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1. 1. CAD for VLSI Design - II Lecture 6 V. Kamakoti and Shankar Balachandran
2. 2. Overview of this Lecture • CMOS Transistor Theory – Delay Issues (Cont’d) • Types and effects of Capacitances on delay
3. 3. Parasitic Capacitance • Switching speeds of MOS systems strongly depend on the parasitic capacitances associated with MOSFETs and interconnections • Total Cload on the output of a CMOS gate is the sum of: – Gate capacitance (Cg) – Junction capacitance due to the source and drain regions and their surroundings (Csb and Cdb) – Interconnect (or routing) capacitance (Cw) • Gate oxide capacitance per unit area, ε ε = 0 ox ox ox C t
4. 4. MOSFET Capacitances xd xd xd
5. 5. Gate Capacitance • Gate capacitance, Cg = Cox WL • Total gate capacitance Cg can be decomposed in two elements: 1. Overlap capacitance: due to the topological structure of the MOSFET. 2. Gate-to-Channel capacitance: due to the interaction between gate voltage and channel charge.
6. 6. Gate Overlap Capacitance • In reality , actual channel length, Leff < drawn length, L (mask length), due to the extension of the source and drain regions somewhat below the oxide by an amount xd, called the lateral diffusion, i.e., Leff = L – 2.xd • xd gives rise to overlap capacitance which is linear and has a fixed value. Co is overlap capacitance per unit transistor width (fF/μm) = = =gso gdo ox d oC C C x W C W
7. 7. Gate-to-Channel Capacitance • It has thee components: Cgs, Cgd and Cgb
8. 8. Average Gate Capacitance Region Cgb Cgs Cgd Cg Cutoff CoxWL eff 0 0 CoxWLeff/2 0 CoxWLeff+2CoW Linear 0 CoxWLeff/2 CoxWLeff+2CoW Saturation (2/3)CoxWLe ff (2/3)CoxWLeff+2C oW
9. 9. Area and Side-wall Capacitance • Area Capacitance (Carea) due to the bottom-plate junction formed by the source (drain) region with doping ND and substrate with doping NA (bottom area 5). • Side-wall (perimeter) Capacitance (Csw) formed by junctions 2, 3, and 4. These are surrounded by the p+ channel-stop implant with doping level NA+ which is usually larger than that of the substrate larger capacitance per unit area. = ,area j s jC C WL Cwhere is junction capacitanceperunitarea ( ) ( ) where is junction side-wall capacitanceperunit length = + ⋅ ′= 2 , sw jsw s j sw jsw jsw j C C W L C C C x
10. 10. MOSFET Capacitance Model = = + = + + + GS GS gs gso GD GD g g gdo bg d C C C C C C C C C C
11. 11. Wire (Routing) Capacitance ( ) ε ε πε ≈ + ≈ + ⋅ 2 log pp o o fri x nge o w x C wl h CC h t
12. 12. Parallel-plate and Fringing Capacitance Total Cap. w/t w t h t/h=1 t/h=0.5 Cpp
13. 13. Modern Interconnect • Inter-layer capacitance increases with decreasing feature sizes. • Multi-layer capacitive interactions result in unwanted coupling among neighboring signals cross talk
14. 14. Impact of Inter-layer Capacitance
15. 15. Capacitances for a 0.25μm Process Capacitance Area Cap (fF/μm2) Perim. Cap (fF/μm) Poly - substrate 0.088 0.041 0.015 n+ diff - substrate 1.660 0.399 n+ overlap cap. -- 0.562 p+ overlap cap. -- 0.630 Cox 5.951 Metal1 - poly 0.017 0.041 1.832 0.038 0.054 Metal1 - substrate 0.047 Metal2 - substrate 0.027 p+ diff - substrate 0.323 Metal2 – metal1 0.054
16. 16. 0.25μm Interconnect Hierarchy • Optimize interconnect structure at each layer. – for local wires, density and low C are important – use dense and thin wiring grid – for global wires in order to reduce delays, use fat, widely spaced wires. • Improve wire delays by using better material (Cu) and low-K dielectrics for insulators. Intracell Intercell Intermodule Global
17. 17. Electrical Wire Models • Ideal Wire - it is simply a line with no attached parameters or parasitics it has no impact on electrical behavior. • Lumped Model – simplified model simple and fast computation, e.g., lumped C, lumped RC or lumped RLC • Distributed Model - Parasitics of a wire are distributed along its length and are not lumped into a single position, distributed C, distributed RC, or distributed RLC Clumped = lwire.cwire
18. 18. Elmore Delay Formula • For an n stage RC chain, the first order time constant is given by, • If Ri = Rj and Ci = Cj for all i and j , (1≤ i, j ≤ n) then, ( ) ( ) n i n i j i j n nC R C R R C R C R R R = = τ = + + + + + + + = ∑ ∑ 1 1 2 1 2 1 2 1 1 ... ... ( )+ τ = 1 2 n n n RC
19. 19. Distributed RC Model for a Wire Using Elmore delay formula we can determine the dominant time constant of the wire, i.e., it is a first-order approximation.
20. 20. Questions and Answers Thank You