The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Mar 29th, 2024, 3:12am
Pages: 1
Send Topic Print
resistor variation over wafer (Read 5110 times)
aaron_do
Senior Fellow
******
Offline



Posts: 1398

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
Back to top
 
 

there is no energy in matter other than that received from the environment - Nikola Tesla
View Profile   IP Logged
SRF Tech
Community Member
***
Offline



Posts: 59
Arizona
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
Back to top
 
 

Excellence in ESD and IO Design
www.srftechnologies.com
View Profile WWW   IP Logged
aaron_do
Senior Fellow
******
Offline



Posts: 1398

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
Back to top
 
 

there is no energy in matter other than that received from the environment - Nikola Tesla
View Profile   IP Logged
loose-electron
Senior Fellow
******
Offline

Best Design Tool =
Capable Designers

Posts: 1638
San Diego California
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.

Back to top
 
 

Jerry Twomey
www.effectiveelectrons.com
Read My Electronic Design Column Here
Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical
View Profile WWW   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2024 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.