Etching of Scandium-Doped Aluminum Nitride using Inductively Coupled Plasma Dry Etch and Tetramethyl Ammonium Hydroxide

Properties such as wide bandgap, higher electromechanical coupling, and low dielectric permittivity have made Scandium-Doped<b> </b>Aluminum Nitride (Sc_xAl_1-xN) a promising material for optoelectronics and RF applications. devices are challenging to fabricate because Sc_xAl_1-xN films are complex to etch, especially with greater scandium concentrations. Our group has developed a process to etch of Sc_xAl_1-xN (x=0.125, 0.20, 0.40) thin films (~ 730 nm thick), which results in vertical sidewalls that approach 90° (±0.2°) and reduces the degree of undercut. As part of this process, etching homogeneity among different Sc compositions is maintained by high-temperature annealing in nitrogen atmosphere, followed by a wet etching in a tetramethyl ammonium hydroxide (TMAH) bath. The wet etch step used a 25% concentrated TMAH (TMAH: Water in 1: 3 ratio) solution at 78°C~82°C to etch the films. The etching rate of Sc_0.125Al_0.875N, Sc_0.20Al_0.80N, and Sc_0.40Al_0.60N was found to be 365 nm/min, 243 nm/min, and 81 nm/min, respectively. An identical etching profile can be obtained by TMAH vapor as well.<br/><br/>Sc_xAl_1-xN etching was found to be independent of the SiO₂ hard mask thickness andwe have analyzed the prospective reasons behind the factors that affect verticality during etching. We also demonstrated how the annealing process significantly repairs the surface damage introduced into the Sc_xAl_1-xN by the ion-bombardment effect caused during the SiO₂ hard mask dry etch step, thereby ultimately prevents lateral etching. We can also reduce sidewall roughness of a post etched Sc_xAl_1-xN film with the combination of inductively coupled plasma (ICP) etch and annealing without affecting the sidewall verticality in all Sc compositions. Preliminary results of ongoing device fabrication that use this developed etch approach will also be presented.