Copper damascene process and interlayer dielectrics with ultra-low permittivity have been introduced for manufacturing future devices with higher function speed. As materials with permittivity values lower than 2.2 are required, several kinds of porous materials have been proposed as candidates. However, these porous materials have been observed to shrink easily during CD (critical dimension) measurements with a CD-SEM. To solve this problem, the mechanism of shrinkage and the solution for damage-free SEM observation condition was studied. The shrinkage caused by different electron beam irradiation conditions in a CD-SEM (S-9260, Hitachi High-Technologies Corporation) was investigated with an atomic force microscope (AFM). The result shows that the shrinkage depends on the energy and the dose of electron irradiation. In addition, the change of chemical states and composition caused by electron beam irradiation was analyzed and the shrinkage mechanism was studied. The optimum electron beam irradiation conditions for damage-free measurement are proposed based on experimental results.