Improving the wet-etching accuracy of Bi2Sr2CaCu2O8+δ crystal chips for high-temperature superconducting terahertz emitters using potassium hydroxide solution

The development of a technology for the microfabrication of Bi2Sr2CaCu2O8+δ (Bi2212) crystals is essential for realizing high-performance terahertz emitting devices based on Bi2212 single crystals. We developed an anisotropic wet-etching method using potassium hydroxide solution to improve the etching accuracy of Bi2212 crystal chips. Etching solutions with potassium hydroxide concentration of 10–13 wt. % and temperatures of approximately 40–45 °C are suitable for sample etching. The developed etching method enabled us to obtain crystal chips with sidewall angles of approximately 90°. In the case of a crystal chip with a thickness of ∼6 μm, the undercuts from the edges of the photomask were ∼1.5 μm, which were significantly shorter than those obtained in previous studies using acidic solutions (∼5–10 μm). The etching rate of the developed solution (0.1 μm/min) was lower than that of the acidic solutions (∼20 μm/min), which provided suitable etching conditions for the samples. Devices using Bi2212 crystal chips, fabricated using the developed technique, exhibited clear terahertz emissions, similar to those reported in previous studies. The enhanced accuracy of the proposed etching process is expected to improve the device characteristics of Bi2212 terahertz emitters, particularly in terms of the emission power and frequency.