Till now i am using fix clock to do CTS. Wat are the methods i need to follow to create a robust clock tree. Also i want to know how to do skew minimization in magma . i am using Magma Blast Fusion 4.2
Skew is a parameter you give the CTS tool to meet, you can specify what ever you want...within reason. As far as robustness, the only thing you can do is fully specify your clocks and any ignore pins, etc. After that it is up to the CTS tool to build a robust clock tree.
I personally am not a Magma user, so I don't know the exact commands, but it seems to me you have a good clock already...unless the design is very small.