  The energy bands and the global density of states are computed for
superconductor / normal-metal superlattices in the clean limit. Dispersion
relations are derived for the general case of insulating interfaces, including
the mismatch of Fermi velocities and effective band masses. We focus on the
influence of finite interface transparency and compare our results with those
for transparent superlattices and trilayers. Analogously to the rapid variation
on the atomic scale of the energy dispersion with layer thicknesses in
transparent superlattices, we find strong oscillations of the almost flat
energy bands (transmission resonances) in the case of finite transparency. In
small-period transparent superlattices the BCS coherence peak disappears and a
similar subgap peak is formed due to the Andreev process. With decreasing
interface transparency the characteristic double peak structure in the global
density of states develops towards a gapless BCS-like result in the tunnel
limit. This effect can be used as a reliable STM probe for interface
transparency.
