Heater

A heater comprising:

• a substrate including an upper surface that is rectangular and elongate in a longitudinal direction;
• a heating resistor formed on the upper surface of the substrate;
• and
• a protective film including a crystallized glass layer covering the heating resistor, and an amorphous glass layer covering the crystallized glass layer, the crystallized glass layer being held in direct contact with the upper surface of the substrate, the amorphous glass layer as a whole being spaced apart from the upper surface of the substrate;
• wherein the protective film further includes a semi-crystalline glass layer in addition to the crystallized glass layer and the amorphous glass layer, the semi-crystalline glass layer intervening between the substrate and a portion of the amorphous glass layer that projects from the crystallized glass layer, the semi-crystalline glass layer having an end that is adjacent to the crystallized glass layer and held in direct contact with the upper surface of the substrate,
• wherein the heating resistor comprises two main portions extending in parallel to each other in the longitudinal direction of the upper surface of the substrate, and a connection portion connecting the two main portions to each other, and
• the crystallized glass layer comprises an upper surface formed with a recess corresponding in position to a location between the two main portions of the heating resistor, the recess being spaced apart from the upper surface of the substrate.