[Magic-dev] help, why capacitance doesn't match?

[wwu at cs.ucr.edu]

Hello, all,

I use magic to extract the capacitance of metal wire, but the value doesn't
match with the cap by calculation with MOSIS parametric results.

The tech I used is "SCMOS: SCN5M_SUBM.15 [to process: TSMC25]", 
technology file is SCN5M_SUBM.15.tech27. 

The MOSIS parametric results is:
http://www.mosis.org/cgi-bin/cgiwrap/umosis/swp/params/tsmc-025/t11y_epi-params.txt

For example, a (3 lambda * 75 lambda) Metal2, the capacitance extracted from
Magic is 171.387, equals to 0.327 *3 *75 + 0.627*(3+75)*2. 
====================  param number in .tech27  ============================
#metal2-sub blocked by 
 overlap (m2,fm2,rm2,m3c,m123c,m234c)/metal2 ~space/w   0.327 ~space/a,~space/m1
 perimc
(m2,fm2,rm2,m2c,pdm12c,ndm12c,psm12c,nsm12c,pm12c,m123c,nwsm12c,m3c,m234c)/metal2
~(m2,fm2,rm2,m2c,pdm12c,ndm12c,psm12c,nsm12c,pm12c,m123c,nwsm12c,m3c,m234c)/metal2
  0.627
===========================================================================  

However, in MOSIS's file, M2-Sub areacap=19af/um^2, fringecap=60af/um.
So total cap = 19 *(3*0.15)(75*0.15) + 60*((3+75)*0.15)*2 = 96.187+1404=1500.187.


So which one is correct, or do I misunderstand the MOSIS tech file?

Thanks so much,

Wei