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Pre-ablation features formed by focusing a femtosecond laser beam with dual axicons to c-Si in vacuum
Applied Surface Science 261 (2012) 174–181
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Pre-ablation features formed by focusing a femtosecond laser beam with dual axicons to c-Si in vacuum Go Odachi, Kento Hara, Ryosuke Sakamoto, Takashi Yagi ∗ Physics Department, Tokai University, 1117 Kita-kaname, Hiratsuka, Kanagawa, Japan
a r t i c l e
i n f o
Article history: Received 24 April 2012 Received in revised form 15 July 2012 Accepted 25 July 2012 Available online 9 August 2012 Keywords: Femtosecond laser Micromachining Axicon Ripples Silicon Nano-humps
a b s t r a c t The focusing of a 786 nm femtosecond laser beam and a second harmonic at 393 nm with dual axicon optics to the surface of c-Si in a vacuum chamber was performed. We demonstrate the importance of avoiding atmospheric effects in order to form and preserve submicron features associated with laser interactions with the solid surface. A submicron hole was formed at the central spot of the Bessel–Gaussian (BG) beam pattern with the fluence just above the ablation threshold. Pre-ablation features consisted of humps and swells on a scale of ∼100 nm and were formed through the surface modification fluence at the first fringe position of the BG pattern. Laser modified zones were always associated with periodic ripple structures with low and high spatial frequencies during the early stages of laser irradiation. This observation demonstrates the role of the ripple formation in commencing the surface removal processes. Hump and ripple formation mechanisms are considered with respect to volume expansion of the amorphous layer and a conventional model involving the surface electromagnetic wave by taking into account the transient refractive index. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Micromachining with ultra-short pulse lasers has experienced two decades of development, reaching the status of practical applications in the semiconductor and optical network device industry. Reducing the machining dimension to the submicron scale in a controlled manner is a current challenge. As the machining size approaches the submicron to micron range with a sufficiently small heat affected zone, laser fluence must be kept close to the ablation threshold. In this process, one obvious major difficulty is to avoid the periodic structures or ripples created within the laser irradiated zone. A number of research works have focused on ripple formation on dielectrics, metals, and semiconductors with a Ti:Sapphire laser [1–6] and several other wavelengths [1]. Recently, the formation of a rippled structure by laser irradiation with a much higher pulse rate was reported, showing features different from single shot irradiation and up to a 1 kHz pulse rate [7,8]. The formation mechanisms for periodic ripple structures on a solid surface have been considered since the 1970s. A theory established by Sipe et al., which was based on the interference of the incident laser beam and surface-excited electromagnetic (EM) wave and which was later improved by including the transient refractive index, is often used to explain experimental results
∗ Corresponding author. Fax: +81 473 50 2013. E-mail address: [email protected] (T. Yagi). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.135
[6,9,10]. In addition, the importance of the Coulomb explosion was pointed out [11]. Laser-induced rippled structures are now categorized mostly into four groups; low-frequency ripples (LSFL) with a period close to the laser wavelength oriented perpendicular to the incident laser polarization (LSFL-perpendicular) [6 and references herein] and in parallel (LSFL-parallel) [7], and high spatial frequency ripples (HSFL) in two orientations (HSFLperpendicular) [2,5,8,12–15] and (HSFL-parallel) [3,16,17]. Despite significant effort, the formation mechanisms of these periodic structures are still controversial. Since the clean performance of machining is expected in a vacuum [18], the micro drilling of c-Si in a vacuum using axicon optics is continued from our previous work done in air [19]. The experimental scheme, improved to increase the working distance, is employed in the present experiment, and successfully focuses the laser beam to a spot of ∼2 m in a vacuum chamber. Our setup achieves hole formation on a submicron scale without contamination from debris, thereby avoiding the deterioration of the focusing optics by ablation products. The sample surface irradiated with a femtosecond laser in a vacuum vividly accompanies LSFL and HSFL leading to the material removal process from ablation phenomena. By comparing the surface morphology formed with a laser fluence close to the multi-pulse ablation threshold in vacuum and in air, several remarkable features appear. It is shown that the microscopic surface morphology offers insight into the elementary physical processes of laser metamorphism at the laser fluence for surface modification.
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Fig. 1. Experimental setup with neutral density filter F, axicons A1 , A2 , convex lens L, Si plate S, window W, vacuum chamber C, and vacuum pump P. The inset is the laser beam pattern on the Si plate.
2. Experimental An optical system for forming a Bessel beam pattern in a vacuum chamber is shown in Fig. 1. A laser beam enters the first axicon A1 with an opening angle of 160◦ and a cone height of 4 mm in order to form a diverging doughnut-shaped beam. The beam is then focused by a single plano-convex lens L with a focal length of 100 mm through the second axicon A2 at a cone angle of 130◦ and a cone height of 13 mm. A beam pattern with a Bessel–Gaussian (BG) intensity distribution is then formed through the window plate of the vacuum chamber with 8 mm thickness fused silica. The window plate is placed just after the tip of A2 so that the diameter of the ring pattern of the laser beam is large enough to avoid non-linear optical effects. The tilt angle of the conical wave front leaving A2 is 8.3◦ with respect to the optical axis. The peak intensity of the central spot of the BG mode varies gradually along the optical axis, and the maximum intensity is found at 3 cm from the tip of A2 . The variation of the peak intensity is found to be ±15% within an interval of 150 m, which facilitates the setting of the target. A magnified beam pattern at the distance where the central spot intensity peaks from the tip of A2 is observed by a microscope objective with a CCD, as shown in the inset of Fig. 1. A crystalline Si (c-Si) plate cut from a wafer with a (1 0 0) surface orientation (Ntype doped with phosphor, surface roughness Ra < 2 nm) is placed in this position perpendicular to the ground in a vacuum at 0.4 Pa. Accurate positioning of the Si plate is performed by observing optical emission from the laser created plasma by reducing the laser intensity to the ablation threshold level. The Si wafer used in the present experiment has an oxide layer with a thickness of a few nanometers, as measured with electron spectroscopy for chemical analysis (ESCA, ULVAC-PHI Quantum 2000). Optical components are arranged on a simple optical bench, remote from the laser source without an anti-vibration mechanism. The laser is operated at a wavelength of 786 nm, with a pulse duration of 176 fs and a pulse repetition rate of 500 Hz. For machining in the ultraviolet wavelengths (UV), the second harmonic (SH) at 393 nm is obtained by focusing a laser beam on a lithium triborate crystal (LBO) with a thickness of 1 mm. The emerging beams from the crystal are passed through a filter for removing the fundamental wavelength. The pulse duration of the SH is measured to be 133 fs. The laser pulse energy is changed with a neutral density (ND) filter in front of A1 . The exposure shot number is controlled with a mechanical shutter (not shown here). The vacuum chamber is initially evacuated before laser irradiation, and moved horizontally by
2 mm after laser irradiation with pre-defined shot numbers. Air is then introduced and the same procedure for laser irradiation as in a vacuum is repeated. In this experiment, no alteration of the optical alignment in vacuum and in air is assured. The modified pattern on the laser irradiated surface, morphology, and the distribution of the ablation products are observed with a field emission scanning electron microscope (FESEM, Hitachi S4800). The elemental analysis of the ablated zone is performed with the Electron Probe Micro Analyzer (EPMA, Shimadzu EPMA-1610), which has spatial and depth resolutions of ∼1.7 m. 3. Results Fig. 2(a) presents the ablated pattern in vacuum formed by 500 shots with a pulse energy of 0.38 J. The polarization of the laser beam is horizontal throughout the experiment with a fundamental laser wavelength. After ablation, a single hole without debris accumulation, except for a small number of round particles, is formed at the central spot of the BG beam. The hole has a size of 1.2 m × 0.8 m and is covered with nearly periodic trenches (LSFL-perpendicular). The average spacing between the trenches is 590 nm. The faint periodically aligned vertical strips with an average spacing of 240 nm are recognized in the square, which is reproduced with higher contrast in the inset. As the pulse energy increases to 1.01 J, ablated patterns appear at the first fringe position as shown in Fig. 2(b). The laser fluence apparently exceeds the multi-shot ablation threshold of ∼0.2 J/cm2 [20] at this location. The LSFL-perpendicular is formed clearly without molten features, Fig. 3(a) shows the ablated pattern formed after 500 laser shots with pulse energy of 0.63 J. The first fringe is subject to the fluence of surface modification, as indicated by the laser affected zone marked by A, where a contrast enhance view is shown in the inset. Since the surface removal process starts here by increasing the laser shots, the laser fluence should be ∼0.15 J/cm2 in this area [21]. Fig. 3(b) shows the result of 2000 laser shots, where vertical trenches are over-written with fine horizontal regular patterns with a spatial period of ∼130 nm (HSFL-parallel) formed at B and C. The trenches are separated by 540 nm, forming LSFL-perpendicular at B. These results suggest that HSFL-parallel appears prior to the formation of LSFL-perpendicular. The fine details of the area A of Fig. 3(a) are presented in Fig. 3(c) with a higher magnification. The round particles distributing over the whole area shown are formed from the ablation of the main crater at the central spot. The laser affected zone is
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Fig. 2. Ablation patterns in vacuum (a) by the central peak of the BG beam with pulse energy of 0.38 J at a786 nm wavelength after 500 shots, where the inset is the contrast enhanced view of the squared area, (b) by the central and the first fringe with a pulse energy of 1.02 J after 2000 shots.
featured with feeler-like patterns at the fringe of the laser affected oval region. These patterns are separated by ∼160 nm and ∼220 nm, as indicated by the vertical and oblique double arrows, respectively. Further increasing the laser shot number to 1000 makes this feature more prominent, as shown in Fig. 3(d). The fronts of the feelers are terminated by humps with cracks at the top. The long axis of the oval humps and the direction of the cracks are mostly horizontal, except for the upper part of the laser affected zone. Dark stripe patterns parallel to the laser polarization are distributed periodically with a spacing of ∼160 nm as shown by the circle D. Fig. 3(e) shows a further magnified view of this area, where dots encircling the laser affected zone in Fig. 3(b) and (d) are the humps with cracks such as the one denoted by E. In addition, dark horizontally long ovals look as though they were formed by peeling “skin” away (F) with the bulge (G), as well. The humps, which are not as prominent as E, also show cracks in the horizontal direction. Fig. 3(f) presents a magnified view of the laser affected zone in Fig. 3(b). Removing surface material mostly begins in the central region. This result shows that removing the surface layer proceeds from the formation of the humps. The borders of the laser affected and non-affected zones in the circled area H of Fig. 3(f) are further magnified in Fig. 3(g), showing how the boundary, as indicated by I, is clearly distinguishable by the entirely different morphologies of both zones. Here, the surface is undulating in the laser affected zone and falls off at the boundary.
The bulges, after the top layer is removed, grow and bunch as the one indicated by J. The performance of the drilling of Si in air at 1 atm is compared with the results in vacuum by introducing air into the vacuum chamber. The ablated pattern with 500 shots of 0.63 J pulse energy is presented in Fig. 4(a), which was obtained with the same laser irradiation conditions as in vacuum, shown in Fig. 3(a). The results in air show a clear contrast to the ablated pattern in vacuum. A significant amount of grain-like debris and ripple patterns with an average spatial period of ∼520 nm perpendicular to the laser polarization appear at the first fringe position. However, grainlike debris accumulated around the ablated hole is less abundant around the rippled area. This situation is evident in the magnified image of the circled area, as shown in Fig. 4(b). The ripple structure of the trenches is formed from a molten surface. Parts A and B of Fig. 5 present the oxygen content covering the ablated area in air (Fig. 4(a)), and in vacuum (Fig. 3(a)), respectively, observed with EPMA. In this figure H and R denote the ablated holes and rippled patterns, respectively. The result in vacuum shows that laser irradiation does not change the composite distribution around the hole. The oxygen level as a background is due to the native oxide layer. The thick accumulation of grain-like debris formed in air is found to contain an oxygen-rich compound. The rippled zone also shows less debris but some sort of oxygen compound. Our separate experiments on the grain-like debris are performed; these involve collecting the debris and measuring these pieces with a transmission electron microscope (TEM) and electron diffraction (EDX). The results show that the grain-like debris consists of polycrystalline Si particles tens of nanometers in size linked with oxygen-rich amorphous material [22]. Irradiating SH on the surface of the c-Si results in the unique features in the laser affected zone. In this experiment, three patched areas, modified with laser irradiation, are formed in the central spot region of ∼2 m in diameter. Fig. 6(a) presents the surface morphology of one of the ablated zones formed by 500 shots of the SH pulses with 1.9 J of pulse energy in vacuum. The pulse energy in this experiment is larger than that using the fundamental wavelength because the optical alignment was not in the optimal conditions for SH. We found that the fluence at the laser affected zone is just above the surface modification threshold, judging from the surface morphology after multi-shot laser irradiation. The laser affected zone is featured with the humps linked with filamentary strings. The humps have a size of ∼30 nm with cracks formed at the top. The links of the humps are arranged perpendicular to the laser polarization, showing a weak regularity with a spatial period of ∼60 nm. A slight increase of the pulse energy to 2.12 J enhances the size of the humps at the center of the pattern as shown in Fig. 6(b). In this figure the ranges of humps show fractal features at the border of the laser affected region. As the laser shots are increased to 4000, the central region, as shown in Fig. 6(c), looks darkened, and is ablated. The surface layer is removed, and a roundish object appears as indicated by A, and what looks like vacant holes, as indicated by the arrows. Fig. 6(d) shows the crater formed in the center of the laser affected zone by 6000 laser shots. The ablation crater reveals ragged features, while Fig. 6(c) and (d) shows that the debris consists of mostly smooth and nearly round particles, and does not resemble the features of the crater. In these observations, the surface between the filamentary strings does not look to have changed from the original surface in the peripheral region, where the effect of laser irradiation is weak.
4. Discussion The various fluence regions are distinguished from the intensity distribution of the BG beam pattern from well above the ablation
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Fig. 3. Ablation patterns by the pulse energy of 0.63 J, with the hole formed by the central peak of the BG beam and the laser affected zone at the first fringe indicated by A after (a) 500 shots (the contrast enhanced view of A is shown in the inset) and (b) 2000 shots. (c) Magnified image of the laser affected zone A of (a). (d) The laser affected zone after 1000 shots. (e) Magnified image of the circled area D of (d). (f) Magnified image of the laser affected zone in (b). (g) Further magnified image of the circled area H of (f).
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threshold at the central spot to the surface modification level in the outer fringes. The laser fluence in the central spot region creates a submicron hole without any debris accumulation in vacuum, as shown in Fig. 2(a). In contrast, a hole is surrounded by a dense accumulation of debris for experiments performed in air, as shown in Fig. 4(a) and our previous work [19]. However, this hole is covered with LSFL-perpendicular, which always appears and is preserved under certain fluence condition. The morphology of the Si surface in the outer fringe, where the laser fluence is just above the surface modification threshold, highlights some elementary processes involved in the laser-matter interaction. The first fringe of the BG beam, labeled by A in Fig. 3(a), formed by 500 shots with a pulse energy of 0.63 J shows nascent surface modification and removal. This area consists of the rim region surrounded by oval humps, and expands with a clear boundary with arrays of humps increasing with laser shot number (Fig. 3(b)). These arrays of dots were reported by other authors for multi-laser shots [3,23,24] and single pulse irradiation on c-Si [25]. The humps exist as isolated islands in the feeler-like structures extending out from the rim of the laser affected zone. Similar feeler-like patterns oriented perpendicular to the laser polarization are seen in the circle of Fig. 2(a). After laser irradiation with an increased cumulative shot number, the tips of the feelers are terminated at humps (Fig. 3(d)). These humps frequently grow to form periodic ripple patterns. Fig. 3(f) and (g) shows that the laser affected zone is covered with a range of humps, clearly marking the border to the virgin surface of the flat plane. As the laser irradiation proceeds, the laser affected zone is filled with swells and humps, covered with cracks. The surface removal process starts from these cracks, as shown in Fig. 3(f). The mechanism required to explain the features of the laser affected pattern in the present results might be due to the combined effects of hot spots, which resemble gray spots in the darker zones in Fig. 3(c) and the feelers, which may indicate the appearance of heat flow. Hot spots can be formed by surface irregularity and/or a locally enhanced EM field, which is supposedly responsible for HSFL formation. Once the hot spots are formed, laser light is absorbed more efficiently. The dark peripheral zones surrounding the hot spots might be due to a change to an amorphous state caused by the thermal energy. In addition, heat flow links the spots to form feeler-like patterns in Fig. 3(c) and (d). There is a possibility that the c-Si under solid SiO2 melts and re-solidifies, forming an amorphous state in the laser affected zone A of Fig. 3(a), since the melting
Fig. 4. (a) Ablated patterns under identical fluence conditions as Fig. 3 after 500 shots in air at 1atm. (b) Magnified image of the circled area of (a).
temperature of a-SiO2 is higher than that of a-Si [25]. The lower density of a-Si compared to c-Si, a ≈ 0.97c [26], should induce a volume expansion of the surface layer after re-solidification, leading to the humps and winkles as seen in Fig. 3(g). The oxide layer cracks while the bulge in the hump grows. The oxide layer is then
Fig. 5. Distribution of an oxygen compound obtained by EPMA, (a) in the vicinity of the ablated hole of Fig. 4(a), and (b) in the vicinity of the ablated hole of Fig. 3(a). H and R indicate holes and ripples, respectively.
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Fig. 6. Ablation patterns in vacuum by the central peak of the BG beam with (a) a pulse energy of 1.9 J at 393 nm wavelength after 500 shots, and with a pulse energy of 2.12 J after (b) 2000 shots, (c) 4000 shots, (d) 6000 shots.
ripped off by the evaporating gas. The importance of the oxide layer in the formation of the humps or dots was pointed out in the earlier work, showing that these dots were not formed without the oxide layer [24]. The bulges show a smooth surface, probably as a result of melting. The phase change from c-Si to a-Si requires some threshold of deposited heat energy, clearly marking the boundary between the laser affected and non-affected zones, as seen in Fig. 3(g). After the skin covering the bulges is ripped off, thin oval patterns parallel to the laser polarization remain to form the periodic ripples. The present results indicate that the periodic ripple structures initiate the surface removal process with the laser modification fluence with a wavelength of 786 nm. The ripples formed after laser irradiation show periods of 590 nm in Fig. 2(a) and 540 nm in Fig. 3(b) (LSFL-perpendicular) and those with ∼130 nm in Fig. 3(b) and ∼160 nm (HSFL-parallel) in Fig. 3(e). Sipe’s theory [9], rearranged by including the transient refractive index by Bonse [6,10], predicts a period for HSFL-parallel of ∼ /n, where n is the transient refractive index of the target, as the free electron density is ∼2 × 1027 m−3 [6]. This free electron density is produced by 0.25 J/cm2 of excitation, as calculated from linear and two-photon excitations [6], which is close to the multi-shot ablation threshold of 0.2 J/cm2 [20]. If we quote n ∼ 3 for this fluence [10], then ∼260 nm is obtained for HSFL-parallel. The period calculated in this scheme does not reproduce the ripple period of 130 nm and 160 nm, even using n ∼ 3.67 for non-excited Si. Instead, ∼/2n [11,27] provides ∼131 nm. The formation of HSFL-parallel along the border of the laser affected zone in the present results is limited to a narrow range of laser fluence and shot number. In contrast, LSFL-perpendicular becomes more prominent as the free electron density is increased. It was previously reported that the spatial period of LSFL-perpendicular decreases with a reduction in fluence but with an increase in the shot number; for example, it is ∼570 nm
at 500 shots of ∼0.2 J/cm2 [28]. The 590 nm period in the central spot (Fig. 2(a)) and 540 nm period in the hole periphery (Fig. 3(b)) are consistent with the results of Ref. [28]. Periodicities similar to LSFL and HSFL of the present results are also reported by other authors [17]. Laser irradiation in air at 1 atm under identical fluence and shot number as in vacuum results in different features in the ablation pattern. LSFL-perpendicular is formed at the first fringe position in air (Fig. 4(a)), while the same fluence induces a phase change without forming similar ripple structures in vacuum (Fig. 3(a)). The surface of the hole edge and rippled zone appear molten, indicating that these surfaces have transformed to a liquid state. As a result, the trenches in the rippled zone are filled with molten material. The chemical composition of the rippled zone formed in air (part A of Fig. 5) shows a weak oxidation, probably due to the formation of an oxide layer after melting and re-solidification of the Si surface in the presence of air. The period of ∼520 nm is consistent with the result obtained by 500 shots of laser irradiation with 0.15–0.25 J/cm2 in Ref. [28]. Increased residual thermal energy in the sample after laser irradiation in air [29–31] might reduce the threshold for ripple formation and also yield the present result. The preference for the formation of ripples on c-Si in air at 1 atm was also reported by other authors [14]. The surface modification by SH at the present fluence shows some unique features. Instead of the feeler-like patterns, thin filamentary strings with small humps are aligned with a ∼60 nm periodicity perpendicular to the laser polarization (HSFLperpendicular in Fig. 6(a)). This result sharply contrasts to that of a ∼160 nm periodicity aligned parallel to the laser polarization formed by the fundamental wavelength. The periodicity can be predicted to be /n [16] or /2n [5,8,27] where n stands for the refractive index of excited c-Si at the wavelength of SH. The
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Fig. 7. Refractive index, extinction coefficient, and reflectivity vs. transient free carrier density. Top: refractive index n, middle: extinction coefficient k, and bottom: refractive index R.
refractive index should be altered from the non-excited c-Si due to the significant fraction of the electrons excited from the valence band to the conduction band during and after laser irradiation. The refractive index n and extinction coefficient k, which compose the complex refractive index as nˆ = n + ik, are calculated from the real εˆ = ε + iε , leadand imaginary parts of the dielectric permittivity
ing to n = ( ε 2 + ε 2 + ε )/2 and k = ( ε 2 + ε 2 − ε )/2. ε and ε are obtained from the contributions from the crystalline and the Drude electron–hole plasma: [32] ε = 1 −
ε =
(εg − 1)(N0 − N) e2 N + N0 ε0 m∗opt me (ω2 + −2 ) e2 N
ε0 m∗opt me ω ω2 2 + 1
+ εg
N0 − N N0
wavelength of 440 nm was reported to be 0.1 J/cm2 , resulting in the reduction of the reflection coefficient from that of the non-excited surface by a factor of 0.97 [35]. The free electron density is calculated as 1.6 × 1027 m−3 from the reduced reflection coefficient for a wavelength of 440 nm. If this electron density is accepted for the excitation by the SH (393 nm), which is indicated by the vertical dotted lines in Fig. 7, then a refractive index of 5.74 is obtained. Hence, the resultant ripple period, calculated by /n, is ∼68 nm, which is in the range of the observed ripple period of ∼60 nm. The primary mechanism of forming the filamentary structures appears to share the same origin as the periodical ripples described by the conventional model based on the interference between incident and surface EM waves. However, the formation mechanism of the fractal features at the border of the laser affected region is not identified. From the present measurements, the early stages of the surface removal process with the surface modification fluence at the SH wavelength is initiated by forming periodically aligned filamentary structures with locally formed nano-humps, terminated with fractal patterns. The central area of the laser affected zone darkens (Fig. 6(c)), indicating a phase change is occurring beneath the surface layer. Since the filamentary strings blur within the darkened area, the peeling of the surface does not seem to start from the cracks on the strings. Instead, surface peeling begins by forming spout holes, presumably due to the evaporation of gas. The material removal process starts at this moment. This removal process does not include the formation of the melt pool, and instead leads to the formation of ragged features on the crater bottom (Fig. 6(d)). However, the debris is mostly round. The larger-sized particles seem to originate from the roundish objects formed under the surface, while the smaller particles come from the tips of the crests after repetitive melting and re-solidification. These morphologies sharply contrast with the rugged shapes of the fine particles shown in our previous work in air [19]. The apparent difference in the early stage of the ablation process with a wavelength of 786 nm and SH is summarized: the surface layer lifts up to form humps, then cracks and the peeling of the surface skin layer initiate the material removal process. The traces resembling heat flow link the humps to the laser affected zone. Under the proper fluence conditions, the periodically formed humps are left as ripples. The SH forms periodically aligned filamentary structures first, which are then obscured during surface metamorphism. Material removal starts from the spout holes formed in the metamorphic region. These different surface responses might be brought about by the thermal effect of phonons due to an indirect transition from the valence to conduction bands in c-Si at a wavelength of 786 nm. This mechanism is less significant for the SH due to the higher photon energy.
(1) 5. Conclusion (2)
where e, me , m∗opt , , ε0 , ω, N0 , and N are the charge and mass of an electron, m∗opt = 0.18, the reduced effective mass of the elec-
tron and hole, ∼1.1 × 10−15 s for the relaxation time of the electron gas, dielectric permittivity of vacuum, 2.5 × 1015 rad/s for the angular frequency of SH, ∼2 × 1029 m−3 for the electron density in the valence band [31–33], and the free electron density excited to the conduction band, respectively. The real and imaginary parts of the dielectric permittivity, εg and εg for non-excited c-Si are 34.1 and 6.0, respectively, calculated from the refractive index and extinction coefficient in Ref. [34]. Fig. 7 presents the calculated n, k, and the reflection coefficient R, R = ((n − 1)2 + k2 )/((n + 1)2 + k2 ), vs. N. The single-shot melting threshold for a thin Si film at an irradiation
The optical arrangement of dual conical axicons successfully produces submicron sized holes in c-Si within a vacuum chamber. The Bessel–Gaussian beam formed by the present optical system yields a variety of elementary processes associated with the modification and removal of surface material, depending on the characteristic fluences at various fringe positions. Laser irradiation in vacuum is meritorious to the formation and preservation of featured patterns resulting from the laser excitation of the surface by avoiding the deteriorating effect of air. The surface corrugation patterns produced by laser irradiation consist of nanometer scale humps and periodic ripples. LSFLperpendicular with a period of ∼590 nm appears prior to the formation of a hole with the laser fluence above the ablation threshold. HSFL-parallel forms with a period of ∼160 nm as scars of
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removed humps at the fluence for surface modification with an excitation wavelength of 786 nm. The humps and wrinkles in the laser affected zone might be due to the volume expansion of the amorphous layer. A surface irradiated with the SH shows HSFLperpendicular with a period of ∼60 nm, consisting of filamentary strings linking small cracked humps before material removal. The corrugated pattern shows fractal features at the border of the laser affected area. The spatial period of HSFL-parallel formed at a wavelength of 786 nm and HSFL-perpendicular by the SH are reasonably explained by /2n and /n, respectively. After multi-shot irradiation by SH, spout holes are formed and a roundish object appears in the opening, leading to material removal and formation of round debris particles. Laser irradiation in air at 1 atm leads to the formation of LSFLperpendicular with molten features at a lower fluence than that for experiments conducted in vacuum. The grain-like debris produced in the ablation is oxygen-rich, and is partially removed where ripples are formed. LSFL-perpendicular formed at the central spot and the outer fringe in vacuum or air seems to reveal the dependence of periodicity on fluence. A smaller period appears with decreasing fluence and increasing shot number [28]. It seems that a large fluence results in a long period; however, the shot number is required to be small to avoid deterioration of the ripple pattern. On the other hand, a reduced fluence results in a short period; in this case, the shot number needs to be large enough to form the periodical trenches. References [1] [2] [3] [4]
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Pre-ablation features formed by focusing a femtosecond laser beam with dual axicons to c-Si in vacuum Go Odachi, Kento Hara, Ryosuke Sakamoto, Takashi Yagi ∗ Physics Department, Tokai University, 1117 Kita-kaname, Hiratsuka, Kanagawa, Japan
a r t i c l e
i n f o
Article history: Received 24 April 2012 Received in revised form 15 July 2012 Accepted 25 July 2012 Available online 9 August 2012 Keywords: Femtosecond laser Micromachining Axicon Ripples Silicon Nano-humps
a b s t r a c t The focusing of a 786 nm femtosecond laser beam and a second harmonic at 393 nm with dual axicon optics to the surface of c-Si in a vacuum chamber was performed. We demonstrate the importance of avoiding atmospheric effects in order to form and preserve submicron features associated with laser interactions with the solid surface. A submicron hole was formed at the central spot of the Bessel–Gaussian (BG) beam pattern with the fluence just above the ablation threshold. Pre-ablation features consisted of humps and swells on a scale of ∼100 nm and were formed through the surface modification fluence at the first fringe position of the BG pattern. Laser modified zones were always associated with periodic ripple structures with low and high spatial frequencies during the early stages of laser irradiation. This observation demonstrates the role of the ripple formation in commencing the surface removal processes. Hump and ripple formation mechanisms are considered with respect to volume expansion of the amorphous layer and a conventional model involving the surface electromagnetic wave by taking into account the transient refractive index. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Micromachining with ultra-short pulse lasers has experienced two decades of development, reaching the status of practical applications in the semiconductor and optical network device industry. Reducing the machining dimension to the submicron scale in a controlled manner is a current challenge. As the machining size approaches the submicron to micron range with a sufficiently small heat affected zone, laser fluence must be kept close to the ablation threshold. In this process, one obvious major difficulty is to avoid the periodic structures or ripples created within the laser irradiated zone. A number of research works have focused on ripple formation on dielectrics, metals, and semiconductors with a Ti:Sapphire laser [1–6] and several other wavelengths [1]. Recently, the formation of a rippled structure by laser irradiation with a much higher pulse rate was reported, showing features different from single shot irradiation and up to a 1 kHz pulse rate [7,8]. The formation mechanisms for periodic ripple structures on a solid surface have been considered since the 1970s. A theory established by Sipe et al., which was based on the interference of the incident laser beam and surface-excited electromagnetic (EM) wave and which was later improved by including the transient refractive index, is often used to explain experimental results
∗ Corresponding author. Fax: +81 473 50 2013. E-mail address: [email protected] (T. Yagi). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.135
[6,9,10]. In addition, the importance of the Coulomb explosion was pointed out [11]. Laser-induced rippled structures are now categorized mostly into four groups; low-frequency ripples (LSFL) with a period close to the laser wavelength oriented perpendicular to the incident laser polarization (LSFL-perpendicular) [6 and references herein] and in parallel (LSFL-parallel) [7], and high spatial frequency ripples (HSFL) in two orientations (HSFLperpendicular) [2,5,8,12–15] and (HSFL-parallel) [3,16,17]. Despite significant effort, the formation mechanisms of these periodic structures are still controversial. Since the clean performance of machining is expected in a vacuum [18], the micro drilling of c-Si in a vacuum using axicon optics is continued from our previous work done in air [19]. The experimental scheme, improved to increase the working distance, is employed in the present experiment, and successfully focuses the laser beam to a spot of ∼2 m in a vacuum chamber. Our setup achieves hole formation on a submicron scale without contamination from debris, thereby avoiding the deterioration of the focusing optics by ablation products. The sample surface irradiated with a femtosecond laser in a vacuum vividly accompanies LSFL and HSFL leading to the material removal process from ablation phenomena. By comparing the surface morphology formed with a laser fluence close to the multi-pulse ablation threshold in vacuum and in air, several remarkable features appear. It is shown that the microscopic surface morphology offers insight into the elementary physical processes of laser metamorphism at the laser fluence for surface modification.
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Fig. 1. Experimental setup with neutral density filter F, axicons A1 , A2 , convex lens L, Si plate S, window W, vacuum chamber C, and vacuum pump P. The inset is the laser beam pattern on the Si plate.
2. Experimental An optical system for forming a Bessel beam pattern in a vacuum chamber is shown in Fig. 1. A laser beam enters the first axicon A1 with an opening angle of 160◦ and a cone height of 4 mm in order to form a diverging doughnut-shaped beam. The beam is then focused by a single plano-convex lens L with a focal length of 100 mm through the second axicon A2 at a cone angle of 130◦ and a cone height of 13 mm. A beam pattern with a Bessel–Gaussian (BG) intensity distribution is then formed through the window plate of the vacuum chamber with 8 mm thickness fused silica. The window plate is placed just after the tip of A2 so that the diameter of the ring pattern of the laser beam is large enough to avoid non-linear optical effects. The tilt angle of the conical wave front leaving A2 is 8.3◦ with respect to the optical axis. The peak intensity of the central spot of the BG mode varies gradually along the optical axis, and the maximum intensity is found at 3 cm from the tip of A2 . The variation of the peak intensity is found to be ±15% within an interval of 150 m, which facilitates the setting of the target. A magnified beam pattern at the distance where the central spot intensity peaks from the tip of A2 is observed by a microscope objective with a CCD, as shown in the inset of Fig. 1. A crystalline Si (c-Si) plate cut from a wafer with a (1 0 0) surface orientation (Ntype doped with phosphor, surface roughness Ra < 2 nm) is placed in this position perpendicular to the ground in a vacuum at 0.4 Pa. Accurate positioning of the Si plate is performed by observing optical emission from the laser created plasma by reducing the laser intensity to the ablation threshold level. The Si wafer used in the present experiment has an oxide layer with a thickness of a few nanometers, as measured with electron spectroscopy for chemical analysis (ESCA, ULVAC-PHI Quantum 2000). Optical components are arranged on a simple optical bench, remote from the laser source without an anti-vibration mechanism. The laser is operated at a wavelength of 786 nm, with a pulse duration of 176 fs and a pulse repetition rate of 500 Hz. For machining in the ultraviolet wavelengths (UV), the second harmonic (SH) at 393 nm is obtained by focusing a laser beam on a lithium triborate crystal (LBO) with a thickness of 1 mm. The emerging beams from the crystal are passed through a filter for removing the fundamental wavelength. The pulse duration of the SH is measured to be 133 fs. The laser pulse energy is changed with a neutral density (ND) filter in front of A1 . The exposure shot number is controlled with a mechanical shutter (not shown here). The vacuum chamber is initially evacuated before laser irradiation, and moved horizontally by
2 mm after laser irradiation with pre-defined shot numbers. Air is then introduced and the same procedure for laser irradiation as in a vacuum is repeated. In this experiment, no alteration of the optical alignment in vacuum and in air is assured. The modified pattern on the laser irradiated surface, morphology, and the distribution of the ablation products are observed with a field emission scanning electron microscope (FESEM, Hitachi S4800). The elemental analysis of the ablated zone is performed with the Electron Probe Micro Analyzer (EPMA, Shimadzu EPMA-1610), which has spatial and depth resolutions of ∼1.7 m. 3. Results Fig. 2(a) presents the ablated pattern in vacuum formed by 500 shots with a pulse energy of 0.38 J. The polarization of the laser beam is horizontal throughout the experiment with a fundamental laser wavelength. After ablation, a single hole without debris accumulation, except for a small number of round particles, is formed at the central spot of the BG beam. The hole has a size of 1.2 m × 0.8 m and is covered with nearly periodic trenches (LSFL-perpendicular). The average spacing between the trenches is 590 nm. The faint periodically aligned vertical strips with an average spacing of 240 nm are recognized in the square, which is reproduced with higher contrast in the inset. As the pulse energy increases to 1.01 J, ablated patterns appear at the first fringe position as shown in Fig. 2(b). The laser fluence apparently exceeds the multi-shot ablation threshold of ∼0.2 J/cm2 [20] at this location. The LSFL-perpendicular is formed clearly without molten features, Fig. 3(a) shows the ablated pattern formed after 500 laser shots with pulse energy of 0.63 J. The first fringe is subject to the fluence of surface modification, as indicated by the laser affected zone marked by A, where a contrast enhance view is shown in the inset. Since the surface removal process starts here by increasing the laser shots, the laser fluence should be ∼0.15 J/cm2 in this area [21]. Fig. 3(b) shows the result of 2000 laser shots, where vertical trenches are over-written with fine horizontal regular patterns with a spatial period of ∼130 nm (HSFL-parallel) formed at B and C. The trenches are separated by 540 nm, forming LSFL-perpendicular at B. These results suggest that HSFL-parallel appears prior to the formation of LSFL-perpendicular. The fine details of the area A of Fig. 3(a) are presented in Fig. 3(c) with a higher magnification. The round particles distributing over the whole area shown are formed from the ablation of the main crater at the central spot. The laser affected zone is
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Fig. 2. Ablation patterns in vacuum (a) by the central peak of the BG beam with pulse energy of 0.38 J at a786 nm wavelength after 500 shots, where the inset is the contrast enhanced view of the squared area, (b) by the central and the first fringe with a pulse energy of 1.02 J after 2000 shots.
featured with feeler-like patterns at the fringe of the laser affected oval region. These patterns are separated by ∼160 nm and ∼220 nm, as indicated by the vertical and oblique double arrows, respectively. Further increasing the laser shot number to 1000 makes this feature more prominent, as shown in Fig. 3(d). The fronts of the feelers are terminated by humps with cracks at the top. The long axis of the oval humps and the direction of the cracks are mostly horizontal, except for the upper part of the laser affected zone. Dark stripe patterns parallel to the laser polarization are distributed periodically with a spacing of ∼160 nm as shown by the circle D. Fig. 3(e) shows a further magnified view of this area, where dots encircling the laser affected zone in Fig. 3(b) and (d) are the humps with cracks such as the one denoted by E. In addition, dark horizontally long ovals look as though they were formed by peeling “skin” away (F) with the bulge (G), as well. The humps, which are not as prominent as E, also show cracks in the horizontal direction. Fig. 3(f) presents a magnified view of the laser affected zone in Fig. 3(b). Removing surface material mostly begins in the central region. This result shows that removing the surface layer proceeds from the formation of the humps. The borders of the laser affected and non-affected zones in the circled area H of Fig. 3(f) are further magnified in Fig. 3(g), showing how the boundary, as indicated by I, is clearly distinguishable by the entirely different morphologies of both zones. Here, the surface is undulating in the laser affected zone and falls off at the boundary.
The bulges, after the top layer is removed, grow and bunch as the one indicated by J. The performance of the drilling of Si in air at 1 atm is compared with the results in vacuum by introducing air into the vacuum chamber. The ablated pattern with 500 shots of 0.63 J pulse energy is presented in Fig. 4(a), which was obtained with the same laser irradiation conditions as in vacuum, shown in Fig. 3(a). The results in air show a clear contrast to the ablated pattern in vacuum. A significant amount of grain-like debris and ripple patterns with an average spatial period of ∼520 nm perpendicular to the laser polarization appear at the first fringe position. However, grainlike debris accumulated around the ablated hole is less abundant around the rippled area. This situation is evident in the magnified image of the circled area, as shown in Fig. 4(b). The ripple structure of the trenches is formed from a molten surface. Parts A and B of Fig. 5 present the oxygen content covering the ablated area in air (Fig. 4(a)), and in vacuum (Fig. 3(a)), respectively, observed with EPMA. In this figure H and R denote the ablated holes and rippled patterns, respectively. The result in vacuum shows that laser irradiation does not change the composite distribution around the hole. The oxygen level as a background is due to the native oxide layer. The thick accumulation of grain-like debris formed in air is found to contain an oxygen-rich compound. The rippled zone also shows less debris but some sort of oxygen compound. Our separate experiments on the grain-like debris are performed; these involve collecting the debris and measuring these pieces with a transmission electron microscope (TEM) and electron diffraction (EDX). The results show that the grain-like debris consists of polycrystalline Si particles tens of nanometers in size linked with oxygen-rich amorphous material [22]. Irradiating SH on the surface of the c-Si results in the unique features in the laser affected zone. In this experiment, three patched areas, modified with laser irradiation, are formed in the central spot region of ∼2 m in diameter. Fig. 6(a) presents the surface morphology of one of the ablated zones formed by 500 shots of the SH pulses with 1.9 J of pulse energy in vacuum. The pulse energy in this experiment is larger than that using the fundamental wavelength because the optical alignment was not in the optimal conditions for SH. We found that the fluence at the laser affected zone is just above the surface modification threshold, judging from the surface morphology after multi-shot laser irradiation. The laser affected zone is featured with the humps linked with filamentary strings. The humps have a size of ∼30 nm with cracks formed at the top. The links of the humps are arranged perpendicular to the laser polarization, showing a weak regularity with a spatial period of ∼60 nm. A slight increase of the pulse energy to 2.12 J enhances the size of the humps at the center of the pattern as shown in Fig. 6(b). In this figure the ranges of humps show fractal features at the border of the laser affected region. As the laser shots are increased to 4000, the central region, as shown in Fig. 6(c), looks darkened, and is ablated. The surface layer is removed, and a roundish object appears as indicated by A, and what looks like vacant holes, as indicated by the arrows. Fig. 6(d) shows the crater formed in the center of the laser affected zone by 6000 laser shots. The ablation crater reveals ragged features, while Fig. 6(c) and (d) shows that the debris consists of mostly smooth and nearly round particles, and does not resemble the features of the crater. In these observations, the surface between the filamentary strings does not look to have changed from the original surface in the peripheral region, where the effect of laser irradiation is weak.
4. Discussion The various fluence regions are distinguished from the intensity distribution of the BG beam pattern from well above the ablation
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Fig. 3. Ablation patterns by the pulse energy of 0.63 J, with the hole formed by the central peak of the BG beam and the laser affected zone at the first fringe indicated by A after (a) 500 shots (the contrast enhanced view of A is shown in the inset) and (b) 2000 shots. (c) Magnified image of the laser affected zone A of (a). (d) The laser affected zone after 1000 shots. (e) Magnified image of the circled area D of (d). (f) Magnified image of the laser affected zone in (b). (g) Further magnified image of the circled area H of (f).
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threshold at the central spot to the surface modification level in the outer fringes. The laser fluence in the central spot region creates a submicron hole without any debris accumulation in vacuum, as shown in Fig. 2(a). In contrast, a hole is surrounded by a dense accumulation of debris for experiments performed in air, as shown in Fig. 4(a) and our previous work [19]. However, this hole is covered with LSFL-perpendicular, which always appears and is preserved under certain fluence condition. The morphology of the Si surface in the outer fringe, where the laser fluence is just above the surface modification threshold, highlights some elementary processes involved in the laser-matter interaction. The first fringe of the BG beam, labeled by A in Fig. 3(a), formed by 500 shots with a pulse energy of 0.63 J shows nascent surface modification and removal. This area consists of the rim region surrounded by oval humps, and expands with a clear boundary with arrays of humps increasing with laser shot number (Fig. 3(b)). These arrays of dots were reported by other authors for multi-laser shots [3,23,24] and single pulse irradiation on c-Si [25]. The humps exist as isolated islands in the feeler-like structures extending out from the rim of the laser affected zone. Similar feeler-like patterns oriented perpendicular to the laser polarization are seen in the circle of Fig. 2(a). After laser irradiation with an increased cumulative shot number, the tips of the feelers are terminated at humps (Fig. 3(d)). These humps frequently grow to form periodic ripple patterns. Fig. 3(f) and (g) shows that the laser affected zone is covered with a range of humps, clearly marking the border to the virgin surface of the flat plane. As the laser irradiation proceeds, the laser affected zone is filled with swells and humps, covered with cracks. The surface removal process starts from these cracks, as shown in Fig. 3(f). The mechanism required to explain the features of the laser affected pattern in the present results might be due to the combined effects of hot spots, which resemble gray spots in the darker zones in Fig. 3(c) and the feelers, which may indicate the appearance of heat flow. Hot spots can be formed by surface irregularity and/or a locally enhanced EM field, which is supposedly responsible for HSFL formation. Once the hot spots are formed, laser light is absorbed more efficiently. The dark peripheral zones surrounding the hot spots might be due to a change to an amorphous state caused by the thermal energy. In addition, heat flow links the spots to form feeler-like patterns in Fig. 3(c) and (d). There is a possibility that the c-Si under solid SiO2 melts and re-solidifies, forming an amorphous state in the laser affected zone A of Fig. 3(a), since the melting
Fig. 4. (a) Ablated patterns under identical fluence conditions as Fig. 3 after 500 shots in air at 1atm. (b) Magnified image of the circled area of (a).
temperature of a-SiO2 is higher than that of a-Si [25]. The lower density of a-Si compared to c-Si, a ≈ 0.97c [26], should induce a volume expansion of the surface layer after re-solidification, leading to the humps and winkles as seen in Fig. 3(g). The oxide layer cracks while the bulge in the hump grows. The oxide layer is then
Fig. 5. Distribution of an oxygen compound obtained by EPMA, (a) in the vicinity of the ablated hole of Fig. 4(a), and (b) in the vicinity of the ablated hole of Fig. 3(a). H and R indicate holes and ripples, respectively.
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Fig. 6. Ablation patterns in vacuum by the central peak of the BG beam with (a) a pulse energy of 1.9 J at 393 nm wavelength after 500 shots, and with a pulse energy of 2.12 J after (b) 2000 shots, (c) 4000 shots, (d) 6000 shots.
ripped off by the evaporating gas. The importance of the oxide layer in the formation of the humps or dots was pointed out in the earlier work, showing that these dots were not formed without the oxide layer [24]. The bulges show a smooth surface, probably as a result of melting. The phase change from c-Si to a-Si requires some threshold of deposited heat energy, clearly marking the boundary between the laser affected and non-affected zones, as seen in Fig. 3(g). After the skin covering the bulges is ripped off, thin oval patterns parallel to the laser polarization remain to form the periodic ripples. The present results indicate that the periodic ripple structures initiate the surface removal process with the laser modification fluence with a wavelength of 786 nm. The ripples formed after laser irradiation show periods of 590 nm in Fig. 2(a) and 540 nm in Fig. 3(b) (LSFL-perpendicular) and those with ∼130 nm in Fig. 3(b) and ∼160 nm (HSFL-parallel) in Fig. 3(e). Sipe’s theory [9], rearranged by including the transient refractive index by Bonse [6,10], predicts a period for HSFL-parallel of ∼ /n, where n is the transient refractive index of the target, as the free electron density is ∼2 × 1027 m−3 [6]. This free electron density is produced by 0.25 J/cm2 of excitation, as calculated from linear and two-photon excitations [6], which is close to the multi-shot ablation threshold of 0.2 J/cm2 [20]. If we quote n ∼ 3 for this fluence [10], then ∼260 nm is obtained for HSFL-parallel. The period calculated in this scheme does not reproduce the ripple period of 130 nm and 160 nm, even using n ∼ 3.67 for non-excited Si. Instead, ∼/2n [11,27] provides ∼131 nm. The formation of HSFL-parallel along the border of the laser affected zone in the present results is limited to a narrow range of laser fluence and shot number. In contrast, LSFL-perpendicular becomes more prominent as the free electron density is increased. It was previously reported that the spatial period of LSFL-perpendicular decreases with a reduction in fluence but with an increase in the shot number; for example, it is ∼570 nm
at 500 shots of ∼0.2 J/cm2 [28]. The 590 nm period in the central spot (Fig. 2(a)) and 540 nm period in the hole periphery (Fig. 3(b)) are consistent with the results of Ref. [28]. Periodicities similar to LSFL and HSFL of the present results are also reported by other authors [17]. Laser irradiation in air at 1 atm under identical fluence and shot number as in vacuum results in different features in the ablation pattern. LSFL-perpendicular is formed at the first fringe position in air (Fig. 4(a)), while the same fluence induces a phase change without forming similar ripple structures in vacuum (Fig. 3(a)). The surface of the hole edge and rippled zone appear molten, indicating that these surfaces have transformed to a liquid state. As a result, the trenches in the rippled zone are filled with molten material. The chemical composition of the rippled zone formed in air (part A of Fig. 5) shows a weak oxidation, probably due to the formation of an oxide layer after melting and re-solidification of the Si surface in the presence of air. The period of ∼520 nm is consistent with the result obtained by 500 shots of laser irradiation with 0.15–0.25 J/cm2 in Ref. [28]. Increased residual thermal energy in the sample after laser irradiation in air [29–31] might reduce the threshold for ripple formation and also yield the present result. The preference for the formation of ripples on c-Si in air at 1 atm was also reported by other authors [14]. The surface modification by SH at the present fluence shows some unique features. Instead of the feeler-like patterns, thin filamentary strings with small humps are aligned with a ∼60 nm periodicity perpendicular to the laser polarization (HSFLperpendicular in Fig. 6(a)). This result sharply contrasts to that of a ∼160 nm periodicity aligned parallel to the laser polarization formed by the fundamental wavelength. The periodicity can be predicted to be /n [16] or /2n [5,8,27] where n stands for the refractive index of excited c-Si at the wavelength of SH. The
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Fig. 7. Refractive index, extinction coefficient, and reflectivity vs. transient free carrier density. Top: refractive index n, middle: extinction coefficient k, and bottom: refractive index R.
refractive index should be altered from the non-excited c-Si due to the significant fraction of the electrons excited from the valence band to the conduction band during and after laser irradiation. The refractive index n and extinction coefficient k, which compose the complex refractive index as nˆ = n + ik, are calculated from the real εˆ = ε + iε , leadand imaginary parts of the dielectric permittivity
ing to n = ( ε 2 + ε 2 + ε )/2 and k = ( ε 2 + ε 2 − ε )/2. ε and ε are obtained from the contributions from the crystalline and the Drude electron–hole plasma: [32] ε = 1 −
ε =
(εg − 1)(N0 − N) e2 N + N0 ε0 m∗opt me (ω2 + −2 ) e2 N
ε0 m∗opt me ω ω2 2 + 1
+ εg
N0 − N N0
wavelength of 440 nm was reported to be 0.1 J/cm2 , resulting in the reduction of the reflection coefficient from that of the non-excited surface by a factor of 0.97 [35]. The free electron density is calculated as 1.6 × 1027 m−3 from the reduced reflection coefficient for a wavelength of 440 nm. If this electron density is accepted for the excitation by the SH (393 nm), which is indicated by the vertical dotted lines in Fig. 7, then a refractive index of 5.74 is obtained. Hence, the resultant ripple period, calculated by /n, is ∼68 nm, which is in the range of the observed ripple period of ∼60 nm. The primary mechanism of forming the filamentary structures appears to share the same origin as the periodical ripples described by the conventional model based on the interference between incident and surface EM waves. However, the formation mechanism of the fractal features at the border of the laser affected region is not identified. From the present measurements, the early stages of the surface removal process with the surface modification fluence at the SH wavelength is initiated by forming periodically aligned filamentary structures with locally formed nano-humps, terminated with fractal patterns. The central area of the laser affected zone darkens (Fig. 6(c)), indicating a phase change is occurring beneath the surface layer. Since the filamentary strings blur within the darkened area, the peeling of the surface does not seem to start from the cracks on the strings. Instead, surface peeling begins by forming spout holes, presumably due to the evaporation of gas. The material removal process starts at this moment. This removal process does not include the formation of the melt pool, and instead leads to the formation of ragged features on the crater bottom (Fig. 6(d)). However, the debris is mostly round. The larger-sized particles seem to originate from the roundish objects formed under the surface, while the smaller particles come from the tips of the crests after repetitive melting and re-solidification. These morphologies sharply contrast with the rugged shapes of the fine particles shown in our previous work in air [19]. The apparent difference in the early stage of the ablation process with a wavelength of 786 nm and SH is summarized: the surface layer lifts up to form humps, then cracks and the peeling of the surface skin layer initiate the material removal process. The traces resembling heat flow link the humps to the laser affected zone. Under the proper fluence conditions, the periodically formed humps are left as ripples. The SH forms periodically aligned filamentary structures first, which are then obscured during surface metamorphism. Material removal starts from the spout holes formed in the metamorphic region. These different surface responses might be brought about by the thermal effect of phonons due to an indirect transition from the valence to conduction bands in c-Si at a wavelength of 786 nm. This mechanism is less significant for the SH due to the higher photon energy.
(1) 5. Conclusion (2)
where e, me , m∗opt , , ε0 , ω, N0 , and N are the charge and mass of an electron, m∗opt = 0.18, the reduced effective mass of the elec-
tron and hole, ∼1.1 × 10−15 s for the relaxation time of the electron gas, dielectric permittivity of vacuum, 2.5 × 1015 rad/s for the angular frequency of SH, ∼2 × 1029 m−3 for the electron density in the valence band [31–33], and the free electron density excited to the conduction band, respectively. The real and imaginary parts of the dielectric permittivity, εg and εg for non-excited c-Si are 34.1 and 6.0, respectively, calculated from the refractive index and extinction coefficient in Ref. [34]. Fig. 7 presents the calculated n, k, and the reflection coefficient R, R = ((n − 1)2 + k2 )/((n + 1)2 + k2 ), vs. N. The single-shot melting threshold for a thin Si film at an irradiation
The optical arrangement of dual conical axicons successfully produces submicron sized holes in c-Si within a vacuum chamber. The Bessel–Gaussian beam formed by the present optical system yields a variety of elementary processes associated with the modification and removal of surface material, depending on the characteristic fluences at various fringe positions. Laser irradiation in vacuum is meritorious to the formation and preservation of featured patterns resulting from the laser excitation of the surface by avoiding the deteriorating effect of air. The surface corrugation patterns produced by laser irradiation consist of nanometer scale humps and periodic ripples. LSFLperpendicular with a period of ∼590 nm appears prior to the formation of a hole with the laser fluence above the ablation threshold. HSFL-parallel forms with a period of ∼160 nm as scars of
G. Odachi et al. / Applied Surface Science 261 (2012) 174–181
removed humps at the fluence for surface modification with an excitation wavelength of 786 nm. The humps and wrinkles in the laser affected zone might be due to the volume expansion of the amorphous layer. A surface irradiated with the SH shows HSFLperpendicular with a period of ∼60 nm, consisting of filamentary strings linking small cracked humps before material removal. The corrugated pattern shows fractal features at the border of the laser affected area. The spatial period of HSFL-parallel formed at a wavelength of 786 nm and HSFL-perpendicular by the SH are reasonably explained by /2n and /n, respectively. After multi-shot irradiation by SH, spout holes are formed and a roundish object appears in the opening, leading to material removal and formation of round debris particles. Laser irradiation in air at 1 atm leads to the formation of LSFLperpendicular with molten features at a lower fluence than that for experiments conducted in vacuum. The grain-like debris produced in the ablation is oxygen-rich, and is partially removed where ripples are formed. LSFL-perpendicular formed at the central spot and the outer fringe in vacuum or air seems to reveal the dependence of periodicity on fluence. A smaller period appears with decreasing fluence and increasing shot number [28]. It seems that a large fluence results in a long period; however, the shot number is required to be small to avoid deterioration of the ripple pattern. On the other hand, a reduced fluence results in a short period; in this case, the shot number needs to be large enough to form the periodical trenches. References [1] [2] [3] [4]
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