A semiconductor device including a semiconductor die in a die stack under-filled with a film. Once the semiconductor die are formed, they may be stacked and interconnected. The interconnection may leave a small space between semiconductor die in the die stack. This space is advantageously completely filled using a vapor deposition process where a coating is deposited as a vapor which flows over all surfaces of the die stack, including into the spaces between the die in the stack. The vapor then deposits on the surfaces between and around the die and forms a film which completely fills the spaces between the die in the die stack. The material used in the vapor deposition under-fill process may for example be a member of the parylene family of polymers, and in embodiments, may be parylene-N.