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:






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:






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:









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:






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









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!