Tensilely-stressed SiN films were produced by reactive sputtering of silicon in a krypton-nitrogen magnetron radio-frequency plasma. These SiN films can be used to produce tight free-standing membranes possessing excellent optical characteristics by using LSI microfabrication techniques. The origins of tensile stress were identified by using optical emission spectroscopy and by analyzing the film structure. Using heavy krypton atoms as a nitrogen plasma diluent, we reduced the peening effects and the mobility of the film constituents and enhanced the shadowing effects on the deposition surface. The production of the tensilely-stressed films are attributed to these heavy atom effects. Rutherford backscattering spectroscopy and electron probe micro-analysis revealed a uniform element distribution and a negligible inclusion of krypton in the films; the film composition was SiN1.21, independent of the Kr density used for processing. X-ray diffraction, infrared, ultraviolet-visible, and thermal desorption spectroscopies showed that the SiN films had an amorphous structure and that the degree of nitridation and the atomic density in the Si-N networks were enhanced by using krypton, resulting in a rigid structure suitable for free-standing membranes.