離子蝕刻製程對高溫超導量子干涉元件特性影響之研究

Abstract

本研究主要探討以鈦酸鍶雙晶基板製作釔鋇銅氧超導量子干涉元件(Superconductor Quantum Interference Device, SQUID)之特性，過程中以離子蝕刻製程對高溫超導釔鋇銅氧微橋的特性影響。 超導量子干涉元件的製程步驟可分成基板準備、薄膜成長、黃光微影與蝕刻製程。在離子蝕刻過程中可能會因離子束能量過大造成薄膜失去超導特性，或是使薄膜過熱與再結晶，讓微橋結構的臨界電流與臨界溫度遠低於蝕刻前的薄膜特性。透過調整離子束能量以及改良冷卻系統讓原本寬度4 μm厚度200 nm的微橋結構臨界電流從1 mA提升至9 mA，且在表面形貌有明顯的差異。另外在製作超導量子干涉元件的過程中，雙晶基板經過原子力顯微鏡掃描後發現表面平整度不如預期，且在雙晶晶界處有約10~200 nm的溝槽深度，因此也改進拋光研磨條件讓基板表面粗糙度降至0.4 ~ 0.8 nm ，且雙晶晶界使用原子力顯微鏡量測顯示與基板其他位置無異，表示雙晶缺陷到達原子力顯微鏡的偵測極限約數個奈米以下。 後續使用鈦酸鍶雙晶基板製作超導量子干涉元件，透過拋光研磨以及改良離子蝕刻解決了因基板雙晶晶界不平整造成電阻－溫度量測上剩餘電阻或是因離子束轟擊過熱失去超導性的情形，成功地做出臨界電流I_c 約80~230 μA，峰值電壓V_pp 約 8~15 μV的超導量子干涉元件。

This study mainly discusses the characteristics of the SQUIDs (Superconductor Quantum Interference Devices) fabricated by the SrTiO_3 bicrystal substrates, and the influence of the ion etching process on the characteristics of the high-Tc YBa2Cu3Oy (YBCO) superconducting microbridge. The process steps of the SQUIDs can divide into substrate preparation, film growth, lithography and etching processes. In the ion etching process, the film may lose superconductivity due to excessive ion beam energy or the film may be overheated and recrystallized, so that the critical current and critical temperature of the microbridge structure are much lower than those of the film before etching. By adjusting the ion beam energy and improving the cooling system, the critical current of the YBCO microbridge structure with a width of 4 μm and a thickness of 200 nm is increased from 1 mA to 9 mA, and the surface morphology is significantly improved. In addition, in the process of fabricating SQUIDs, the atomic force microscopy(AFM) indicated that the surface flatness of bicrystal substrates was not good as expected, and there was a groove depth of about 10~200 nm at the grain boundaries(GBs). Thus improving the polishing process is needed. Therefore, the polishing condition is optimized to reduce the surface roughness of the substrate to be 0.4 ~ 0.8 nm, and the measurement of the bicrystal grain boundaries by AFM is the same as other positions of the substrates without GBs, indicating that the grain boundary defects reach the detection limit of the AFM, being about several nanometers below. Subsequently, using the SrTiO3 bicrystal substrates to fabricate SQUIDs, through the new polishing process and improved ion etching to solve the problems of residual resistance due to the groove of the bicrystal GBs in substrates and the loss of superconductivity due to the ion beam bombardment overheating observed in the resistance-temperature measurement. Finally, the SQUIDs were successfully fabricated and show that the critical current Ic was about 80 to 230 μA and the peak to peak voltage Vpp was about 8 to 15 μV.