We present critical-sink routing tree (CSRT) constructions which exploit available critical-path information to yield high-performance routing trees. Our CS-Steiner and "global slack removal" algorithms together modify traditional Steiner tree constructions to optimize signal delay at identified critical sinks. We further propose an iterative Elmore routing tree (ERT) construction which optimizes Elmore delay directly, as opposed to heuristically abstracting linear or Elmore delay as in previous approaches. Extensive timing simulations on industry IC and MCM interconnect parameters show that our methods yield trees that significantly improve (by averages of up to 67%) over minimum Steiner routings in terms of delays to identified critical sinks. ERTs also serve as generic high-performance routing trees when no critical sink is specified: for 8-sink nets in standard IC (MCM) technology, we improve average sink delay by 19% (62%) and maximum sink delay by 22% (52%) over the minimum Steiner routing. These approaches provide simple, basic advances over existing performance-driven routing tree constructions. Our results are complemented by a detailed analysis of the accuracy and fidelity of the Elmore delay approximation; we also exactly assess the suboptimality of our heuristic tree constructions. In achieving the latter result, we develop a new characterization of Elmore-optimal routing trees, as well as a decomposition theorem for optimal Steiner trees, which are of independent interest.

We povide a new theoretical framework for constructing Steiner routing trees with minimum Elmore delay. Earlier work [3, 13] has established Elmore delay as a high fidelity estimate of "physical", i.e., SPICE-computed, signal delay. Previously, however, it was not known how to construct an Elmore delay-optimal Steiner tree. Our main theoretical result is a generalization of Hanan's theorem [11] which limited the number of possible locations of Steiner nodes in an optimal delay rectilinear Steiner tree. Another theoretical result establishes a new decomposition theorem for constructing optimal-delay Steiner trees. We develop a branch-and-bound method, called BB-SORT-C, which exactly minimizes any linear combination of Elmore sink delays; BB-SORT-C is practical for routing small nets and for delimiting the space of achievable routing solutions with respect to Elmore delay.

We present two critical-sink routing tree (CSRT) constructions which exp!oit criticai-path information that becomes availab[e during timing-driven layout.Our CS-Steiner heuristics with "Giobai Slack Removal" modify traditional Stetner constructions and produce routing trees wtth szgntjicanthj lower criticalsink delays compared with existing performance-driven methods.We also propose a new class of Elrnore routing tree (ERT) constructions, which deratively add tree edges to minimize Elmore delay.This direct optimization of Elmore delay yields trees that improve delays to identified crztical sinks by up to 69% over minimum Steiner routtngs.ERTs also improve performance over such recent methods as [1] [6] when no critical stnks are specified.

We address the efficient construction of interconnection trees with near-optimal delays. We study the accuracy and fidelity of easily-computed delay models with respect to detailed simulation (e.g., SPICE-computed delays). We show that Elmore delay minimization (W.C. Elmore, 1948) is a high-fidelity interconnect objective for IC interconnect technologies, and propose a greedy low delay tree (LDT) heuristic which for any monotone delay function can efficiently minimize maximum delay. For comparison, we also generate optimal routing trees (ORTs) with respect to Elmore delay, using branch-and-bound search. Experimental results show that the LDT heuristic approximates ORTs very accurately: for nets with up to seven pins, LDT trees have a maximum sink delay within 2.3% of optimum on average. Moreover, compared with minimum spanning tree constructions, the LDT achieves average reductions in delay of up to 35% depending on the net size.>