CSE477 VLSI Digital Circuits Fall 2001 Lecture 09 Resistance Vijay Narayanan www cse psu edu cg477 Adapted in part from Rabaey s Digital Integrated Circuits Prentice Hall 1995 CSE477 L09 Resistance 1 Irwin Vijay PSU 2001 CMOS Inverter Dynamic Characteristics VDD Last lecture s focus Vout 0 CL Rn Vin V DD CSE477 L09 Resistance 2 tpHL f Rn CL Today s focus Irwin Vijay PSU 2001 Review Sources of Capacitance Vin Vout2 CG4 M2 Vin CGD12 M4 CDB2 CDB1 M1 Vout Vout2 Cw M3 CG3 intrinsic MOS transistor capacitances extrinsic MOS transistor capacitances wiring capacitance CSE477 L09 Resistance 3 Irwin Vijay PSU 2001 Review Components of C L 0 25 micron Expression Value fF Value fF H L L H 0 23 0 23 C Term CGD1 2 Co n W n CGD2 2 Cop W p 0 61 0 61 CDB1 KeqnADnCj KeqswnPDnCjsw 0 66 0 90 CDB2 KeqpADpCj KeqswpPDpCjsw 1 5 1 15 CG3 2 Con W n CoxW nL n 0 76 0 76 CG4 2 Cop W p CoxW pLp 2 28 2 28 Cw from extraction 0 12 0 12 CL 6 1 6 0 CSE477 L09 Resistance 4 Irwin Vijay PSU 2001 Sources of Resistance q MOS structure resistance Ron q Source and drain resistance q Wiring resistance CSE477 L09 Resistance 5 Irwin Vijay PSU 2001 MOS Structure Resistance q Assume Ron is a constant and linear resistance Req q Use the average value of the resistances at the endpoints of the transition VDD and VDD 2 Req Ron t1 Ron t2 Req VDD IDSAT 1 7 9 VDD where IDSAT k W L VDD VT VDSAT V2DSAT 2 CSE477 L09 Resistance 6 Irwin Vijay PSU 2001 Equivalent Resistance q q Inversely proportional to W L Doubling the transistor width halves the resistance For VDD VT VDSat 2 resistance independent of VDD Once the supply voltage approaches VT resistance increases dramatically CSE477 L09 Resistance 7 7 x10 5 for VGS VDD VDS VDD VDD 2 6 Req Ohm q 5 4 3 2 1 0 0 5 1 1 5 2 2 5 VDD V VDD V NMOS k PMOS k 1 35 115 1 5 19 55 2 15 38 2 5 13 31 Irwin Vijay PSU 2001 Source and Drain Resistance G D S RS RD RS D LS D W Rr RC where RC is the contact resistance LS D is the length of the source or drain diffusion Rr is the sheet resistance of the drain source diffusion 20 to 100 r q More pronounced with scaling since junctions are shallower and contact openings smaller CSE477 L09 Resistance 8 Irwin Vijay PSU 2001 Contact Resistance q Transitions between routing layers contacts add extra resistance to a wire l l q keep signals wires on a single layer whenever possible reduce contact resistance by making contact holes larger beware of current crowding Typical contact resistances minimum size l l 5 to 20 for metal or poly to n p diffusion and metal to poly 1 to 5 for metal to metal contacts CSE477 L09 Resistance 9 Irwin Vijay PSU 2001 Wiring Capacitances Poly Al1 Al2 Al3 Al4 Al5 Field 88 54 30 40 13 25 8 9 18 6 5 14 5 2 12 Interwire Cap Active 41 47 15 27 9 4 19 6 8 15 5 4 12 Poly 57 54 17 29 10 20 7 15 5 4 12 Al1 Al2 Al3 Al4 pp in aF m2 fringe in aF m 36 45 15 27 8 9 18 6 6 14 41 49 15 27 9 1 19 35 45 14 27 38 52 Poly Al1 Al2 Al3 Al4 Al5 40 95 85 85 85 115 per unit wire length in aF m CSE477 L09 Resistance 10 Irwin Vijay PSU 2001 Wire Resistance L H L R A Sheet Resistance Rr R1 L HW R2 W Material Silver Ag Copper Cu Gold Au Aluminum Al Tungsten W CSE477 L09 Resistance 11 m 1 6 x 10 8 1 7 x 10 8 2 2 x 10 8 2 7 x 10 8 5 5 x 10 8 Material Sheet Res r n p well diffusion 1000 to 1500 n p diffusion 50 to 150 n p diffusion 3 to 5 with silicide polysilicon 150 to 200 polysilicon with 4 to 5 silicide Aluminum 0 05 to 0 1 Irwin Vijay PSU 2001 Overcoming Interconnect Resistance q Selective technology scaling l q scale W while holding H constant Use better interconnect materials l lower resistivity materials like copper Wire delay is proportional to its R resistance times C capacitance thus reducing C reduces signal delay improves signal integrity and lowers power consumption Unfortunately as processes shrink wires get shorter reducing C but they get closer together increasing C and narrower increasing R So RC wire delay increases and capacitive coupling gets worse Copper has about 40 lower resistivity than aluminum so copper wires can be thinner reducing C without increasing R l q use silicides Use more interconnect layers l reduces the average wire length L but beware of extra contacts CSE477 L09 Resistance 12 Irwin Vijay PSU 2001 Wire Spacing Comparisons Intel P858 Al 0 18 m Intel P856 5 Al 0 25 m 0 07 0 05 IBM CMOS 8S CU 0 18 m M6 M5 0 08 0 12 M4 0 17 M5 M4 0 33 M3 0 49 M3 0 33 M2 0 49 M2 1 11 Scale 2 160 nm CSE477 L09 Resistance 13 M1 1 00 M1 0 10 M7 0 10 M6 0 50 M5 0 50 M4 0 50 M3 0 70 M2 0 97 M1 From MPR 2000 Irwin Vijay PSU 2001 Comparison of Wire Delays 1 Normalized Wire Delay 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 Al SiO2 Cu SiO2 Cu FSG Cu SiLK From MPR 2000 CSE477 L09 Resistance 14 Irwin Vijay PSU 2001 Wire Delay Models q Lumped RC model l l q different fractions of circuit parasitics R and C are lumped into a single circuit element good model for short wires pessimistic and inaccurate for long wires Distributed RC model l circuit parasitics are distributed along the length of the wire Elmore delay formula used to model the wire delay q Step response points of interest Voltage Range Lumped RC Distributed RC 0 50 tp 0 69 RC 0 38 RC 0 63 RC 0 5 RC 10 90 tr 2 2 RC 0 9 RC 0 90 2 3 RC 1 0 RC CSE477 L09 Resistance 15 Time to reach the 50 point is t ln 2 0 69 Time to reach the 90 point is t ln 9 2 2 Irwin Vijay PSU 2001 Lumped Wire Model q Circuit parasitics distributed along a wire are lumped into single elements R and C Driver RDriver cwire Clumped Lcwire capacitance per unit length q As long as the resistive component is small and the switching frequency is low only have to consider the capacitive component CSE477 L09 Resistance 16 Irwin Vijay PSU 2001 Distributed RC Chain Model R1 1 Vin C1 R2 …