Wide bandgap materials in thermal management of electronic structures

Abstract

The need for optimized thermal management in microelectronic devices derives from several sources, of which the ever-increasing miniaturization is only one. Power dissipation needs in certain designs are up to 40 W/cm/sup 2/, while 100 W/cm/sup 2/ is said to be required in the not too distant future. New multilayer ceramic integrated circuits (MCIC's) contain many "buried heat sources" in the form of resistors, capacitors, and inductors, the volume of which are increasing as well. Microelectronic packages and circuits and particularly high power microelectronics, require both temperature and temperature gradient control. The first primarily assures that the components are kept below a certain temperature threshold, providing reliability. Keeping the circuit between specific low and high temperature boundaries, primarily effects performance and structural considerations. Where other solutions are not available due to size, thermal load, or other systems considerations, a cold plate-a substrate equipped with liquid flow channels-could be resorted to. In most cases, however, the complexity that cold plates introduce, cannot be tolerated if for no reason other than cost. Heat exchange via a high thermal conductivity substrate connected to an appropriate heat sink must, in most cases, be relied on. Electrically conducting (metallic) substrates have become available but most applications require it to be a dielectric, exemplified by the most widely used "electronic" alumina (Al/sub 2/O/sub 3/, 96%) with an average thermal conductivity value of 20 W/mK. The last five years or so have seen an increase in the use of polycrystalline AlN (k/spl cong/175 W/mK). Its application has also been limited due to cost. Substrates based on yet another (polycrystalline) wide bandgap material, SiC (250 W/mK), are again too costly for most applications and are conductive. Furthermore, SiC occurs in several polymorphic forms and many polytypes i.e., it is hard to obtain phase pure, potentially causing property variations. SiC as well as diamond are available in single crystal thin film form and various quality (conductivity) grades. Commercially available polycrystalline thin film diamond ranges in TC value between 750 and 1500 W/mK. The as-deposited material displays a high surface roughness requiring, in most cases, extensive and thus expensive polishing to "planarize" it to receive the electronic circuitry.