Period expansion of Co/C and CoN/CN soft x-ray multilayers has been investigated by x-ray diffraction and Raman spectroscopy. Below the anneal temperature of 400 °C, the period expansion (< 12&percent;) of Co/C multilayers is mainly caused by the graphitization of the amorphous carbon layers. By 500 °C, the crystallization and agglomeration of Co layers induce an enormous period expansion (∼40&percent;). The period expansion of CoN/CN multilayers is only 4&percent; at 400 °C, which is much smaller than that of Co/C multilayers. The interface patterns of the CoN/CN multilayers still exist even if they were annealed at 700 °C. The Raman spectroscopy analyses indicate that the formation of thesp3bonding can be suppressed effectively by doping N atoms, and thus the period expansion is decreased considerably at annealing temperatures below 600 °C. The significant suppression of grain growth above 600 °C is believed to be attributed to the coexistence of hcp and fcc Co structures induced by interstitial N atoms, which cause the high-temperature period expansion decrease. The results also imply that the structural stability of Co/C soft x-ray multilayers can be significantly improved through doping N atoms.©1997 American Institute of Physics.