Piezoelectric aluminum nitride thin-films: A review of wet and dry etching techniques

Aluminum nitride (AlN) is a technologically relevant material that can be deposited at low temperatures in the form of thin-films while preserving most of its physical properties. Consequently, it is widely adopted in microelectromechanical systems (MEMS), especially in acoustic wave devices (RF filters), optoelectronics, sensors and energy harvesters. For many of these applications, the micro-patterning of the AlN thin-film is necessary. This review compiles the different wet and dry etching methods reported to date, facilitating the implementation of the material in a variety of MEMS processes. First, wet etch processes involving phosphoric acid (H₃PO₄), tetra methyl ammonium hydroxide (TMAH) and potassium hydroxide (KOH) are introduced. Then, dry etching techniques are reviewed, namely ion beam etching (IBE), reactive ion etching (RIE), and inductively-coupled RIE (ICP-RIE). In each case, the etch mechanisms and chemical reactions are provided so that the processes can be better understood and tuned. The dependency of the processes on the temperature, material polarity and surface oxidation are discussed whenever the information is available. Furthermore, the effects of the etch parameters on material selectivity and resulting surface roughness, anisotropy, undercut profile and sidewall angle are considered.