if I got top 100 can i get free fee in any iit Institute
No, there is no provision for fee reduction on the basis of your JEE Rank. Whatever rank you get, you have to pay the required fee to the institute you are taking admission into. However, if you are a SC/ST category student then there is some scholarship/ discount on institute's fee. There is also a provision for fee reduction for students on the basis of their family income.
Hope it helps.