Method and apparatus for hermetic sealing of assembled die

An optically transparent, hermetic coating is locally formed on die and other components already assembled in an existing nonhermetic structure. The local hermetic sealing of an OSA includes providing an OSA having at least one exposed optoelectronic component, and providing a paralene source. The method provides for positioning a mask between the paralene source and the OSA, the mask including an opening aligned with a first region of the OSA. The first region includes at least one of the exposed optoelectronic components. The paralene source is then caused to generate paralene such that a permanent paralene coating is deposited through the opening and on the first region of the OSA. The permanent paralene coating essentially hermetically seals the first region. The permanent paralene coating is chosen to be transparent to the wavelength of light emitted and/or used in the optical sub-assembly.