Integrated AlN/diamond heat spreaders for silicon device processing

Abstract

Diamond with its very high thermal conductivity is an excellent choice for spreading heat produced during the operation of all electronic devices and in particular, high power and high frequency devices. Hitherto, diamond heat spreaders have been bonded to silicon devices using metallization and soldering layers. However, the interfacial thermal resistances at the interfaces decrease the effective thermal conductivity of the bonded heat spreader. Although direct growth of diamond on Si is expected to reduce the interfacial resistance, it has not been attempted. Contamination of the device wafers from carbon, oxygen and other impurities by diffusion during the growth of diamond could be a limiting factor. Moreover, diamond oxidizes above 600°C in the presence of oxygen and may not be stable during oxidation and other high temperature steps in silicon device processing. Growth of diamond by CVD is not defect free and contains voids that decrease the thermal conductivity. Hence, a buffer layer of AlN that fills these voids and thereby reduce the thermal resistance is thought to be beneficial.

The growth and characterization of AlN and diamond films on the backside of Si (100) wafer with silicon nitride on the device side is investigated. AlN films were deposited by pulsed DC reactive magnetron sputtering at 600°C. Diamond film was deposited by microwave plasma chemical vapor deposition at 900°C. The films were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) for crystalline quality, by scanning electron microscopy (SEM) for morphology, and by infrared thermography for heat spreading characteristics. The heat spreading characteristics of the wafer with the composite AlN/diamond films were found to be superior to that of wafers with no heat spreaders or to the wafers with either single layer diamond or single layer AlN heat spreaders. Deep level transient spectroscopy (DLTS) and secondary ion mass spectroscopy (SIMS) were performed on the samples with and without the heat spreader for determining the concentration of the impurities. The results showed that the purity of the wafers is not altered. The device characteristics were studied by fabrication of Schottky diodes on the wafers with the composite AlN/diamond heat spreader and compared with that of devices on wafers with no heat spreader. The device characteristics were found to be similar and unaffected by integration with AlN/diamond heat spreader. Thus, the integration of AlN/diamond heat spreaders with silicon device processing has been shown to be successful.