Thermoelectric Coolers:

System Level Modeling of Thin-Film Thermoelectric Coolers
Avram Bar-Cohen – PI
Bao Yang – Co-PI
Peng Wang – Research Assistant
Viatcheslav Litvinovitch – Undergraduate Research Assistant

Objective:
Define and quantify the role of thermoelectric energy conversion in the thermal control of high heat flux regions on semiconductor chips.

Background:
Thermal control of localized, on-chip “hot spots,” or regions of high heat fluxes that occur due to non-uniform heat generation across the macrocells constituting an advanced microprocessor, require the development of new and more aggressive thermal management techniques. Solid-state energy conversion devices, such as thermoelectric or thermionic micro-coolers, can be embedded in, or bonded to the back of, the silicon chip absorbing heat at the localized “hot spot” region and transporting it via electrons to a cooler region.

Current Effort:
A current project, in collaboration with UC Santa Cruz, involves the use of Si/SiGe superlattice micro-coolers that have demonstrated a cooling power density exceeding 500 W/cm² and surface temperature reductions of several degrees in an isolated configuration. In this study, we are numerically investigating the integration and packaging of such micro-coolers with IC chips using finite-element modeling and simulations with ANSYS. This parametric study of micro-cooler behavior explores the effect of chip thickness, chip electro-thermal properties, and micro-cooler characteristics of the temperature of single and multiple combinations of microcoolers and “hot spots” or “fireballs.”

Recent Publications:

Journal Papers:

P. Wang, and A. Bar-Cohen, “On-Chip Hot Spot Cooling Using Silicon Thermoelectric Microcooler”, Journal of Applied Physics, Accepted in May 2007.

B. Yang, P. Wang, A. Bar-Cohen, “Mini-Contact Enhanced Thermoelectric Cooling of Hot Spot in High Power Devices”, IEEE Transactions on Components and Packaging Technologies, 2007 (in press).

P. Wang, A. Bar-Cohen and B. Yang, “Analytical Modeling of Silicon Thermoelectric Microcooler”, Journal of Applied Physics, vol.100, no.1, pp.14501-1-13, 2006.

Conference Papers:

P. Wang, A. Bar-Cohen, and B. Yang, “Enhanced Thermoelectric Cooler for On-Chip Hot Spot Cooling”, Proceedings of Pacific Rim/ASME International Electronic Packaging Technical Conference and Exhibition (InterPack’07), Vancouver, Canada, July 8 -12, 2007. Paper Number: IPACK2007-33798.

Abstract: Due to shrinking feature size and increasing transistor density, combined with the performance demanded from next generation microprocessors, on-chip hot spots, with their associated high heat fluxes and sharp temperature gradients, have emerged as the primary driver for thermal management of today’s IC technology. This paper describes the novel use of thermoelectric coolers for on-chip hot spot cooling through the use of a copper mini-contact pad, which connects the thermoelectric cooler and the silicon chip thus concentrating the thermoelectric cooling power. A package-level numerical simulation is developed to predict the local on-chip hot spot cooling performance which can be achieved with such minicontacts.  Attention is focused on the hot spot temperature reduction associated with variations in mini-contact size and the thermoelectric element thickness, as well as the parasitic effect of the thermal contact resistance introduced by the minicontact enhanced TEC. This numerical model and simulation results are validated by comparison to spot cooling experiments with a uniformly heated chip serving as the test vehicle. The experimental results demonstrate that a copper mini-contact pad can improve spot cooling performance by 80 ~ 115% on a 500m thick silicon chip under optimum operating conditions and that larger power dissipation on the chip leads to better spot cooling performance.

P. Wang, and A. Bar-Cohen, “Simplified Thermal Model of Silicon Microcooler for On-Chip Hot Spot Remediation”, Proceedings of Pacific Rim/ASME International Electronic Packaging Technical Conference and Exhibition (InterPack’07), Vancouver, Canada, July 8 -12, 2007, Paper Number: IPACK2007-33940.

Abstract: Thermal management of on-chip hot spots has become an increasing challenge in recent years because such localized high flux hot spots can not be effectively removed by conventional cooling techniques. The authors have recently explored the novel use of the silicon chip itself as a solid state thermoelectric microcooler (µTEC) for hot spot thermal management. This paper describes the development and application of a thermo-electric design tool based on closed form equations for the primary variables. This tool can be used to effectively reduce the complexity and required time for the design and optimization of the silicon microcooler geometry and material properties for on-chip hot spot remediation.

P. Wang, A. Bar-Cohen and B. Yang “Analytical Modeling of On-Chip Hot Spot Cooling Using Thermoelectric Microcooler”, Proceedings of ASME International Mechanical Engineering Congress & Exposition (IMECE’06), Chicago, Illinois, November 5-10, 2006, Paper Number: IMECE2006-14383.

Abstract: Thermal management of microprocessors has become an increasing challenge in recent years because of localized high flux hotspots which can not be effectively removed by conventional cooling techniques. This paper describes the novel use of the silicon chip itself as thermoelectric microcooler to suppress the hotspot temperature. A three-dimensional analytical thermal model of the silicon chip, including localized silicon thermoelectric cooling, thermoelectric heating, Joule heating, hotspot heating, background heating, and conductive/convective cooling on the back of the silicon chip, is developed and used to predict the on-chip hotspot cooling performance. The effects of  chip thickness, microcooler size, doping concentration and parasitic Joule heating from the electric contact resistance on hotspot cooling are investigated in details.

P. Wang, A. Bar-Cohen and B. Yang, “Impact of Thermal Contact Resistance on Hot Spot Cooling Using Enhanced Thermoelectric Coolers”, Proceedings of IMAPS Thermal Management’06, Palo Alto, California, September 10-13, 2006 (CDROM).

P. Wang, A. Bar-Cohen and B. Yang, “Multiple Silicon-Based Thermoelectric Microcoolers for Hot Spot Thermal Management”, Proceedings of the 13th International Heat Transfer Conference (IHTC-13), Sydney, Austria, August 13-18, 2006. Paper Number: CND-09.

B. Yang, P. Wang, A. Bar-Cohen. “Thermoelectric Mini-Contact Cooler for Hot Spot Removal in High Power Devices”, Proceedings of the 56th Electronic Components and Technology Conference (ECTC-56), San Francisco, California, May 30 - June 2, 2006, pp. 997-1002.

Abstract: Cooling hot-spots with high heat flux (e.g., >1000W/cm2) is becoming one of the most important technical challenge facing today’s IC industry. More aggressive thermal solutions, than would be required for uniform heating, are highly desired. Solid state thermoelectric coolers (TECs) have received recent attention for hot-spot thermal management. However, present day TECs typically have cooling flux much lower than heat flux in the hot-spots. In this work, we reported an innovative technique–TE Mini-contact- to significantly increase cooling flux of TECs for the application in hot-spot cooling. A chip package featuring a TE Minicontact cooler integrated with conventional integrated heat spreader (IHS) and heat sink is designed. The cooling performance of such chip package has been investigated by using a 3-D numeric model. It is found that the cooling in the hot-spot (1250W/cm2, 400µm by 400µm) can be about 19oC better in the proposed package than that achieved in the conventional chip package without TEC. The effects of trench, die thickness, and TEC misalignment on the cooling of the hot-spot are also discussed.

Y. Zhang, G. Zeng, A. Shakouri, P. Wang, and A. Bar-Cohen, “Optimization of Doping Concentration for Three-Dimensional Bulk Silicon Microrefrigerator”, Proceedings of the 22nd Semiconductor Thermal Measurement, Modeling, and Management Symposium (Semi-Therm-22), Dallas, Texas, March 14-16, 2006, pp. 232-236.

P. Wang, A. Bar-Cohen, B. Yang, and A. Shakouri, “Thermoelectric Microcooler for Hot Spot Thermal Management”, Proceedings of the Pacific Rim/ASME International Electronic Packaging Technical Conference and Exhibition (InterPack’05), San Francisco, California, July 17 -22, 2005. Paper No: IPACK2005-73244.

Y. Zhang, G. Zeng, A. Shakouri, P. Wang, and A. Bar-Cohen, “Experimental Demonstration of Microrefrigerator Flip-chip Bonded with IC Chips for Hot Spot Thermal Management”, Proceedings of the Pacific Rim/ASME International Electronic Packaging Technical Conference and Exhibition (InterPack’05), San Francisco, California, July 17 -22, 2005. Paper Number: IPACK2005-73466.

Solbrekken, G., Bar-Cohen, A., and Shakouri, A.. “Experimental Demonstration of Thermal Management Using Thermoelectric Energy Conversion.” Proceedings, IEEE ITHERM 2004. Las Vegas, NV. June, 2004.