The effects of 3 metals with the same sheet resistance in a circuit

[swaroopa]

In the project i am carrying out,M1,M2 & M3 have the same sheet resistance.That means that all the 3 metals are having the same resistance.Then where is the question of M3 having the least resistance and M1 having maximum resistance?

What effect will this have on the supply?

This is an analog project.

 

[A.Anand Srinivasan]

Re: Sheet resistance

please post your circuit.... the idea that you are posing is not clear....

 

[swaroopa]

Re: Sheet resistance

The sheet resistance values are given in the PDK. This is supposed to be followed for all the ckts. I am just wondering because normally the trend is M3 wil have the least resistance compared to M1.So we use M3for Power routings.and M1 for internal routings.Now,M3and M1 are having the same sheet resistance and same thickness.Doesn't that mean that they are havin the same resistance???How wil this effect the power supply?Why is the PDK saying that M3 ,M2 and M1 have the same thickness and same sheet resistance?

 

[A.Anand Srinivasan]

Re: Sheet resistance

Sheet resistance is measured in ohms/(square distance), and is applicable to two-dimensional systems where the thin film is considered to be a two dimensional entity i.e.thickness is very low and hence neglected. It is equivalent to resistivity as used in three-dimensional systems.

so the thickness of M1 and M3 doesn't come into picture here.... it is only the length and width.... i think there must be a difference in those two quantities... please check on it....

 

[swaroopa]

Re: Sheet resistance

In this project,we have the minimum width of M1=M2=M3 .also,isn't the length a routing parameter? Also, is the resistance of M3< M2 because the sheet resistance of M3< M2??

I mean in a general process,Can two different metals have the same sheet resistance? if both of them have the same sheet resistance, and if i follow a minum width of the metal wont the resistance be the same?Then how can i call them as M1 and M2 as seperate names?

 

[shiv_emf]

Sheet resistance

did u check thinkness of metal layer???????

 

[A.Anand Srinivasan]

Re: Sheet resistance

sheet resistance of a material is given by ρ/t where ρ is the resistivity and t is the thickness... since you say M3 and M2 need not have the same resistance just because they have the same sheet resistance.... their ρ value can be changed by doping and t value changed by fabrication.....

 

[swaroopa]

Re: Sheet resistance

Hi,
Probably I'll make the question more clear...

I am working on an analog project (single poly – 3 metal) & am involved in the top-level integration of the entire chip. Currently, I am laying out the power grid for the chip.

As seen in the image provided, MET1, MET2 & MET3 have the same thickness i.e. 0.6550um. The sheet resistance values are also the same, Minimum = 50, Nominal = 60, Maximum = 70 mohm/square.

Since the sheet resistance is the same for all 3 metals, does this mean that the resistance of all 3 metals is also the same per square (for the same L & W)? & hence all 3 metals are the same?

R = Rs (L/W)

Normally, when we do power planning for a particular technology (e.g. single poly – 6 metal), the sheet resistances for the metals are different i.e. Metal1 will be most resistive & Metal6 will be the least resistive. Hence we use Metal5 & Metal6 for power planning so that there is a uniform distribution of power/gnd throughout the chip i.e. IR drop is reduced. Therefore, the 6 metals are different & not the same.

In my case, if all 3 metals are the same then would it make a difference with what metal I use to create the power grid?

 

[sekapr]

Re: Sheet resistance

more than one metal can have the same sheet resistance. It depends on the material used for metallization. since the signals connect transistors, it is preferred to use the lower metals for signal routing so that you will have minimum number of vias in the signal path.

 

[A.Anand Srinivasan]

Re: Sheet resistance

ya... they must be the same if you consider the resistance per square of it.... but for power routing as far as heard more area is used... i'm not that strong in this but just know what is what perspective....

different metals can have same sheet resistance only if their thickness is different or their resistivity is same... i dont think there are metals with same resistivity but maybe close ones....

 

[forkschgrad]

Re: Sheet resistance

that's new to me. it's usually metal3 which has the least resistance and usually the widest since it's normally connected to pad.