Thermal bubble formation is a fundamental process in nucleate boiling heat transfer and in many microelectromechanical thermal systems. Here, we report an experimental study of the dynamic and thermal behavior of bubbles generated at a single site, that is, a microcavity filled with alumina particles. The thermal process associated with the bubble departure from the isolated cavity, in particular, was shown to be different from that of macroscale boiling. The bubble departure diameter remains constant in a low superheat (or Jakob number) regime which is solely determined by the balance of interfacial tension and buoyancy. In addition, the bubble departure frequency increases along the bubble size as the substrate temperature rises. The further-increased frequency of bubbles emerging from the cavity causes multiple bubbles to coalesce before the preceding bubble completely detaches from the substrate, thus, leading to the decrease of apparent departure frequency with the increase of substrate temperature.