It is a big challenge to weld two materials with large differences in coefficients of thermal expansion and melting points. Here we report that the welding between fused silica (softening point, 1720°C) and SiC wafer (melting point, 3100°C) is achieved with a near infrared femtosecond laser at 800 nm. Elements are observed to have a spatial distribution gradient within the cross section of welding line, revealing that mixing and inter-diffusion of substances have occurred during laser irradiation. This is attributed to the femtosecond laser induced local phase transition and volume expansion. Through optimizing the welding parameters, pulse energy and interval of the welding lines, a shear joining strength as high as 15.1 MPa is achieved. In addition, the influence mechanism of the laser ablation on welding quality of the sample without pre-optical contact is carefully studied by measuring the laser induced interface modification.
© 2017 Optical Society of America
Welding of transparent materials is one of the indispensable manufacturing processes in various industries, including the precision machinery, healthcare, and optoelectronic industries [1,2]. Traditional joining methods, including anodic bonding, brazing and adhesive bonding, usually have drawbacks of premature aging, degassing, photo-bleaching and low accuracy, which puts forward challenges in meeting the ever-increasing demand in microminiaturization and environmental effectiveness. Laser microwelding is a superior method for bonding materials since it has many advantages such as high precision and high space-selectivity . However, due to the linear absorption mechanism, a light-absorbing intermediate layer usually needs to be inserted at the interface between transparent materials before conventional laser welding. This would bring additional complexity to the welding processes.
Unlike traditional laser used for microwelding, ultrashort pulsed laser, with intrinsic feature of high peak intensities, could be well applied in direct microwelding of transparent materials without inserting an intermediate layer, taking advantage of the unique nonlinear absorption mechanism [4,5]. When ultrashort pulsed laser is focused onto the interface of transparent materials with optical contact, nonlinear absorption would occur at the focal region [6–10]. Then the localized material will undergo melting and rapid resolidification, and a strong bonding strength will eventually form at the interface [4,11]. This interface modification based joining technology has drawn considerable attention in recent years. For instance, T. Tamaki et al. have first demonstrated the direct welding of silica glass substrates using 1-kHz, 85-fs, 800-nm laser pulses with pressure assistance . K. Sugioka and S. Wu et al. further proposed a new strategy using double-pulse ultrafast laser irradiation to improve the sample bonding strength [12,13]. Especially, the welding between dissimilar transparent materials, fused silica and borosilicate glass, was reported by W. Watanabe et al. using 1-kHz femtosecond laser pulses , showing potential applications for joining of materials with different thermal expansion coefficients. Then Y. Ozeki et al. demonstrated the direct femtosecond laser welding of copper and non-alkali glass whose melting points are 1083 °C and 705 °C, respectively . The join strength could be as high as >16MPa. Moreover, D. Hélie et al. demonstrated the femtosecond laser welding of fused silica and BK7 glass, and a maximum joint strength of 5.25 MPa was achieved . Then other combinations of dissimilar materials such as metal-glass, silicon-glass, etc. were subsequently reported to be welded together with ultrashort pulsed laser [16–19]. However, to date, there has no report on welding of dissimilar materials with large difference both in coefficients of thermal expansion and melting point using ultrashort lasers, which has important application in aerospace and optoelectronic industries.
In addition, ultrashort laser welding of transparent materials usually require pre-optical contact (<100 nm), otherwise laser ablation which is usually considered to have negative influence on welding quality will occur at the sample interface . This puts forward strict requirements on welding process and surface flatness of the sample. Therefore, relaxing the strict requirements has important practical significance for promoting the industrial application of this welding technique. K. Cvecek et al. have recently demonstrated the joining of glass plates across gaps of up to 1μm taking advantage of thermal volume expansion induced by the focused 1-MHz, 10-ps, 1064-nm laser beam . However, this approach is subject to the thermal coefficients of materials and needs to precisely control the focal position with an accuracy of better than 1 µm. Given that the influence mechanism of laser ablation on welding quality of transparent materials has not been carefully discussed, research on this issue would be meaningful and advantageous to relax the strict requirements described above.
In this work, fused silica and SiC, which have large difference both in coefficients of thermal expansion and melting point, are successfully welded together by femtosecond laser. Through characterizing the morphology and composition across the sample interface, the laser-matter interaction in the welding region is carefully investigated. The welding quality is optimized by varying the pulse energy and interval of the welding lines. Furthermore, the influence mechanism of laser ablation on the welding quality of fused silica and SiC without pre-optical contact is carefully discussed.
2. Experimental procedure
The single crystal SiC wafer and fused silica used in this experiment have a size of 10 × 8 × 0.3 mm3 and 30 × 15 × 4 mm3, respectively. The coefficient of thermal expansion and melting point of SiC at zero pressure are about 5.0 × 10−6 /°C and 3100 °C, respectively , while those for fused silica are 5.9 × 10−7 /°C and 1720 °C [11,22]. All the samples are polished to achieve flatness below λ/4. The roughness of SiC wafer is below 1.0 nm. Schematic diagram of the experimental setup for laser welding is shown in Fig. 1. Fused silica and SiC wafer are clamped together to achieve optical contact over an area of several square millimeters through the clamp before the laser welding process, with the gap between the sample less than λ/4. The sample with a gap value of about λ is achieved by inserting two microfibers with diameters of about 1 μm between SiC wafer and fused silica. Then the samples are mounted onto a computer controlled XYZ motion stage that allows translation parallel or perpendicular to the laser propagation axis. An amplified Ti:sapphire laser system producing 240-fs, 800-nm, 50-kHz pulses are used in the experiment. The laser beam having a Gaussian intensity profile is focused onto the sample interface through fused silica by a 20 × Mitutoyo microscope objective (working distance 20 mm, nominal numerical aperture 0.42). The diameter of the focused spot is about 2.5 μm at the 1/e2 level. The pulse energy is adjusted by rotating half wave plate (HWP) in front of a thin film polarizer (TFP). The laser fluence F is expressed by F = E/πR2, where E is the pulse energy and R is the radius of the laser beam at the 1/e2 level beam waist.
The setup used for measuring the shear joining strength of sample is described in our previous work . The thrust force in a downward direction is applied onto the SiC wafer through the indenter and increase until the welded sample is cleaved into two substrates. The shear joining strength is defined by dividing the critical thrust force by the sealing area (area sealed by welding lines).
3. Results and discussion
3.1 Characterization on morphology and composition
Fused silica and SiC with optical contact are directly welded together by 50-kHz, 240-fs, 800-nm laser pulses and the results are shown in Fig. 2(a). The welding velocity and pulse energy are 0.5 mm/s and 1.0 μJ, respectively. The corresponding laser fluence is about 20.4 J/cm2, well above the ablation threshold of fused silica and SiC [23,24]. A sealed region with a size of 3 × 4 mm2 is formed in the optical contacting region by the welding lines. The sample is characterized by optical microscope as shown in Fig. 2(b). Cracks are not found within the irradiated region. The width of welding line is measured to be about 9.8μm. The interval between the centers of adjacent welding lines is about 20 μm. The shear bonding strength is characterized to be as high as 10.5 MPa.
The sample after shear joining strength testing is characterized by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Figure 3(a) shows the SEM image of welding lines on SiC surface, with a zoomed region shown in Fig. 3(b). It could be seen that considerable fused silica residuals remain on SiC surface after shear-force separation, indicating that a strong bonding has formed between SiC and fused silica in the irradiated region. No silica particle is found in the unirradiated area in the intervals between adjacent welding lines. The composition of the residuals is also verified by EDS. The material in region 1 is characterized to be SiC as shown in Fig. 3(c). The residuals in region 2 are characterized to be made of O and Si as shown in Fig. 3(d). Note that the detected Au element is due to the spray gold process during the characterization.
We characterized the morphology and chemical composition of cross section of the welded sample. The cross section of welding lines with 20 μm spacing is shown in Fig. 4(a), and the magnified region is given in the upper right corner. It could be seen that the inner gap between materials is obviously eliminated after laser irradiation. The irradiated region shows in a curved structure, revealing that melting and rapid resolidification have occurred there [17,25]. The melting and subsequent resolidification is usually considered as the cause for forming covalent bonds at the interface [11,26]. The laser-materials interaction is found in a confined region near the interface, with a depth of about 3 μm along the propagation direction of laser. The melting region in SiC wafer is much smaller than that in silica due to its very high melting temperature and thermal conductivity. The similar phenomenon is also observed in the femtosecond laser welding of BK7 glass and fused silica . The coefficient of thermal diffusivity and melting point are key parameters for melting. For fast and high energy deposition with femtosecond laser pulses, lower thermal diffusivity and lower melting point is contributed to a large melting region. In our case, fused silica has low thermal diffusivity and low soften point, its melting depth is about 4 times of SiC for the same thermal source.
The element distribution within the cross section of welding line is obtained by line analysis with EDS as shown in Fig. 4(b). The constituent elements of O and Si exhibit apparent concentration gradient along the longitudinal direction within the irradiated region, indicating that mixing and interdiffusion of materials have occurred during laser irradiation. Once irradiated by the femtosecond laser pulse, the electrons within the focal region will be rapidly heated, and then transfer their energies to the lattice before considerable hydrodynamic movement, quasi at constant volume [27,28]. Isochoric heating would occur, leading to phase transitions and plasma evolution for both materials. Buildup of a strong pressure within the focal region  will also happen, which then releases via rapid expansion, leading to the mixing of local materials in liquid phase. The materials interdiffusion is considered proceeding through the heating and cooling process. The subsequent laser pulses will further enhance the mixing and interdiffusion processes. Thus, the sample is well welded together. We conceive that the mixed material near the interface acts as a buffer, which could effectively solve the mismatch of thermal expansion and melting points between SiC and fused silica.
3.2 Dependence of shear joining strength on pulse energy and interval
The welding parameters are optimized as follows. For a fixed sealing area at 3 mm × 4 mm, SiC and fused silica with optical contact are welded with pulse energies of 0.5 μJ to 2 μJ, corresponding to fluences of 10.2 J/cm2 to 40.7 J/cm2, and intervals of 15 μm to 80 μm. The dependence of shear joining strength on pulse energy and interval is depicted in Fig. 5, respectively. Note that the values of each point in the curves take an average of three measurement results to reduce the errors caused in the welding processes and measurements.
Figure 5(a) represents the dependence of shear joining strength on pulse energy. For the energy scan, the interval is fixed at 20 μm. The shear joining strength increases with the increase of pulse energy in the range of 0.5 μJ to 1.8 μJ, but the increase rate gradually slows down, giving a saturated value of 13.8 MPa at pulse energy of 1.8μJ. When the pulse energy increases to 2 μJ, the shear joining strength appears to slightly decrease. We expect that both laser ablation and effective welding area have influence on the shear joining strength, which will be discussed below.
Figure 5(b) represents the relationship between pulse energy and width of welding line. It could be seen that the width of welding line has a similar variation trend with shear joining strength. The width of welding line increases with the increase of pulse energy in the range of 0.5 μJ to 1.8 μJ, and the increase rate gradually slows down. Thus, the effective welding area would increase with the increase of pulse energy, leading to a similar increasing tendency of shear joining strength in this range. Note that effective welding area refers to the total area of welding lines. When the pulse energy reaches above 1.8 μJ, the width of welding line continues to increase, differing from shear joining strength. It can be explained that effective welding area is not the only factor influencing the shear joining strength. Laser ablation is considered as another critical factor affecting the welding quality. For low pulse energy, laser ablation is deemed to be well suppressed within the laser irradiated region, having negligible negative effect on shear joining strength. However, when the pulse energy reaches above the critical value, laser ablation could no longer be well suppressed in the focal region. Therefore, despite the increase in effective welding area, the shear joining strength appears to slightly decrease when the pulse energy reaches above 1.8 μJ.
Figure 5(c) shows the dependence of shear joining strength on the interval. For the interval scan, the pulse energy is fixed at 1.8 μJ. Note that, for the sample with interval of 15 μm, the fused silica within the sealing region is ripped off after shear-force separation, adhering well on the surface of SiC wafer. Thus, the actual shear joining strength of sample would be greater than the measured value of 15.1 MPa. It could be seen that the shear joining strength monotonously decreases with the increase of interval in the range of 15 μm to 80 μm. This is attributed to the decrease in effective welding area. When the interval is increased, the number of welding lines in the sealing area will decrease. Thus, the effective welding area will decrease with the increase of interval, resulting in the decrease of shear joining strength.
3.3 Influence of laser ablation on welding quality
The laser ablation induced on interface of the samples without pre-optical contact is usually considered to conduct negative influence on welding quality. However, the influence mechanism has not been carefully studied. In this work, the influence of laser ablation on welding quality is studied through enlarging the inner gap between SiC wafer and fused silica. The sample with optical contact and non-optical contact (gap value of about λ) are welded under pulse energy of 1.8 μJ and speed of 0.5 mm/s, respectively. The shear joining strength of the sample with non-optical contact is measured to be only about 5.7 MPa, significantly less than that of the sample with optical contact (13.8 MPa).
The surface morphology of sample with optical contact and non-optical contact after post-joining separation is characterized by SEM. For the sample with non-optical contact, there is no chunk fused silica residuals remained on SiC surface after shear-force separation as shown in Fig. 6(a). The average width of welding line is measured to be about 15.3 μm. In addition, apparent sign of laser ablation is observed on SiC surface. Considerable nanoparticles in droplet shape are found locating in the unirradiated area between adjacent welding lines in Figs. 6(b) and 6(c). The phenomenon of particles coalescence is also observed, revealing that the ejected particles have experienced melting and quick resolidifying process. However, for the sample with optical contact, laser ablation is well suppressed in the focal region. There are considerable chunk fused silica remained on SiC surface after shear-force separation as shown in Fig. 6(d), revealing that a strong bonding strength has formed between SiC and fused silica for optical contact. The average width of welding line is measured to be about 13.6 μm, apparently less than that of the sample with non-optical contact.
Therefore, the influence mechanism of laser ablation on welding quality can be concluded as follow. When the sample inner gap is enlarged, laser ablation could no longer be well suppressed in the focal region. The melt materials will be fragmented and expelled away from the heated focal region by the laser induced strong pressure. This will consume energy and hamper the mixing and interdiffusion processes proceeding in the focal region, going against to form a covalent band. In addition, the expelled materials will also quickly cool down and resolidify in places away from the heated region due to their high melting points and thermal conductivities, which is not conducive to form a strong band between the surfaces of the two materials. Therefore, the shear joining strength of the sample with non-optical contact appears significantly less than that of the sample with optical contact. Based on the above analysis, appropriately expanding the heat-effected zone, which could be realized through increasing the pulse repetition rate, pulse width or laser absorptivity, might be an available approach of relaxing the strict requirement for optical contacting during laser welding. Because this could ensure the expelled materials still locate in a high temperature environment, benefiting the formation of covalent band between the samples. Note that, J. Chen et al. have recently demonstrated the joining of borosilicate glass and fused silica with pre-existing gap of up to 3 μm through optimizing the laser power and focal position using 400-kHz, 5.9-ps, 1030-nm laser pulses . Given its important practical significance, relaxing the strict requirements for the welding of materials with large difference in thermal coefficients through expanding the heat-effected zone will be our further research issue.
In summary, fused silica and SiC, two materials with large difference both in coefficients of thermal expansion and melting point, are successfully welded together by femtosecond laser pulses. After optimizing the welding parameters, a shear joining strength as high as 15.1 MPa is achieved. The morphology and element spatial distribution along the cross section of the bond region are characterized by SEM and EDS. The laser induced mixing and interdiffusion of materials, occurring within the focal region, is considered as the main reason for the welding. The laser ablation is proved to have significant influence on behaviors of materials at the interface irradiated by focused femtosecond laser pulse, which also determines the welding quality.
National Natural Science Foundation of China (NSFC) (61378019, 61223007).
The corresponding author thanks Prof. Hong Chang and Dr. Maojie Yang in Institute of Earth Environment, CAS for SEM imaging.
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