A novel technique has been developed to measure temperatures with a spatial resolution less than 10 &mgr;m. This method uses the temperature‐sensitive time decay fluorescence of a phosphor as a surface sensor. The spatial resolution is obtained by using an electron beam to excite individual phosphor grains deposited on the surface of interest. The phosphor selected for its dynamic range and chemical stability is the inorganic compound magnesium fluorogermanate activated with manganese. The time decay constant of its fluorescence decreases monotonically with temperature between −200 °C to 450 °C. This method was verified on a cross‐sectioned Zener diode. A description of the technique and a temperature map of the sectioned area of an operating diode are presented.