Optimization of Thermal and Electrical Properties in Amorphous Carbon Thin Films for Phase Change Memory Electrodes

Phase-change memory (PCM) has emerged as a promising next-generation non-volatile memory technology, offering advantages such as scalability, fast programming speed, and reliability. However, efficient heat confinement during PCM operation remains a critical challenge. A significant portion of the heat generated through electrical pulses, which should be used to induce phase transitions in the active layer, escapes through the bottom electrode when using conventional electrode materials such as TiN or W. To address this issue, amorphous carbon (a-C) thin films, which possess intrinsically low thermal conductivity and tunable electrical properties, are an attractive option for PCM electrodes.

This study investigates the influence of sputtering pressure, post-annealing treatment, and nitrogen or boron doping on the thermal and electrical properties of a-C thin films, with the aim of optimizing their performance as PCM electrodes. DC sputtering was employed to deposit a-C thin films, with sputtering pressures systematically varied (2.5, 5, and 25 mTorr) to analyze their impact on the film properties. Thermal conductivity measurements were conducted via time domain thermoreflectance (TDTR). We revealed that increasing the sputtering pressure led to a decrease in thermal conductivity, while the electrical resistivity increased significantly with higher sputtering pressures. X-ray photoelectron spectroscopy (XPS) analysis indicated higher sp² bonding fractions at higher pressures, resulting in a more open structure prone to impurity incorporation. Optimal electrical performance was achieved at 2.5 mTorr, with a resistivity of 0.44 Ω cm and thermal conductivity of 1.47 W m^-1 K^-1 at room temperature. Post-annealing treatments at 400 °C for 30 minutes slightly decreased thermal conductivity and significantly reduced electrical resistivity, which can be attributed to increased sp² clustering and reduced impurity concentration. Additionally, both nitrogen and boron doping resulted in lower thermal conductivity and decreased resistivity compared to the pristine a-C film. We have observed that as the sp² ratio increases due to nitrogen or boron doping, the sound velocity decreases. Consequently, our results may indicate a significant reduction in the thermal conductivity of propagons. Post-annealing of nitrogen or boron doped a-C film further improved both thermal and electrical characteristics.

Our findings demonstrate that controlling sputtering pressure, post-annealing treatments, and nitrogen or boron doping can effectively optimize a-C thin film properties for PCM electrodes. The research yielded films with thermal conductivity below 1 W m^-1 K^-1 and adequate electrical conductivity, ideal for PCM electrode applications. These optimized a-C films show great promise in addressing heat management challenges in PCM devices, potentially enhancing efficiency and reliability in next-generation memory technologies.