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Self-organized fractal-like structures formation on the silicon wafer surface using the femtosecond laser pulses
Optics and Lasers in Engineering 128 (2020) 106008
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Self-organized fractal-like structures formation on the silicon wafer surface using the femtosecond laser pulses R. Goodarzi, F. Hajiesmaeilbaigi∗, E. Bostandoost Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
a r t i c l e Keywords: Femtosecond Laser Silicon Fractal Spikes Ripples
i n f o
a b s t r a c t In this paper interaction of a different number of femtosecond laser pulses and various fluences with silicon wafer has been studied. Effects of the pulse number and the laser fluence on self-organized structures created during femtosecond laser pulses processing are investigated. The scanning electron microscopy images of the sample show that by increasing the number of laser pulses, spike-like structures form on the surface, which gradually transform to hills and holes structure. The increment of the laser fluence causes the microstructures to grow in diameter. However, the increase in diameter is limited by surface stability. When the micro-structures are at their maximum size and a stable geometrical shape, further fluence increment can cause new self-organized nanostructures to be created over the microstructures’ surface. The hierarchical micro and nanostructures created after femtosecond laser pulse interaction have a similar shape. Thus fractal-like structures formed in the femtosecond pulse laser processing can be useful in the surface nano-structuring applications.
1. Introduction The ultra-short and ultra-intense lasers are among the most advanced lasers, which have found special applications in the laser processing field [1,2]. The ablation in the absence of heat-affected zones (HAZ) and without shock wave effects are the essential advantages of femtosecond laser pulse processing [3] which is due to the fast nonlinear ablation. Besides the controlled processing, the ultra-short laser pulses can be implemented in self-organized surface patterning [4]. Nowadays, the primary conditions to control the shape of the micro, and nano selforganized structures are under discussions [5–7]. Although this phenomenon has been widely researched, there are many ambiguities that need to be studied in more details. The spike-like structures, laser-induced periodic surface structures (LIPSS) and ripples are the most substantial self-organized structures that are reported massively in various experimental investigations [8– 11]. The formation of the spikes on the silicon surface with different conditions was studied by Her and Mazur; they showed that height and diameter of the spikes could be extended from submicrometer to micrometer [12–14]. The spike-like structures as the black silicon that are formed by using femtosecond laser pulses have special usage in solar cell technology [15]. Bonse et al. were among the first researchers who noticed the self-organized periodic structures can be formed on the silicon surface after irradiating the surface by femtosecond laser pulses [16]. Bonse showed that the surface plasmon polaritons play a dominant role in the initial stage of LIPSS formation and the spatial periods of such
∗
structures can be smaller than the laser wavelength [17] which was confirmed by Derrien et al. [18]. Also, Bonse indicated that by increasing the number of interacting pulses, mean spatial period would decrease to submicrometer dimensions [19]. The nonlinear increase of the ripples’ depth relative to the fluence of irradiation is investigated by Tan et al. [20]. Liu et al. observed that ripple orientation is co-determined by the laser polarization direction [4]. There are also some experimental results which report 100-nm periodic structures upon femtosecond laser irradiation of silicon in liquid environments such as water [21]. It is worth to mention that there are lots of innovative and valuable experimental and theoretical results are obtained last years which emphasized on the importance of these facts [22–24]. Vorobyev et al. firstly showed that nanostructures accompanying with microstructures is a natural consequence of femtosecond laser ablation [25]. Despite nanostructures observation in previous studies, based on our knowledge, there are not any reports to show that similar micro and nanostructures can be formed simultaneously on the surface. Furthermore, the geometrical properties of the self-organized structures are not studied intensively that makes this aspect unclear. The hierarchical micro and nanostructures can be assumed as fractals. Due to their distinctive geometrical shape, the fractal-like structures are crucial subjects in surface processing which can be interesting in unusual patterning of matter [26–28]. Fractals are encountered ubiquitously in nature, they are geometrical objects in which the same patterns occur repeatedly at different scales and sizes. Also, the fractals are created by repeating a process over and over in an ongoing feedback loop; they exhibit similar
Corresponding author. E-mail address: [email protected] (F. Hajiesmaeilbaigi).
https://doi.org/10.1016/j.optlaseng.2020.106008 Received 22 September 2019; Received in revised form 2 December 2019; Accepted 4 January 2020 0143-8166/© 2020 Elsevier Ltd. All rights reserved.
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 1. Schematic of the experiment setup.
Fig. 3. a) EDX analysis result of the silicon wafer surface after interaction with single femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
Fig. 2. SEM image of the silicon wafer surface after interaction with a single shot femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
patterns at extremely small scales called self-similarity that is a neverending pattern. However, there are some natural phenomena with the repeated structure in various scales for a limited number of times that somehow are similar to fractals which can be called fractal-like structures [29,30]. In this study, the combined formation of micro and nanostructures is investigated in terms of the hierarchical or fractal-like structure
Table 1 The percentage of the elements in the silicon wafer surface after interaction with single femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
formation. The morphology of the silicon wafer surface after irradiation was studied by using scanning electron microscopy (SEM) images to compare various effects of femtosecond laser pulses. The ablation threshold was measured for the later experiments. The primary conditions for ripple and spike formation were studied by using various number of pulses and different laser fluences. Furthermore, the procedure that results in the formation of self-organized nanostructures over the self-organized microstructures is studied briefly. 2. Experimental details The experimental setup is illustrated schematically in Fig. 1. The experiments were performed at room temperature and atmospheric Table 2 The percentage of the elements found on the silicon surfaces after interaction by ten pulses with fluences of a) 𝐹 = 0.35 J.cm−2 and b) 𝐹 = 0.36 J.cm−2 . a)
b)
Element
Weight%
Atomic%
Element
Weight%
Atomic%
Element
Weight%
Atomic%
C O Si Total
32.70 0.89 66.42 100.00
52.94 1.08 45.98
C O Si Total
35.98 1.33 62.69 100.00
56.41 1.56 42.03
C O Si Total
39.83 6.81 53.36 100.00
58.78 7.54 33.68
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 4. SEM images of the ripple formation on the silicon surface after interaction with five number of femtosecond laser pulses with fluence of a) 𝐹 = 0.3 J.cm−2 and b) 𝐹 = 0.34 J.cm−2 .
pressure. The 40 fs pulses of Ti:sapphire laser with a chirped pulse amplification system was used for surface processing. The upper surface of the target was irradiated perpendicularly by linearly and horizontally polarized Gaussian laser pulses, with the repetition rate of 10 Hz at a central wavelength of 790 nm after focusing by an objective lens (OLYMPUS, UPlanFI, 4X/0.1). The different laser pulse energy was achieved using a zero-order half-wave plate coupled to a Glan-Taylor polarizer. The duration of the incident pulses just after the compressor chamber of the chirped pulse amplification (CPA) system and right before the objective lens was measured by intensity and interferometric autocorrelator systems. The mean value of the pulse duration on the target surface is around 40 ± 2 fs. It is worth mentioning that the beam diameter before the focusing lens is considerably large, so the intensity of the laser pulse during the propagation is below than the critical power of self-focusing. Furthermore, the pulse broadening due
to the group velocity dispersion is resolved by pre-chirping of the laser pulse. Therefore, the nonlinear effect, such as the Kerr lens focusing and pulse chirping, are neglected in our experiments. In this research, the n-doped silicon wafer with a single-side polished surface (111) with 500 μm thickness was used. The silicon samples were cleaned by using an ultrasonic bath with acetone around fifteen minutes; then, rinsed with methanol and dried with hot air. The sample was tightened to a two-axis translation motorized stage with PC control for careful positioning. In order to study the effects of a different numbers of pulses on the surface, the speed of the translation stage was changed in each experiment. Also, different fluence was obtained by increment and decrement of pulse energy while the spot size of the laser pulse is invariant. Considering the different focal conditions, we adjusted the position of the focal plane by moving the sample along the optical axis in order to obtain the minimum focal positions in the experiments. Also,
Fig. 5. SEM images of the a) hill-like structures and b)-h) nano grooves formation on the silicon surface after interaction with ten number of femtosecond laser pulses with fluence of a) 𝐹 = 0.35 J.cm−2 , b) 𝐹 = 0.36 J.cm−2 , c) 𝐹 = 0.37 J.cm−2 , d) 𝐹 = 0.38 J.cm−2 , e) 𝐹 = 0.39 J.cm−2 , f) 𝐹 = 0.4 J.cm−2 , g) 𝐹 = 0.41 J.cm−2 and h) 𝐹 = 0.42 J.cm−2 .
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 6. SEM images of the bead-like structures on the silicon surface after interaction with fifty number of femtosecond laser pulses with fluence of a) 𝐹 = 0.45 J.cm−2 , b) 𝐹 = 0.47 J.cm−2 , c) 𝐹 = 0.49 J.cm−2 , d) 𝐹 = 0.51 J.cm−2 , e) 𝐹 = 0.53 J.cm−2 , f) 𝐹 = 0.55 J.cm−2 , g) 𝐹 = 0.57 J.cm−2 , h) 𝐹 = 0.59 J.cm−2 , i) 𝐹 = 0.61 J.cm−2 , j) 𝐹 = 0.63 J.cm−2 , k) 𝐹 = 0.65 J.cm−2 and l) 𝐹 = 0.67 J.cm−2 .
the focal plane was determined by finding the smallest diameter of the craters. Morphology of the silicon surface after the interaction was obtained by using the field emission scanning electron microscope (FE-SEM ZEISS SIGMA, EHT = 15.00 KV) and the scanning electron microscope (SEM ZEISS Gemini, EHT = 25.00 KV). The image processing analyses were utilized to study the structures that created on the surface. 3. Results and discussions The ablation threshold of the silicon wafer was calculated by using the well-known experimentally relation 𝐷2 = 2𝑤20 𝑙𝑛( 𝐹𝐹 ) [31]. Where D 𝑡ℎ
is diameter of the ablated area which was found by image processing,
w0 is the laser spot size, F is the laser fluence, and Fth is the ablation threshold fluence. In Fig. 2, the SEM image of silicon wafer surface illustrates the effect of a single shot of femtosecond laser with a fluence of 𝐹 = 3 J.cm−2 on the surface. After measuring the ablated diameters for various laser fluences the ablation threshold is calculated around 0.26 J.cm−2 , which is in agreement with many other articles [16,22,32]. The purity of the silicon wafer is considered 99% approximately, according to the company claim by using the nanopurification method [33]. The composition analysis shows that due to the interaction of femtosecond laser pulses, some other elements will be trapped in the surface. The energy dispersive X-ray (EDX) analysis of Fig. 2 is illustrated in Fig. 3; this figure shows that some oxygen atoms from the air are trapped in the surface. The carbon atoms are also caught in the sur-
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 7. SEM images of the hierarchical structures; a) the monotonous hexagonal formation on the surface and b) the nanostructures formed on the microstructure.
face massively. The carbon atoms come from the contamination of the air and surface of the sample. The percentages of the main elements are presented in Table 1. In spite of that, the ablation can change the surface composition, but still the density of silicon atoms is dominant. The first self-ordered pattern that has been studied was the laserinduced periodic surface structure (ripples) [11,34]. The regular structures were not observed in an experiment with a single laser pulse. But, after increasing the number of femtosecond laser pulses to five, the ripple pattern appeared. The SEM images of the samples after interaction by five femtosecond laser pulses with 0.3 and 0.34 J.cm−2 fluence are depicted in Fig. 4 (a) and (b), respectively.
As illustrated in Fig. 4 the self-structure formation due to the multiple pulse interaction indicates that a feed-back mechanism exists [16]. The ripples formation is well explained in the literature which show that, long after the absorption of femtosecond laser pulse energy by the electrons at the thermally non-equilibrium condition, due to the diffraction of light by nanoroughness, which is randomly and sparsely produced on the surface, the periodic structures are imprinted on the surface [6]. Also, during the periodic structure creation by multi-pulse, the first pulse produces random nanostructures which can couple the surface plasma polaritons (SPPs) to the scattered light that may act as a periodic spatial modulation of the deposited laser energy [6]. In this way,
Fig. 8. SEM images of the silicon surface after interaction with one hundred number of femtosecond laser pulses with fluence of a) 𝐹 = 0.5 J.cm−2 , b) 𝐹 = 0.55 J.cm−2 , c) 𝐹 = 0.6 J.cm−2 , d) 𝐹 = 0.65 J.cm−2 , e) 𝐹 = 0.7 J.cm−2 , f) 𝐹 = 0.8 J.cm−2 , g) 𝐹 = 0.9 J.cm−2 and h) 𝐹 = 1 J.cm−2 .
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 9. a) SEM images of the silicon surface after interaction with five hundred number of femtosecond laser pulses with fluence of 𝐹 = 1 J.cm−2 and b) the zoom out image of the specified area.
spatially modulated heating of the surface leads to formation of ripples on the surface. This research illustrated that by increasing the fluence of the laser pulses, the ripples borders become clear. This result can be inferred from the fact that the higher fluence of laser pulse creates a deeper ablated zone. Therefore, by increasing the laser fluence, the ripples will growth efficiently, and the feed-back mechanism has enough time to construct the appropriate structure. The hill-like structures are appeared on the silicon surface after increasing the number of interacted pulses. According to the experiment, ten pulses with a fluence of 𝐹 = 0.35 J.cm−2 can create the hill-like structures. Fig. 5 shows the hill-like structures for various laser fluence from 𝐹 = 0.35 − 0.42 J.cm−2 . As can be seen, the diameters of the hills are between 20 to 2000 nanometer. Also, by increasing the laser fluence, the hills get some grooves, and by further increasing, the grooves will cover all of the interaction areas on the surface. We suppose that the formation of the nano grooves is the response of the hill-like structures to an ionization below the surface created by breakdown which results in a sphere-to-plane formation [35]. Each hill has some nano grooves on its surface that will be caused a fractal-like structure formed on the silicon surface. As depicted in Fig. 5, the hill-like structures are appeared by increasing the number of pulses to ten, one can expect that the duration of the melting phase on the surface is longer with respect to the ripples formation condition. The EDX analysis of the structures for comparing elements contribution of the samples which are shown in Fig. 5 (a) and (b) are listed in Table 2. The elemental analysis indicates that by increasing the number of interacting pulses from five to ten, the weights of the carbon and oxygen percent are increased. The carbon and oxygen atoms have more time to be captured on the surface due to their longer melting time. Furthermore, by increasing the laser fluence, the carbon and oxygen percent is also increased due to the deeper ablation depth and an effective ablation [5]. When the fifty pulses impinged to the silicon surface, the well-known bead-like structures that are firstly introduced by Shen et al. [14] observed. Fig. 6 shows the SEM images of the silicon surface after interaction with fifty femtosecond laser pulses with various fluences of 𝐹 = 0.45 − 0.67 J.cm−2 . As can be seen, by increasing the laser fluence, the borders between created structures become clear, and the diameter of the hills increase considerably. The evolution of the bead-like structures only by increasing the laser fluence may be interpreted as a feeding mechanism which means the laser pulse energy provides the necessary
supply for the creation of the micro-structures on the surface [13]. In this way, spatially modulated heating of the surface leads to the formation of ripples on the surface. Role of the absorbed laser pulse energy for controlling the self-organized structure is due to the so-called Radiation Remnants (RR) that is capable of extracting energy from the incident radiation and transferring it to the material [36]. The laser fluence plays an important role in the size of the beadlike structures. The diameters of the self-organized surface structures strongly determine the structures’ stability. Moreover, the increase in the diameters of surface structures has a limitation. By increasing the laser fluence, the melted area will increase while the surface tension cannot preserve the structure stability. As illustrated in Fig. 6, by increasing the diameter of the bead-like structures gradually, their geometric shape changes from round to monotonous hexagonal which is more stable. Fig. 7 (a) shows the created monotonous hexagonal structures on the surface. In nature, honey bees use this fact to make their hive. Therefore, the size of the micro-structures is under control. Another important observation is that along with the increasing of the micro-structures diameter; some nanostructures are formed over them. Fig. 7 (b) shows the nanostructures formed on the microstructures by zoom out on the previously displayed structure in Fig. 6 k). The created hierarchical structure contains nanostructures that are formed on the microstructures so the micro and nanostructures have the same surface pattern. It seems that a fractal-like structure is gradually formed on the surface. In this research, a more investigation was performed using the one hundred number of pulses interacting with the silicon surface. The spikelike structures created with a hundred number of pulses is shown in Fig. 8 for different laser fluence of 𝐹 = 0.5 − 1 J.cm−2 . Fig. 8 indicates that by increasing the laser fluence, the spikes diameters are increased and also the deep holes are created gradually. These deep grooves in higher laser fluences regime may be created due to the progressive aggregation of nanoparticles generated during the ablation process which can decorating the structure by further modulation of the deposited energy [37]. Finally, by further increasing the number of pulses to 500 with a fluence of 𝐹 = 1 J.cm−2 , the hills and holes are created efficiently on the silicon surface. The holes creation on the surface has repeated in different lines while getting away from the center of the laser pulse focusing area, as can be seen in Fig. 9, the diameter of the created holes and hills decreases rapidly. This observation gives two different points. First, due
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 10. SEM images of the a) and b) holes at the edge and center of laser focusing area respectively and c) and d) are the hills at the edge and center of laser focusing area respectively which they are created on the silicon surface after interaction with five hundred number of femtosecond laser pulses with fluence of 𝐹 = 1.5 J.cm−2 .
to the Gaussian shape of the laser pulses, while the laser fluence decrease in the edge of the focusing area, the holes’ diameter decrease too. Second, a different line of ordered holes on the surface shows that an energy distribution on the surface is responsible for the position of created holes. We suppose that this modulation on laser energy comes from the diffraction patterns created by various optical elements which laser pulse has to pass from them such as pin-hole, lens, and mirrors. The more magnified pictures of the hills and holes in Fig. 9 (b) show that the sub-structures with nanometric scales are created on the hills and holes. Fig. 10 shows the two apparent kinds of holes and hills formed on the silicon surface by using 500 number of pulses with a fluence of 𝐹 = 1.5 J.cm−2 . The two kinds of holes are created at the edge and the center of the laser focusing area are shown in Fig. 10 a) and b) respectively. These holes have a layered wall. The number of layers and craters decrease by increasing the distances from the center of the pulse. The most important parameter which is responsible for the number of layers and the opening diameter is the laser fluence. This issue is repeated for the hills created in the same conditions besides the holes. Fig. 10 (c) and (d) indicate the hills formed at the edge and center of laser focusing area respectively. The hills have similar hill-like structures on their surface which are denser at the center. Thus, again the hierarchical or fractal-like structures are fabricated on the silicon surface using multiple numbers of femtosecond laser pulses. 4. Conclusions In this research, effects of the laser fluence and the number of femtosecond laser pulses interacting with the silicon surface were investigated. The morphological studies show that by increasing the number of pulses from five to ten, the structure of the surface gradually changes from ripples to spikes. At the beginning of the formation of the spikes,
the hills and holes have bead-like structures and smoothly separated from each other by the random borders. Moreover, by increasing the number of interacting pulses from ten to one hundred, at a laser fluence above ablation threshold, the hills and holes boundaries become clear and spike-like structures appear on the silicon surface. The slow modification of the surface caused by the increasing the number of interacting pulses indicates that there is a feedback mechanism that strongly depends on the surface-induced patterns. The induced pattern can be a result of light interference on the surface, or it may be due to the balance between the pondermotive force and the stress-strain [22,38,39]. The shape of the induced pattern is determined by the number of interacting laser pulses. From this aspect, the surface modification is strongly dependent on the number of laser pulses have been irradiated on the surface. In the self-induced surface structures created during the femtosecond laser pulse irradiation, if the number of interacting laser pulses is fixed, the hills and holes diameter is enhanced by increasing the laser pulse fluence. Diameter of the micro-structures grows while their stability puts a constraint on their size. While the laser pulse energy increases, the deposit of their energy in the surface increase too. The self-induced structures cannot react to the additional amount of energy by increasing their diameter. Thus, the accumulation of laser energy must appear in another way. Our results indicate that when the laser fluence is increased enough, a new group of self-induced structures is created over the microstructures formed before. The size of the new structures is on the order of tens of nanometers while their pattern is similar to the existing microstructures. These new nano-structures and the previously formed micro-structures have the same geometrical shape. In this hierarchical structures, the microstructures beneath the nanostructures make a fractal-like or a self-similar structure on the surface which are created
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Self-organized fractal-like structures formation on the silicon wafer surface using the femtosecond laser pulses R. Goodarzi, F. Hajiesmaeilbaigi∗, E. Bostandoost Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
a r t i c l e Keywords: Femtosecond Laser Silicon Fractal Spikes Ripples
i n f o
a b s t r a c t In this paper interaction of a different number of femtosecond laser pulses and various fluences with silicon wafer has been studied. Effects of the pulse number and the laser fluence on self-organized structures created during femtosecond laser pulses processing are investigated. The scanning electron microscopy images of the sample show that by increasing the number of laser pulses, spike-like structures form on the surface, which gradually transform to hills and holes structure. The increment of the laser fluence causes the microstructures to grow in diameter. However, the increase in diameter is limited by surface stability. When the micro-structures are at their maximum size and a stable geometrical shape, further fluence increment can cause new self-organized nanostructures to be created over the microstructures’ surface. The hierarchical micro and nanostructures created after femtosecond laser pulse interaction have a similar shape. Thus fractal-like structures formed in the femtosecond pulse laser processing can be useful in the surface nano-structuring applications.
1. Introduction The ultra-short and ultra-intense lasers are among the most advanced lasers, which have found special applications in the laser processing field [1,2]. The ablation in the absence of heat-affected zones (HAZ) and without shock wave effects are the essential advantages of femtosecond laser pulse processing [3] which is due to the fast nonlinear ablation. Besides the controlled processing, the ultra-short laser pulses can be implemented in self-organized surface patterning [4]. Nowadays, the primary conditions to control the shape of the micro, and nano selforganized structures are under discussions [5–7]. Although this phenomenon has been widely researched, there are many ambiguities that need to be studied in more details. The spike-like structures, laser-induced periodic surface structures (LIPSS) and ripples are the most substantial self-organized structures that are reported massively in various experimental investigations [8– 11]. The formation of the spikes on the silicon surface with different conditions was studied by Her and Mazur; they showed that height and diameter of the spikes could be extended from submicrometer to micrometer [12–14]. The spike-like structures as the black silicon that are formed by using femtosecond laser pulses have special usage in solar cell technology [15]. Bonse et al. were among the first researchers who noticed the self-organized periodic structures can be formed on the silicon surface after irradiating the surface by femtosecond laser pulses [16]. Bonse showed that the surface plasmon polaritons play a dominant role in the initial stage of LIPSS formation and the spatial periods of such
∗
structures can be smaller than the laser wavelength [17] which was confirmed by Derrien et al. [18]. Also, Bonse indicated that by increasing the number of interacting pulses, mean spatial period would decrease to submicrometer dimensions [19]. The nonlinear increase of the ripples’ depth relative to the fluence of irradiation is investigated by Tan et al. [20]. Liu et al. observed that ripple orientation is co-determined by the laser polarization direction [4]. There are also some experimental results which report 100-nm periodic structures upon femtosecond laser irradiation of silicon in liquid environments such as water [21]. It is worth to mention that there are lots of innovative and valuable experimental and theoretical results are obtained last years which emphasized on the importance of these facts [22–24]. Vorobyev et al. firstly showed that nanostructures accompanying with microstructures is a natural consequence of femtosecond laser ablation [25]. Despite nanostructures observation in previous studies, based on our knowledge, there are not any reports to show that similar micro and nanostructures can be formed simultaneously on the surface. Furthermore, the geometrical properties of the self-organized structures are not studied intensively that makes this aspect unclear. The hierarchical micro and nanostructures can be assumed as fractals. Due to their distinctive geometrical shape, the fractal-like structures are crucial subjects in surface processing which can be interesting in unusual patterning of matter [26–28]. Fractals are encountered ubiquitously in nature, they are geometrical objects in which the same patterns occur repeatedly at different scales and sizes. Also, the fractals are created by repeating a process over and over in an ongoing feedback loop; they exhibit similar
Corresponding author. E-mail address: [email protected] (F. Hajiesmaeilbaigi).
https://doi.org/10.1016/j.optlaseng.2020.106008 Received 22 September 2019; Received in revised form 2 December 2019; Accepted 4 January 2020 0143-8166/© 2020 Elsevier Ltd. All rights reserved.
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 1. Schematic of the experiment setup.
Fig. 3. a) EDX analysis result of the silicon wafer surface after interaction with single femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
Fig. 2. SEM image of the silicon wafer surface after interaction with a single shot femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
patterns at extremely small scales called self-similarity that is a neverending pattern. However, there are some natural phenomena with the repeated structure in various scales for a limited number of times that somehow are similar to fractals which can be called fractal-like structures [29,30]. In this study, the combined formation of micro and nanostructures is investigated in terms of the hierarchical or fractal-like structure
Table 1 The percentage of the elements in the silicon wafer surface after interaction with single femtosecond laser pulse with fluence of 𝐹 = 3 J.cm−2 .
formation. The morphology of the silicon wafer surface after irradiation was studied by using scanning electron microscopy (SEM) images to compare various effects of femtosecond laser pulses. The ablation threshold was measured for the later experiments. The primary conditions for ripple and spike formation were studied by using various number of pulses and different laser fluences. Furthermore, the procedure that results in the formation of self-organized nanostructures over the self-organized microstructures is studied briefly. 2. Experimental details The experimental setup is illustrated schematically in Fig. 1. The experiments were performed at room temperature and atmospheric Table 2 The percentage of the elements found on the silicon surfaces after interaction by ten pulses with fluences of a) 𝐹 = 0.35 J.cm−2 and b) 𝐹 = 0.36 J.cm−2 . a)
b)
Element
Weight%
Atomic%
Element
Weight%
Atomic%
Element
Weight%
Atomic%
C O Si Total
32.70 0.89 66.42 100.00
52.94 1.08 45.98
C O Si Total
35.98 1.33 62.69 100.00
56.41 1.56 42.03
C O Si Total
39.83 6.81 53.36 100.00
58.78 7.54 33.68
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 4. SEM images of the ripple formation on the silicon surface after interaction with five number of femtosecond laser pulses with fluence of a) 𝐹 = 0.3 J.cm−2 and b) 𝐹 = 0.34 J.cm−2 .
pressure. The 40 fs pulses of Ti:sapphire laser with a chirped pulse amplification system was used for surface processing. The upper surface of the target was irradiated perpendicularly by linearly and horizontally polarized Gaussian laser pulses, with the repetition rate of 10 Hz at a central wavelength of 790 nm after focusing by an objective lens (OLYMPUS, UPlanFI, 4X/0.1). The different laser pulse energy was achieved using a zero-order half-wave plate coupled to a Glan-Taylor polarizer. The duration of the incident pulses just after the compressor chamber of the chirped pulse amplification (CPA) system and right before the objective lens was measured by intensity and interferometric autocorrelator systems. The mean value of the pulse duration on the target surface is around 40 ± 2 fs. It is worth mentioning that the beam diameter before the focusing lens is considerably large, so the intensity of the laser pulse during the propagation is below than the critical power of self-focusing. Furthermore, the pulse broadening due
to the group velocity dispersion is resolved by pre-chirping of the laser pulse. Therefore, the nonlinear effect, such as the Kerr lens focusing and pulse chirping, are neglected in our experiments. In this research, the n-doped silicon wafer with a single-side polished surface (111) with 500 μm thickness was used. The silicon samples were cleaned by using an ultrasonic bath with acetone around fifteen minutes; then, rinsed with methanol and dried with hot air. The sample was tightened to a two-axis translation motorized stage with PC control for careful positioning. In order to study the effects of a different numbers of pulses on the surface, the speed of the translation stage was changed in each experiment. Also, different fluence was obtained by increment and decrement of pulse energy while the spot size of the laser pulse is invariant. Considering the different focal conditions, we adjusted the position of the focal plane by moving the sample along the optical axis in order to obtain the minimum focal positions in the experiments. Also,
Fig. 5. SEM images of the a) hill-like structures and b)-h) nano grooves formation on the silicon surface after interaction with ten number of femtosecond laser pulses with fluence of a) 𝐹 = 0.35 J.cm−2 , b) 𝐹 = 0.36 J.cm−2 , c) 𝐹 = 0.37 J.cm−2 , d) 𝐹 = 0.38 J.cm−2 , e) 𝐹 = 0.39 J.cm−2 , f) 𝐹 = 0.4 J.cm−2 , g) 𝐹 = 0.41 J.cm−2 and h) 𝐹 = 0.42 J.cm−2 .
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 6. SEM images of the bead-like structures on the silicon surface after interaction with fifty number of femtosecond laser pulses with fluence of a) 𝐹 = 0.45 J.cm−2 , b) 𝐹 = 0.47 J.cm−2 , c) 𝐹 = 0.49 J.cm−2 , d) 𝐹 = 0.51 J.cm−2 , e) 𝐹 = 0.53 J.cm−2 , f) 𝐹 = 0.55 J.cm−2 , g) 𝐹 = 0.57 J.cm−2 , h) 𝐹 = 0.59 J.cm−2 , i) 𝐹 = 0.61 J.cm−2 , j) 𝐹 = 0.63 J.cm−2 , k) 𝐹 = 0.65 J.cm−2 and l) 𝐹 = 0.67 J.cm−2 .
the focal plane was determined by finding the smallest diameter of the craters. Morphology of the silicon surface after the interaction was obtained by using the field emission scanning electron microscope (FE-SEM ZEISS SIGMA, EHT = 15.00 KV) and the scanning electron microscope (SEM ZEISS Gemini, EHT = 25.00 KV). The image processing analyses were utilized to study the structures that created on the surface. 3. Results and discussions The ablation threshold of the silicon wafer was calculated by using the well-known experimentally relation 𝐷2 = 2𝑤20 𝑙𝑛( 𝐹𝐹 ) [31]. Where D 𝑡ℎ
is diameter of the ablated area which was found by image processing,
w0 is the laser spot size, F is the laser fluence, and Fth is the ablation threshold fluence. In Fig. 2, the SEM image of silicon wafer surface illustrates the effect of a single shot of femtosecond laser with a fluence of 𝐹 = 3 J.cm−2 on the surface. After measuring the ablated diameters for various laser fluences the ablation threshold is calculated around 0.26 J.cm−2 , which is in agreement with many other articles [16,22,32]. The purity of the silicon wafer is considered 99% approximately, according to the company claim by using the nanopurification method [33]. The composition analysis shows that due to the interaction of femtosecond laser pulses, some other elements will be trapped in the surface. The energy dispersive X-ray (EDX) analysis of Fig. 2 is illustrated in Fig. 3; this figure shows that some oxygen atoms from the air are trapped in the surface. The carbon atoms are also caught in the sur-
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 7. SEM images of the hierarchical structures; a) the monotonous hexagonal formation on the surface and b) the nanostructures formed on the microstructure.
face massively. The carbon atoms come from the contamination of the air and surface of the sample. The percentages of the main elements are presented in Table 1. In spite of that, the ablation can change the surface composition, but still the density of silicon atoms is dominant. The first self-ordered pattern that has been studied was the laserinduced periodic surface structure (ripples) [11,34]. The regular structures were not observed in an experiment with a single laser pulse. But, after increasing the number of femtosecond laser pulses to five, the ripple pattern appeared. The SEM images of the samples after interaction by five femtosecond laser pulses with 0.3 and 0.34 J.cm−2 fluence are depicted in Fig. 4 (a) and (b), respectively.
As illustrated in Fig. 4 the self-structure formation due to the multiple pulse interaction indicates that a feed-back mechanism exists [16]. The ripples formation is well explained in the literature which show that, long after the absorption of femtosecond laser pulse energy by the electrons at the thermally non-equilibrium condition, due to the diffraction of light by nanoroughness, which is randomly and sparsely produced on the surface, the periodic structures are imprinted on the surface [6]. Also, during the periodic structure creation by multi-pulse, the first pulse produces random nanostructures which can couple the surface plasma polaritons (SPPs) to the scattered light that may act as a periodic spatial modulation of the deposited laser energy [6]. In this way,
Fig. 8. SEM images of the silicon surface after interaction with one hundred number of femtosecond laser pulses with fluence of a) 𝐹 = 0.5 J.cm−2 , b) 𝐹 = 0.55 J.cm−2 , c) 𝐹 = 0.6 J.cm−2 , d) 𝐹 = 0.65 J.cm−2 , e) 𝐹 = 0.7 J.cm−2 , f) 𝐹 = 0.8 J.cm−2 , g) 𝐹 = 0.9 J.cm−2 and h) 𝐹 = 1 J.cm−2 .
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 9. a) SEM images of the silicon surface after interaction with five hundred number of femtosecond laser pulses with fluence of 𝐹 = 1 J.cm−2 and b) the zoom out image of the specified area.
spatially modulated heating of the surface leads to formation of ripples on the surface. This research illustrated that by increasing the fluence of the laser pulses, the ripples borders become clear. This result can be inferred from the fact that the higher fluence of laser pulse creates a deeper ablated zone. Therefore, by increasing the laser fluence, the ripples will growth efficiently, and the feed-back mechanism has enough time to construct the appropriate structure. The hill-like structures are appeared on the silicon surface after increasing the number of interacted pulses. According to the experiment, ten pulses with a fluence of 𝐹 = 0.35 J.cm−2 can create the hill-like structures. Fig. 5 shows the hill-like structures for various laser fluence from 𝐹 = 0.35 − 0.42 J.cm−2 . As can be seen, the diameters of the hills are between 20 to 2000 nanometer. Also, by increasing the laser fluence, the hills get some grooves, and by further increasing, the grooves will cover all of the interaction areas on the surface. We suppose that the formation of the nano grooves is the response of the hill-like structures to an ionization below the surface created by breakdown which results in a sphere-to-plane formation [35]. Each hill has some nano grooves on its surface that will be caused a fractal-like structure formed on the silicon surface. As depicted in Fig. 5, the hill-like structures are appeared by increasing the number of pulses to ten, one can expect that the duration of the melting phase on the surface is longer with respect to the ripples formation condition. The EDX analysis of the structures for comparing elements contribution of the samples which are shown in Fig. 5 (a) and (b) are listed in Table 2. The elemental analysis indicates that by increasing the number of interacting pulses from five to ten, the weights of the carbon and oxygen percent are increased. The carbon and oxygen atoms have more time to be captured on the surface due to their longer melting time. Furthermore, by increasing the laser fluence, the carbon and oxygen percent is also increased due to the deeper ablation depth and an effective ablation [5]. When the fifty pulses impinged to the silicon surface, the well-known bead-like structures that are firstly introduced by Shen et al. [14] observed. Fig. 6 shows the SEM images of the silicon surface after interaction with fifty femtosecond laser pulses with various fluences of 𝐹 = 0.45 − 0.67 J.cm−2 . As can be seen, by increasing the laser fluence, the borders between created structures become clear, and the diameter of the hills increase considerably. The evolution of the bead-like structures only by increasing the laser fluence may be interpreted as a feeding mechanism which means the laser pulse energy provides the necessary
supply for the creation of the micro-structures on the surface [13]. In this way, spatially modulated heating of the surface leads to the formation of ripples on the surface. Role of the absorbed laser pulse energy for controlling the self-organized structure is due to the so-called Radiation Remnants (RR) that is capable of extracting energy from the incident radiation and transferring it to the material [36]. The laser fluence plays an important role in the size of the beadlike structures. The diameters of the self-organized surface structures strongly determine the structures’ stability. Moreover, the increase in the diameters of surface structures has a limitation. By increasing the laser fluence, the melted area will increase while the surface tension cannot preserve the structure stability. As illustrated in Fig. 6, by increasing the diameter of the bead-like structures gradually, their geometric shape changes from round to monotonous hexagonal which is more stable. Fig. 7 (a) shows the created monotonous hexagonal structures on the surface. In nature, honey bees use this fact to make their hive. Therefore, the size of the micro-structures is under control. Another important observation is that along with the increasing of the micro-structures diameter; some nanostructures are formed over them. Fig. 7 (b) shows the nanostructures formed on the microstructures by zoom out on the previously displayed structure in Fig. 6 k). The created hierarchical structure contains nanostructures that are formed on the microstructures so the micro and nanostructures have the same surface pattern. It seems that a fractal-like structure is gradually formed on the surface. In this research, a more investigation was performed using the one hundred number of pulses interacting with the silicon surface. The spikelike structures created with a hundred number of pulses is shown in Fig. 8 for different laser fluence of 𝐹 = 0.5 − 1 J.cm−2 . Fig. 8 indicates that by increasing the laser fluence, the spikes diameters are increased and also the deep holes are created gradually. These deep grooves in higher laser fluences regime may be created due to the progressive aggregation of nanoparticles generated during the ablation process which can decorating the structure by further modulation of the deposited energy [37]. Finally, by further increasing the number of pulses to 500 with a fluence of 𝐹 = 1 J.cm−2 , the hills and holes are created efficiently on the silicon surface. The holes creation on the surface has repeated in different lines while getting away from the center of the laser pulse focusing area, as can be seen in Fig. 9, the diameter of the created holes and hills decreases rapidly. This observation gives two different points. First, due
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
Optics and Lasers in Engineering 128 (2020) 106008
Fig. 10. SEM images of the a) and b) holes at the edge and center of laser focusing area respectively and c) and d) are the hills at the edge and center of laser focusing area respectively which they are created on the silicon surface after interaction with five hundred number of femtosecond laser pulses with fluence of 𝐹 = 1.5 J.cm−2 .
to the Gaussian shape of the laser pulses, while the laser fluence decrease in the edge of the focusing area, the holes’ diameter decrease too. Second, a different line of ordered holes on the surface shows that an energy distribution on the surface is responsible for the position of created holes. We suppose that this modulation on laser energy comes from the diffraction patterns created by various optical elements which laser pulse has to pass from them such as pin-hole, lens, and mirrors. The more magnified pictures of the hills and holes in Fig. 9 (b) show that the sub-structures with nanometric scales are created on the hills and holes. Fig. 10 shows the two apparent kinds of holes and hills formed on the silicon surface by using 500 number of pulses with a fluence of 𝐹 = 1.5 J.cm−2 . The two kinds of holes are created at the edge and the center of the laser focusing area are shown in Fig. 10 a) and b) respectively. These holes have a layered wall. The number of layers and craters decrease by increasing the distances from the center of the pulse. The most important parameter which is responsible for the number of layers and the opening diameter is the laser fluence. This issue is repeated for the hills created in the same conditions besides the holes. Fig. 10 (c) and (d) indicate the hills formed at the edge and center of laser focusing area respectively. The hills have similar hill-like structures on their surface which are denser at the center. Thus, again the hierarchical or fractal-like structures are fabricated on the silicon surface using multiple numbers of femtosecond laser pulses. 4. Conclusions In this research, effects of the laser fluence and the number of femtosecond laser pulses interacting with the silicon surface were investigated. The morphological studies show that by increasing the number of pulses from five to ten, the structure of the surface gradually changes from ripples to spikes. At the beginning of the formation of the spikes,
the hills and holes have bead-like structures and smoothly separated from each other by the random borders. Moreover, by increasing the number of interacting pulses from ten to one hundred, at a laser fluence above ablation threshold, the hills and holes boundaries become clear and spike-like structures appear on the silicon surface. The slow modification of the surface caused by the increasing the number of interacting pulses indicates that there is a feedback mechanism that strongly depends on the surface-induced patterns. The induced pattern can be a result of light interference on the surface, or it may be due to the balance between the pondermotive force and the stress-strain [22,38,39]. The shape of the induced pattern is determined by the number of interacting laser pulses. From this aspect, the surface modification is strongly dependent on the number of laser pulses have been irradiated on the surface. In the self-induced surface structures created during the femtosecond laser pulse irradiation, if the number of interacting laser pulses is fixed, the hills and holes diameter is enhanced by increasing the laser pulse fluence. Diameter of the micro-structures grows while their stability puts a constraint on their size. While the laser pulse energy increases, the deposit of their energy in the surface increase too. The self-induced structures cannot react to the additional amount of energy by increasing their diameter. Thus, the accumulation of laser energy must appear in another way. Our results indicate that when the laser fluence is increased enough, a new group of self-induced structures is created over the microstructures formed before. The size of the new structures is on the order of tens of nanometers while their pattern is similar to the existing microstructures. These new nano-structures and the previously formed micro-structures have the same geometrical shape. In this hierarchical structures, the microstructures beneath the nanostructures make a fractal-like or a self-similar structure on the surface which are created
R. Goodarzi, F. Hajiesmaeilbaigi and E. Bostandoost
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