Similarity deviation in capacitively coupled RF O₂ plasmas driven by nonlinear collisional processes

Discharge similarity laws provide an important basis for designing and scaling plasma sources across different spatial and temporal scales, but they are challenged in electronegative gases with complex reaction kinetics. In this work, we use a one-dimensional particle-in-cell/Monte Carlo collision (PIC/MCC) method to systematically study similarity behavior in capacitively coupled radio-frequency (RF) O₂ plasmas over 6–50 Pa and 10–30 MHz. We focus on how nonlinear collision processes, such as electron detachment, ion–ion mutual neutralization, and electron–ion recombination, affect the validity of classical similarity scaling. The results show that when the relevant nonlinear mechanisms are removed, key quantities such as plasma density and electron power absorption follow the classical similarity scaling. In contrast, when the full reaction set is included, clear deviations from similarity appear. These findings indicate that nonlinear reaction kinetics are the primary factor limiting the applicability of similarity laws in electronegative O₂ discharges, and the dominant mechanism depends on the discharge mode. This work clarifies how complex chemistry modifies similarity behavior under practical process conditions.