High-energy focused proton beam irradiation has been used to controllably blueshift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Irradiation results in an increased resistivity, hence a locally reduced rate of anodization. Irradiated regions are consequently thinner and of a higher refractive index than unirradiated regions, and the microcavity blueshift arises from a net reduction in the optical thickness of each porous layer. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-color reflection images over the full visible spectrum.

Perfectly, crystalline, 55 nm thick silicon membranes have been fabricated over several square millimeters and used to observe transmission ion channeling patterns showing the early evolution of the axially channeled beam angular distribution for small tilts away from the [011] axis. The reduced multiple scattering through such thin layers allows fine angular structure produced by the highly non-equilibrium transverse momentum distribution of the channeled beam during its initial propagation in the crystal to be resolved. The membrane crystallinity and flatness were measured by using proton channeling measurements and the surface roughness of 0.4 nm using atomic force microscopy. V

We have developed a process to fabricate arbitrary-shaped, three-dimensional microstructures in 0.4 cm p-type silicon using focused high-energy proton beam irradiation, followed by electrochemical anodization. This has enabled us to produce free-standing complex microstructures such as arrays or long wires, grids, wheels, vertically stacked wires and wires which can be controllably bent upward and downward in the vertical plane. The two most important factors which determine the wire cross-section dimensions and depth are the irradiation ion fluence and energy. We can controllably vary the width of wires from 1 to 5 μm by varying the fluence of 1 MeV protons and the depth of wires from 2 to 15 μm by varying the proton energy. By using a combination of multiple energy proton irradiation over a range of 200-1000 keV, and gray-scale masks, different ion penetration depths and multilevel free-standing three-dimensional silicon structures can be obtained in a single etch step.

We have studied the effect of oxidation on the propagation loss and surface roughness of silicon-on-oxidized-porous-silicon strip waveguides fabricated using proton-beam irradiation and electrochemical etching. A thin thermal oxide is formed around the core of the waveguide, enabling the symmetric reduction of core size and roughness on all sides. Significant loss reduction from about 10 dB/cm to 1 dB/cm has been obtained in TE and TM polarizations after oxidation smoothening of both the bottom and the sidewalls by 20 nm. This corresponds well with simulations using the beam-propagation method that show significant contributions from both surfaces.

We have fabricated spherical and cylindrical concave micro-mirrors in silicon with dimensions from 20 microm to 100 microm. The fabrication process involves standard photolithography followed by large area ion beam irradiation and electrochemical anodisation in a HF electrolyte. After thermal oxidation the silicon surface roughness is less than 2 nm. We also present a multilayer porous silicon distributed Bragg reflector fabricated on concave silicon surfaces which selectively reflect and focus a band of wavelengths from a parallel beam of incident white light. Development of such low roughness concave microstructures opens up new applications in areas such as silicon photonics and quantum information science.