Gradient Effects - common centroidmethods

[vlsi.rajesh]

Gradient Effects

Hi,
I have read in Art of Analog layout book about matching and he has written that we r going for Common centroid matching to cancel the effects of Stress and Thermal gradient.for thermal gradient he told about power device present in out circuit as example and stress gradient comes while package shift.
Questions?
1. If there is no power device in our chip, is it necessary to do common centroid matching for other devices.If Yes, Please explain about it.
2.Is there any possiblity to occur stress gradient in our chip (other than package shift and also before fabrication) due to some device(or device operation) present in our circuit.Any example please?.
3.We are doing common centroid matching due to this two gradient effects or any other effects are there.., which will try to change the device operation.

Please reply for my questions and if u have any material related to this other than Art of Analog Layout please send to vlsi.rajesh@gmail.com or please post in edaboard, that will be helpfull to All.

Thanks ...

regards,
Rajesh

 

[dick_freebird]

Re: Gradient Effects

There will be some stress / strain on any mounted die but
whether there is a topside gradient, I doubt.

Even without "power devices" you can still see significant
localization of power dissipation. Consider an op amp with a
10mA output rating and a 3V supply; maybe 25mW or so, seems
insignificant but with modern geometries this might be in 100x100um
or less area and on a par with the power densities seen in "real"
power devices. Plus, with power concentrated at the topside,
you will see more lateral thermal conduction as a percentage
of total power, than in a larger area (or, larger lateral extent
than wafer thickness) power device. Lateral thermal conduction
is directly going to make a lateral thermal gradient. The more
geometries shrink, the more power densities rise and the more
variation in gradient will be localized.

You can address this somewhat by adding "cooling zone" extent to
your layouts, preventing too-tight packing of heater against "receiver".
Centroid layout is good for when you do not know a priori the
final orientation of your sensitive pair against the thermocline
(as in a standard op amp cell for general use). Back in the olden
times when an op amp was the whole product, we would just
"point the bow into the waves" and call it good. Hard to do a good
centroided layout in single layer metal, with minimum transistors
almost as big as bond pads....

 

[vlsi.rajesh]

Re: Gradient Effects

Thanks freebird, i have understand a little since iam new to Analog layout. If u have any other deep and basic answer's, please let me know. Any solution from other's....