Fig. 1. Schematic diagram of experimental setup

Fig. 2. Ultra-depth-of-field microscopy images of two-factor experiments. (a)(b)(c) No structure; (d)(e)(f) cone; (g)(h)(i) square column

Fig. 3. Time-frequency spectra of two-factor experiments based on STFT . (a)(b)(c) No structure; (d)(e)(f) cone; (g)(h)(i) square column

Fig. 4. Time-domain waveforms of acoustic signal with different intervals. (a) 20 μm; (b) 50 μm; (c) 80 μm

Fig. 5. Acoustic signals. (a) Time-domain waveform; (b) frequency spectrum

Fig. 6. Surface morphologies of silicon wafer under 15 W laser power (magnification is 500). (a) 5 times ablation; (b) 10 times ablation; (c) 16 times ablation; (d) local zoom of Fig. 6(a); (e) local zoom of Fig. 6(b); (f) local zoom of Fig. 6(c)

Fig. 7. Variation curves of MSD value and sound intensity with processing times under different laser powers.(a) 12 W; (b) 15 W; (c) 18 W

Fig. 8. MSD values and sound intensity reordered by height

Fig. 9. Variation curves of MSD value and intensity ratio with processing times under different laser powers. (a) 12 W; (b) 15 W; (c) 18 W

Fig. 10. MSD values and intensity ratio of acoustical signal reordered by depth

Fig. 11. Reflectivity for each sample

Fig. 12. Architecture of BP neural network

Fig. 13. Confusion matrix for classification of reflectivity classification. (a) Training set; (b) verification set; (c) testing set; (d) all