3-D micro surface profilometry employing novel Mirau-based lateral scanning interferometry

Abstract

An innovative 3-D surface imaging methodology for reconstructing micro surface profiles with a long depth measuring range and a nano-scale resolution was developed using the newly developed Mirau-based lateral scanning interferometry (LSI). The current measuring field of view (FOV) of conventional white light interferometers is limited by microscopic views of the existing interferometric objectives, such as those in Michelson, Mirau or Linnik designs. Moreover, the vertical scanning operation required for acquiring volumetric interferometric data is extremely time-consuming and makes white light vertical scanning interferometry (VSI) infeasible for automatic optical inspection (AOI) of micro 3-D structures. To resolve this, a newly developed white light LSI method based on Mirau’s optical configuration was developed by controlling the tilting angle of the reference mirror in the Mirau interferometric objective. With the proposed optical configuration, the surface is inspected at a tilting angle with respect to the maximum coherence plane of the interferometric system along its lateral scanning direction when the objective lies perpendicular to the tested surface. In addition, a system calibration method was developed to establish an accurate mathematical mapping model between the object depth and the lateral axis. To evaluate the feasibility of the methodology, a calibrated step height was measured for evaluating the accuracy and repeatability. Some industrial samples, such as photon spacers and other microstructures fabricated by nano-imprinting processes, were measured to verify the actual performance on real components. It was found that the measurement repeatability was controlled less than 60 nm within one standard deviation for a maximum measurable depth of 27.21 µm.

On-machine and in-process surface metrology for precision manufacturing

“…Optical microscopy is the only full-field technology that can be used for 3D measurement of surface texture on the microscopic scale under on-machine conditions [20] [144]. Most of the existing 3D optical microscopy techniques [94] [134], such as interference Fig.…”

An insight on optical metrology in manufacturing

“…As shown in appendix A, various Mirau-related systems and methods have been further developed and patented to meet diverse demands for better interferometric optical design, vibration-resistant optical design and algorithms, thickness measuring methods, and dynamic measuring methods and strategies. WLI microscopes [140][141][142][143][144][145] have been widely used for measuring film thickness and monitoring surfaces with discontinuities of more than several wavelengths. In contrast to laser phaseshifting interferometry (PSI) employing a light source with a long coherence length, WLI uses a broadband light source with a short coherence length of approximately 1 µm [146].…”

A new multiple air beam approach for in-process form error optical measurement

“…It is also noted that, for a given sensor used in a surface profiler for y(x, z), if there is a mismatch between the area covered by the sensor's single measurement and the surface area (x, z) (figure 4) to be measured, scan motions in the x and z plane will be needed. This applies to the three basic types of sensor: point sensor [4,17,20,27,28,[32][33][34], line sensor [35] and area sensor [11,12,36].…”