An electron tube having a hole or "window" covered by a thin, electron permeable membrane is provided with means for minimizing stress concentration in the membrane adjacent the periphery of the hole, thereby relieving stress concentrations which would otherwise occur when the membrane is forced inwardly into the hole by atmospheric or other pressure on the exterior of the tube. In a manufacturing method, a polymeric ring may be provided between the membrane and the exterior wall of the tube. The polymeric ring, desirably a polyimide has a glass transition temperature less than the elevated temperature used to expel volatile materials from the interior of the tube. The polymeric ring substantially relieves stresses induced by differential thermal expansion or contraction at temperatures between the glass transition temperature and the elevated temperature, as during cooling following the volatile removal step.