Nanoscale Thermal Management With Gas-Cooling

Nanoscale thermal management (NSTM) will become imperative as power density of IC’s increases further or as 3-dimensional IC’s are introduced. Such NSTM solutions must be integrated with the IC’s to be effective. It can be envisioned that an ideal NSTM solution will involve two-phase flow, liquid flow and gas flow. This paper focuses on gas flow, both as a fundamental thermal management technique in any future NSTM solution, and also as a basis for understanding more complex liquid and two-phase flow techniques that may also be involved in any NSTM solution. Heat removal by gaseous flow in any NSTM solution may be broken up in four fundamental processes: (1) bulk flow through micro-scale ducts where there may or may not be any heat transfer through the side-walls, (2) interaction of gaseous molecules with duct walls which may be at the same or a different temperature, (3) interaction between gas molecules and any enhanced surfaces such as carbon nanotubes (CNTs) as pin fins, and (4) thermal conduction and distribution in the device substrate. This paper touches upon all four fundamental process of heat removal by gaseous flow in an NSTM solution. Preliminary results from the first three processes are presented whereas a preliminary design and fabrication solution is presented for the fourth process. Preliminary computational results for pressure-driven flow of helium as the coolant gas through a micro-channel duct with a backward facing step are presented as an example for the first process. Backward facing step simulates any change in flow cross-section that may be unavoidable or desirable in any practical NSTM solution. Computation is based on direct simulation Monte Carlo (DSMC), where statistical noise due to low-speed flow is greatly eliminated through the use of the IP technique. Pressure boundary conditions are used in this simulation as they would be more realistic to represent an NSTM application. Preliminary results for computation of tangential momentum accommodation coefficient (TMAC) for helium-wall collisions are presented as an example for the second process. Molecular dynamics (MD) simulation of collisions between helium atoms and copper atoms in a crystallographically perfect copper wall are performed for different ratios between kinetic energy of helium atoms and Lenard-Jones energy of He-Cu interactions. The results are compared to limited experimental results that are available. Preliminary computational results for energy transfer between hydrogen molecules/atoms and a single wall carbon nanotube (SWNT) are presented as an example for the third process. Here MD simulation is employed where high temperature hydrogen molecules are allowed to collide with a SWNT that is initially at a lower temperature. As time progresses, the amount of net energy transfer from the hydrogen molecules to the SWNT is monitored. The result is normalized with the nominal surface area of the SWNT and the driving temperature differential to form coefficient for thermal energy transport (CTET). The values obtained are compared against macroscopic correlations. Deposition of nano-crystalline diamond is proposed as a solution for thermal spreading in the fourth process. Some examples of deposition and the corresponding atomic structure are presented.