3D silicon micromachining

3D silicon machining gives the ability to produce patterned structures on or beneath the wafer surface, allowing the fabrication of components which are important in many fields like micro/nano-electro-mechanical systems (M/NEMS), silicon photonics, microelectronics, biology etc. Fabricating such micro- and nano-scale structures such as wires, waveguides, bridges or cantilevers is an important constituent component for these research fields and much effort has been devoted for device design, fabrication and integration for developing future micro/nanotechnologies. Fabricating such complex, arbitrary-shaped silicon structures in three dimensions (3D) is of great importance for different mechanical, electronic and optical devices. Various techniques have been described in the literature for 3D silicon micro/nanofabrication. However, Making complex structures, such as arbitrary free-standing 3D structures at certain depths or multilevel structures in bulk silicon are highly challenging using standard fabrication processes.

Figure 1. (a) Schematic of defect density versus depth for 1 MeV protons in silicon. (b) Schematic showing hole current deflected around end-of-range regions owing to their higher defect density.

Figure 2 shows a schematic of different types of 3D free-standing structures which can be fabricated using this approach. Figures 3,4,5 give examples of each of these processes.

Figure 2. Schematics of the processes involved in fabricating (i) flat, (ii) curved and (iii) multilevel free-standing 3D wires using (a) 1 MeV and 250 keV proton irradiation for wires and 2 MeV protons with a high line fluence for supporting walls, (b) anodization to produce PSi around the cores and (c) removal of the PSi leaving a free-standing structure.

Figure 3. SEM images of 3D micromachined arrays of free-standing cores forming various shapes, supported by heavily irradiated silicon walls. (a), (b) Wires and their close-up views. (c) 3D-machined wheel with a pillar for support. (d) Grid formed by irradiation with 250 keV protons.

Figure 4. SEM images of micromachined (a),(b) curved 3D wires

Figure 5. (a) Two-level silicon wire arrays created with different irradiation combinations of line fluence and proton energy.