The decrease in signal processing speed due to increased resistance and capacitance delay resulting from aggressive miniaturisation of logic and memory devices is a major obstacle for continued down scaling of electronics.1-3 Particularly, minimizing the dimensions of interconnects ？ metal wires that connect different device components on the chip ？ is crucial for device scaling. The interconnects are isolated from each other by non-conducting or dielectric layers. Much of the recent research has focused on decreasing the resistance of scaled interconnects because integration of dielectrics using complementary metal oxide semiconductor (CMOS) compatible processes has proven to be exceptionally challenging. The key requirements for interconnect isolation materials are that they should possess low relative dielectric constants (referred to as ？？values), serve as diffusion barriers against migration of interconnect metals such cobalt into semiconductors and be thermally, chemically and mechanically stable. In 2005, the International Roadmap for Devices and Systems (IRDS) recommended dielectrics with ？-values of < 2.2 and the most recent report recommends dielectric values of ≤ 2 by 2028.4 Despite this, state-of-the-art low-？ materials, such as silicon oxide derivatives, organic compounds, and aerogels exhibit ？ values > 2 and possess poor thermo-mechanical properties.5 Here, we report a dielectric thin film with ultra-low ？ values of？1.78 and 1.16 ？ close to that of air (？ = 1) ？ at 100 kHz and 1 MHz, respectively, in amorphous boron nitride (a-BN) obtained using CMOS compatible low temperature process. We demonstrate that 3 nm thin a-BN is mechanically and electrically robust with breakdown strength of 7.3 MV/cm ？ exceeding requirements. Cross-sectional transmission electron microscopy reveals that a-BN is able to prevent diffusion of cobalt interconnect atoms into silicon under very harsh accelerated conditions ？ in contrast with reference barriers. Our results demonstrate that the amorphous counterpart of two-dimensional hexagonal BN possesses the ideal low-？ dielectric characteristics for high-performance electronics.