Dual-wavelength vertical scanning low-coherence interference microscopy

摘要

Particularly in optical industries and in micro systems technology white-light interferometry has become a standard tool for highly accurate topography measurement. Our work is based on a modified commercial white-light interferometer with a tube lens of a rather short focal length. This allows a compact design and a large field of view without influencing the numerical aperture of the objective. Furthermore, a LED illumination is used, which is a precondition for our approach. The short focal length of the tube lens requires a proper optical correction in order to avoid measuring errors caused by aberrations. Nevertheless, spherical surfaces with relatively large local surface tilts or MEMS with sharp edges often give rise to systematic measuring errors. These are caused by diffraction and dispersion effects, which finally lead to deviations between height values obtained from the envelope's maximum of a white-light interference signal and those values obtained from the signal's phase. For certain cases this may result in ghost steps in the measured topography. In order to identify these steps we use a second phase evaluation at a different center wavelength. During the depth scan images are taken for both center wavelengths. A special evaluation enables us to clearly identify the appearing phase steps and to correct the results in a second step. The main application of this technique is the measurement of curved or structured specular surfaces with high resolution, which until now is limited by the occurring effects. In addition, it might be possible to use low-cost optics in combination with the dual-wavelength technique in order to correct the measuring errors resulting from optical aberrations.