A thermal inkjet printhead is disclosed having improved reliability and thermal efficiency. The printhead includes heating resistors and thin protective regionspositioned above the heating resistors for protecting the heating resistors fromcorrosion and cavitation damage from the ink. The heating resistors are positioned substantially within the geometric confines of the portion of the protective regions exposed to the ink. Positioned between the heating resistors and the underlying printhead structure are field oxide regions that thermally insulate the heating resistors from the underlying printhead structure. Positioned between the heating resistors and the underlying printhead structure is a passivation layer consisting of a middle layer of phosphorous glass, an underglaze between the middle layer and the underlying printhead structure, and glass mesas between the middle layer and theheating resistors. The underglaze helps prevent phosphorus from the middle layerfrom contaminating the underlying printhead structure. Similarly, the glass mesas prevent phosphorus from the middle layer from contaminating the heating resistors.