THEORETICAL LIMITS FOR SIGNAL REFLECTIONS DUE TO INDUCTANCE FOR ON-CHIP INTERCONNECTIONS F. Huret, E. Paleczny, P. Kennis F. Huret, E. Paleczny, P. Kennis Institut d ’Electronique et de Microélectronique du Nord, UMR CNRS 9929 D. Deschacht, G. Servel D. Deschacht, G. Servel Laboratoire d’Informatique, de Robotique Laboratoire d’Informatique, de Robotique et de Microélectronique, UMR CNRS et de Microélectronique, UMR CNRS SLIP ’2000, San Diego, April 8-9th. SLIP ’2000, San Diego, April 8-9th.

OUTLINE OF THE TALK Introduction Theoretical limits Electromagnetic analysis : - Methodology - Application Limits between RLC and RC models Illustration of the theoretical limits : - in frequency-domain - in time domain Comparison with previous work Conclusion

INTRODUCTION  0.7µm, 2 metal layers  Up to 100,000 devices on a chip  Typical CPU frequency 50MHz  0.25µm, 6 metal  Up to 10,000,000 devices on a chip  Typical CPU frequency 400 MHz IC 10 years of evolution evolution

INTRODUCTION With the continued scaling down of technology, increased die aera : * cross-section decreases * interconnect length increases interconnections : blocking point of performances improvement Introduction of new materials such as Cu inclusion of inductance ?

INTRODUCTION Interconnect delay dominates gate delay in current deep submicronic VLSI circuits. More accurate interconnect models and signal propagation characterization are required. With faster on-chip rise times inductance is becoming more important. Electromagnetic analysis is needed.

THEORETICAL LIMITS L ong lines : Static hypothesis Short lines : Traveling wave

Range of lengths for inductance inclusion : We have to determine  attenuation factor  phase factor x : attenuation coefficient THEORETICAL LIMITS

Propagation parameters of the waveguide INTERCONNECTION = WAVEGUIDE   Phase factor rad/cm Attenuation factor dB/cm ou Np/cm Zc Characteristic impedance ELECTROMAGNETIC ANALYSIS Methodology Full wave analysis Finite Element Method

ELECTROMAGNETIC ANALYSIS Methodology Definitons of the voltage-current matrices used in this analysis

ELECTROMAGNETIC ANALYSIS Methodology Vin(t) Vout(t) Vin(freq)  (freq)  (freq) Zc(freq) Matched Load Impedances Chain Matrix Vout(freq) F.F.T * F.F.T = Fast Fourier Transform F.F.T.

ELECTROMAGNETIC ANALYSIS Application Interconnection geometry and environment 0.8  m M5 0.8  m 2.4  m M5 7.3  m passivation SiO2 Si bulk 7  cm 500  m 2.4  m M5 7.3  m passivation SiO2 2 nd configuration 1 st configuration 3 rd configuration Metal 5 : W=1  m T= 1  m Aluminium or Copper

ELECTROMAGNETIC ANALYSIS Application Frequency behavior of the attenuation factors

ELECTROMAGNETIC ANALYSIS Application Frequency behavior of the phase factors

ELECTROMAGNETIC ANALYSIS Application Attenuation determination Traveling wave Attenuation value of the wave, for 10 GHz, versus interconnection length

Theoretical limits : We have determined  To determine x : comparison output signal between RC and RLCG models ELECTROMAGNETIC ANALYSIS Application

OUTLINE OF THE TALK Introduction Theoretical limits Electromagnetic analysis : - Methodology - Application Limits between RLC and RC models Illustration of the theoretical limits : - in frequency-domain - in time domain Conclusion

LIMIT BETWEEN RLC AND RC MODELS The RLCG line model deduced from the electromagnetic analysis :

These calculated values are used to build the distributed  RC model LIMIT BETWEEN RLC AND RC MODELS n cells COMPARISON BETWEEN : HSPICE simulations : RC model Electromagnetic analysis : RLC model

LIMIT BETWEEN RLC AND RC MODELS Waveform of input and output signals in the range of lengths with inductance effect

LIMIT BETWEEN RLC AND RC MODELS Waveform of input and output signals in the range of lengths with inductance effect

LIMIT BETWEEN RLC AND RC MODELS Attenuation determination : Limit : the amplitude of the reflected wave is sufficiently low to give the reflection effect negligible

OUTLINE OF THE TALK Illustration of the theoretical limits : - in frequency-domain - in time domain Theoretical limits : We have determined  x

ILLUSTRATION OF THEORETICAL LIMITS in the frequency-domain

Frequency Time domain ILLUSTRATION OF THEORETICAL LIMITS in the frequency-domain

ILLUSTRATION OF THEORETICAL LIMITS in the time-domain

OUTLINE OF THE TALK Introduction Theoretical limits Electromagnetic analysis : - Methodology - Application Limits between RLC and RC models Illustration of the theoretical limits : - in frequency-domain - in time domain Comparison with previous work Conclusion

COMPARISON WITH PREVIOUS WORK The two figures of merit can be combined into a two sided inequality that determines the range of the length of interconnect in which inductance effects are significant : « Figures of Merit to characterize the Importance of On-chip Inductance » DAC 98, June st configuration : R =  /m C = 170 pF/m L = 490 nH/m G # 0 2 nd configuration : R =  /m C = 63.6 pF/m L = 655 nH/m G # 0

COMPARISON WITH PREVIOUS WORK

COMPARISON WITH PREVIOUS WORK

CONCLUSION A full-wave electromagnetic analysis have been presented to build accurate interconnect models, including inductance effects. New limits for signal reflections due to inductance for on-chip interconnections have been proposed.

CONCLUSION These limits have been illustrated with typical interconnection geometries, for Al and Cu wires. This study shows evidence demonstrating that a range exists for which inductance effects cannot be neglected and requires a transmission line model.

CONCLUSION FUTURE WORK : Interconnect coupling : taking into account not only the coupling capacitance, but also the impact of inductance and mutual inductance.