I have one reference on Youngs modulus of silicon nitride:
Sensors and Actuators 20(1989) pp. 135-141. They have measured
Youngs modulus of nitrid to:

290 GPa for LPCVD nitride
210 GPa for Plasma-CVD nitride

Youngs modulus will be dependent on the deposition parameters as these
numbers show.

Best regards

Ole Sondergard
Danfoss A/S
DK-6430 Nordborg
Denmark