BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIGS. 1A and 1B are cross-sectional views showing modes of a memory element before and after a voltage is applied; FIGS. 2A to 2C are cross-sectional views showing how a bubble is generated; FIG. 3 is a TEM photograph of a portion in which a bubble is formed; FIGS. 4A and 4B are a diagram of a passive matrix type memory device and a d