- Email: [email protected]
Fabrication of microelectrodes deeply embedded in LiNbO3 using a femtosecond laser
Applied Surface Science 254 (2008) 7018–7021
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of microelectrodes deeply embedded in LiNbO3 using a femtosecond laser Yang Liao a,b, Jian Xu a,b, Haiyi Sun a, Juan Song a,b, Xinshun Wang a,b, Ya Cheng a,* a State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 December 2007 Received in revised form 7 May 2008 Accepted 8 May 2008 Available online 13 May 2008
We present a novel technique to fabricate deeply embedded microelectrodes in LiNbO3 using femtosecond pulsed laser ablation and selective electroless plating. The fabrication process mainly consists of four steps, which are (1) micromachining of microgrooves on the surface of LiNbO3 by femtosecond laser ablation; (2) formation of AgNO3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modification of the inner surfaces; and (4) electroless copper plating. The void-free electroless copper plating is obtained with appropriate cross section of microgrooves and uniform initiation of the autocatalytic deposition on the inner surface of grooves. The dimension and shape of the microelectrodes could be accurately controlled by changing the conditions of femtosecond laser ablation, which in turn can control the distribution of electric field inside LiNbO3 crystal for various applications, opening up a new approach to fabricate three-dimensional integrated electro-optic devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 42.62.Cf 77.84.s 78.20.Jq 81.16.Mk Keywords: Selective electroless plating Femtosecond pulsed laser Microelectrode Lithium niobate
1. Introduction LiNbO3 is ferroelectric material with large optical nonlinearity and pockels effects. Electro-optic components based on LiNbO3 waveguiding structures have gained significant importance, such as electro-optic switches [1], modulators [2–4] and electro-optic tuned quasi-phase-matched (QPM) devices [5,6]. In these components, it is crucial to design and fabricate electrodes, which have been realized commonly by use of lithographic methods. Though the lithographic process is well developed for mass production in the semiconductor industry and allows for achieving microscopic dimensions and great complexity at low cost, due to the inherently planar nature of the lithographic process, this technology is limited in its capability to produce three-dimensional structures. Reich et al. [7] have reported topographical electrodes for poling lithium niobate by laser ablation as a simple patterning method superior to conventional surface electrodes. However, the topographical electrodes must be completely contacted with liquid
* Corresponding author. Tel.: +86 21 69918546; fax: +86 21 69918021. E-mail address: [email protected] (Y. Cheng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.122
electrolyte, which limited its application in integrated devices. As a simple and cost-effective technique, laser-induced selective electroless deposition has been widely studied over the past two decades [8–10]. In particular, the rapid development of ultrafast laser technology has enabled generation of intense ultrashort laser pulses with durations approaching a single optical cycle, leading to many interesting nonlinear optical phenomena such as high-order harmonic generation [11], above threshold ionization (ATI) [12], and so on. Owing to its ultra-high peak power and ultra-short pulse duration, femtosecond laser has become an ideal tool for local modification of transparent materials through nonlinear optical processes [13]. Recently, we reported selective metallization on insulator substrates using femtosecond laser direct-writing followed by electroless copper plating [14]. Improved selectivity of plating and good conductivity of metal microstructures have been demonstrated. However, the ability to produce three-dimensional metal microstructures using a femtosecond laser has not yet been shown. In this paper, we present a novel manufacturing technique to fabricate microelectrodes deeply embedded in LiNbO3 using femtosecond pulsed laser ablation and femtosecond laser induced selective electroless plating. It is well known that the viability of
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
7019
Fig. 2. A cross-sectional optical micrograph of the copper microstructures deposited in spindle section grooves, opening width: 20 mm; depth: 150 mm; plating time: 4 h. Fig. 1. Schematic illustration of the fabrication process for embedded microelectrodes in LiNbO3.
femtosecond laser machining [15–19] and good via and trench filling ability of electroless copper deposition [20,21] have been proved. By combining the advantages of these two techniques, geometry-controllable and void-free microelectrodes deeply embedded in LiNbO3 are demonstrated. The three-dimensional geometries of embedded electrodes could offer more controllable and homogeneous electric field across a buried waveguide sandwiched into two electrodes, which advances the design opportunities for new integrated LiNbO3 devices. 2. Experimental The fabrication process mainly consists of four steps illustrated in Fig. 1, including (1) micromachining of microgrooves on the surface of LiNbO3 by femtosecond laser ablation; (2) formation of AgNO3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modification of the inner surfaces; and (4) electroless copper plating. Commercially available un-doped congruent LiNbO3 substrates of size 10 5 3 mm3 were used for the study. The surface was polished to an optical grade on all six sides of substrate. For the laser ablation and modification, a Ti:Sapphire laser system (Legend USP, Coherent Inc.) with an operating wavelength of 800 nm, a pulse width of 40 fs, and a repetition rate of 1 kHz was used for laser direct-writing. The laser beam was focused on the samples with a 20 microscope objective (NA = 0.45). A half-wave plate and a polarizer were used to adjust laser power continuously. The LiNbO3 sample was placed on a computer-controlled XYZ stage, and scanning speeding and pattern could be controlled precisely. After femtosecond laser ablation, the substrates were sonicated in acetone and then in distilled water for 5 min each. The AgNO3 films were prepared by immersing the substrates into a 0.5 mol/L AgNO3 solution and withdrawing the substrates at an approximate speed of 1 mm/s. Subsequently, the substrates were naturally dried in the dark at room temperature for 12 h. The thickness of the AgNO3 films could be controlled by adjusting the withdraw speed. Prior to electroless plating, the substrates were cleaned with distilled water in ultrasonic bath for removal of unirradiated AgNO3 films. The composition of the electroless copper plating solution was CuSO45H2O (5.0 g/L), C10H16N2O8 (EDTA; 14.0 g/L), HCHO (5.0 g/L) as a reducing agent, 2,20 -dipyridine (2 mg/L) as a stabilizer, polyethylene glycol (4000 Mw 50 mg/L) and bis (3-
sulfopropyl) disulfide (SPS; 0.1 mg/L) as surface activators. The pH value of the plating bath was adjusted to approximately 12.5 using tetramethylammonium hydroxide (TMAH) and the bath temperature was maintained at 40 8C. 3. Results and discussion Fig. 2 shows a cross-sectional optical micrograph of copper microstructures deposited in femtosecond-laser-ablated grooves. The spindle-like grooves of 150 mm depth were fabricated by repetitively inscribing 20 times using focused femtosecond laser beam at a scan speed of 200 mm/s and pulse energy of 60 mJ. The sidewalls of grooves coated with AgNO3 films were modified by femtosecond laser direct-writing at slightly higher pulse energy and the same focus depth and writing speed. By this way, metallic Ag particles can be generated from the decomposition of AgNO3 films on the irradiation area, which can serve as seeds for in-situ selective copper depositing in the following electroless plating process [14]. It was observed that uniform electroless copper deposition was initiated on the sidewalls of grooves. Therefore, it could be deduced that Ag particles were deposited uniformly on the entire surface inside grooves by femtosecond laser directwriting, which can be interpreted by taking account of the reflection of the laser beam on the sidewalls of grooves [22]. At the same time, we observed the ‘‘necking’’ at the top of the grooves due to the narrow openings, which resulted in seams and voids in the microelectrodes. In order to avoid the formation of necking, a matrix ablation technique was used to control the section shape of grooves. Fig. 3 shows an illustration of the matrix ablation and an example of obtained U-groove. The desired section pattern of the groove is applied to the substrate by writing adjacent lines, which define a matrix in the transverse (X, Z) plane [23]. The U-groove with length 200 mm, width 40 mm and depth 50 mm was made by inscribing a volume consisting of a matrix of (20 10) lines at a speed of 1 mm/ s and pulse energy of 2 mJ, and matrix density parameters (DX, DZ) were chosen to be respectively 2 mm and 1 mm. After femtosecond laser ablation, chippings in the groove could be removed by sonicleaning in distilled water. It could be observed that the groove is clean and homogeneous, except that the end of groove in the substrate edge is broader than other portions as a result of edge effect.
7020
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
Fig. 3. (a) Illustration of fabrication of grooves by femtosecond pulsed laser ablation: by moving the substrate in three-dimensions. (b–d) Optical micrographs of a fabricated groove: (b) end view, (c) top view and (d) bottom view.
Fig. 4 shows two examples of the copper microstructures deposited in the U-grooves. Two different depth grooves were respectively fabricated by inscribing volumes consisting of matrices of (20 10) and (20 40) lines and other parameters were the same as those of the example shown in the Fig. 3. The inner surfaces of grooves coated with AgNO3 films were modified by femtosecond laser direct-writing on the bottom surface of grooves at a speed of 200 mm/s and pulse energy of 2 mJ. As shown in the Fig. 2, attributed to the good uniformity of seed layers, the uniform electroless copper deposition on the inner surface of Ugrooves was obtained. However, the increasing openings of U-
grooves resulted in the disappearance of ‘‘necking’’. The thickness of 3-h deposited films was about 6 mm, equivalent to a deposition rate of 2 mm/h. In addition, the lateral growth of copper beside the grooves was also observed. The dependence of groove-filling characteristics on plating time is shown in Fig. 5. One could see that copper deposition rate on the bottom of grooves is higher than that on the surface, which could attribute to addition of the surface activators into the electroless plating solution [21]. Due to the overgrowth on the grooves, as shown in Fig. 5(c), the mechanical polishing of substrate surface was performed after electroless plating. Fig. 6 shows optical
Fig. 4. Optical micrographs of the copper microstructures deposited in U-grooves: (a) end view and (c) top view of a groove of 40 mm width and 42 mm depth, (b) end view of a groove of 40 mm width and 95 mm depth, plating time of the two grooves: 3 h.
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
7021
Fig. 5. Cross-sectional optical micrographs of the copper microstructures deposited in U-grooves with plating time: (a) 10 h, (b) 15 h and (c) 30 h, opening width: 40 mm; depth: 75 mm and 85 mm, respectively.
and cost-effective fabrication technique of deeply embedded microelectrodes in LiNbO3 wafer. The shape and dimension of the microelectrodes could be accurately controlled by changing the conditions of femtosecond pulsed laser ablation, which in turn can control the distribution of electric field inside LiNbO3 crystal. By modifying the inner surface of grooves coated with AgNO3 film using femtosecond laser, the uniform autocatalytic copper deposition on the inner surface of grooves was obtained, and the grooves with a depth of up to 110 mm and increasing opening can therefore be filled up by electroless plating. Since optical waveguides can also be directly written inside LiNbO3 crystal using femtosecond laser pulses [24], the embedded microelectrodes can therefore be easily integrated with the buried optical waveguides, opening up a new approach to fabricate 3D integrated electro-optic devices. Acknowledgements The authors thank Fei he, Zenghui Zhou and Chen Wang at SIOM for their contribution to this work. This research was financially supported by National Basic Research Program of China (Grants No. 2006CB806000). Ya Cheng acknowledges the support of 100 Talents Program of the Chinese Academy of Sciences. References
Fig. 6. Optical micrographs showing (a) top view and (b) end view of two polished microelectrodes embedded in a LiNbO3 wafer, opening width: 40 mm; depth: 100 mm and 110 mm, respectively.
micrographs of polished copper microelectrodes embedded in a LiNbO3 wafer. These micrographs clearly indicate almost no voids and good uniform filling for the deep grooves.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
4. Conclusion Combining femtosecond laser ablation with femtosecond laserinduced selective electroless plating, we have presented a simple
[21] [22] [23] [24]
M. Papuchon, Y. Combemale, X. Mathieu, et al. Appl. Phys. Lett. 27 (1975) 289. O.G. Ramer, IEEE J. Quantum Electron. 18 (1982) 386. G.K. Gopalakrishnan, C.H. Bulmer, W.K. Burns, Electron. Lett. 28 (1992) 826. E.L. Wooten, K.M. Kissa, A. Yi-Yan, et al. IEEE J. Sel. Top. Quant. 6 (2000) 69. Y.Q. Lu, J.J. Zheng, Y.L. Lu, Appl. Phys. Lett. 74 (1999) 123. Y.Q. Lu, Z.L. Wang, Y.X. Xi, et al. Appl. Phys. Lett. 77 (2000) 3719. M. Reich, F. Korte, C. Fallnich, et al. Opt. Lett. 23 (1998) 1817. G.A. Shafeev, Quamtum Electron. 27 (1997) 1104. K. Korda´s, J. Be´ke´si, R. Vajtai, et al. Appl. Surf. Sci. 172 (2001) 178. D.S. Chen, Q.H. Lu, Y. Zhao, Appl. Surf. Sci. 253 (2006) 1573. H. Xiong, R.X. Li, Z.N. Zeng, et al. Phys. Rev. A 75 (2007) 051802. L.H. Bai, J.T. Zhang, Z.Z. Xu, et al. Phys Rev. Lett. 97 (2006) 193002. K. Itoh, W. Watanabe, S. Nolte, et al. MRS Bull. 31 (2006) 620. J. Xu, Y. Liao, H.D. Zeng, et al. Opt. Express 15 (2007) 12743. Y. Cheng, K. Sugioka, K. Midorikawa, et al. Opt. Lett. 28 (2003) 1144. K. Sugioka, Y. Cheng, K. Midorikawa, Appl. Phys. A 81 (2005) 1. Y. Cheng, K. Sugioka, K. Midorikawa, Opt. Lett. 28 (2003) 55. Y. Cheng, K. Sugioka, K. Midorikawa, Opt. Lett. 29 (2004) 2007. H.Y. Sun, F. He, Z.H. Zhou, et al. Opt. Lett. 32 (2007) 1536. V.M. Dubin, Y. Shacham-Diamand, B. Zhao, et al. J. Electrochem. Soc. 144 (1997) 898. Z. Wang, O. Yaegashi, H. Sakaue, et al. J. Electrochem. Soc. 151 (2004) 781. K. Korda´s, J. Be´ke´si, R. Vajtai, et al. J. Electron. Mater. 30 (2001) 21. Y. Bellouard, A. Said, M. Dugan, et al. Opt. Express 12 (2004) 2120. J. Burghoff, C. Grebing, S. Nolte, et al. Appl. Phys. Lett. 9 (2006) 081108.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of microelectrodes deeply embedded in LiNbO3 using a femtosecond laser Yang Liao a,b, Jian Xu a,b, Haiyi Sun a, Juan Song a,b, Xinshun Wang a,b, Ya Cheng a,* a State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 December 2007 Received in revised form 7 May 2008 Accepted 8 May 2008 Available online 13 May 2008
We present a novel technique to fabricate deeply embedded microelectrodes in LiNbO3 using femtosecond pulsed laser ablation and selective electroless plating. The fabrication process mainly consists of four steps, which are (1) micromachining of microgrooves on the surface of LiNbO3 by femtosecond laser ablation; (2) formation of AgNO3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modification of the inner surfaces; and (4) electroless copper plating. The void-free electroless copper plating is obtained with appropriate cross section of microgrooves and uniform initiation of the autocatalytic deposition on the inner surface of grooves. The dimension and shape of the microelectrodes could be accurately controlled by changing the conditions of femtosecond laser ablation, which in turn can control the distribution of electric field inside LiNbO3 crystal for various applications, opening up a new approach to fabricate three-dimensional integrated electro-optic devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 42.62.Cf 77.84.s 78.20.Jq 81.16.Mk Keywords: Selective electroless plating Femtosecond pulsed laser Microelectrode Lithium niobate
1. Introduction LiNbO3 is ferroelectric material with large optical nonlinearity and pockels effects. Electro-optic components based on LiNbO3 waveguiding structures have gained significant importance, such as electro-optic switches [1], modulators [2–4] and electro-optic tuned quasi-phase-matched (QPM) devices [5,6]. In these components, it is crucial to design and fabricate electrodes, which have been realized commonly by use of lithographic methods. Though the lithographic process is well developed for mass production in the semiconductor industry and allows for achieving microscopic dimensions and great complexity at low cost, due to the inherently planar nature of the lithographic process, this technology is limited in its capability to produce three-dimensional structures. Reich et al. [7] have reported topographical electrodes for poling lithium niobate by laser ablation as a simple patterning method superior to conventional surface electrodes. However, the topographical electrodes must be completely contacted with liquid
* Corresponding author. Tel.: +86 21 69918546; fax: +86 21 69918021. E-mail address: [email protected] (Y. Cheng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.122
electrolyte, which limited its application in integrated devices. As a simple and cost-effective technique, laser-induced selective electroless deposition has been widely studied over the past two decades [8–10]. In particular, the rapid development of ultrafast laser technology has enabled generation of intense ultrashort laser pulses with durations approaching a single optical cycle, leading to many interesting nonlinear optical phenomena such as high-order harmonic generation [11], above threshold ionization (ATI) [12], and so on. Owing to its ultra-high peak power and ultra-short pulse duration, femtosecond laser has become an ideal tool for local modification of transparent materials through nonlinear optical processes [13]. Recently, we reported selective metallization on insulator substrates using femtosecond laser direct-writing followed by electroless copper plating [14]. Improved selectivity of plating and good conductivity of metal microstructures have been demonstrated. However, the ability to produce three-dimensional metal microstructures using a femtosecond laser has not yet been shown. In this paper, we present a novel manufacturing technique to fabricate microelectrodes deeply embedded in LiNbO3 using femtosecond pulsed laser ablation and femtosecond laser induced selective electroless plating. It is well known that the viability of
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
7019
Fig. 2. A cross-sectional optical micrograph of the copper microstructures deposited in spindle section grooves, opening width: 20 mm; depth: 150 mm; plating time: 4 h. Fig. 1. Schematic illustration of the fabrication process for embedded microelectrodes in LiNbO3.
femtosecond laser machining [15–19] and good via and trench filling ability of electroless copper deposition [20,21] have been proved. By combining the advantages of these two techniques, geometry-controllable and void-free microelectrodes deeply embedded in LiNbO3 are demonstrated. The three-dimensional geometries of embedded electrodes could offer more controllable and homogeneous electric field across a buried waveguide sandwiched into two electrodes, which advances the design opportunities for new integrated LiNbO3 devices. 2. Experimental The fabrication process mainly consists of four steps illustrated in Fig. 1, including (1) micromachining of microgrooves on the surface of LiNbO3 by femtosecond laser ablation; (2) formation of AgNO3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modification of the inner surfaces; and (4) electroless copper plating. Commercially available un-doped congruent LiNbO3 substrates of size 10 5 3 mm3 were used for the study. The surface was polished to an optical grade on all six sides of substrate. For the laser ablation and modification, a Ti:Sapphire laser system (Legend USP, Coherent Inc.) with an operating wavelength of 800 nm, a pulse width of 40 fs, and a repetition rate of 1 kHz was used for laser direct-writing. The laser beam was focused on the samples with a 20 microscope objective (NA = 0.45). A half-wave plate and a polarizer were used to adjust laser power continuously. The LiNbO3 sample was placed on a computer-controlled XYZ stage, and scanning speeding and pattern could be controlled precisely. After femtosecond laser ablation, the substrates were sonicated in acetone and then in distilled water for 5 min each. The AgNO3 films were prepared by immersing the substrates into a 0.5 mol/L AgNO3 solution and withdrawing the substrates at an approximate speed of 1 mm/s. Subsequently, the substrates were naturally dried in the dark at room temperature for 12 h. The thickness of the AgNO3 films could be controlled by adjusting the withdraw speed. Prior to electroless plating, the substrates were cleaned with distilled water in ultrasonic bath for removal of unirradiated AgNO3 films. The composition of the electroless copper plating solution was CuSO45H2O (5.0 g/L), C10H16N2O8 (EDTA; 14.0 g/L), HCHO (5.0 g/L) as a reducing agent, 2,20 -dipyridine (2 mg/L) as a stabilizer, polyethylene glycol (4000 Mw 50 mg/L) and bis (3-
sulfopropyl) disulfide (SPS; 0.1 mg/L) as surface activators. The pH value of the plating bath was adjusted to approximately 12.5 using tetramethylammonium hydroxide (TMAH) and the bath temperature was maintained at 40 8C. 3. Results and discussion Fig. 2 shows a cross-sectional optical micrograph of copper microstructures deposited in femtosecond-laser-ablated grooves. The spindle-like grooves of 150 mm depth were fabricated by repetitively inscribing 20 times using focused femtosecond laser beam at a scan speed of 200 mm/s and pulse energy of 60 mJ. The sidewalls of grooves coated with AgNO3 films were modified by femtosecond laser direct-writing at slightly higher pulse energy and the same focus depth and writing speed. By this way, metallic Ag particles can be generated from the decomposition of AgNO3 films on the irradiation area, which can serve as seeds for in-situ selective copper depositing in the following electroless plating process [14]. It was observed that uniform electroless copper deposition was initiated on the sidewalls of grooves. Therefore, it could be deduced that Ag particles were deposited uniformly on the entire surface inside grooves by femtosecond laser directwriting, which can be interpreted by taking account of the reflection of the laser beam on the sidewalls of grooves [22]. At the same time, we observed the ‘‘necking’’ at the top of the grooves due to the narrow openings, which resulted in seams and voids in the microelectrodes. In order to avoid the formation of necking, a matrix ablation technique was used to control the section shape of grooves. Fig. 3 shows an illustration of the matrix ablation and an example of obtained U-groove. The desired section pattern of the groove is applied to the substrate by writing adjacent lines, which define a matrix in the transverse (X, Z) plane [23]. The U-groove with length 200 mm, width 40 mm and depth 50 mm was made by inscribing a volume consisting of a matrix of (20 10) lines at a speed of 1 mm/ s and pulse energy of 2 mJ, and matrix density parameters (DX, DZ) were chosen to be respectively 2 mm and 1 mm. After femtosecond laser ablation, chippings in the groove could be removed by sonicleaning in distilled water. It could be observed that the groove is clean and homogeneous, except that the end of groove in the substrate edge is broader than other portions as a result of edge effect.
7020
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
Fig. 3. (a) Illustration of fabrication of grooves by femtosecond pulsed laser ablation: by moving the substrate in three-dimensions. (b–d) Optical micrographs of a fabricated groove: (b) end view, (c) top view and (d) bottom view.
Fig. 4 shows two examples of the copper microstructures deposited in the U-grooves. Two different depth grooves were respectively fabricated by inscribing volumes consisting of matrices of (20 10) and (20 40) lines and other parameters were the same as those of the example shown in the Fig. 3. The inner surfaces of grooves coated with AgNO3 films were modified by femtosecond laser direct-writing on the bottom surface of grooves at a speed of 200 mm/s and pulse energy of 2 mJ. As shown in the Fig. 2, attributed to the good uniformity of seed layers, the uniform electroless copper deposition on the inner surface of Ugrooves was obtained. However, the increasing openings of U-
grooves resulted in the disappearance of ‘‘necking’’. The thickness of 3-h deposited films was about 6 mm, equivalent to a deposition rate of 2 mm/h. In addition, the lateral growth of copper beside the grooves was also observed. The dependence of groove-filling characteristics on plating time is shown in Fig. 5. One could see that copper deposition rate on the bottom of grooves is higher than that on the surface, which could attribute to addition of the surface activators into the electroless plating solution [21]. Due to the overgrowth on the grooves, as shown in Fig. 5(c), the mechanical polishing of substrate surface was performed after electroless plating. Fig. 6 shows optical
Fig. 4. Optical micrographs of the copper microstructures deposited in U-grooves: (a) end view and (c) top view of a groove of 40 mm width and 42 mm depth, (b) end view of a groove of 40 mm width and 95 mm depth, plating time of the two grooves: 3 h.
Y. Liao et al. / Applied Surface Science 254 (2008) 7018–7021
7021
Fig. 5. Cross-sectional optical micrographs of the copper microstructures deposited in U-grooves with plating time: (a) 10 h, (b) 15 h and (c) 30 h, opening width: 40 mm; depth: 75 mm and 85 mm, respectively.
and cost-effective fabrication technique of deeply embedded microelectrodes in LiNbO3 wafer. The shape and dimension of the microelectrodes could be accurately controlled by changing the conditions of femtosecond pulsed laser ablation, which in turn can control the distribution of electric field inside LiNbO3 crystal. By modifying the inner surface of grooves coated with AgNO3 film using femtosecond laser, the uniform autocatalytic copper deposition on the inner surface of grooves was obtained, and the grooves with a depth of up to 110 mm and increasing opening can therefore be filled up by electroless plating. Since optical waveguides can also be directly written inside LiNbO3 crystal using femtosecond laser pulses [24], the embedded microelectrodes can therefore be easily integrated with the buried optical waveguides, opening up a new approach to fabricate 3D integrated electro-optic devices. Acknowledgements The authors thank Fei he, Zenghui Zhou and Chen Wang at SIOM for their contribution to this work. This research was financially supported by National Basic Research Program of China (Grants No. 2006CB806000). Ya Cheng acknowledges the support of 100 Talents Program of the Chinese Academy of Sciences. References
Fig. 6. Optical micrographs showing (a) top view and (b) end view of two polished microelectrodes embedded in a LiNbO3 wafer, opening width: 40 mm; depth: 100 mm and 110 mm, respectively.
micrographs of polished copper microelectrodes embedded in a LiNbO3 wafer. These micrographs clearly indicate almost no voids and good uniform filling for the deep grooves.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
4. Conclusion Combining femtosecond laser ablation with femtosecond laserinduced selective electroless plating, we have presented a simple
[21] [22] [23] [24]
M. Papuchon, Y. Combemale, X. Mathieu, et al. Appl. Phys. Lett. 27 (1975) 289. O.G. Ramer, IEEE J. Quantum Electron. 18 (1982) 386. G.K. Gopalakrishnan, C.H. Bulmer, W.K. Burns, Electron. Lett. 28 (1992) 826. E.L. Wooten, K.M. Kissa, A. Yi-Yan, et al. IEEE J. Sel. Top. Quant. 6 (2000) 69. Y.Q. Lu, J.J. Zheng, Y.L. Lu, Appl. Phys. Lett. 74 (1999) 123. Y.Q. Lu, Z.L. Wang, Y.X. Xi, et al. Appl. Phys. Lett. 77 (2000) 3719. M. Reich, F. Korte, C. Fallnich, et al. Opt. Lett. 23 (1998) 1817. G.A. Shafeev, Quamtum Electron. 27 (1997) 1104. K. Korda´s, J. Be´ke´si, R. Vajtai, et al. Appl. Surf. Sci. 172 (2001) 178. D.S. Chen, Q.H. Lu, Y. Zhao, Appl. Surf. Sci. 253 (2006) 1573. H. Xiong, R.X. Li, Z.N. Zeng, et al. Phys. Rev. A 75 (2007) 051802. L.H. Bai, J.T. Zhang, Z.Z. Xu, et al. Phys Rev. Lett. 97 (2006) 193002. K. Itoh, W. Watanabe, S. Nolte, et al. MRS Bull. 31 (2006) 620. J. Xu, Y. Liao, H.D. Zeng, et al. Opt. Express 15 (2007) 12743. Y. Cheng, K. Sugioka, K. Midorikawa, et al. Opt. Lett. 28 (2003) 1144. K. Sugioka, Y. Cheng, K. Midorikawa, Appl. Phys. A 81 (2005) 1. Y. Cheng, K. Sugioka, K. Midorikawa, Opt. Lett. 28 (2003) 55. Y. Cheng, K. Sugioka, K. Midorikawa, Opt. Lett. 29 (2004) 2007. H.Y. Sun, F. He, Z.H. Zhou, et al. Opt. Lett. 32 (2007) 1536. V.M. Dubin, Y. Shacham-Diamand, B. Zhao, et al. J. Electrochem. Soc. 144 (1997) 898. Z. Wang, O. Yaegashi, H. Sakaue, et al. J. Electrochem. Soc. 151 (2004) 781. K. Korda´s, J. Be´ke´si, R. Vajtai, et al. J. Electron. Mater. 30 (2001) 21. Y. Bellouard, A. Said, M. Dugan, et al. Opt. Express 12 (2004) 2120. J. Burghoff, C. Grebing, S. Nolte, et al. Appl. Phys. Lett. 9 (2006) 081108.