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Tungsten microcone growth by laser irradiation
Applied Surface Science 218 (2003) 175–187
Tungsten microcone growth by laser irradiation Yuji Kawakami*, Eiichi Ozawa1 Gas Deposition Division, Vacuum Metallurgical Co., Ltd., 516 Yokota, Sanbu-machi, Sanbu-gun, Chiba 289-1297, Japan Received 9 August 2002; received in revised form 3 March 2003; accepted 11 April 2003
Abstract Nanosecond pulsed-Nd:YAG laser irradiation of a target material with second-harmonic waves (wavelength ¼ 532 nm) under various gas atmospheres resulted in the formation of tungsten microcone arrays with a high aspect ratio protruding from the initial surface of the target material. The peak laser fluence was changed from 3.0 to 19.1 J/cm2. A tungsten substrate was irradiated with between 1 and 48 000 laser shots. The number of laser shots strongly influenced the growth and morphology of the microcones: several tens or hundreds of pulses created only rough surfaces, and further pulses created the microcones. The microcones grew to heights of up to 20 mm and narrowed to tips of about 1.5 mm in diameter. Our observations reveal that tungsten microcone can be generated in various ambient gases. We experimentally evaluated the microcone morphology in order to determine the mechanism of microcone formation in ambient atmospheres of helium, SF6, and air. The microcone formation appears to be a function of the repeated melting and solidification of the tungsten tips of the structures by the pulsed laser irradiation on the top surface. The morphology of the microcone depends on the ambient gas atmosphere used. # 2003 Elsevier Science B.V. All rights reserved. PACS: 79.20.Ds; 81.65.Cf Keywords: Nanosecond pulsed-Nd:YAG laser; Microcone growth; High aspect ratio; High-purity tungsten; Inert gas atmosphere; Chemical gas atmosphere
1. Introduction The micro-processing of solid surfaces by laser ablation has attracted considerable attention over the last two decades, with marked progress achieved in forming surface modifications and patterns such as laser-induced periodic surface structures (LIPSSs) [1–3], coherent arrays of ultrafine particles [4–7], *
Corresponding author. Tel.: þ81-475-89-3816; fax: þ81-475-89-1469. E-mail address: [email protected] (Y. Kawakami). 1 Present address: Nanotechnology Researchers Network Center of Japan, Toranomon 30 Mori Bldg. 2F, 3-2-2 Toranomon, Minatoku, Tokyo 105-0001, Japan. Tel.: þ81-3-5404-3280; fax: þ81-3-5404-3290.
and microcone and microcolumn arrays [8–15]. Many investigations focused on the fabrication of microcones and microcolumns, structures thought to offer good utility as photonic materials and field emitters. The target materials used in these studies have included silicon (Si) [8–13], a polymer (polyimide) [14], oxides (YBa2 Cu3 O7x , ZrO2) [15–17], nitride (Si3N4) [17], and metals (Co, Ti, W) [18–20]. A KrF (248 nm) excimer laser and a Ti: sapphire (800 nm) laser have been used on Si substrates to investigate the influence of ambient gas species such as SF6 [8–11,13], Cl2 [13], and air [8,11,12]. The growth of microcones on Si substrates is thought to be driven by a chemical reaction with the surrounding gases, based on earlier results showing an absence of microcone formation in an inert
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00615-9
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(Ar) gas [8], and an absence of sharp conical spikes in a vacuum [13]. In the case of polymer, Dyer et al. [14] reported that the cones were produced by a shielding effect of the particulate impurities. Similarly, the impurity shielding mechanism for cone formation was observed for multi-component materials such as YBa2 Cu3 O7x . The yttrium concentration increased at the cone tip during the initial stage of microcone growth; and this yttrium-rich shell appeared to suppress laser vaporization [15,16]. In a detailed investigation of the mechanism of Si3N4 microcone formation, Miyamoto and Maruo proposed the following relation between the cone formation and debris particles: (1) the debris particles are initially produced and adhere to the laser-irradiated surface; (2) the ablation around the debris particles produces small protuberances; (3) the increase in the surface area reduces the laser fluences on the sides of the protuberances; and as a consequence, (4) conical protuberances with a high aspect ratio are generated [17]. In the case of metals, however, the mechanism of cone formation has not been completely understood. Two mechanisms have been proposed. Based on experiments on the generation of Co microcones, Usoskin et al. [18] proposed ‘a growth model’. They conjectured that the initial laser irradiation formed ‘nuclei’ of metallic cones which later assembled into a surface relief (initial surface patterns) and grew through the ablation of ‘hot’ melted valleys at the focal points of the electromagnetic energy from the light waves. In an investigation of titanium microcones, Dolgaev et al. [19] suggested that the nuclei were formed by ‘capillary waves’ induced when the difference in the surface tension of the melted material generated the distribution of reflective index over the surface. Since the bottom of the wave was ablated more than the top, they conjectured that the evaporation produced a metal vapor that deposited on the top of the wave. When this occurred, the cones were produced by the ‘lift-up’ of the liquid and vapor to the cone tips. Both of the foregoing studies suggested that microcones are generated by the distribution of the ablation rate between the tips and the valleys. However, no detailed analyses have been performed to clarify the density of the cones, the density of the irregular arrays, or other factors involved in the formation mechanism and growth rate of metallic microcones.
We recently discovered the coherent arrays of ultrafine particles around a laser-irradiated mark on a single crystal of tungsten [4–7]; however, we could not produce them with an aspect ratio greater than 1.5. We next tried to grow tungsten microcones on a single crystal tungsten (99.99 mass%) [20] and on very-highpurity tungsten (99.99999 mass%) in a helium background gas. In doing so, however, we did not examine the relationship between the microcone morphology and experimental conditions in detail. In this study, we attempted to synthesize tungsten microcones with a high aspect ratio by investigating the growth mechanism of microcones with varying experimental parameters such as laser pulses, peak laser fluences, and ambient gas atmosphere.
2. Experimental The experimental apparatus used in this study (Chamber; ULVAC Materials Technology, Chiba, Japan) is schematically illustrated in Fig. 1(a), and an overhead view of the laser irradiation chamber is shown in Fig. 1(b). A nanosecond pulsed-Nd:YAG laser (Spectra-Physics, INDI-50, CA, 532 nm wavelength, 4.5–5.5 ns pulse duration) was used. The spatial profile of this laser beam is nearly Gaussian. As shown in Fig. 1(a), the beam was introduced into the chamber through a fused silica window and focused onto the tungsten surface with a plano-convex singlet lens (BK7, focal length 495.7 mm, CVI Laser Corporation, New Mexico). We used a round aperture to generate an area of high intensity. The diameter of the damage area, D, is related to the peak laser fluence, F0, as [21]: F0 D2 ¼ 2o20 ln (1) Fth where Fth is the threshold fluence and o0 the 1/e2 Gaussian beam radius (¼107 mm). In the case of the Gaussian beam, the peak laser fluence is obtained from laser pulse energy, Epulse, as F0 ¼
2Epulse po20
(2)
The peak laser fluence was varied between 3.0 and 19.1 J/cm2 by changing the distance from the lens (f ¼ 495:7 mm) to the substrate. The angle of
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Fig. 1. (a) Schematic diagram of the experimental apparatus and (b) photograph of laser irradiation experiment.
incidence, y, was 108, and the same region of the tungsten substrate was irradiated by 1–48 000 laser shots repeated at a frequency of 20 Hz. A high-purity tungsten substrate (Vacuum Metallurgical, Chiba, Japan, 99.99999 mass%, f10 mm t3 mm) was used as the laser ablation target. The highpurity and polycrystalline tungsten were produced by the chemical vapor deposition of decomposed WF6 vapor. Under X ray diffraction (XRD), the substrate surface exhibited preferred crystal orientation to the h1 0 0i direction [22]. The substrate surface was mirror-polished using silicon carbide polishing paper and diamond paste, then washed by acetone and petroleum benzine using a supersonic wave cleaner. The follow-
ing four atmospheres were used to investigate how the ambient gas affected the microcone formation: (1) lowpressure, high-purity helium gas (99.9999 mass%), (2) low-pressure SF6 gas (99.999 mass%), (3) air, and (4) vacuum. Before the experiments, the chamber was evacuated to a pressure of less than 1 104 Pa. The morphology of the laser-irradiated surface was observed by scanning electron microscopy (SEM; JEOL Ltd., JSM-6300F, Tokyo, Japan). The accelerating voltage of the electron beam was 5 kV. The substrates were observed at normal incident (08), and also with the substrate inclined 458 between the direction of the electron beam and the normal direction to the substrate. Surface-height profiles were
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measured using a Dektak IIA profilometer. Depth profiles of the microcones were also analyzed using auger electron spectroscopy (AES; Physical Electronics Industries, PHI-660, Minneapolis, USA).
3. Results and discussion 3.1. Influence of the number of laser shots SEM was used to observe the morphology of the tungsten surface irradiated by the nanosecond pulsedNd:YAG laser. Fig. 2 shows scanning electron micrographs of tungsten microcones after laser irradiation with a peak laser fluence of 5.2 J/cm2, with the number of laser shots varied from (a) a single shot to (g) 24 000 shots. The initial laser shot produced only a rough surface with craters, as shown in Fig. 2(a). Thirty laser shots created melted/solidified tungsten hemispheres (Fig. 2(b)). The reduction in the number of hemispheres was due to capillarity among the random protrusions (Fig. 2(b) and (c)). The large hemispheres absorbed the smaller ones, resulting in a reduction of the surface energy. Over several hundred laser shots, the tungsten hemisphere then acquired a tip structure similar in form to a mushroom (Fig. 2(h)). The microcone array (Fig. 2(d)) was generated from the hemispheres, which resembled nuclei in structure. As shown in Fig. 2(e)–(f), the number of laser shots strongly influenced the morphology of the microcone. As the number of laser shots increased, the height of the cone rose from 10 to 20 mm, whereas the diameter of the spheres generated at the tips of the microcones remained almost constant. We also found that more than 2400 shots were needed to produce a cone with a high aspect ratio of greater than 6 (the ratio of the diameter to the height of the cone). This tendency was also observed in previous reports on microcolumns [8,9,12,15]. Microcone growth did not occur over any part of the laser-irradiated area, primarily due to disturbances in the laser spot resulting from thermal strain waves, thermal expansion, temperature gradients, non-uniformity in temperature due to the intensity distribution of the Gaussian beam, and other factors. The density of the microcones decreased above 2400 laser shots (Fig. 2(e) and (f)). We observed similar results for single crystal tungsten [20]. At 24 000 laser shots, the cones melted and disappeared (Fig. 2(g)).
To investigate the generation and melting of the cones, we measured the ablation rates by measuring the change of the mass of the target due to the laser irradiation. The ablation rate is plotted in Fig. 3 as a function of the number of laser shots when the peak laser fluence is 5.2 J/cm2. The initial slope of the ablation rate was almost proportional to the number of pulses up to 10 000, and the ablation rate was calculated at about 70 ng/pulse in areas where microcone growth was observed. However, when the number of laser shots rose above 10 000, the ablation rate dropped by a factor of 2 to about 35 ng/pulse. This change came from the decline in the laser fluence due to the increase in the surface area accompanying the higher surface roughness and slope. 3.2. Influence of the peak laser fluence The cone formation process is known to be significantly affected by the laser fluence [16]. From the calculation of the absorptivity, the laser ablation threshold of tungsten at 532 nm/5 ns is approximately 1 J/cm2 [23]. When measured experimentally, the threshold of fluence for tungsten was 380 mJ/cm2 with an excimer laser (193 nm, 10 ns) [24] and 1.58 J/cm2 with an Nd:YAG laser (1064 nm, 9 ns) [25]. These predicted and observed thresholds of the laser fluence are basically valid when the laser beam exhibits a flattop profile. When the special profile of the laser beam is nearly Gaussian, we should employ the peak laser fluence defined by Eq. (2). Thus, we examined the effect of the laser fluence by setting the minimum peak laser fluence at 3.0 J/cm2, and then increasing it up to 19.1 J/cm2. The corresponding ‘average’ laser fluence (¼laser intensity/spot area) was between about 1.5 and 9.6 J/cm2. Fig. 4 shows scanning electron micrographs of the tungsten microcone obtained by irradiation at the peak laser fluences of (a) 3.0, (b) 7.6, (c) 10.6, and (d) 19.1 J/cm2 after 1200 laser shots. Microcones were formed with the irradiation even at lowest peak laser fluences of 3.0 J/cm2. Microholes distributed irregularly at sites between the cones were observed when the peak laser fluences were 7.6 and 10.8 J/cm2. At the highest peak laser fluences (Fig. 4(d)), the tungsten tips were judged to be melted due to the strong laser irradiation, resulting in a complete disappearance of the microcones. We observed a ripple structure at the peak laser fluence of 19.1 J/cm2. The ripples were
Y. Kawakami, E. Ozawa / Applied Surface Science 218 (2003) 175–187 Fig. 2. SEM microphotograph of tungsten microcone obtained after irradiation at the peak laser fluence of 5.2 J/cm2 in 4 kPa of He gas: (a) 1 shot, (b) 30 shots, (c) 300 shots, (d) 600 shots, (e) 2400 shots, (f) 7200 shots, (g) 24 000 shots, and (h) mushroom structure (1200 shots) (d–h: 458 SEM views).
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Fig. 3. Relationship between ablation amount and laser shots at peak laser fluence of 5.2 J/cm2 in 4 kPa of He gas.
spaced at intervals of about 3 mm, and each was independent from the laser wavelength. This may have been related to some thermal effect stemming from the periodic laser thermal shock. At the peak laser fluence of 19.1 J/cm2, similar ripple structures were observed when the number of laser shots was far below 1200. Next, we investigated the action of the laser pulses. Fig. 5 shows scanning electron micrographs of the
tungsten surface obtained by the laser irradiation with a peak laser fluence of 19.1 J/cm2 after 30, 100, 200, and 12 000 pulses. Tungsten tips (nuclei) were observed (Fig. 5(a) and (b)), but instead of growing into microcones, they assembled into a ripple pattern (Fig. 5(c)), and then a ‘canyon’ structure (Fig. 5(d)). These structures are thought to have been generated by preferential ablation. Fig. 6 shows the ablation rate at a peak laser fluence of 19.1 J/cm2.
Fig. 4. SEM micrographs of tungsten microcone obtained after irradiation at various peak laser fluences after 1200 shots in 4 kPa of He gas: (a) 3.0 J/cm2, (b) 7.6 J/cm2, (c) 10.6 J/cm2, and (d) 19.1 J/cm2.
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Fig. 5. SEM microphotograph of tungsten microcone obtained after irradiation at the peak laser fluence of 19.1 J/cm2 in 4 kPa of He gas: (a) 30 shots, (b) 100 shots, (c) 200 shots, and (d) 12 000 shots (d: 458 SEM views).
In comparison with Fig. 3, we see that the extent of ablation increases in proportion without changing the slant, as the ablation takes place like a drilling process, with no observable cone growth. At the lower fluence of 10.6 J/cm2, we observed a mixture of microcones and solidified tungsten. Thus, the critical peak laser fluence for the transition from microcone arrays to the random structure was about 10 J/cm2 under the present experimental conditions.
Table 1 summarizes the various surface morphologies under an He gas atmosphere as the peak laser fluence changes. 3.3. Influence of the gaseous atmosphere To investigate the influence of the gas species on the formation of microcones, we measured the surface profile of laser-irradiated areas by a profilometer.
Fig. 6. Relationship between ablation amount and laser shots at peak laser fluence of 19.1 J/cm2 in 4 kPa of He gas.
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Table 1 Various surface morphologies by peak laser fluence Peak laser fluence (J/cm2) Surface morphology
3–10 Microcone
10–12 Microcone and smooth surface by melting
12–20 Ripple pattern structure
Fig. 7 shows the surface profiles of tungsten microcones formed in helium at 4 kPa after (a) 450 shots and (b) 600 shots at the fluence of 10.6 J/cm2. The height of the microcones protruding from the original tungsten surface was about 7–12 mm, and it increased with the number of laser shots. Thus, the re-deposition of the ablated material is clearly one of the dominant mechanisms of microcone growth. We can summarize the mechanism of microcone growth in an He gas as follows. First, the initial laser pulse produces a rough surface with crater structures. Over the next several tens of pulses, tungsten hemispheres undergo repeated stages of melting and solidification. The protuberance and dome structures of the rough surface are formed during the first of the foregoing melting stages, and subsequently the dome structures become fixed as the cone morphology. The number of protuberances is large in the nucleation stage, but decreases over repeated laser pulses until a stable surface is obtained. During the next several hundred pulses, the protuberances change into mushroom-like structures due to the preferential ablation around the normal microcones. The top and bottom
area of each cone are easily melted, since the top absorbs heat very efficiently and the bottom has a higher ablation rate than the sides. Once the mushroom structure has appeared, the top of the mushroom shields the sides. Evaporated tungsten atoms and clusters from the bottom area are then deposited on the top and sides of the cone, increasing the cone sizes. In this process, the melting and solidification of the tungsten tips play a very important role in producing the microcone structure. Fig. 7 revealed that tungsten microcones could be generated in the high-purity inert gas system. To investigate the effect of surrounding gas species, we used SF6 as the ambient gas. Fig. 8 shows scanning electron micrographs of tungsten microcones formed in SF6 at 50 kPa after (a) 100, (b) 300, (c) 600, and (d) 2400 shots at 5.2 J/cm2. The microcones in the SF6 gas were similar to the microcones in the He gas in morphology, but they had different surface profiles under the same conditions. These differences might have been caused by the etching effect and different growth mechanism. Based on the measurement of the surface-height profiles (Fig. 9(a)), the etching rate of the substrate in SF6 at 4 kPa and 9.3 J/cm2 was estimated at about 7.3 nm/ shots, or nearly double the etching rate in helium (about 3.6 nm/pulse at 4 kPa). These results show that the etching of the substrate by SF6 gas strongly influenced the ablation of the tungsten substrate. In the comparison of surface profiles generated in helium and SF6 gas, the aspect ratios were nearly the same but the surfaces generated in helium had deeper ditches
Fig. 7. Surface profile of the tungsten microcone height obtained after irradiation in He gas at 4 kPa with 10.6 J/cm2: (a) 450 shots, (b) 600 shots.
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Fig. 8. SEM microphotograph of tungsten microcone obtained after irradiation in SF6 at 50 kPa with 5.2 J/cm2: (a) 100 shots, (b) 300 shots, (c) 600 shots, and (d) 2400 shots (458 SEM views).
and a lower cone height. On this basis, we believe that the etching rate exceeds the deposition rate at the top of the microcones. Tungsten sulfide and tungsten fluoride might be formed by the chemical reaction between the tungsten and SF6. Since tungsten fluoride is in the vapor phase at room temperature, tungsten sulfide, a material with a lower melting point than pure tungsten, should be a nuclei of the microcone growth when the VLS (vapor–liquid–solid) mechanism is dominant.
Thus, the tungsten and Si microcone formations in SF6 seem to differ, in that the former depends not on the VLS mechanism, but the etching and re-deposition processes. To investigate these mechanisms further, we analyzed the chemical component of the cone against its depth using Auger depth-profiling analysis. The spot size, electron voltage, electron current, sputter ion voltage, and sputter rate were 500 nm, 10 kV, 200 nA, 2 kV, and about 8 nm/min at 308 of incidence
Fig. 9. Surface profile of the tungsten microcone height obtained after irradiation after 1200 shots with 9.3 J/cm2. (a) SF6 at 4 kPa; (b) air.
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Fig. 10. Auger depth-profiling analysis of microcone surface: (a) in SF6 at 4 kPa and (b) in air.
of the ions, respectively. Fig. 10(a) shows the tungsten and sulfur concentrations when SF6 was used as a surrounding gas at the top spheres and centers of the cones as schematically indicated in Fig. 10(a). Sulfur was only detectable in very thin surface layers (sputter time < 1 min; depth < 8 nm) deposited on both the top spheres and centers of the cones. The synthesized sulfide might have suppressed the re-deposition at the tops of the cones, and the VLS mechanism prevented the cones from acting as nuclei of growth due to the VLS mechanism. Such a suppression of re-deposition could also explain why the microcones in SF6 were blunt even though the etching rate in the valley area was higher than that in He. We can summarize that tungsten is etched at a higher rate in an SF6 gas atmosphere than in He. To clarify further, we need to consider the how tungsten reacts chemically with sulfur and fluorine to form tungsten sulfide and tungsten fluoride, respectively. These reaction processes may increase the etching rate. In the reaction between tungsten and fluoride, the vapor pressure of fluoride is so high that the reaction acts only as an etching process. However, if sulfide is deposited on the surfaces of the cones, the cones can be expected to suppress the etching and increase the viscosity of the melt caused by the laser pulses. The VLS growth prevents the cones from serving a nuclei. Microcones in the SF6 gas therefore acquire a blunted
form, and the valleys deepen due to the high etching rate resulting from the fluoride formation. When air was used as a surrounding gas, tungsten oxide formed a thin layer at the tops and middles of the microcones, as shown in the Auger depth-profiles in Fig. 10(b). The thickness of the oxide layers was more than 2 mm (sputter time > 200) at the centers of the cones, and almost no impurities were observed at the tops. These findings imply that the VLS mechanism has no direct influence on the microcone formation in air, since the top tungsten sphere plays no role as a nucleus of the VLS growth. After 10 min of Auger depth-profiling analysis of the top layer, the oxygen content dropped to a very low level approaching that of noise. Therefore, we terminated the analysis at 50 min. The vapor pressure of tungsten oxide is so high that air acts as a highly efficient etching agent. However, the oxide vapor generated has no influence on the microcone formation. The oxidation was confined to the top area, where it left a deposition of oxide after the laser pulses ceased. Fig. 11 shows scanning electron micrographs of tungsten microcones obtained by irradiation at a peak laser fluence of 9.3 J/cm2 after 1200 pulses in (a) SF6, (b) air, and (c) helium, respectively. In the SF6 atmosphere (Fig. 11(a)), blunt spike microcones were formed, and surface cracking was observed. The cracking was considered to be induced by the sulfide
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Fig. 11. SEM microphotograph of the tungsten microcone obtained by irradiation at the peak laser fluence of 9.3 J/cm2 after 1200 shots in various ambient gas atmospheres: (a) SF6 at 4 kPa, (b) air, and (c) He at 4 kPa (458 SEM views).
generated on the cone surface via the process previously illustrated in Fig. 10(a). In the samples irradiated in air (Fig. 11(b)), the microcone surfaces were very rough with abundant cracking. This roughness
and cracking may have been due to the formation of tungsten oxide (Fig. 10(b)), a material with a different thermal expansion coefficient from tungsten. Fig. 12 shows the ablation rate at peak laser fluence between
Fig. 12. Relationship between ablation rate and peak laser fluence after 1200 shots in various ambient gas atmosphere.
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5.2 and 19.1 J/cm2 after 1200 pulses in various ambient gas atmospheres. At low-pressure, the ablation rate increases as the peak laser fluence rises. This can be explained by the increase of the energy density against the target. In contrast, the ablation rate decreases as the peak laser fluence rises under an atmospheric condition (air) or in SF6 at 50 kPa. These findings were thought to result from a chemical reaction of oxygen and SF6. As even the minimum peak laser fluence (5.2 J/cm2) exceeds the threshold fluence for tungsten, the increase in the ablated area due to surface modification is not negligible when the fluence rises to high levels. It also appeared that the absorption of the laser to the plume might have been very small, since the plasma plume was very small at all laser fluences observed. To evaluate the optimum laser fluence for the formation of the microcones, we varied the peak laser fluence between 3.0 and 19.1 J/cm2. The microcones were formed at peak laser fluences of 3.0–10 J/cm2 in He, 5.0–10 J/cm2 in air, and 3.0–12 J/cm2 in SF6. At peak laser fluences above those levels, the irradiated surfaces were so heavily damaged that the microcones themselves melted down, resulting in a complete elimination of microcone formation. Earlier in Section 1, we stated that several reactive gases played important roles in determining the morphologies of the Si [8–13], polymer [14], YBa2 Cu3 O7x [15,16] and Si3N4 On the other hand, in recent experiments targeting single-crystal Ge [19], polycrystalline Ti [19], and single-crystal tungsten [20], impurities had no effect on the growth of the microcones. Under our experimental conditions, the etching and re-deposition of ablated materials contributed more to the growth of tungsten microcones than the VLS.
4. Conclusions Our group has formed a series of tungsten microcones with aspect ratios greater than 6 by laser ablating a substrate of high-purity tungsten in various ambient gas atmospheres. In doing so, we have reached the following conclusions. 1. Microcone growth strongly depended on the number of laser pulses and the changes in surface
morphology from crater, tip, and mushroom structures to cones. 2. The influence of the peak laser fluence on the morphology of the microcone arrays was confirmed. Microcones formed at a peak laser fluence of about 3.0–10 J/cm2 in helium. The critical peak laser fluence needed to change the morphology from microcones to a random structure was about 10 J/cm2 under our experimental conditions. 3. The influence of ambient gas species on the morphology of the microcone arrays was clarified. Microcones were formed in He and air under our experimental conditions. In SF6, the etching effect from the chemical reaction between the gas and tungsten contributed to the formation of microcones with blunt spikes; re-deposition could not occur on the top portions of the microcones. The tungsten tips were also etched/vaporized as a result of the dense SF6 gas around the top sections of the microcones. These factors limited the height of the microcones. 4. The formation and growth mechanism of the microcones seems to differ from the VLS mechanism. Since nucleation and elimination processes are involved, the capillarity effect may operate through to the surface energy of the molten tungsten.
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Tungsten microcone growth by laser irradiation Yuji Kawakami*, Eiichi Ozawa1 Gas Deposition Division, Vacuum Metallurgical Co., Ltd., 516 Yokota, Sanbu-machi, Sanbu-gun, Chiba 289-1297, Japan Received 9 August 2002; received in revised form 3 March 2003; accepted 11 April 2003
Abstract Nanosecond pulsed-Nd:YAG laser irradiation of a target material with second-harmonic waves (wavelength ¼ 532 nm) under various gas atmospheres resulted in the formation of tungsten microcone arrays with a high aspect ratio protruding from the initial surface of the target material. The peak laser fluence was changed from 3.0 to 19.1 J/cm2. A tungsten substrate was irradiated with between 1 and 48 000 laser shots. The number of laser shots strongly influenced the growth and morphology of the microcones: several tens or hundreds of pulses created only rough surfaces, and further pulses created the microcones. The microcones grew to heights of up to 20 mm and narrowed to tips of about 1.5 mm in diameter. Our observations reveal that tungsten microcone can be generated in various ambient gases. We experimentally evaluated the microcone morphology in order to determine the mechanism of microcone formation in ambient atmospheres of helium, SF6, and air. The microcone formation appears to be a function of the repeated melting and solidification of the tungsten tips of the structures by the pulsed laser irradiation on the top surface. The morphology of the microcone depends on the ambient gas atmosphere used. # 2003 Elsevier Science B.V. All rights reserved. PACS: 79.20.Ds; 81.65.Cf Keywords: Nanosecond pulsed-Nd:YAG laser; Microcone growth; High aspect ratio; High-purity tungsten; Inert gas atmosphere; Chemical gas atmosphere
1. Introduction The micro-processing of solid surfaces by laser ablation has attracted considerable attention over the last two decades, with marked progress achieved in forming surface modifications and patterns such as laser-induced periodic surface structures (LIPSSs) [1–3], coherent arrays of ultrafine particles [4–7], *
Corresponding author. Tel.: þ81-475-89-3816; fax: þ81-475-89-1469. E-mail address: [email protected] (Y. Kawakami). 1 Present address: Nanotechnology Researchers Network Center of Japan, Toranomon 30 Mori Bldg. 2F, 3-2-2 Toranomon, Minatoku, Tokyo 105-0001, Japan. Tel.: þ81-3-5404-3280; fax: þ81-3-5404-3290.
and microcone and microcolumn arrays [8–15]. Many investigations focused on the fabrication of microcones and microcolumns, structures thought to offer good utility as photonic materials and field emitters. The target materials used in these studies have included silicon (Si) [8–13], a polymer (polyimide) [14], oxides (YBa2 Cu3 O7x , ZrO2) [15–17], nitride (Si3N4) [17], and metals (Co, Ti, W) [18–20]. A KrF (248 nm) excimer laser and a Ti: sapphire (800 nm) laser have been used on Si substrates to investigate the influence of ambient gas species such as SF6 [8–11,13], Cl2 [13], and air [8,11,12]. The growth of microcones on Si substrates is thought to be driven by a chemical reaction with the surrounding gases, based on earlier results showing an absence of microcone formation in an inert
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00615-9
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(Ar) gas [8], and an absence of sharp conical spikes in a vacuum [13]. In the case of polymer, Dyer et al. [14] reported that the cones were produced by a shielding effect of the particulate impurities. Similarly, the impurity shielding mechanism for cone formation was observed for multi-component materials such as YBa2 Cu3 O7x . The yttrium concentration increased at the cone tip during the initial stage of microcone growth; and this yttrium-rich shell appeared to suppress laser vaporization [15,16]. In a detailed investigation of the mechanism of Si3N4 microcone formation, Miyamoto and Maruo proposed the following relation between the cone formation and debris particles: (1) the debris particles are initially produced and adhere to the laser-irradiated surface; (2) the ablation around the debris particles produces small protuberances; (3) the increase in the surface area reduces the laser fluences on the sides of the protuberances; and as a consequence, (4) conical protuberances with a high aspect ratio are generated [17]. In the case of metals, however, the mechanism of cone formation has not been completely understood. Two mechanisms have been proposed. Based on experiments on the generation of Co microcones, Usoskin et al. [18] proposed ‘a growth model’. They conjectured that the initial laser irradiation formed ‘nuclei’ of metallic cones which later assembled into a surface relief (initial surface patterns) and grew through the ablation of ‘hot’ melted valleys at the focal points of the electromagnetic energy from the light waves. In an investigation of titanium microcones, Dolgaev et al. [19] suggested that the nuclei were formed by ‘capillary waves’ induced when the difference in the surface tension of the melted material generated the distribution of reflective index over the surface. Since the bottom of the wave was ablated more than the top, they conjectured that the evaporation produced a metal vapor that deposited on the top of the wave. When this occurred, the cones were produced by the ‘lift-up’ of the liquid and vapor to the cone tips. Both of the foregoing studies suggested that microcones are generated by the distribution of the ablation rate between the tips and the valleys. However, no detailed analyses have been performed to clarify the density of the cones, the density of the irregular arrays, or other factors involved in the formation mechanism and growth rate of metallic microcones.
We recently discovered the coherent arrays of ultrafine particles around a laser-irradiated mark on a single crystal of tungsten [4–7]; however, we could not produce them with an aspect ratio greater than 1.5. We next tried to grow tungsten microcones on a single crystal tungsten (99.99 mass%) [20] and on very-highpurity tungsten (99.99999 mass%) in a helium background gas. In doing so, however, we did not examine the relationship between the microcone morphology and experimental conditions in detail. In this study, we attempted to synthesize tungsten microcones with a high aspect ratio by investigating the growth mechanism of microcones with varying experimental parameters such as laser pulses, peak laser fluences, and ambient gas atmosphere.
2. Experimental The experimental apparatus used in this study (Chamber; ULVAC Materials Technology, Chiba, Japan) is schematically illustrated in Fig. 1(a), and an overhead view of the laser irradiation chamber is shown in Fig. 1(b). A nanosecond pulsed-Nd:YAG laser (Spectra-Physics, INDI-50, CA, 532 nm wavelength, 4.5–5.5 ns pulse duration) was used. The spatial profile of this laser beam is nearly Gaussian. As shown in Fig. 1(a), the beam was introduced into the chamber through a fused silica window and focused onto the tungsten surface with a plano-convex singlet lens (BK7, focal length 495.7 mm, CVI Laser Corporation, New Mexico). We used a round aperture to generate an area of high intensity. The diameter of the damage area, D, is related to the peak laser fluence, F0, as [21]: F0 D2 ¼ 2o20 ln (1) Fth where Fth is the threshold fluence and o0 the 1/e2 Gaussian beam radius (¼107 mm). In the case of the Gaussian beam, the peak laser fluence is obtained from laser pulse energy, Epulse, as F0 ¼
2Epulse po20
(2)
The peak laser fluence was varied between 3.0 and 19.1 J/cm2 by changing the distance from the lens (f ¼ 495:7 mm) to the substrate. The angle of
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Fig. 1. (a) Schematic diagram of the experimental apparatus and (b) photograph of laser irradiation experiment.
incidence, y, was 108, and the same region of the tungsten substrate was irradiated by 1–48 000 laser shots repeated at a frequency of 20 Hz. A high-purity tungsten substrate (Vacuum Metallurgical, Chiba, Japan, 99.99999 mass%, f10 mm t3 mm) was used as the laser ablation target. The highpurity and polycrystalline tungsten were produced by the chemical vapor deposition of decomposed WF6 vapor. Under X ray diffraction (XRD), the substrate surface exhibited preferred crystal orientation to the h1 0 0i direction [22]. The substrate surface was mirror-polished using silicon carbide polishing paper and diamond paste, then washed by acetone and petroleum benzine using a supersonic wave cleaner. The follow-
ing four atmospheres were used to investigate how the ambient gas affected the microcone formation: (1) lowpressure, high-purity helium gas (99.9999 mass%), (2) low-pressure SF6 gas (99.999 mass%), (3) air, and (4) vacuum. Before the experiments, the chamber was evacuated to a pressure of less than 1 104 Pa. The morphology of the laser-irradiated surface was observed by scanning electron microscopy (SEM; JEOL Ltd., JSM-6300F, Tokyo, Japan). The accelerating voltage of the electron beam was 5 kV. The substrates were observed at normal incident (08), and also with the substrate inclined 458 between the direction of the electron beam and the normal direction to the substrate. Surface-height profiles were
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measured using a Dektak IIA profilometer. Depth profiles of the microcones were also analyzed using auger electron spectroscopy (AES; Physical Electronics Industries, PHI-660, Minneapolis, USA).
3. Results and discussion 3.1. Influence of the number of laser shots SEM was used to observe the morphology of the tungsten surface irradiated by the nanosecond pulsedNd:YAG laser. Fig. 2 shows scanning electron micrographs of tungsten microcones after laser irradiation with a peak laser fluence of 5.2 J/cm2, with the number of laser shots varied from (a) a single shot to (g) 24 000 shots. The initial laser shot produced only a rough surface with craters, as shown in Fig. 2(a). Thirty laser shots created melted/solidified tungsten hemispheres (Fig. 2(b)). The reduction in the number of hemispheres was due to capillarity among the random protrusions (Fig. 2(b) and (c)). The large hemispheres absorbed the smaller ones, resulting in a reduction of the surface energy. Over several hundred laser shots, the tungsten hemisphere then acquired a tip structure similar in form to a mushroom (Fig. 2(h)). The microcone array (Fig. 2(d)) was generated from the hemispheres, which resembled nuclei in structure. As shown in Fig. 2(e)–(f), the number of laser shots strongly influenced the morphology of the microcone. As the number of laser shots increased, the height of the cone rose from 10 to 20 mm, whereas the diameter of the spheres generated at the tips of the microcones remained almost constant. We also found that more than 2400 shots were needed to produce a cone with a high aspect ratio of greater than 6 (the ratio of the diameter to the height of the cone). This tendency was also observed in previous reports on microcolumns [8,9,12,15]. Microcone growth did not occur over any part of the laser-irradiated area, primarily due to disturbances in the laser spot resulting from thermal strain waves, thermal expansion, temperature gradients, non-uniformity in temperature due to the intensity distribution of the Gaussian beam, and other factors. The density of the microcones decreased above 2400 laser shots (Fig. 2(e) and (f)). We observed similar results for single crystal tungsten [20]. At 24 000 laser shots, the cones melted and disappeared (Fig. 2(g)).
To investigate the generation and melting of the cones, we measured the ablation rates by measuring the change of the mass of the target due to the laser irradiation. The ablation rate is plotted in Fig. 3 as a function of the number of laser shots when the peak laser fluence is 5.2 J/cm2. The initial slope of the ablation rate was almost proportional to the number of pulses up to 10 000, and the ablation rate was calculated at about 70 ng/pulse in areas where microcone growth was observed. However, when the number of laser shots rose above 10 000, the ablation rate dropped by a factor of 2 to about 35 ng/pulse. This change came from the decline in the laser fluence due to the increase in the surface area accompanying the higher surface roughness and slope. 3.2. Influence of the peak laser fluence The cone formation process is known to be significantly affected by the laser fluence [16]. From the calculation of the absorptivity, the laser ablation threshold of tungsten at 532 nm/5 ns is approximately 1 J/cm2 [23]. When measured experimentally, the threshold of fluence for tungsten was 380 mJ/cm2 with an excimer laser (193 nm, 10 ns) [24] and 1.58 J/cm2 with an Nd:YAG laser (1064 nm, 9 ns) [25]. These predicted and observed thresholds of the laser fluence are basically valid when the laser beam exhibits a flattop profile. When the special profile of the laser beam is nearly Gaussian, we should employ the peak laser fluence defined by Eq. (2). Thus, we examined the effect of the laser fluence by setting the minimum peak laser fluence at 3.0 J/cm2, and then increasing it up to 19.1 J/cm2. The corresponding ‘average’ laser fluence (¼laser intensity/spot area) was between about 1.5 and 9.6 J/cm2. Fig. 4 shows scanning electron micrographs of the tungsten microcone obtained by irradiation at the peak laser fluences of (a) 3.0, (b) 7.6, (c) 10.6, and (d) 19.1 J/cm2 after 1200 laser shots. Microcones were formed with the irradiation even at lowest peak laser fluences of 3.0 J/cm2. Microholes distributed irregularly at sites between the cones were observed when the peak laser fluences were 7.6 and 10.8 J/cm2. At the highest peak laser fluences (Fig. 4(d)), the tungsten tips were judged to be melted due to the strong laser irradiation, resulting in a complete disappearance of the microcones. We observed a ripple structure at the peak laser fluence of 19.1 J/cm2. The ripples were
Y. Kawakami, E. Ozawa / Applied Surface Science 218 (2003) 175–187 Fig. 2. SEM microphotograph of tungsten microcone obtained after irradiation at the peak laser fluence of 5.2 J/cm2 in 4 kPa of He gas: (a) 1 shot, (b) 30 shots, (c) 300 shots, (d) 600 shots, (e) 2400 shots, (f) 7200 shots, (g) 24 000 shots, and (h) mushroom structure (1200 shots) (d–h: 458 SEM views).
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Fig. 3. Relationship between ablation amount and laser shots at peak laser fluence of 5.2 J/cm2 in 4 kPa of He gas.
spaced at intervals of about 3 mm, and each was independent from the laser wavelength. This may have been related to some thermal effect stemming from the periodic laser thermal shock. At the peak laser fluence of 19.1 J/cm2, similar ripple structures were observed when the number of laser shots was far below 1200. Next, we investigated the action of the laser pulses. Fig. 5 shows scanning electron micrographs of the
tungsten surface obtained by the laser irradiation with a peak laser fluence of 19.1 J/cm2 after 30, 100, 200, and 12 000 pulses. Tungsten tips (nuclei) were observed (Fig. 5(a) and (b)), but instead of growing into microcones, they assembled into a ripple pattern (Fig. 5(c)), and then a ‘canyon’ structure (Fig. 5(d)). These structures are thought to have been generated by preferential ablation. Fig. 6 shows the ablation rate at a peak laser fluence of 19.1 J/cm2.
Fig. 4. SEM micrographs of tungsten microcone obtained after irradiation at various peak laser fluences after 1200 shots in 4 kPa of He gas: (a) 3.0 J/cm2, (b) 7.6 J/cm2, (c) 10.6 J/cm2, and (d) 19.1 J/cm2.
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Fig. 5. SEM microphotograph of tungsten microcone obtained after irradiation at the peak laser fluence of 19.1 J/cm2 in 4 kPa of He gas: (a) 30 shots, (b) 100 shots, (c) 200 shots, and (d) 12 000 shots (d: 458 SEM views).
In comparison with Fig. 3, we see that the extent of ablation increases in proportion without changing the slant, as the ablation takes place like a drilling process, with no observable cone growth. At the lower fluence of 10.6 J/cm2, we observed a mixture of microcones and solidified tungsten. Thus, the critical peak laser fluence for the transition from microcone arrays to the random structure was about 10 J/cm2 under the present experimental conditions.
Table 1 summarizes the various surface morphologies under an He gas atmosphere as the peak laser fluence changes. 3.3. Influence of the gaseous atmosphere To investigate the influence of the gas species on the formation of microcones, we measured the surface profile of laser-irradiated areas by a profilometer.
Fig. 6. Relationship between ablation amount and laser shots at peak laser fluence of 19.1 J/cm2 in 4 kPa of He gas.
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Table 1 Various surface morphologies by peak laser fluence Peak laser fluence (J/cm2) Surface morphology
3–10 Microcone
10–12 Microcone and smooth surface by melting
12–20 Ripple pattern structure
Fig. 7 shows the surface profiles of tungsten microcones formed in helium at 4 kPa after (a) 450 shots and (b) 600 shots at the fluence of 10.6 J/cm2. The height of the microcones protruding from the original tungsten surface was about 7–12 mm, and it increased with the number of laser shots. Thus, the re-deposition of the ablated material is clearly one of the dominant mechanisms of microcone growth. We can summarize the mechanism of microcone growth in an He gas as follows. First, the initial laser pulse produces a rough surface with crater structures. Over the next several tens of pulses, tungsten hemispheres undergo repeated stages of melting and solidification. The protuberance and dome structures of the rough surface are formed during the first of the foregoing melting stages, and subsequently the dome structures become fixed as the cone morphology. The number of protuberances is large in the nucleation stage, but decreases over repeated laser pulses until a stable surface is obtained. During the next several hundred pulses, the protuberances change into mushroom-like structures due to the preferential ablation around the normal microcones. The top and bottom
area of each cone are easily melted, since the top absorbs heat very efficiently and the bottom has a higher ablation rate than the sides. Once the mushroom structure has appeared, the top of the mushroom shields the sides. Evaporated tungsten atoms and clusters from the bottom area are then deposited on the top and sides of the cone, increasing the cone sizes. In this process, the melting and solidification of the tungsten tips play a very important role in producing the microcone structure. Fig. 7 revealed that tungsten microcones could be generated in the high-purity inert gas system. To investigate the effect of surrounding gas species, we used SF6 as the ambient gas. Fig. 8 shows scanning electron micrographs of tungsten microcones formed in SF6 at 50 kPa after (a) 100, (b) 300, (c) 600, and (d) 2400 shots at 5.2 J/cm2. The microcones in the SF6 gas were similar to the microcones in the He gas in morphology, but they had different surface profiles under the same conditions. These differences might have been caused by the etching effect and different growth mechanism. Based on the measurement of the surface-height profiles (Fig. 9(a)), the etching rate of the substrate in SF6 at 4 kPa and 9.3 J/cm2 was estimated at about 7.3 nm/ shots, or nearly double the etching rate in helium (about 3.6 nm/pulse at 4 kPa). These results show that the etching of the substrate by SF6 gas strongly influenced the ablation of the tungsten substrate. In the comparison of surface profiles generated in helium and SF6 gas, the aspect ratios were nearly the same but the surfaces generated in helium had deeper ditches
Fig. 7. Surface profile of the tungsten microcone height obtained after irradiation in He gas at 4 kPa with 10.6 J/cm2: (a) 450 shots, (b) 600 shots.
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Fig. 8. SEM microphotograph of tungsten microcone obtained after irradiation in SF6 at 50 kPa with 5.2 J/cm2: (a) 100 shots, (b) 300 shots, (c) 600 shots, and (d) 2400 shots (458 SEM views).
and a lower cone height. On this basis, we believe that the etching rate exceeds the deposition rate at the top of the microcones. Tungsten sulfide and tungsten fluoride might be formed by the chemical reaction between the tungsten and SF6. Since tungsten fluoride is in the vapor phase at room temperature, tungsten sulfide, a material with a lower melting point than pure tungsten, should be a nuclei of the microcone growth when the VLS (vapor–liquid–solid) mechanism is dominant.
Thus, the tungsten and Si microcone formations in SF6 seem to differ, in that the former depends not on the VLS mechanism, but the etching and re-deposition processes. To investigate these mechanisms further, we analyzed the chemical component of the cone against its depth using Auger depth-profiling analysis. The spot size, electron voltage, electron current, sputter ion voltage, and sputter rate were 500 nm, 10 kV, 200 nA, 2 kV, and about 8 nm/min at 308 of incidence
Fig. 9. Surface profile of the tungsten microcone height obtained after irradiation after 1200 shots with 9.3 J/cm2. (a) SF6 at 4 kPa; (b) air.
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Fig. 10. Auger depth-profiling analysis of microcone surface: (a) in SF6 at 4 kPa and (b) in air.
of the ions, respectively. Fig. 10(a) shows the tungsten and sulfur concentrations when SF6 was used as a surrounding gas at the top spheres and centers of the cones as schematically indicated in Fig. 10(a). Sulfur was only detectable in very thin surface layers (sputter time < 1 min; depth < 8 nm) deposited on both the top spheres and centers of the cones. The synthesized sulfide might have suppressed the re-deposition at the tops of the cones, and the VLS mechanism prevented the cones from acting as nuclei of growth due to the VLS mechanism. Such a suppression of re-deposition could also explain why the microcones in SF6 were blunt even though the etching rate in the valley area was higher than that in He. We can summarize that tungsten is etched at a higher rate in an SF6 gas atmosphere than in He. To clarify further, we need to consider the how tungsten reacts chemically with sulfur and fluorine to form tungsten sulfide and tungsten fluoride, respectively. These reaction processes may increase the etching rate. In the reaction between tungsten and fluoride, the vapor pressure of fluoride is so high that the reaction acts only as an etching process. However, if sulfide is deposited on the surfaces of the cones, the cones can be expected to suppress the etching and increase the viscosity of the melt caused by the laser pulses. The VLS growth prevents the cones from serving a nuclei. Microcones in the SF6 gas therefore acquire a blunted
form, and the valleys deepen due to the high etching rate resulting from the fluoride formation. When air was used as a surrounding gas, tungsten oxide formed a thin layer at the tops and middles of the microcones, as shown in the Auger depth-profiles in Fig. 10(b). The thickness of the oxide layers was more than 2 mm (sputter time > 200) at the centers of the cones, and almost no impurities were observed at the tops. These findings imply that the VLS mechanism has no direct influence on the microcone formation in air, since the top tungsten sphere plays no role as a nucleus of the VLS growth. After 10 min of Auger depth-profiling analysis of the top layer, the oxygen content dropped to a very low level approaching that of noise. Therefore, we terminated the analysis at 50 min. The vapor pressure of tungsten oxide is so high that air acts as a highly efficient etching agent. However, the oxide vapor generated has no influence on the microcone formation. The oxidation was confined to the top area, where it left a deposition of oxide after the laser pulses ceased. Fig. 11 shows scanning electron micrographs of tungsten microcones obtained by irradiation at a peak laser fluence of 9.3 J/cm2 after 1200 pulses in (a) SF6, (b) air, and (c) helium, respectively. In the SF6 atmosphere (Fig. 11(a)), blunt spike microcones were formed, and surface cracking was observed. The cracking was considered to be induced by the sulfide
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Fig. 11. SEM microphotograph of the tungsten microcone obtained by irradiation at the peak laser fluence of 9.3 J/cm2 after 1200 shots in various ambient gas atmospheres: (a) SF6 at 4 kPa, (b) air, and (c) He at 4 kPa (458 SEM views).
generated on the cone surface via the process previously illustrated in Fig. 10(a). In the samples irradiated in air (Fig. 11(b)), the microcone surfaces were very rough with abundant cracking. This roughness
and cracking may have been due to the formation of tungsten oxide (Fig. 10(b)), a material with a different thermal expansion coefficient from tungsten. Fig. 12 shows the ablation rate at peak laser fluence between
Fig. 12. Relationship between ablation rate and peak laser fluence after 1200 shots in various ambient gas atmosphere.
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5.2 and 19.1 J/cm2 after 1200 pulses in various ambient gas atmospheres. At low-pressure, the ablation rate increases as the peak laser fluence rises. This can be explained by the increase of the energy density against the target. In contrast, the ablation rate decreases as the peak laser fluence rises under an atmospheric condition (air) or in SF6 at 50 kPa. These findings were thought to result from a chemical reaction of oxygen and SF6. As even the minimum peak laser fluence (5.2 J/cm2) exceeds the threshold fluence for tungsten, the increase in the ablated area due to surface modification is not negligible when the fluence rises to high levels. It also appeared that the absorption of the laser to the plume might have been very small, since the plasma plume was very small at all laser fluences observed. To evaluate the optimum laser fluence for the formation of the microcones, we varied the peak laser fluence between 3.0 and 19.1 J/cm2. The microcones were formed at peak laser fluences of 3.0–10 J/cm2 in He, 5.0–10 J/cm2 in air, and 3.0–12 J/cm2 in SF6. At peak laser fluences above those levels, the irradiated surfaces were so heavily damaged that the microcones themselves melted down, resulting in a complete elimination of microcone formation. Earlier in Section 1, we stated that several reactive gases played important roles in determining the morphologies of the Si [8–13], polymer [14], YBa2 Cu3 O7x [15,16] and Si3N4 On the other hand, in recent experiments targeting single-crystal Ge [19], polycrystalline Ti [19], and single-crystal tungsten [20], impurities had no effect on the growth of the microcones. Under our experimental conditions, the etching and re-deposition of ablated materials contributed more to the growth of tungsten microcones than the VLS.
4. Conclusions Our group has formed a series of tungsten microcones with aspect ratios greater than 6 by laser ablating a substrate of high-purity tungsten in various ambient gas atmospheres. In doing so, we have reached the following conclusions. 1. Microcone growth strongly depended on the number of laser pulses and the changes in surface
morphology from crater, tip, and mushroom structures to cones. 2. The influence of the peak laser fluence on the morphology of the microcone arrays was confirmed. Microcones formed at a peak laser fluence of about 3.0–10 J/cm2 in helium. The critical peak laser fluence needed to change the morphology from microcones to a random structure was about 10 J/cm2 under our experimental conditions. 3. The influence of ambient gas species on the morphology of the microcone arrays was clarified. Microcones were formed in He and air under our experimental conditions. In SF6, the etching effect from the chemical reaction between the gas and tungsten contributed to the formation of microcones with blunt spikes; re-deposition could not occur on the top portions of the microcones. The tungsten tips were also etched/vaporized as a result of the dense SF6 gas around the top sections of the microcones. These factors limited the height of the microcones. 4. The formation and growth mechanism of the microcones seems to differ from the VLS mechanism. Since nucleation and elimination processes are involved, the capillarity effect may operate through to the surface energy of the molten tungsten.
Acknowledgements The authors gratefully thank Prof. Isamu Miyamoto of Osaka University and Dr. Takafumi Seto of the National Institute of Advanced Industrial Science and Technology (AIST) for valuable discussions and Mr. Toshinobu Yoshida of Vacuum Metallurgical Co., Ltd., for surface profile measurements. This work was supported by the R&D Institute for Photonics Engineering (RIPE) entrusted from the Advanced Photon Processing and Measurement Technologies Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] P.M. Fauchet, A.E. Siegman, Appl. Phys. Lett. 40 (1982) 824. [2] J.F. Young, J.E. Sipe, J.S. Preston, H.M. van Driel, Appl. Phys. Lett. 41 (1982) 261.
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