- Email: [email protected]
Fabrication of microlens arrays on soda-lime glass using a laser direct-write technique and a thermal treatment assisted by a CO2 laser
Optics and Lasers in Engineering 73 (2015) 1–6
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Fabrication of microlens arrays on soda-lime glass using a laser direct-write technique and a thermal treatment assisted by a CO2 laser Tamara Delgado, Daniel Nieto n, María Teresa Flores-Arias Microoptics and GRIN Optics Group, Applied Physics Department, Faculty of Physics, University of Santiago de Compostela, Santiago de Compostela E15782, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 13 January 2015 Received in revised form 25 March 2015 Accepted 31 March 2015
A low-cost method for fabricating microlens arrays on commercial soda-lime glass is presented. The hybrid technique is composed by a laser direct writing technique and a laser assisted post-thermal treatment. In particular we use a nanosecond Q-Switch Nd:YVO4 laser for fabricating the initial structure of microposts on soda-lime glass substrates and a CO2 laser combined with a furnace for reshaping and improving its morphological and optical qualities. This new fabrication approach lets us obtain a high quality microlenses array with a diameter of 50 μm, sag 1.5 μm, focal length 1 mm and a spot size of 7.8 μm. Furthermore, the proposed technique preserves the advantages of the laser direct-write technique in terms of design flexibility, simplicity, fast prototyping, low cost and so on; while the alternative laser assisted thermal treatment lets us overcome the bounding problems presented in other conventional thermal treatments. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser assisted thermal treatment Glass microstructuring Microlenses Nanosecond Q-switch laser CO2 laser
1. Introduction The technology to fabricate small optical components has become increasingly important due to their wide application in photonics and optoelectronics. In particular, microlens arrays are basic components of many optical devices and systems. These are used in several applications such as: fiber couplers in optical communication systems [1], laser beam shaping elements [2], sensors [3] or image processing [4]. A huge variety of fabrication methods have been developed: lithographic approaches [5,6], methods based on pressure difference [7,8], or surface-tensiondriven techniques consisting of melt-reflow [9,10] are some of the most commonly used methods. While these techniques demonstrate the ability to produce microlens arrays with uniform surface profiles and great quality, they present high-cost or require long fabrication times. Hence, the ability to generate microlens arrays in a rapid and cost-effective manner still results an important challenge. The use of the laser ablation for material processing is not new, excimer and short-pulse lasers have been reported in literature for this purpose [11,12]. This technique is, in general, not currently able to produce microlenses with a comparable quality of the
n
Corresponding author. E-mail addresses: [email protected] (D. Nieto), maite.fl[email protected] (M.T. Flores-Arias). http://dx.doi.org/10.1016/j.optlaseng.2015.03.026 0143-8166/& 2015 Elsevier Ltd. All rights reserved.
above mentioned methods; however, it offers some advantages that can be of significant practical importance, such as: direct write and contactless etching nature, flexibility in terms of surface shapes or simplicity of the fabrication setup. For transparent materials, laser ablation should ideally be performed with ultraviolet radiation because of the nonlinear absorption in this wavelength range. Nevertheless, non-linear coupling of highintensity laser pulses in the near-IR range with subpicosecond duration has shown several advantages [13–15]. Micromachining with nanosecond and ultrafast pulse lasers has been reported in the literature as a good method for obtaining quality micro-optical elements [16,17]. The use of a CO2 laser has been also published to fabricate microlenses and microlens arrays by a thermal reflow method [18]. The temperature of the surface of the glass rises until the material glass transition temperature and the surface tensions on the melted material result into shape modification. The use of glass with thermostable properties results crucial for this technique, and this fact makes the fabrication of microlenses with ordinary optical glasses to be difficult [19]. This paper presents an improved method for fabricating microlens arrays on commercial soda-lime glass. This consists of a combination of an ablation-laser direct writing technique, for fabricating the initial microstructures, and a thermal treatment assisted by a CO 2 laser combined with a furnace [21] for reshaping and improving the morphological and optical qualities of the generated microlenses. In a previous publication
2
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
[20] a similar method for fabricating microlens arrays was described. In brief, this method used the direct-write laser technique for microstructuring the soda-lime glass and a subsequent thermal treatment applied by a mufla furnace in order to improve the optical properties of the microlens arrays. In this paper we present an alternative thermal treatment that highly improves the quality of the microlens arrays fabricated, this employs a CO 2 laser combined with a furnace. In this occasion the use of the CO 2 laser allows us to melt only the top surface of our samples, while the glass volume is heated to lower temperature. In this way, we reduce the damage generated on the glass throughout the laser ablation process, without stressing the material and achieving a change in the surface profile, improving the optical quality of the microlenses; as we were able to do in previous works, but avoiding this time the bounding problems presented in conventional thermal treatments. The presented two steps method permits us to fabricate microlens arrays with comparable imaging and stray light properties as those obtained by other methods [22], preserving the advantages of the laser direct-write technique: flexibility in terms of design, simplicity, fast prototyping, low cost; and overcoming the bounding problems presented in conventional thermal treatments through the proposed alternative laser assisted thermal treatment. In addition, the use of a nanosecond pulsed laser for performing the ablative direct-write process, includes the benefits of using lasers commonly implemented for laser processing of materials, which makes the presented technique highly competitive for the industrial sector. In Section 2 the employed materials and methods for fabricating the microlens arrays are presented. In Section 3 we present the characteristics of the soda-lime glass used as a substrate and explain the two steps of the fabrication process. Section 4 analyzes the fabricated microlenses in terms of their physical and optical characteristics. Section 5 presents the conclusions.
2. Methods and materials
The microlens spot size and the irradiance of the focus were analyzed using a beam profiler BP-109UV. This is a high-precision instrument that can analyze the power distribution of laser beams with diameters from 10 μm to 9 mm. The glass used as a substrate for fabricating the microlens arrays was a commercial soda-lime glass, provided by a local supplier. The composition of this glass was determined by using a scanning electron microscope (SEM) Zeiss FESEM-ULTRA Plus. In addition, the soda-lime glass was also characterized by its transmission spectrum, obtained with a Perkin Elmer Lambda 25 spectrometer with a spectral range between 200 and 1100 nm.
3. Fabrication procedure We present a two-steps technique for fabricating spherical microlenses based on the laser ablation of a soda-lime glass substrate, with a beam laser of circular Gaussian profile; followed by a thermal treatment assisted by a CO2 laser. The soda-lime glass used as a substrate was analyzed in terms of its transmission spectrum (Fig. 1), in order to determine the material absorption at the two different wavelengths of the employed laser systems. As we can observe, the soda-lime glass results transparent at the fundamental wavelength of the Nd:YVO4 laser (1064 nm); while this absorbs the wavelengths higher than 5000 nm, resulting in a material absorbent at the 10.6 μm wavelength of the CO2 laser. In Table 1 we present the compositional analysis of our glass. It reveals the existence of tin (Sn) only in one side of the samples. This fact is related to the float technique used for producing window glasses, where the diffusion of tin into the glass surface in contact with a Sn bath during the molding process takes place [24]. Hence, the employed soda-lime glass presents two distinct faces and the presence of impurities of Sn in one of them is crucial
Table 1 Chemical composition of soda-lime glass.
In order to perform the microstructuring of the glass a Rofin model Nd:YVO4 laser was used. This is a solid-state laser, operating at 1064 nm wavelength and using a Q-Switch regime, with pulses of 20 ns. Subsequently, an Easy Mark CO2 laser system with a fundamental wavelength of 10.6 μm was employed to apply the thermal treatment; providing a maximum power of 124 W. A confocal microscope SENSOFAR PLμ 2300 allowed us to perform topographic measurements on the surface and to obtain 3D images of the generated microlenses. The results presented were acquired using a 20 EPI microscope objective, with a numerical aperture of 0.45 and a working distance of 4.5 mm.
Front side
Rare side
Element
Weight%
Atomic%
Element
Weight%
Atomic%
O Na Mg Al Si Ca Sn TOTAL
50.25 9.08 2.19 0.54 33.08 4.87 – 100.00
63.51 7.99 1.82 0.40 23.82 2.45 – 100.00
O Na Mg Al Si Ca Sn TOTAL
48.97 9.14 2.24 0.49 32.34 4.91 1.90 100.00
63.00 8.18 1.90 0.37 23.70 2.52 0.33 100.00
100
T%
80 60 40 20 200
400
600
800
1000
2500
3000
4000 5000
10000
λ(nm) Fig. 1. Transmission spectrum of the soda-lime glass. (a) Range from 200 to 1100 nm determined with the Perkin-Elmer Lambda 25 spectrometer and (b) typical spectrum at medium infrared wavelengths [23].
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
in the laser ablation. These impurities act as absorption centers and allow us to start the ablation process that leads to the microstructuring of the glass [25]. 3.1. Laser microstructuring The laser setup employed for microstructuring the soda-lime glass substrates was a nanosecond Q-Switch Nd:YVO4 laser operating at the fundamental wavelength of 1064 nm, combined with a galvanometer system for addressing the output laser beam. A flatfield lens with a focal length of 100 mm was used to focus the laser beam, providing a uniform irradiance distribution on glass substrate over a working area of 80 80 mm2. Fig. 2a shows the hexagonal pattern designed for the packing of arrays of microposts. Fig. 2b shows the micropost array arrangements in terms of diameter and pitch. In Fig. 2c we can see a diagram of the experimental setup and the fabrication process of the initial microposts. As we can observe in Fig. 2c, each cylindrical micropost is obtained by the ablation of a circular trench formed through the movement of the laser beam using the galvanometer mirror system, while the sample is maintained in the same position. The optimal laser parameters for fabricating the initial microstructures were: average power of 7 W, repetition rate of 9 kHz and scan speed of 70 mm/s. 3.2. Laser assisted thermal treatment With the aim of improving the quality of the originally generated microlenses, the initial glass posts were subjected to a thermal treatment. A roller furnace provided with a CO2 laser (Fig. 3) was employed to perform this thermal treatment [21]. In this case a flat-field lens with a focal length of 1 m was used to focus the CO2 laser beam, obtaining a working area of 120 120 mm2. The laser parameters used were: wavelength of 10.6 μm, repetition rate of 10 kHz and a scan speed of 350 cm/s. The samples were gradually heated by using a roller furnace. The system allows us to pass our samples through different regions of heating and cooling, with a selected speed of 1000 mm/h for the roller furnace. When the samples reach the central part of the setup they achieve a temperature of 500 1C, which is founded below the transition temperature of the glass, so the samples are not melted. However,
3
in this way we are able to avoid the thermal shock and possible cracks when the CO2 laser interacts with the glass substrates. The energy of the CO2 laser is deposited into the samples in a perpendicular direction of their movement (as showed in Fig. 3), subsequently the glass transition temperature of the soda-lime glass is achieved only on the top surface of our samples [21]. The glass reduces its viscosity and consequently the surface tension of the melted material induces the modification of the surface shape. The transformation of the posts into
Fig. 3. Experimental setup of the thermal treatment.
Before the thermal treatment After the thermal treatment
Redistribution of the material from the top to the bottom
Fig. 4. Profile of one post and the generated microlens after the thermal treatment.
Fig. 2. (a) Hexagonal pattern. (b) Microlens array arrangements: diameter and pitch and (c) Experimental setup and fabrication process of the cylindrical.
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
microlenses starts through the redistribution of the material from the top sides of the post to the bottom of the crater, forming spherical microlenses with good optical quality, as we can see in Fig. 4. To conclude with the thermal treatment, the samples were gradually cooled throughout the last regions of the roller furnace. The thermal treatment applied with the CO2 laser combined with the roller furnace allows us to reshape and improve the optical properties of the generated microlenses by a laser ablation process and shows the advantages of overcoming the bounding problems presented in other conventional thermal treatments, where the whole piece is melted and consequently bounded to the mold where this is deposited [26].
4. Characterization and results In this section we present an analysis of the fabricated microlenses in terms of their geometrical and morphological characteristics and their optical properties. In Fig. 5 we show 3D confocal images of the final generated microlenses, performed with a SENSOFAR PLμ 2300 confocal microscope. The modification suffered by the shape of the microlenses, according to the characteristics of the applied thermal treatment, is can be appreciated. Fig. 5a depicts a set of generated microposts by the laser direct-write technique. In Fig. 5b we can observe that working at a duty cycle of the CO2 laser of 25%, which is equivalent to a power of 68 W; the cylindrical posts continue with a flattop surface, having almost no refractive power. For a duty cycle of 35% (90 W) we obtain microlenses with a parabolic profile (Fig. 5c) that provides us a good focusing power, with good optical properties; as we will see in following presented results. If we continue increasing the value of the applied power by the CO2 laser, finally the initial cylindrical posts become flat and the microlenses obtained by the laser direct-write consequently disappear, as we can see in Fig. 5d. The diameter of the microlenses is maintained
almost constant over the range of the used powers, while their sag decreases due to the redistribution of the material from the top sides of the post to the bottom of the crater. Fig. 6 shows the profile after being subjected to different thermal treatments of one random microlens chosen at one microlens array. The shape of the surface of the microlens presents a flat-top profile with some irregularities after the laser ablation process. Part of these irregularities still remains when we apply a thermal treatment with a low power of the CO2 laser. As the applied power is increased these irregularities tend to disappear and the shape of the microlens starts to change from a flat-top post to a parabolic profile. For high power values the microlenses become flat and their refractive power disappears. The optical properties of the fabricated microlenses were analyzed in terms of their focal length and spot size. The
After ablation
4 3
Height [μm]
4
25%
2 35%
1 45% 0
0
5
10
15
20
25
30
35
40
45
50
X [μm] Fig. 6. Cross-sectional profile of one microlens at different Duty Cycles of the CO2 laser, measured with the confocal microscope SENSOFAR PLμ 2300.
Fig. 5. Confocal 3D images of the fabricated microlenses at a duty cycle of the CO2 laser of: (a) without thermal treatment (b) 25%, (c) 35% and (d) 45%; and a repetition rate of 10 kHz.
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
experimental setup for determining the focal length of the microlenses consisted of a He–Ne laser (λ ¼632.8 nm), and a 20 microscope objective (Fig. 7). In order to determine the focal distance of the tested microlenses, we focused the microscope objective on the surface of the microlenses. Then, this was moved until observing the focal plane of the microlens array, the difference between the two positions gives us the focal distance of the fabricated microlenses [27]. The focus spot size was determined calculating the width at 1/e2 with a beam profiler BP-109UV. The focal distance and the focal spot size of the fabricated microlenses are shown in Table 2. In addition, through the topographical data provided by the confocal microscope we obtain the geometrical and morphological characteristics of the microlenses in terms of their diameter and sag (Table 2).
5
As it can be appreciated the diameter remains almost constant while the sag decreases as applied power increases due to the displacement of the melted material. Variations on the microlenses sag are related to the focalization capabilities, for this reason we observe an increase on the focal length as the sag decreases. Finally, we determined the quality of the obtained focuses on the focal plane by measuring their irradiance distribution with a beam profiler BP-109UV. Fig. 8 shows the irradiance distribution of one random microlens within the corresponding spots array. For a low energy of the CO2 laser during the thermal treatment, the obtained focuses (Fig. 8a and d) show that although our microlenses are able to focus the light, part of this is crossing the microstructures without being focusing. When the microposts are subjected to a high power of the CO2 laser and as a consequence these are almost flattened, the focusing power is lost and the obtained focuses are scattered (Fig. 8c and f). Finally, working with a duty cycle of 35% of the CO2, we obtained microlenses with a good focusing capability, as we can see in Fig. 8b. In addition, in Fig. 8e, obtained for a power of 90 W, the homogeneity of the microlenses focuses at the focal plane as well as the similarity with the PSF aberrations free can be observed.
Table 2 Optical and geometrical characteristics of the fabricated microlenses. Features of the microlenses Duty cycle
Without thermal treatment
25%
35%
45%
Diameter [mm] Sag [mm] Focal length [mm] Focal spot size [mm] Roughness [nm]
50 7 – –
50 6 0.7 9.6 13.2
51 1.5 1 7.8 8
53 0.25 3 28.8 7.6
Fig. 7. Setup for measuring the microlens focal length.
Fig. 8. Irradiance distribution of one random microlens, pictures (a), (b) and (c); within the microlens array with the corresponding focal plane at pictures (d), (e) and (f).
6
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
5. Conclusions We have presented an improved low-cost method for fabricating microlens arrays on soda-lime glass. This technique is based on the combination of an ablation-laser direct writing process and a laser assisted thermal treatment. A Q-Switched Nd:YVO4 laser combined with a galvanometer system was employed for microstructuring the glass and fabricating in this way the initial microposts. This is a rapid and simple method that allows us to fabricate inexpensive microlenses. The thermal treatment applied by a furnace provided with a CO2 laser was used for reshaping and improving the optical qualities of the generated microlenses. This alternative thermal process allows us to melt only the top surface of our samples, avoiding the bounding problems presented in other conventional thermal treatments. Different powers of the CO2 laser were tested for applying the thermal treatment, fabricating different microlenses. We observed that the diameter of a microlens is maintained almost constant, while their sag decreases as we increase the energy given by the CO2 laser. For a range of powers between 68 and 116 W, microlenses with a diameter around 50 μm and sags between 7 and 0.25 μm were fabricated. The optical quality of the generated microlenses was determined in terms of their focal length and focal spot size. We obtained microlenses with a focal length between 0.7 and 3 mm and with a focal spot size from 28.8 to 7.8 μm, for different thermal treatments. Acknowledgments This work has been supported by Consellería de Cultura, Xunta de Galicia, under the contract EM2012/019 Spain. References [1] Smith PJ, Taylor CM, McCabe EM, Selviah DR, Day SE, Commander LG. Switchable fiber coupling using variable-focal-length microlenses. Rev Sci Instrum 2001;72:3132–4. [2] Wippermann F, Zeitner UD, Dannberg P, Bräuer A, Sinzinger S. Beam homogenizers based on chirped microlens arrays. Opt Express 2007;15:6218–31. [3] Artzner G. Microlens arrays for Shack–Hartmann wavefront sensor. Opt Eng 1992;316:1311–22. [4] Arimoto H, Javidi B. Integral three-dimensional imaging with digital reconstruction. Opt Lett 2001;26:157–9. [5] Kunnavakkam MV, Houlihan FM, Schlaz M, Liddle JA, Kolodner P, Nalamasu O, Rogers JA. Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process. Appl Phys Lett 2003;82:1152–4.
[6] Wu MH, Park C, Whitesides GM. Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photolithography. Langmuir 2002;18:9312–8. [7] Pan CT, Wu TT, Chen MF, Chang YC, Lee CJ, Huang JC. Hot embossing of microlens array on bulk metallic glass. Sens Actuators A: Phys 2008;141:422–31. [8] Chang CY, Yang SY, Sheh JL. A roller embossing process for rapid fabrication of microlens arrays on glass substrates. Microsyst Technol 2006;12:754–9. [9] Ashraf M, Gupta C, Chollet F, Springham SV, Rawat RS. Geometrical characterization techniques for microlenses made by thermal reflow of photoresist cylinder. Opt Lasers Eng 2008;46:711–20. [10] O'Neill FT, Sheridan JT. Photoresist reflow of microlens production part I: backgraound and experiments. Optik 2002;113:391–404. [11] Naessens K, Van Daele P, Baets RG. Excimer-laser-ablation based microlens fabrication for optical fiber coupling purposes. Proc SPIE 2003;4941:133–9. [12] Ben-Yakar A, Byer RL. Femtosecond laser ablation properties of borosilicate glass. J Appl Phys 2004;96:5316–23. [13] Fan CH, Longtin JP. Modeling optical breakdown in dielectrics during ultrafast laser processing. Appl Opt 2001;40:3124–31. [14] Stuart BC, Feit MD, Rubenchik AM, Shore BW, Perry MD. Laser-induced damage in dielectrics with nanosecond and subpicosecond pulses. Phys Rev Lett 1995;74:2248–51. [15] Karnakis DM, Knowles MRH, Alty KT, Schlaf M, Snelling HV. Comparison of glass processing using high repetition femtosecond (800 nm) and UV (255 nm) nanosecond pulsed lasers. In: Proceedings of the international society for optics and photonics MOEMS-MEMS micro & nanofabrication. 2005. p. 216–227. [16] Zoubir A, Shah L, Richardson K, Richardson M. Practical uses of femtosecond laser micro-materials processing. Appl Phys 2003;77:311–5. [17] Sayah A, Parashar VK, Gijs MAM. Micro-replication of optical lenses in glass using a novel sol–gel technology. In: Proceedings of the IEEEMEMS'02 IEEE. 2002. p. 516–519. [18] Wakaki M, Komachi Y, Kanai G. Microlenses and microlens arrays formed on a glass plate by use of a CO2 laser. Appl Opt 1998;37:627–31. [19] Veiko VP, Yakovlev EB, Frolar VV, Chujko VA, Kromin AK, Abbakumov MO, Shakola AT, Fomichov PA. Laser heating and evaporation of glass and glassboring materials and its application for creating MOC. In: Roychoudhuri CS, Veldkamp WB, editors. Proceedings of the SPIE miniature and micro-optics: fabrication and system applications.;1544 1991. p. 152–163. [20] Nieto D, Arines J, Gómez-Reino C, O'Connor GM, Flores-Arias MT. Fabrication and characterization of microlens arrays on soda-lime glass using a combination of laser direct-write and thermal reflow technique. J Appl Phys 2011;110 (2):023108. [21] Estepa LC & De la Fuente GF. Continuous furnace with coupled laser for the surface treatment of materials. Patent No 00560; (2006March). [22] Ottevaere H, Cox R, Herzig HP, Miyashita T, Naessens K, Taghizadeh M, Völkel R, Woo HJ, Thienpont H. Comparing glass and plastic refractive microlenses fabricated with different technologies. J Opt A: Pure App Opt 2006;8:S407. [23] Narottam PB, Doremus RH. Handbook of Glass Properties. Orlando, FL: Academic Press, Inc.; 1986. [24] Pilkington LAB. Pilkington brothers limited, St Helens, Lancashire. the float glass process. Proc Roy Soc Lond A 1969;314:1–25. [25] Nieto D, Arines J, Flores-Arias MT. Fluence ablation threshold dependence on tin impurities in commercial soda-lime glass. Appl Opt 2014;53:5416–20. [26] Heckele M, Schomburg WK. Review on micro molding of thermoplastic polymers. J Micromech and Microeng 2004;14:R1. [27] Sayah A, Parashar VK, Gijs MAM. Micro-replication of optical lenses in glass using a novel sol gel technology. In: Proceedings of the IEEE MEMS'02 2002, p. 516–519.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Fabrication of microlens arrays on soda-lime glass using a laser direct-write technique and a thermal treatment assisted by a CO2 laser Tamara Delgado, Daniel Nieto n, María Teresa Flores-Arias Microoptics and GRIN Optics Group, Applied Physics Department, Faculty of Physics, University of Santiago de Compostela, Santiago de Compostela E15782, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 13 January 2015 Received in revised form 25 March 2015 Accepted 31 March 2015
A low-cost method for fabricating microlens arrays on commercial soda-lime glass is presented. The hybrid technique is composed by a laser direct writing technique and a laser assisted post-thermal treatment. In particular we use a nanosecond Q-Switch Nd:YVO4 laser for fabricating the initial structure of microposts on soda-lime glass substrates and a CO2 laser combined with a furnace for reshaping and improving its morphological and optical qualities. This new fabrication approach lets us obtain a high quality microlenses array with a diameter of 50 μm, sag 1.5 μm, focal length 1 mm and a spot size of 7.8 μm. Furthermore, the proposed technique preserves the advantages of the laser direct-write technique in terms of design flexibility, simplicity, fast prototyping, low cost and so on; while the alternative laser assisted thermal treatment lets us overcome the bounding problems presented in other conventional thermal treatments. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser assisted thermal treatment Glass microstructuring Microlenses Nanosecond Q-switch laser CO2 laser
1. Introduction The technology to fabricate small optical components has become increasingly important due to their wide application in photonics and optoelectronics. In particular, microlens arrays are basic components of many optical devices and systems. These are used in several applications such as: fiber couplers in optical communication systems [1], laser beam shaping elements [2], sensors [3] or image processing [4]. A huge variety of fabrication methods have been developed: lithographic approaches [5,6], methods based on pressure difference [7,8], or surface-tensiondriven techniques consisting of melt-reflow [9,10] are some of the most commonly used methods. While these techniques demonstrate the ability to produce microlens arrays with uniform surface profiles and great quality, they present high-cost or require long fabrication times. Hence, the ability to generate microlens arrays in a rapid and cost-effective manner still results an important challenge. The use of the laser ablation for material processing is not new, excimer and short-pulse lasers have been reported in literature for this purpose [11,12]. This technique is, in general, not currently able to produce microlenses with a comparable quality of the
n
Corresponding author. E-mail addresses: [email protected] (D. Nieto), maite.fl[email protected] (M.T. Flores-Arias). http://dx.doi.org/10.1016/j.optlaseng.2015.03.026 0143-8166/& 2015 Elsevier Ltd. All rights reserved.
above mentioned methods; however, it offers some advantages that can be of significant practical importance, such as: direct write and contactless etching nature, flexibility in terms of surface shapes or simplicity of the fabrication setup. For transparent materials, laser ablation should ideally be performed with ultraviolet radiation because of the nonlinear absorption in this wavelength range. Nevertheless, non-linear coupling of highintensity laser pulses in the near-IR range with subpicosecond duration has shown several advantages [13–15]. Micromachining with nanosecond and ultrafast pulse lasers has been reported in the literature as a good method for obtaining quality micro-optical elements [16,17]. The use of a CO2 laser has been also published to fabricate microlenses and microlens arrays by a thermal reflow method [18]. The temperature of the surface of the glass rises until the material glass transition temperature and the surface tensions on the melted material result into shape modification. The use of glass with thermostable properties results crucial for this technique, and this fact makes the fabrication of microlenses with ordinary optical glasses to be difficult [19]. This paper presents an improved method for fabricating microlens arrays on commercial soda-lime glass. This consists of a combination of an ablation-laser direct writing technique, for fabricating the initial microstructures, and a thermal treatment assisted by a CO 2 laser combined with a furnace [21] for reshaping and improving the morphological and optical qualities of the generated microlenses. In a previous publication
2
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
[20] a similar method for fabricating microlens arrays was described. In brief, this method used the direct-write laser technique for microstructuring the soda-lime glass and a subsequent thermal treatment applied by a mufla furnace in order to improve the optical properties of the microlens arrays. In this paper we present an alternative thermal treatment that highly improves the quality of the microlens arrays fabricated, this employs a CO 2 laser combined with a furnace. In this occasion the use of the CO 2 laser allows us to melt only the top surface of our samples, while the glass volume is heated to lower temperature. In this way, we reduce the damage generated on the glass throughout the laser ablation process, without stressing the material and achieving a change in the surface profile, improving the optical quality of the microlenses; as we were able to do in previous works, but avoiding this time the bounding problems presented in conventional thermal treatments. The presented two steps method permits us to fabricate microlens arrays with comparable imaging and stray light properties as those obtained by other methods [22], preserving the advantages of the laser direct-write technique: flexibility in terms of design, simplicity, fast prototyping, low cost; and overcoming the bounding problems presented in conventional thermal treatments through the proposed alternative laser assisted thermal treatment. In addition, the use of a nanosecond pulsed laser for performing the ablative direct-write process, includes the benefits of using lasers commonly implemented for laser processing of materials, which makes the presented technique highly competitive for the industrial sector. In Section 2 the employed materials and methods for fabricating the microlens arrays are presented. In Section 3 we present the characteristics of the soda-lime glass used as a substrate and explain the two steps of the fabrication process. Section 4 analyzes the fabricated microlenses in terms of their physical and optical characteristics. Section 5 presents the conclusions.
2. Methods and materials
The microlens spot size and the irradiance of the focus were analyzed using a beam profiler BP-109UV. This is a high-precision instrument that can analyze the power distribution of laser beams with diameters from 10 μm to 9 mm. The glass used as a substrate for fabricating the microlens arrays was a commercial soda-lime glass, provided by a local supplier. The composition of this glass was determined by using a scanning electron microscope (SEM) Zeiss FESEM-ULTRA Plus. In addition, the soda-lime glass was also characterized by its transmission spectrum, obtained with a Perkin Elmer Lambda 25 spectrometer with a spectral range between 200 and 1100 nm.
3. Fabrication procedure We present a two-steps technique for fabricating spherical microlenses based on the laser ablation of a soda-lime glass substrate, with a beam laser of circular Gaussian profile; followed by a thermal treatment assisted by a CO2 laser. The soda-lime glass used as a substrate was analyzed in terms of its transmission spectrum (Fig. 1), in order to determine the material absorption at the two different wavelengths of the employed laser systems. As we can observe, the soda-lime glass results transparent at the fundamental wavelength of the Nd:YVO4 laser (1064 nm); while this absorbs the wavelengths higher than 5000 nm, resulting in a material absorbent at the 10.6 μm wavelength of the CO2 laser. In Table 1 we present the compositional analysis of our glass. It reveals the existence of tin (Sn) only in one side of the samples. This fact is related to the float technique used for producing window glasses, where the diffusion of tin into the glass surface in contact with a Sn bath during the molding process takes place [24]. Hence, the employed soda-lime glass presents two distinct faces and the presence of impurities of Sn in one of them is crucial
Table 1 Chemical composition of soda-lime glass.
In order to perform the microstructuring of the glass a Rofin model Nd:YVO4 laser was used. This is a solid-state laser, operating at 1064 nm wavelength and using a Q-Switch regime, with pulses of 20 ns. Subsequently, an Easy Mark CO2 laser system with a fundamental wavelength of 10.6 μm was employed to apply the thermal treatment; providing a maximum power of 124 W. A confocal microscope SENSOFAR PLμ 2300 allowed us to perform topographic measurements on the surface and to obtain 3D images of the generated microlenses. The results presented were acquired using a 20 EPI microscope objective, with a numerical aperture of 0.45 and a working distance of 4.5 mm.
Front side
Rare side
Element
Weight%
Atomic%
Element
Weight%
Atomic%
O Na Mg Al Si Ca Sn TOTAL
50.25 9.08 2.19 0.54 33.08 4.87 – 100.00
63.51 7.99 1.82 0.40 23.82 2.45 – 100.00
O Na Mg Al Si Ca Sn TOTAL
48.97 9.14 2.24 0.49 32.34 4.91 1.90 100.00
63.00 8.18 1.90 0.37 23.70 2.52 0.33 100.00
100
T%
80 60 40 20 200
400
600
800
1000
2500
3000
4000 5000
10000
λ(nm) Fig. 1. Transmission spectrum of the soda-lime glass. (a) Range from 200 to 1100 nm determined with the Perkin-Elmer Lambda 25 spectrometer and (b) typical spectrum at medium infrared wavelengths [23].
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
in the laser ablation. These impurities act as absorption centers and allow us to start the ablation process that leads to the microstructuring of the glass [25]. 3.1. Laser microstructuring The laser setup employed for microstructuring the soda-lime glass substrates was a nanosecond Q-Switch Nd:YVO4 laser operating at the fundamental wavelength of 1064 nm, combined with a galvanometer system for addressing the output laser beam. A flatfield lens with a focal length of 100 mm was used to focus the laser beam, providing a uniform irradiance distribution on glass substrate over a working area of 80 80 mm2. Fig. 2a shows the hexagonal pattern designed for the packing of arrays of microposts. Fig. 2b shows the micropost array arrangements in terms of diameter and pitch. In Fig. 2c we can see a diagram of the experimental setup and the fabrication process of the initial microposts. As we can observe in Fig. 2c, each cylindrical micropost is obtained by the ablation of a circular trench formed through the movement of the laser beam using the galvanometer mirror system, while the sample is maintained in the same position. The optimal laser parameters for fabricating the initial microstructures were: average power of 7 W, repetition rate of 9 kHz and scan speed of 70 mm/s. 3.2. Laser assisted thermal treatment With the aim of improving the quality of the originally generated microlenses, the initial glass posts were subjected to a thermal treatment. A roller furnace provided with a CO2 laser (Fig. 3) was employed to perform this thermal treatment [21]. In this case a flat-field lens with a focal length of 1 m was used to focus the CO2 laser beam, obtaining a working area of 120 120 mm2. The laser parameters used were: wavelength of 10.6 μm, repetition rate of 10 kHz and a scan speed of 350 cm/s. The samples were gradually heated by using a roller furnace. The system allows us to pass our samples through different regions of heating and cooling, with a selected speed of 1000 mm/h for the roller furnace. When the samples reach the central part of the setup they achieve a temperature of 500 1C, which is founded below the transition temperature of the glass, so the samples are not melted. However,
3
in this way we are able to avoid the thermal shock and possible cracks when the CO2 laser interacts with the glass substrates. The energy of the CO2 laser is deposited into the samples in a perpendicular direction of their movement (as showed in Fig. 3), subsequently the glass transition temperature of the soda-lime glass is achieved only on the top surface of our samples [21]. The glass reduces its viscosity and consequently the surface tension of the melted material induces the modification of the surface shape. The transformation of the posts into
Fig. 3. Experimental setup of the thermal treatment.
Before the thermal treatment After the thermal treatment
Redistribution of the material from the top to the bottom
Fig. 4. Profile of one post and the generated microlens after the thermal treatment.
Fig. 2. (a) Hexagonal pattern. (b) Microlens array arrangements: diameter and pitch and (c) Experimental setup and fabrication process of the cylindrical.
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
microlenses starts through the redistribution of the material from the top sides of the post to the bottom of the crater, forming spherical microlenses with good optical quality, as we can see in Fig. 4. To conclude with the thermal treatment, the samples were gradually cooled throughout the last regions of the roller furnace. The thermal treatment applied with the CO2 laser combined with the roller furnace allows us to reshape and improve the optical properties of the generated microlenses by a laser ablation process and shows the advantages of overcoming the bounding problems presented in other conventional thermal treatments, where the whole piece is melted and consequently bounded to the mold where this is deposited [26].
4. Characterization and results In this section we present an analysis of the fabricated microlenses in terms of their geometrical and morphological characteristics and their optical properties. In Fig. 5 we show 3D confocal images of the final generated microlenses, performed with a SENSOFAR PLμ 2300 confocal microscope. The modification suffered by the shape of the microlenses, according to the characteristics of the applied thermal treatment, is can be appreciated. Fig. 5a depicts a set of generated microposts by the laser direct-write technique. In Fig. 5b we can observe that working at a duty cycle of the CO2 laser of 25%, which is equivalent to a power of 68 W; the cylindrical posts continue with a flattop surface, having almost no refractive power. For a duty cycle of 35% (90 W) we obtain microlenses with a parabolic profile (Fig. 5c) that provides us a good focusing power, with good optical properties; as we will see in following presented results. If we continue increasing the value of the applied power by the CO2 laser, finally the initial cylindrical posts become flat and the microlenses obtained by the laser direct-write consequently disappear, as we can see in Fig. 5d. The diameter of the microlenses is maintained
almost constant over the range of the used powers, while their sag decreases due to the redistribution of the material from the top sides of the post to the bottom of the crater. Fig. 6 shows the profile after being subjected to different thermal treatments of one random microlens chosen at one microlens array. The shape of the surface of the microlens presents a flat-top profile with some irregularities after the laser ablation process. Part of these irregularities still remains when we apply a thermal treatment with a low power of the CO2 laser. As the applied power is increased these irregularities tend to disappear and the shape of the microlens starts to change from a flat-top post to a parabolic profile. For high power values the microlenses become flat and their refractive power disappears. The optical properties of the fabricated microlenses were analyzed in terms of their focal length and spot size. The
After ablation
4 3
Height [μm]
4
25%
2 35%
1 45% 0
0
5
10
15
20
25
30
35
40
45
50
X [μm] Fig. 6. Cross-sectional profile of one microlens at different Duty Cycles of the CO2 laser, measured with the confocal microscope SENSOFAR PLμ 2300.
Fig. 5. Confocal 3D images of the fabricated microlenses at a duty cycle of the CO2 laser of: (a) without thermal treatment (b) 25%, (c) 35% and (d) 45%; and a repetition rate of 10 kHz.
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
experimental setup for determining the focal length of the microlenses consisted of a He–Ne laser (λ ¼632.8 nm), and a 20 microscope objective (Fig. 7). In order to determine the focal distance of the tested microlenses, we focused the microscope objective on the surface of the microlenses. Then, this was moved until observing the focal plane of the microlens array, the difference between the two positions gives us the focal distance of the fabricated microlenses [27]. The focus spot size was determined calculating the width at 1/e2 with a beam profiler BP-109UV. The focal distance and the focal spot size of the fabricated microlenses are shown in Table 2. In addition, through the topographical data provided by the confocal microscope we obtain the geometrical and morphological characteristics of the microlenses in terms of their diameter and sag (Table 2).
5
As it can be appreciated the diameter remains almost constant while the sag decreases as applied power increases due to the displacement of the melted material. Variations on the microlenses sag are related to the focalization capabilities, for this reason we observe an increase on the focal length as the sag decreases. Finally, we determined the quality of the obtained focuses on the focal plane by measuring their irradiance distribution with a beam profiler BP-109UV. Fig. 8 shows the irradiance distribution of one random microlens within the corresponding spots array. For a low energy of the CO2 laser during the thermal treatment, the obtained focuses (Fig. 8a and d) show that although our microlenses are able to focus the light, part of this is crossing the microstructures without being focusing. When the microposts are subjected to a high power of the CO2 laser and as a consequence these are almost flattened, the focusing power is lost and the obtained focuses are scattered (Fig. 8c and f). Finally, working with a duty cycle of 35% of the CO2, we obtained microlenses with a good focusing capability, as we can see in Fig. 8b. In addition, in Fig. 8e, obtained for a power of 90 W, the homogeneity of the microlenses focuses at the focal plane as well as the similarity with the PSF aberrations free can be observed.
Table 2 Optical and geometrical characteristics of the fabricated microlenses. Features of the microlenses Duty cycle
Without thermal treatment
25%
35%
45%
Diameter [mm] Sag [mm] Focal length [mm] Focal spot size [mm] Roughness [nm]
50 7 – –
50 6 0.7 9.6 13.2
51 1.5 1 7.8 8
53 0.25 3 28.8 7.6
Fig. 7. Setup for measuring the microlens focal length.
Fig. 8. Irradiance distribution of one random microlens, pictures (a), (b) and (c); within the microlens array with the corresponding focal plane at pictures (d), (e) and (f).
6
T. Delgado et al. / Optics and Lasers in Engineering 73 (2015) 1–6
5. Conclusions We have presented an improved low-cost method for fabricating microlens arrays on soda-lime glass. This technique is based on the combination of an ablation-laser direct writing process and a laser assisted thermal treatment. A Q-Switched Nd:YVO4 laser combined with a galvanometer system was employed for microstructuring the glass and fabricating in this way the initial microposts. This is a rapid and simple method that allows us to fabricate inexpensive microlenses. The thermal treatment applied by a furnace provided with a CO2 laser was used for reshaping and improving the optical qualities of the generated microlenses. This alternative thermal process allows us to melt only the top surface of our samples, avoiding the bounding problems presented in other conventional thermal treatments. Different powers of the CO2 laser were tested for applying the thermal treatment, fabricating different microlenses. We observed that the diameter of a microlens is maintained almost constant, while their sag decreases as we increase the energy given by the CO2 laser. For a range of powers between 68 and 116 W, microlenses with a diameter around 50 μm and sags between 7 and 0.25 μm were fabricated. The optical quality of the generated microlenses was determined in terms of their focal length and focal spot size. We obtained microlenses with a focal length between 0.7 and 3 mm and with a focal spot size from 28.8 to 7.8 μm, for different thermal treatments. Acknowledgments This work has been supported by Consellería de Cultura, Xunta de Galicia, under the contract EM2012/019 Spain. References [1] Smith PJ, Taylor CM, McCabe EM, Selviah DR, Day SE, Commander LG. Switchable fiber coupling using variable-focal-length microlenses. Rev Sci Instrum 2001;72:3132–4. [2] Wippermann F, Zeitner UD, Dannberg P, Bräuer A, Sinzinger S. Beam homogenizers based on chirped microlens arrays. Opt Express 2007;15:6218–31. [3] Artzner G. Microlens arrays for Shack–Hartmann wavefront sensor. Opt Eng 1992;316:1311–22. [4] Arimoto H, Javidi B. Integral three-dimensional imaging with digital reconstruction. Opt Lett 2001;26:157–9. [5] Kunnavakkam MV, Houlihan FM, Schlaz M, Liddle JA, Kolodner P, Nalamasu O, Rogers JA. Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process. Appl Phys Lett 2003;82:1152–4.
[6] Wu MH, Park C, Whitesides GM. Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photolithography. Langmuir 2002;18:9312–8. [7] Pan CT, Wu TT, Chen MF, Chang YC, Lee CJ, Huang JC. Hot embossing of microlens array on bulk metallic glass. Sens Actuators A: Phys 2008;141:422–31. [8] Chang CY, Yang SY, Sheh JL. A roller embossing process for rapid fabrication of microlens arrays on glass substrates. Microsyst Technol 2006;12:754–9. [9] Ashraf M, Gupta C, Chollet F, Springham SV, Rawat RS. Geometrical characterization techniques for microlenses made by thermal reflow of photoresist cylinder. Opt Lasers Eng 2008;46:711–20. [10] O'Neill FT, Sheridan JT. Photoresist reflow of microlens production part I: backgraound and experiments. Optik 2002;113:391–404. [11] Naessens K, Van Daele P, Baets RG. Excimer-laser-ablation based microlens fabrication for optical fiber coupling purposes. Proc SPIE 2003;4941:133–9. [12] Ben-Yakar A, Byer RL. Femtosecond laser ablation properties of borosilicate glass. J Appl Phys 2004;96:5316–23. [13] Fan CH, Longtin JP. Modeling optical breakdown in dielectrics during ultrafast laser processing. Appl Opt 2001;40:3124–31. [14] Stuart BC, Feit MD, Rubenchik AM, Shore BW, Perry MD. Laser-induced damage in dielectrics with nanosecond and subpicosecond pulses. Phys Rev Lett 1995;74:2248–51. [15] Karnakis DM, Knowles MRH, Alty KT, Schlaf M, Snelling HV. Comparison of glass processing using high repetition femtosecond (800 nm) and UV (255 nm) nanosecond pulsed lasers. In: Proceedings of the international society for optics and photonics MOEMS-MEMS micro & nanofabrication. 2005. p. 216–227. [16] Zoubir A, Shah L, Richardson K, Richardson M. Practical uses of femtosecond laser micro-materials processing. Appl Phys 2003;77:311–5. [17] Sayah A, Parashar VK, Gijs MAM. Micro-replication of optical lenses in glass using a novel sol–gel technology. In: Proceedings of the IEEEMEMS'02 IEEE. 2002. p. 516–519. [18] Wakaki M, Komachi Y, Kanai G. Microlenses and microlens arrays formed on a glass plate by use of a CO2 laser. Appl Opt 1998;37:627–31. [19] Veiko VP, Yakovlev EB, Frolar VV, Chujko VA, Kromin AK, Abbakumov MO, Shakola AT, Fomichov PA. Laser heating and evaporation of glass and glassboring materials and its application for creating MOC. In: Roychoudhuri CS, Veldkamp WB, editors. Proceedings of the SPIE miniature and micro-optics: fabrication and system applications.;1544 1991. p. 152–163. [20] Nieto D, Arines J, Gómez-Reino C, O'Connor GM, Flores-Arias MT. Fabrication and characterization of microlens arrays on soda-lime glass using a combination of laser direct-write and thermal reflow technique. J Appl Phys 2011;110 (2):023108. [21] Estepa LC & De la Fuente GF. Continuous furnace with coupled laser for the surface treatment of materials. Patent No 00560; (2006March). [22] Ottevaere H, Cox R, Herzig HP, Miyashita T, Naessens K, Taghizadeh M, Völkel R, Woo HJ, Thienpont H. Comparing glass and plastic refractive microlenses fabricated with different technologies. J Opt A: Pure App Opt 2006;8:S407. [23] Narottam PB, Doremus RH. Handbook of Glass Properties. Orlando, FL: Academic Press, Inc.; 1986. [24] Pilkington LAB. Pilkington brothers limited, St Helens, Lancashire. the float glass process. Proc Roy Soc Lond A 1969;314:1–25. [25] Nieto D, Arines J, Flores-Arias MT. Fluence ablation threshold dependence on tin impurities in commercial soda-lime glass. Appl Opt 2014;53:5416–20. [26] Heckele M, Schomburg WK. Review on micro molding of thermoplastic polymers. J Micromech and Microeng 2004;14:R1. [27] Sayah A, Parashar VK, Gijs MAM. Micro-replication of optical lenses in glass using a novel sol gel technology. In: Proceedings of the IEEE MEMS'02 2002, p. 516–519.