Deep SiO2 etching with Al and AlN masks for MEMS devices

Silicon oxide-based materials such as quartz and silica are widely used in Microelectromechanical Systems (MEMS). One way to enhance capability of their deep plasma etching is to increase selectivity by the use of hard masks. Though this approach was studied previously, information on the use of hard masks for etching of silicon-oxide based materials on 200 mm substrates is scarce. We present results of etching process development for amorphous silicon oxide using Al and AlN masks with a view of application of results for etching of silica and quartz. Three gas chemistries (C4F8/O2, CF4 and SF6) and their mixtures were compared in an industrial reactive ion etch (RIE) chamber with two plasma sources. It was established that pure SF6 is the best etchant and AlN is better mask than Al for providing higher selectivity and sidewall angle close to vertical. A range of etching parameters for micromasking-free etching was established and etched structures of up to 4:1 aspect ratio were created in 21mthick oxide using the process with etch rate of 0.32-0.36 m/min and selectivity to AlN mask of (38-49):1.