Nano In-Process Measurement of 3D Micro-Profile Using Optical Inverse Scattering

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A new optical measurement method, which can be applied to in-process measurement of three-dimensional micro-profiles with accuracy in the nanometer order, is proposed in this paper. The proposed method is called optical inverse scattering phase method and offers the advantage of measuring a three-dimensional profile within the whole area illuminated by the laser beam simultaneously. This means that no scanning process is required. The optical inverse scattering phase method is based on two principles, one is optical Fourier transform, the other is iterative Fourier phase retrieval algorithm. Employing Fourier phase retrieval algorithm, three-dimensional micro-profiles are reconstructed from only the Fraunhofer diffraction intensity measured by Fourier transform optical system. Computer simulations as well as actual measurements were performed for the verification of our proposed method. First, the simulation results suggested the capability for reconstructing three-dimensional micro-profiles with accuracy in the nanometer order. As a strategy to avoid stagnation of iterative algorithm, a new concept based on the design model of the workpiece and the Gaussian profile of the laser beam was also proposed. Next, in order to verify the feasibility of the optical inverse scattering phase method, the automated optical measuring system consisting of Ar ion laser, Fourier transform lens, CCD linesensor, EWS computer, etc., was developed and the measurement experiments were carried out for the ultra-precision grid pattern with the depth of 44nm and the pitch of 10μm. The experimental results showed that the proposed method makes it possible to reconstruct a micro-profile within the whole illumination area equivalent to the laser beam diameter at one time without scanning.