Ga+ FIB implantation and selective wet etching for 3D nanostructures
Masking effects in silicon during wet chemical and dry etching can be utilized by means of silicon doping with high concentration as well as by silicon surface modification with ion beams. The masking effect occurs on p+-layers when silicon is doped by a sufficiently high concentration of boron. The same effect has been found for gallium which is also a p-type dopant in silicon.
For maskless nano-patterning of silicon gallium is of special interest because it is used in most focused ion beam (FIB) tools. Ga+ FIB irradiation of semiconductors for different purposes is a well established technique and the beam diameter can be optimized nowadays down to 10 nm. Combining Ga+ FIB implantation into (100)-oriented silicon with subsequent selective and anisotropic wet chemical etching (e.g. in KOH), 3D silicon structures on the nanometer scale can be fabricated due to etch rate retardation at Ga+ FIB-treated silicon areas when the Ga concentration in Si exceeds 5 x 1019 cm-3.
Figure: Ga implantation profiles in Si as a function of fluence. At a Ga concentration above a critical value of 5 x 1019 cm-3 a selective etch behaviour can be found.
Figure: Ga doped (p++) Si nanowires between contact pads, which have a width of smaller than 200 nm and a thickness of approx. 30-50 nm (left and middle). Nano nozzles, after FIB point irradiation and etching of a thin Si membrane (right). The thickness of the nozzle walls is smaller than 50 nm.
Nanomachining & investigation of freestanding Si nanowires
In the modern silicon based technology the monolithic integration of nano-electro-mechanical systems (NEMS) with microelectronic components plays an increasing role. Structures, like nanowires, bridges or cantilevers are important constituents for this purpose. NEMS show an enormous sensitivity to temperature, force or to added mass. Therefore, device design, materials processing, fabrication and integration are big challenges for future nanotechnologies. The high exibility of this patterning process and direct design at a spatial resolution in theorder of 10 nm achieved in modern FIB systems is a big advantage for the fabrication of freely suspended semiconductor nanostructures compared to other surface nanomachining and bulk micromachining processes.
Local Ga+ FIB implantation has been applied to define Si nanowires' dimensions. After subsequent etching the nanowires are suspended and unsupported over an air gap between the nanowire and the buried oxide of a silicon-on-insulator (SOI) substrate.
Figure: Schematic illustration of the nanowire fabrication process.
The line fluence ΦL was varied between 1011 and 4 x 1012 cm-1 leading to different nanowire widths due to the beam profile of the FIB. The thickness of the nanowire can be estimated to about 54 nm from the depth of the implanted profile for 30 keV Ga+ into Si, and from the critical Ga concentration for etch retardation. The estimated value of 54 nm is in a good agreement with FIB cross-section measurements delivering 55 nm. Wire dimensions as small as 20 to 200 nm in width can be achieved.
Figure: SEM image of nanowires implanted with an ion beam current of 10 pA. ΦL decreases from 4 x 1012 to 1011 cm-1 from left to right, apparent by the decrease of the nanowire width.
The nanowire width is increasing with the implanted line fluence ΦL. The qualitative behaviour of the wire width responding to ΦL can be separated into two regions: up to ΦL of about 1012 cm-1 the width increases rapidly; beyond this line fluence the correlation becomes nearly linear.
Figure: Width of the Si nanowires as a function of the implanted line fluence. The experimental data were obtained using different FIB currents and spot sizes.
Combining the approximation of the FIB intensity profile by a bi-Gaussian function with SRIM/TRIM simulations, the response of the wire width to the implanted line fluence can be reproduced (solid line in the figure above) by the calculation of implantation profiles and the subsequent evaluation of the exceedance of the threshold concentration for etch retardation.
Figure: Cross-section of the implantation profile of a Si nanowire showing the Ga concentration. The solid line shows the Ga concentration of about 4 x 1019 cm-3 for etch retardation.
Two terminal measurements of the nanowires were performed at different temperatures. The nanowires showed an semiconductive behaviour in the range of -15 to 95 °C (figure (a)). The resistivity decreases rapidly for increasing ΦL (due to surface to volume ratio) and fades to saturation for higher ΦL (figure (b)). The resistivity increase rapidly for lower temperatures down to 50 K (figure (c)).
Figure: Electrical properties of the nanowires. (a) I-V characteristics in the range from -15 to 95 °C. (b) Wire resistivity as a function of implanted line fluence. (c) Resistance as a function of temperature for lower temperatures.
Thus, the resistivity of the nanowires can be tailored by process parameters.