Silicon nitride thin films used for transistor spacer are usually under large tensile residual stresses. When a film is thick enough, it can crack by channel cracking mechanism. The toughness of these films, used in models for failure criteria, can be obtained by determining the critical thickness for channel cracking of blanket films with known residual stresses. However, depositing films of a range of different thickness to determine the critical thickness is tedious, and this process also introduces risks of producing particles in the deposition chamber when the film exceeds the critical thickness. An alternative technique is to deposit films of thickness below critical thickness and then to apply additional tensile stresses to the film by bending the substrate. A critical film stress can be determined for channel cracking from which the film toughness is obtained. This paper describes the experiments performed for a new SiN film and presents the results.