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

Abstract

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.