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Structural Characterization of Laser Bonded Sapphire Wafers Using a Titanium Absorber Thin Film
Journal of Materials Science & Technology 31 (2015) 484e488
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Journal of Materials Science & Technology journal homepage: www.jmst.org
Structural Characterization of Laser Bonded Sapphire Wafers Using a Titanium Absorber Thin Film € che A. de Pablos-Martín*, S. Tismer, Th. Ho Fraunhofer Institute for Mechanics of Materials IWM, Walter-Hülse-Str. 1, 06120 Halle, Germany
a r t i c l e i n f o Article history: Received 23 October 2014 Received in revised form 24 November 2014 Accepted 3 December 2014 Available online 20 March 2015
Two sapphire substrates were tightly bonded by irradiation with a 1064 nm nanosecond laser and using a sputtered 50 nm-titanium thin film as an absorbing medium. Upon laser irradiation, aluminum from the upper substrate is incorporated into the thin film, forming TieAleO compounds. While the irradiated region becomes transparent, the bond quality was evaluated by scanning acoustic microscopy. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Key words: Laser welding Absorbing film Sapphire wafer bonding
1. Introduction Laser bonding represents an advantageous technique for joining dissimilar materials[1], not only for its process speed, but also for the highly localized bonding area. Due to the latter, the induced thermal stresses are minimized. Laser bonding of silicon with silicon has become very popular in the field of micromechanics and power electronics[2]. However, joining dissimilar materials has received increasing interest in the last years. Good examples are the joining of silicon and glass[3e5], metals[6,7], metals and plastic[8,9], glass and metals[10], and various other substrates using absorber layers in-between are being widely investigated[11,12]. Sapphire is intensively used in the field of optical components and micromechanical devices[13e15], due to its favorable mechanical properties and chemical durability. Moreover, it is heatresistant; has a high ablation threshold; and exhibits optical transparency over a large spectral range[16]. Its applications include windows for high-pressure optical cells and high-pressure devices[17e19]. Sapphire substrates have been bonded to other materials, like gold, silver by manifold techniques and using various solder materials like metallic fillers[20e22]. Titanium, on the other hand, has attracted much attention as a structural material for its high mechanical endurance, high corrosion resistance, which is very important in harsh environments[23], as well as in biomaterials due to its bio-compatibility. Titanium has
been involved in microelectromechanical systems (MEMS)[24,25], like microfluidics and bio-chips[26]. Titanium alloys have been reported to be welded through laser irradiation in particularly for biomedical applications[27e29]. Last but not least, titanium has been used as substrate for thin films[30]. A good example, is the work of Zhang et al.[31], describing a Ti wafer bonded with a soda lime glass wafer for MEMS devices, like resonators. Thin-film preparation plays an important role in device miniaturization imposed by current technology requirements[32,33]. Titanium has of course been prepared as thin film[34,35], yielding the advantage of a surface of higher quality than that in polished bulk metal. Titanium thin films have been deposited on glass[36], quartz, and several other substrates[25,37e39]. Hoffmann et al.[40] reported titanium thin films for its applicability as advanced memory concepts based on single electron transistors (SETs). Titanium has been also used as absorber between two transparent materials for biocompatible joints[41,42]. Wissinger et al.[43] reported siliconesilicon wafer bonding through a titanium/copper thin film, as well as titanium/gold[44] as absorber for the laser irradiation. Based on the above-mentioned advantages of the laser-welding technique and of sapphire substrates and Ti thin films, the aim of this work is to site-specifically join two sapphire substrates by the absorbing action of a titanium thin film due to local heating caused by a pulsed 1064 nm Nd:YAG laser.
* Corresponding author. Ph.D.; Tel.: þ49 345 5589 227; Fax: þ49 345 5589 101. E-mail address: [email protected] (A. de Pablos-Martín). http://dx.doi.org/10.1016/j.jmst.2014.12.007 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
A. de Pablos-Martín et al. / Journal of Materials Science & Technology 31 (2015) 484e488
2. Experimental 2.1. Set-up Titanium thin films of 50 nm thickness were deposited on 430 mm thick, polished 11 mm 13 mm c-sapphire substrate by magnetron sputtering. Secondly, uncoated identical sapphire substrate of the same dimensions was placed on top of the Ti-coated sapphire substrate, leaving the Ti thin film in-between, acting as an absorbing medium. Before laser joining, the uncoated and the Ti-coated substrates were thoroughly cleaned with ethanol and de-ionized water and dried with an air gun. In order to bring both substrates in close proximity, a custom-made sample holder was used, adjusting a contact pressure of ~7 MPa. 2.2. Laser bonding The laser source was a pulsed 1064 nm nanosecond Nd:YAG laser (Xiton Photonics implemented into a microSTRUCT C micromachining workstation by 3D-Micromac AG), operated at 50 kHz. Using a refraction objective with 100 mm focal length, the laser beam was focused precisely on the interface between the upper substrate and the titanium thin film. While sapphire is transparent at the wavelength used, the titanium thin film absorbs it partially. The laser machining parameters were optimized to achieve the best welding conditions: 50 kHz, 0.5 W, corresponding to a laser fluence of 2 J/cm2 on the sample and a markspeed of 10 mm/s, which corresponds to a 99.5% of pulse overlapping, and one single pass in each line. A pattern consisting of horizontal lines forming the typical Chevron geometry was irradiated using a distance between irradiated lines optimized to 20 mm. The length of the irradiated structure, W was 6.65 mm and the width B was 10 mm. The Chevron notch had an angle of 90 . The processing time of the whole pattern was about 15 min. 2.3. Light microscopy The microstructure of the bonds and the luminescence of the samples were evaluated with an optical microscope Leica DM RXE650H. The following Leica Leitz Wetzlar objective lenses were used: a PL FLUOTAR with 5x magnification and numerical aperture (NA) of 0.12; and a PL with 1.6x magnification and NA of 0.04. 2.4. Scanning acoustic microscopy (SAM) The bonded area of the bonds was investigated through SAM. This non-destructive technique has been used as pre-examination in the characterization of bonded wafers before any mechanical test and it is a good test to visualize and quantify the bonded area[45]. The acoustic microscope employed here was an SAM400 (PVA TePla Analytical Systems GmbH, Westhausen, Germany) with a modified remote Pulser RPH3 of an PPR500 Pulser (JSRUltrasonics Imaginant Inc., Pittsford, USA) in combination with an ultrasonic transducer (SIEGERT TFT GmbH, Hermsdorf, Germany) with 175 MHz center frequency and a focal length of 5.9 mm in water. For acoustic coupling the samples were submerged in de-ionized and degassed water at 21 C. The coupling medium is required for matching the acoustic impedances of the sample surface and the front of the acoustic lens. To improve acoustical imaging within solid samples, the focus of the ultrasonic transducer was positioned at the bonding interface inside the sample, what caused an actual distance of 1700 mm between lens and sample surface. The acoustic micrographs were computed offline from the recorded echo
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signals. A sliding window algorithm was used to determine the correct interface echo time delay and for optimization purposes two dimensional image filtering was performed. The contrast of the acoustic images was also adjusted, thresholded and binarized to derive the effective bonded area. All signal and data processing was performed using a custom-made analysis software based on MATLAB (The MathWorks, Natick, MA, USA). 2.5. (Scanning) transmission electron microscopy ((S)TEM) Cross sections of the stack consisting of both sapphire substrates joined through the titanium thin film were examined by TEM/ STEM. In order to obtain electron-transparent sections, the stack was cut in the middle, parallel to the laser irradiated lines. Then, plane-parallel discs were sliced perpendicular to the laser writing direction, ground plane-parallel and a wedge-polishing route was developed: using a specific tripod sample holder in combination with a multi-functional grinding and polishing tool (MultiPrep, Allied Company), where a very thin, electron-beam transparent wedge was generated, by polishing it under a defined, very small angle (z1.6 ). The sample was then subjected to double-sided, ionbeam milling using low-energy (2.5 keV) Arþ ions under a small angle of incidence (6 ) until the central part of the samples became electron transparent (precision ion-polishing system, PIPS, Gatan, Inc.). In order to avoid charging of the non-conducting cross-section samples under electron irradiation, selective carbon coating was accomplished[46]. Bright field images in TEM mode and STEM images were recorded with an FEI Tecnai G2 F20 microscope operating at 200 kV. EDXS point analysis of Al (K line), Ti (K line) was recorded in STEM mode at 200 kV in the same microscope. 3. Results A Chevron structure consisting of horizontal lines was irradiated over the whole package by focusing on the titanium interface (Fig. 1(a)). Both sapphire substrates were successfully bonded through the titanium intermediate thin film, by the heat imposed by laser irradiation. The appearance of laser-irradiated region changes in comparison with that before the laser welding process, since it appears to be rather transparent (Fig. 1(a)). This is a first indication that the composition of the initially Ti-thin film has changed during laser processing. There is no damage to be detected on the sapphire surfaces after laser processing as shown in Fig. 1(a). The interface was also observed under the optical microscope by focusing in the intermediate Ti layer (Fig. 1(b, c)). The laser irradiated lines are clearly observed. The overlap of the lines is also shown, which ensures optimal welding conditions. Moreover, a waving structure of the lines is observed. In order to evaluate the bond quality and detect delamination, SAM was applied to the package, as shown in Fig. 2. Only a minor non-bonded area is discerned at the bottom of the structure. The rest represents a fully bonded area. In order to investigate the sapphire/titanium/sapphire bonded interface, a cross-section of the irradiated Chevron structure was prepared and the TEM micrographs are shown in Fig. 3. Fig. 3(a) shows the cross-section of the bond. The continuous intermediate sealant layer is clearly observed between both sapphire substrates, without the appearance of fractures or cracks, which is in good agreement with the fully bonded area observed by SAM (Fig. 2). One side of the sealant is flat while the other side shows a more waving profile, with the appearance of some bumps in the direction of the left sapphire substrate. A closer observation to this interface (Fig. 3(b, c)) shows that in fact the sealant layer is constituted by three different layers of different thickness. Particles are
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homogenously distributed over the three layers. STEM micrographs in Fig. 4(a, b) display the cross-section, where it is clearly observed that the layer on the left contains mostly enlarge particles, while the middle and the right layers contain round particles. The EDX line scan along the entire cross-section (Fig. 4(a)) shows that the composition is not the same in the entire interface, where in fact exists an elemental distribution, especially for titanium. Fig. 4(b) shows an EDX line scan across one of the enlarge particles, which contains Ti and Al. 4. Discussion
Fig. 1. Optical micrograph of the irradiated pattern: (a) overview, (b, c) interface.
Fig. 2. SAM micrograph of the laser irradiated pattern in Fig. 1.
Vlǎdoiu et al.[47] reported the thermal and optical properties of titanium, giving an absorption coefficient a (1064 nm) ¼ 1.3$106, an optical penetration depth of 7.7 nm and a thermal penetration depth at the used ns regime (5 ns pulses) of 0.4 mm. By other side, the higher reflectivity of metals in IR range prevents efficient coupling of the pulse energy to the metals surface[47]. In the present case, the reflected radiation from titanium would increase the temperature of the upper sapphire, contributing to the heat accumulation in the interface. However, the reflectivity of metals, in general, decreases with increasing temperature[48]. Thus, the pulse overlap (giving by the applied laser scan speed) contributes to increasing the temperature and thus the reflectivity from the titanium thin film would be lower than that expected. The partial absorption together with the reflectivity, the low value of thermal conductivity (0.2 W/(cm K)) and a pulse overlap of 99.5% favor the heat accumulation in the titanium thin film, leading to its melting. Resolidification of the molten interface leads to the welding of both sapphire substrates. The irradiated pattern shows a high degree of transparency, as displayed in Fig. 1(a). This clearly indicates that the composition of the sealant between both sapphire substrates has changed from the original Ti thin film. It was previously reported that by laser irradiation of similar systems, like sapphireesapphire welded through a glass sealant[49], the upper sapphire substrate was ablated during interaction with the laser and aluminum from it was incorporated into the intermediate sealant[49]. Thus, an incorporation of aluminum and oxygen into the Ti thin film is plausible, and this new composition containing TieAleO would be transparent at the given thickness, as shown in Fig. 1(a). Also, the oxidation of Ti to TiO2 after the laser process would contribute to the transparency of the final thin film, since TiO2 thin films of ca.150 nm in thickness exhibit transmittances as high as 91% in the visible range as reported by Vishwas et al.[50]. In the same work, it is stated that the crystallization of the TiO2 thin film by annealing increases its transmittance[50]. The irradiated region consists in overlapping lines with a wavy structure, which highlight the melting of the intermediate sealant under laser interaction and subsequent resolidification. From the SAM micrograph in Fig. 2, it is stated that both sapphire substrates were successfully bonded through the laser welding procedure, developing a fully bonded area from the overlapping of the irradiated lines. Through TEM investigation in Fig. 3(b), it is proven that the flat side of the sealant corresponds to the Ti-coated substrate, since it is at the bottom during the laser procedure and is less influenced by the heating input. This asymmetry of the heat-affected zone (HAZ) has been ascribed to different laser absorption along the beam propagation path[51] (and references therein). It is stated that the upper part of the focal volume is heated stronger than the material at the lower part, giving rise to the asymmetry in the HAZ[51]. On the contrary, in the right side of the sealant in Fig. 3(b, c), some bumps are observed. It can be discerned that the intermediate layer is homogeneously divided into three similar parallel parts of
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Fig. 3. TEM micrographs of the cross-section of the bond: (a) overview, (b, c) detailed micrographs.
different thickness (Fig. 3(b, c)), probably also due to a different heat distribution from the laser irradiation. A great increase of the intermediate layer is observed, from the initially 50 nm Ti, to more than 300 mm after the laser welding. This expansion of the molten material in the direction of the laser irradiation, with the formation of bumps has been also reported in Si/Si bond with metallic interface acting as absorber, like Ti and Au thin films[43]. In our previous work[49], it was reported that the upper sapphire substrate was ablated because of laser irradiation and molten-solder material filled the ablated region, forming similar bumps and leading to a solder material containing aluminum form the upper substrate. In the present study, a similar case seems to occur, since the formed bump occupies the ablated volume of the upper sapphire substrate.
From the point of view of mechanical robustness, the resulting interlocking could be of advantage over a plain interface. Fig. 4(a, b) displays the STEM micrographs of the cross-section, which offer a different contrast in comparison with the bright field micrographs in Fig. 3. Round and enlarge particles are formed, appearing in bright contrast. The EDX line scan across the interface (Fig. 4(a)) reveals that the composition, that is, the Al:Ti ratio changes across the interface and that exists a titanium enrichment in the middle part of the cross-section. Al is distributed along the entire interface, decreasing its concentration from the left side (initially non coated substrate) to the right side (initially Ti-coated substrate), agreeing with an ablation of the upper substrate, which distributes the Al in the whole solder.
Fig. 4. STEM micrograph and EDX line scan across the cross-section of the bond (a) and across one of the particles (b). The orange line indicates the direction of the EDX line scans. The red square in (b) indicates the position of the EDXS point analysis. The yellow square indicates the reference image for the drift correction.
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Enlarge particles are distributed in the Al-enriched side of the solder (Fig. 4(b)). An EDX line scan across one of these enlarge particles (Fig. 4(b)) indicates that it contains Al, Ti and O. EDXS point analyses at one of the round particles distributed in the middle and right side of the solder (marked in STEM micrograph in Fig. 4(b)) reveals that it contains also both, Al and Ti, in a Ti:Al ratio approximate to 2:1. To the best of our knowledge no AleTieO phases with this stoichiometry were published. A similar one, Ti3AlOx, was reported by Jones et al.[52]. Selverian et al.[53] and Zalar et al.[54] described reactions between a titanium thin film deposited over a sapphire substrate. Here, Ti reduces the sapphire surface, leading to Al0 precipitation. It was stated that Ti near the interface formed bonds predominantly with O. Ti also bonded with Al in the top layer of the sapphire, forming a mixed bonding involving Al, O, and Ti. As time progressed, the interface mixed; metallic TieAl bonds formed as the sapphire decomposed into Al and O; and O diffused into the Ti film. The formation of two layers was reported: Ti3Al[O], adjacent to the sapphire, which refers to Ti3Al with oxygen in solid solution, and Ti0.67 [O0.33] at the free surface[53]. The formation of the above mentioned compounds in the interface, together with the resulting interlocking from the bump formation, would lead to the observed full bonded area in Fig. 2. The mechanical characterization of this bond is in progress. 5. Conclusion Two sapphire substrates were joined through a laser lightabsorbing titanium thin film deposited by sputtering on one of the substrates. The bonding was accomplished by irradiating a predefined pattern using a 1064 nm nanosecond laser and improving the laser parameters to obtain the best welding conditions. The irradiated part possessed a high degree of transparency. Upon laser irradiation, aluminum from the upper substrate was incorporated in the titanium intermediate layer, forming a threelayer structure where different AleTieO compounds co-exist. The formation of these compounds would favor the joining of both sapphire substrates. The mechanical properties of the bond are part of the future work. Acknowledgments The authors acknowledge financial support of FhG Internal Programs (Grant No. 692 280). The authors are very grateful to Nico Teuscher (Fraunhofer Institute for Mechanics of Materials IWM, Halle, Germany) for the Titanium sputtering on the sapphire substrates. References [1] R.M. Carter, J.Y. Chen, J.D. Shephard, R.R. Thomson, D.P. Hand, Appl. Opt. 53 (2014) 4233e4238. [2] U. Gosele, Q.Y. Tong, A. Schumacher, G. Krauter, M. Reiche, A. Plossl, P. Kopperschmidt, T.H. Lee, W.J. Kim, Sens. Actuator A-Phys. 74 (1999) 161e168. [3] S. Theppakuttai, D.B. Shao, S.C. Chen, in: 2003 ASME International Mechanical Engineering Congress on Electronic and Photonic Packing, Electrical Systems and Photonic Design, Washington, DC, 2003, pp. 107e112. [4] M.J. Wild, A. Gillner, R. Poprawe, Sens. Actuator A-Phys. 93 (2001) 63e69. [5] W. Watanabe, S. Onda, T. Tamaki, K. Itoh, J. Nishii, Appl. Phys. Lett. 89 (2006) 021106. [6] Y. Abe, T. Watanabe, H. Tanabe, K. Kagiya, Adv. Mater. Res.-Switz. 15‒17 (2007) 393e397. [7] V. Dragoi, E. Cakmak, E. Pabo, Rom. J. Inform. Sci. Technol. 13 (2010) 65e72. [8] S. Katayama, Y. Kawahito, Scripta Mater. 59 (2008) 1247e1250. [9] Y. Kawahito, Y. Niwa, T. Terajima, S. Katayama, Mater. Trans. 51 (2010) 1433e1436. [10] D. Faidel, W. Behr, S. Gross, U. Reisgen, in: Laser Assisted Net Shape Engineering 6, Proceedings of the Lane 2010, Part 2, 2010, pp. 153e162.
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Journal of Materials Science & Technology journal homepage: www.jmst.org
Structural Characterization of Laser Bonded Sapphire Wafers Using a Titanium Absorber Thin Film € che A. de Pablos-Martín*, S. Tismer, Th. Ho Fraunhofer Institute for Mechanics of Materials IWM, Walter-Hülse-Str. 1, 06120 Halle, Germany
a r t i c l e i n f o Article history: Received 23 October 2014 Received in revised form 24 November 2014 Accepted 3 December 2014 Available online 20 March 2015
Two sapphire substrates were tightly bonded by irradiation with a 1064 nm nanosecond laser and using a sputtered 50 nm-titanium thin film as an absorbing medium. Upon laser irradiation, aluminum from the upper substrate is incorporated into the thin film, forming TieAleO compounds. While the irradiated region becomes transparent, the bond quality was evaluated by scanning acoustic microscopy. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Key words: Laser welding Absorbing film Sapphire wafer bonding
1. Introduction Laser bonding represents an advantageous technique for joining dissimilar materials[1], not only for its process speed, but also for the highly localized bonding area. Due to the latter, the induced thermal stresses are minimized. Laser bonding of silicon with silicon has become very popular in the field of micromechanics and power electronics[2]. However, joining dissimilar materials has received increasing interest in the last years. Good examples are the joining of silicon and glass[3e5], metals[6,7], metals and plastic[8,9], glass and metals[10], and various other substrates using absorber layers in-between are being widely investigated[11,12]. Sapphire is intensively used in the field of optical components and micromechanical devices[13e15], due to its favorable mechanical properties and chemical durability. Moreover, it is heatresistant; has a high ablation threshold; and exhibits optical transparency over a large spectral range[16]. Its applications include windows for high-pressure optical cells and high-pressure devices[17e19]. Sapphire substrates have been bonded to other materials, like gold, silver by manifold techniques and using various solder materials like metallic fillers[20e22]. Titanium, on the other hand, has attracted much attention as a structural material for its high mechanical endurance, high corrosion resistance, which is very important in harsh environments[23], as well as in biomaterials due to its bio-compatibility. Titanium has
been involved in microelectromechanical systems (MEMS)[24,25], like microfluidics and bio-chips[26]. Titanium alloys have been reported to be welded through laser irradiation in particularly for biomedical applications[27e29]. Last but not least, titanium has been used as substrate for thin films[30]. A good example, is the work of Zhang et al.[31], describing a Ti wafer bonded with a soda lime glass wafer for MEMS devices, like resonators. Thin-film preparation plays an important role in device miniaturization imposed by current technology requirements[32,33]. Titanium has of course been prepared as thin film[34,35], yielding the advantage of a surface of higher quality than that in polished bulk metal. Titanium thin films have been deposited on glass[36], quartz, and several other substrates[25,37e39]. Hoffmann et al.[40] reported titanium thin films for its applicability as advanced memory concepts based on single electron transistors (SETs). Titanium has been also used as absorber between two transparent materials for biocompatible joints[41,42]. Wissinger et al.[43] reported siliconesilicon wafer bonding through a titanium/copper thin film, as well as titanium/gold[44] as absorber for the laser irradiation. Based on the above-mentioned advantages of the laser-welding technique and of sapphire substrates and Ti thin films, the aim of this work is to site-specifically join two sapphire substrates by the absorbing action of a titanium thin film due to local heating caused by a pulsed 1064 nm Nd:YAG laser.
* Corresponding author. Ph.D.; Tel.: þ49 345 5589 227; Fax: þ49 345 5589 101. E-mail address: [email protected] (A. de Pablos-Martín). http://dx.doi.org/10.1016/j.jmst.2014.12.007 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
A. de Pablos-Martín et al. / Journal of Materials Science & Technology 31 (2015) 484e488
2. Experimental 2.1. Set-up Titanium thin films of 50 nm thickness were deposited on 430 mm thick, polished 11 mm 13 mm c-sapphire substrate by magnetron sputtering. Secondly, uncoated identical sapphire substrate of the same dimensions was placed on top of the Ti-coated sapphire substrate, leaving the Ti thin film in-between, acting as an absorbing medium. Before laser joining, the uncoated and the Ti-coated substrates were thoroughly cleaned with ethanol and de-ionized water and dried with an air gun. In order to bring both substrates in close proximity, a custom-made sample holder was used, adjusting a contact pressure of ~7 MPa. 2.2. Laser bonding The laser source was a pulsed 1064 nm nanosecond Nd:YAG laser (Xiton Photonics implemented into a microSTRUCT C micromachining workstation by 3D-Micromac AG), operated at 50 kHz. Using a refraction objective with 100 mm focal length, the laser beam was focused precisely on the interface between the upper substrate and the titanium thin film. While sapphire is transparent at the wavelength used, the titanium thin film absorbs it partially. The laser machining parameters were optimized to achieve the best welding conditions: 50 kHz, 0.5 W, corresponding to a laser fluence of 2 J/cm2 on the sample and a markspeed of 10 mm/s, which corresponds to a 99.5% of pulse overlapping, and one single pass in each line. A pattern consisting of horizontal lines forming the typical Chevron geometry was irradiated using a distance between irradiated lines optimized to 20 mm. The length of the irradiated structure, W was 6.65 mm and the width B was 10 mm. The Chevron notch had an angle of 90 . The processing time of the whole pattern was about 15 min. 2.3. Light microscopy The microstructure of the bonds and the luminescence of the samples were evaluated with an optical microscope Leica DM RXE650H. The following Leica Leitz Wetzlar objective lenses were used: a PL FLUOTAR with 5x magnification and numerical aperture (NA) of 0.12; and a PL with 1.6x magnification and NA of 0.04. 2.4. Scanning acoustic microscopy (SAM) The bonded area of the bonds was investigated through SAM. This non-destructive technique has been used as pre-examination in the characterization of bonded wafers before any mechanical test and it is a good test to visualize and quantify the bonded area[45]. The acoustic microscope employed here was an SAM400 (PVA TePla Analytical Systems GmbH, Westhausen, Germany) with a modified remote Pulser RPH3 of an PPR500 Pulser (JSRUltrasonics Imaginant Inc., Pittsford, USA) in combination with an ultrasonic transducer (SIEGERT TFT GmbH, Hermsdorf, Germany) with 175 MHz center frequency and a focal length of 5.9 mm in water. For acoustic coupling the samples were submerged in de-ionized and degassed water at 21 C. The coupling medium is required for matching the acoustic impedances of the sample surface and the front of the acoustic lens. To improve acoustical imaging within solid samples, the focus of the ultrasonic transducer was positioned at the bonding interface inside the sample, what caused an actual distance of 1700 mm between lens and sample surface. The acoustic micrographs were computed offline from the recorded echo
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signals. A sliding window algorithm was used to determine the correct interface echo time delay and for optimization purposes two dimensional image filtering was performed. The contrast of the acoustic images was also adjusted, thresholded and binarized to derive the effective bonded area. All signal and data processing was performed using a custom-made analysis software based on MATLAB (The MathWorks, Natick, MA, USA). 2.5. (Scanning) transmission electron microscopy ((S)TEM) Cross sections of the stack consisting of both sapphire substrates joined through the titanium thin film were examined by TEM/ STEM. In order to obtain electron-transparent sections, the stack was cut in the middle, parallel to the laser irradiated lines. Then, plane-parallel discs were sliced perpendicular to the laser writing direction, ground plane-parallel and a wedge-polishing route was developed: using a specific tripod sample holder in combination with a multi-functional grinding and polishing tool (MultiPrep, Allied Company), where a very thin, electron-beam transparent wedge was generated, by polishing it under a defined, very small angle (z1.6 ). The sample was then subjected to double-sided, ionbeam milling using low-energy (2.5 keV) Arþ ions under a small angle of incidence (6 ) until the central part of the samples became electron transparent (precision ion-polishing system, PIPS, Gatan, Inc.). In order to avoid charging of the non-conducting cross-section samples under electron irradiation, selective carbon coating was accomplished[46]. Bright field images in TEM mode and STEM images were recorded with an FEI Tecnai G2 F20 microscope operating at 200 kV. EDXS point analysis of Al (K line), Ti (K line) was recorded in STEM mode at 200 kV in the same microscope. 3. Results A Chevron structure consisting of horizontal lines was irradiated over the whole package by focusing on the titanium interface (Fig. 1(a)). Both sapphire substrates were successfully bonded through the titanium intermediate thin film, by the heat imposed by laser irradiation. The appearance of laser-irradiated region changes in comparison with that before the laser welding process, since it appears to be rather transparent (Fig. 1(a)). This is a first indication that the composition of the initially Ti-thin film has changed during laser processing. There is no damage to be detected on the sapphire surfaces after laser processing as shown in Fig. 1(a). The interface was also observed under the optical microscope by focusing in the intermediate Ti layer (Fig. 1(b, c)). The laser irradiated lines are clearly observed. The overlap of the lines is also shown, which ensures optimal welding conditions. Moreover, a waving structure of the lines is observed. In order to evaluate the bond quality and detect delamination, SAM was applied to the package, as shown in Fig. 2. Only a minor non-bonded area is discerned at the bottom of the structure. The rest represents a fully bonded area. In order to investigate the sapphire/titanium/sapphire bonded interface, a cross-section of the irradiated Chevron structure was prepared and the TEM micrographs are shown in Fig. 3. Fig. 3(a) shows the cross-section of the bond. The continuous intermediate sealant layer is clearly observed between both sapphire substrates, without the appearance of fractures or cracks, which is in good agreement with the fully bonded area observed by SAM (Fig. 2). One side of the sealant is flat while the other side shows a more waving profile, with the appearance of some bumps in the direction of the left sapphire substrate. A closer observation to this interface (Fig. 3(b, c)) shows that in fact the sealant layer is constituted by three different layers of different thickness. Particles are
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homogenously distributed over the three layers. STEM micrographs in Fig. 4(a, b) display the cross-section, where it is clearly observed that the layer on the left contains mostly enlarge particles, while the middle and the right layers contain round particles. The EDX line scan along the entire cross-section (Fig. 4(a)) shows that the composition is not the same in the entire interface, where in fact exists an elemental distribution, especially for titanium. Fig. 4(b) shows an EDX line scan across one of the enlarge particles, which contains Ti and Al. 4. Discussion
Fig. 1. Optical micrograph of the irradiated pattern: (a) overview, (b, c) interface.
Fig. 2. SAM micrograph of the laser irradiated pattern in Fig. 1.
Vlǎdoiu et al.[47] reported the thermal and optical properties of titanium, giving an absorption coefficient a (1064 nm) ¼ 1.3$106, an optical penetration depth of 7.7 nm and a thermal penetration depth at the used ns regime (5 ns pulses) of 0.4 mm. By other side, the higher reflectivity of metals in IR range prevents efficient coupling of the pulse energy to the metals surface[47]. In the present case, the reflected radiation from titanium would increase the temperature of the upper sapphire, contributing to the heat accumulation in the interface. However, the reflectivity of metals, in general, decreases with increasing temperature[48]. Thus, the pulse overlap (giving by the applied laser scan speed) contributes to increasing the temperature and thus the reflectivity from the titanium thin film would be lower than that expected. The partial absorption together with the reflectivity, the low value of thermal conductivity (0.2 W/(cm K)) and a pulse overlap of 99.5% favor the heat accumulation in the titanium thin film, leading to its melting. Resolidification of the molten interface leads to the welding of both sapphire substrates. The irradiated pattern shows a high degree of transparency, as displayed in Fig. 1(a). This clearly indicates that the composition of the sealant between both sapphire substrates has changed from the original Ti thin film. It was previously reported that by laser irradiation of similar systems, like sapphireesapphire welded through a glass sealant[49], the upper sapphire substrate was ablated during interaction with the laser and aluminum from it was incorporated into the intermediate sealant[49]. Thus, an incorporation of aluminum and oxygen into the Ti thin film is plausible, and this new composition containing TieAleO would be transparent at the given thickness, as shown in Fig. 1(a). Also, the oxidation of Ti to TiO2 after the laser process would contribute to the transparency of the final thin film, since TiO2 thin films of ca.150 nm in thickness exhibit transmittances as high as 91% in the visible range as reported by Vishwas et al.[50]. In the same work, it is stated that the crystallization of the TiO2 thin film by annealing increases its transmittance[50]. The irradiated region consists in overlapping lines with a wavy structure, which highlight the melting of the intermediate sealant under laser interaction and subsequent resolidification. From the SAM micrograph in Fig. 2, it is stated that both sapphire substrates were successfully bonded through the laser welding procedure, developing a fully bonded area from the overlapping of the irradiated lines. Through TEM investigation in Fig. 3(b), it is proven that the flat side of the sealant corresponds to the Ti-coated substrate, since it is at the bottom during the laser procedure and is less influenced by the heating input. This asymmetry of the heat-affected zone (HAZ) has been ascribed to different laser absorption along the beam propagation path[51] (and references therein). It is stated that the upper part of the focal volume is heated stronger than the material at the lower part, giving rise to the asymmetry in the HAZ[51]. On the contrary, in the right side of the sealant in Fig. 3(b, c), some bumps are observed. It can be discerned that the intermediate layer is homogeneously divided into three similar parallel parts of
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Fig. 3. TEM micrographs of the cross-section of the bond: (a) overview, (b, c) detailed micrographs.
different thickness (Fig. 3(b, c)), probably also due to a different heat distribution from the laser irradiation. A great increase of the intermediate layer is observed, from the initially 50 nm Ti, to more than 300 mm after the laser welding. This expansion of the molten material in the direction of the laser irradiation, with the formation of bumps has been also reported in Si/Si bond with metallic interface acting as absorber, like Ti and Au thin films[43]. In our previous work[49], it was reported that the upper sapphire substrate was ablated because of laser irradiation and molten-solder material filled the ablated region, forming similar bumps and leading to a solder material containing aluminum form the upper substrate. In the present study, a similar case seems to occur, since the formed bump occupies the ablated volume of the upper sapphire substrate.
From the point of view of mechanical robustness, the resulting interlocking could be of advantage over a plain interface. Fig. 4(a, b) displays the STEM micrographs of the cross-section, which offer a different contrast in comparison with the bright field micrographs in Fig. 3. Round and enlarge particles are formed, appearing in bright contrast. The EDX line scan across the interface (Fig. 4(a)) reveals that the composition, that is, the Al:Ti ratio changes across the interface and that exists a titanium enrichment in the middle part of the cross-section. Al is distributed along the entire interface, decreasing its concentration from the left side (initially non coated substrate) to the right side (initially Ti-coated substrate), agreeing with an ablation of the upper substrate, which distributes the Al in the whole solder.
Fig. 4. STEM micrograph and EDX line scan across the cross-section of the bond (a) and across one of the particles (b). The orange line indicates the direction of the EDX line scans. The red square in (b) indicates the position of the EDXS point analysis. The yellow square indicates the reference image for the drift correction.
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Enlarge particles are distributed in the Al-enriched side of the solder (Fig. 4(b)). An EDX line scan across one of these enlarge particles (Fig. 4(b)) indicates that it contains Al, Ti and O. EDXS point analyses at one of the round particles distributed in the middle and right side of the solder (marked in STEM micrograph in Fig. 4(b)) reveals that it contains also both, Al and Ti, in a Ti:Al ratio approximate to 2:1. To the best of our knowledge no AleTieO phases with this stoichiometry were published. A similar one, Ti3AlOx, was reported by Jones et al.[52]. Selverian et al.[53] and Zalar et al.[54] described reactions between a titanium thin film deposited over a sapphire substrate. Here, Ti reduces the sapphire surface, leading to Al0 precipitation. It was stated that Ti near the interface formed bonds predominantly with O. Ti also bonded with Al in the top layer of the sapphire, forming a mixed bonding involving Al, O, and Ti. As time progressed, the interface mixed; metallic TieAl bonds formed as the sapphire decomposed into Al and O; and O diffused into the Ti film. The formation of two layers was reported: Ti3Al[O], adjacent to the sapphire, which refers to Ti3Al with oxygen in solid solution, and Ti0.67 [O0.33] at the free surface[53]. The formation of the above mentioned compounds in the interface, together with the resulting interlocking from the bump formation, would lead to the observed full bonded area in Fig. 2. The mechanical characterization of this bond is in progress. 5. Conclusion Two sapphire substrates were joined through a laser lightabsorbing titanium thin film deposited by sputtering on one of the substrates. The bonding was accomplished by irradiating a predefined pattern using a 1064 nm nanosecond laser and improving the laser parameters to obtain the best welding conditions. The irradiated part possessed a high degree of transparency. Upon laser irradiation, aluminum from the upper substrate was incorporated in the titanium intermediate layer, forming a threelayer structure where different AleTieO compounds co-exist. The formation of these compounds would favor the joining of both sapphire substrates. The mechanical properties of the bond are part of the future work. Acknowledgments The authors acknowledge financial support of FhG Internal Programs (Grant No. 692 280). The authors are very grateful to Nico Teuscher (Fraunhofer Institute for Mechanics of Materials IWM, Halle, Germany) for the Titanium sputtering on the sapphire substrates. References [1] R.M. Carter, J.Y. Chen, J.D. Shephard, R.R. Thomson, D.P. Hand, Appl. Opt. 53 (2014) 4233e4238. [2] U. Gosele, Q.Y. Tong, A. Schumacher, G. Krauter, M. Reiche, A. Plossl, P. Kopperschmidt, T.H. Lee, W.J. Kim, Sens. Actuator A-Phys. 74 (1999) 161e168. [3] S. Theppakuttai, D.B. Shao, S.C. Chen, in: 2003 ASME International Mechanical Engineering Congress on Electronic and Photonic Packing, Electrical Systems and Photonic Design, Washington, DC, 2003, pp. 107e112. [4] M.J. Wild, A. Gillner, R. Poprawe, Sens. Actuator A-Phys. 93 (2001) 63e69. [5] W. Watanabe, S. Onda, T. Tamaki, K. Itoh, J. Nishii, Appl. Phys. Lett. 89 (2006) 021106. [6] Y. Abe, T. Watanabe, H. Tanabe, K. Kagiya, Adv. Mater. Res.-Switz. 15‒17 (2007) 393e397. [7] V. Dragoi, E. Cakmak, E. Pabo, Rom. J. Inform. Sci. Technol. 13 (2010) 65e72. [8] S. Katayama, Y. Kawahito, Scripta Mater. 59 (2008) 1247e1250. [9] Y. Kawahito, Y. Niwa, T. Terajima, S. Katayama, Mater. Trans. 51 (2010) 1433e1436. [10] D. Faidel, W. Behr, S. Gross, U. Reisgen, in: Laser Assisted Net Shape Engineering 6, Proceedings of the Lane 2010, Part 2, 2010, pp. 153e162.
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