Fabrication of uniaxially strained silicon nanowires S.F. Feste a, ⁎, J. Knoch a,b, ⁎, D. Buca a , S. Mantl a a Institute of Bio- and Nanosystems, Forschungszentrum Jülich, D-52425 Jülich, Germany b IBM Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland abstract article info Available online 24 August 2008 Keywords: Nanowire Strained silicon Strain relaxation Uniaxial strain In this letter we propose a method for the fabrication of suspended strained silicon nanowires. Tensile uniaxially strained silicon is obtained by elastic strain relaxation of patterned tensile biaxially strained silicon on insulator layer. Electron beam lithography, reactive ion etching and oxidation are employed to write and transfer the nanowires down to a dimension of 15 nm in diameter. Surface smoothing and wire diameter reduction are controlled by an oxidation process. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The performance improvement of CMOS circuits has become ex- tremely difficult and technologically challenging to achieve by down- scaling of the conventional bulk silicon MOSFETs. A major difficulty for nanoscale devices is the control of short channel effects. Therefore, two different routes are pursued in order to realize an increase of device performance: i) multigate architectures such as FinFETs or tri-gate MOSFETs that enable a further reduction of device dimensions and ii) high mobility channel materials such as strained silicon that yield a performance increase without scaling. In respect of the first approach nanowires (NWs) and nanotubes have recently attracted a great deal of interest due to their geometrical smallness which facilitates the fabrication of gate-all-around devices with an optimal electrostatic gate control. In particular, silicon NWs grown by the vapor–liquid–solid mechanism have received a lot of attention [1]. On the other hand, the traditional top–down approach for fabricating NWs offers the advan- tages of an accurate control of the position and allows to adjust the crystalline direction of the NWs. Concerning the second approach ii), strained silicon as channel material is of great technological interest because it can easily be incorporated into existing fabrication lines and provides substantially increased electron mobilities. Employing biaxial, tensile strain, electron mobilities twice as high as in silicon can be achieved which, however, requires substantial strain levels of about 1 GPa. A similar increase of the hole mobility can be observed. This increase of hole mobility is lost at larger vertical fields, though [2,3]. On the other hand, uni- axial tensile strain is beneficial for the electron mobility at much lower strain levels and furthermore theoretical calculations based on measured bulk silicon piezoelectric coefficients predict for tensile uniaxial strain an enhance- ment for both, electron and hole mobilities [4]. In this paper we present a method for the fabrication of tensely, uniaxially strained silicon nanowires that allows to combine the advantages of the nanowire architecture with the electrical transport properties of uniaxially strained silicon. The nanowires have a circular cross-section and are suspended between two broader silicon areas. Wires down to a diameter of b 15 nm and a length of 700 nm have been realized. 2. Uniaxially strained silicon nanowires It is known from the elasticity theory that nanometer scale structures patterned out of strained layers can relax (e.g. via lateral expansion) to achieve a lower strain state: for instance, small squares relax completely and larger squares relax to a lower strain level. Narrow, long rectangular islands exhibit a different strain in the two island directions [5]. An extreme case of such a rectangular island is a nanowire which we will concentrate on in the following. We have performed a two dimensional finite element simulation of the change in the strain state of rectangular structures with dif- ferent aspect ratios. For the simulation the elastic constants of bulk silicon have been used. Biaxial tensile strain has been modeled by applying the same force per unit length on all four sides of the rectan- gular structure. To obtain the strain state in the relaxed structure, the straining force was removed from the long sides of the rectangles and boundary conditions were chosen to let this side move freely while the tensile force on the short sides of the rectangle was kept constant and the boundary of the structure was fixed in position [6]. As can be seen from Fig. 1a the tensile strain in the y-direction quickly gets homogenous over the structure with increasing length to width ratio, keeping most of the initial biaxial tensile strain. Fig. 1b shows the strain distribution in the x-direction for a rectangle with an aspect ratio of 4:1. While the pure tensile strain in the y-direction already is homogenous over the middle part of the structure, the strain in Thin Solid Films 517 (2008) 320–322 ⁎ Corresponding authors. Feste is to be contacted at Institute of Bio- and Nanosystems, Forschungszentrum Jülich, D-52425 Jülich, Germany. Knoch, IBM Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland. Tel.: +49 2461614505; fax: +49 2461 614673. E-mail addresses: s.feste@fz-juelich.de (S.F. Feste), jkn@zurich.ibm.com (J. Knoch). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.08.141 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf