Use of Dummy Metals in Layout

[jediknight_redmond]

Hello there..

I have seen some layouts with floating metals or dummy metals in between 2 parallel metals. I am wondering why they do such a thing. Can anyone enlighten me out there?

Thanks in advance..

 

[protonixs]

Hello there..

I have seen some layouts with floating metals or dummy metals in between 2 parallel metals. I am wondering why they do such a thing. Can anyone enlighten me out there?

Thanks in advance..

They are used to reduce the coupling capacitance between the parallel metals.

 

[octocreation]

Also there are used for to reduce density issues...

 

[jediknight_redmond]

Thanks protonix, you are my idol. But is it okey to use minimum width for these dummy metals? Or their width should be equal to the width of the two metals near it?

 

[Fom]

Floating metal between two metal lines will increase capacitance coupling between these metal lines.

 

[k_90]

Never leave metal foating is not a good idea for both design and manufacturing.

If a designer wants to isolate a signal from either a noisy/quite such as clocks and bias lines a few methods would be used such as spacing the metal further apart, changing layers and connecting them to a quite signal using proper shielding.

Adding a floating metal would increase the capacitance between the two nets and no-one would know which signal the floating metal would take reference from increasing the fringe capacitance!

So using my experiance as a senior layout engineer I would never have floating metals and niether have I ever came accoss it in layout design.

 

[Fom]

So using my experiance as a senior layout engineer I would never have floating metals and niether have I ever came accoss it in layout design

But you as a senior layout engineer should know that some foundry's design rules requires add dummy metal to the top metal layer. They call that is for stress relief.

 

[k_90]

your right what I meant to say was in my current job we don't have the need for floating metal, in my last job we used it as a security barrier to stop light attacks.
Anyway i still would'nt put it in between two signals, of course the flip side of the coin is if its to meet fab rules and they didtnt have a choice....

 

[dchaphle]

Hi,

Dummy should be added to meet metal density requirement, but one should take care of parasitics into account. however if signal is noisy or sensitive then go for metal shielding

 

[selvaraja]

is used for sheild signal from noisy enviroment and the dummy must connected to ground.

 

[JoannesPaulus]

Everybody,
floating metals are only added to meet metal density rules (on all metal layers). They do increase the parasitic capacitance but in most cases the increase is negligible.

 

[rfsystem]

The path is clear:

1. Floating density patterns for digital applications. The intend is to reduce the loading
of the logic signal.

2. Grounded density pattern for anlog and bias signals. The shielding is the most important aspect. For inductive coupling also double twisted folded lines are used for differential signals.

There are some logic layer which define different ectraction rules. For instance analog, digital , sram etc. These could be used to drive a differnt pattern and check the island property of the density pattern.

 

[JoannesPaulus]

1. Floating density patterns for digital applications. The intend is to reduce the loading of the logic signal.

How can you reduce loading introducing more metal? The floating metal does not add capacitance, in my humble opinion.

 

[Fom]

The floating metal does not add capacitance, in my humble opinion.

Actually the floating metall can increase existing coupling capacitance.
Introducing floating metal you will replace one capacitance by two in series. But capacitance of each of these two will be increased by factor more than 2.

 

[rauol]

Top metal layers are sometimes added for security reasons in Microcontrollers so as to protect the memory contents from external probing

 

[rfsystem]

Yes, putting lateral dummy fill metal in series act like many caps in series and have higher capacitance than w/o any metal

But

that is not allowed because of the rule restrictions

But

the questions was to connect to ground or let floating. And here series caps give less load than grounded fill metal!!!

 

[drDOC]

I would connect all caps to some voltage. Personally I really do not trust any floating caps, especially not in an harsh environment. Oxide is quite thick between metals, so the caps could be charged to potentially dangerous break through voltages.

And if it is a rf circuit or some clocked, auto-zeroed chopper stabilized precision amplifier, then I would do a parasitic extraction to simulate the full design with all caps again after layout. That should be a save way to simulate the influence of the caps.

 

[Azmi]

based on my experience, dummy metal used to reduce process variation issue during metallization process. maybe among the issues are misallignment, flash which will make metal shorted. Another purpose is to ensure that the edge of the metal is flat/straight line (no saw shape under SEM). However, the trade-off is this dummy metal will increase the existing coupling capacitance.