# Quantitative Evaluation of Subsurface Damage by Improved Total Internal Reflection Microscopy
## 4 Experimental Results and Discussion  
Under the total internal reflection illumination, the upper surface of the sample #1 is scanned rapidly by the 10\(\times\) objective lens. The regions containing severe subsurface damage are located. Typical images of SSD with magnification of 50\(\times\) are shown in Figure 8. The imaging areas of Figure 8a,b are identical. There are several linear features in Figure 8b under total internal reflection illumination on the top of SSD. However, they have disappeared in Figure 8a, having been taken under bright field illumination of the microscope system. This indicates that these linear features are subsurface damage. By the micro-focusing operation, 65 images of SSD are captured by the CCD. The definition curves of the subsurface damage within the red ellipses are calculated using equation (1). Figure 8c is the definition curve of the short crack in Figure 8b. The normalized E rises or falls rapidly before or after the turning points, as shown in Figure 4b. There are 7 points between the two turning points. The microscope moves in the z direction with a step of 1 \(\upmu\)m. The refractive index of fused silica glass is 1.46 at 532 nm. Due to the refraction effect during total internal reflection imaging, the distance between the two turning points needs to be multiplied by the material refractive index n when calculating the SSD depth. Therefore, the depth of the short crack is about 13.14 \(\upmu\)m under the polished layer. The normalized E of points between the two turning points float up and down significantly. This phenomenon may be caused by the crack structure or the environmental vibration. The definition curve of the long crack within the red ellipse is shown in Figure 8d. There are 3 points between the two turning points. The depth is about 7.3 \(\upmu\)m. The depth of the long crack is smaller than that of the short one. The normalized E of points between the two points drop quickly, and we can also verify this. Figure 8: _Cont._  
Figure 9 is the result of sample #2 under 50\(\times\) magnification. The total internal reflection image shown in Figure 9b indicates that much fewer SSD has remained after longer periods of polishing. The linear feature shown in Figure 9b is not shown in Figure 9a, captured under the bright field illumination. Therefore, the linear feature is subsurface damage. The definition curve of the most obvious region of the crack is shown in Figure 9c. There are 4 points between the turning points. Its depth is about 8.76 \(\upmu\)m, and that is much shallower than the short crack's depth in Figure 8b. Figure 8: Images of subsurface damage for sample #1. (**a**) Image under bright field illumination; (**b**) Image under total internal reflection illumination; (**c**) Definition curve of the short crack; (**d**) Definition curve of the long crack. Figure 9: Images of subsurface damage for sample #2. (**a**) Image under bright field; (**b**) Image under total internal reflection illumination; (**c**) Definition curve of the crack. The depth resolution of the improved TIRM is mainly decided by two factors: The objective's DOF, and the positioning accuracy of the translation stage for the microscope motion along the depth direction. If the SSD depth is smaller than the DOF, and the step of the microscope is close to DOF, the definition curve will be a unimodal structure as shown in Figure 3, and it couldn't be resolved effectively. Generally, a higher magnification objective is accompanied with a larger N.A., while the DOF is much smaller. The 100\(\times\) objective of the optical microscope has a DOF of 0.66 \(\upmu\)m (N.A. = 0.90). When the 100\(\times\) objective is used, the depth resolution could be improved. However, environmental vibrations will prevent the microscope from maintaining stable imaging under higher magnification. Images will be blurred and misplaced. A very good vibration isolation platform is required.