IC Counterfeiting is becoming serious issue. The approach discussed here is to use Electromagnetic (EM) input to an IC and measure its EM input output response. The idea is to extract a signature from EM response which should be unique to one IC. The main purpose of this work is to show that it is possible to authenticate electronic chips from a non-intrusive method, based on the use of RF waves. IC authentication can be performed using Physical Unclonable Function (PUF). PUF are based on process variation inherent to semiconductor fabrication process. EM based authentication is also based on the same principle. Nevertheless, unlike PUF such a method does not need dedicated circuitry and thus may have lower cost of implementation and may be easier to industrialize. This work first focuses on FPGA which are a common target of counterfeiting. We first prove that FPGAs are sensitive to EM excitations and find the optimum configuration using a lightweight marker not as complex as PUF to optimize the sensitivity to EM excitation. Finally, a post processing is performed on the EM measurement to get the FPGA signature which is later used for authentication. The post-processing operations are being developed in order to deal with aging effects and other measurements issues commonly seen with RF measurement.