In this paper, we explore the electrical characteristics of ring oscillator (RO) by optimizing fabrication inline windows of 16-nm high- κ /metal-gate bulk FinFET devices. Key process parameters are ranked according to ROs' performance, including effective capacitance (Ceff), effective resistance (Reff), and integrated circuit quiescent current (IDDQ). Process-dependence factors are then extracted and classified to reveal the actual root cause of each cluster. The findings of this paper indicate the dual gate spacer, the source/drain (S/D) proximity, the S/D depth, and the S/D implant affect Ceff, Reff, and IDDQ significantly, but the variation source of these parameters is the thickness of the dual gate spacer. Furthermore, impacts of the ON-state current ratio of N/P devices on delay and IDDQ of RO are examined by replacing dual spacers with single ones, the uniformity of implantation can be enhanced. Thus, the fluctuation of IDDQ is seven times reduced (from 252 to 37 nA) and RO characteristic can fit to designing target.