Three-dimensional confocal reflectance microscopy for surface metrology

Abstract

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.

Differential confocal microimaging with non-uniform reflection suppression

“…With the gradual popularization of microelectronic computer applications and the development of modern optical technology, laser application technology has become increasingly significant in the precision machining industry, particularly in areas such as machining, optical processing, and electronic processing [1] . As a result, three-dimensional morphology measurement technology has gained immense importance [2][3] .…”

Dark-field line confocal imaging with point confocality and extended line field for bulk defects detection

“…To further expand the confocal dimension, one way is to distribute multiple slits axially to extend multiple confocal lines at discrete depths [10] . An alternative is to focus the different wavelengths of a broadband line source at different depths to create sequential chromatic confocal lines [11][12][13] . However, two issues emerge.…”

“…However, two issues emerge. One is that confocal rejection of the out-of-focus light is effective only in the slit width direction, leading to cross talk in the slit length direction and degradation of the optical sectioning ability [13] . The other is that the inherent constraint on light collection angle and field of view of the objective hinders the simultaneous realization of high collection efficiency of the scattered light and large confocal line field.…”

“…Spectrally encoded line confocal imaging tried to address the issues by using a grating to disperse a broadband source into its component wavelengths in the transverse direction [13][14][15] . Since the specific transverse position corresponds to the specific wavelength, a spectrometer is used to pick up spectral lines separately to reduce the cross talk.…”

Curved surface measurement method using chromatic confocal sensor and tilt scanning

“…Most optical measurements essentially use different methods to obtain the relative or absolute height of the specimen surface. In addition, a linear slide and a rotation stage are regularly used to drive the specimen or optical sensor to obtain their three-dimensional surface information [8,9]. Commonly used optical measurement methods include white light interferometry [10], focus variation measurement [11],…”