DIFFERENT TYPES OF CELLS IN VLSI

Well taps (Tap Cells): They are traditionally used so that Vdd or GND are connected to substrate or n-well respectively. This is to help tie Vdd and GND which results in lesser drift and prevention from latchup.

End cap Cells: The library cells do not have cell connectivity as they are only connected to power and ground rails, thus to ensure that gaps do not occur between well and implant layer and to prevent the DRC violations by satisfying well tie-off requirements for core rows we use end-cap cells.

Decap Cells: They are temporary capacitors which are added in the design between power and ground rails to counter the functional failure due to dynamic IR drop. Dynamic IR Drop happens at the active edge of the clock at which a high current is drawn from the power grid for a small duration. If power source is far from a flop the chances are there that flop can go into metastable state. To overcome decaps are added, when current requirement is high this decaps discharge and provide boost to the power grid.

decap cell.

Tie Cells: Tie-high and Tie-Low cells are used to connect the gate of the transistor to either power or ground. In Lower technology nodes, if the gate is connected to power/ground the transistor might be turned on/off due to power or ground bounce. These cells are part of standard-cell library. The cells which require Vdd (Typically constant signals tied to 1) connect to Tie high cells The cells which require Vss/Gnd (Typically constant signals tied to 0) connect to Tie Low cells.

 Filler cells: Filler cells are used to establish the continuity of the N- well and the implant layers on the standard cell rows, some of the small cells also don’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 standalone LVS check failed on some cells).

Spare cells: These are just that. They are extra cells placed in your layout in anticipation of a future ECO. When I say future, I mean after you taped out and got your silicon back. After silicon tests complete, it might become necessary to have some changes to the design. There might be a bug, or a very easy feature that will make the chip more valuable. This is where you try to use the existing “spare” cells in your design to incorporate the design change. For example, if you need a logic change that requires addition of an AND cell, you can use an existing spare AND to make this change. This way, you are ensuring that the base layer masks need no regeneration. The metal connections have changed, and hence only metal masks are regenerated for the next fabrication.

Kinds of spare cells: There are many variants of spare cells in the design. Designs are full of spare inverters, buffers, nand, nor and specially designed configurable spare cells.

 

Inserting Spare Cells

Spare cells need to added while the initial implementation. There are two ways to do this.

                 The designer adds separate modules with the required cells. You start your PnR with spare cells included, and must make sure that the tool hasn't optimized them away. There can be more than one such spare modules, and they will be typically named spare* or some such combination. The inputs are tied to power or ground nets, as floating gates shouldn't be allowed in the layout. The outputs are left unconnected. 

            Spare cells can also be added to design by including cells in Netlist itself.