Semiconductor: Dissolved Ozone Measurement in Semiconductor Ultrapure Water Systems

Ozonation is used to disinfect and sanitize ultrapure water as it passes through final filtering and distribution equipment. The goal is to control bacteria levels of the ultrapure water transported to the fab. Biological growth would generate organics or particles that would detrimentally affect product yields. Ozonation typically occurs prior to the UPW storage tank, in the distribution return piping, or may be diffused directly into the storage vessel itself. Levels are typically maintained in the range 5 to 50 ppb but then must be eliminated using ozone destructing UV light. Ozone measurements are made to insure proper dosage control is maintained after injection and then to insure residual ozone is eliminated by UV exposure. At this point any residual ozone becomes an unwanted contaminant and contributes to increased oxygen levels, which can affect the performance of a downstream de-gassification process. For this scenario, as Diagram A shows, there are two points of measurement required.