Sacrificial Materials and Release Etchants for Metal MEMS That Reduce or Eliminate Hydrogen-Induced Residual Stress Change

• Published in: Journal of microelectromechanical systems 2021-06, Vol.30 (3), p.426-432
• Main Authors: Ni, Longchang, de Boer, Maarten P.
• Format: Article
• Language: English
• Subjects: Aluminum nitride; Compressive properties; Conductivity; copper; Etchants; Etching; Ferric chloride; Hydrogen; hydrogen incorporation; III-V semiconductor materials; Low temperature; Metals; Microelectromechanical systems; Micromechanical devices; Oxidation; Residual stress; Room temperature; Silica; Silicon dioxide; Stress; Tantalum
• ISSN: 1057-7157; 1941-0158
• DOI: 10.1109/JMEMS.2021.3069397

Metal microelectromechanical systems (MEMS) provide attractive properties such as high reflectivity and high conductivity. They are typically processed using an organic sacrificial layer that is removed by an oxygen ash. This enables low-temperature processing but presents other problems. For example, ashing is not applicable to metals that are susceptible to oxidation. However, if an inorganic sacrificial silicon dioxide is used, its removal commonly involves acids, exposure to which can be detrimental to metals. For example, metallic films are susceptible to hydrogen incorporation, resulting in large compressive stress. In recent work with structural tantalum and sacrificial silicon dioxide as a model system, we demonstrated that hydrogen injected during the release process can induce 1200 MPa compressive stress. This work explores AlN and Cu as alternative sacrificial materials in which release etching of structural Ta by AZ 400K developer and FeCl 3 solution, respectively, is hydrogen free. Isotropic release etching is observed for Cu at room temperature and for AlN above 60 °C. In comparison with 1200 MPa, the uniaxial stress change after structure release is 80 MPa compressive with Cu and is unchanged after AlN sacrificial etch. [2021-0005]