Contact windows in CMOS Layout

[ukint]

cmos layout contact current density

Hi All.

Does contact reduce the resistance in the given diffusion or does it increase the resistance?.I read in some article saying more the number of contacts more the resistance.I was of the view that it was otherwise(more the contacts ,lesser the resistance).Can anyone explain me the real use of contact?

Regards,
ukint

 

[Litz]

Hi ukint,

Contacts or vias, these layers define cuts in the insulation layer that seperate conducting layers and allow the upper layer to contact down through the cut or contact hole.

Depending on aplication, the contacts may increase the resistance but in case of connectivity contacts reduce the resistance because more contacts means more path for the electron to flow from one layer to the other. Consider connecting a 20um metals, lets say for example metal1 and metal2 and you use a single contact of 5um, as compared for example when you use 4 contacts of the same size, the resistance of the latter is lesser compared to the first case.

Depending on the technology you do, a single big contact or more smaller contacts have both advanatage and disadvantage. We have a term called "perimeter to area ratio" which when you consider has an impact in terms of resistance.

Best regards,
Litz

 

[gharuda]

............____________________ metal1
............_|__________________ metal2


..........._____________________
...........__|__|__|___|__|___|__


if u see in the first case their is only one via between metal 1 and metal2 ,when current flows from metal2 to metal1 or vice versa . The concentration of electron or current density is more at the junction b/w M1 and M2. As the concentration increases the juction might be damaged. Since their is only one path for the current to flow from M1 toM2 the resistence increases.

in the second case more vias is used so . current flow is even disturbuted b/w M1 and M2.

 

[ukint]

Thanks litz and gharuda.That was pretty useful information.

 

[Litz]

Hi Ukint,

Yeah you are right, that's what we call "electromigration".

Regards,
Litz

Added after 1 minutes:

Ukint,

If you dont mine, may I ask how long you've been doing layout?

Litz

 

[ukint]

Litz,
I have just started doing layouts.