Removing Heat from Si with a “Thermal Circuit”: An Ab-Initio Study

Electronic devices generate unwanted heat. The removal of this heat often involves a layer of a high-thermal-conductivity material X deposited at a strategic location on the Si chip. The layer is then cooled using a mechanical fan or more sophisticated means. The authors present here the results of ab-initio molecular-dynamics simulations which examine the thermal interactions between heat generated in Si and three materials X: Ge, C, and Si itself. The geometry is a very thin wire of atoms X located within a Si slab. Heat is periodically removed from the wire as if it were connected to a heat sink. The wire then absorbs heat from the warmer Si slab, thus cooling it. The rate at which the Si temperature drops depends on how efficiently heat crosses the Si|X boundary and then is absorbed by the wire. The interactions involve the coupling of Si-Si vibrational modes with Si-X (interface) and then X-X (layer) modes. The worst performance occurs for XC as lower-frequency Si-Si modes must decay into higher-frequency Si-C then CC modes. This involves slow two- (or more) phonon processes. The fastest cooling rate occurs when the wire is made of Si itself: there is no Si|Si interface and the vibrational interactions involve only Si bulk modes which couple to each other resonantly. The Ge wire performs quite well, as the Si-Si modes easily decay into lower-frequency Si-Ge and Ge-Ge modes.