Inspection tools for nano-particle contamination on a planar substrate surface is a critical problem in micro-electronics. The present solutions are either expensive and slow or inexpensive and fast but have low sensitivity because of limitations due to diffraction. Most of them are also substrate specific. In this article we report how Coherent Fourier Scatterometry is used for detection of particles smaller than ?/4. Merits of the technique, especially, the procedures to improve SNR, its flexibility and its robustness on rough surfaces are discussed with simulated and experimental results.