Signal integrity effects are becoming ever more visible for geometries of 0.13 micron and below. Routing complexity has increased dramatically and as a result more layers of metal are being added to silicon. To control the die size, the width of the metal is continuously being decreased whereas to keep the metal resistance low, the height of the metal wires is being increased. Furthermore, metal wire lengths are now longer than ever.
For all these reasons, wires have now become longer and thinner, and as a result the wire-to-wire capacitance between two conductors (Cw) has increased. Moreover, with more and more interconnect layers on a chip, the distance from the higher metal layers to the substrate layer increases, thereby decreasing the substrate capacitance component of the total net capacitance (Cs). Consequently, the wire-to-wire capacitance now dominates over the substrate capacitance in 0.13 micron and below, and the overall result is an increased likelihood of signal corruption due to capacitive coupling between signals.
To better illustrate the effects of crosstalk, let us consider the circuit shown in the figure. This simple circuit shows three nets (Net A, Net B and Net C) which are coupled to each other via capacitors (CC1 and CC2). The concept of crosstalk requires that we define a "victim" and "aggressor" net.
A "victim" is defined as the net on which delays are being computed (Net B). An "aggressor" is a net that has a significant coupling capacitance to the victim net and whose transitions can affect the timing of the victim net (Net A and Net C).
Due to capacitive coupling, a transition that occurs on the aggressor net may be partially transmitted to the victim net. This may cause the expected signal transition waveform on the victim to be different from the waveform if there was no coupling capacitance. The result is a change in the measured delays for the victim and may also include a change in the logic transition for the victim.
Depending on the direction of the transitions on the victim and aggressor, the effect can be one of the following:
- An increase in delay if the victim and aggressor are switching in the opposite direction.
- A decrease in the delay if the victim and aggressor are switching in the same direction.
- No change in the delay if the change in the victim cannot be observed at delay or slew measurement trip points.