Roughness Characterization of ultra-smooth surfaces using common-path interferometry

The spatial frequency response, height sensitivity and system noise of a scanning common-path interferometer have been studied. The impulse response function and the frequency transfer function of the system are obtained analytically using Fresnel diffraction integral. Experimental results are also given. The results show that the system covers a broad spatial frequency range from 2x10(-5)/mu m to 3/mu m. The height sensitivity of the system is better than 0.01 Angstrom. System stationary noise less than 0.1 Angstrom RMS is achievable without additional noise reduction post-process. Measurement of an ultra-smooth silicon substrate with a sub-angstrom roughness is successfully demonstrated. Measurement of a smooth glass substrate is also shown.