Silicon nanowire AFM tips grown on released scanning probe cantilevers from stencil-deposited catalysts

We present a parallel, full wafer technique for deposition of catalyst on released scanning probe bodies for the growth of individual high aspect-ratio Si nanowire tips. 1-D probes are necessary for imaging high aspect-ratio nano-patterns, such as deep and narrow trench geometries, or nanometer lateral resolution in single cell probing [1]. Together with carbon nanotubes, Si nanowires are excellent candidates for providing such tips. The main challenge remains the efficient integration at full-wafer scale of these 1D nano-objects with the scanning probe bodies, while still maintaining their good mechanical properties as scanning tips [2]. We are integrating here NW growth with a shadow mask technique, a unique approach providing the capability of parallel nanopatterning on top of 3D substrates [3-4]. In this work stencil lithography is used to deposit nano-catalysts for Si nanowire growth at controlled positions on released cantilever bodies. We started by fabricating two wafers: the substrate with tip-less Si scanning probes connected only by a thin Si bridge to the wafer body, and the stencil, with 100 nm thin low-stress SiN membranes containing apertures, as shown in Fig. 1. A customized SUSS MA/BA6 stencil aligner was used to align the two and mechanically clamp them. In the aligned position, each aperture from one membrane corresponds to a position close to the tip of the cantilevers. The clamped set was introduced in an evaporator and 20 nm Au was deposited on the tip of the cantilevers through the 300 nm diameter circular stencil apertures, as illustrated in Fig. 2. Epitaxial Si nanowires were then grown from the Au catalysts by the vapor-liquid-solid method at 530 ºC with a hydrogen diluted SiH4 precursor with a partial pressure of 0.2 mbar for 5 minutes. Fig. 3 shows an example of Si nanowire grown at the tip of the cantilever. The grown nanowires were well suited for tapping mode atomic force microscopy measurements. We scanned Si nanotrenches fabricated by e-beam lithography, 100 nm wide, 275 nm apart, and 600 nm deep. The comparison between scans of this sample done with a super-sharp tip and with a Si nanowire tip is shown in Fig. 4. The high aspect-ratio of the nanowire tips provided a clear advantage for this geometry: the side walls of the trenches are much sharper when scanned with the nanowire tip, while a deconvolution technique has to be used if attempting to obtain the same information from the super-sharp tip image. We thus proved for the first time the use of stencil lithography for the deposition of catalysts at controlled positions on released micro-structures at full-wafer scale. Si nanowires were grown from the deposited catalysts on cantilever tips. These nanowires were used as scanning probes for 1:6 aspect-ratio Si nanotrenches and showed to perform much better than commercially-available super-sharp tips.