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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 20 — Oct. 2, 2006
  • pp: 9544–9550
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Nonlinear enhancement of femtosecond laser ablation efficiency by hybridization with nanosecond laser

J. S. Yahng, B. H. Chon, C. H. Kim, S. C. Jeoung, and H. R. Kim  »View Author Affiliations


Optics Express, Vol. 14, Issue 20, pp. 9544-9550 (2006)
http://dx.doi.org/10.1364/OE.14.009544


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Abstract

Synchronization of femtosecond laser with nanosecond (~250 ns) laser results in a large enhancement in laser ablation efficiency of the Si wafer 12 times more than that with an independent laser exposure. Transient changes in the status of target material due to the proceeding nanosecond laser increase the femtosecond laser ablation efficiency.

© 2006 Optical Society of America

1. Introduction

The needs for high-speed material processing with higher precision and lower mechanical and thermal damage have increased in the fields of mass production for modern device [1

1. N. Bärsch, K. Körber, A. Ostendorf, and K. H. Tönshoff, “Ablation and cutting of planar silicon devices using femtosecond laser pulses,” Appl. Phys. A 77, 237–242 (2003).

4

4. M. Park, B. H. Chon, H. S. Kim, S. C. Jeoung, D. Kim, J. I. Lee, H. Y. Chu, and H. R. Kim, “Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application,” Opt. Lasers Eng. 44, 138–146 (2006). [CrossRef]

]. While traditional mechanical processes like diamond sawing, drilling, or scribing have come to their limitations in the course of size reduction with less damages, laser based material processing has been presented to offer an alternative method to tailor microelectronic and micromechanical components. However, even nanosecond laser is not free from any thermal as well as mechanical damage to the material. Micromachining based on ultrafast lasers can be used to process biological samples, including live cell and thin films as well as bulk materials for many fields including micro-optics, electronics, and even biology [5

5. J. Li and G. K. Ananthasuresh, “A quality study on the excimer laser micromachining of electro-thermal-compliant micro devices,” J. Micromech. Microeng. 11, 38–47 (2001). [CrossRef]

11

11. A. Vogel, J. Noack, G. Hüttman, and G. Paltauf, “Mechanisms of femtosecond laser nanosurgery of cells and tissues,” Appl. Phys. B 81, 1015–1047 (2005). [CrossRef]

].

Ultrafast laser techniques have been known to exhibit potential applications in high precision processing because of their minimizing heat affected zone (HAZ), as compared to the nanosecond laser technologies [12

12. A. Zoubir, L. Shah, K. Richardson, and M. Richardson, “Practical uses of femtosecond laser micro-materials processing,” Appl. Phys. A 77, 311–315 (2003).

16

16. C. Y. Chien and M. C. Gupta, “Pulse width effect in ultrafast laser processing of materials,” Appl. Phys. A 81, 1257–1263 (2005). [CrossRef]

]. Other processing technology based on the high energy particle such as an electron or beam ion, which needs a special environment like vacuum, shows a limitation in its application [17

17. S. J. Ahn, D. W. Kim, H. S. Kim, K. H. Cho, and S. S. Choi, “Laser fabrication of micron-size apertures for electron beam microcolumns,” Appl. Phys. A 69, S527–S530 (1999). [CrossRef]

]. So, ultrashort pulse laser techniques have been investigated widely to develop the counterpart techniques. However, the ultrafast laser techniques have a very weak point in terms of the processing speed, which is much slower than that of the traditional mechanical method currently adapted in the device production. It is urgent to overcome those issues for real industrial applications of ultrafast laser processing. Meanwhile, adaptive optics, which is generally used in nanosecond laser processing, may be applied to overcome the limitation of low processing speed [18

18. H. Haferkamp and D. Seebaum, “Beam delivery by adaptive optics for material processing applications using high-power CO2 lasers,” Proc. SPIE 2207, 156–164 (1994) [CrossRef]

]. However, it is not convenient to directly apply adaptive optics to ultrafast laser processing due to the deterioration of the performance caused by a possible change in the ultrafast laser beam characteristics.

In this report, we present a novel optical processing technology to improve the processing performance of femtosecond micromachining. The precise coupling of ns laser pulse into an ultrafast laser pulse both in the spatial and temporal domain, in short, hybridization processing, results in remarkably higher processing speeds compared to the case where each laser is used in processing by itself. Furthermore, we have observed better performance in material processing with minimizing the surface roughness, which is usually observed in ultrafast laser processing [19

19. T. H. Her, R. J. Finlay, C. Wu, and E. Mazur, “Femtosecond laser-induced formation of spikes on silicon,” Appl. Phys. A 70, 383–385 (2000). [CrossRef]

,20

20. S. C. Jeoung, H. S. Kim, M. I. Park, J. Lee, C. S. Kim, and C. O. Park, “Preparation of room-temperature photoluminescent nanoparticles by ultrafast laser processing of single-crystalline Ge,” Jpn. J. Appl. Phys. 44, 5278–5281 (2005). [CrossRef]

], as well as mechanical stress caused by the illumination of nanosecond laser pulses with high fluence on a substrate [21

21. F. Benitez, F. Sanchez, V. Trtik, M. Varela, M. Bibes, B. Martinez, and J. Fontcuberta, “Laser irradiation of SrTiO3 single crystals,” Appl. Phys. A 69, S501–S504 (1999). [CrossRef]

,22

22. M. S. Amer, M. A. El-Ashry, L. R. Dosser, K. E. Hix, J. F. Maguire, and B. Irwin, “Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers,” Appl. Surf. Sci. 242, 162–167 (2005). [CrossRef]

].

2. Experiments

Figure 1 shows a schematic diagram of the experimental setup. The femtosecond amplifier system consists of a mode-locked Ti:Sapphire oscillator and a regenerative amplifier, which delivers 150 fs laser pulses at 810 nm with a repetition rate of 1 kHz [23

23. M. I. Park, C. S. Kim, C. O. Park, and S. C. Jeoung, “XRD studies on the femtosecond laser ablated single-crystal germanium in air,” Opt. Lasers. Eng. 43, 1322–1329 (2005) [CrossRef]

]. The nanosecond laser has a pulse width of 250 ns with a repetition rate of 1 kHz at 532 nm. A delay generator was used to vary the time delay between the nanosecond laser and femtosecond laser pulse.

By using the stabilization system consisting of the polarization beam splitter and the half wave plate, we achieved better long-term stability in nanosecond laser output. A variable neutral density filter was used to attenuate the femtosecond laser power. A p-doped Si (100) wafer with a thickness of 650 µm was mounted on the motorized translation stage that was used to translate the sample with a constant speed of 6 mm/sec. The distance between laser spot is 6 µm/pulse. Both the femtosecond and nanosecond laser were focused on the wafer with a bi-convex lens with a focal length of 150 mm, and the beam diameters at the focal plane were 40 µm and 80 µm for the femtosecond laser and nanosecond laser, respectively.

The ablation depth, as well as surface morphology, was characterized by atomic force microscopy (AFM, XE-120, PSIA, Korea) and scanning electron microscopy (SEM W-800, Hitachi, Japan)

Fig. 1. Schematic diagram of the hybridization laser processing setup (DM: dichroic mirror, BS: beam splitter, PD: photo diode, PBS: polarization beam splitter, HWP: half wave plate, ND: neutral density filter)

3. Results and discussions

Figure 2 exhibits the images of AFM and SEM of silicon surface processed at three different time delays (-300, 0, and 300 ns) in the hybridization configuration in addition to ns- and fs-laser pulses only. The time delay is defined in terms of the time difference between the femtosecond laser and the nanosecond laser. At the negative delay, the nanosecond laser follows the femtosecond laser, and the nanosecond laser arrives before the femtosecond laser at the positive time delay. The fluences of the nanosecond laser and femtosecond laser pulses are kept at 1.0 J/cm2 and 0.13 J/cm2, respectively. The fluence of both lasers are close to the damage threshold of the silicon substrate as shown in Fig. 2(a) and (b) [24

24. W. Marine, N. M. Bulgakova, L. Patrone, and I. Ozerov, “Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling,” Appl. Phys. A 79, 771–774 (2004). [CrossRef]

,25

25. H. O. Jeschke, M. E. Garcia, M. Lenzner, J. Bonse, J. Krüger, and W. Kautek, “Laser ablation thresholds of silicon for different pulse durations: theory and experiment,” Appl. Surf. Sci. 197–198, 839–844 (2002). [CrossRef]

]. First, we have processed the silicon wafer with the nanosecond laser or femtosecond laser by itself (we have denoted this as ‘ns only’ or ‘fs only’, respectively) to compare the performance of the hybridization processing to that of normal microprocessing. It should be noted that the apparent features of a micro crack can be observed in the case of the ns laser pulse only exposure. The micro-crack extends longer than about 100 µm from the center of laser spot. Optical microscopic images on the processed area also clearly show the appearance of the micro crack. Meanwhile, in case of the fs laser only, the laser illumination results in a prominent surface roughness with neither actual ablation of the silicon substrate nor the micro crack.

Fig. 2. AFM and SEM images of the processed Si wafer surfaces for (a) the nanosecond laser only and (b) the femtosecond laser only, and hybridization processing with a time delay of (c) -300, (d) 0, (e) +300 ns.

In hybridization processing, the ablation of the Si wafer was investigated with varying time delay from -450 ns to 350 ns. For the time delay of -300 ns, as in the ns only and fs only cases, the ablation depth is only about 0.2 µm. It is interesting to note, however, that both the surface roughening and micro-crack formation is substantially lowered even if the two laser pulses are completely isolated from each other in the temporal domain (Fig. 2(c)). When one sets the time delay between the two different laser pulses to be zero while keeping all the other exposure conditions including laser fluence constant, the valley-like morphology appears in both the AFM and SEM images with a remarkable enhancement of the ablation depth to about 3.0 µm (Fig. 2(d)). An additional interesting observation is the substantial reduction of the surface roughness and micro-crack. With a further increase of the time delay to 300 ns, the ablation depth slightly decreased to about 1.8 µm with a little, but observable, micro-crack on the processed surface of the silicon as shown in Fig. 2(e). It should be noted that the width of the processed area is almost constant, even though the depth of processing is strongly dependent on the time delay.

In order to have more quantitative information on the observed large enhancement of the ablation depth in hybridization processing, we have measured the cross-sectional profile of the processed regime by AFM and show the results in Fig. 3(a) at the various time delays. The pulse trails observed in a digital oscilloscope are also shown in Fig. 3(b). When the delay time is set to zero, we observed the largest ablation depth. To estimate the process efficiency quantitatively, we have measured the ablation area from the cross-sectional depth profile and denote the results as a function of delay time in Fig. 4. The ablation efficiency at a delay time of zero is more than 12 times that of the femtosecond laser only. At the negative delay, the dependence of the ablation efficiency on the time delay between the fs- and ns laser pulses is very close to the temporal profile of the ns laser. At the positive time delay, however, we observed an apparent delayed decaying component with the time to decrease to 50 % of the peak of several hundreds of nanosecond in the ablation efficiency curve.

Fig. 3. (a) AFM cross sectional depth profile of the processed surface of a silicon wafer for various time delays. (b) Pulse trail of the laser pulses used in the hybridization process of a silicon wafer.

No apparent enhancement in the ablation efficiency at the negative time delay reveals that the precedent fs-laser illumination on the silicon substrate does not affect on the ablation by the following ns-laser pulse. This observation could be rationalized in terms of the different ablation mechanisms of thermal and nonthermal processes for ns laser and ultrafast laser pulses, respectively. Furthermore, the ablation threshold of 0.8 J/cm2 for 10 ns laser pulses is about four times higher than that for 100 fs laser pulses. [24

24. W. Marine, N. M. Bulgakova, L. Patrone, and I. Ozerov, “Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling,” Appl. Phys. A 79, 771–774 (2004). [CrossRef]

,25

25. H. O. Jeschke, M. E. Garcia, M. Lenzner, J. Bonse, J. Krüger, and W. Kautek, “Laser ablation thresholds of silicon for different pulse durations: theory and experiment,” Appl. Surf. Sci. 197–198, 839–844 (2002). [CrossRef]

] Therefore, the precedent fs laser pulse energy close to the ablation threshold scarcely alters the local temperature of the materials, and then dose not result in any prominent changes in the overall processing efficiency.

Fig. 4. Cross-sectional ablation area of a silicon wafer as a function of delay time. The temporal feature of nanosecond laser pulse (dotted line) is shown for comparison.

The temporal coupling of the femtosecond laser and nanosecond laser induces the local temperature change on the sample during processing, which produces a remarkable decrease in the ablation threshold fluence and increases the processing speed accompanied of the surface metallization of Si(100). Increasing of the nanosecond laser energy causes a change in the temperature of the target material. The energy of the nanosecond laser is controlled so as not to cause the irreversible change with the nanosecond laser fluence by itself. At the same time, the femtosecond laser pulse, which is synchronized in the time domain, irradiates the same space and irreversibly ablates a large amount of material with a lower fluence. Such a progress in femtosecond laser micro-processing makes it possible to maximize the processing speed and reduce the processing threshold energy, which decreases the various high order nonlinear effects which are confronted when we focus high-power femtosecond laser pulses on the target under atmospheric conditions [30

30. D. J. Hwang, C. P. Grigoropoulos, and T. Y. Choi, “Efficiency of silicon micromachining by femtosecond laser pulses in ambient air,” J. Appl. Phys. 99, 083101 (2006). [CrossRef]

].

4. Conclusions

We have demonstrated a novel hybridization method to overcome the inherent lower processing speed of ultrafast laser micromachining. While keeping the good characteristics of the femtosecond laser micromachining, such as a reduction of the thermal and mechanical damage area, we can improve the processing speed by transiently changing the nature of the target substrates before the femtosecond laser shot. By synchronizing the nanosecond laser with the femtosecond laser in the temporal and spatial plane, it is possible to increase processing performance with a small amount of femtosecond laser fluence.

Acknowledgements

This work was financially supported by the Ministry of Commerce, Industry and Energy of Korea.

References and links

1.

N. Bärsch, K. Körber, A. Ostendorf, and K. H. Tönshoff, “Ablation and cutting of planar silicon devices using femtosecond laser pulses,” Appl. Phys. A 77, 237–242 (2003).

2.

C. Li, S. Nikumb, and F. Wong, “An optimal process of femtosecond laser cutting of NiTi shape memory alloy for fabrication of miniature devices,” Opt. Lasers Eng. 44, 1078–1087 (2006). [CrossRef]

3.

T. Matsumura, A. Kazama, and T. Yagi, “Generation of debris in the femtosecond laser machining of a silicon substrate,” Appl. Phys. A 81, 1393–1398 (2005). [CrossRef]

4.

M. Park, B. H. Chon, H. S. Kim, S. C. Jeoung, D. Kim, J. I. Lee, H. Y. Chu, and H. R. Kim, “Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application,” Opt. Lasers Eng. 44, 138–146 (2006). [CrossRef]

5.

J. Li and G. K. Ananthasuresh, “A quality study on the excimer laser micromachining of electro-thermal-compliant micro devices,” J. Micromech. Microeng. 11, 38–47 (2001). [CrossRef]

6.

J. H. Klein-Wiele, J. Bekesi, and P. Simon, “Sub-micron patterning of solid materials with ultraviolet femtosecond pulsesm,” Appl. Phys. A 79, 775–778 (2004). [CrossRef]

7.

A. P. Joglekar, H. Liu, G. J. Spooner, E. Meyhöfer, G. Mourou, and A. J. Hunt, “A study of the deterministic character of optical damage by femtosecond laser pulses and applications to nanomaching,” Appl. Phys. B 77, 25–30 (2003). [CrossRef]

8.

T. C. Chen and R. B. Darling, “Parametric studies on pulsed near ultraviolet frequency tripled Nd:YAG laser micromachining of sapphire and silicon,” J. Mat. Proc. Tech. 169, 214–218 (2005). [CrossRef]

9.

J. S. Yahng, S. C. Jeoung, D. S. Choi, D. Cho, J. H. Kim, H. M. Choi, and J. S. Paik, “Fabrication of microfluidic devices by using a femtosecond laser micromachining technique and µ-PIV studies on its fluid dynamics,” J. Korean Phys. Soc. 47, 977–981 (2005).

10.

M. A. Seo, D. S. Kim, H. S. Kim, D. S. Choi, and S. C. Jeoung, “Formation of photoluminescent germanium nanostructures by femtosecond laser processing on bulk germanium: role of ambient gases,” Opt. Express 14, 4908–4914 (2006). [CrossRef] [PubMed]

11.

A. Vogel, J. Noack, G. Hüttman, and G. Paltauf, “Mechanisms of femtosecond laser nanosurgery of cells and tissues,” Appl. Phys. B 81, 1015–1047 (2005). [CrossRef]

12.

A. Zoubir, L. Shah, K. Richardson, and M. Richardson, “Practical uses of femtosecond laser micro-materials processing,” Appl. Phys. A 77, 311–315 (2003).

13.

P. Rudolph, J. Bonse, J. Krüger, and W. Kautek, “Femtosecond- and nanosecond-pulse laser ablation of bariumalumoborosilicate glass,” Appl. Phys. A 69, S763–S766 (1999). [CrossRef]

14.

J. Jandeleit, A. Horn, R. Weichenhain, E. W. Kreutz, and R. Poprawe, “Fundamental investigations of micromachining by nano- and picosecond laser radiation,” Appl. Surf. Sci. 127–129, 885–891 (1998). [CrossRef]

15.

M. S. Amer, M. A. El-Ashry, L. R. Dosser, K. E. Hix, J. F. Maguire, and B. Irwin, “Femtosecond versus nanosecond laser machining: comparison of induced stress and structural changes in silicon wafers,” Appl. Surf. Sci. 242, 162–167 (2005). [CrossRef]

16.

C. Y. Chien and M. C. Gupta, “Pulse width effect in ultrafast laser processing of materials,” Appl. Phys. A 81, 1257–1263 (2005). [CrossRef]

17.

S. J. Ahn, D. W. Kim, H. S. Kim, K. H. Cho, and S. S. Choi, “Laser fabrication of micron-size apertures for electron beam microcolumns,” Appl. Phys. A 69, S527–S530 (1999). [CrossRef]

18.

H. Haferkamp and D. Seebaum, “Beam delivery by adaptive optics for material processing applications using high-power CO2 lasers,” Proc. SPIE 2207, 156–164 (1994) [CrossRef]

19.

T. H. Her, R. J. Finlay, C. Wu, and E. Mazur, “Femtosecond laser-induced formation of spikes on silicon,” Appl. Phys. A 70, 383–385 (2000). [CrossRef]

20.

S. C. Jeoung, H. S. Kim, M. I. Park, J. Lee, C. S. Kim, and C. O. Park, “Preparation of room-temperature photoluminescent nanoparticles by ultrafast laser processing of single-crystalline Ge,” Jpn. J. Appl. Phys. 44, 5278–5281 (2005). [CrossRef]

21.

F. Benitez, F. Sanchez, V. Trtik, M. Varela, M. Bibes, B. Martinez, and J. Fontcuberta, “Laser irradiation of SrTiO3 single crystals,” Appl. Phys. A 69, S501–S504 (1999). [CrossRef]

22.

M. S. Amer, M. A. El-Ashry, L. R. Dosser, K. E. Hix, J. F. Maguire, and B. Irwin, “Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers,” Appl. Surf. Sci. 242, 162–167 (2005). [CrossRef]

23.

M. I. Park, C. S. Kim, C. O. Park, and S. C. Jeoung, “XRD studies on the femtosecond laser ablated single-crystal germanium in air,” Opt. Lasers. Eng. 43, 1322–1329 (2005) [CrossRef]

24.

W. Marine, N. M. Bulgakova, L. Patrone, and I. Ozerov, “Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling,” Appl. Phys. A 79, 771–774 (2004). [CrossRef]

25.

H. O. Jeschke, M. E. Garcia, M. Lenzner, J. Bonse, J. Krüger, and W. Kautek, “Laser ablation thresholds of silicon for different pulse durations: theory and experiment,” Appl. Surf. Sci. 197–198, 839–844 (2002). [CrossRef]

26.

L. Gavioli, M. G. Betti, and C. Mariani, “Dynamics-induced surface metallization of Si(100),” Phys. Rev. Lett. 77, 3869–3872 (1996). [CrossRef] [PubMed]

27.

V. R. Dhanak, A. Santoni, and L. Petaccia, “A high temperature X-ray absorption and valence band spectroscopy study of the Si(100) surface,” J. Electron Spectrosc. Relat. Phenom. 114–116, 471 (2001).

28.

L. V. Zhigilei and B. J. Garrison, “Mechanisms of laser ablation from molecular dynamics simulations: dependence on the initial temperature and pulse duration,” Appl. Phys. A 69, S75–S80 (1999).

29.

J. S. Yahng, J. R. Nam, and S. C. Jeoung are preparing a manuscript to be called “Temperature dependence of ultrafast laser ablation threshold of crystalline silicone”.

30.

D. J. Hwang, C. P. Grigoropoulos, and T. Y. Choi, “Efficiency of silicon micromachining by femtosecond laser pulses in ambient air,” J. Appl. Phys. 99, 083101 (2006). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers

ToC Category:
Ultrafast Optics

History
Original Manuscript: June 30, 2006
Revised Manuscript: September 18, 2006
Manuscript Accepted: September 22, 2006
Published: October 2, 2006

Citation
J. S. Yahng, B. H. Chon, C. H. Kim, S. C. Jeoung, and H. R. Kim, "Nonlinear enhancement of femtosecond laser ablation efficiency by hybridization with nanosecond laser," Opt. Express 14, 9544-9550 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-20-9544


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References

  1. N. Bärsch, K. Körber, A. Ostendorf, and K. H. Tönshoff, "Ablation and cutting of planar silicon devices using femtosecond laser pulses," Appl. Phys. A 77, 237-242 (2003).
  2. C. Li, S. Nikumb, and F. Wong, "An optimal process of femtosecond laser cutting of NiTi shape memory alloy for fabrication of miniature devices," Opt. Lasers Eng. 44, 1078-1087 (2006). [CrossRef]
  3. T. Matsumura, A. Kazama, and T. Yagi, "Generation of debris in the femtosecond laser machining of a silicon substrate," Appl. Phys. A 81, 1393-1398 (2005). [CrossRef]
  4. M. Park, B. H. Chon, H. S. Kim, S. C. Jeoung, D. Kim, J. I. Lee, H. Y. Chu, H. R. Kim, "Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application," Opt. Lasers Eng. 44, 138-146 (2006). [CrossRef]
  5. J. Li and G. K. Ananthasuresh, "A quality study on the excimer laser micromachining of electro-thermal-compliant micro devices," J. Micromech. Microeng. 11,38-47 (2001). [CrossRef]
  6. J. H. Klein-Wiele, J. Bekesi, and P. Simon, "Sub-micron patterning of solid materials with ultraviolet femtosecond pulsesm," Appl. Phys. A 79, 775-778 (2004). [CrossRef]
  7. A. P. Joglekar, H. Liu, G. J. Spooner, E. Meyhöfer, G. Mourou, and A. J. Hunt, "A study of the deterministic character of optical damage by femtosecond laser pulses and applications to nanomaching," Appl. Phys. B 77, 25-30 (2003). [CrossRef]
  8. T. C. Chen and R. B. Darling, "Parametric studies on pulsed near ultraviolet frequency tripled Nd:YAG laser micromachining of sapphire and silicon," J. Mater. Process. Technol. 169, 214-218 (2005). [CrossRef]
  9. J. S. Yahng, S. C. Jeoung, D. S. Choi, D. Cho, J. H. Kim, H. M. Choi, and J. S. Paik, "Fabrication of microfluidic devices by using a femtosecond laser micromachining technique and μ-PIV studies on its fluid dynamics," J. Korean Phys. Soc. 47, 977-981 (2005).
  10. M. A. Seo, D. S. Kim, H. S. Kim, D. S. Choi, and S. C. Jeoung, "Formation of photoluminescent germanium nanostructures by femtosecond laser processing on bulk germanium: role of ambient gases," Opt. Express 14, 4908-4914 (2006). [CrossRef] [PubMed]
  11. A. Vogel, J. Noack, G. Hüttman, and G. Paltauf, "Mechanisms of femtosecond laser nanosurgery of cells and tissues," Appl. Phys. B 81, 1015-1047 (2005). [CrossRef]
  12. A. Zoubir, L. Shah, K. Richardson, and M. Richardson, "Practical uses of femtosecond laser micro-materials processing, " Appl. Phys. A 77, 311-315 (2003).
  13. P. Rudolph, J. Bonse, J. Krüger, and W. Kautek, "Femtosecond- and nanosecond-pulse laser ablation of bariumalumoborosilicate glass," Appl. Phys. A 69, S763-S766 (1999). [CrossRef]
  14. J. Jandeleit, A. Horn, R. Weichenhain, E. W. Kreutz, and R. Poprawe, "Fundamental investigations of micromachining by nano- and picosecond laser radiation," Appl. Surf. Sci. 127-129, 885-891 (1998). [CrossRef]
  15. M. S. Amer, M. A. El-Ashry, L. R. Dosser, K. E. Hix, J. F. Maguire, and B. Irwin, "Femtosecond versus nanosecond laser machining: comparison of induced stress and structural changes in silicon wafers," Appl. Surf. Sci. 242, 162-167 (2005). [CrossRef]
  16. C. Y. Chien and M. C. Gupta, "Pulse width effect in ultrafast laser processing of materials," Appl. Phys. A 81, 1257-1263 (2005). [CrossRef]
  17. S. J. Ahn, D. W. Kim, H. S. Kim, K. H. Cho, and S. S. Choi, "Laser fabrication of micron-size apertures for electron beam microcolumns, " Appl. Phys. A 69, S527-S530 (1999). [CrossRef]
  18. H. Haferkamp, and D. Seebaum, "Beam delivery by adaptive optics for material processing applications using high-power CO2 lasers," in Laser Materials Processing: Industrial and Microelectronics Applications, E. Beyer, M. Cantello, A. V. La Rocca, L. D. Laude, F. O. Olsen, G. Sepold, eds., Proc. SPIE 2207, 156-164 (1994). [CrossRef]
  19. T. H. Her, R. J. Finlay, C. Wu, and E. Mazur, "Femtosecond laser-induced formation of spikes on silicon," Appl. Phys. A 70, 383-385 (2000). [CrossRef]
  20. S. C. Jeoung, H. S. Kim, M. I. Park, J. Lee, C. S. Kim, and C. O. Park, "Preparation of room-temperature photoluminescent nanoparticles by ultrafast laser processing of single-crystalline Ge," Jpn. J. Appl. Phys. 44, 5278-5281 (2005). [CrossRef]
  21. F. Benitez, F. Sanchez, V. Trtik, M. Varela, M. Bibes, B. Martinez, and J. Fontcuberta, "Laser irradiation of SrTiO3 single crystals," Appl. Phys. A 69, S501-S504 (1999). [CrossRef]
  22. M. S. Amer, M. A. El-Ashry, L. R. Dosser, K. E. Hix, J. F. Maguire, and B. Irwin, "Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers," Appl. Surf. Sci. 242, 162-167 (2005). [CrossRef]
  23. M. I. Park, C. S. Kim, C. O. Park, and S. C. Jeoung, "XRD studies on the femtosecond laser ablated single-crystal germanium in air," Opt. Lasers. Eng. 43, 1322-1329 (2005) [CrossRef]
  24. W. Marine, N. M. Bulgakova, L. Patrone, and I. Ozerov, "Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling," Appl. Phys. A 79, 771-774 (2004). [CrossRef]
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