d) Formation Time (sec).... Number Density ( /cm ) - - Current Density (A/cm ) Electric Field (MV/cm) Fig. : I-V characteristics of a MOS capacitor using a SiO film with a thickness of nm.

5 Summary Nanocrystalline silicon has been synthesized by ICP-CVD with LIAs. Plan-view TEM images showed that spatially isolated nc-si was synthesized and that the diameter and the standard deviation of nc-si could be controlled not only with a substrate temperature, gas pressure, RF power and process time but also with pre/post plasma treatments. The resultant trend suggests that radical precursors and reactive nucleation sites on the SiO surface have an important role in the synthesis of nc-si. The diameter of almost all nc-si under the present conditions was less than nm. In particular, under conditions of the substrate temperature of o C and. Pa with oxygen plasma pretreatment and hydrogen plasma posttreatment, the mean diameter and surface density of nc-si were. ±. nm and. cm -, respectively. This result is suitable for quantum effect device applications. In addition, the electrical properties of the present SiO film indicate the capability of the thin dielectric layer of nc-si devices. Acknowledgments The authors would like to thank Y. Setsuhara of the Osaka University with many useful discussions about the ICP-CVD method, and EMD Corporation for the technical assistance. References [] R. Muralidhar, R.F. Steimle, M. Sadd, R. Rao, C.T. Swift, E.J. Prinz, J. Yater, L. Grieve, K. Harber, B. Hradsky, S. Straub, B. Acred, W. Paulson, W. Chen, L. Parker, S.G.H. Anderson, M. Rossow, T. Merchant, M. Paransky, T. Huynh, D. Hadad, Ko-min Chang and B.E. White Jr., IEDM Tech. Dig., p. (). [] L. T. Canham, Appl. Phys. Lett., (). [] S. Miyazaki, Proc. of Thin Film Materials & Devices Meeting,, Nara, p.. [] Y. Setsuhara, S. Miyake, Y. Sakawa and T. Shoji, Jpn. J. Appl. Phys., (). [] Y. Setsuhara, J. Plasma Fusion Res., (). [] S. Miyazaki, Y. Hamamoto, E. Yoshida, M. Ikeda and M. Hirose, Thin Solid Dilms, ().