It is important to develop a high-yielding and reproducible fabrication process of submicron Nb/AlO/sub x//Nb Josephson junctions to improve the integration level and the operating speed of Josephson LSI circuits. For this purpose, we have developed a junction fabrication process by introducing an electron cyclotron resonance (ECR) plasma etching technique with CF/sub 4/ gas. In the ECR plasma etching technique, highly anisotropic etching of Nb was achieved. Over-etching was reduced by 86%. We have successfully fabricated Nb/AlO/sub x//Nb junctions with critical current density of 10/sup 4/ A/cm/sup 2/ using the cross-line patterning (CLIP) method and the electron beam (EB) lithography technique, where the size of the junctions was varied from 2 /spl mu/m to 0.5 /spl mu/m at 0.1 /spl mu/m intervals. High-quality submicron junctions for integrated circuits with small spread of critical current Ic was obtained. High uniformity of Ic was achieved. The characteristics of the fabricated junctions are discussed and compared with the junctions fabricated by RIE technique.