Advanced Modelling and Fabrication of Suspended Silicon Nano-Structures Using Multi-Species Focused Ion Beam Implantation

In this work, we present recent advances in the development and modelling of silicon nanostructures defined by focused ion beam (FIB) technologies. The development of FIB columns equipped with liquid metal alloy ion sources (LMAIS) opens new opportunities for FIB-based processing, enabling multi-species patterning [1]. Here, we report the use of focused Au+ and Ga+ ion implantation for nanolithography on silicon, focusing on the modelling of the implantation and etching processes to control the thickness and height/depth to the ultimate sub-nm range. Our aim is to develop a predictive model for the etching behavior of FIB-implanted volumes, enabling the design of corrugated and 3D suspended silicon structures with tailored dimensions. 2. Process overview Previous studies have demonstrated that Ga+ ion implantation in silicon via FIB serves as an efficient and straightforward resistless lithography technique [2-5]. This approach facilitates the fabrication of nanometer-scale structures on silicon and other materials. The implanted Ga+ volume enhances silicon's resistance to both wet chemical etching, enabling its use as an etching hard mask (Figure 1). The resolution of the structures is largely determined by the focused beam's diameter and the ion penetration depth and straggling. Several applications of this method have been reported, including the fabrication of suspended silicon nanowires [6] and single-electron devices [7]. 3. Modelling We model the process to precisely control the vertical dimensions (z-direction) of silicon nanostructures, including both suspended and non-suspended configurations. Process-calibration experiments were conducted for Ga+ using a 30 keV Crossbeam 550L system (Zeiss) and for Au+ using a 35 keV Velion FIB-SEM system (Raith). Fluence values ranged from 1·10¹⁴ at/cm² to 1·10¹⁷ at/cm², avoiding lower doses (insufficient) and higher doses (entering milling regimes) [5]. The implanted samples were etched using tetramethylammonium hydroxide (TMAH) at 25% concentration and 80°C. Atomic force microscopy (AFM) was used to characterize the structures post-implantation and etching. After calibration, we developed algorithms in Matlab software to predict the etching rates in TMAH (Figure 2) and the final etching depth (Figure 3) for both Au+ and Ga+ ion species as a function of implantation dose. This information is crucial for fine-tuning the dimensions of corrugated and 3D suspended silicon structures. Figure 4 illustrates a corrugated silicon surfaces defined by Ga+ implantation and figure 5 shows suspended nanowires (25-30 nm width, 20 nm thickness) defined by Au+ implantation. 4. Conclusions In this study, we successfully demonstrated the development and modelling of silicon nanostructures using FIB implantation with both Ga and Au ion species. Through the detailed process-calibration experiments and the development of predictive algorithms, we achieved precise control over the etching behavior of ion-implanted silicon in TMAH, enabling the fabrication of highly defined, suspended 3D nanostructures. Our results show that the combination of FIB implantation and wet etching provides an effective route for creating complex silicon nanostructures, opening new possibilities for the fabrication of nanoscale devices in electronics and sensing.