Density measurements of PECVD hydrogenated silicon nitride films, Si(x
)N(y)H(z), are reponed. Two methods are investigated: the critical ang
le of reflection of x rays and direct calculations with atomic composi
tion deduced from infrared chemical bond analysis. Two kinds of sample
are studied: as-deposited films with different atomic compositions an
d films with given composition post-annealed up to 1000-degrees-C. On
the first kind of sample both methods confirm the increase of density
of the films when they are prepared in nitrogen-rich plasma. Moreover,
from x-ray measurements, we deduce a maximum density of 3 +/- 0.1, do
se to the high-temperature CVD silicon nitride value. In the second ki
nd of sample, no significant change is observed in the specific mass,
and this agrees with stability properties of the low-hydrogen-content
almost stoichiometric silicon nitride reported recently.