Study on High Thermal Conductivity AlN Thin Films Sputtered at Low Temperature

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1. Research Background of Low-Temperature Sputtering Deposition of Submicron Aluminum Nitride Thin Film

The generation of heat affects the performance and lifespan of almost all modern electronic devices. This is particularly true in high-density integrated circuits and power or radio frequency (RF) electronic devices, where high temperatures can reduce the performance of transistors, increase leakage, and ultimately shorten the lifespan of the devices. A temperature higher than the optimal operating range of only 5 ° C can halve the lifespan of certain equipment.

Thermal management can be achieved by actively adjusting heat flux and managing thermal transients, such as using emerging thermal transistors and diodes. Passive methods include using thin films to block or remove heat from the hotspots of electronic devices. Such heat sinks must have high thermal conductivity, but they usually must be electrical insulators to prevent interference between components, so only a few materials (such as aluminum nitride (AlN), boron nitride (BN), and diamond) have these characteristics.

AlN has attracted widespread attention due to its large bandgap (about 6.1 eV) and excellent thermal conductivity. In fact, the thermal conductivity of AlN films has been proven to be several hundred and several micrometers thick, but AlN films are typically deposited above 1200 ° C. Moreover, integrated electronics will also benefit from thinner microscale AlN films, whose thermal conductivity has not yet been optimized, and its thermal limits are also poorly known.

2. High Thermal Conductivity AlN Thin Films by Low-Temperature Sputtering Deposition

A research team has made the latest progress in developing AlN thin films with low temperature deposition and high thermal conductivity. AlN is one of the few electrical insulation materials with excellent thermal conductivity, but high-quality films typically require extremely high deposition temperatures (>1000°C). For thermal management applications in dense or high-power integrated circuits, it is important to deposit heat sinks at low temperatures (<500°C) without affecting the underlying electronic devices.

The team demonstrated 100 nm to 1.7 um AlN films obtained by low-temperature (<100°C) sputtering. The relationship between the thermal properties, grain size, and interface quality of AlN thin films was analyzed by X-ray diffraction, transmission X-ray microscopy, Raman and Auger spectroscopy. By controlling the deposition conditions of the reaction, an ~3× variation in thermal conductivity (~36–104 W m^–1 K^–1)  of a ~600 nm AlN thin film was achieved, and its upper limit represents one of the highest values of AlN thin film thickness at room temperature, especially at deposition temperatures below 100°C.

Fig. 1 Experimental overview for sputtered high thermal conductivity AlN thin films

Fig. 2 Compared with the literature results, the room temperature planar thermal conductivity of the AlN film is a function of (a) deposition temperature and (b) thickness.

Fig. 3 Correlation between thermal conductivity, grain size, and gas composition of ~600 nm AlN thin films

Fig. 4 Transmission electron micrographs show the microstructure of sputtered AlN film on c-Al2O3 under different conditions

Fig. 5 HRTEM and FFT analyses of AlN films’ crystallization at the substrate interface deposited on c-Al2O3 under different conditions

Fig. 6 Measurement of boundary conductivity of AlN thin films deposited on Si (111) and c-Al2O3

Fig. 7 Relationship between thermal conductivity and (a) film thickness of the AlN films and (b) deposition/synthesis temperature

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