Microelectronics Journal 36 (2005) 629–633 www.elsevier.com/locate/mejo
Silicon periodic nano-structures obtained by laser exposure of nano-wires K. Kakushimaa, T. Bourouinab,*, T. Sarnetc, G. Kerrienc, D. De´barrec, J. Boulmerc, H. Fujitaa a CIRMM Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan ESIEE, Ecole Supe´rieure d’Inge´nieurs en Electrotechnique et Electronique, Equipe Syste`mes de Communication et Microsyste`mes, ESYCOM—EA 2552, 2 Bd Blaise Pascal, 93162 Noisy-le-Grand, France c IEF, Institut d’Electronique Fondamentale CNRS UMR 8622, Baˆtiment 220, Universite´ Paris-Sud, 91405 Orsay, France
b
Received 30 November 2004; received in revised form 22 March 2005; accepted 1 April 2005 Available online 6 June 2005
Abstract Silicon nano-wires were fabricated using thin Silicon on Insulator (SOI) wafers and a combination of anisotropic wet etching by TetraMethyl Ammonium Hydroxide (TMAH) and Local Oxidation of Silicon (LOCOS). These nano-wires were submitted to laser exposure using gas immersion laser doping (GILD). The result was the formation of either periodic nano-structures or silicon balls. Since the process uses very short laser pulses, it involves rapid melting and solidification of silicon. Hence, the observed periodicity is ascribed to Rayleigh instability, which involves surface tension effects in melted silicon. q 2005 Elsevier Ltd. All rights reserved. Keywords: Silicon; Nano-wire; Nano-structure; Periodic; GILD; Excimer laser
1. Introduction Laser-based processes find more and more applications in micro- and nano-technologies. Among these processes, Gas Immersion Laser Doping (GILD) is a very promising technology mainly for applications in advanced Complimentary Metal Oxide Semiconductor (CMOS) technology. Indeed, it appears from the International Technology Roadmap for Semiconductors (ITRS) that major issues for Metal Oxide Semiconductor Field Effect Transistors (MOSFETS) transistors around year 2016 include the ability to produce drain and source doping with ultrashallow junctions (around 5 nm), high doping concentrations (up to 2!1020 cmK3) and high lateral extension abruptness (down to 1 nm/decade). Beside these applications to integrated circuits, GILD was recently applied by our group [1–4] for the fabrication of * Corresponding author. Tel.: C33 1 45 92 66 92; fax: C33 1 45 92 66 99. E-mail address:
[email protected] (T. Bourouina).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.04.034
silicon bridge mechanical resonators having unique characteristics: (i) ultra-heavy boron-doping, up to 3!1021 atoms/ cm3, that is about 10 times above the solid solubility of boron in silicon; (ii) thickness down to 90 nm due to ultra-shallow doping; and (iii) 10 times increase of resonance frequencies due to the high tensile stress related to the extremely high boron concentration. In this paper, we present new results obtained using the GILD experimental setup. They relate to the fabrication of periodic nano-structures. 2. Experimental results 2.1. Fabrication of silicon nano-wires The silicon nano-wires are formed on a thin (200 nm) silicon on insulator (SOI) wafer. We used SOI wafers from SOITEC Company. They result from the original UNIBONDw and SMARTCUTw technologies [5]. Using such wafers, the basic fabrication sequence for the fabrication of a silicon nano-wire is depicted in Fig. 1. Using a silicon base with (100) orientation, the fabrication process consists of a combination of silicon nitride (SiN) deposition by Chemical Vapor Deposition (CVD) and
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Fig. 1. Fabrication sequence of a silicon nano-wire.
patterning along a h100i direction, Fig. 1(a); a local oxidation (LOCOS) step, Fig. 1(c); between two TMAH etching steps Fig. 1(b) and (d). This fabrication process was originally proposed by Hashiguchi et al. [6,7]. By this method, the shape and the dimensions (down to 50 nm) of the wires are determined precisely by the thickness of the top silicon layer of the SOI wafer, independently of the resolution limitations of conventional UV lithography. The cross-section of the wire is also precise: it is triangular. Indeed, it is formed by two (111) planes and the (100) base, which make an angle of 54.78. Moreover, because these wires lie with (111) planes, namely along h110i directions on (100) wafers, their orientation is also very well controlled. In particular, orthogonal twin wires, which make an angle of exactly 908 (according to the crystalline orientation), are obtainable, Fig. 2(a). More complex arrangements can also be obtained, like the ‘p’-shape shown in Fig. 2(b). 2.2. Laser exposure using GILD doping experiments Laser doping takes place during laser induced melting/ solidification cycles, in the presence of a precursor gas (BCl3 in our case). Due to the very high solidification rate, segregation processes are greatly reduced, and doping concentrations may exceed the solubility limit (Fig. 3). Consequently, the laser-doped layer is limited to the lasermelted layer only, and high density, box-like and wellactivated doped layers may be realized (Fig. 3). The experiments are performed in a high vacuum chamber (base pressure z10K7 mbar) using an XeCl excimer laser (lZ308 nm, with typically a pulse duration
Fig. 2. Silicon nano-wires with triangular cross-section: (a) two orthogonal wires forming a V-shape; (b) three wires forming a p-shape.
z30 ns, 0.2 J/pulse and a repetition rate up to 25 Hz). The laser beam is normal to the surface and is made uniform with a beam homogenizer. Laser fluence on the sample surface may be adjusted with a variable attenuator in the range of 0.3–0.8 J/cm2. Beam homogeneity (%G5%) and optical alignment are controlled with two CCD cameras optically conjugated with the Si surface. Each laserprocessed area is submitted to a number (up to 200) of sequences of one gas injection pulse followed by one laser pulse. Laser doping by the GILD method is a very attractive solution to realize highly doped box-like ultra-shallow junctions for the sub-0.1 mm CMOS technologies in place of conventional techniques [8–10]. It also found applications in micro- and nano-electromechanical systems (MEMS/ NEMS) [1–4]. 2.3. Application of GILD to silicon nano-wires For the experiments dealt with in this paper, silicon nanowires were first fabricated on a 200 nm-thick SOI wafer.
K. Kakushima et al. / Microelectronics Journal 36 (2005) 629–633
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Fig. 5. Silicon balls resulting from excimer laser exposure of a 200 nmthick, 1500 nm-wide silicon nano-wire on SiO2 (SOI wafer) with a dose of 600 mJ/cm2. Sample B.
Fig. 3. Doping profiles obtained by Secondary Ion Mass Spectrometry (SIMS). Measurements performed on B-doped Si samples after 10, 50, and 200 sequences of one gas injection pulse followed by one laser pulse. The dotted line indicates the boron solubility limit at thermal equilibrium.
Then, the GILD technique was used to expose the top surface of the samples to the excimer laser. Three samples were prepared. The first one and the third one (sample A and sample C) have a width of 200 nm and the second (sample B) has a width of 1500 nm. Sample A was exposed with a dose of 100 mJ/cm2 while sample B was
Fig. 4. 1D periodic silicon nano-structure resulting from excimer laser exposure of a 200 nm-thick, 200 nm-wide silicon nano-wire on SiO2 (SOI wafer) with a dose of 100 mJ/cm2. Sample A.
exposed with a dose of 600 mJ/cm2. The corresponding results are shown in Figs. 4 and 5. As can be seen in Fig. 4, the exposure of sample A has led to the formation of a wavy periodicity of about 400 nm on the silicon nano-wire. Fig. 5 illustrates another case in which the exposure was high enough to cause the formation of droplets, which turned into silicon balls after solidification. The ball diameter is nearly 1 mm for the experiment shown in Fig. 5, corresponding to a wire width of 1500 nm. Since the diameter is related not only to the SOI thickness but also to the width of the nanowire, smaller balls are also obtainable from the 200 nmwide nano-wires. The result of laser exposure can be also an aperiodic structure as shown in Fig. 6. 3. Discussion The transformation of the nano-wires into periodic nanostructures is ascribed to Rayleigh instability, a phenomenon
Fig. 6. Aperiodic silicon nano-structure resulting from excimer laser exposure of a 200 nm-thick, 200 nm-wide silicon nano-wire on SiO2 (SOI wafer). Sample C. Process conditions led here to an aperiodic structure.
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Fig. 7. Schematic model for Rayleigh instability in a cylindrical nano-wire.
that is widely used for the explanation of droplet formation in liquids [11]. This phenomenon involves surface tension effects. As illustrated in Fig. 7, a constriction in a nano-wire (of cylindrical shape) reveals two radii of curvature R1 and R2 having opposite signs. The result can lead to a change of the nature of the surface stress 1 1 s Z 2g C (1) R1 R2 g is the surface tension (of the liquid). This stress can become positive under certain conditions, leading to a collapse of a cylinder into spheres. The Rayleigh instability occurs when [11] lR 2pR0
(2)
l and R0 being the length and equivalent radius of the nanowire, respectively. The corresponding period l of the nanostructure is then: l Z 9R0 =2
(3)
It is noteworthy that a detailed analysis of this problem shows that the instability can arise for a set of other values of R0 than the one mentioned in Eq. (2), (higher order eigenvalues and mode shapes). When considering the triangular cross-section (due to the (111) sidewalls), similar calculations that led to relation (3) give the following formula for the period l lZ
18p pffiffiffi3 t z2:77t 1C 3
leading to droplet formation. Then, silicon quickly turns solid. The shape of the nano-wire is then frozen after this second phase transformation is completed. The high doping level of silicon (up to 10 21 boron atoms/cm3) may play a non-negligible role on the periodicity because silicon behaves similarly to a metal. Indeed, quantum effects on stability have already been revealed on metallic nano-wires as an effect of the free electrons in the material [12]. The effect of these electrons interferes with the instable states derived with the Rayleigh classical model mentioned above. This can also lead under certain conditions, to a complete suppression of the instability.
4. Conclusion A method is proposed for obtaining periodic silicon nano-structures including uniformly spaced balls. This method uses excimer laser pulses to shine silicon nanowires. The resulting period is related to the dimensions and shape of the nano-wire. A theoretical model was proposed and compared to experimental results. The proposed method can be used as an alternative to lithography-based methods of nano-patterning. Examples of applications for the periodic silicon nano-structures proposed in this work include photonics (1D photonic crystals). The particular case of the silicon nano-balls may find applications in quantum-dots based components such as single electron transistors (SET) as well as DNA filters.
Acknowledgements This research project is collaboration between The University of Tokyo (U.T.), IEF and ESIEE in the framework of CIRMM. It has a support from the Centre National de la Recherche Scientifique (CNRS) in France and the French embassy in Tokyo. CIRMM is the Center for International Research on Micro-Mechatronics. It is a part of The U.T., extending its connections worldwide in a network of laboratories involved in the field of micro-mechatronics.
(4)
where t is the thickness of the SOI wafer used for the silicon nano-wire fabrication. Using Eq. (4) one obtains a theoretical value of lZ554 nm. The experimental value which can be obtained from the SEM observation in Fig. 4 is lZ460 nm. Therefore, there is a rather good agreement between the theoretical value and the experimental one. Since our fabrication uses very short laser pulses, it involves rapid melting and solidification process. After melting due to a laser pulse, silicon behaves as a liquid for which one can apply the general laws of hydrodynamics. In particular, Rayleigh instability can occur in a nano-wire
References [1] G. Kerrien, K. Kakushima, T. Sarnet, J. Boulmer, D. De´barre, D. Bouchier, A. Bosseboeuf, T. Bourouina, MEMS applications of laser-induced ultra-shallow and ultra-heavy boron-doping of silicon above the solid-solubility, IEEE/LEOS Optical MEMS’2002 Conference, 20–23 August 2002, Lugano, Switzerland, 2002 pp. 129–130. [2] K. Kakushima, T. Noguchi, T. Sarnet, G. Kerrien, J. Boulmer, D. De´barre, T. Bourouina, H. Kawakatsu, H. Fujita, Fabrication of heavily boron-doped mechanical resonators, The 12th International Conference on Solid-State Sensors, Actuators and Microsystems TRANSDUCERS’03, Boston, MA, June 8–12, USA, 2003 pp. 8–12.
K. Kakushima et al. / Microelectronics Journal 36 (2005) 629–633 [3] K. Kakushima, T. Bourouina, G. Kerrien, S. Petitgrand, T. Sarnet, D. De´barre, A. Bosseboeuf, E. Dufour-Gergam, J. Boulmer, Micro and nano-resonators obtained by laser doping, E-MRS 2003 Spring Meeting (European Materials Research Society), Symposium N: New Materials in Sensor Technologies. Strasbourg, France, June 10–13, 2003. [4] T. Sarnet, G. Kerrien, N. Yaakoubi, A. Bosseboeuf, E. DufourGergam, D. De´barre, J. Boulmer, K. Kakushima, C. Laviron, M. Hernandez, J. Venturini, T. Bourouina, Laser doping for microelectronics and microtechnology, SPIE, vol. 5448, HighPower Laser Ablation, Taos, New Mexico, USA, 25–30 April, 2004. [5] SOITEC web site: http://www.soitec.com/products.htm [6] G. Hashiguchi, H. Mimura, Fabrication of silicon quantum wires using separation by implanted oxygen wafer, Jpn. J. Appl. Phys. 33 (1994) L1649–L1650. [7] G. Hashiguchi, M. Mita, H. Toshiyoshi, D. Kobayashi, M. Goto, Y. Wada, J. Endo, H. Fujita, Fabrication of facing nano-wire probes
[8] [9] [10]
[11]
[12]
633
for in-situ observation of tunneling phenomena with ultra-high resolution TEM, TRANSDUCERS’99 Sendai, Japan, June 7–10, 1999 pp. 862–865. E.C. Jones, E. Ishida, Shallow junction doping technologies for ULSI, Mater. Sci. Eng. R24 1–2 (1) (1998) 1–80. International Technology Roadmap for Semiconductors, http://public. itrs.net G. Kerrien, J. Boulmer, D. De´barre, D. Bouchier, A. Grouillet, D. Lenoble, Ultra-shallow, super-doped and box-like junctions realized by laser-induced doping, Appl. Surf. Sci. 186 (2002) 45–51. J. McCuan, Retardation of plateau-Rayleigh instability: a distinguishing characteristic among perfectly wetting fluids. arXiv : math.AP/9701214v1 Report-no. MSRI 1997-011, 1997. F. Kassubek, C.A. Stafford, H. Grabert, R.E. Goldstein, Quantum suppression of the Rayleigh instability on nanowires, Nonlinearity 14 (2001) 167–177.
