An investigation into the phonon contamination of x‐ray sensitive superconducting tunnel junctions arising from the x‐ray photoabsorption in various substrates has been conducted. Results are presented on the design of a superconducting tunnel junction (STJ) which substantially reduces or even eliminates phonon induced noise from the substrate. Such noise is the predominant feature in x‐ray spectra from junctions due to the bulk of the photons being absorbed in the substrate rather than in the thin superconducting film. The design involves the choice of a suitable buffer sandwich between the substrate and the STJ. Such a buffer would appear not only to attenuate the phonons created in the x‐ray photoabsorption in the substrate but also to scatter the phonons inelastically, introducing a frequency down‐conversion. Such a process ensures that few phonons of energy sufficient to break Cooper pairs in the superconducting film of the STJ enter the junction. ©1996 American Institute of Physics.