Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics(1-3). Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (kappa values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends(4) the development of dielectrics with kappa values of less than 2 by 2028. Existing low-kappa materials (such as silicon oxide derivatives, organic compounds and aerogels) have kappa values greater than 2 and poor thermo-mechanical properties(5). Here we report three-nanometre-thick amorphous boron nitride films with ultralow kappa values of 1.78 and 1.16 (close to that of air, kappa = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-kappa dielectric characteristics for high-performance electronics.