Parasitics and InterconnectsImpact of Interconnect WiresDealing with ResistanceResistance EstimationSheet Resistance“Channel” ResistanceResistance of Non-rectangular ShapesResistance Considerations in LayoutProblems of Interconnect ResistanceElectromigrationElectromigration (contd.)Slide 12Resistance & Reliability - Ohmic Voltage DropResistance & Reliability - Ohmic Voltage Drop (contd.)Solutions for Ohmic-voltage DropsPower and Ground DistributionDealing with CapacitanceCapacitance EstimationThe Parallel Plate ModelTypical Wiring Capacitance ValuesFringing CapacitanceFringing Capacitance ValuesCapacitance vs. W/HInterwire CapacitanceSlide 25Impact of Interwire CapacitanceCapacitance CrosstalkCombating Capacitive CrosstalkDriving Large CapacitancesReducing the SwingPrecharged BusCharge Redistribution AmplifierBipolar vs. MOSTristate BuffersTechnology ScalingIncreased Impact of InterconnectScaling BehaviorScaling TransistorsScaling InterconnectTechnology Scaling & InterconnectNature of InterconnectTechnology Scaling & Interconnect (contd.)Wire Length Estimation via Rent’s RuleWhat can be done?Polycide Gate MOSFETModern InterconnectDealing with InductanceInductive Effects in ICsInductance & Performance - Transmission Line EffectsTransmission LineLossless Transmission Line ParametersPropagation along Loss less Transmission LineWave Propagation SpeedTerminationTransmission Line with Terminating ImpedancesWave Reflection for Different TerminationsTransmission Line Response (RL = )Lattice DiagramHandling Transmission Line EffectsOutput Buffer ModelOutput Buffer ResponseSimultaneous Switching of DriversL di/dtL di/dt SimulationSolving the Ldi/dt ProblemSolving the Ldi/dt Problem (contd.)Selecting the Right PinSlide 68Decoupling CapacitorPackagingPackaging RequirementsClassifying PackagingInterconnect LevelsWire BondingTape Automated Bonding (TAB)Flip-Chip BondingPackage-to-Board InterconnectPackage TypesTypical L & C Values of Packaging and Bonding StylesMulti-Chip ModulesWhen to consider interconnect parasitics?When to consider interconnect parasitics? (contd.)Finally...Curt SchurgersUCLA - EE DepartmentEmail: [email protected]: 310-206-4465 Copyright 2002 Mani SrivastavaParasitics and InterconnectsEE116B (Winter 2002): Lecture #72Copyright 2002 Mani SrivastavaImpact of Interconnect WiresInterconnects introduce parasitic effects that are absent in the ideal wiresaffect performance due to increase in delaysreduce reliability due to increase in noiseThe impact of these parasitics increase as devices shrink and dies get largerCauses of parasiticsResistiveCapacitiveInductive3Copyright 2002 Mani SrivastavaDealing with ResistanceEstimation and impact4Copyright 2002 Mani SrivastavaResistance EstimationResistance of a slab of conducting material isR = (/ t) * (L/W) = Rs * (L/W)where Rs = sheet resistance in / squareFollowing have same resistances:LWWWL Ltt5Copyright 2002 Mani SrivastavaSheet ResistanceTypical values in a 1 m process are:Poly (t=0.33 m) = 10, M1/M2 = .07, Silicide = 3n+ & p+ Diffusion = 10, N-WELL = 1K to 1.5KFunction of:thicknesse.g. upper metal layers are usually thickere.g. memory processes have thinner metals to reduce vertical topology jumps to improve yieldresistivity, which in turn depends on•density of impurities (in case of poly & diffusion)•extent of chemical change (in case of silicide)6Copyright 2002 Mani Srivastava“Channel” ResistanceUseful approximation for performance estimationRc = k*(L/W) where k = 1 / Cox(Vgs-Vt) in linear regionFor both NMOS & PMOS, typical values of k are 1000 to 30000 / squareMobility and threshold Vt depend on temperature channel resistance depends on temperature switching-time and power consumption vary tooIncrease in Rc is about +0.25% per Centigradeincrease in Rs is about +0.3% / Celsius for metal & poly, about 1% / Celsius for well diffusion7Copyright 2002 Mani SrivastavaResistance of Non-rectangular ShapesSHAPE RATIO RESISTANCEA 1 1A 5 5B 1 2.5B 1.5 2.55B 2 2.6B 3 2.75C 1.5 2.1C 2 2.25C 3 2.5C 4 2.65Measured Resistances8Copyright 2002 Mani SrivastavaResistance Considerations in LayoutMetal preferred for long interconnectsPolysilicon only for local interconnectsDiffusion (n+, p+) and poly have comparable Rshowever, diff has larger caps & associated RC delaysTransitions between layers add contact resistancetypical values min sized contacts in a 1 m process are: 21 for M1 to n+,p+, or poly; 2 for M1 to M2 avoid excess contacts & viaspossible to reduce contact resistance by making larger holes… but limited due to current crowding (current concentrates around perimeter)9Copyright 2002 Mani SrivastavaProblems of Interconnect ResistanceRC delays of the wiresSee lecture on DelayResistive (ohmic) voltage dropsRelated problem: Electromigration10Copyright 2002 Mani SrivastavaElectromigrationCurrent density (current per unit area) in a metal wire is limited due to electromigrationa direct current in a metal wire running over a long time period causes a transport of metal ions•causes wire to break or to short circuit to another wiresignal wires carry AC, and are less susceptible•bidrectional electron flow anneals crystalline structureRate of electromigration depends on:temprature, crystal structure, current density•only current density can be controlled by VLSI designersSolutions?keeping current below 0.5 to 1 mA/m helps•this can be used to determine minimal wire widthsadding alloying elements (Cu,Tu) prevents movement of Al ions11Copyright 2002 Mani SrivastavaElectromigration (contd.)Limits dc-current to 1 mA/m[from http://infopad.eecs.berkeley.edu/~icdesign/. Copyright 1996 UCB and Prentice hall 1995]12Copyright 2002 Mani SrivastavaElectromigration (contd.)[from http://infopad.eecs.berkeley.edu/~icdesign/. Copyright 1996 UCB and Prentice hall 1995]13Copyright 2002 Mani SrivastavaResistance & Reliability - Ohmic Voltage DropCurrent flowing through a resistive wire results in olhmic voltage drop that degrades signal levelsimportant in power distribution networks where current levels reach 100 mA and even amperesExample: a 15 mm long Vdd or GND wire with a current of 1 mA per m widththis is about max Al wire can sustain without electromigrationwith sheet resistance of 0.07 /square, the
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