Keywords

1 Introduction: High-Energy States in Liquid for Laser Processing

The formation of local high-energy states upon focused irradiation of laser pulses is a basic phenomenon in processing of various materials with lasers [1]. Upon absorbing the energy of laser light, the irradiated part is instantaneously heated; thus, the formed local high-energy state results in the removal of materials by melting, evaporation, or ablation. The recent development of high-power lasers with kW-classes has contributed to high-speed processing in the industry. Moreover, laser light can be focused into a small spot at the micrometer scale. Therefore, the precise micromachining of solid material surfaces is executable. This type of laser microprocessing can be applied to various materials ranging from hard materials such as metals and ceramics, to soft materials, such as organic materials and biological tissues.

Similar phenomena can be induced on the surfaces of liquids. Laser ablation from the liquid surface has been studied to obtain a better understanding of the mechanisms of laser ablation in more complex solid systems [2, 3]. Material ejection from the free surfaces of liquid has been observed similarly to solid materials when the fluence of the irradiation exceeds the threshold [4,5,6]. Laser ablation of liquid was investigated using a nanosecond imaging technique, as shown in Fig. 10.1. The threshold was evaluated at 100 mJ/cm2 for benzene irradiated by ultraviolet (UV) pulses of a KrF excimer laser [4]. The difference in threshold values for several benzene derivatives was revealed to be correlated to the efficiency of photochemical bond dissociation [5]. On the other hand, the ablation of diluted solutions of benzene derivatives showing relatively larger thresholds was discussed based on the photothermal mechanism [6]. Thus, the high-energy states used in laser material processing can be formed on the surface of liquids. Note, laser ablation of laser-absorbing liquids should be distinguished from laser-induced breakdown in transparent liquids, which are often used for laser-induced breakdown spectroscopy (LIBS) measurements for the elemental analysis [7].

Fig. 10.1
figure 1

Copyright 1994 American Chemical Society

Nanosecond images of laser ablation of liquid toluene. The scale bar represents 2 mm. Delay time: a 0 ns, b 35 ns, c 100 ns, d 500 ns, e 1 μs, f 5 μs, g 10 μs, and h 100 μs. Reprinted with permission from Ref [5],

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When a substrate that can transmit laser light is set on the laser-absorbing liquid, a high-energy state can be formed at the solid–liquid interface by irradiating the liquid through the transparent substrates (Fig. 10.2). More precisely, a high-energy state is formed in the thin liquid layer in contact with the transparent materials. When the liquid presents a higher absorbance at the laser wavelength, the liquid layer where the laser light is absorbed becomes thinner, resulting in a higher energy density. This high-energy state can act on the backside surface of the substrate and etch the surface gently. Wang et al. developed a surface micromachining technique employing this phenomenon and named it “Laser-induced backside wet etching (LIBWE)” [8].

Fig. 10.2
figure 2

Schematic drawing of a direct and b indirect laser processes

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2 Laser-Induced Backside Wet Etching (LIBWE) in the Initial Study

LIBWE is a technique for the laser surface micromachining of transparent materials, such as glasses. Because transparent materials are generally hard and brittle, precise micromachining is challenging; for example, cracks are easily formed. The same issue exists for laser processing. Moreover, the transparency of the materials makes laser processing inherently difficult, which is based on the energy deposition upon the optical absorption of materials. One important characteristic of LIBWE is the crack-free micromachining of the transparent materials. The indirect action of a laser enables precise micromachining without crack formation; the laser energy is not absorbed directly by the transparent materials to be etched, but instead by the laser-absorbing liquid in contact with the transparent materials. A diagram of the LIBWE apparatus is shown in the Fig. 10.3.

Fig. 10.3
figure 3

Diagram of LIBWE apparatus used in Ref. [8]. Reprinted by permission from Springer: Ref [8], Copyright 1999.

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In a study by Wang et al. [8], fused silica was etched by a KrF excimer laser (λ = 248 nm, FWHM 30 ns, pulse repetition 2 Hz) by employing an acetone solution containing pyrene with a concentration of 0.4 mol/dm3. The laser light was patterned by a metal stencil mask and projected at the liquid–solid interface. The laser energy passing through the fused silica plate was absorbed by the solution at the interface. The concentration of the solution was nearly saturated, demonstrating a high absorbance at the laser wavelength. The optical penetration depth was evaluated as 0.7 μm. Etching showed the threshold for applied fluence similarly to the laser ablation; etching was observed at a fluence higher than the threshold of 240 mJ/cm2. Therefore, the LIBWE has often been described as the etching based on the laser ablation of liquid. The threshold of LIBWE is significantly lower than the fluence required for direct laser ablation (>14 J/cm2) [9]. The surface of the irradiated area was etched pulse by pulse above the threshold, as shown in Fig. 10.4. The etch rate was ranged from 5 to 25 nm/pulse for the applied laser fluence ranging from 400 to 1300 mJ/cm2. Additionally, the fabrication of crack-free and debris-free microstructures was demonstrated on patterned irradiation, as shown in Fig. 10.5.

Fig. 10.4
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Etch depths increased linearly with the number of applied laser pulses. Reprinted by permission from Springer: Ref [8], Copyright 1999.

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Fig. 10.5
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Micrometer-scale lines and spaces pattern fabricated on the surface of fused silica. Reprinted by permission from Springer: Ref [8], Copyright 1999.

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Several research groups have developed the LIBWE technique. The important characteristics of this technique are as follows:

  1. (1)

    Micrometer-scale structures can be fabricated on the surface of hard and brittle transparent materials without debris or crack formation.

  2. (2)

    Etching can be performed at a significantly lower fluence compared to that required for the direct ablation of materials.

  3. (3)

    The depth of etching increases linearly with the applied pulse numbers, enabling the fabrication of structures with high aspect ratios, whose depths can be precisely controlled.

  4. (4)

    A smooth etched surface can be obtained.

  5. (5)

    The apparatus used for general laser processing can be applied to this technique.

  6. (6)

    Stable and reproducible processing can be executed upon accumulation of pulse irradiation.

  7. (7)

    Various lasers and laser-absorbing liquids can be applied for the processing of various transparent materials.

Utilization of the indirect action of lasers is an effective method for the crack-free micromachining of hard and brittle transparent materials. Another indirect processing techniques utilizing the action of laser-induced plasma have been developed as the laser-induced plasma-assisted ablation (LIPAA) technique [10,11,12,13,14]. Additionally, laser etching at a surface-adsorbed layer (LESAL) [15] and laser-induced backside dry etching (LIBDE) [14, 16, 17] have been developed based on studies regarding LIBWE.

3 Variation in Combination of Lasers and Materials

The LIBWE process is based on the formation of the laser-induced high-energy state at the liquid–solid interface and its action on transparent materials. This simple concept of LIBWE has resulted in a wide variety of laser and material selection. However, a substantial limitation is that the laser energy sufficient for etching must reach the interface. Therefore, the optical transmittance at the laser wavelength is a key factor in the LIBWE process. Figure 10.6 presents the optical transmittance spectra of several transparent materials with indications of the wavelengths of typical lasers.

Fig. 10.6
figure 6

Optical transmission spectra of representative transparent materials: a CaF2 (2 mm), b fused silica (2 mm), c Corning gorilla glass, aluminosilicate glass (1 mm), d Corning Pyrex, borosilicate glass (2 mm), e soda-lime glass (1.75 mm); thickness indicated in parentheses

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In a study by Wang et al. [8], a KrF excimer laser and fused silica plates were employed as the light source and the material to be etched, respectively. This combination came from the experiments on laser ablation of polymeric materials enhanced by dye molecules [18], where the transparent polymeric films containing dye molecules were ablated by UV pulses of a KrF excimer laser. Pyrene, a popular dye molecule used by photochemists, was chosen as the dye molecule for the experiments. Transparent polymeric films containing pyrene were prepared on fused silica substrates. When the polymer film was unintentionally irradiated through the substrate, the fused silica plate was unexpectedly damaged, despite the irradiated fluence being significantly below the damage threshold. To apply this phenomenon to the etching of fused silica, polymer films were replaced by an acetone solution containing pyrene. By employing the solution, the liquid–solid interface can be regenerated after each laser pulse irradiation, and deeper etching can be attained by repeated pulse irradiation. Acetone was selected as a solvent to dissolve pyrene at high concentrations. Note, similar damage formation had already been realized in an early study regarding the laser ablation of molecular liquids [2]. In this section, the combinations of lasers and materials are discussed. Lasers, laser-absorbing liquids, and materials to be etched are listed with the types of experimental setups in Table 10.1. Note, the studies reporting the first usage of the combination are listed.

Table 10.1 Combinations of lasers, laser-absorbing liquids, materials to be etched, and types of the experimental setups in the LIBWE processes
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Precedent to the naming of LIBWE, there were several reports regarding the laser machining of transparent materials assisted by liquid media. Ikeno et al. had reported the etching of fused silica using a Nd:YAG laser (λ = 1064 nm, τ = 1 ms, pulse energy 3 J/pulse) with a NiSO4 aqueous solution (~2 mol/l) [19]. Hole drilling at the millimeter to sub-millimeter scale was performed by the focused irradiation of near-infrared (NIR) laser pulses. Dolgaev et al. reported the drilling of sapphire using a copper-vapor laser (λ = 510 nm, τ = 10 ns, pulse repetition 8 kHz) [20, 21] combined with aqueous solutions of CrO3, KMnO3, and FeCl3 or a highly dispersed suspension of carbon in toluene as a laser-absorbing liquid. These drilling processes originate from the high-energy states formed in the liquid layer. In these studies, it was concluded that the first action of the high-energy state is not the etching of the transparent materials, but the formation of absorbing oxide layers on the surface of the transparent materials [21]. The contributions of the deposited and/or modified layers for the LIBWE process are presented in a later section. Recently, similar processes were reported as LIBWE processes employing aqueous solutions of inorganic compounds combined with fundamental or second harmonic generation (SHG) pulses of diode-pumped solid-state (DPSS) lasers [22–28].

After the naming of LIBWE, etching employing excimer lasers (ArF (λ = 193 nm), KrF (λ = 248 nm), XeCl (λ = 308 nm), and XeF (λ = 351 nm)) has been reported by several research groups. The number of studies employing a KrF excimer laser is larger than that using other excimer lasers, which may be due to the fact that the optical absorption ascribed to the π-π* transition of the benzene ring is located at approximately 250 nm, and a wide variety of molecules containing benzene rings can be used as absorbers. The materials to be etched were fused silica, quartz (SiO2 crystal), sapphire, and fluoride crystals (CaF2, MgF2, and BaF2). These materials can transmit laser pulses at UV wavelengths. Wang et al. reported the etching of a UV-transparent fluoropolymer (FEP) film [49]. One exceptional case is the study by Cheng et al., who reported the etching of Borofloat33 (Schott), which is a type of borosilicate glass, with the fourth harmonic generation (FHG) pulses (λ = 266 nm) of a DPSS laser [43]. As shown in Fig. 10.6, pyrex classified to borosilicate glasses presents an absorption edge at approximately 350 nm, and no transmittance at 266 nm. It was reported that a Borofloat33 glass with a thickness of 0.7 mm absorbed 50% of the laser energy. Thus, the remaining 50% reached the interface and was used for the crack-free fabrication of microtrenches. This study revealed that the LIBWE of the materials that weakly absorbs the laser energy is executable if sufficient energy reaches the interface without causing damage to the substrates. When a XeF excimer laser that emits laser pulses with the wavelength of 351 nm is used for LIBWE, the etching of borosilicate glasses (Corning, Pyrex, 7059, Schott D263, AF45) can be performed as a processing of highly transparent materials [54].

Pyrene is a major dye molecule used in LIBWE. Several organic solvents such as acetone, tetrahydrofuran, toluene, cyclohexane, and tetrachloroethylene (C2Cl4) are used to dissolve it. Because molecules with benzene rings present an optical absorption near 250 nm, toluene and other substituted benzenes can be used as laser-absorbing dye molecules [35, 36, 43–45]. Cheng et al. investigated the threshold fluences of LIBWE for 11 organic laser-absorbing liquids and confirmed that the unit length absorbance, which is related to the optical penetration lengths, correlated with the obtained threshold values [43]. Vass et al. used a solution of naphthalene dissolved in methylmethacrylate for LIBWE with ArF [29] and KrF [38] excimer lasers as well as FHG pulses of the Nd:YAG laser [46,37,48]. Fujito et al. employed a solution of phenolphthalein dissolved in N-methyl-2-pyrrolidone for the etching of sapphire [37]. Aqueous dye solutions have adapted for LIBWE by Ding et al.; pyranine [39] and naphthalene-1,3,5-trisulfonic acid trisodium salt (Np(SO3)3Na3) [40] were employed as dye molecules. Thus, a wide variety of dye molecules can be used in the LIBWE employing the UV lasers.

The green emission of SHG pulses of solid lasers in visible range (λ = 515, 527, and 532 nm) can be used for LIBWE. As shown in Fig. 10.6, all transparent materials present a sufficient transmittance at this wavelength. Cheng et al. developed the LIBWE employing SHG pulses and named it visible LIBWE [59]. Soda-lime glasses, the most commonly available glass used for windows in buildings, whose absorption edges are located near 380 nm, can be etched by employing the visible wavelengths. Tsvetkov et al. employed an aqueous solution containing Amaranth, a red food dye [23, 58]. Moreover, polyethylene glycol (PEG) was added to the solution to reduce the surface tension of the liquid, which affected the cavitation behavior, which will be described later.

Liquid metal was also used for etching. Etching using liquid-phase gallium combined with a KrF excimer laser was first reported by Zimmer et al., which presents one-order higher etch rates than the LIBWE employing an organic dye solution [41]. The low melting point of gallium (29.76 ºC) enables its use in the liquid phase upon heating. Although high reflectivity causes energy loss, the penetration depth of light is relatively small. Therefore, metals in the liquid phase can be good laser-absorbing liquids for the LIBWE process, which can be used for a wide range of wavelengths. Zimmer et al. reported the etching of fused silica by fundamental laser pulses of the Nd:YAG laser (λ = 1064 nm) [68]. Studies employing mercury [42] and molten tin [17] have been reported. Eutectic indium/gallium alloy (In–Ga) can be used for visible LIBWE without heating [64, 66, 67, 69].

4 Variation in Experimental Setups for LIBWE

In addition to the combination of lasers and materials, the simple concept of LIBWE also results in variation of the setups for irradiation. In this section, the experimental setups for LIBWE are reviewed.

4.1 Mask Projection with Excimer Laser

Because excimer lasers can emit laser pulses with large areas, they can be used for patterned irradiation by mask projection. This technique is similar to that used in the photolithography processes for semiconductor device production. Therefore, setups for patterned irradiation with homogeneous light intensity have been established and enable precise surface micromachining. It was reported that smooth surface with low roughness of 50 nm could be obtained by employing this type of apparatus [52, 70]. The fabrication of micro-optical devices has been demonstrated [32, 52, 70, 71].

Initially, we used simple experimental setups equipped by ourselves with a metal stencil mask and a planoconvex lens for each experiment. Although the fabrication of microstructures can be demonstrated, the quality of fabrication is occasionally not exceptional [51]. Beam shaping and the precise positioning of samples to match the liquid–solid interface with the imaging plane of the optical system enable high-quality machining, as shown in Fig. 10.7; for fabricating the structure, Böhme et al. used an irradiation setup with projection optics having a resolution of 5.0 and 1.5 μm [32].

Fig. 10.7
figure 7

Surface structure of fused silica fabricated by Böhme et al. Reprinted from Ref. [32], Copyright (2002), with permission from Elsevier.

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We introduced a machining setup composed of beam homogenizer, dielectric attenuator, chromium-on-quartz-masks, as well as an × 8 demagnification objective (MicroLas Lasersystem GmbH), and a motorized x–y-z stage for the sample positioning (Fig. 10.8). The resolution limit of this system was specified as 3 μm lines and spaces. The quality and reproducibility of microfabrication improved by using this system [36]. A precise control of the sample position within ± 15 μm to match the liquid–solid interface with the imaging plane of the optical system is required for precise etching [72]. We developed a system to compensate for the inclination of the surface and the displacement induced by the surrounding temperature change; the fabrication of microstructures with homogeneous quality over a wide area on the fused silica plates was then attained. Based on the precise positioning of samples, the fabrication of deep microtrenches and holes with high aspect ratios was performed (Fig. 10.9) [72,73,74,75,76,77]. This type of fabrication can be achieved by locating dynamically the etch front, which in this case was the bottom of the trenches, at the imaging plane of the optical system.

Fig. 10.8
figure 8

a An image and b layout of the new setup for the mask projection LIBWE

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Fig. 10.9
figure 9

Cross-sectional SEM image of deep microtrench fabricated with adjusting dynamically the sample position for matching the etch front to the imaging plane of the optical system. Reprinted from Ref. [74], Copyright (2006), with permission from Elsevier.

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4.2 Etching on Interference

The spatial resolution of the mask projection systems is limited by optical systems. To achieve a higher resolution, the interference of laser beams was applied for etching. Zimmer et al. reported the fabrication of a surface relief grating with a period of 780 nm by phase grating projection using a Schwarzschild objective [33]. The sub-micron resolution of LIBWE was proven in this study. Vass et al. demonstrated the fabrication of surface relief grating with a shorter period by a two-beam interference of FHG pulses of the Nd:YAG laser with incident angles up to 60° [46]. They named this method TWIN-LIBWE and fabricated surface relief gratings with various periods, the smallest period was 104 nm (Fig. 10.10) [47, 48, 78,79,80,81].

Fig. 10.10
figure 10

Surface relief grating with period of 104 nm fabricated by the immersion TWIN-LIBWE. Reprinted by permission from Springer: Ref [78], Copyright 2007.

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4.3 Direct-Writing by Scanning with High Repetition

Laser-Direct-Write (LDW) is an attractive technique that enables versatile material processing with patterns in scales ranging from nanometers to millimeters [82], where lasers with a high pulse repetition (~100 kHz) are employed for computer-controlled two- and three-dimensional pattern formation in a serial fashion. LDW with LIBWE was first demonstrated by Cheng et al. using computer numerical control (CNC) stages and a DPSS laser operated with a pulse repetition of 4.5 kHz [43]. Microchannel structures for microfluidic chips were fabricated on borosilicate glass. A direct write with beam scanning by a galvanometer-based point scanning system was demonstrated by Niino et al. [44, 83] (Fig. 10.11), where a DPSS laser was operated at a 10 kHz pulse repetition and used for flexible mask-less processing. This type of LIBWE can be used for the laser marking of glass materials (Fig. 10.12) [44, 63, 84].

Fig. 11
figure 11

Reprinted from Ref. [83], Copyright (2007), with permission from Elsevier.

Setup for LDW LIBWE employing the galvanometer-based point scanning system with a DPSS laser.

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5 Various Studies Regarding the Elucidation of the LIBWE Mechanisms

While the basic concept of the LIBWE is simple, the mechanisms involved are complex. The mechanism of material removal has not yet been fully understood. Several processes would be involved in the etching phenomena, and their contributions may be altered by the applied materials and conditions of laser irradiation. The wide variety of materials and conditions for LIBWE makes the situation more complex. Discussions and diagnostic studies addressing the elemental phenomena are overviewed.

5.1 Estimation of Temperature Rise

In the initial studies, the temperature increase in the thin liquid layers was evaluated using the absorbed laser energy, liquid volume that absorbed laser light obtained from the optical penetration depth, and the heat capacity of the solvent [8, 52]. The maximum temperatures of the liquid layer were evaluated as 1900–2300 K, which is above the softening and melting temperatures of fused silica. Wang et al. discussed on the involvement of super-heated liquid [31] produced by heating within a pulse duration based on a cyclic multiphotonic absorption mechanism [85]. Note, the transient imaging of the interface indicated that the initial flat bubbles before expansion were observed at a delay time of 100 ns [35, 40, 86]. Organic molecules are decomposed at such high temperature. Partial decomposition of organic molecules was indicated by emission of gas bubbles with the distinctive odor of sulfur-containing gas [39], or by production of carbon soot from photolyzed toluene [35]. Note, the emitted gas bubbles should be distinguished from the cavitation bubbles that will be described later. Kopitkovas et al. reported that the difference in the threshold fluences obtained for the LIBWE by KrF and XeCl lasers can be explained by the calculated induced temperatures reaching the melting point of quartz (1880 K) as shown in Fig. 10.13 [52]. However, as indicated by Vass et al., the temperature of the solid target was not calculated in these studies. They evaluated the temperature of a solid target by numerical calculations using the method of finite differences [29, 86]. The surface of the solid target was concluded to reach the melting point; the depths of the melting layers formed upon irradiation at various fluences were also evaluated (Fig. 10.14). The difference between etch depth and calculated maximum depth of the melted layer was ascribed to the partial removal of the melted fused silica. On the other hand, an analytical solution of the laser-induced temperature distribution was evaluated by Zimmer [87]. He argued that the temperature of the surface of the solid target did not reach the melting point without the existence of the interface absorption. The existence of a modified layer with interface absorption was proven, which will be described later.

Fig. 10.12
figure 12

Reproduced from Ref. [84] with permission from Japan Laser Processing Society.

Laser marking on glass by LIBWE with a micropit array structures for writing b a QR code on a soda-lime glass plate.

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Fig. 10.13
figure 13

a Etch rates dependent on the laser fluence presenting two-stage behaviors in the LIBWE employing the KrF and XeCl laser. b Calculated temperature at the quartz-liquid interface as a function of the laser fluence. Reprinted from Ref. [52], Copyright (2003), with permission from Elsevier.

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5.2 Etch Rates and Threshold Values

The threshold fluence and the etch rate have been focused in the studies on LIBWE as the parameters characterizing the processes. Böhme et al. first realized that the etch rates dependent on the fluence were fitted by two lines in different fluence ranges [32]. As shown in Fig. 10.13, similar two-stage behaviors were confirmed in different studies [40, 52, 53, 70, 86]. This appears to be a general behavior for LIBWE, indicating that different mechanisms are involved in the etching process. Note, the etch rate of LIBWE can be varied with spot size for irradiation, as shown in Fig. 10.15 [88]. The experimental results obtained under different conditions should be treated with care.

Fig. 10.14
figure 14

(left) Etch rates versus laser fluence for LIBWE employing an ArF excimer laser and naphthalene/methylmethacrylate, (right) the maximum depth of the melted fused silica layer calculated by the method of finite differences. Reprinted from Ref. [29], Copyright (2004), with permission from Elsevier.

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The incubation effect in the LIBWE is shown in Fig. 10.15. The existence of this effect was first reported by Yasui et al. [51] in the etching of fused silica with a XeCl excimer laser at the fluence of 0.4 J/cm2; etching was not discerned upon irradiation with 3000 pulses, while etching with the depth of 0.4 μm was observed upon irradiation with 5000 pulses. Similar behavior was observed for the etching of quartz by a XeCl excimer laser at low fluences (1.18 J/cm2), just above the threshold; start of the etching was observed upon irradiation with more than 150 laser pulses [53]. The existence of the incubation effect revealed the involvement of an initial process causing the generation of absorbing sites, such as defects. Ding et al. confirmed that the etching of sapphire started after the etched area was covered with black particles of amorphous carbon [36]. On the other hand, the etching from a single laser pulse was confirmed at a sufficiently high fluence [53]. Single-shot etching was also confirmed in LIBWE employing focused laser pulses of DPSS lasers [84, 89].

5.3 Characterization of Etched Surface.

Etched surfaces were evaluated using Rutherford backscattering spectroscopy (RBS) [90], X-ray photoelectron spectroscopy (XPS) [90,91,92], Raman scattering spectroscopy [90, 91], fiber-tip ATR-IR spectroscopy [91], and ellipsometry [92]. The Raman spectra of the etched surface indicated the presence of amorphous carbon deposits, while chemical modification of fused silica was indicated by the XPS and fiber-tip ATR-IR results. Thus, not only the adhered amorphous carbon deposit layer, but also the formation of a modified layer in LIBWE was indicated. RBS measurements indicated the generation of an amorphization layer on quartz crystals with a thickness of up to 60 nm. The formation of a modified layer on fused silica with a thickness of 10–30 nm with an absorption coefficient of 100,000–180,000 cm−1 was indicated by ellipsometry. Zimmer et al. evaluated the absorption coefficient of the modified layer by local absorption measurements for a surface prepared by the ion beam beveling technique (Fig. 10.16) [93]. The existence of a modified layer on fused silica with a thickness less than 25 nm with a high absorption coefficient of 40 × 104 cm−1 was indicated. The thickness of a few tens nm was indicated in several observations and coincident with the typical etch rates obtained for LIBWE with excimer lasers. Based on the confirmation of the existence of a modified layer, they proposed a new mechanism of LIBWE composed of the formation of a modified layer and its direct ablation [94].

Fig. 10.15
figure 15

a Averaged etch rate in dependent on the applied pulse number for difference spot sizes. b Calculated “real” etch rate per laser pulse. Reprinted from Ref. [88], Copyright (2005), with permission from Elsevier.

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5.4 Diagnostic Studies on LIBWE

To address the mechanism of LIBWE, diagnostic studies using several in situ measurements have been applied. As previously indicated, a rapid temperature increase in the liquid layer causes cavitation in the liquid. The cavitation behaviors were observed by transient imaging using the shadow graph technique, as shown in Fig. 10.17 [35, 40, 86, 95,96,97], and in situ reflectivity measurements [61, 98]. Based on the dynamics of a bubble, the pressure inside of a bubble can be evaluated based on the following physical model [99]:

Fig. 10.16
figure 16

(left) Schematic illustration of the preparation of the sample by ion beam beveling (#1) and the near-surface local absorption measurement (#2). (right) Measured etch depth and absorption at different positions of beveling. Reprinted from Ref. [93] with permission from AIP Publishing.

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$$p\left(R, \frac{dR}{dt}\right)=\left({p}_{stat}+\frac{2\sigma }{{R}_{n}}\right){\left(\frac{{R}_{n}}{R}\right)}^{3\kappa }-\frac{2\sigma }{R}-\frac{4\mu }{R}\frac{dR}{dt}$$
(10.1)

where R is the radius, Rn is the equilibrium radius of the bubble, pstat is the statistic ambient air pressure, σ is the surface tension, μ is the dynamic shear viscosity, and κ is the polytropic exponent of the solvent. Vass et al. calculated an initial high pressure of approximately 70 MPa, which decreased instantly with the expansion of the bubble [86]. This initial high pressure was considered to remove the melted material from the surface. The generated initial high pressure propagates in the liquid as a shockwave, which can be detected using a piezoelectric pressure gauge [44, 58, 84, 89, 96, 97]. The initial pressure of 65 MPa deduced from the distance dependence was in the same order with the pressure obtained from the cavitation dynamics [97]. Meanwhile, cavitation dynamics indicate that the high-pressure is generated once again when cavitation bubble collapsed. Pairs of pressure signals corresponding to the formation and collapse of the cavitation bubble were observed (Fig. 10.18) [44, 84, 89]. The single-shot etching behavior was confirmed to be correlated to the cavitation behavior [84]. A micropit with a diameter of approximately 30 μm and the maximum depth up to 0.35 μm was fabricated by single-pulse irradiation. The maximum depths of micropits fabricated by two laser pulses with different time intervals are shown in Fig. 10.19; the etching with depth of approximately 0.6 μm was confirmed at the time interval larger than 125 μs, while etch depths became the half of them at the time interval less than 60 μs. Because the lifetime of cavitation bubble was evaluated as 63.2 ± 0.9 μs for the applied pulse energy of 44.0 μJ/pulse, these results revealed that the etching by the second pulse irradiated before collapsing of the cavitation bubble was completely suppressed. Moreover, decreased etch depths at the time intervals between 70 and 120 μs indicated partial suppression due to the formation of the second bubble [95]. The suppression of the etching by the second pulse in the successive two pulses was clearly observed in the etching with beam scanning. Figure 10.20 shows the micropit array fabricated with pulse repetition and scan speed of (a) 10 kHz and 750 mm/s and (b) 20 kHz and 1500 mm/s, respectively; laser pulses were delivered at every 75 μm in both cases. When the pulse repetition was 20 kHz, time interval of laser pulses of 50 μs was shorter than the lifetime of the cavitation bubble. As shown in Fig. 10.17, the bubble expands to out of the irradiated area. Thus, the suppression of the etching by every second pulse was induced (Fig. 10.20). Given the results that the self-assembled monolayer (SAM) on the fused silica around the irradiated area kept their function as reported by Ding et al. [100, 101], gas phase chemicals inside the bubble induced minor effect on the surface around the irradiated areas.

Fig. 10.17
figure 17

Time-resolved optical images of the cavitation bubble formed by a KrF excimer laser irradiation on the interface between toluene and fused silica at the delay time of a 100 ns, b 500 ns, c 1.2 μs, d 10 μs, e 50 μs, and f 100 μs. Reprinted from Ref. [35], Copyright (2003), with permission from Elsevier.

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Fig. 10.18
figure 18

Transient pressure signals observed in LIBWE using toluene and DPSS UV lasers at a 1 kHz (fluence: 520 mJ cm−2, laser spot: 50 μm) and b 20 kHz (fluence: 5.7 J cm−2, laser spot: 12 μm). Reprinted from Ref. [44], Copyright (2006), with permission from Elsevier.

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Fig. 10.19
figure 19

Maximum depths of the micropits fabricated with two laser pulses with different time intervals. Reproduced from Ref. [84] with permission from Japan Laser Processing Society.

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Although the results on single-shot LIBWE indicated the required action of liquid for etching, the involvement of the surface modification was not ruled out. The modified layer generation within the pulse duration was detected by time-resolved reflection measurements, as shown in Fig. 10.21 [94]. An increase in reflectivity observed within 15 ns indicated the generation of the modified layer within the pulse duration. Meanwhile, the etch depth of up to 0.35 μm in the single-shot LIBWE [84] would be larger than the thickness of the modified layer, that are coincident with the typical etch rates of a few tens nanometer. There should be additional mechanisms for material removal. In a similar single-shot LIBWE, the decrease in etch rates by adding PEG to the laser-absorbing liquid was confirmed [58]. The addition of PEG as a surfactant reduces the surface tension of the liquid and results in a decrease in the cavitation pressure as indicated by the measured pressure signal. The results indicate that the removal of melted material by the initial high pressure of cavitation is a possible additional mechanism. In the same study, the authors discussed the role of etching with supercritical water. However, given the etching is relatively slow, an etch rate of ca. 1 μm/min was observed in the etching of glass with supercritical water [102], the role of etching with supercritical water would be minor.

Fig. 10.20
figure 20

Micropit array structures fabricated with pulse repetition and scan speed of a 10 kHz and 750 mm/s, b 20 kHz and 1500 mm/s, respectively. Reproduced from Ref. [84] with permission from Japan Laser Processing Society.

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Fig. 10.21
figure 21

Time-resolved reflection measurements at 248 nm for LIBWE and the backside ablation (BSA) of a 100-nm thick photoresist film on fused silica (PR) and LIBWE-modified fused silica (mFS). Reprinted by permission from Springer: Ref [94], Copyright 2010.

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The blue light plasma emission, including the atomic lines of Ca and Al, was detected by Cheng et al., which indicated the direct ablation of glass materials [103]. Meanwhile, in the visible LIBWE of PMMA with In–Ga, the plasma emission of In and Ga was observed [67], which indicated plasma formation from the liquid media. The etching of transparent materials assisted by laser-induced plasma has been studied as LIPAA. In these studies, it was confirmed that the major role of plasma in LIPAA was the creation of an absorption site at the surface of the material [13, 14]. In the LIBWE process, the laser-induced plasma may contribute to the generation of an absorption site.

6 Summary

In this chapter, studies regarding the LIBWE process, that is the laser processing based on the action of high-energy states formed in liquid layer, are reviewed. This process achieves crack-free and debris-free precise micromachining of transparent materials. Therefore, it can be applied to fabricate micro-optical and microfluidic devices. The simple concept of this technique for utilizing the high-energy state in liquid allows a variety of combinations of lasers and materials as well as a variety of experimental setups for processing. While the concept for processing is simple, the mechanism of material removal, the action of the generated high-energy states, is rather complex. Several studies have been conducted to elucidate the actions of the high-energy states for material removal, which revealed that several mechanisms with different contribution ratios are involved in this process. One important mechanism is that the surface layer of the materials is modified by the chemical species formed in the high-energy state, and direct laser ablation of the laser-absorbing modified layer is induced. This phenomenon can be induced within a single-pulse duration. This process would be a major mechanism in the fabrication of microstructures on silica-based transparent materials with organic laser-absorbing liquid that shows a low etch rate up to a few tens of nanometers per pulse. For the etching of sapphire, the adhered amorphous carbon layer would be act as the interface absorbing layer. However, it is not clear that similar mechanism is involved in the etching of fluoride crystals or polymeric materials, or the etching employing liquid metallic absorbers. In addition to the direct ablation of the modified layer, the mechanical removal of melted material by the high pressure induced in the initial stage of cavitation would be involved in the etching process induced at a higher fluence. An increased contribution of the latter mechanism was observed in the LDW-type LIBWE demonstrating incubation-free etching.