Thermal characterization of nanostructured superlattices of TiN/TaN: Applications as electrodes in Phase Change Memory

Phase Change Memory (PCM) technology relies on the contrast in electrical resistance between the amorphous and crystalline states of a chalcogenide active material. Electrical PCM devices use Joule heating by short pulses of current to induce phase change, such that the amount of heat injected into the active material and the rate of cooling determine the final state of material formed. In this paper, we explore the possibility of replacing commonly used TiN electrodes by nanostructured superlattices of TiN/TaN that have lower through-plane thermal conductivity, in order to improve the confinement of heat within the phase change material and achieve a reduction in the device programming current. TiN(m)/TaN(n) superlattices were grown on Si substrates using physical vapor deposition, m and n representing the intra-period thicknesses of TiN and TaN layers respectively (m, n: 5 - 25 nm). The through-plane thermal conductivity of these superlattices was measured using time-domain thermoreflectance (TDTR), and was found to be in the range 1.5 - 2 W/m-K, a reduction from the bulk conductivity of TiN (~ 19 W/m-K) by up to a factor of 10. Transmission Electron Microscopy (TEM) was used to characterize film morphology, pointing to additional sources of carrier scattering that might lead to this reduction in conductivity, and suggesting avenues for optimization of growth parameters. The low thermal conductivity of the superlattice films opens up the possibility of using them as bottom electrodes in PCM, towards the goal of reducing power consumption and improving device packing density. A simplified 1D thermal model predicts that reductions in programming current b y ~75% are possible.