Gold-assisted lithography for suspended nanostructures

Direct lithography techniques are widely used for rapid R&D studies and prototyping. The flexibility offered by these fabrication approaches can follow the creativity in the design of device layouts. Tra-ditionally, direct patterning is suitable for binary structures, but when the third dimension is consid-ered, the process can become very complex. This work investigates a novel direct patterning technique that simplifies the fabrication process of suspended structures, by using gold ions as a mask for the pattern transfer. Nowadays, Focused Ion Beam (FIB) systems have been improved with new types of sources that can open up new applications. Systems equipped with a liquid metal alloy ion source (LMAIS), such as the Velion FIB from Raith, allow the researchers to use different ions in a single instrument. This study was carried out on a FIB with a AuGeSi source, where the effect of gold ion implantation in Silicon has been investigated in relation to its influence on the etch resistance of Si in tetramethylammonium hydroxide (TMAH) solution. Similarly to previous works already carried out with Gallium ions [1,2], a certain con-centration of gold ions on the surface of a silicon sample can act as a direct mask for the pattern trans-fer in wet solution for the anisotropic etching of TMAH as schematically represented in Figure 1. This effect occurs when the ion fluence reaches a certain threshold value. As depicted in Figure 2, by spanning the values of Au+ ion fluences with an energy of 35 keV, it is possible to observe three regimes of this process. In the first (below the threshold) there is no masking effect, i.e. the implanted regions are not stable enough to resist etching. When a threshold value is reached (1x1015 ions/cm2), gold atoms are embedded in the surface layer and dramatically slow down the etching rate of these areas. Lastly, at high fluences, the sputtering regime of the gold ions becomes relevant, so that the layout is transferred along the z-direction, resulting in a kind of implanted dwells. As the second regime, the implanted regions are also stable in wet etching, while the surrounding is removed anisotropically. Optimization of the entire process, from ion implantation to etching, has led to complete control of this fabrication technique. The examples in Figure 3 show how this approach can be used to produce a variety of structures with challenging properties. Such as suspended silicon nanowires (SiNWs) 32 nm wide, tens of microns long, with variable spacing down to 100 nm and complex structures such as suspended meshes. This study has shown that this unconventional way of patterning silicon is flexible and can assist in the fabrication of complex suspended structures that would be difficult to create using other traditional techniques. The extreme geometric parameters produced by precise ion implantation via FIB make this approach promising for gas sensing and NEMS application. Similarly to what shown with gallium-ap-proach [3, 4], the suspended structures can be functionalized or metallized according to the sensing method required for the devices, widening the potential applications of this platform.