Micro-ablation on silicon by femtosecond laser pulses focused with an axicon assisted with a lens

Applied Surface Science 257 (2010) 476–480

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Micro-ablation on silicon by femtosecond laser pulses focused with an axicon assisted with a lens R. Inoue, K. Takakusaki, Y. Takagi, T. Yagi ∗ Physics Department, Tokai University, 1117 Kita-kaname, Hiratsuka, Kanagawa 259-1292, Japan

a r t i c l e

i n f o

Article history: Received 11 April 2010 Received in revised form 5 July 2010 Accepted 6 July 2010 Available online 14 July 2010 PACS: 42.62−b 42.65Re 78.40Fy 78.47+p 81.20Wk

a b s t r a c t Micro-ablation of crystalline silicon was performed by irradiating a silicon substrate with femtosecond laser pulses of wavelengths 786 nm or 393 nm focused using a conical axicon assisted with a convex lens. Focusing the laser beam close to the tip of the axicon by means of the lens significantly improved the efficiency of concentration of laser pulse energy at the central spot of the resulting Bessel–Gaussian intensity distribution. As a result, micron-sized holes were formed with the diameter determined by the ablation threshold in the calculated fluence profile. It is possible to predict hole size from the laser pulse energy and the wavelength. Crystalline particles, a few tens of nanometers in size, were formed near the ablated zone. © 2010 Elsevier B.V. All rights reserved.

Keywords: Femtosecond laser Micromachining Axicon Debris Clusters Silicon

1. Introduction Compact femtosecond lasers are now established as important machining tools for forming micron-size structures on solid surfaces, owing to the stability of the pulse operation and large throughput. Various materials such as glass [1], metals, and semiconductors [2,3], have been tested for machining purposes by focusing a laser beam with a lens, yielding success in cutting and drilling on the micron scale. Several studies have evaluated the performance of submicron drilling on a silicon substrate, ever since a femtosecond laser beam was successfully focused using a Schwarzschild objective [4]. Machining on length scales smaller than 100 nm was demonstrated by exploiting the controllability of the ablation threshold, adapted to the Gaussian intensity profile with a submicron spot size on the metal [5]. An alternative approach is to use a conical axicon, which produces a Bessel mode beam to ablate a hole with a large aspect ratio on the micron scale [6,7]. In addition to the optimization of the focusing optics, the progression speed of drilling in a bulk or a thin-film material has been inves-

∗ Corresponding author. Tel.: +81 463 58 1211; fax: +81 463 50 2013. E-mail address: [email protected] (T. Yagi). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.016

tigated in several experimental studies [2,3,8–13]. This effect has become predictable using the waveguide model of light propagating through a hole formed deep into the solid and the appropriate single-shot ablation coefficients [14]. There have also been continuous attempts to gain a fundamental understanding of the material removal processes such as excitation and relaxation of carriers, and the mechanisms leading to the lattice decomposition in semiconductors (see [15–18] and references therein). Owing to the ultra-short duration of the pulses, the decomposition process in the target dielectric or semiconductor materials is rather sharply categorized as resulting from the disruption of covalent bonds by the optical excitation of electrons from the valence band to the conduction band, and successive ultra-fast disordering leading to the material ablation. In the present work, we examine the formation of holes on a silicon substrate using femtosecond laser pulses with wavelengths 786 nm and 393 nm, the second harmonics (SH), focused by a conical axicon assisted with a convex lens. We demonstrate that our simple optical arrangement can be used to form a hole on the submicron scale with pulses of energy below 1 ␮J, at a distance just a few millimeters from the tip of the axicon. The formation of debris is also considered briefly in the light of the material decomposition process.

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Fig. 1. Experimental setup with neutral density filter F, convex lenses L1, L2, and L3, LBO C, bending mirrors M1 and M2, electrical shutter S, axicon A, and target T.

2. Experimental Fig. 1 shows the experimental setup used for drilling with the SH. The laser pulses, of duration 110 fs and repetition rate of 500 Hz are focused to LBO with thickness of 1 mm to produce SH pulses. These pulses are collimated and focused with a lens of focal length 1.5 m. A conical axicon made of BK7 with an opening angle of 160◦ and a cone height of 4 mm is placed with its tip 3 mm in front of the beam waist of the focused laser beam. The silicon surface with (1 0 0) orientation is placed on the output side of the axicon, 3 mm in front of the tip, at the center of the Bessel beam where the fluence is highest. The optical elements used for generating the SH are removed for drilling with the 786-nm wavelength laser beam. The laser pulse energy is adjusted with a neutral density (ND) filter so that the laser fluence at the center of the Bessel beam can be a few times the ablation threshold. The convex lens serve to condense the laser pulse energy to the central spot of the Bessel beam pattern, giving a better efficiency than with the axicon alone. The drilled holes and debris distribution on the silicon substrate are observed with FESEM (Hitachi S-4800) unless otherwise specified. (Wet SEM (Hitachi S-3200N) was used in some figures.) 3. Model calculation When a fundamental Gaussian beam is focused with the lens and enters an axicon with negligible thickness and an opening angle close to 180◦ , the electric field at a radial distance r0 from the optical axis and within the plane containing the axicon tip is:



EG (r0 , z) = E0 exp −



r0 2 ω(z)2

 

z + r0 −

exp ik

r02 2R(z)



(1)

where z is the distance of the axicon tip from the Gaussian beam waist, ω is the spot size, R is the radius of curvature of the incident wave front,  is the tilt angle of the conical wave front given by  = (n − 1)(90◦ − /2) for a refractive index n and axicon opening angle . The electric field at an arbitrary distance r from the optical axis on the observation plane is calculated as:



E(r, s0 , z) = 0

∞

 ik  s0



× exp −

EG (r0 , z)r0 J0

ik(r 2 + r02 ) 2s0

Fig. 2. Calculated fluence distributions on the sample plane, compared with the Gaussian distribution multiplied by 100 (G × 100).

mined spot sizes (the 1/e width of the E field distribution) of 170 ␮m and 100 ␮m, and with the pulse energies of 2.0 ␮J and 0.92 ␮J for the 786-nm and the 393-nm wavelength beams, respectively. Fig. 2 shows the fluence distributions of BGB, calculated numerically at a distance of 3 mm from the tip of the axicon. The fluence distribution of the Gaussian fundamental beam at the beam waist focused with the lens alone is shown magnified 100 times for comparison. These results confirm that the fluence of the central peak of the BGB is two orders of magnitude greater than that of the Gaussian beam at r = 0. 4. Results Fig. 3 shows the observed beam pattern of the BGB formed on the sample surface after 1000 shots of 786-nm wavelength irradiation with a pulse energy of 68 ␮J. The concentric ring pattern shows grain-like debris accumulating between the ablated gullies. The solid and dashed lines indicate the contours of maximum and zero fluence, respectively, obtained from the calculated profile plotted in Fig. 2. These results show that the fluence model predicts the fringe positions accurately, for the present experimental setup. The holes formed at the center of the BGB after 250 and 1000 laser shots with a pulse energy of 2.0 ␮J are shown in Fig. 4(a) and (b), respectively. These results show the formation of a hole 4 ␮m in diameter. As the pulse accumulation is increased as in Fig. 4(b), three dark regions appear around the hole, as shown by the arrows. They are located on the innermost ring of the BGB. They are the

 krr 

 dr0

0

s0 (2)

where J0 is the zeroth order Bessel function and s0 the distance of the observation plane from the axicon tip [19–21]. The energy fluence distribution is calculated from (2) as F = ε0 |E|2 ct/2, where ε0 is the dielectric constant, c is the speed of light, and t the duration of the laser pulses. The fluence distributions of the Bessel–Gaussian beam (BGB) are calculated with E0 , which is derived from the experimentally deter-

Fig. 3. An ablation pattern produced by 1000 shots of exposure with pulse energy of 68 ␮J at 786 nm wavelength. Solid lines: peak intensity contours. Dashed lines: zero-intensity contours.

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Fig. 4. Holes formed by the central peak of the BGB with pulse energy of 2 ␮J at 786 nm wavelength after (a) 250 shots, (b) 1000 shots (arrows indicating effect of laser radiation), (c) 500 shots, (d) magnified view of debris 10 ␮m from the hole center in (c).

regions that have been little affected by the laser, then serve to determine the threshold fluence for multiple-shot ablation. Fig. 4(c) is a magnified view of the hole after 500 shots, where a significant thermal effect is not seen on the edge or in the vicinity of the hole but covered with grain-like debris. The debris, 10 ␮m from the center of the hole, is mostly aggregated to form small particles, as shown in Fig. 4(d). Close to the central hole in Fig. 4(b) and (c), periodically spaced vertical lines, with a spacing of 610–700 nm, are faintly visible. The polarization of the laser beam is horizontal in these figures. A much smaller hole size is expected when the 393-nm wavelength laser is used. A snapshot of the hole produced by 5000 shots of the pulse energy 0.92 ␮J is given in Fig. 5(a). The circles drawn as white solid lines indicate the peak intensity of the fringes calculated from the model. The dark dotted circle marks the position where the intensity in the central spot equals the peak intensity of the first ring. The debris takes two distinct appearances, depending on its location. It resembles cotton wool along the edge of the hole and the outer peak intensity circles. Its fine features in these regions could not be resolved with the highest magnification of the FESEM (100,000×). In contrast, the debris takes the form of fine grain-like particles between the peak intensity fringes. Fig. 5(b) shows that, 15 ␮m from the central hole, the debris are predominantly particlelike, and accumulate between the peak intensity fringes, while the wool-like debris is not seen. A stronger magnification of the particulate debris is shown in Fig. 5(c). The particles have a rugged shape, looking roughly hexagonal or square, where indicated by the arrows, and their average size is slightly less than 30 nm. As the number of shots is reduced, the hole drilled at the location of the central spot of the BGB is clearly seen. Fig. 6(a) and (b), which was obtained by Wet SEM, shows the ablated patterns after 250 and 1000 shots, respectively. The hole in Fig. 6(a) is further magnified in Fig. 6(c). The holes created in the present experiment

have the diameter of around 1 ␮m and display a sharp edge and the peripheral embossment less than a few tenths of micrometers in height. The line pattern formed below the hole has a periodicity of 250–290 nm.

5. Discussion Focusing a femtosecond laser beam with a simple optical arrangement (a conical axicon preceded by a focusing lens) produces sufficient fluence at the center of the resulting Bessel beam to ablate a hole 1 ␮m in size with a laser pulse energy of 1 ␮J. In the present experiment with a 393-nm wavelength laser, the central spot of the BGB, the region contained within the first zero-intensity contour, contains an energy of 2 × 10−8 J per pulse. The efficiency, defined as the fraction of the total laser energy utilized for drilling the hole, is 2%. This small number is understandable because the intensity distribution on the target, of overall size 100 ␮m, contains 50 circular fringes that share the laser pulse energy nearly equally. The efficiency can be improved much further by reducing the spot size with a lens of shorter focal length and by using a smaller pulse energy. The edges of the holes in Fig. 4(a) and (b) were produced by a fluence of 0.3 J/cm2 , which makes reasonable agreement to the ablation threshold ∼0.2 J/cm2 for wavelength 786 nm [16]. The circular zone corresponding to the innermost ring of the BGB in Fig. 4(b), which seems to be affected weakly by the laser radiation, will provide the threshold of 0.15 J/cm2 to start ablation at the smallest rate in three categories of the ablation rates. The formation of the hole and the surface modification in the peripheral ring are considered to be due to rate of ablation being the two orders of magnitude greater in the central region than in the ring [14]. The ablation patterns in Fig. 6(a) and (b), produced with

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Fig. 5. Ablation pattern after 5000 shots of exposure with pulse energy 0.98 ␮J at 393 nm wavelength. (a) Over all view. Solid lines: peak intensity contours. Dark dotted line: contour with the same intensity as the first fringe peak. (b) Magnified image 15 ␮m away from the central hole, with white curves indicating fringe peak contours. (c) Further magnification of (b) with arrows indicating hexagonal (solid arrow) and square (dotted arrow) particles.

393-nm wavelength irradiation, show the surface material having been removed in the second fringe, and affected by the laser radiation down to the fifth fringe positions in increasing the laser shot numbers. The threshold fluence for this process is estimated to be 0.05 J/cm2 , while the fluence at the edge of the hole (1 ␮m from the center) in Fig. 6(b) is 0.3 J/cm2 . The larger difference between the fluences at the edge of the hole and the peripheral zone, for 393 nm as opposed to 786 nm radiation, is due to the lower melting threshold for the shorter wavelength. These effects need to be considered when using the short wavelength for achieving clean microdrilling with the axicon optics. The area with the diameter of 2 ␮m around the hole edge, shown in Fig. 6(c), appears to be molten and embossed by the pressure of the ablated gas.

Fig. 6. Holes formed by the central spot of the BGB with pulse energy of 0.98 ␮J at 393 nm wavelength after (a) 250 shots and (b) 1000 shots. (c) Magnified view of the hole in (a) observed at the angle of 45◦ to the surface normal.

The microscopic holes formed by this method have a sharp edge and a weak embossment, which results from the BGB intensity profile. It is possible that the peripheral zone around the central peak, where the fluence exceeds the melting threshold but lies below the ablation threshold, is very narrow and produces this sharp edge. It is possible, thanks to these features, to predict the hole diameter, as it delineates the ablation threshold in the fluence profile of the central spot.

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The periodic array of vertical lines (ripples), seen in Figs. 4(b) and 6(c) and formed by the 786 nm and 393 nm irradiations have a periodicity of 610–700 nm and 250–290 nm, respectively and a polarization perpendicular to them. Such ripples are frequently formed in the ablated circular zones. We consider that these ripple formations share the same origin as those formed at the focused spot with the fluence close to the ablation threshold. The debris formed in the drilling experiment, with the fluence set to nearly three times the ablation threshold at 786 nm wavelength, appears wool-like around the hole, as seen in Fig. 4(c). It is similar in appearance to that in our previous work, where debris was produced in air or in a rare gas with focused Gaussian laser pulses [22]. However, it consists predominantly of fine particles a few tens of nanometers in size in the remote region ∼10 ␮m from the hole as shown in Fig. 4(d). We note that the ablated material forms an atomic vapor, clusters, or droplets, depending on its depth below the surface of the target [23]. These particles of varied size are distributed on the sample surface according to their kinetic energy before they interact with air. The atomic vapor condenses to form fine clusters, which ultimately gather to produce the wool-like debris around the hole. Machining with the 393-nm wavelength laser produces a similar type of debris around the hole, also resembling cotton wool, and fine particles further away as shown in Fig. 5(a). Since the wool-like debris accumulates along the gullies caved out by the laser pulses, it should be composed of aggregated atoms and ultrafine clusters carried by the ablation gas. On the other hand, the particles are denser between the peaks in Fig. 5(b) and have a variety of shapes, hexagonal or square, as seen in the magnified view in Fig. 5(c). This is not seen with the 786 nm wavelength. The mechanism of formation of these particles has not been identified and requires further research. The disappearance of the particles at the peaks in the fringe pattern is considered to be due to evaporation following laser irradiation, for which the threshold is smaller than for solid silicon. 6. Conclusion Focusing a laser beam close to the tip of an axicon with a simple lens successfully improves the efficiency to condense the laser pulse energy to the central spot of the BGB. This will provide a

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