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Design >> Mixed-Signal Design >> why should be metal layers in alternate directions?
https://designers-guide.org/forum/YaBB.pl?num=1297495478

Message started by seahs on Feb 11^th, 2011, 11:24pm

Title: why should be metal layers in alternate directions?
Post by seahs on Feb 11^th, 2011, 11:24pm

For example, if metal 2 is horizontal and metal 3 should be vertical.
Is it because of parasitic capacitance?

Title: Re: why should be metal layers in alternate directions?
Post by rfidea on Feb 12^th, 2011, 12:31am

No, it is just a way for simplify routing. It is not by a coincident it is called Manhattan routing. (Except for Broadway that is...)

Title: Re: why should be metal layers in alternate directions?
Post by loose-electron on Feb 13^th, 2011, 1:39pm

Mostly its for simple routing, alternate N-S paths with E-W paths. (or up-down, left right)

Consequently, if you want to go anywhere on the chip, in theory, you only need to go thru one set of vias.

The reality of the matter is that lower metal layers tend to be used for routing inside a functional block, and upper metals get used between blocks in the system.

Vertical thickness of oxides is intentionally kept thick to minimize coupling between vertical layers, but some coupling does exist nonetheless.

Title: Re: why should be metal layers in alternate directions?
Post by love_analog on Mar 3^rd, 2011, 11:39am

Another reason is to prevent coupling. If M2 is horizontal and M3 is also horizontal and right on top of M2, you could capacitive coupling between the 2 metals.