Vapor Phase Decomposition (VPD)
Vapor phase decomposition is a technique by which trace elements on the surface of a silicon wafer are collected into a liquid sample. The silicon wafer is exposed to hydrofluoric acid vapor in a sealed chilling chamber. The hydrofluoric acid vapor forms a condensate on the chilled wafer surface. This condensate etches the oxide layer off of the wafer surface along with any trace metals that are present. The condensate is then collected by rolling a drop of scan solution (a dilute mixture of hydrogen peroxide, nitric acid, and hydrofluoric acid) across the surface of the wafer. The drop is transferred from the wafer surface into a clean sample vial. The liquid sample is then analyzed for trace metals using HR-ICP-MS. The VPD technique is capable of measuring metallic contaminants at concentrations ranging from 1E6 to 1E14 atoms/cm2. It is particularly useful in measuring light elements (e.g., lithium, beryllium, boron, sodium, magnesium and aluminum) on bare silicon or in hydrofluoric acid soluble thin films.