  We present a theory of surface plasma waves in metal films with arbitrary
electronic collision rate $\tau$. Both \textit{tangential} and \textit{normal}
modes are investigated. A universal self-amplification channel for these waves
is established as a result of the unique interplay between ballistic electronic
motions and boundary effects. The channel is shown to be protected by a general
principle and its properties independent of $\tau$. The effects of film
thickness and surface roughness are also calculated. Experimental implications,
such as Ferrel radiation, are discussed.
