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What is the use of adding filler cells to a design?

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What are the use(s) of adding filler cells to the design? Are they definitely essential for even low frequency ICs?
 

filler cell

Filler cells are used to fill any spaces between regular library cells to avoid planarity problems. They are need when the density of the required metal or layer has not meet the foundry or fab requirement. Thus, you need to add it whether it is low or high frequency.
 
fill cap cells

Filling 100% of the area with regular cells are generally impossible
=> spaces are need in a digital layout to improve routing
Filler cells are used to fill theses spaces between regular library cells to
connect power rail
 
what are filler cells

Use of filler cells create symmetrical metal density that is manufacturable. If we do not use filler cells, we'll land up in a non-working chip, because during fabrication, the next transistor that comes after a long distance from another of the same type, will have improper layout.
 

filltie cell

alok,

If you explain briefly that will be good.

Prithivi.
 

filler cells design

forkschgrad said:
can it be used as a protection elements?
Click to expand...

What type of protection? ESD?

Some filler cells have caps in them for decoupling cap.
 

why are filler cells added in the design

i dont't believe it can protect the devices from esd. what im asking is aside from non-linearities. maybe parasitics.
 

filler cell with metal

tfwee said:
Filler cells are used to fill any spaces between regular library cells to avoid planarity problems. They are need when the density of the required metal or layer has not meet the foundry or fab requirement. Thus, you need to add it whether it is low or high frequency.
Click to expand...

Hi twfee,

Can you also tell me what exactly are these FILLCAP, FILLTIE cells. Why and where are they used. I have seen these cells in one of our libraries.

Thanks in advance
Chethan
 

why filler cells asic

:D

Filler cells are used to establish the continuity of the N- well and the implant layers
on the standard cell rows, This is one of the Fab constraints, for ease in the generation of the masks.

D-cap cells are quite different from the filler cells , while these can as Decoupling capacitances , Filler cells cannot introduce any functionality ...they have only the power and ground rails and shielding if any and the NWELL, P-IMP ,N- IMP layers as per the design rules,

as you ask the SoC-encounter tool to place the filler cells, it fills the gaps in the design in std cell rows with diiferent variety of filler cells as per the gaps availabilty, however there will be overlaps, you can delete the overlapping ones.


I don't really think that filler cells can solve any metal density issues.....which is usually solved by metal fill option in the tool.

Metal density rule arises out of an other fab constraint, to physically support the substrate minimum metal density should be maintained throughout the die area on all layers.
 
filler cell and metal fill

There are also IO filler cells. They are used for PAD ring (predriver and postdriver rings) continuity. This way the ESD protection (remember only in the case of IO filler cells, NOT the std. cell filler cell) of the chip improves.

For the std cell region filler cells, "rkadarla" has explained nicely..... In addition to that, some of the small cells also doesn't have the bulk connection (substrate connection) because of their small size (thin cells). In those cases, the abutment of cells through inserting filler cells can connect those substrates of small cells to the power/ground nets. i.e. those thin cells can use the bulk connection of the other cells (this is one of the reason why you get stand alone LVS check failed on some cells)

Of course some of the filler cells are used to make up the poly density (if that filler cell is having any poly structure inside), but certainly not for metal density.

BTW, it's always better to fill the gaps with fatter filler cells first then thinner filler cells.

Some libraries have spare cells embedded inside filler cells. Good for ECO....
 
eco filler cells

If we place too many Cap_Filler, is there any transcient issue (out of ESD or gate leakage)?
 

need for filler cells

hi,

fillers cells are used to fill the gaps between the std cells and to make the metal density uniform from the fab point of view.
they are also useful for eco-engineering change order, u can configure them as a functional cells to make a samll functionality chage or to cope with setup or hold violations.

it may help u.

thanks..

HAK..
 

why use filler cells

If you place too many decap cells it will reduce dynamic IR issues as well as noise.But the only issue is if you find any timing violations needs to be fixed or some ECO needs to be implemented in that region post base PG is completed, you may not be able to find suitable filler cells to replace them.
 
filler cells

Decoupling cells may increase leakage current.
 

I do see filler cells in standard cell arrays that occasionally provide constant high and/or low outputs for other cells. What is the reason for that?
At a previous company I used to work for, we were mandated to use those cells to provide "soft constants" instead of tying the inputs to the rails directly. The reason was ESD protection.
Can anyone confirm the validity of this?
Thanks,
Zoltan
 

Yes, you are right. There are special cells (tie-high and tie-low) that provide constant values for the regular cells. The reason is ESD.
 

Filler cells can be used for ECO in case BASE PG is done.
If we need to do an ECO after Base PG we can use these filler cells for ECO.
 

Thanks for the confirmation. Would you happen to know of any publicly available reference that would explain the details? I have done a relatively extensive Google search, but I could not dig up any.
oratie said:
Yes, you are right. There are special cells (tie-high and tie-low) that provide constant values for the regular cells. The reason is ESD.
Click to expand...
 

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