Issues to be taken care in Digital Layout

[Prasanna Kumar]

Hi All,

I shall be greatful if any one can tell me what are the issues that are to be taken care in Digital Layout.

If I am not mistaken, as in Analog layout we keep in mind about the following which are important

1. Matching of Transistors,Capacitors.
2. Common Centriod methods.
3. Latch-up Prevention.

I wanted to know if there are any such issues in Digital Layout also. If so,Please let me Know.

Thanks in Advance.

 

[Vamsi Mocherla]

When you do digital layout, you really do not have to worry about matching. What you should worry is to layout in as much compact area as possible. Usually in any standard cell layout, one of the dimensions of the cell is fixed. For example, you do no t vary the height but expand in the X direction when you increase the drive. Of course you have to take care of the thickness of the VDD and VSS lines. And of course you do put some contacts for the Well and the substrate to take care of latchup. But unlike analog circuits you really not have to form gaurd rings.

 

[Prasanna Kumar]

Thanks a lot Vamsi !!!

Can U send me some links where i can get information on " ElectroMigration, Hot Electrons " and the effect of these on Layout stuff like that.

Thanks in advance

Added after 41 minutes:

Thanks a lot Vamsi .

I was thinking that there are issues like what we have in analog design, in digital also. Now it has been cleared.

Thanks a lot

 

[Vamsi Mocherla]

Well, electromigration occurs when you have a conductor(wire) carrying a higher current than it is supposed to. Actually electromigration is due to the fact that electrons are forcibly pulled from a metal for conduction creating hotspots. This will lead to the destruction of the metal. If you want to read about layout issues, read "the art of analag layout" or Baker Li Boyce book.

 

[Humungus]

Consider that electromigration is a temperature-dependent issue. As any conductor may self heat, this increases the electromigration phonomena, which in turn reduces the cross section of the conductor, so increasing self heating and so on.

Also in digital layout you must consider crostalk as this reduces the noise marging of your gates, which could cause undesired siwtching.

 

[jordan76]

Hi Vamsi Mocherla,

I agree with you for most part, but doubt about the following:

... But unlike analog circuits you really not have to form gaurd rings.

For digital circuits you also need to watch out for latch-up. Therefore you may want to add guard rings for safety if possible(that is a good practice). Especially when large currents may occur you have to add enough guard rings.

Pls correct me if I am wrong.

regards,
jordan76

 

[bg21359]

besides EM, you also need to consider coupling noise from adjacent wires;

 

[Sunrising]

Also in digital layout you must consider crostalk as this reduces the noise marging of your gates, which could cause undesired siwtching.

What's the general techniques for crostalk protection in digital layout? Same as that for substrate noise isolation? Or shileding as well? Or anything else?