Y. S. Ow

Controlled blueshift of the resonant wavelength in porous silicon microcavities using ion irradiation

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.

Effects of oxide formation around core circumference of silicon-on-oxidized-porous-silicon strip waveguides

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.

Fabrication of large-area ultra-thin single crystal silicon membranes

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.

Fabrication of three dimensional porous silicon distributed Bragg reflectors

Three-dimensional distributed Bragg reflectors, which reflect all incident wavelengths, have been fabricated with micrometer dimensions in porous silicon, resulting in white reflective surfaces when viewed over a wide angular range. Large area arrays of several mm2 containing many individual micrometer-size pixellated reflectors that can be tuned to reflect a narrow or wide range of wavelengths are designed to appear either as constant or changing reflective images to the naked eye. This work opens avenues in controlling the reflection of light in all directions for applications in wide-angle displays, broadband reflective surfaces for resonant white light emission from semiconductor nanocrystals, and three-dimensional microcavities.

Modification of Porous Silicon Formation by Varying the End of Range of Ion Irradiation

A silicon micromachining process based on high-energy ion beam irradiation and electrochemical anodization to form porous silicon (PSi) has been used to fabricate patterned PSi and silicon microstructures, such as patterned distributed Bragg reflectors, 1 microturbines, 2 and concave silicon profiles. 3 Protons or helium ions, with energies of 250 keV to 2 MeV are focused to a beam spot of a few hundred nanometers for direct patterned irradiation on ptype silicon wafers. Irradiation causes localized increase in the resistivity arising from the point defects created along the ion trajectories. 4,5 Increased resistivity reduces the electrical hole current flowing through these regions during subsequent electrochemical anodization, 4 slowing down the PSi formation. PSi may then be easily removed with potassium hydroxide (KOH) to reveal underlying silicon microstructures. With high irradiation fluences, PSi formation ceases completely. A more recent development involves using standard ultraviolet photolithography to create patterned photoresist (PR) masks for shielding irradiation from a uniform broad ion beam. 3,6 This greatly improves irradiation in terms of irradiated area, time required and uniformity of fluence compared to using a focused ion beam. The experiments described here were performed using this method of irradiation. In addition, instead of simply stopping ions, PR with varying thicknesses were also used to selectively move the end-of-range region nearer to the silicon surface. The endof-range region was then used to fabricate silicon lines with nanosized tips as well as buried PSi channels.

On the Dependence of the Surface Roughness of Electrochemically Anodized Silicon on Ion Irradiation Fluence

We have studied the dependence of the surface roughness of electrochemically anodized p-type silicon on ion irradiation fluence.For moderate resistivity wafers, the surface roughness reduces with fluence, consistent with the dominant effect being a reducedanodization rate. However, for low resistivity wafers, the surface roughness increases with fluence. This is explained by showinghow irradiation converts the low wafer resistivity, which tends to form mesoporous silicon with low associated roughness, into amoderate resistivity, which tends to form microporous silicon with high associated roughness. This result explains why theanomalous behavior of surface roughness and photoluminescence intensity is observed.© 2010 The Electrochemical Society. DOI: 10.1149/1.3481769 All rights reserved.Manuscript submitted June 28, 2010; revised manuscript received August 2, 2010. Published August 31, 2010.

Effects of focused MeV ion beam irradiation on the roughness of electrochemically micromachined silicon surfaces

The authors compare the effects of focused and broad MeV ion beam irradiation on the surface roughness of silicon wafers after subsequent electrochemical anodization. With a focused beam, the roughness increases rapidly for low fluences and then slowly decreases for higher fluences, in contrast to broad beam irradiation where the roughness slowly increases with fluence. This effect is important as it imposes a limitation on the ability to fabricate smooth surfaces using focused ion beam irradiation. For a given fluence, small variations in the resistivity of an irradiated area may arise due to fluctuations of the focused beam current during irradiation. These small variations in resistivity then give rise to an increased roughness during the electrochemical etching. The roughness may be reduced by increasing the scan speed, which alters the way in which the fluctuations in fluence are averaged out over the irradiated surface.

Novel types of silicon waveguides fabricated using proton beam irradiation

In this work, we describe the use of a combination of proton beam irradiation and electrochemical etching to fabricate high index-contrast waveguides directly in silicon without the need for silicon-on-insulator substrate. Various types of waveguides with air or porous silicon cladding have been demonstrated. We show that porous silicon (PS) is a flexible cladding material due to the tunability of its refractive index and thickness. The Si/PS waveguide system also possesses better transmittance in the ranges of 1.2-9 and 23-200 μm, compared to Si/SiO2 waveguides. This is potentially important for mid and far-IR applications. Since it is compatible with conventional CMOS technology, this process can be used for fabrication of integrated optoelectronics circuits.

Electrochemical Anodization of Silicon-on-Insulator Wafers Using an AC

Electrochemical anodization of bulk silicon has applications in many micromachining processes. However, its use for silicon photonics is limited because silicon-on-insulator (SOI) wafers cannot be anodized using a conventional process because of the buried oxide. We overcome this using an alternating potential to induce an ac across an SOI wafer, treating it as a capacitative structure. The resultant surface roughness is comparable to that obtained using conventional anodization, and uniform etching across a 6 mm exposed surface is obtained with a minimum patterned linewidth of 2.5 μm in the device layer.