Metallization of High Thermal Conductivity Materials
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Metallization of High Thermal Conductivity Materials
Yoshihiko Imanaka and Michael R. Notis Introduction Great progress in LSI (large-scale integration) has been made over the past 10 years, and more rapid progress is expected in the future. In recent years, the increased circuit density in LSI devices has led to remarkable heat generation, thereby creating thermalmanagement issues for microelectronics packaging.1,2 In order to develop highperformance LSI effectively, packaging and system-level configuration of the computer must be modified to handle the heat. There are two paths for heat dissipation from an integrated circuit (IC) through the chip back side and through the substrate, as shown in Figure 1. With respect to chip back-side cooling, a heatsink is attached to the back side of the LSI device to remove the heat efficiently, because solders are damaged by generated heat (see Figure 1a). The primary requirements for the sink material are high thermal conductivity, a coefficient of thermal expansion (CTE) close to that of Si, and low specific gravity. When LSI chips are placed directly on the electrode, heat dissipation is by means of the substrate (see Figure 1b). In addition to needing high thermal conductivity, and a CTE close to that of Si, the substrate then needs to have high electrical resistance and a low dielectric constant. The processing conditions of metallization and the characteristics of metallized materials strongly control LSI packaging performance and reliability for microelectronics applications because metallization provides contact with such advantageous characteristics as good electrical conductivity, high thermal conductivity, and strong adherence to ceramic substrates. Metallization is therefore the key technology in microelectronics packaging. In this article, we describe recent developments involving
MRS BULLETIN/JUNE 2001
materials with high thermal conductivity for use in the field of microelectronics packaging and the metallization technology of related materials.
High Thermal Conductivity Materials for Heatsink Applications The characteristics of high thermal conductivity materials, including metals, metal alloys, ceramic/metal composites, and ceramics, are listed in Table I. Cu and Al are most often used for heatsink materials.
Among metal alloys, Cu-W has been the standard for metal heatsinks in microelectronics packaging because W added to Cu provides a lower CTE than Cu alone, without markedly lowering the thermal conductivity.3 Cu-Mo alloys have been developed to reduce the density of the composites.4 Fe-Ni alloys, examples of which are 42 alloy, Invar, and Kovar, are also commonly used for applications requiring low thermal expansion, but these alloys also have low thermal conductivity. An Fe-Ni-Ag composite has been developed through infiltration of molten silver into a porous Fe-Ni metal preform. The material has advantages in cost and density, compared with refractory-based metal composites.5 Al-SiC composites have been used for their reduced density. Both liquid-m
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