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resistor variation over wafer (Read 4344 times)
aaron_do
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resistor variation over wafer
Feb 16th, 2007, 3:22am
 
Hi all,

I know that a resistor's value can vary by as much as 50% from its modelled value, and resistors can be matched to a high degree of accuracy. However, I was wondering what the typical variation in a  resistance value is across the entire wafer assuming no matching is used. In particular the nwell resistor but general guidelines would be great too.

From my own experience, I found very little variation across a wafer for a unsalicided npoly resistor....less than 5% without any matching.

thanks for the help,
Aaron
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Re: resistor variation over wafer
Reply #1 - Feb 16th, 2007, 7:11am
 
Aaron,
 A lot really depends on your process.  Example, If you are using an Nwell resistor on a Bulk Silicon process, generally the nwell has an STI built between the two contact ends of the resistor.  I have seen nwell resistors vary dramatically because of this, as the STI depth is not a tightly controlled dimension, creating a lot of variation.  I personally have seen nwells change by >50% on some processes, within the same die.  Again it was because the STI depth was not tighly controlled at all.
 Poly resistors tend to be much better because their physical dimensions are more tightly controlled, as is the doping profile.

Ultimately you need to trust what the foundary is telling you about the resistors, because they know which factors affect the resistances and how well they control those factors.  Of course the caveat here is I have seen foudaries that did not even model temp-co for their resistance (that was a pain).

Your good experience with the npoly unsalicide resistor, may very well be unique to that process, I would not base work you do on other processes on that one experience.

Hope this helps,
Stephen
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aaron_do
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Re: resistor variation over wafer
Reply #2 - Feb 20th, 2007, 2:53am
 
Hi Stephen,

exactly the answer i was looking for. Thank you very much,

Aaron
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Re: resistor variation over wafer
Reply #3 - May 23rd, 2007, 6:47pm
 
Contact resistance can be very variale and the common strategy there is to size resistors so that contact resistance is a small part of the total resistance. (small value resistance becomes lots of in parallel resistors due to that)

Rule of thumb +/- 20% due to process and wafer to wafer.

Across the wafer I have seen 7-8% with no care in matching, and 4 or 6 inch wafers.

Dont take data from the foundry on faith. Question all of it, request the source data.
I have been on both sides of that issue many times.

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