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Although the stacking of multiple strata to produce three-dimensional (3D) integrated circuits (ICs) improves interconnect length and hence reduces power and latency, it also results in the exacerbation of the thermal management challenge owing to the increased power density. There is a need for design tools to understand and optimise the trade-off between electrical and thermal design at the device and block levels. This study presents results from thermal-electrical co-optimisation for block-level floorplanning in a multi-die 3D IC under various manufacturing and physical design constraints. A method for temperature computation based on linearity of the governing energy equation is presented. This method is combined with previously reported electrical delay models for 3D ICs to simultaneously optimise both the maximum temperature and the interconnect length. It is shown that co-optimisation of thermal and electrical objectives results in a floorplan that is attractive from both perspectives. Physical design constraints because of cost-effective 3D manufacturing such as using fully or partly identical dies using reciprocal design symmetry (RDS), differentiated technology in each die and thinned die/wafer are discussed and their impact on the thermal-electrical co-optimisation is investigated. In some cases, the cheapest manufacturing choice, such as using identical die, for each layer may not result in optimal thermal and electrical design. Results presented in this work highlight the need for thermal and electrical co-design in multi-strata microelectronics, and for reconciling manufacturing and design considerations in order to develop practical design tools for 3D ICs.