  The main result of the present paper consists in a quantitative estimate of
unique continuation at the boundary for solutions to the wave equation. Such
estimate is the sharp quantitative counterpart of the following strong unique
continuation property: let $u$ be a solution to the wave equation that
satisfies an homogeneous Robin condition on a portion $S$ of the boundary and
the restriction of $u_{\mid S}$ on $S$ is flat on a segment $\{0\}\times J$
with $0\in S$ then $u_{\mid S}$ vanishes in a neighborhood of $\{0\}\times J$.
