  Silicon nitride (Si3N4) has emerged as a promising material for integrated
nonlinear photonics and has been used for broadband soliton microcombs and
low-pulse-energy supercontinuum generation. Therefore understanding all
nonlinear optical properties of Si3N4 is important. So far, only stimulated
Brillouin scattering (SBS) has not been reported. Here we observe, for the
first time, backward SBS in fully cladded Si3N4 waveguides. The Brillouin gain
spectrum exhibits an unusual multi-peak structure resulting from hybridization
with with high-overtone bulk acoustic resonances (HBARs) of the silica
cladding. The reported intrinsic Si3N4 Brillouin gain at 25 GHz is estimated as
7x10^-13 m/W. Moreover, the magnitude of the Si3N4 photoelastic constant is
estimated as |p12| = 0.047 +/- 0.004. Since SBS imposes an optical power
limitation for waveguides, our results explain the capability of Si3N4 to
handle high optical power, central for integrated nonlinear photonics.
