Improving strength of LPCDV Si3N4 freestanding thin films by nanometer thick compressive adlayer

Si3N4 is a material widely used in MEMS technology. High mechanical strength makes Si3N4 attractive for applications where there is a need for ultrathin, yet robust, freestanding films such as nanometer thick X-ray and TEM windows. In this work, mechanical properties of single layer Si3N4 and bilayer Si3N4+B membranes were studied by bulge test method. Burst pressure and corresponding membrane stress in Si3N4 layer were found to be significantly increased when a 3nm thick Boron layer is deposited on the top side of 25nm thick Si3N4 membranes, whereas a Boron layer applied to the bottom side of the membranes has no effect on the membrane strength. Using FEM simulations we show that the Boron layer deposited at the top side decreases the maximum tensile stress in Si3N4 near the membrane edge, where a significant contribution to the total stress comes from bending. From this we conclude that failure in single layer Si3N4 membranes during bulge test is dominated by fracture at the edge. The burst pressure of bilayer Si3N4+B membranes was found to be higher for membranes with lower (more compressive) residual stress in Boron, which indicates that failure of bilayer membranes is caused by fracture in the Boron layer.