Three-dimensional confocal reflectance microscopy for surface metrology

Confocal microscopy uses a confocal aperture in front of a detector to eliminate out-of-focus blur, providing optical sectioning, high spatial resolution, and high contrast. It enables precise three-dimensional (3D) reconstruction of a sample surface. Using objective lenses with high numerical aperture and short wavelength illumination, confocal microscopy offers high spatial resolution in both lateral and axial directions in a non-destructive and non-contact manner. These qualities make confocal microscopy an ideal tool for the inspection and measurement of microscopic samples. One of the limitations of confocal microscopy for 3D surface metrology, however, is its speed. Both lateral and axial scanning is needed to produce a stack of two-dimensional images along the height, and this takes more time. While high-end confocal laser scanning microscopy can acquire a 3D sample surface within a few seconds, an even faster imaging speed is required in some applications, such as in in-line inspection. Various techniques have been proposed to speed up 3D confocal measurements, such as chromatic confocal microscopy, differential confocal microscopy, dual-detection confocal microscopy, and direct-view confocal microscopy. Here, we review the basic principles, theories, scanning methods, and progress of confocal microscopy for surface metrology, and we discuss the latest progress in the field of ultrafast 3D surface measurement. We anticipate that confocal microscopy, which has become one of the standard metrology tools, will continue to evolve for 3D surface measurement.