The physical system under study is a bonded semiconductor wafer into which hydrogen ions have been implanted within a planar zone, typical of a configuration used in forming semiconductor-on-insulator compliant substrates by wafer splitting. Under the assumption that splitting is a consequence of crack growth driven by hydrogen gas pressure, a lower bound estimate of the implant density required for large area crack growth is obtained which, for an ideal gas, depends only on the cohesive strength of the material and on temperature. ©1997 American Institute of Physics.