We have studied the influence of nitrogen plasma exposure of the InP surface on the electrical characteristics of electron cyclotron resonance deposited Al/SiNx:H/InP devices. Nitrogen plasma exposure was performed just before the SiNx:H deposition without vacuum breaking. A 30 s plasma treatment at low microwave power (60 W) sharply reduces the minimum of the interface trap density, from 4x10(12) to 1.6x10(12) eV(-1) cm(-2) (obtained by the high-low frequency capacitance method). We explain this reduction due to the formation of P-N and/or In-P-N complexes at the InP surface during the plasma-exposure step. These bonds are broken thermally after a rapid thermal annealing of the device. The minimum of the interface trap density, obtained at the optimum plasma exposure conditions, is among the lowest ones reported in the Literature and similar to those obtained with more complicated processing technologies of the SiNx:H/InP structure. This is a clear indication that plasma exposure in N-2 atmosphere seems to be a valuable and simple surface conditioning method.