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When to use Poly routing ?

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rahultcd

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While doing layouts, I observed one comment on schematic ,
"Route by poly only" for one of the net connection.

That connection was connectiong two gates. When asked for explanation , designer said metals are more leaky than poly..

But I didnt understand it . Could someone tell exact reason why they cud have used poly for routing instead of metal?

Thanx
 

You can use poly connection for a short distance and it's possible to do that. But i don't agree with the reasoning of your designer. You know as a layout designer that poly is highly resistive than metal.
 

I'm with fixrouter4400.

Metal is less resistive so better for routing, thats not to say it could'nt be used back in the old days when we only had poly & metal routing you had no choice....I think I was a baby then though lol!

I would have thought using poly would pickup more noise from the substrate being closer to it than metal. I might be a good idea to ask the designer what he meant it dosn't do any harm to ask.
 

Gates in very close vicinity - sometimes better be routed using poly. The trade-off is total resistance between the gates. It might happen so - to connect them using metal1 - you need to insert at least two contacts, and do consider the the poly-metal1 contact resistance. Check for the specified distance between the MOS-gates if the 2 * poly-cont-res + metal parasitic res is less or more than just the parasitic poly-interconnect-res value.

Also keep in mind, you also waste space [no matter how small] to have poly-extensions of each gate + poly-cont-enclosure rule, and then probability of failure in every contact - insert double cont to reduce the probability.
There was a concept of LI (Local Interconnect) back in history [which is still in use - might not be in 90nm and below I guess].
 

Thanx to all for reply..

Actually the distance between two gate is not less. Its quite big in length, where in normal case we can use metal. But specifically here they have asked to rote by poly. Also there is not a area problem.

What I cud think, is , as poly is more resistive, there wud be more resistance to electrons/current to LEAK out of poly, i.e. if some other meatal paths/route are running near to poly path , leakage (by capacitive/inductive coupling) would be less as compared to metal. If we use metal instead of poly, there can chances of
leakage (by capacitive/inductive coupling). And there may be specific requirement for that route/signal to have less leakage.

Let m know if I am correct ....

thanx
 

As related to layout perspective, we all know to avoid poly routing, bcz poly is more sensitive to noise since it is closer to substrate and even it is more resistive.

Ya for routing do not use poly only, use metal one with contacts to connect gates

if your designer is saying to use poly then its really interesting and i suggest you to get clarification from your design engineer and even let me know the answer

Don't forget this thumb rule, "Don't be afraid to ask question to circuit designer".
 

I have already asked designer..

& they said metal leaks more than poly..

BUt I disn't understand it properly.. so thts why I need help..
 

I've never heard of that expression "metal leaks more than poly" I think you'll need to clarify what he/she means exactly. All parasitics are creating by the layout designer but can be minimized by good planning and communiction.

Perhaps they are refering to fringe capacitance?
Why not shield the net?
 

Was this in a non-volatile memory based on stored charge?

In this case I could understand taking extra precautions about leakage but normally I wouldn't expect metal routing to be leaky, or leaky enough to matter.
 

Hi..leakage,gate siging,paracatics..all r challenging thing's ...future also...but in the routing mainly consider thing is reduce poly.As my knowledge that is the basic thing in layout design first consideration.May be it will effect on fab for more space.Think and search again....must....best of luck..
 

I can imagine that a tiny metal line driven by a clock can work like a transmitter and that therefore a "tiny metal trace is more leaky" because of some cap coupling towards another metal line.

Anyway some designers, like me, are sometimes paranoid about these things. You can ask your designer if something is unclear.

"Actually the distance between two gate is not less. Its quite big in length"

Probably this is not the intention of the designer!
 

I accept the opinions of all and me also suggest u to ask designer once more....
 

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