  Mechanical properties of a nanomechanical resonator have a significant impact
on the performance of a resonant Nano-electromechanical system (NEMS) device.
Here we study the mechanical properties of suspended membranes fabricated out
of low-pressure chemical vapor deposited silicon nitride thin films.
Doubly-clamped membranes of silicon nitride with thickness less than 50 nm and
length varying from 5 um to 60 um were fabricated. The elastic modulus and
stress in the suspended membranes were measured using Atomic Force Microscope
(AFM)-based nanomechanical spectroscopy. The elastic modulus of the suspended
membranes was found to be significantly higher than those of corresponding thin
films on the substrate. A reduction in the net stress after the fabrication of
suspended membrane was observed and is explained by estimating the
contributions of thermal stress and intrinsic stress. We establish a
mathematical model to calculate the normalized elastic modulus of a suspended
membrane. Lastly, we study the capillary force-gradient between the SiNx
suspended membrane-Si substrate that could collapse the suspended membrane.
