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Large volume ablation of Sapphire with ultra-short laser pulses
Applied Surface Science 270 (2013) 763–766
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Large volume ablation of Sapphire with ultra-short laser pulses A. Shamir a,b,∗ , A.A. Ishaaya a a b
Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel Electro-optics Unit, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel
a r t i c l e
i n f o
Article history: Received 5 August 2012 Received in revised form 6 December 2012 Accepted 18 January 2013 Available online 29 January 2013 Keywords: Sapphire etching Femtosecond pulse laser ablation
a b s t r a c t The superior optical and mechanical properties of Sapphire (Al2 O3 ) are highly desirable in various optoelectronics and micro-mechanical applications. However, Sapphire’s intrinsic hardness and resistance to most chemicals result in significant processing difficulties. Laser micro-machining is emerging as a promising technology, in particular, the use of ultra-short pulses for material ablation. In this work we investigate and characterize experimentally large volume ablation of Sapphire with femtosecond pulses, and compare the results to previously reported drilling and cutting experiments. We manage to identify optimized parameters for overcoming deleterious thermal effects and debris scattering, and demonstrate high quality 180 m-deep ablation of 1 mm × 15 mm area in Sapphire. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sapphire is widely used in the field of optical and micromechanical devices owing to its useful optical, mechanical, and electrical properties. However, Sapphire is mechanically and chemically difficult to process. Its hardness causes conventional techniques, such as mechanical sawing, to be inefficient when precision cutting is required, due to surrounding mechanical damage and excessive tool wear. As Sapphire is inert to most types of wet chemicals and dry etcing, other methods such as laser assisted etching [1], enhanced etching by ion implementation [2] and plasma etching [3] have been investigated, yet none has produced satisfactory results. Laser ablation has also been proposed as a potential machining technique. The interaction of laser pulses with Sapphire and other materials has been investigated for many years as a promising technology. The use of lasers for material processing enables fine precision work with minimal, localized, thermal effects such as cracks, sample melting and heat affected zone. Material removal by laser ablation depends on the laser parameters such as wavelength, pulse duration, pulse energy, and on the material itself. Ablation of Sapphire was demonstrated using nano-second and pico-second pulsed lasers in the UV spectrum [4,5]. Recently, femtosecond (fs) laser ablation has attracted considerable attention because of its ability to produce precise, well-defined micrometer-sized structures in materials that cannot be achieved
∗ Corresponding author at: Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel. Tel.: +972 8 6461841/8 6428464; fax: +972 8 6472949. E-mail address: [email protected] (A. Shamir). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.153
using nano-second pulsed lasers. The use of fs pulses enables high precision machining with minimal collateral damage due to two main reasons [6,7]. First, the damage mechanism, which involves ionization through nonlinear absorption, is limited to the focal volume only. Second, the ionization process is faster than heat conductance to the bulk. Considerable work has been reported on the characterization of Sapphire drilling and cutting using ultra-short pulses. A comparison in performance between sub-picosecond and picoseconds pulses was reported, showing an improved quality for sub-ps pulses [8]. Furthermore, the effect of the number of pulses and their energy on the surface damage threshold of Sapphire and on the depth and diameter of drilled holes has been investigated [9,10]. In addition, an investigation of the laser scan speed and focal position on the quality of cutting, was also reported [11]. The latter results indicate that formation of micro-cracks are unavoidable and increases with scan speed. Here we focus on investigation of large area (more than 10 mm2 ), deep (more than 100 m) ablation of Sapphire using fs laser pulses. To the best of our knowledge, such large volume ablation of Sapphire with fs pulses has not been addressed in previous work, which mainly concentrated on hole drilling and line cutting. Large volume ablation requires relatively low pulse energies and long run times. This results in severe thermal load that typically leads to cracking of the Sapphire substrate, while scattered debris constantly modify the surface so that the effects of various parameters are significantly changed. We experimentally measure the ablation depth as function of pulse energy and scan speed, and manage to provide adequate heat removal from the substrate to avoid macro-cracks. We demonstrate uniform 180 m deep ablation of 1 mm × 15 mm area in Sapphire. This capability can be readily implemented in various applications that require
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44
Fig. 1. Experimental setup.
deep ablation, for example, micro-patterning around waveguides on Sapphire.
Penetraion Depth [µm]
42
40
38
36
34
2. Experimental setup 32
The experimental setup is shown in Fig. 1. Experiments were performed with an amplified Ti:Sapphire laser (Coherent Legend Elite; 800 nm central wavelength, 1 kHz repetition rate, 35 fs pulse duration) operating at full chirp mode with an output beam diameter of 8 mm. The pulse energy is adjusted using a half wave plate and a polarizer, and the average power is measured with a power meter (Ophir 30A-P-DIF-V1) before the focusing lens. The focusing lens has a focal length of f = 200 mm and is mounted on a linear translation stage to adjust the focal position. This focal length results in a 35 m focal spot diameter and a Rayleigh range of more than 1.5 mm in Sapphire, eliminating the need to move the lens during the ablation process. The substrate is a double-side polished C-plane Sapphire wafer, 2-in. in diameter and 430 m in thickness, positioned horizontally on a precision motorized X–Y stage (PI motors model M-111.1DG). All experiments where done in normal atmospheric condition, without the assist of gas, and at energies below the air ionization threshold.
30
0
50
100
150
200
Number of Laser Passes [#] Fig. 2. Penetration depth as a function of the number of laser passes for ablation of a 350 m long line with final line width of approximately 40 m.
3. Results and discussion In order to demonstrate the effect of debris scattering and surface texture modification, we first present results of depth penetration for multiple passes over a short line (length: 350 m, final line width of approximately 40 m). Our pulse energy is 100 J and we repeatedly scan the line at velocity of approximately 90 m/s. Preliminary experiments using pulse energies above 300 J resulted in air ionization, macro-cracking, and non uniformities. As can be seen from Fig. 2 the penetration depth is saturated after about sixty passes at a relatively low penetration depth of 40 m. We believe that the beam inability to penetrate deeper is due to debris, truncation and scattering from the line edges. We will show further on, that in order to avoid the saturation depth for large area ablation a special scan pattern should be chosen. Fig. 3 shows high magnification images of the ablated lines after two and after fifty laser passes. As evident, the heat affected zone around the ablated lines is wider as the number of repetitions increase, and cracks originating from the ablated lines are visible. In our experiments, deep ablation of large area requires long hours of processing under changing surface conditions. When ablating the first layer of the sample the surface is highly polished, yet the following layers of the surface are not, and thus have higher heat absorption and ablation rate. The Gaussian beam tail, which has little contribution to the ablation process, has greater thermal effect on an unpolished surface as opposed to the polished edges along an ablated line where it is truncated. These effects must be taken into consideration when a certain ablation depth is desired. For these reasons, our first attempts with pulse energies of 30–100 J resulted in broken wafers and cracks that originated from the ablated area after either minutes or a few hours of work, depending on the applied pulse energy. To reduce the thermal
Fig. 3. Heat affected zone (HAZ) after two passes (left) and fifty passes (right).
heat load, a copper mask consisting of two plates, each of 1 mm thickness and slots at the desired area size, was prepared. The Sapphire sample was sandwiched in between the plates after covering them with a thermally conductive compound. The mask significantly improves heat dissipation from the sample and the slots block the beam tail around the area edges thus limiting unnecessary heat absorption. In the following experiments this mask helped in obtaining large volume ablation without visible cracks with run times of up to 14 h. We tried to obtain large area ablation with different patterning schemes. We first used a configuration which included four repetitions over the same line followed by a perpendicular movement of 15 m. This resulted in a broken wafer after a short operation time. Next, we tried another multi-scan scheme resembling a “snake” pattern, in which the time between successive ablations of a specific area is relatively long, and the entire area is ablated layer by layer. The basic step in our snake scheme is shown in Fig. 4. It consists of a horizontal leg approximately 15 mm long followed by a vertical
Fig. 4. A basic step of the “snake” ablation pattern scheme.
A. Shamir, A.A. Ishaaya / Applied Surface Science 270 (2013) 763–766
765
0 −20
V Vx2 Vx3 Vx4
−40
Depth [µm]
−60 −80 −100 −120 −140 −160 −180 −200
0
200
400
600
800
1000
Width [µm] Fig. 5. Measured depth profile of consecutive snake pattern repetitions at pulse energies of 100 J (solid lines) and 200 J (dotted lines).
Fig. 6. Depth profile for slow scan speeds.
50 Vx6 Vx8 0
Depth [[µm]
leg approximately 15 m long, and then a 15 mm horizontal leg in the opposite direction followed by another 15 m vertical leg. The vertical displacement was chosen as a compromise between processing time and surface smoothness. We found that with our spot size, an area overlap of 2/3 between the successive horizontal scan lines provides acceptable smoothness. The horizontal and vertical scan velocities were kept at ratio of 1:3 in order to minimize thermal effects at the vertical turning points, where we typically observed initial cracks formation. With this ratio, macro-cracks did not appear (no optimization was made due to the negligible effect of this setting on the processing time). Applying 35 consecutive basic steps resulted in an ablated area of approximately 1 mm width. We first characterized the ablation depth after multiple passes over a smaller area of 1 mm × 2.7 mm (instead of 1 mm × 15 mm) at a scan speed of approximately 90 m/s. The ablation depth was measured with Veeco Dectak 8 machine using a 200 nm tip. The results are shown in Fig. 5, where each consecutive pass is denoted with a different color. As evident, the ablation has penetrated deep, well beyond the saturation depth for line ablation (Fig. 1), even though the number of passes on a specific spot along the scan line is significantly lower. The first ablated layer has a depth of approximately 20 m. As expected, the ablation rate of the first polished layer is considerably lower than that of subsequent layers (due to the surface modification). Increasing the applied pulse energy by a factor of two has a minor effect on the first layer but results in a substantially higher ablation rate for the successive, unpolished layers. The depth profiles at the higher pulse energy are slightly bowed along the ablated area. This may be attributed to higher thermal load than our copper mask could handle. The depth standard deviation was calculated for each pass and pulse energy along 0.5 mm in the central part. At pulse energy of 100 J we found it to be approximately 2 m for the first three passes and 3.5 m for the forth pass. At pulse energy of 200 J the surface roughness is approximately 8 m for the last three passes. We next performed deep ablation for the desired area of 1 mm × 15 mm. The runtime for four passes at a speed of 90 m/s over such a large area was over 13 h. In order to reduce the run time we preformed ablation with the same snake pattern and four passes (100 J pulse energy) at different (faster) scan speeds. The resulting depth profiles were measured at three locations along the channel width and was found to be similar. The measured depth profiles are shown in Figs. 6 and 7. The depth standard deviation was calculated in the same manner as for Fig. 5, and was found to be 3–6 m for Fig. 6 profiles and over 10 m
−50
−100
−150
−200
0
200
400
600
800
1000
Width [µm] Fig. 7. Depth profile for fast scan speeds.
for Fig. 7. As evident from Fig. 6, deep and rather smooth ablation is obtained at low scan speed (90 m/s). Increasing the scan speed to Vx2 results in slightly reduced ablation depth. Although increasing the scan speed further (Vx3, Vx4) should be analogous to reducing the average pulse number on a specific spot, no significant effect on the depth profile is observed. For faster scan speeds of Vx6 and Vx8, shown in Fig. 7, the non-uniformity of the ablated surface is significant and visible to the eye, and macro cracks were observed. The quality of the ablation is clearly better for slow scan speeds as the average number of pulses on each spot is higher. The left side of the depth profile plots correspond to the mask edge while the right side correspond to the end of the snake cycle and shows the effect of the beam on the edge. Note that the ablation depth for four passes over an area of this size is 30 m deeper than that achieved for the smaller area of Fig. 5 (solid black line), at the same conditions. This result repeated over several trials, and is not due to the statistical variance of the ablation depth. Apparently, there is a relation between the total area being ablated to the penetration depth. 4. Conclusion We have demonstrated uniform and deep ablation of a 1 mm × 15 mm area in a Sapphire substrate using fs pulses. We
766
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characterized large volume ablation in terms of applied pulse energy, scan speed and ablation rate. We have shown that with an appropriate heat removal mask and ablation pattern, macrocracks can be avoided, and under the right conditions, a significantly deeper ablation can be achieved, compared with hole drilling or line ablation (for a given pulse energy). This can be attributed to debris scattering and severe thermal load in the case of hole drilling and line ablation leading to saturation of the depth. In contradiction to cutting applications, the scan speed has minor effect on the ablation depth, however large scan speeds reduce the uniformity of the ablated surface. The deepest ablation and best uniformity is obtained at low scan speeds with a snake-like ablation pattern. Acknowledgement This research was partially supported by the Israel Science Foundation (Grant Nos. 1205/08 and 1626/08).
References [1] S.I. Dolgaev, A.A. Lyalin, A.V. Simakin, G.A. Shafeev, Applied Surface Science 96–98 (1996) 491. [2] X. Dongzhu, Z. Dezhang, P. Haochang, X. Hongjie, R. Zongxin, Journal of Physics D: Applied Physics 31 (1998) 1647. [3] D.W. Kim, C.H. Jeong, K.N. Kim, H.Y. Lee, H.S. Kim, Y.J. Sung, G.Y. Yeom, Applied Surface Science 435 (2003) 242. [4] T.C. Chen, R.B. Darling, Journal of Materials Processing Technology 169 (2005) 214. [5] A.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Applied Physics Letters 55 (1989) 2045. [6] R.R. Gattass, E. Mazur, Nature Photonics 2 (2008) 219. [7] P.S. Banks, B.C. Stuart, A.M. Komashko, M.D. Feit, A.M. Rubenchik, M.D. Perry, Proceedings of SPIE 3934 (2000) 14. [8] D. Ashkenasi, A. Rosenfeld, H. Varel, M. Wähmer, E.E.B. Campbell, Applied Surface Science 120 (1997) 65. [9] X.C. Wang, G.C. Lim, H.Y. Zheng, F.L. Ng, W. Liu, S.J. Chua, Applied Surface Science 120 (2004) 221. [10] S.H. Kim, I.B. Sohn, S. Jeong, Applied Surface Science 225 (2009) 9717. [11] X.C. Wang, H.Y. Zheng, P.L. Chu, J.L. Tan, K.M. Teh, T. Liu, B.C.Y. Ang, G.H. Tay, Optics and Lasers in Engineering 48 (2010) 657.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Large volume ablation of Sapphire with ultra-short laser pulses A. Shamir a,b,∗ , A.A. Ishaaya a a b
Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel Electro-optics Unit, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel
a r t i c l e
i n f o
Article history: Received 5 August 2012 Received in revised form 6 December 2012 Accepted 18 January 2013 Available online 29 January 2013 Keywords: Sapphire etching Femtosecond pulse laser ablation
a b s t r a c t The superior optical and mechanical properties of Sapphire (Al2 O3 ) are highly desirable in various optoelectronics and micro-mechanical applications. However, Sapphire’s intrinsic hardness and resistance to most chemicals result in significant processing difficulties. Laser micro-machining is emerging as a promising technology, in particular, the use of ultra-short pulses for material ablation. In this work we investigate and characterize experimentally large volume ablation of Sapphire with femtosecond pulses, and compare the results to previously reported drilling and cutting experiments. We manage to identify optimized parameters for overcoming deleterious thermal effects and debris scattering, and demonstrate high quality 180 m-deep ablation of 1 mm × 15 mm area in Sapphire. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sapphire is widely used in the field of optical and micromechanical devices owing to its useful optical, mechanical, and electrical properties. However, Sapphire is mechanically and chemically difficult to process. Its hardness causes conventional techniques, such as mechanical sawing, to be inefficient when precision cutting is required, due to surrounding mechanical damage and excessive tool wear. As Sapphire is inert to most types of wet chemicals and dry etcing, other methods such as laser assisted etching [1], enhanced etching by ion implementation [2] and plasma etching [3] have been investigated, yet none has produced satisfactory results. Laser ablation has also been proposed as a potential machining technique. The interaction of laser pulses with Sapphire and other materials has been investigated for many years as a promising technology. The use of lasers for material processing enables fine precision work with minimal, localized, thermal effects such as cracks, sample melting and heat affected zone. Material removal by laser ablation depends on the laser parameters such as wavelength, pulse duration, pulse energy, and on the material itself. Ablation of Sapphire was demonstrated using nano-second and pico-second pulsed lasers in the UV spectrum [4,5]. Recently, femtosecond (fs) laser ablation has attracted considerable attention because of its ability to produce precise, well-defined micrometer-sized structures in materials that cannot be achieved
∗ Corresponding author at: Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel. Tel.: +972 8 6461841/8 6428464; fax: +972 8 6472949. E-mail address: [email protected] (A. Shamir). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.153
using nano-second pulsed lasers. The use of fs pulses enables high precision machining with minimal collateral damage due to two main reasons [6,7]. First, the damage mechanism, which involves ionization through nonlinear absorption, is limited to the focal volume only. Second, the ionization process is faster than heat conductance to the bulk. Considerable work has been reported on the characterization of Sapphire drilling and cutting using ultra-short pulses. A comparison in performance between sub-picosecond and picoseconds pulses was reported, showing an improved quality for sub-ps pulses [8]. Furthermore, the effect of the number of pulses and their energy on the surface damage threshold of Sapphire and on the depth and diameter of drilled holes has been investigated [9,10]. In addition, an investigation of the laser scan speed and focal position on the quality of cutting, was also reported [11]. The latter results indicate that formation of micro-cracks are unavoidable and increases with scan speed. Here we focus on investigation of large area (more than 10 mm2 ), deep (more than 100 m) ablation of Sapphire using fs laser pulses. To the best of our knowledge, such large volume ablation of Sapphire with fs pulses has not been addressed in previous work, which mainly concentrated on hole drilling and line cutting. Large volume ablation requires relatively low pulse energies and long run times. This results in severe thermal load that typically leads to cracking of the Sapphire substrate, while scattered debris constantly modify the surface so that the effects of various parameters are significantly changed. We experimentally measure the ablation depth as function of pulse energy and scan speed, and manage to provide adequate heat removal from the substrate to avoid macro-cracks. We demonstrate uniform 180 m deep ablation of 1 mm × 15 mm area in Sapphire. This capability can be readily implemented in various applications that require
764
A. Shamir, A.A. Ishaaya / Applied Surface Science 270 (2013) 763–766
44
Fig. 1. Experimental setup.
deep ablation, for example, micro-patterning around waveguides on Sapphire.
Penetraion Depth [µm]
42
40
38
36
34
2. Experimental setup 32
The experimental setup is shown in Fig. 1. Experiments were performed with an amplified Ti:Sapphire laser (Coherent Legend Elite; 800 nm central wavelength, 1 kHz repetition rate, 35 fs pulse duration) operating at full chirp mode with an output beam diameter of 8 mm. The pulse energy is adjusted using a half wave plate and a polarizer, and the average power is measured with a power meter (Ophir 30A-P-DIF-V1) before the focusing lens. The focusing lens has a focal length of f = 200 mm and is mounted on a linear translation stage to adjust the focal position. This focal length results in a 35 m focal spot diameter and a Rayleigh range of more than 1.5 mm in Sapphire, eliminating the need to move the lens during the ablation process. The substrate is a double-side polished C-plane Sapphire wafer, 2-in. in diameter and 430 m in thickness, positioned horizontally on a precision motorized X–Y stage (PI motors model M-111.1DG). All experiments where done in normal atmospheric condition, without the assist of gas, and at energies below the air ionization threshold.
30
0
50
100
150
200
Number of Laser Passes [#] Fig. 2. Penetration depth as a function of the number of laser passes for ablation of a 350 m long line with final line width of approximately 40 m.
3. Results and discussion In order to demonstrate the effect of debris scattering and surface texture modification, we first present results of depth penetration for multiple passes over a short line (length: 350 m, final line width of approximately 40 m). Our pulse energy is 100 J and we repeatedly scan the line at velocity of approximately 90 m/s. Preliminary experiments using pulse energies above 300 J resulted in air ionization, macro-cracking, and non uniformities. As can be seen from Fig. 2 the penetration depth is saturated after about sixty passes at a relatively low penetration depth of 40 m. We believe that the beam inability to penetrate deeper is due to debris, truncation and scattering from the line edges. We will show further on, that in order to avoid the saturation depth for large area ablation a special scan pattern should be chosen. Fig. 3 shows high magnification images of the ablated lines after two and after fifty laser passes. As evident, the heat affected zone around the ablated lines is wider as the number of repetitions increase, and cracks originating from the ablated lines are visible. In our experiments, deep ablation of large area requires long hours of processing under changing surface conditions. When ablating the first layer of the sample the surface is highly polished, yet the following layers of the surface are not, and thus have higher heat absorption and ablation rate. The Gaussian beam tail, which has little contribution to the ablation process, has greater thermal effect on an unpolished surface as opposed to the polished edges along an ablated line where it is truncated. These effects must be taken into consideration when a certain ablation depth is desired. For these reasons, our first attempts with pulse energies of 30–100 J resulted in broken wafers and cracks that originated from the ablated area after either minutes or a few hours of work, depending on the applied pulse energy. To reduce the thermal
Fig. 3. Heat affected zone (HAZ) after two passes (left) and fifty passes (right).
heat load, a copper mask consisting of two plates, each of 1 mm thickness and slots at the desired area size, was prepared. The Sapphire sample was sandwiched in between the plates after covering them with a thermally conductive compound. The mask significantly improves heat dissipation from the sample and the slots block the beam tail around the area edges thus limiting unnecessary heat absorption. In the following experiments this mask helped in obtaining large volume ablation without visible cracks with run times of up to 14 h. We tried to obtain large area ablation with different patterning schemes. We first used a configuration which included four repetitions over the same line followed by a perpendicular movement of 15 m. This resulted in a broken wafer after a short operation time. Next, we tried another multi-scan scheme resembling a “snake” pattern, in which the time between successive ablations of a specific area is relatively long, and the entire area is ablated layer by layer. The basic step in our snake scheme is shown in Fig. 4. It consists of a horizontal leg approximately 15 mm long followed by a vertical
Fig. 4. A basic step of the “snake” ablation pattern scheme.
A. Shamir, A.A. Ishaaya / Applied Surface Science 270 (2013) 763–766
765
0 −20
V Vx2 Vx3 Vx4
−40
Depth [µm]
−60 −80 −100 −120 −140 −160 −180 −200
0
200
400
600
800
1000
Width [µm] Fig. 5. Measured depth profile of consecutive snake pattern repetitions at pulse energies of 100 J (solid lines) and 200 J (dotted lines).
Fig. 6. Depth profile for slow scan speeds.
50 Vx6 Vx8 0
Depth [[µm]
leg approximately 15 m long, and then a 15 mm horizontal leg in the opposite direction followed by another 15 m vertical leg. The vertical displacement was chosen as a compromise between processing time and surface smoothness. We found that with our spot size, an area overlap of 2/3 between the successive horizontal scan lines provides acceptable smoothness. The horizontal and vertical scan velocities were kept at ratio of 1:3 in order to minimize thermal effects at the vertical turning points, where we typically observed initial cracks formation. With this ratio, macro-cracks did not appear (no optimization was made due to the negligible effect of this setting on the processing time). Applying 35 consecutive basic steps resulted in an ablated area of approximately 1 mm width. We first characterized the ablation depth after multiple passes over a smaller area of 1 mm × 2.7 mm (instead of 1 mm × 15 mm) at a scan speed of approximately 90 m/s. The ablation depth was measured with Veeco Dectak 8 machine using a 200 nm tip. The results are shown in Fig. 5, where each consecutive pass is denoted with a different color. As evident, the ablation has penetrated deep, well beyond the saturation depth for line ablation (Fig. 1), even though the number of passes on a specific spot along the scan line is significantly lower. The first ablated layer has a depth of approximately 20 m. As expected, the ablation rate of the first polished layer is considerably lower than that of subsequent layers (due to the surface modification). Increasing the applied pulse energy by a factor of two has a minor effect on the first layer but results in a substantially higher ablation rate for the successive, unpolished layers. The depth profiles at the higher pulse energy are slightly bowed along the ablated area. This may be attributed to higher thermal load than our copper mask could handle. The depth standard deviation was calculated for each pass and pulse energy along 0.5 mm in the central part. At pulse energy of 100 J we found it to be approximately 2 m for the first three passes and 3.5 m for the forth pass. At pulse energy of 200 J the surface roughness is approximately 8 m for the last three passes. We next performed deep ablation for the desired area of 1 mm × 15 mm. The runtime for four passes at a speed of 90 m/s over such a large area was over 13 h. In order to reduce the run time we preformed ablation with the same snake pattern and four passes (100 J pulse energy) at different (faster) scan speeds. The resulting depth profiles were measured at three locations along the channel width and was found to be similar. The measured depth profiles are shown in Figs. 6 and 7. The depth standard deviation was calculated in the same manner as for Fig. 5, and was found to be 3–6 m for Fig. 6 profiles and over 10 m
−50
−100
−150
−200
0
200
400
600
800
1000
Width [µm] Fig. 7. Depth profile for fast scan speeds.
for Fig. 7. As evident from Fig. 6, deep and rather smooth ablation is obtained at low scan speed (90 m/s). Increasing the scan speed to Vx2 results in slightly reduced ablation depth. Although increasing the scan speed further (Vx3, Vx4) should be analogous to reducing the average pulse number on a specific spot, no significant effect on the depth profile is observed. For faster scan speeds of Vx6 and Vx8, shown in Fig. 7, the non-uniformity of the ablated surface is significant and visible to the eye, and macro cracks were observed. The quality of the ablation is clearly better for slow scan speeds as the average number of pulses on each spot is higher. The left side of the depth profile plots correspond to the mask edge while the right side correspond to the end of the snake cycle and shows the effect of the beam on the edge. Note that the ablation depth for four passes over an area of this size is 30 m deeper than that achieved for the smaller area of Fig. 5 (solid black line), at the same conditions. This result repeated over several trials, and is not due to the statistical variance of the ablation depth. Apparently, there is a relation between the total area being ablated to the penetration depth. 4. Conclusion We have demonstrated uniform and deep ablation of a 1 mm × 15 mm area in a Sapphire substrate using fs pulses. We
766
A. Shamir, A.A. Ishaaya / Applied Surface Science 270 (2013) 763–766
characterized large volume ablation in terms of applied pulse energy, scan speed and ablation rate. We have shown that with an appropriate heat removal mask and ablation pattern, macrocracks can be avoided, and under the right conditions, a significantly deeper ablation can be achieved, compared with hole drilling or line ablation (for a given pulse energy). This can be attributed to debris scattering and severe thermal load in the case of hole drilling and line ablation leading to saturation of the depth. In contradiction to cutting applications, the scan speed has minor effect on the ablation depth, however large scan speeds reduce the uniformity of the ablated surface. The deepest ablation and best uniformity is obtained at low scan speeds with a snake-like ablation pattern. Acknowledgement This research was partially supported by the Israel Science Foundation (Grant Nos. 1205/08 and 1626/08).
References [1] S.I. Dolgaev, A.A. Lyalin, A.V. Simakin, G.A. Shafeev, Applied Surface Science 96–98 (1996) 491. [2] X. Dongzhu, Z. Dezhang, P. Haochang, X. Hongjie, R. Zongxin, Journal of Physics D: Applied Physics 31 (1998) 1647. [3] D.W. Kim, C.H. Jeong, K.N. Kim, H.Y. Lee, H.S. Kim, Y.J. Sung, G.Y. Yeom, Applied Surface Science 435 (2003) 242. [4] T.C. Chen, R.B. Darling, Journal of Materials Processing Technology 169 (2005) 214. [5] A.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Applied Physics Letters 55 (1989) 2045. [6] R.R. Gattass, E. Mazur, Nature Photonics 2 (2008) 219. [7] P.S. Banks, B.C. Stuart, A.M. Komashko, M.D. Feit, A.M. Rubenchik, M.D. Perry, Proceedings of SPIE 3934 (2000) 14. [8] D. Ashkenasi, A. Rosenfeld, H. Varel, M. Wähmer, E.E.B. Campbell, Applied Surface Science 120 (1997) 65. [9] X.C. Wang, G.C. Lim, H.Y. Zheng, F.L. Ng, W. Liu, S.J. Chua, Applied Surface Science 120 (2004) 221. [10] S.H. Kim, I.B. Sohn, S. Jeong, Applied Surface Science 225 (2009) 9717. [11] X.C. Wang, H.Y. Zheng, P.L. Chu, J.L. Tan, K.M. Teh, T. Liu, B.C.Y. Ang, G.H. Tay, Optics and Lasers in Engineering 48 (2010) 657.