Morphological inspection is a significant step in the manufacturing process of precision machinery, integrated circuits, micro-optical devices, etc. Typical representatives are internal combustion engine fuel injectors, etched patterns on semiconductor silicon wafers, and micro-gratings. The presence of step-type structures on the surface of these components achieves certain special functions, which place nanometer-level requirements on the high measurement resolution of surface 3D morphology. Interferometric testing is a high-precision non-destructive measurement method for surface 3D morphology, which is combined with microscopic imaging technology to achieve microscopic morphology measurements. Depending on the magnification of the microscope, the lateral resolution varies from a few microns to hundreds of nanometers, and the field-of-view covers ~0.1 mm×0.1 mm to ~10 mm×10 mm. When the size of the part to be measured is large, the three-dimensional morphology measurement should be completed using stitching test. For some larger size parts to be measured, the lateral scale of the step-type structure is often larger, and the lateral resolution requirement of the measurement is reduced, which is no longer limited to the microscopic imaging method, and the reduced imaging scheme can be used to achieve direct measurement in a large field-of-view range. The goal of this study is to design and build a broadband optical interferometer with a large field-of-view to meet the inspection needs of large-size and step-type structure samples.

For the detection needs of special samples, this study selects a 1 inch detector and sets an imaging magnification target of 0.25×. According to this setting, we determine the field-of-view, numerical aperture, system focal length, and other system parameters of the entire system. After comprehensive consideration of the system's interferometric needs, this study uses the Köhler illumination system, double telecentric imaging structure, and sets a double flat tilted form of interference, which retains the characteristics of the Mirau-type structure with an equal light range and achieves a similar unobstructed effect as the Fizeau structure [Fig.1 (c)]. Subsequently, we select the appropriate initial structure using Zemax for the system design. After the system is built, the resolution plate and standard stage are selected for test verification and evaluation of the system.

The large-field broadband optical interferometer designed and built in this study operates in the 480‒750 nm band and uses a 1 inch detector to form a measurement field-of-view of 47.60 mm×35.76 mm. This system consists of an illumination collimator, an interferometric cavity, and an imaging lens. The illumination collimator adopts the Köhler illumination, providing a uniform illumination object field-of-view with NA=0.015 and Φ=59.6 mm; the interferometer cavity retains the advantages of the Mirau-type equal-range interference and the Fizeau-type no-center-obstruction feature, and consists of a spectroscopic plate tilted at 1.5° and a reference plate tilted at 3° (Fig. 2); the imaging lens and collimator form a double telecentric imaging optical path with an imaging magnification of 0.25×, and the aberration correction reaches 0.24% in the wide spectral range (Fig. 8). The completed broadband optical interferometer was used to test the USAF1951 resolution plate with a system resolution of up to 14 lp/mm(Fig.12), and step plates with calibration heights of 7.805 μm and 46.554 μm were tested with the results of 7.833 μm and 46.552 μm, respectively. The height deviation for the step-type structure measurement was better than 0.4% (Figs.13‒14).

To address the problem that the existing interferometric inspection system is difficult to meet the efficient measurement of three-dimensional morphology of large-size step-type structures, this study begins with the basic optical path selection, then determines the optical structure form of double telecentric and Köhler illumination, and adopts a new interferometric cavity structure form that retains the advantages of the Mirau-type equal-range interference and the Fizeau-type no-center-obstruction feature. After designing the system using Zemax, the broadband optical interferometer system with a large field-of-view was obtained. The interferometer works in the wavelength range of 480‒750 nm and uses a 1 inch detector to achieve an imaging magnification of 0.25×, a measurement field-of-view of 47.60 mm×35.76 mm, and an aberration correction of 0.24% over a wide spectral range. The completed broadband optical interferometer was used to test the USAF1951 resolution plate, consisting of two step plates whose calibration heights were 7.805 μm and 46.554 μm. The test results show that the system’s measurement resolution can up to 14 lp/mm, and the height deviation of the step-type structure measurement is better than 0.4%. This broadband optical interferometer can achieve both larger field-of-view measurement and step-type structure measurements while retaining very high vertical resolution. It avoids the problem of low measurement efficiency caused by stitching when using the traditional interferometer to detect large-size step-type structure samples, which has a greater application prospect in the precision measurement of precision machinery, integrated circuits, and micro-optical devices.