The prominent features of plasma flash evaporation may be caused by cluster deposition under high net flux of radicals. In order to clarify such features quantitatively, the atomic oxygen flux (Gamma(o)) and cluster size (d) in 200 Torr RF thermal Ar-O-2 plasma were measured on the substrate. It was found that Gamma(o) higher than 10(19) atoms/cm(2)-s can be easily achieved in this processing and this value corresponds to frozen atomic oxygen fraction 7%. The cluster size of YBCO was estimated to be about 0.3 to 10 nm. 0.8 nm cluster deposition under Gamma(o) higher than 6 x 10(18) atoms/cm(2)-s made it possible to deposit fine epitaxial YBCO films with Tc = 90K on SrTiO3 (100) with extremely high deposition rate around 2.2 mu m/min.