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Why to avoid FILL1 in design

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hiral.kotak

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Hi All,

Can you pls explain, in design why to avoid filler FILL1 in design? What is the effect of min drive strenght FILL1 cell in design?

Thanks
HAK
 

hi HAK,

We usually start from top to bottom. Means try to fill first FILL4 or more then that as per the design specification and then FILL3 and FILL2 and in the last FILL1. The reason 2 FILL1 == FILL2 ( as per space concern).
 

Filler are used for N-well continuity. As birdy mentioned, filler cells are inserted in a top to bottom fashion at the end of physical design flow. Filler cells are just N-well implant. They are physical only cells and doesn't have any timing information or drive strength associated with them.
 

The number associated to the FILL name only indicate how many std-cell site (based unit) this filler could fill.
You could also used DCAP to fill the empty space.
 

You may want to avoid fillter 1 to avoid abrupt vt changes, suppose, what I am thinking is a LVT filler1 doesn't sit in between two HVt Cells as it creates, the vt abrupt changes.

Ofcourse, avoiding filler1 is not the hard rule, but , better to avoid.

Thanks,
Jack

---------- Post added at 18:10 ---------- Previous post was at 18:02 ----------

Filler cells are only N-well implants. They are used for N-well continuity. Now a day you have multi-Vt libraries. Variation in threshold voltage is obtained by varying channel dopping. So for each Vt an additional fabrication mask is required. Different Vt have their own well continuity as they have different dopping levels and masks. So you have different fillers cells for different VT. Filler doesn't have any threshold voltage.
 
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