Temperature measurement during laser heating of thin films using a temperature-sensitive phosphor

A phosphor with temperature-dependent lifetime has been used to measure the temperature distribution produced by laser heating of a thin film surface. A gold thin film deposited on a quartz substrate is coated with 40 m film of the phosphor material. A cw argon ion laser (476 nm) beam is split into two beams, with the more intense beam focused to 15 m (1/e² radius) to heat the film through the quartz substrate. The weaker probe beam is chopped and focused tightly using a microscope objective to excite the phosphor from the other side. The spatial variation in lifetime, and hence the temperature distribution, is obtained by scanning the probe beam over the heated region. The temperature distribution measured for different film thickness's is compared with calculations using a finite element model. The calculated temperatures at the gold surface near the laser beam are higher than the experimentally measured values, and agree only when the heat-sinking effect of the phosphor material is taken into account. The results suggest that a phosphor layer thinner than a micron will be required (for 15 m laser spot size) so as not to perturb the temperature of the gold layer.