With the vigorous development of technology in optics and semiconductors, transparent devices with smooth surfaces such as high-precision optical glass are widely employed in semiconductor and other fields. During grinding and polishing, optical glass inevitably produces a large number of scratches, pockmarks, bubbles, pollution particles, microcracks, and other defects in the subsurface. Micron/nanoscale subsurface defects will reduce the physical properties of transparent samples such as optical components, and seriously affect the development of processing and manufacturing technologies in optics and semiconductors. How to detect the subsurface defects of transparent optical components with high precision and provide key parameters for the high-precision preparation of transparent optical components has become an urgent problem in optical inspection. Subsurface defect detection technologies include destructive and non-destructive ones. Destructive detection technologies are simple to operate, and can intuitively and effectively observe the detection results, but they will make the test defects and the actual defects different. Therefore, the existing subsurface defect detection methods mainly focus on non-destructive detection technologies, including total internal reflection microscopy (TIRM), optical coherence tomography (OCT), and laser confocal scanning microscopy (CLSM), but these detection technologies cannot take into account both resolution and detection speed. Through-focus scanning optical microscopy (TSOM) is a model-based optical computational imaging method that can achieve non-contact, non-destructive, and fast measurement of three-dimensional nanostructures. TSOM features high sensitivity, simple hardware system, and sensitivity to nanoscale size changes, and it is not limited by the optical diffraction limit and can conduct online detection. To quickly and non-destructively detect subsurface defects of transparent samples, we propose a new method for detecting micronscale defects in the subsurface of optical components by TSOM and explore it in detail.

The incident light from the halogen lamp source is irradiated to the subsurface of the sample. Scattering occurs where a defect exists and the scattering light is imaged by the objective lens to the CCD target detector. This method is based on traditional light microscopy and equipped with a high-precision piezoelectric ceramic displacement stage to control the Z movement of the sample, with the movement positioning accuracy of 1 nm. A series of optical images of the subsurface defects are obtained at a certain range of defocus positions from above to below the focus point by scanning along the propagation direction of the light field (Z direction). The images series are stacked according to spatial positions to form an image cube (TSOM cube). Then, the image cube is sliced along the Z direction to generate the TSOM image. The TSOM image is processed through data analysis algorithms to obtain three-dimensional information such as size, shape, and position of micronscale and nanoscale structures, and the target is located by the maximum gray value.

The method can be adopted to detect and locate micronscale defects (Fig. 5). As the refractive index of scattered light is different in different materials, compensation and correction of the refractive index are necessary to obtain the actual depth of the defects (Fig. 7). According to the refraction law, the compensation and correction formula for the refractive index can be derived [Eq. (1)]. After TSOM scanning, the actual depth of the subsurface defects can be calculated based on Eq. (1). Experimental comparison and simulation (Fig. 10) show that larger subsurface defects exhibit volume effects. The position of the maximum light intensity corresponding to the defect in TSOM scanning is point p at the intersection of the radius parallel to the optical axis and its tangent (Fig. 11). To accurately determine the depth from the sample surface to the center of the defect, we should add the defect radius to the depth calculated in the TSOM scanning. After the radius correction, the average depth of the defect is 2000.3 μm, with a standard deviation of 2.4 μm and a relative standard deviation of 0.12%. Compared with other measurement methods, the depth deviation is 1.8 μm (Table 2).

The TSOM method can be employed to detect micronscale subsurface defects in transparent glass and locate the defect depth with a relative standard deviation of up to 0.12%. Theoretically, when the absolute depth of subsurface defects is reduced to hundred-microns, the standard deviation of subsurface defect location is only sub-microns. When TSOM is utilized to locate the depth of subsurface defects at the micrometer scale, it is necessary to compensate and correct the refractive index to further improve the accuracy of defect depth location. When the size of the subsurface defect is large, both the simulation and experiment show that the scattering light intensity distribution of TSOM is affected by the volume effect of the defect itself, which has an important influence on the depth of locating the center of micronscale defects.