We analyze the influence of rapid thermal annealing on Al/SiNx:H/Si structures with a nitrogen to silicon ratio of 1.55 in the insulator bulk. The SiNx:H is deposited by the electron cyclotron resonance plasma method and the films were annealed at temperatures ranging from 300 to 1050 degrees C. We have related the changes of the interface trap density with those of the insulator bulk density of dangling bonds, resistivity, and breakdown field. A sharp dip in the dangling bond density is observed at moderate annealing temperatures, from 1.8 X 10(18) cm(-3) for the as-deposited film down to 9.6 X 10(16) cm(-3) at the point of inversion of the trend between 500 and 600 degrees C. The density of interface states is also reduced in this range of temperatures from 3.6 X 10(11) to 1.2 X 10(11) eV(-1) cm(-2). Resistivity and breakdown held are maintained in the range 5 X 10(14)-5 X 10(15) Ohm cm and 6.4-6.6 MV/cm, respectively, up to a temperature of 600 degrees C. We attribute the improvement of the interface characteristics and the decrease of dangling bonds to a thermal relaxation and reconstruction of the SiNx:H lattice and its interface with the silicon substrate. For temperatures above this threshold the electrical properties suddenly deteriorate and the density of dangling bonds increase. At even higher temperatures (above 900 degrees C) a release of hydrogen from N-H bonds takes place.