Thermal and Mechanical Analysis of Embedded Liquid Cooling with Microchannel and Pin-fin Structures

Abstract

Despite the fact that computing performance requirement is increasing exponentially with the rapid growth of AI technology, CMOS scaling is approaching its physical limit. As such, advanced packaging is expected to be a leading technology for the further growth of semiconductor industry. 3D heterogeneous integration of multi-functional chips offers significant advantages over conventional packages. However, thermal management of 3D IC is a critical issue that needs to be addressed, since its complex architecture exacerbates the difficulty of heat removal. Chip embedded liquid cooling is gaining its attention as an effective solution for large heat dissipation due to its high heat transfer performance. It utilizes silicon micro-etching process to fabricate microchannel or micro pin-fin structure within the backside of a chip. This approach enables direct liquid cooling of a chip with low thermal resistance. While providing the high heat dissipation, electrical connection between the adjacent chip is guaranteed by Through Silicon Via (TSV) integrated within the microchannel/pin-fin. Therefore, those microstructures play crucial role not only to determine the thermal performance, but also to have effect on the reliability of electrical interconnect. Thermo-mechanical stress and strain induced during high-temperature process could damage delicate microchannel/pin-fin with size of a few tens of microns, and consequently deteriorate the reliability. There have been various studies on embedded liquid cooling from thermal perspective, but there are only few study that evaluates its mechanical stability performance. In this study, full-3D model of two chips stacked package with embedded liquid cooling is developed and investigated numerically from both thermal and mechanical perspectives. Mechanical analysis using FEA simulation is conducted to evaluate the stress/strain distribution the across the system. Microchannel and pin-fin structures are compared against the bare chip without microstructure as a baseline, and the results showed that both microchannel and pin-fin exhibit higher local stress with 1.22x, and 2.22x larger than bare chip respectively. Substantial stress concentration was occurred at the corner of the bottom chip especially for the pin-fin case, which could lead the potential defect of the system. For the thermal analysis, Computational Fluid Dynamics (CFD) simulation is performed to evaluate heat transfer and pressure drop characteristics of each microstructure. Pin-fin demonstrated superior heat dissipation but with the expense of higher pressure drop than microchannel. Based on those simulation results, novel design with hybrid structure of microchannel and pin-fin are proposed to achieve balanced performance for both thermal and mechanical aspects. Pin-fins are placed at the center line of the chip avoiding the corner area where the maximum local stress would occur, whereas microchannel are integrated in the remaining area. Such novel architecture combines the thermal benefits of pin-fin as well as the mechanical strength of microchannel. It is numerically demonstrated to achieve high heat transfer while maintaining sufficient mechanical reliability simultaneously. Furthermore, the ratio of microchannel/pin-fin area is also investigated to maximize the thermal and mechanical performances. Those results provide the insight for the optimal design of novel hybrid structure and presented its expanded capabilities as thermal solution for 3D ICs.