Ultrahigh Resolution Titanium Deep Reactive Ion Etching.

Titanium (Ti) represents a promising new material for microelectromechanical systems (MEMS) because of its unique properties. Recently, this has been made possible with the advent of processes that enable deep reactive ion etching (DRIE) of high-aspect-ratio (HAR) structures in bulk Ti substrates. However, to date, these processes have been limited to minimum feature sizes (MFS) ≥750 nm. Although this is sufficient for many applications, MFS reduction to the deep submicrometer range opens potential for further device miniaturization and an opportunity for endowing devices with unique functionalities that are derived from precisely defined structures within this length scale regime. Herein, we report results from studies seeking to create means for realizing such opportunities through extension of Ti DRIE to the deep submicrometer scale. The effects of key process parameters on etch performance were investigated, and the understanding gained from these studies formed the development of a new ultrahigh resolution (UHR) Ti DRIE process. Using this process, we demonstrate, for the first time, fabrication of HAR structures in bulk Ti substrates with 150 nm MFS, smooth vertical sidewalls (88°), good etch rate (587 nm/min), and mask selectivity (11.1). This represents a fivefold or greater improvement in MFS relative to our previously reported processes and a 29-fold or greater improvement over more recent processes reported by others. As such, the UHR Ti DRIE process extends the state-of-the-art considerably, and it opens important new opportunities for Ti MEMS, particularly in the implantable medical device realm.