Effects of mode conversion on parasitic coupling in high-speed VLSI circuits

The mode conversion means that a modification of the electromagnetic field configuration occurs, generally, after discontinuities. In deep submicron digital ULSI circuits, the mode conversion analysis is indispensable to identify the signal return path, the return current distribution and therefore, for an accurate inductance modelling which remains a challenging problem (Y. I. Ismael and E. G. Friedman, 2000). On the other hand, switching activity of high speed CMOS circuit may produce large current derivatives in wires (crosstalk) and substrate. These current transients can generate large potential surges and coupled noise. In this mind, a reduction of the mode conversion phenomenon decreases noise in high speed ULSI circuits (Y. Quere et al., 2003). We have investigated the mode conversion, in the frequency domain, for multiple-line inter-layer transitions in CMOS devices. The signal integrity analysis in time domain proved the detrimental effects of mode conversion. Finally, we confirmed that our design rule reduces the mode conversion phenomenon in the case of transition with multiple coupled lines.