Micro-Electro-Discharge Machining Technologies for MEMS

Advances in micromachining techniques have led to the evolution of micro-electromechanical systems (MEMS). These techniques are typically based on semiconductor manufacturing processes, which offer various advantages such as batch manufacturing of miniaturized devices and monolithic integration of microelectronics with the devices. Surface micromachining has been used to construct complex microstructures, but since the structural geometries of these microstructures are two-dimensional, their mechanical abilities are often limited. This constraint has been addressed by the use of bulk micromachining techniques that involve etching and deposition processes. Anisotropic wet etching (Sato et al., 1998) and deep reactive ion etching (Laermer & Urban, 2005) have been widely used to create three-dimensional (3-D) geometries in MEMS. However, these processes are severely limited in their material options. As for deposition, electroplating is widely used to form 3-D metallic microstructures, but practical materials are limited to selected metals and alloys. In contrast, certain stainless steels and shape memory alloys have been commonly used for a variety of biomedical and implant devices such as stents and surgical devices. These materials have not been leveraged as much as silicon in MEMS, however, largely because they are not compatible with MEMS fabrication processes. As these examples indicate, there is an explicit gap between the diversity of engineering materials and the ability to use them in the design/ fabrication of MEMS; bridging this gap is expected to create new opportunities in the field. Micro-electro-discharge machining (μEDM) is a powerful bulk micromachining technique, as it is applicable to any type of electrical conductor, including all kinds of metals and alloys as well as doped semiconductors. μEDM is a non-contact machining technique, hence it can be easily applied to thin, fragile, and/ or soft materials regardless of their mechanical properties. Complex 3-D shapes can be achieved through numerical control (NC) systems with high-precision positioning stages. These unique features and the extensive material base available to μEDM have led to the process being leveraged for industrial applications, such as ink-jet nozzle fabrication (Allen & Lecheheb, 1996), micromachining of magnetic heads for digital VCRs (Honma et al., 1999), and micromechanical tooling (Wada & Masaki, 2005). In recent years, the technique has been increasingly utilized for MEMS fabrication to exploit a broad range of engineering materials that are incompatible with standard MEMS processes, overcoming the common constraint in MEMS, i.e., lack of diversity of bulk materials available for their fabrication (Takahata & Gianchandani, 2007).