Micromachining of silicon with a proton microbeam

In the latest years the fabrication of sensors and actuators devices on a microscopic scale and their integration with electronic devices (micro-electromechanical systems - MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion micro beam is a suitable tool for such purpose. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate definite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main effect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with different doping levels (p+, p, n+) and irradiating regions with different area (from 200x200 m m2 to 25x25 um2) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures.