the specification discloses an isolation trench formed in a semiconductor body . a stress relief layer of oxide is formed on the interior walls of the trench , the layer being sufficiently thin to prevent stressing of the lower corners of the trench . a masking layer of nitride is formed over the layer . an isolation body of oxide or polysilicon then refills the remainder of the trench and a cap oxide and layer of field oxide is formed over the semiconductor body and the filled trench .