Etching of scandium-doped aluminum nitride using inductively coupled plasma dry etch and tetramethyl ammonium hydroxide

Abstract

Properties such as wide bandgap, higher electromechanical coupling, and low dielectric permittivity have propelled Sc_xAl_1−xN as an advantageous material for optoelectronics and RF applications. Sc_xAl_1−xN devices are challenging to fabricate because ScAlN films are complex to etch, especially with greater scandium concentrations. Our group has developed a procedure to etch Sc_xAl_1−xN (x = 0.125, 0.20, 0.40) thin films (~ 730 nm thick), which results in vertical sidewalls that approach 85–90° (± 0.4°) and reduce the degree of the undercut. To maintain uniformity between various Sc compositions during the etching process, a nitrogen atmosphere was employed for high-temperature annealing, followed by immersion in a bath of tetramethyl ammonium hydroxide (TMAH) for wet etching. A 25% concentrated TMAH solution was used at 78–82 °C to etch down the Sc_xAl_1−xN film layer. The etching rate of ${\mathrm{Sc}}_{0.125}{\mathrm{Al}}_{0.875}\mathrm{N}$, ${\mathrm{Sc}}_{0.20}{\mathrm{Al}}_{0.80}\mathrm{N}$, and ${\mathrm{Sc}}_{0.40}{\mathrm{Al}}_{0.60}\mathrm{N}$ was found to be approximately, 365 nm/min, 243 nm/min, and 81 nm/min, respectively. Experimental results demonstrated that an identical etching profile can be obtained by TMAH vapor as well. We demonstrated how the annealing process recovers the deterioration introduced into the Sc_xAl_1−xN by the ion-bombardment effect developed during the SiO₂/SiN_x hard mask dry etch step, ultimately preventing lateral etching. We have also reduced sidewall roughness of a post-etched Sc_xAl_1−xN film with the combination of inductively coupled plasma etch, high-temperature annealing, and wet etching without affecting the sidewall verticality. Preliminary results of ongoing device fabrication that use this developed etch approach are also presented herewith to give an overview of the ongoing work.

Graphical abstract

Our group has developed a state-of-the-art procedure to etch Sc_xAl_1−xN (x = 0.125, 0.20, 0.40) thin films (~ 730 nm thick), which results in vertical sidewalls that approach 85–90° (± 0.4°) and reduce the degree of the undercut. To maintain uniformity between various Sc compositions during the etching process, a nitrogen atmosphere was employed for high-temperature annealing, followed by immersion in a bath of tetramethyl ammonium hydroxide (TMAH) for wet etching. A 25% concentrated TMAH solution was used at 78–82 °C to etch down the Sc_xAl_1−xN film layer. Experimental results demonstrated that an identical etching profile can be obtained by TMAH vapor as well. We demonstrated how the annealing process recovers the deterioration introduced into the Sc_xAl_1−xN by the ion-bombardment effect developed during the SiO₂/SiN_x hard mask dry etch step, ultimately preventing lateral etching. We have also reduced sidewall roughness of a post-etched Sc_xAl_1−xN film with the combination of inductively coupled plasma (ICP) etch, high-temperature annealing, and wet etching without affecting the sidewall verticality.