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Advanced Routing in Changing Technology Landscape Hardy Leung, Magma ISPD 2003.

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Presentation on theme: "Advanced Routing in Changing Technology Landscape Hardy Leung, Magma ISPD 2003."— Presentation transcript:

1 Advanced Routing in Changing Technology Landscape Hardy Leung, Magma ISPD 2003

2 Overview Introduction - Problem solved? Routing Challenges (selected) - Complex spacing rules - Transitional pitches - Process antenna rules Opportunities (selected) - Redundant vias - Wire spreading, widening, and filling - OPC- and PSM-aware routing Conclusion

3 Introduction The Routing Flow - Global routing  Focused on congestion, capacity, prediction (for placement)  Global view, supply/demand-based - Detailed routing  Focused on design-rule correctness (DRC)  Local view, model complex design rules, pins - Track routing (optional)  Bridge the gap between GR and DR  Optimization opportunity for timing, noise - Other combinations  Simultaneous GR-TR-DR, hierarchical routing, …

4 Introduction (cont.) Academic Focus - Net-based topology generation - Congestion analysis - Global routing Industrial Focus - Detailed routing with complex design rules - Very high capacity (10M gates and beyond) The disconnect - Industry has not benefited from academia for a while in this area - The classical DR problem is perceived as “solved” (only grunt work or implementation details remain)

5 Introduction (cont.) Classical Detailed Routing obstacle pin

6 Introduction (cont.) Classical Detailed Routing - Mapped into a graph problem - Solved with Dijkstra’s algorithm or variants - Common thought  nanometer design rules merely implementation details Reality – Devil’s in the Detail - 90nm brings about a new set of challenges - New rules - Tightening of existing rules - Capacity - Prototype of a router may take 3 months; Maturity will take 5 years - Appreciation for nanometer routing problem is much needed  Design rules are KEY  Must handle high capacity

7 Overview Introduction - Problem solved? Routing Challenges (selected) - Complex spacing rules - Transitional pitches - Process antenna rules Opportunities (selected) - Redundant vias - Wire spreading, widening, and filling - OPC- and PSM-aware routing Conclusion

8 Routing Challenges Complex Spacing Rules - Width-dependency, length-dependency, halo Transitional Pitches - Drastic difference in pitches and consequences Process Antenna Rules - Complex rules, tightened constraints Routing in Uncertainty (physical OCV) - How to reduce sensitivity to process variation Interaction Between Complex Design Rules - Redundant via addition  antenna violations - Antenna fixing  timing closure issues (due to addition of vias) - Transitional pitches  Hard to fix antenna with jumpers … (many more) …

9 Complex Spacing Rules Evolution of Spacing Rules - Per-layer constant - “Fatwire” spacing  Special spacing for very fat wire (20X minimum width) - Width-dependent spacing  Spacing is expressed as a function of max(W1, W2) - WW-dependent spacing  Spacing is expressed as a function of W1, W2 - WL-dependent spacing  Spacing is expressed as a function of L, max(W1, W2)

10 Complex Spacing Rules (cont.) Evolution of Spacing Rules (II) - Parameters are tighter  0.18u  Default width is 0.4u, fatwire is 10.0u (ratio = 25X)  0.13u  Default width is 0.2u, fatwire is 0.3u (ratio = 1.5X) - Additional modifiers are introduced  Halo (disconnected vs. connected)  Parallel run-length (and how it should be measured)

11 Complex Spacing Rules (cont.) Consequence - Fatwire will be created and complex spacing rules will be triggered by signal router  No longer a static concept - Notch and hole filling cannot be post-processed  Must be modeled, detected, and filled (or avoided) during routing - Advanced polygon analysis is needed during routing  False positive and false negative are both unacceptable - Failure to avoid and/or detect fatwire correctly may cause non-convergence  Route  DRC detected  Route again  DRC again  …

12 Complex Spacing Rules (cont.) Case I – Notch Filling - Need polygon-based fatwire analysis with halo - Actual violation is far away from the notch notch fatwirehalo Fatwire DRC

13 Complex Spacing Rules (cont.) Case II – Tricky Pin Geometries - Pins may be designed just one notch shy of fatwire - Any non-trivial connection may result in spacing violations

14 Complex Spacing Rules (cont.) Case III – Fatwire Created During Routing - Individual wires and vias look clean - Not if combined 0.9u M4 M5 M6 M5 M4 0.9u 1.8u

15 Complex Spacing Rules (cont.) Classical Detailed Routing obstacle pin

16 Complex Spacing Rules (cont.) Pure Graph-based Approach has Limitation - Case I – Notch filling  Not easy to model - Case II – Tricky pin geometries  Can “worst-case” it by disabling non-trivial connection - Case III – Fatwire created during routing  Not easy to model

17 Transitional Pitches Definition of routing grids - Typically at least the line-to-”via” spacing - In general, line-to-upvia != line-to-downvia Local layers Intermediate layers Global layers

18 Transitional Pitches (cont.) In case of major pitch change - Recommend use of line-to-downvia for pitch efficiency - Global, track, detailed routers must understand and manage the pitch transition

19 Transitional Pitches (cont.) Understand and model upvias - Global Router  Minimize the use of upvias on transitional layers - Track Router  Align upvias to the same tracks if possible - Detailed Router  Change in ripup/reroute algorithms  Shift vias so that it blocks 2 tracks instead of 3 tracks - Antenna Fixer  Judicious use of jumpers between layer groups

20 Process Antenna Rules Design Requirement - Total charge accumulated on metal connected to a polysilicon gate during any stage of metalization cannot exceed a certain threshold - Usually expressed as:  WA / GA < ratio - The antenna fixing effect of diffusion (a discharge path) can be model as:  WA d / GA d < ratio d  WA d – diffusion reduces per-unit-area charge accumulation  GA d – diffusion increases the effective gate area  Ratio d – higher tolerance when diffusion is present

21 Process Antenna Rules (cont.) Antenna Fixing - Jumpers (or bridges) break long wire - Diodes introduce diffusion G G jumper

22 Process Antenna Rules (cont.) Antenna Fixing - Jumpers (or bridges) break long wire - Diodes introduce diffusion G G Diode (diffusion)

23 Process Antenna Rules (cont.) Antenna Fixing - Jumpers (or bridges) break long wire - Diodes introduce diffusion - Buffering (a way to break long wires) - Sizing (to increase the gate area)

24 Process Antenna Rules (cont.) How Difficult is the Problem? - Let gate-strength(g, L) be the maximum length of a wire with minimum width on layer L that can be connected to the gate g without causing antenna violation - In other words, gate-strength(g, L) = ratio * g / width L - A related concept, diffusion-strength(d, g, L) G

25 Process Antenna Rules (cont.) Gate and Diffusion Strengths are Functions of - process + library - foundry (different modeling, conservatism) Difficulty of Antenna Fixing - Gate and diffusion strengths are useful metrics to measure how difficult it is to fix antenna violation u  Gate strength ~ 1000u (trivial to fix)  Infinite diffusion strength u / 90nm  Gate strength ~ 100u  Worst case, 15u (very poor cell design)  Limited diffusion strength

26 Process Antenna Rules (cont.) Advance in Process Technology … - Reduced gate strength  Process antenna effect very easy to be violated  Limited degree of freedom in antenna fixing - Reduced diffusion strength  Diffusion no longer a panacea  Accurate blackbox abstraction needed (can’t waive) - Transitional pitches  surgical jumper may not be feasible due to fat overhang - Pervasive power mesh for IR-drop  surgical jumper may not be feasible since upper layers blocked

27 Process Antenna Rules (cont.) How to Fix it Then? - More powerful jumper techniques - Antenna-aware global routing  WARNING, most preventive implementation will not work - Hierarchical antenna checking and fixing - Buffering and sizing with antenna-awareness (in addition to timing, noise, crosstalk, EM, …)

28 Overview Introduction - Problem solved? Routing Challenges (selected) - Complex spacing rules - Transitional pitches - Process antenna rules Opportunities (selected) - Redundant vias - Wire spreading, widening, and filling - OPC- and PSM-aware routing Conclusion

29 Redundant Vias Single-cut via  Double-cut via - Improve yield and reliability - Based on post-processing

30 Redundant Vias (cont.) Many Different Choices - 1x2, 2x1, centered, biased - 70% to 80% coverage even for very congested designs Observations - Need room on only one of the two adjacent layers - Rare to see congestion on all layers everywhere Additional Degree of Freedom - DR creates local detour, or enforce double vias in uncongested regions - TR assigns track in redundant-via friendly ways - GR avoids congestion on both layers whenever possible (good to do so for routability anyway)

31 Redundant Vias (cont.) More Aggressive Redundant Vias - Can achieve 90% coverage Caveats - Foundry technology may negatively impact feasibility  If redundant vias have fat via overhang - Redundant vias may introduce antenna violation  Dramatically tightened antenna rules on via layers - Redundant vias will change timing  Speed up some paths, slow down some  Therefore, it needs to be done within P&R framework

32 Wire Spreading and Widening Again, for Yield and Reliability - Detailed router (or post-processing) can spread wires whenever possible - Better result if global router and track router spread the wires in a more global scope - Potential consequence in timing, crosstalk, …  Therefore, it needs to be done within P&R framework

33 Wire Spreading and Widening (cont.) Spreading and Widening

34 Metal Filling Objective - Satisfy metal density requirement Evolution of Density Requirements u – 20% to 80%, whole chip u – 20% to 80%, sliding window of 300u x 300u (150u step size) - 90nm – 25% to 75%, 300u x 300u - 90nm – 30% to 70%,1000u x 1000u - 90nm – 45% to 50%, whole chip

35 Metal Filling (cont.) More Requirements - Metal filler (transitively) tied to power/ground  No floating metal - Shallow ties  no big branch dangling from power/ground mesh  Big branches behave like floating metal - Metal filling with minimal impact on timing  Stay away from signal geometries whenever possible - Other Idea  Metal fillers as additional power mesh for better IR-drop? Hmm…

36 Metal Filling (cont.) Hard Problem (much harder than before) - Density requirement hard to be fulfilled  Customers complained that foundry-recommended dummy floating patterns will FAIL their requirements  Low-density area is easy to fill  High-density area already satisfies the requirements  Medium-density, fragmented regions are problematic - Need  Adaptive density-driven (PG-tied) metal filling

37 Metal Filling (cont.)

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42 OPC- and PSM-Aware Routing Impact of Lithography on Design Rules - Many tricky design-rules  Guardbands, workarounds to discourage or prohibit undesirable features, and to allow effective application of RET  Rules are more and more complicated - Rules are absolute  DRC deck – pass or fail  But what about “recommended rules”? - Opportunity  Recommended rules Redundant vias, wire spreading  Understanding of yield-and-manufacturability within routers or post-processors  produce high-quality layout (added value)

43 OPC- and PSM-Aware Routing (cont.) Routing with PSM Consideration - Not a concern in 0.13u, arguable in 90nm - Will be needed in 65nm - Can the router help?  Maybe - Idea – Most routing are done in preferred direction  How about extra width and spacing requirement in the preferred direction so that there is never a need to phase-shift any geometry in non-preferred direction?  Extra resource consumption may be negligible

44 OPC- and PSM-Aware Routing (cont.) Direction-dependent Width and Spacing

45 Overview Introduction - Problem solved? Routing Challenges (selected) - Complex spacing rules - Transitional pitches - Process antenna rules Opportunities (selected) - Redundant vias - Wire spreading, widening, and filling - OPC- and PSM-aware routing Conclusion

46 Problem Solved? Not Challenges? Definitely - Complex spacing rules - Transitional pitches - Process antenna rules - … (many more) … Opportunities? Plenty - Redundant vias - Wire spreading, widening, and filling - OPC- and PSM-aware routing - … (many more) …

47 Conclusion (cont.) How can Academia Help? - Build real routing system  New techniques  Must handle non-trivial capacity  Must handle at least the most basic rules – spacing, width, vias - Understanding and appreciation of nanometer rules  Observation – GR written by researchers with DR experience are in general of much higher quality - Take advantage of routing as a yield optimization technique  Significant opportunity and added value

48 Thank You


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