Prépublication 04-40, IRMAR, UMR-CNRS 6625, Université de Rennes 1 (Août 2004)This article is a continuation of previous works by Bovier-Eckhoff-Gayrard-Klein, Bovier-Gayrard-Klein and Helffer-Klein-Nier. It is concerned with the analysis of the exponentially small eigenvalues of a semiclassical Witten Laplacian. We consider here the case of riemanian manifolds with boundary with a Dirichlet realization of the Witten Laplacian. A modified version of this preprint has been published in Mémoires de la SMF vol. 105, (2006