Abstract
Properties such as wide bandgap, higher electromechanical coupling, and low dielectric permittivity have propelled ScxAl1−xN as an advantageous material for optoelectronics and RF applications. ScxAl1−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 ScxAl1−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 ScxAl1−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 ScxAl1−xN by the ion-bombardment effect developed during the SiO2/SiNx hard mask dry etch step, ultimately preventing lateral etching. We have also reduced sidewall roughness of a post-etched ScxAl1−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 ScxAl1−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 ScxAl1−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 ScxAl1−xN by the ion-bombardment effect developed during the SiO2/SiNx hard mask dry etch step, ultimately preventing lateral etching. We have also reduced sidewall roughness of a post-etched ScxAl1−xN film with the combination of inductively coupled plasma (ICP) etch, high-temperature annealing, and wet etching without affecting the sidewall verticality.
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Acknowledgments
This work was performed, in part, at the Center for High Technology Materials (CHTM) and Center for Integrated Nanotechnologies (CINT), an Office of Science User Facility operated for the U.S. Department of Energy (DOE), and Office of Science Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
Funding
This research was partially funded by the Department of Energy, Sandia Laboratories Academic Alliance. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
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ASMZS participated in the methodology, investigation, silicon oxide deposition and etching, annealing, acid cleaning, experiment and formal analysis, and writing of the original draft. IS participated in the methodology, investigation, experiments, lithography, silicon oxide mask preparation, formal analysis, SEM analysis, and reviewing and editing of the manuscript; RKC participated in the lithography, sample dicing, metallization, mask design, wet etching, formal analysis, and reviewing and editing of the manuscript; AA participated in the wet etching, acid cleaning, SEM analysis, and reviewing and editing of the manuscript; GE was materials sponsor and participated in the reviewing and editing of the manuscript; AS was a sponsor and participated in the reviewing and editing of the manuscript; TB participated in the conceptualization, idea, project and student supervision, review and editing of the manuscript, and funding acquisition. All authors have read and agreed to the published version of the manuscript.
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Shifat, A.S.M.Z., Stricklin, I., Chityala, R.K. et al. Etching of scandium-doped aluminum nitride using inductively coupled plasma dry etch and tetramethyl ammonium hydroxide. MRS Advances 8, 871–877 (2023). https://doi.org/10.1557/s43580-023-00601-6
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DOI: https://doi.org/10.1557/s43580-023-00601-6