The effects of the aluminum nitride coating thickness on molybdenum emitter tips were investigated by an in situ I-V measurement technique inside a typical magnetron sputtering system. ALN was deposited on Mo tips using a de-modulated 1 kW power source at 200 degrees C. Each I/V measurement was carried out immediately following a 15 a ALN deposition. Significantly improved field emission was observed as well as a strong emission thickness dependence, which we attribute to the influence of space charge.