Titanium is a promising new material system for the bulk micromachining of microelectromechanical ͑MEMS͒ devices. Titaniumbased MEMS have the potential to be used for a number of applications, including those which require high fracture toughness or biocompatibility. The bulk titanium etch rate, TiO 2 mask etch rate, and surface roughness in an inductively coupled plasma ͑ICP͒ as a function of various process parameters are presented. Optimized conditions are then used to develop the titanium ICP deep etch ͑TIDE͒ process. The TIDE process is capable of producing high aspect ratio structures with smooth sidewalls at etch rates in excess of 2 m/min, providing a new means for the microfabrication of titanium-based MEMS devices.

Recent process developments have permitted the highly anisotropic bulk micromachining of titanium microelectromechanical systems (MEMS). By using the metal anisotropic reactive ion etching with oxidation (MARIO) process, arbitrarily high-aspect-ratio structures with straight sidewalls and micrometre-scale features have been bulk micromachined into titanium substrates of various thicknesses, ranging from 0.5-mm sheet down to 10-microm free-standing titanium foils. Bulk micromachined structures are generally free of residual stresses and are preferred when large, rigid, flat and/or high-force actuators are desired. However, so far there has been a limited ability to select materials on the basis of specific application in bulk micromachining, primarily because of the predominance of MEMS processes dedicated to single-crystal silicon, such as silicon deep reactive ion etching. The MARIO process permits the creation of bulk titanium MEMS, which offers potential for the use of a set of material properties beyond those provided by traditional semiconductor-based MEMS. Consequently, the MARIO process enables the fabrication of novel devices that capitalize on these assets to yield enhanced functionalities that would not be possible with traditional micromechanical material systems.

This letter describes a simple method for three-dimensional microfabrication of complex, high-aspect-ratio structures with arbitrary surface height profiles in bulk silicon. The method relies on the exploitation of reactive ion etching lag to simultaneously define all features using a single lithographic masking step. Modulation of the mask pattern openings used to define the features results in etch depth variation across the pattern, which is then translated into surface height variation through removal of the superstructure above the etched floors. Utilization of a nonisotropic superstructure removal method based on sacrificial oxidation enables definition of high-aspect-ratio structures with vertical sidewalls and fine features. The utility of the approach is demonstrated in the fabrication of a sloping electrode structure for application in a hybrid micromirror device.

Current components of heavy-ion-induced transient currents in a 0.2-m fully-depleted SOI MOSFET are analyzed in the time domain. The analysis demonstrates that the transient currents have another slow-decay current component that is different from the two conventional current components: a prompt discharge current and a slow-decay parasitic bipolar current. The slow-decay component revealed here is a flow of deposited carriers stored in the body region to maintain quasi-neutrality, and it drastically widens the transient pulse. Index Terms-Heavy ions, parasitic bipolar transistor, silicon on insulator (SOI) technology, single event transients, time domain analysis, transient currents.