Fabrication Technique of Single Flux Quantum ALU by Using Nb Trilayer

Nb trilayer process has been serving as the most stable fabrication process of the Josephson junction integrated circuits for more than two decades. Fast development of semiconductor fabrication technology has been possible with the recent advancement of the fabrication equipment. In this work, we took an advantage of those advanced fabrication equipments in developing a superconducting arithmetic logic unit (ALU) by using Nb trilayers. We used DC magnetron sputtering technique for metal depositions and RF sputtering technique for SiO₂ depositions. Various dry etching techniques were used to define Josephson junction areas and various patterns of metallic and insulating layers. Our Nb films were stress-free and had the T_c´s above 9 K. To enhance the step coverage of Nb films we used reverse bias DC magnetron sputtering technique. The fabricated 1-bit, 2-bit, and 4-bit ALU circuits were tested at a few kilo-hertz clock frequency as well as a few tens gigahertz clock frequency. Our 1-bit ALU operated correctly at up to 40 GHz clock frequency, and the 4-bit ALU operated at 5 GHz clock frequency.