Help needed: Interconnect capacitance using HFSS/Sonnet/any other EM simulator

Hi all,
I am trying to simulate the following interconnect structure from ptm website:

http://ptm.asu.edu/images/local_electrical.gif

The specs are: W = 0.10 um, s = 0.10 um, l = 100 um, t = 0.20 um, h = 0.20 um, K = 2.2.

I want to calculate C_ground and C_couple.

To calculate C_ground first, I implemented the structure in sonnet with only one interconnect(instead of 3 shown) but it gives a negative value of capacitance. Please correct me if i am wrong but the capacitance value given by sonnet is series capacitance(as mentioned in Sonnet) but what i want is the capacitance between bottom surface of interconnect and lower ground plane.
I used thick metal model(Copper) in Sonnet for the interconnect.

Now the problems are:
1. What does series capacitance exactly mean in reference to the structure implemented(url above)?
2. How can i calculate capacitance between interconnect and ground plane instead of series capacitance?
3. Why is the capacitance coming out to be negative?

I'm a newbie with EM simulators.
Any kind of help would be greatly appreciated.
Also, please tell if there is any way i can upload my sonnet implemented file for analysis or finding errors in implementation.
Thank you in advance.

Here is the implementation of the interconnect in Sonnet EM simulator.
I would be grateful if someone could analyse the same for errors.

Thank you.

View attachment Sonnet_implementation.rar