Design-model-based in-process measuring method of a three-dimensional microprofile by employing the laser inverse scattering phase-reconstruction method

As the micromachining techniques are developed, in-process measurement and evaluation technique for the 3D micro- profile in the microparts production system are required. In this paper, we propose a new optical measuring method which can be applied to in-process measurement of 3D micro-profile with an accuracy of nanometer order. The laser inverse scattering method has a possibility to measure 3D micro- profile in the whole area of a workpiece which is illuminated by the laser beam simultaneously without scanning. The 3D micro-profile is reconstructed by measuring only Fraunhofer diffraction intensities. The principle of measurement is based on the optical Fourier transform and phase-retrieval technique. The hybrid phase-retrieval algorithm as a fusion of the logarithmic Hilbert transform with the Fourier series expansion and the iterative algorithm is developed. We proposed the process error evaluation system by employing the laser inverse scattering method with introducing design model based scheme and the computer simulation system of measurement procedure is built up. In order to verify the validity of the proposed method, the computer simulations for the 3D profile such as microparts fabricated by photolithography process are performed. We show that the proposed method is effective for evaluating defective 3D micro-profile result from process error.