Circuit Performance Variability Decomposition Michael Orshansky, Costas Spanos, and Chenming Hu Department of Electrical Engineering and Computer Sciences, University of California at Berkeley

Motivation: need to know sources of circuit variability New considerations: –Increasing role of interconnect –Importance of intra-field variability Variability modeling –Critical path circuit –Analytic delay calculation –Realistic linewidth variation model Analysis of modeling results Conclusions Overview

Causal decomposition to find major sources of variability Designing for reduced sensitivity Reducing the amount of statistical information Reducing the simulation effort by scaling statistical input space dimensionality Maximizing yield: more accurate limits of acceptable parameter spreads Emphasis in this project: device vs interconnect variability decomposition Circuit Performance Variability Decomposition: Motivation

New Considerations: Interconnect Traditionally: device variability is dominant Deep Sub-Micron: interconnect delay contribution grows Perhaps interconnect variability dominates Realistic analysis has to consider device and interconnect variability jointly Need to distinguish global and local interconnect –Average wirelength of local (lower level) metal lines scales down as gate length is reduced –Global (higher level) lines become longer as the chip size increases

New Considerations: Within-Field Variability Traditionally: between-field variability is dominant Deep Sub-Micron: within-field gate CD variability is significant Within-field variability is deterministic –Can be treated as random for simplicity

Need a representative circuit for high-speed CMOS designs such as microprocessor Critical path circuit: –14-stage gate chain –2-input NANDs with average FO=2.5 Gate stages separated by local interconnect lines There is one global interconnect line buffered by repeaters Critical Path Circuit N=14

Analytic Delay Modeling Sakurai’s delay model: D=0.4R w C w +0.7(R d C l +R d C w +R w C l ) Device behavior: R d =V dd /I d We also use Sakurai’s analytical 2-D capacitance models RdRw ClCl CwCw

Variability Modeling Variability model: Assume linear delay response: Variability of each source is Improved model of global interconnect linewidth variation –This model predicts smaller variability of effective W in global lines

Can define. Decomposition depends on: –Technology parameters (nominal and statistical) –Circuit characteristics (typical) Case study: L gate =0.18 –Device Parameters: T ox =40A, V dd =1.8V –Current drive: I d (L g )=0.56mA-1.9(L g -0.18) Interconnect Parameters: Impact of Technological and Circuit Choices A Case Study of 0.18um Technology

Sensitivity of Delay to Local Interconnect Sensitivity is Sensitivity to gate CD is highest and still increases in this range Conditions: L global =1.15 cm

Sensitivity of Delay to Global Interconnect At large L global sensitivity to Wg and Tg is comparable to L gate Increase of sensitivity due to Wg and Tg is drastic for small pitch Conditions:, 3 repeaters used

Effect of Buffering on Delay Sensitivity Increasing the number of repeaters by 2 reduces sensitivity to global interconnect parameters Wg and Tg by 30% Use of repeaters both reduces delay and delay sensitivity

Effect of Reducing Metal Pitch Sensitivity to global parameters drastically increases for smaller pitches Sensitivity to L gate is still highest (L global =1.15cm)

Improved Interconnect Variability Model Model predicts reduction of and increase of sensitivity with L global Consider a buffer driving a line with no repeaters and At large L global, delay variability due to Wg is strongly attenuated

Variability Decomposition: an Example Need to assume specific variance values Values depend on design/technology choices “good design”: wide pitch for global interconnect, min pitch for local connections; Assumed max (3-sigma) parameter deviations (normalized): SW T H

Variability Decomposition: Results Only for poor designs, a sizable portion of variability (35%) is due to interconnect For good designs interconnect variability contribution is small (12%) Improved model accounts for reduction of in large L global

Summary and Conclusions Analytical circuit performance variability decomposition proposed to assess interconnect and device contributions Need to consider device and interconnect variability jointly Exact decomposition is specific for circuit and technology Optimal designs lead to better performance and smaller interconnect variability contribution Use of repeaters leads to reduction of interconnect contribution Device variability remains the dominant source of overall variability in circuit performance (about 90%)