Cracks that propagate with near-perfect sinusoidal form are reported in amorphous silicon-rich silica films deposited onto (001) silicon substrates by plasma-enhanced chemical vapour deposition and subjected to thermal annealing. The cracks are shown to result from high tensile stresses that develop in the film during thermal annealing at temperatures in the range up to 700 degrees C, a process shown to be correlated with the loss of hydrogen from the films. Two distinct modes of crack propagation are reported: straight cracks that propagate along directions parallel to [100] cube-edge directions in the substrate, and oscillating cracks that propagate with sinusoidal form parallel to [110] diagonal directions. Sections through the cracks show that the oscillating cracks have a complex three-dimensional structure that extends through the glassy film and into the underlying silicon substrate. This involves a correlated oscillation between the crystal-lographic orientation of the crack in the surface plane and that of the crack extension into the substrate. Whereas a complete theoretical treatment of this behaviour would be extremely complicated, a simple theory is developed to demonstrate that an oscillating crack has a minimum energy per unit length for a particular wavelength and amplitude that depends upon the physical parameters of both film and substrate. The energy at this minimum is shown to be lower than that of a straight crack for certain parameter ranges so that the oscillating geometry is preferred.