Non-destructive Na content analysis in bonded silicon-glass microcavities using laser-induced breakdown spectroscopy

Silicon-glass microcavities have been widely used as a functional packaging method for many applications since its founding. During the process, sodium ion (Na+) gains mobility due to the high temperature and moves towards the cathode, where it receives an electron and further moves outside the glass, forming metal liquid at the glass/cathode interface, since the melting point of Na is 97.79 ºC. Naturally, a part of Na will stay at the cathode after the wafer is removed, and, without a proper cleaning, it accumulates. This allows liquid Na droplets to be blown inside the silicon/glass interface with a gas flow at the later bonding process, which can strongly influence the sensitive silicon elements. Standard methods such as Raman or mass spectroscopy are not appropriate for this application, because the contamination is either not detectable or the cavity will be destroyed. In this study, we experimentally analyzed the closed system using laser induced breakdown spectroscopy (LIBS). With a high-intensity laser, a gas breakdown was generated inside the cavity and measured via optical emission spectrum. The study was performed in two steps: first, the minimal dimension of the cavity was determined in order to not damage the walls; second, the system was fabricated according to the results from last step, and the measurement concept was proved.