yi.luo

Problems of post-layout simulation using Calibre LVS/PEX

Discussion created by yi.luo on Jan 15, 2014
Latest reply on Feb 11, 2014 by karen_chow

Hi all:

 

I'm facing a problem about the Calibre LVS/PEX, I will describe it using  this simple current mirror shown below, X and Y are same MOSFETs with  width=16, nfing=2, m=1:

 

 

1.png

 

 

When I just do the layout with only one finger for both X and Y (w=16,  nfing=1, m=1), and do the post-layout simulation, I injected 100uA to  node

Vb, it has no problem seen from the post-layout schematic: X will absorb 100uA and Y will source 100uA.

 

But when I use 2 fingers, the problem comes. The layout is:

 

 

2.png

 

 

In this layout, the X and Y are in a single transistor which has w=32  and nfing=4,  that means, X/Y are w=16, nfing=2 which is the same with  the schematic above. ------ This is in my mind what is correct.

 

But when we do the LVS, it failed and showed property errors that X/Y  has 2 fingers in the schematic but has 4 fingers in the layout:

 

 

 

3.png

 

 

 

 

If change some MOS properties in the schematic, nfing is changed from 2  to 4 (layout doesn't change), then hen the LVS passes, which we think is  not correct (but it passes!). I used this extracted CalibreView file to  do the post-layout simulation, the current reduced to a half as seen  the comparison below:

 

 

Pre-simulation, both X and Y have 100uA current:

 

 

7.png

 

 

Post-simulation, the current reduced to only 50uA:

 

 

8.png

 

 

 

 

 

Does anybody know what would be the problem is and give some help? Thank  you SO MUCH for any suggestions cuz I've been on this problem for days.  Thank you!

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