Micromachining of Silicon via ION Irradiation with Porous Silicon Formation

Abstract

Silicon and porous silicon micromachining was performed with high-energy ion beam irradiation using MeV protons or helium ions. Ion irradiation is able to locally increase the resistivity for p-type silicon wafers in both the lateral and vertical sense by creating point defects to the silicon lattice. During the subsequent electrochemical anodization in hydrofluoric acid solution to form porous silicon, the electric hole current, which is necessary for the formation of porous silicon, is reduced at the irradiated regions. Porous silicon formation rate hence reduces down with increased ion fluence. At a high enough fluence, hole current flow may be completely stopped and the formation of porous silicon may cease completely at irradiated regions. By controlling the locations of the defects and limiting the porous silicon formation, different two or three dimensional porous silicon structures can be machined. Unanodized silicon forms the inverse structures of the porous silicon and hence two or three dimensional silicon microstructures may be machined as well simply by removing the porous silicon with a solution of diluted potassium hydroxide. Ion irradiation in this thesis was preformed using two different methods. The first method is carried out using direct focused ion irradiation, also known as Proton Beam Writing (PBW), on silicon wafers. The second method of irradiation uses a uniform and large area ion beam which was defocused by a nuclear microprobe setup. In the latter method, irradiation is achieved by a thick, patterned photoresist on the wafer surface acting as a mask to shield the underlying silicon from incoming ions. This method of large area irradiation was developed as part of this thesis and demonstrates the ability to upscale our technique for mass production. Many different kinds of micromachined silicon or porous silicon structures were successfully fabricated for this thesis. They include tunable porous silicon based multilayer Bragg reflectors, freestanding three dimensional structures, silicon based waveguides, concave silicon micro mirrors, and thin silicon films. Silicon was also machined into a high resolution recording medium for computer generated holograms. This technique of machining silicon using ion irradiation and porous silicon formation is definitely not limited to only fabricating these structures and devices. It is really up to one's creativity in utilizing this direct, simple and efficient way of micromachining silicon to fabricate more complex structures for different applications.