BulkMicromachining

Bulk micromachining is a MEMS fabrication approach that removes a substantial portion of a substrate to form microstructures, in contrast to surface micromachining, which builds features from thin films atop a substrate. It yields thick, robust elements such as cavities, membranes, and bulk features suitable for mechanical strength and sealing.

The technique commonly employs wet chemical etchants, notably potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH), which etch silicon selectively based on crystal orientation. Anisotropic etchants reveal specific crystallographic planes (for example, {111} planes), producing defined geometries and smooth surfaces. Isotropic etchants can also be used for general shaping. Front-side or backside patterning with a protective mask (oxide or nitride) enables selective removal of bulk material to create cavities or through-silicon features. Bulk etching is often followed by bonding steps to seal formed cavities or channels.

Dry micromachining methods, such as deep reactive ion etching (DRIE) and XeF2 isotropic etching, provide high-aspect-ratio structures and precise control when wet etching is insufficient. These dry processes are compatible with silicon and other substrates and can be used to create complex internal geometries that are difficult to achieve with wet chemistry alone.

Typical process flows include mask deposition and patterning, bulk etching from a wafer side to form the desired feature, release or cleaning steps, and bonding to seal cavities where necessary. Bulk micromachining is widely used for MEMS devices that require thick, mechanically robust components and defined internal volumes.

Applications include pressure sensors, accelerometers, microfluidic channels, resonators, and other MEMS structures where substantial material removal and structural integrity are essential. Limitations involve crystallographic constraints, surface roughness potential, process complexity, and alignment and bonding requirements.