Appl Phys A (2008) 93: 189–192 DOI 10.1007/s00339-008-4672-2
Display glass cutting by femtosecond laser induced single shot periodic void array Farid Ahmed · Man Seop Lee · Hitoshi Sekita · Tetsumi Sumiyoshi · Masanao Kamata
Received: 12 October 2007 / Accepted: 9 April 2008 / Published online: 3 June 2008 © Springer-Verlag 2008
Abstract We propose an idea of fast cutting a display glass plate where the sample is pre-processed micromachining single shot rear-surface and internal void arrays aligned on working plane prior to glass cleaving. Single shot void morphology is investigated varying input pulse energy, focusing depth, and scanning speed. A femtosecond laser with pulse duration of 172 fs, central wavelength of 780 nm, and repetition rate of 1 kHz is used to fabricate voids.
force in cleaving phase may result in a cutting edge that is not perpendicular to the glass surface. Hence, an additional array of single shot bulk voids at suitable depth on the same plane (Fig. 1b) facilitates easy slicing with an edge right angle to the glass surface. Finally, the pre-processed glass plate is separated in a cleaving phase as exemplified in Fig. 1c.
PACS 42.62.Cf · 42.65.Jx
2 Experimental
1 Introduction
The experiments were carried out using a commercial femtosecond laser (IFRIT of Cyber Laser), which emits ultrashort pulses at 780 nm with pulse duration of 172 fs, and repetition rate of 1 kHz. Focusing was done with achromatic lenses under incident angles of zero degree into the sample. Numerical aperture (NA) of 0.55 and 0.70 were put into play to fabricate periodic void structures and investigate their morphology. The sample (Corning Eagle 2000, thickness: 700 µm) was mounted on a computer controlled 3-axis stage to introduce voids at suitable depth on working plane. Afterwards, mechanical pressure was deposited to cleave the glass plate. Samples were analyzed using optical microscope (OM) and scanning electron microscope (SEM).
Femtosecond pulse filamentation offers promising and innovative means for material micro-processing [1–10]. This article demonstrates a technique for cutting of flat panel display (FPD) glass plate applying the idea of femtosecond pulse filamentation discussed extensively in the literature [1–6]. As explained in [2, 6], alternative Kerr self-focusing and defocusing by self generated plasma self-guide the laser pulses through a narrow channel. Therefore, as shown in Fig. 1a, by focusing laser beam near the rear surface instead of front surface, most of the pulse energy can be used efficiently to fabricate periodically aligned long and narrow voids that reach the rear surface. Since void height is noticeably smaller compared to glass thickness, an imbalanced F. Ahmed () · M.S. Lee Optical Network and Systems Lab., Information and Communications University, 119, Munjiro, Yuseong-gu, 305-714, Daejeon, Korea e-mail:
[email protected] H. Sekita · T. Sumiyoshi · M. Kamata Cyber Laser Inc., Telecom Center Bldg., Tokyo, Japan
3 Result and discussion Cutting performance is separately investigated for sample pre-processing by single shot void array fabricated at rear surface and for the case where an additional single shot void array is introduced following the same plane at certain depth beneath the front surface.
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Fig. 1 Schematic diagram showing cutting steps: (a) Micromachining rear surface void array, (b) Fabricating bulk void array, (c) Cleaving process
3.1 Single-line void array Figure 2 illustrates single shot rear surface void structures micromachined due to filamentation when pulses are tightly focused near rear surface through NA of 0.70. Critical power 3.77λ2 for self-focusing is Pc = 8πn , where λ is the central 2 n0 wavelength of the pulse, n0 is the linear index of refraction, and n2 is the coefficient of the Kerr nonlinear index of refraction [7]. Peak power of the self-focused pulses is E Pp ≈ 0.88 τp , where Ep is pulse energy and τ is pulse width [8]. Due to tight focusing through NA of 0.70 and Kerr selffocusing, wave front of the input pulse converges in a narrow transverse region. According to peak power equation, temporal reduction in pulse duration increases peak power much higher than critical power for self-focusing. Consequently, narrow and long voids can be micromachined at rear surface through the working line. For certain focusing depth, fabricated voids reach the rear surface with their maximum heights (Fig. 2). For NA of 0.70, optimal focal point is 140–150 µm up from the rear side surface, while is it 70–75 µm up from the rear surface when focused through NA of 0.55. Significant deviation from the suggested ranges may result in voids that are shorter or do not reach the rear surface. Onward plasma pressure drives out material easily from the micromachined rear surface voids. Therefore, clean and long voids can be micromachined at the rear surface of glass plate through the working line. Once the voids are micromachined at the rear surface, cutting ends up in a cleaving phase. Figure 3 points out the outcomes when pulses are relatively loosely focused (NA = 0.55) to introduce rear surface voids. In loose focusing condition, pulse width is relatively high at focal point due to degraded self-focusing. Therefore, peak power of the input pulse decreases and spot size increases. Moreover, longer pulse duration and bigger spot size accelerate heat diffusion in the bulk of the material. Consequently, sharp filamentation of laser pulses is impeded and diffusion of excess heat expands void width. As shown in Fig. 3, when pulse energy lifts up,
Fig. 2 Microscope image of cleaved surface of single shot voids in rear side surface introduced by NA = 0.70 at speed of 10 mm/s for pulse energy of 20 µJ (top), and 30 µJ (bottom)
Fig. 3 Dependence of void height and width on pulse energy when focused through NA of 0.55
width of the voids increases and height decreases. Since the strength of self-focusing depends on peak power of the incident laser pulses [9], plasma induced defocusing
Display glass cutting by femtosecond laser induced single shot periodic void array
Fig. 4 Single shot void periods of (a) 10 µm, (b) 15 µm, (c) 20 µm, and (d) 25 µm for pulse energy of 15 µJ, and NA of 0.70
dominates over self focusing in loose focusing condition. For this reason, an excess increase in pulse energy accelerates heat diffusion. While focus through NA of 0.55, voids with better height and morphology can be micromachined at rear surface for pulse energy of 15–25 µJ which have diameter of 3–4 µm as noted in the graph of Fig. 3. Rear surface glass cutting technique faithfully works at maximum speed of 10 mm/s for pulse repetition rate of 1 kHz and average pulse energy of 40 µJ. Further raise in scanning speed may increase unreliability of glass cutting through projected working plane. As scanning speed increases, period between consecutive voids also increases. As shown in Fig. 4, periods between successive cavities are about 10 µm, 15 µm, 20 µm, and 25 µm for speeds: 10 mm/s, 15 mm/s, 20 mm/s, and 25 mm/s, respectively. Experimental evidence shows that it is not easy to cut the sample through the working line when void period is larger than 10 µm. Besides, for extended void period, an imbalanced force in cleaving phase may deviate cutting route from the working line. However, in case of very low scanning speed, sample contamination dominates due to over shooting of laser pulses. Figure 5 shows the SEM image of sliced sample surface processed at speed of 10 mm/s and average pulse energy of 40 µJ. 3.2 Double-line void array In the previous scheme, cutting of glass plate through the working line turns out to be unreliable if translation speed increases beyond 10 mm/s or focal point deviates from its optimal value. Besides, horizontal deviation of glass plate may misplace rear surface voids. These drawbacks can be compensated by introducing an array of single shot bulk voids at suitable depth on the same plane in addition to rear
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Fig. 5 SEM image of sliced sample surface processed with pulse energy of 40 µJ and scanning speed of 10 mm/s when focused through NA of 0.70
Fig. 6 OM image of single shot bulk voids fabricated at different depths for average pulse energy of 20 µJ and NA of 0.70
surface void array. However, according to Fig. 6, void height decreases towards front surface which is also reported in the literature [10]. The reason is when focal point is shift up from rear surface towards front surface, self-focusing is impeded which in turn reduces peak power. As investigated, void array fabricated 50–60% down to the front surface increases reliability of glass cutting. Figure 7 shows SEM image of sliced surface in double line void array scheme where the bulk voids are fabricated 420 µm beneath the front surface. Although this scheme requires two passes of laser beam over the sample, it is reliable in comparison to singleline void array scheme.
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works faithfully if beam axis maintains right angle to sample surface. However, double-line cavity scheme has more tolerance on incident angle deviation. Besides, bulk voids help easy and smooth slicing in cleaving phase with an edge right angle to the surface. Reliable maximum processing speeds for single and double line void schemes are 10 mm/s and 15 mm/s, respectively, for average pulse energy of 40 µJ and repetition rate of 1 kHz. Acknowledgements The authors acknowledge the support of PHOCO Co., Ltd. Korea for making available the femtosecond laser system and optical microscope. This work is supported by Optical Internet Research Center (OIRC), Information and Communications University, South Korea.
Fig. 7 SEM image of sliced glass plate processed with pulse energy of 40 µJ, NA of 0.70, and scanning speed of 15 mm/s
4 Summary In conclusion, we investigated and demonstrated the feasibilities of proposed methods for fast and efficient cutting of display glass (Corning Eagle 2000). Rear surface glass cutting is carried out by micromaching single shot void array at rear surface through working line and post glass cleaving. An additional array of voids 50–60% down to the front surface on the same plane with rear surface void array increases reliability of glass cutting. Single-line void array scheme
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