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Picosecond UV laser induced scribing of polyethylene terephthalate (PET) films for the enhancement of their flexibility
Optics & Laser Technology 82 (2016) 183–190
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Full length article
Picosecond UV laser induced scribing of polyethylene terephthalate (PET) films for the enhancement of their flexibility Min Gi Kang a, Changhwan Kim b, Yong Joong Lee a, Sung Yeol Kim a,n,1, Ho Lee a,n,1 a b
School of Mechanical Engineering, Kyungpook National University, Dague 702-701, Republic of Korea School of Mechanical Design & Manufacturing, Busan Institute of Science and Technology, Busan 616-737, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2015 Received in revised form 5 February 2016 Accepted 1 March 2016
Flexible devices has received a great attention due to their high portability, lightness, and ease of shape reconfiguration. To achieve high flexibility, controlling the mechanical properties of the substrate materials is of importance. In this paper, we controlled the local flexibility of PET films via UV laser scribing. The bending test of the films revealed that their bending curvatures, the associated mechanical damages, and the required bending forces could be successfully tuned by controlling the number and the depth of the scribed lines. Our simple strategy of using laser scribing will find its usefulness in flexible device applications where high flexibility and mechanical stability are required. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Laser scribing PET film Flexible electronics substrate Flexibility Bending property
1. Introduction Flexible devices are currently attracting considerable interest due to their advantages such as high portability, lightness, and ease of shape reconfiguration. Unlike the conventional devices, they can be conformably fitted onto any curved surfaces and folded and unfolded for storage and packaging [1–6]. As such, new applications are emerging in a variety of fields, which include wearable health care devices, flexible displays, conformal antenna arrays, flexible solar cells, flexible batteries and radio-frequency identification (RFID) tags [7–14]. Flexible devices are typically based on organic substrates [15– 18]. The substrates impart flexibility to the devices and control the overall mechanical properties [19], and therefore, the selection of substrate materials and their processing determines the degree of flexibility of the systems. Paper substrates made of a natural polymer-cellulose are promising substrates to achieve high flexibility and foldability. However, they lose their stability upon contacting water or other liquids [20,21], so their applications can be limited. Polyethylene terephthalate (PET) is a synthetic polymer with a good chemical stability against the liquids, and it is among the most widely used organic substrates for most flexible device applications [22–24]. However, this polymer substrates cannot n
Corresponding authors. E-mail addresses: [email protected] (S.Y. Kim), [email protected] (H. Lee). 1 These corresponding authors contributed equally to this paper.
http://dx.doi.org/10.1016/j.optlastec.2016.03.001 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
render high flexibility compared with the paper substrates. For example, when a PET substrate with a typical thickness of 80 mm is used, the bending curvature more than 0.8 mm 1 (i.e. the radius of curvature less than 1.25 mm) cannot be obtained without plastic deformation. Besides, folding or creasing PET films cannot be easily achieved without mechanical instability, which makes it challenging to develop highly flexible device with foldability using the polymer [25–28]. To achieve foldability or high local flexibility, the local bending curvature of the hinges (or bending parts) of the substrates should be high, while the other regions should be mechanically stiffer than the hinges to maintain their shapes. In addition, the repulsive force against folding these substrates should be small enough so that they don't unintentionally spring back to their unfolded shape. We believe these requirements can be achieved by controlling the local flexibility of the substrate by changing its local bending stiffness. If there exist a method to process the polymer substrates to tune their thickness, shape or surface, their local bending stiffness can be controlled to achieve high flexibility and foldability. UV Laser is a powerful tool to scribe a fine feature or pattern on polymer substrates at desired location. The bond breaking interaction between laser and the substrates called photo-ablation is the basis of the laser scribing, and therefore the feature sizes are only limited by the resolution of the laser ( a few hundred nanometers), and their shape and locations can be precisely controlled. Laser-scribing of polymers has been actively investigated
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for its use in various applications including an optical waveguides, micro-machined parts, and micro-channels and vias for flexible electronics [29–32]. However, it is difficult to find a study using laser scribing to control the local mechanical properties of the polymer films, and its effects on the bending properties of the polymer substrates has not been thoroughly investigated. Herein, we controlled the flexibility of PET films by scribing multiple groove lines on them using UV picosecond laser. The changes of bending property was investigated depending on the laser scribing parameters such as the number of lines and the depth of groove by measuring the curvatures of the films. Interestingly, plastic deformations and mechanical instability due to the stress concentration that could occur near the scribed region were significantly reduced by increasing the number lines. Our study revealed that controlling the number of groove lines and their positions on PET films could tune both their overall and local flexibility. We believe that laser scribing can be as an effective tool for controlling the local mechanical properties of polymer films, which can expand the use of the synthetic polymer substrates for the application of foldable devices.
2. Method 2.1. Laser system and PET sample preparation We used a Duetto THG picosecond laser (12 ps, Time-Bandwidth Inc.) to cut and scribe PET films. The laser wavelength of 355 nm, the repetition rate of 50 kHz was selected to minimize heat accumulation and possible heat-driven bulge of the polymer. The scanning speed was 2 mm/s. The numerical aperture and the magnification factor was 0.16 and 10 , respectively. The beam size of the laser, focused on the PET film, is 1.3 mm and is calculated by employing the following equation,
4M 2λf M 2:beam quality, λ:wave length, f:focal length, πD
(
D:entrance beam diameter ( 1/e2)
)
To prepare PET samples for bending test, a PET film (10 mm 10 mm 80 mm ) was attached onto a glass slide by double sided tape, and the level (or flatness) of the film were carefully monitored and adjusted using a CCD camera. Single or multiple lines (3, 7, and 13 lines) were scribed at the center of the top surface of the film by scanning the laser along their lengths 10 times each. (Fig. 1). The interval between the lines were fixed at 100 mm. The scribed PET was then sectioned equally into four slices (10 mm 2.5 mm) by single edge blades. Three of the four PET specimens were used to perform the bending experiment while the remaining one was reserved for SEM imaging. 2.2. Bending tester Fig. 2(a) shows the schematic of the bending stage and PET sample. One of the two L-shape holder was fixed, while the other
one attached to an actuator (BDC 101, Thorlabs, Inc.) move along the rail up to 3 mm to bend the PET sample. The bent state of the PET film was maintained for 0.3 s, and then decompressed back to the original state until any stress induced by the actuator is released. For stability test, the PET film was bent cyclically 1000 times by the actuator. Fig. 2(b) is the optical image of the tester. An optical microscope (Edmond Inc.) was used to capture the shape of bent PET film in real time. An XY stage is connected to the optical microscope for the ease of alignment and monitoring the PET sample. Fig. 2(c) is the magnified image of the holder and the PET sample.
3. Result and discussion 3.1. Controlling the scribing depth of groove line on PET films using UV-picosecond laser We controlled the scribing depth by adjusting the power of a 355 nm picosecond laser. The other processing parameters such as repetition rate, scan speed, the number of scanning, and the beam size focused on the PET film were fixed to 50 kHz, 2 mm/s, 10, 1.3 mm, respectively. Based on the laser beam size and the scan speed that we have used, the spot overlap can be calculated by v employing the following equation,1 − fD , becoming an important factor to form a clear scribed line shape (v: scanning speed, f: repetition rate, D: beam diameter). The calculated value of the spot overlap was found to be 0.97. Fig. 3(a) shows the SEM images of PET samples scribed with the laser using different laser powers from 3 mW to 17 mW. All the samples showed a V-shaped groove line due to the Gaussian intensity profile of the emitted laser beam, and the depth of the groove line was found to be linearly increasing from 15 mm to 55 mm with increasing power (Fig. 3(b)). The scribing width on the top of PET almost remained constant at 6 mm for every laser power tested. The calculated corresponding ablation depth per pulse, for each of the laser power levels used, was 0.045 μm, 0.076 μm, 0.12 μm, and 0.17 μm respectively, assuming that an individual pulse overlapped with a spot area during line scanning of 10 times (330 pulses) is made to ablate as the identical depth. In addition, bulging (i.e. the indication of heat damage) was not noticeable at the periphery of the scribed area with the laser power up to 17 mW, which demonstrates that we can successfully control the depth of scribing (or the aspect ratio) without significant heat damage. These results can be attributed to the fact that the polymer is ablated before the considerable heat caused by laser beam spreads to the periphery of the irradiated part because of the short pulse duration (12 ps) of the ultrashort pulse laser, the low heat diffusion rate of the polymer material and, trivial heat generation in the UV wavelength (355 nm) , meaning that the PET film could be ablated as the beam size inflicted on the PET film. This results in the scribed shape having a high aspect ratio through repeated line scanning [33–35].
Fig. 1. The schematic diagram of the picosecond laser apparatus (a), and the scribed PET sample (b).
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Fig. 2. Schematic diagrams showing the home-made bending tester (a). Optical image of the tester and optical microscope (b), and sample under loading and sample holders in high magnification (c).
Fig. 3. (a) SEM images of laser-scribed grooves on PET according to variations in laser power. (b) The depth of scribing as a function of laser power.
3.2. The bending property of PET films with single-line scribing After confirming that we could control the scribing depth without severe heat damage, we investigated the bending properties of the scribed PET depending on the number of groove lines and the scribing depth. First, a single groove line was scribed on the center of a bare PET film and its bending shape was measured by an optical microscopy. Fig. 4(a) compares the overall bending state of the PET sample with the scribing depth ranging from 0 to 55 mm. In general, the bending of the PET became more severe at the center while it became relieved at the edges for all the samples tested. As the scribing depth increased, the bending curvature at the center increased while the side curvature near the edges decreased, indicating that the deformation and the corresponding stress became more concentrated at the center of the sample. Particularly, the PET sample with a deep scribing, which corresponded to the
scribing depth 40 and 55 mm in Fig. 4(a), showed abrupt changes of the curvature at the center where the groove line existed. Shown in the Fig. 4(b) are the high magnification images near the center that clearly shows this trend. When the scribing depth was small (i.e. 15 mm), the local deformation at the groove line was almost identical to that of the bare PET without scribing. However, the deformation became severe with the increasing the scribing depth: the bending became no longer smooth and the top width of the groove broadened up to 100 mm when the scribing depth was 55 mm. It is obvious that this large local deformation can damage the samples resulting in permanent (i.e. plastic) deformation or even fracture. The images of the PET samples after the bending test (Fig. 4(d)) shows that the PET samples with scribing do not return back to their original flat state (Fig. 4(c)) and they were abruptly bent at the scribed line, indicating that the permanent deformation occurred at the scribed area. And as expected, the degree of this permanent damage (i.e.
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Fig. 4. (a) Low and (b) high magnification optical images of the scribed PET samples with single line scribing under bending. Yellow and red arrow indicates side and center curvature, respectively. The scribing depth tested was 0 (i.e. no scribing), 15, 40, and 55 mm. Optical image of the sample (c) before and (d) after the bending test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
abrupt warpage) on the PET was more severe when the scribing depth was deeper due to the stress concentration and the local deformation at the scribed area. These results show that scribing single line may not be appropriate for achieving high local flexibility of PET films because it tends to produce mechanical damages on the films such as permanent deformation and fracture. 3.3. The bending property of PET films with multi-line scribing To control the bending properties of PET films without causing the mechanical damages, we controlled the number of scribed lines on the PET films and studied their bending properties. Shown in Fig. 5 are the digital images of the bending state of PET with multi-line scribing, along with close-up image and overall shape of the sample. Only the PET samples with the scribing depth of 25 and 40 mm were shown here because the sample with scribing depth of 15 mm did not show any difference in shape and curvature regardless of the number of scribed line, and the PET samples with the scribing depth of 55 mm easily fractured during the bending test. In general, as the number of scribing line on PET film increased, the curvature at the center (dotted red area Fig. 5) decreased, and the center groove line (the inset in the middle) showed less broadening of the top width. Comparison between the samples with three-line and a thirteen-line scribing of 40 mm depth (Fig. 5 (a) D ¼40 mm vs. 5(c) D ¼40 mm) noticeably showed this difference: the abrupt bending curvature became smoother one by increasing the number of scribing lines. This decrease in the curvature and local deformation can be attributed to the fact that the stress and deformation localized in a single groove line is distributed among multiple lines, diminishing the local stress concentration and the severe plastic deformation that could damage
the samples. Note that the variation in the shape of groove lines was negligible among the multi-lines in the same sample (see Fig. 5(c) thirteen-lines sample), indicating that the degree of deformation was similar (and small) for all the lines regardless of their locations in the sample. Fig. 5(d) summarizes the curvatures of the PET films depending on the number and the depth of the scribed lines, which also shows the areas depending on the occurrence of the mechanical damages. To minimize the mechanical damages while achieving high local curvature, selecting the scribed depth more than 25 mm and the number of line more than three are preferred (N Z3 line DZ25 mm) as indicated by the yellow area in the figure. The scribed PET films associated with this region exhibit higher flexibility than that of non-scribed films: they show higher curvature and can be bent with less force due to lower bending stiffness. Although it was possible to achieve higher curvature values by scribing a single line with the depth more than 25 mm, it typically resulted in large plastic deformation and the fracture of the PET samples (gray area). When the depth of the scribed line was less than 15 mm, there was no noticeable increase in the curvature of the film compared with those of non-scribed PET films. Based on the results of experimental work that we performed earlier, the optimal option for flexibility enhancement within the mechanical stability zone can be found. We expect that as the number of scribed lines is increased, the bending force (i.e. flexibility) decreases while the scribed depth is deeper and the bending force falls within the mechanical stability zone. Thus, the sample (D ¼40 mm, N ¼13) would be the best option for maximal flexibility enhancement within the tested range. However, should the mechanical stability be considered in combination with the flexibility, the sample (D ¼25 mm, N¼ 13) would be the best option within the tested range, meaning that an optimal condition could
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Fig. 5. Optical images of the bending state of PET samples with (a) three, (b) seven and (c) thirteen scribing lines. (d) Center curvature as a function of number of scribing lines. Curvature was measured from the left lower insets of Fig. 5(a–c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (a–c) Optical images of the scribed PET samples after the bending test. All the external forces were unloaded. SEM images of the PET samples with multiple lines (d) before and (e) after stability test and (f) PET sample with single line, where crack propagation occurred.
be different depending on the purpose of utilizing the scribed PET film samples within the mechanical stability zone. This result demonstrates that we can successfully tune the local bending properties and flexibility of PET films by controlling the laser scribing parameters. 3.4. Bending stability We studied the cyclic stability of the scribed PET films. The bending test was performed up to 1000th cycle and the shape of the samples before and after the test was compared. (Fig. 6) The scribed PET samples behaved nearly elastically when the scribing depth was small (D ¼15 mm), regardless of the number of the lines; the samples returned back to their original flat shape after removing the external loading. However, when the scribing depth increased to 40 mm or more, the PET samples with a single line did not returned to their original flat shape (Fig. 4(d)), while the samples with multiple lines with same depth nearly returned back to their initial shape except for the case N ¼3. We found that the samples with multiple groove lines did not fracture even after 1000th cycles of bending test. Moreover, the shape of the groove lines where the most of deformation occurs did not change at all even after the test, indicating severe damages such as large plastic deformation and crack propagation did not occur (Fig. 6(d–e)). However, the PET samples with single groove line were susceptible to fracture during the test. (see optical images shown in Fig. 6(f)). The warpage, the residual curvature after the bending test, represents the degree of plastic deformation accumulated during the test, and therefore the permanent deformation on the samples can be measured qualitatively by measuring the curvature. Fig. 7 shows that the center curvature increase with increasing the number of bending cycle, and the initial increase for the first 100 cycles takes up more than 80% of the total curvature change. This result indicates that the plastic deformation (or damage) accumulates with increase of the bending cycles, and the PET sample undergoes most of plastic deformation during the first 100 cycles. As expected, when the scribing depth was fixed, scribing seven or
Fig. 7. Bending curvature at the center of the scribed PET films as a function of the bending cycles. The curvature was measured after the bending test when the PET film was in its relaxed state without external loading. The depth of the groove line was fixed at 40 mm.
thirteen lines compared to three (or single) lines significantly reduced overall curvature (i.e. plastic deformation). These results confirm that scribing multiple groove lines also reduce the plastic deformation accumulation and improve the cyclic bending stability. 3.5. Bending force and flexibility The bending stiffness of the PET films decreases with increasing the depth or the number the scribing lines, and therefore the bending force required to bend a PET film is expected to decrease correspondingly. Fig. 8 compares the overall shapes of the PET sample with or without scribing lines, which are fully bent to study the required bending force and its flexibility. The PET film with multiple scribing lines (Fig. 8a, D ¼40 mm, N ¼13) showed the local bending (or folding) at the scribed region with high curvature
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Fig. 8. Schematic diagrams of overall bending shape of a PET film (a) with (D ¼ 40 mm, N ¼ 13) and (b) without multiple groove lines. F1 and F2 represent the force required to bend the PET film, and R1 and R2 represent the radius of curvature at the center. Inset shows the corresponding digital image of the PET film.
(i.e. small radius of Curvature: 500 mm engaged in the range of radius of curvature employed in recent studies on foldable device [23,26,36]), while its side regions remained relatively un-deformed, showing that laser scribing allowed high local curvature at the scribed area by reducing its local bending stiffness. However, the sample without scribing (Fig. 8 b) showed lower curvature (i.e. higher radius of curvature) at the center, while its sides also deformed. Moreover, the scribed sample required more than six times lower bending force than the non-scribed sample. These results demonstrate that scribing PET films can impart high local flexibility to the films: higher local curvature can be achieved with less bending force. Combined with our previous results of bending stability, we anticipate our strategy of using laser scribing may find its usefulness in flexible device applications where the control of local flexibility and mechanical stability are required.
4. Conclusion In conclusion, we studied the effects of laser scribing on the bending properties of PET films by measuring their bending curvature. As the scribing depth increased, the PET sample with single groove line tended to fail during the bending test due to stress concentration and severe plastic deformation at the scribed region. However, increasing the number of scribing lines prevented the mechanical failure of the PET films by alleviating the two causes. Cyclic bending test revealed that the plastic deformation of the film accumulated with each cycle, and the deformation was decreased by scribing multiple lines on PET films. Even after 1000th cycles, we could not find any observable damage at the scribed region, when the number of lines increased to seven and thirteen. The bending force required to bend the PET film was also found to decrease with the number of groove lines, and therefore flexibility was enhanced with laser scribing. Moreover, the bending occurred mainly at the scribed region, so the scribed PET films showed higher local curvature at the desired location with less bending force. These advantages of laser scribing may find its usefulness in flexible or foldable devices where achieving high flexibility and controlling local mechanical properties are required.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A1038346). Also, this research was conducted under the industrial infrastructure
program of laser industry support, which is funded by the Ministry Of Trade, Industry & Energy (MOTIE, Korea, N0000598).
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Full length article
Picosecond UV laser induced scribing of polyethylene terephthalate (PET) films for the enhancement of their flexibility Min Gi Kang a, Changhwan Kim b, Yong Joong Lee a, Sung Yeol Kim a,n,1, Ho Lee a,n,1 a b
School of Mechanical Engineering, Kyungpook National University, Dague 702-701, Republic of Korea School of Mechanical Design & Manufacturing, Busan Institute of Science and Technology, Busan 616-737, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2015 Received in revised form 5 February 2016 Accepted 1 March 2016
Flexible devices has received a great attention due to their high portability, lightness, and ease of shape reconfiguration. To achieve high flexibility, controlling the mechanical properties of the substrate materials is of importance. In this paper, we controlled the local flexibility of PET films via UV laser scribing. The bending test of the films revealed that their bending curvatures, the associated mechanical damages, and the required bending forces could be successfully tuned by controlling the number and the depth of the scribed lines. Our simple strategy of using laser scribing will find its usefulness in flexible device applications where high flexibility and mechanical stability are required. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Laser scribing PET film Flexible electronics substrate Flexibility Bending property
1. Introduction Flexible devices are currently attracting considerable interest due to their advantages such as high portability, lightness, and ease of shape reconfiguration. Unlike the conventional devices, they can be conformably fitted onto any curved surfaces and folded and unfolded for storage and packaging [1–6]. As such, new applications are emerging in a variety of fields, which include wearable health care devices, flexible displays, conformal antenna arrays, flexible solar cells, flexible batteries and radio-frequency identification (RFID) tags [7–14]. Flexible devices are typically based on organic substrates [15– 18]. The substrates impart flexibility to the devices and control the overall mechanical properties [19], and therefore, the selection of substrate materials and their processing determines the degree of flexibility of the systems. Paper substrates made of a natural polymer-cellulose are promising substrates to achieve high flexibility and foldability. However, they lose their stability upon contacting water or other liquids [20,21], so their applications can be limited. Polyethylene terephthalate (PET) is a synthetic polymer with a good chemical stability against the liquids, and it is among the most widely used organic substrates for most flexible device applications [22–24]. However, this polymer substrates cannot n
Corresponding authors. E-mail addresses: [email protected] (S.Y. Kim), [email protected] (H. Lee). 1 These corresponding authors contributed equally to this paper.
http://dx.doi.org/10.1016/j.optlastec.2016.03.001 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
render high flexibility compared with the paper substrates. For example, when a PET substrate with a typical thickness of 80 mm is used, the bending curvature more than 0.8 mm 1 (i.e. the radius of curvature less than 1.25 mm) cannot be obtained without plastic deformation. Besides, folding or creasing PET films cannot be easily achieved without mechanical instability, which makes it challenging to develop highly flexible device with foldability using the polymer [25–28]. To achieve foldability or high local flexibility, the local bending curvature of the hinges (or bending parts) of the substrates should be high, while the other regions should be mechanically stiffer than the hinges to maintain their shapes. In addition, the repulsive force against folding these substrates should be small enough so that they don't unintentionally spring back to their unfolded shape. We believe these requirements can be achieved by controlling the local flexibility of the substrate by changing its local bending stiffness. If there exist a method to process the polymer substrates to tune their thickness, shape or surface, their local bending stiffness can be controlled to achieve high flexibility and foldability. UV Laser is a powerful tool to scribe a fine feature or pattern on polymer substrates at desired location. The bond breaking interaction between laser and the substrates called photo-ablation is the basis of the laser scribing, and therefore the feature sizes are only limited by the resolution of the laser ( a few hundred nanometers), and their shape and locations can be precisely controlled. Laser-scribing of polymers has been actively investigated
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for its use in various applications including an optical waveguides, micro-machined parts, and micro-channels and vias for flexible electronics [29–32]. However, it is difficult to find a study using laser scribing to control the local mechanical properties of the polymer films, and its effects on the bending properties of the polymer substrates has not been thoroughly investigated. Herein, we controlled the flexibility of PET films by scribing multiple groove lines on them using UV picosecond laser. The changes of bending property was investigated depending on the laser scribing parameters such as the number of lines and the depth of groove by measuring the curvatures of the films. Interestingly, plastic deformations and mechanical instability due to the stress concentration that could occur near the scribed region were significantly reduced by increasing the number lines. Our study revealed that controlling the number of groove lines and their positions on PET films could tune both their overall and local flexibility. We believe that laser scribing can be as an effective tool for controlling the local mechanical properties of polymer films, which can expand the use of the synthetic polymer substrates for the application of foldable devices.
2. Method 2.1. Laser system and PET sample preparation We used a Duetto THG picosecond laser (12 ps, Time-Bandwidth Inc.) to cut and scribe PET films. The laser wavelength of 355 nm, the repetition rate of 50 kHz was selected to minimize heat accumulation and possible heat-driven bulge of the polymer. The scanning speed was 2 mm/s. The numerical aperture and the magnification factor was 0.16 and 10 , respectively. The beam size of the laser, focused on the PET film, is 1.3 mm and is calculated by employing the following equation,
4M 2λf M 2:beam quality, λ:wave length, f:focal length, πD
(
D:entrance beam diameter ( 1/e2)
)
To prepare PET samples for bending test, a PET film (10 mm 10 mm 80 mm ) was attached onto a glass slide by double sided tape, and the level (or flatness) of the film were carefully monitored and adjusted using a CCD camera. Single or multiple lines (3, 7, and 13 lines) were scribed at the center of the top surface of the film by scanning the laser along their lengths 10 times each. (Fig. 1). The interval between the lines were fixed at 100 mm. The scribed PET was then sectioned equally into four slices (10 mm 2.5 mm) by single edge blades. Three of the four PET specimens were used to perform the bending experiment while the remaining one was reserved for SEM imaging. 2.2. Bending tester Fig. 2(a) shows the schematic of the bending stage and PET sample. One of the two L-shape holder was fixed, while the other
one attached to an actuator (BDC 101, Thorlabs, Inc.) move along the rail up to 3 mm to bend the PET sample. The bent state of the PET film was maintained for 0.3 s, and then decompressed back to the original state until any stress induced by the actuator is released. For stability test, the PET film was bent cyclically 1000 times by the actuator. Fig. 2(b) is the optical image of the tester. An optical microscope (Edmond Inc.) was used to capture the shape of bent PET film in real time. An XY stage is connected to the optical microscope for the ease of alignment and monitoring the PET sample. Fig. 2(c) is the magnified image of the holder and the PET sample.
3. Result and discussion 3.1. Controlling the scribing depth of groove line on PET films using UV-picosecond laser We controlled the scribing depth by adjusting the power of a 355 nm picosecond laser. The other processing parameters such as repetition rate, scan speed, the number of scanning, and the beam size focused on the PET film were fixed to 50 kHz, 2 mm/s, 10, 1.3 mm, respectively. Based on the laser beam size and the scan speed that we have used, the spot overlap can be calculated by v employing the following equation,1 − fD , becoming an important factor to form a clear scribed line shape (v: scanning speed, f: repetition rate, D: beam diameter). The calculated value of the spot overlap was found to be 0.97. Fig. 3(a) shows the SEM images of PET samples scribed with the laser using different laser powers from 3 mW to 17 mW. All the samples showed a V-shaped groove line due to the Gaussian intensity profile of the emitted laser beam, and the depth of the groove line was found to be linearly increasing from 15 mm to 55 mm with increasing power (Fig. 3(b)). The scribing width on the top of PET almost remained constant at 6 mm for every laser power tested. The calculated corresponding ablation depth per pulse, for each of the laser power levels used, was 0.045 μm, 0.076 μm, 0.12 μm, and 0.17 μm respectively, assuming that an individual pulse overlapped with a spot area during line scanning of 10 times (330 pulses) is made to ablate as the identical depth. In addition, bulging (i.e. the indication of heat damage) was not noticeable at the periphery of the scribed area with the laser power up to 17 mW, which demonstrates that we can successfully control the depth of scribing (or the aspect ratio) without significant heat damage. These results can be attributed to the fact that the polymer is ablated before the considerable heat caused by laser beam spreads to the periphery of the irradiated part because of the short pulse duration (12 ps) of the ultrashort pulse laser, the low heat diffusion rate of the polymer material and, trivial heat generation in the UV wavelength (355 nm) , meaning that the PET film could be ablated as the beam size inflicted on the PET film. This results in the scribed shape having a high aspect ratio through repeated line scanning [33–35].
Fig. 1. The schematic diagram of the picosecond laser apparatus (a), and the scribed PET sample (b).
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Fig. 2. Schematic diagrams showing the home-made bending tester (a). Optical image of the tester and optical microscope (b), and sample under loading and sample holders in high magnification (c).
Fig. 3. (a) SEM images of laser-scribed grooves on PET according to variations in laser power. (b) The depth of scribing as a function of laser power.
3.2. The bending property of PET films with single-line scribing After confirming that we could control the scribing depth without severe heat damage, we investigated the bending properties of the scribed PET depending on the number of groove lines and the scribing depth. First, a single groove line was scribed on the center of a bare PET film and its bending shape was measured by an optical microscopy. Fig. 4(a) compares the overall bending state of the PET sample with the scribing depth ranging from 0 to 55 mm. In general, the bending of the PET became more severe at the center while it became relieved at the edges for all the samples tested. As the scribing depth increased, the bending curvature at the center increased while the side curvature near the edges decreased, indicating that the deformation and the corresponding stress became more concentrated at the center of the sample. Particularly, the PET sample with a deep scribing, which corresponded to the
scribing depth 40 and 55 mm in Fig. 4(a), showed abrupt changes of the curvature at the center where the groove line existed. Shown in the Fig. 4(b) are the high magnification images near the center that clearly shows this trend. When the scribing depth was small (i.e. 15 mm), the local deformation at the groove line was almost identical to that of the bare PET without scribing. However, the deformation became severe with the increasing the scribing depth: the bending became no longer smooth and the top width of the groove broadened up to 100 mm when the scribing depth was 55 mm. It is obvious that this large local deformation can damage the samples resulting in permanent (i.e. plastic) deformation or even fracture. The images of the PET samples after the bending test (Fig. 4(d)) shows that the PET samples with scribing do not return back to their original flat state (Fig. 4(c)) and they were abruptly bent at the scribed line, indicating that the permanent deformation occurred at the scribed area. And as expected, the degree of this permanent damage (i.e.
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Fig. 4. (a) Low and (b) high magnification optical images of the scribed PET samples with single line scribing under bending. Yellow and red arrow indicates side and center curvature, respectively. The scribing depth tested was 0 (i.e. no scribing), 15, 40, and 55 mm. Optical image of the sample (c) before and (d) after the bending test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
abrupt warpage) on the PET was more severe when the scribing depth was deeper due to the stress concentration and the local deformation at the scribed area. These results show that scribing single line may not be appropriate for achieving high local flexibility of PET films because it tends to produce mechanical damages on the films such as permanent deformation and fracture. 3.3. The bending property of PET films with multi-line scribing To control the bending properties of PET films without causing the mechanical damages, we controlled the number of scribed lines on the PET films and studied their bending properties. Shown in Fig. 5 are the digital images of the bending state of PET with multi-line scribing, along with close-up image and overall shape of the sample. Only the PET samples with the scribing depth of 25 and 40 mm were shown here because the sample with scribing depth of 15 mm did not show any difference in shape and curvature regardless of the number of scribed line, and the PET samples with the scribing depth of 55 mm easily fractured during the bending test. In general, as the number of scribing line on PET film increased, the curvature at the center (dotted red area Fig. 5) decreased, and the center groove line (the inset in the middle) showed less broadening of the top width. Comparison between the samples with three-line and a thirteen-line scribing of 40 mm depth (Fig. 5 (a) D ¼40 mm vs. 5(c) D ¼40 mm) noticeably showed this difference: the abrupt bending curvature became smoother one by increasing the number of scribing lines. This decrease in the curvature and local deformation can be attributed to the fact that the stress and deformation localized in a single groove line is distributed among multiple lines, diminishing the local stress concentration and the severe plastic deformation that could damage
the samples. Note that the variation in the shape of groove lines was negligible among the multi-lines in the same sample (see Fig. 5(c) thirteen-lines sample), indicating that the degree of deformation was similar (and small) for all the lines regardless of their locations in the sample. Fig. 5(d) summarizes the curvatures of the PET films depending on the number and the depth of the scribed lines, which also shows the areas depending on the occurrence of the mechanical damages. To minimize the mechanical damages while achieving high local curvature, selecting the scribed depth more than 25 mm and the number of line more than three are preferred (N Z3 line DZ25 mm) as indicated by the yellow area in the figure. The scribed PET films associated with this region exhibit higher flexibility than that of non-scribed films: they show higher curvature and can be bent with less force due to lower bending stiffness. Although it was possible to achieve higher curvature values by scribing a single line with the depth more than 25 mm, it typically resulted in large plastic deformation and the fracture of the PET samples (gray area). When the depth of the scribed line was less than 15 mm, there was no noticeable increase in the curvature of the film compared with those of non-scribed PET films. Based on the results of experimental work that we performed earlier, the optimal option for flexibility enhancement within the mechanical stability zone can be found. We expect that as the number of scribed lines is increased, the bending force (i.e. flexibility) decreases while the scribed depth is deeper and the bending force falls within the mechanical stability zone. Thus, the sample (D ¼40 mm, N ¼13) would be the best option for maximal flexibility enhancement within the tested range. However, should the mechanical stability be considered in combination with the flexibility, the sample (D ¼25 mm, N¼ 13) would be the best option within the tested range, meaning that an optimal condition could
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Fig. 5. Optical images of the bending state of PET samples with (a) three, (b) seven and (c) thirteen scribing lines. (d) Center curvature as a function of number of scribing lines. Curvature was measured from the left lower insets of Fig. 5(a–c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (a–c) Optical images of the scribed PET samples after the bending test. All the external forces were unloaded. SEM images of the PET samples with multiple lines (d) before and (e) after stability test and (f) PET sample with single line, where crack propagation occurred.
be different depending on the purpose of utilizing the scribed PET film samples within the mechanical stability zone. This result demonstrates that we can successfully tune the local bending properties and flexibility of PET films by controlling the laser scribing parameters. 3.4. Bending stability We studied the cyclic stability of the scribed PET films. The bending test was performed up to 1000th cycle and the shape of the samples before and after the test was compared. (Fig. 6) The scribed PET samples behaved nearly elastically when the scribing depth was small (D ¼15 mm), regardless of the number of the lines; the samples returned back to their original flat shape after removing the external loading. However, when the scribing depth increased to 40 mm or more, the PET samples with a single line did not returned to their original flat shape (Fig. 4(d)), while the samples with multiple lines with same depth nearly returned back to their initial shape except for the case N ¼3. We found that the samples with multiple groove lines did not fracture even after 1000th cycles of bending test. Moreover, the shape of the groove lines where the most of deformation occurs did not change at all even after the test, indicating severe damages such as large plastic deformation and crack propagation did not occur (Fig. 6(d–e)). However, the PET samples with single groove line were susceptible to fracture during the test. (see optical images shown in Fig. 6(f)). The warpage, the residual curvature after the bending test, represents the degree of plastic deformation accumulated during the test, and therefore the permanent deformation on the samples can be measured qualitatively by measuring the curvature. Fig. 7 shows that the center curvature increase with increasing the number of bending cycle, and the initial increase for the first 100 cycles takes up more than 80% of the total curvature change. This result indicates that the plastic deformation (or damage) accumulates with increase of the bending cycles, and the PET sample undergoes most of plastic deformation during the first 100 cycles. As expected, when the scribing depth was fixed, scribing seven or
Fig. 7. Bending curvature at the center of the scribed PET films as a function of the bending cycles. The curvature was measured after the bending test when the PET film was in its relaxed state without external loading. The depth of the groove line was fixed at 40 mm.
thirteen lines compared to three (or single) lines significantly reduced overall curvature (i.e. plastic deformation). These results confirm that scribing multiple groove lines also reduce the plastic deformation accumulation and improve the cyclic bending stability. 3.5. Bending force and flexibility The bending stiffness of the PET films decreases with increasing the depth or the number the scribing lines, and therefore the bending force required to bend a PET film is expected to decrease correspondingly. Fig. 8 compares the overall shapes of the PET sample with or without scribing lines, which are fully bent to study the required bending force and its flexibility. The PET film with multiple scribing lines (Fig. 8a, D ¼40 mm, N ¼13) showed the local bending (or folding) at the scribed region with high curvature
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Fig. 8. Schematic diagrams of overall bending shape of a PET film (a) with (D ¼ 40 mm, N ¼ 13) and (b) without multiple groove lines. F1 and F2 represent the force required to bend the PET film, and R1 and R2 represent the radius of curvature at the center. Inset shows the corresponding digital image of the PET film.
(i.e. small radius of Curvature: 500 mm engaged in the range of radius of curvature employed in recent studies on foldable device [23,26,36]), while its side regions remained relatively un-deformed, showing that laser scribing allowed high local curvature at the scribed area by reducing its local bending stiffness. However, the sample without scribing (Fig. 8 b) showed lower curvature (i.e. higher radius of curvature) at the center, while its sides also deformed. Moreover, the scribed sample required more than six times lower bending force than the non-scribed sample. These results demonstrate that scribing PET films can impart high local flexibility to the films: higher local curvature can be achieved with less bending force. Combined with our previous results of bending stability, we anticipate our strategy of using laser scribing may find its usefulness in flexible device applications where the control of local flexibility and mechanical stability are required.
4. Conclusion In conclusion, we studied the effects of laser scribing on the bending properties of PET films by measuring their bending curvature. As the scribing depth increased, the PET sample with single groove line tended to fail during the bending test due to stress concentration and severe plastic deformation at the scribed region. However, increasing the number of scribing lines prevented the mechanical failure of the PET films by alleviating the two causes. Cyclic bending test revealed that the plastic deformation of the film accumulated with each cycle, and the deformation was decreased by scribing multiple lines on PET films. Even after 1000th cycles, we could not find any observable damage at the scribed region, when the number of lines increased to seven and thirteen. The bending force required to bend the PET film was also found to decrease with the number of groove lines, and therefore flexibility was enhanced with laser scribing. Moreover, the bending occurred mainly at the scribed region, so the scribed PET films showed higher local curvature at the desired location with less bending force. These advantages of laser scribing may find its usefulness in flexible or foldable devices where achieving high flexibility and controlling local mechanical properties are required.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A1038346). Also, this research was conducted under the industrial infrastructure
program of laser industry support, which is funded by the Ministry Of Trade, Industry & Energy (MOTIE, Korea, N0000598).
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