# Quantitative Evaluation of Subsurface Damage by Improved Total Internal Reflection Microscopy
## 3 Experimental Setup and Sample Preparation  
The schematic diagram of the improved TIRM setup is shown in Figure 6. The incident light emitted from a diode-pumped solid-state laser operating at a wavelength of 532 nm passes through half-wave plate 1, the polarization beam splitter PBS, and half-wave plate 2. Half-wave plate 1 is rotated to maximize the power of p-polarized light output from the PBS. Rotating half-wave plate 2, s-polarized light is obtained. The s-polarized beam is directed into the face of the inverted prism by two turning mirrors: Mirror 1 and Mirror 2. Its incident angle is adjusted by tilting and positioning the Mirror 2 vertically. The material of the 90\({}^{\circ}\) prism is consistent with that of the sample. Index matching fluid is filled between the backside of the sample and the prism surface. In order to implement a micro-focusing operation, the optical microscope (HUVITZ-HRM 3000), driven by a motor, could move exactly along the z-axis with a step of 1 \(\upmu\)m. The microscope is equipped with five objective lenses of different magnifications (5\(\times\), 10\(\times\), 20\(\times\), 50\(\times\), 100\(\times\)). Its eyepiece magnification is 10\(\times\). The resolution of the CCD is 2448 \(\times\) 2048. The pixel size is 3.45 \(\upmu\)m. The sample and prism are placed on the X-Y translation stage. The upper surface of the sample is raster-scanned. The incident light spot on the upper surface of the sample has a diameter of 3 mm. Figure 7 shows our improved TIRM setup. The image in the upper right corner is the prism and the sample under test.  
In order to quickly locate SSD and quantitative measurement, the 10\(\times\) and 50\(\times\) objective lenses are frequently used in our experiments. Because of the relatively wide field of view and large DOF (5.9 \(\upmu\)m), the 10\(\times\) objective is employed to scan the surface and subsurface of the sample, and locate the region which includes the SSD. The 50\(\times\) objective is applied to obtain the SSD images at a different depth, and measure the depth of SSD with its smaller DOF (0.84 \(\upmu\)m). The microscope has bright field illumination. The image captured under bright-field illumination is used to distinguish surface defects from any SSD. The SSD does not appear in the bright-field image, and surface defects can be imaged.  
Figure 6: Schematic of the improved total internal reflection microscopy (TIRM) setup.  
Figure 7: Photograph of our improved TIRM.  
Two pieces of fused silica glass are tested in our experiments. Their size is 50 mm in diameter, and 5 mm in thickness. The two samples are grinded firstly with loose abrasive emery #280 (the particle size is about 40\(\sim\)50 \(\upmu\)m). They then both have extra loose abrasive grinding with both emery W28 and W14 in sequence. The particle sizes of W28 and W14 are 20\(\sim\)28 \(\upmu\)m and 10\(\sim\)14 \(\upmu\)m, respectively. The amount of removal in the latter step is 2 to 3 times the particle size of the abrasive particles used in the previous step. The subsurface damage generated in the former step is removed. Finally, the two samples are polished with cerium oxide polishing solution. Under the same processing conditions, the polishing time of sample #1 is shorter than the polishing time of sample #2.