Ground vs floating metal fill effect on timing

[pradeeppiskala]

How does ground and floating metal fill affect timing? Will have the same affect on timing in non-si and si session?

Thanks

 

[OhaAmo]

1. what do you mean by "ground"?

2. Floating metal fill can slightly increase cross-capacitance of regular nets, therefore slows them. SI and non SI results should be +- the same.

 

[rca]

Metal filling should not impact the timing if you have enough space between nets and the metal filling.
Connected the metal filling to ground or power could help to reduce IR-drop.

 

[birdy123]

I am not sure how good you are in understanding the Metal Fill Concept. As you have asked about the timing impact in case of Ground and Floating Metal fill, Let me first add a brief note about the Metal Fills.

During the Manufacturing Process, CMP process effect the dielectric thickness variation. Because of this variation, Yield and performance effects a lot. As the Die size increases, effects also increases. To reduce this effect (which is negative from manufacturing point of view), foundry suggested the Minimum Metal density Rule. Now It's obvious that during the design cycle, designer main goal is to route the design and they can't route the design keeping in mind the minimum Metal density at each and every point. That's the reason dummy metal comes into picture and the process used to fil the gaps with dummy metal to satisfy the Metal density Rule is know as Metal Fill.

Now, you can leave these metal as floating (means not connected to any thing) or you can connect these Metal fills with Ground (0V) or Power (VDD). In both the case the effect is different and as per the effect you have to close the timing and all.
Note: If metal left floating -> Those metal fills know as floating Metal fills
If Metal connected to Ground/power -> Metal Fills are termed as Grounded Metal Fills.

Now Come to your original question? How its going to impact the Timing? Let me ask one question - Do you thing it will impact the Parasitic capacitance of the design? If you think, It will .. then always remember .. If parasitic capacitance changes then Timing also changes.
If you think .. it's not then please have a look in the basics of capacitance.

There are 2 metal plates apart "d" distance from each other. then The capacitance between them is C. Now if you insert a metal plate between them at d/2, then please calculate the Overall capacitance.
calculate the cap by keeping the extra metal plate
1) without connecting with anything and
2) Connecting with Ground.

After Calculating, you will yourself come to know the effect on the Timing in both the case.

Note: I am giving you a direction to think, Think and reply if you come to some conclusion, else i will help you further ... (Sorry this is my approach to help people - No spoon Feeding )

 

[pradeeppiskala]

Cap change

1) with metal connected to ground

In this case, the coupling cap will be double as the distance between parallel plate is halved. The timing impact would be almost same (si). 2*old_coupling cap = new ground cap.

2) With metal floating

Don't know what the cap would be in this case. it will not be x2. But somehow the floating cap should be reduced to ground. How is this done?

P