Unmanufactured CAD sub layers

Lesson learnt

Make sure that your LVS deck only uses manufactured layers to provide connectivity. Otherwise the LVS result may not be valid.

The tale

We had got to within hours of tape out. The chip needed to go, for my sanity if for no other reason. It was LVS clean, but we still had a few unexplained DRC errors in the padring. We could not see anything wrong in the proximity of the reported DRC errors. I was keen to sign it off and happy to provide my own blood as ink. Others wanted another 24 hours to investigate. They were right.

In this next twenty-four hours we found that a second layer metal (M2) connection was made using an unused M2 CAD sub-layer number. A sub layer that was not used in manufacture. The LVS deck “generously” used all M2 CAD sub layers for connection. The DRC deck correctly did not “see” this unused, unmanufactured M2 sub layer at all.

The result of this – if I had signed it off with my blood – would have been an open circuit and silicon failure.

So make sure the LVS deck is not “generous” and instead uses only manufactured sub layers to make connections. There are usually only a few valid sub-layers; maybe one for normal wires and one for metal fill. The fill is put on a different sub-layer so that it does not have to be subjected to post tape out, pre mask making processing such as the very compute intensive optical proximity correction (OPC).

Where did this piece of unused M2 sub layer come from? We believe it came from a manual addition of metal straps in the P&R tool with some typos on the sub layer form.