  We prepared thin layers of amorphous silicon by deposition of a liquid-phase
polysilane precurser on glass substrate. Raman scattering provides evidence for
residual tensile stress in the silicon, which is evaluated quantitatively.
Under treatment with hydrofluoric acid, this stress leads to spiral cracks in
the silicon. We explain the process of crack formation and examine this
phenomenon both analytically and numerically, the latter with the finite
element method (FEM). The FEM yields the geometry correction factor for such
spiral cracks in terms of the Griffith criterion. This allows for the first
time the determination of fracture toughness of amorphous silicon, which is
greatly enhanced in comparison with crystalline silicon.
