SacrificialLayers

Sacrificial layers are temporary films deposited on a substrate to be selectively removed later in the fabrication process, enabling the creation of structures that would be difficult to form in a single deposition sequence. They are a central concept in surface micromachining and other microfabrication techniques, where they support the buildup of complex, free-standing, or released features before being dissolved or etched away.

In surface micromachining, a sacrificial layer is deposited and patterned, while a structural material is deposited atop it. After the desired geometry is defined, the sacrificial layer is removed by a selective etch that leaves the functional structures intact. The choice of material and etchant is governed by selectivity, compatibility with subsequent steps, and the mechanical requirements of the final device.

Common sacrificial materials include inorganic oxides such as silicon dioxide (SiO2) and various polymeric films such as polyimide or soluble resists. The removal methods vary: oxide layers are typically dissolved with fluoride-containing etchants, while polymer sacrificial layers may be dissolved in appropriate solvents or removed by plasma or temperature-based processes. Dry or vapor-phase methods can also be used to minimize capillary forces during release.

Applications span MEMS devices, including cantilevers, diaphragms, and micro-gears, as well as microfluidic channels and other finely released structures. Important considerations include etch selectivity, residual stress, adhesion between layers, risk of stiction during release, and compatibility with drying techniques such as critical point drying to preserve released features.