Concept of ultra clean (UC) dry-process proposed by present authors in 1990s has been widely used in various thinˆlm fabrication elds from the view point of research as well as volume production stages of electronic devices. Original basic concept for UC-process sometimes are lead to another wrong concept direction, usually caused by a compromise for temporal trial, space limitation, facility, and so on. In this paper, UC technological items originally proposed is brie‰y reviewed. In‰uence of vacuum atmosphere by chamber baking, moisture content in process gas lines, outgasing of co-sputtering cathode equipped with magnetic ‰uid, and particle sources in fore-pump. are re-checked again in research process for perpendicular magnetic recording media.