Question about laying a 40Kohm resistor in IC design

[duron999]

Resistor Problem !

Hello, I'm a new hand of IC design,
I want to ask the question is,
Can I lay a 40Kohm resistor ??? Is it possible/suitable ?
Because I find that the process sheet resistance of Poly is 7.5 ohm/square ,
N+ Diffusion is 300 ohm/square.
Are the layout will very very large ???

Another question is:

Is "ohm/square" mean ohm per square micron ??? The unit is in um ???

Thanks !

 

[wee_liang]

Re: Resistor Problem !

Yes, you can. 40kohm resistor is a reasonable size.

But your sheet resistivity seems abit low for this particular process you're using. Are you using a digital process? Consider using nwell resistor if accuracy is not a major concern.

Basically, ohms/square means the resistance can be measured from the effective number of squares you can equate from a line. For eg. if you draw a L=4.5um and W=1.5um for a 10ohm/sq material, the resistance is 4.5/1.5 * 10 = 30ohms.
(i.e. 3 squares).

 

[duron999]

Re: Resistor Problem !

Yes, you can. 40kohm resistor is a reasonable size.

But your sheet resistivity seems abit low for this particular process you're using. Are you using a digital process? Consider using nwell resistor if accuracy is not a major concern.

Basically, ohms/square means the resistance can be measured from the effective number of squares you can equate from a line. For eg. if you draw a L=4.5um and W=1.5um for a 10ohm/sq material, the resistance is 4.5/1.5 * 10 = 30ohms.
(i.e. 3 squares).

Thank ! I'm doing a analog design and using Cadence GPDK Technology,
In the manual, the sheet resistance table only show:
Layer (in ohm/square)
Poly 7.5
N+ Diffusion 300
Metal 1 0.1
Metal 2 0.1
Metal 3 0.1
Metal 4 0.1
Metal 5 0.1
Metal 6 0.1
Poly Contact 5
Diffusion Contact 7.5

Is it mean I can't use nwell resistor ? Can I use these parameter
to layout an accuracy resistor ?

thanks !

 

[j2005]

Re: Resistor Problem !

Hello,

Assuming that your length=1um, then the width=133.3um. Size-wise, I don't think it's going to be that much of a problem if you lay it out in multiples of 3 or 4. It might be worth checking with the foundry if you are allowed other types of resistors. Manuals aren't always up to date and Cadence GPDK may be limited to a few examples.

 

[purefen]

Re: Resistor Problem !

Dear:
the 40k ohm will occupy large area. As the result, the area of the active device will be samller than the area of the resistors. Why don't u think about the mos resistor to reduce ur area.

 

[duron999]

Re: Resistor Problem !

Hello,

Assuming that your length=1um, then the width=133.3um. Size-wise, I don't think it's going to be that much of a problem if you lay it out in multiples of 3 or 4. It might be worth checking with the foundry if you are allowed other types of resistors. Manuals aren't always up to date and Cadence GPDK may be limited to a few examples.

What is the function of "multiples of 3 or 4" ?

Added after 3 minutes:

Dear:
the 40k ohm will occupy large area. As the result, the area of the active device will be samller than the area of the resistors. Why don't u think about the mos resistor to reduce ur area.

I'm afraid that using MOS transistor to replace the resistor will affect
the bias current. Will that change the bias current ??? thanks !

 

[j2005]

Re: Resistor Problem !

Multiples means that you break up the length into equal sections and lay them out side by side. The total length results in the total resistance. It doesn't affect the area much, only makes it look more compact.

 

[cetc1525]

Resistor Problem !

ohm/square is the unit ohm.
maybe you can use a implant resistor.it will be 1k/square.

 

[jianbinma]

Re: Resistor Problem !

Multiples means that you break up the length into equal sections and lay them out side by side. The total length results in the total resistance. It doesn't affect the area much, only makes it look more compact.

It's right ,and it's a good way to lay out .