I. Introduction
High-aspect-ratio (HAR) microstructures on single crystalline silicon (Si), such as deep trenches and holes, are among the core structures of microelectromechanical systems (MEMS) [1]. Dry etch and wet etch are the two major categories of fabrication methods for HAR structures on Si. A higher aspect ratio of these structures is beneficial for the performance of MEMS devices but brings challenges to available fabrication methods. For the dry etch methods, most research and development are based on cryogenic etch or Bosch etch [2]. Due to the difficulty in the mass transport of the ions in plasma, maximum aspect ratio by dry etch have typically been reported as around 30:1, although that of 98:1 [3] and 105:1 [4] has been demonstrated in few cases. Wet etch methods are able to fabricate HAR structures on Si due to the etching selectivity between different crystalline planes of Si. However, traditional wet etch methods, such as KOH etching or anodic etching, are limited to a few types of Si substrates [5] with compromised pattern-transfer fidelity in lateral dimensions [6].