For stability against the thermal budget of the CMOS BEOL process, we developed a new solid-electrolyte switch that uses a SiO2-Ta 2O5 composite as the electrolyte. This switch has high thermal stability because thermal diffusion of Cu+ ions is suppressed in the composite. Moreover, its switching characteristics after thermal annealing are similar to those of a Ta2O5 switch without annealing. The switch with the SiO2-Ta2O5 composite electrolyte has good ON-state durability against DC current stress; its durability is comparable to that of a single via in interconnects. The switch can be implemented in the local interconnection layers of LSIs.