Wednesday 7 October 2015

Signal Integrity Effects

Signal integrity is the ability of an electrical signal to carry information reliably and resist the effects of high-frequency electromagnetic interference from nearby signals.
There are various kind signal integrity violation, In Order to do successful tapeout, one need to overcome all these violation.

Some of the signal integrity effects might occur in your design

1. Crosstalk

Crosstalk is the undesirable electrical interaction between two or more physically adjacent nets due to capacitive cross-coupling. As integrated circuit technologies advance toward smaller geometries, crosstalk effects become increasingly important compared to cell delays and net delays.

Cross talk noise effects:

A signal should be constant for some time. But during the transition in adjacent signal causes anoise bump / glitch on constant signal. If the glitch is significantly high , it can cause incorrect logic to be propagated.

How To fix Crosstalk Issue

Double spacing=>more spacing=>less capacitance=>less cross talk
Multiple vias=>less resistance=>less RC delay
Shielding=> constant cross coupling capacitance =>known value of crosstalk
Buffer insertion=>boost the victim strength

2. Electromigration and its issues prevention

3.Antennae Effect

The antenna effect, more formally plasma induced gate oxide damage, is an effect that can potentially cause yield and reliability problems during the manufacture of MOS integrated circuits. Fabs normally supply antenna rules, which are rules that must be obeyed to avoid this problem. A violation of such rules is called an antenna violation. The word antenna is something of a misnomer in this context—the problem is really the collection of charge, not the normal meaning of antenna, which is a device for converting electromagnetic fields to/from electrical currents. Occasionally the phrase antenna effect is used in this context, but this is less common since there are many effects, and the phrase does not make clear which is meant.

How to Prevent Antennae Effect

  • Change the order of the routing layers. If the gate(s) immediately connects to the highest metal layer, no antenna violation will normally occur.
  • Add vias near the gate(s), to connect the gate to the highest layer used. This adds more vias, but involves fewer changes to the rest of the net.
  • Add diode(s) to the net. A diode can be formed away from a MOSFET source/drain, for example, with an n+ implant in a p-substrate or with a p+ implant in an n-well. If the diode is connected to metal near the gate(s), it can protect the gate oxide. This can be done only on nets with violations, or on every gate (in general by putting such diodes in every library cell). The "every cell" solution can fix almost all antenna problems with no need for action by any other tools. However, the extra capacitance of the diode makes the circuit slower and more power hungry.


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