A joining material like an In film having a low melting point forms a
surface oxide film when melted. If the oxide film is thick, an uneven
surface shape will remain as it is, and can cause vacuum leakage during
vacuum sealing. This problem is solved, for example, by placing particles
of a refractory material inside the low melting joining material. The
particles of the refractory material serve as a holding member for the
low melting material and break the oxide film during a joining operation
to bring the seal bonding with the low melting material to perfection.