CFD Simulation Analysis of Inter/Intra Chip Liquid Cooling for 3D Stacked ICs

Abstract

With the rapid advancement of high-density packaging technology, 3D chip stack has emerged as a viable solution to realize a higher performance of the electronic device [1,2]. Vertical integration with TSV or microbumps provides advantages of shorter interconnection length, which results in reduction of signal latency and power consumption. Moreover, increased integration density of multiple functional chips benefits in system design flexibility and small form factor compared to the conventional 2D/2.5D packages. Despite the promising advantages, thermal management has become a major bottleneck of 3D integrated system. 3D chip stacking architecture exacerbated the difficulty of removing the heat from high heat flux due to the limited heat path. Conventional indirect (remote) cooling such as air-based heat sink or liquid cold plate appears to be insufficient for exponentially increasing demand of heat dissipation of 3D ICs. Therefore, embedded liquid cooling has been gaining attention as a prominent solution to tackle the severe thermal issues. Extensive research on unique designs of embedded liquid cooling have been reported recently [3-7]. However, there are few studies on general comparison of the different types of embedded cooling structure. This paper presents a fundamental study on embedded liquid cooling for 3D IC with two processor chips stacked vertically. Two types of cooling solutions are introduced including inter-chip cooling and intra-chip cooling. Inter-chip cooling approach utilize a microgap space between chips stacked vertically as a coolant fluid path, whereas intra-chip cooling approach utilize silicon etching process to fabricates the microchannel within the backside of the chip. Numerical simulation of full-scale 3D model has been developed with Computational Fluid Dynamics (CFD) to analyze the thermohydraulic performance of each cooling methods. Basic structure of the model consists of Lid (heat spreader), Thermal Interface Material (TIM), two stacked chips, interconnect, and substrate, and those were applied to both intra/inter-chip cooling structures. Conjugate heat transfer problem was solved to evaluate the heat transfer and pressure drop characteristics of each structure. Temperature distribution path through a heat source was investigated to explore the effect of each component in terms of thermal resistivity. In addition, the effect of geometric parameter of each cooling structure, including channel/fin width, height, and microgap size, as well as the effect of flow direction of each stacked layer were studied. Furthermore, the effect of structural difference between two types of cooling methods on mechanical stress have also been investigated using FEM simulation. Comparisons of thermal and mechanical perspective were discussed to provide better understanding of the optimal cooling solution for 3D ICs.