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In this paper, we present an agglomeratively multilevel floorplanning/placement framework based on the B -tree representation called MB- tree to handle the floorplanning and packing for large-scale building modules. The MB-tree adopts a two-stage technique, i.e., clustering followed by declustering. The clustering stage iteratively groups a set of modules based on a cost metric guided by area utilization and module connectivity and at the same time establishes the geometric relations for the newly clustered modules by constructing a corresponding B -tree for them. The declustering stage iteratively ungroups a set of the previously clustered modules (i.e., perform tree expansion) and then refines the floorplanning/placement solution by using a simulated annealing scheme. In particular, the MB-tree preserves the geometric relations among modules during declustering, which makes the MB-tree an ideal data structure for the multilevel floorplanning/placement framework. Experimental results show that the MB-tree obtains significantly better silicon area and wirelength than previous works. Further, unlike previous works, the MB-tree scales very well as the circuit size increases.