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Abstract
We demonstrate joining polymethyl methacrylate (PMMA) substrates by a dendrite pattern of a quenched melt using ultrashort laser pulses. Laser pulses from a 250-fs fiber laser at a repetition rate of 1 MHz were focused at the interface of the two PMMA substrates with an air gap of approximately 14 μm and direct laser joining was accomplished between two pieces of PMMA. Melted PMMA from the laser-irradiated region spread within a gap between the substrates and dendrite morphology of the melt spread outside the direct laser irradiated area of square spiral contour and increased the joining strength. The joint strength was 11 MPa for tensile and 21 MPa for the shear stress. Ultrashort laser pulses are useful to directly join PMMA substrates using localized melting and resolidification with a gap between the substrates.
© 2017 Optical Society of America
1. Introduction
Focused ultrashort laser pulses inside a transparent medium induce a material modification via nonlinear absorption around the focal volume. Ultrashort laser micromachining in polymer materials, including poly(methyl methacrylate) (PMMA), has been demonstrated including the fabrication of waveguide devices [1–6], grating [7–10], and optical memory [11–13]. Welding of polymers is of interest for integration of optical elements and sealing of micro-fluidic and micro-total analysis systems (TAS). Direct laser welding using ultrashort lasers [14–17] has been applied to welding of polymer materials [18–20]. When ultrashort laser pulses are focused at the interface of two polymer substrates, localized melting of the two substrates occurs around the focal volume initiated by nonlinear absorption and a subsequent resolidification of the melted polymer leads to welding/joining of the two substrates. The ultrashort laser welding technique of polymers does not need intermediate light-absorbing layer and directly welds polymer substrates. Tamaki et al. demonstrated the welding of PMMA substrates using 1-kHz, 100-fs, and 800-nm laser pulses [18]. Volpe et al. reported on the joining of PMMA materials using 5-MHz, 360-fs, and 1045-nm laser pulses [19]. Using a 1028 nm laser with pulse duration of 220 fs at a repetition rate of 571 kHz, welding of cycloolefin copolymers has been demonstrated [20]. In those previous reports, the gap between the two polymer substrates was less than a few micrometers. In Ref. 19, two samples were clamped in order to achieve an air gap of a few micrometers range between the substrates. In Ref. 20, spacing between two substrates was monitored by the appearance of Newton ring interference pattern. In the ultrafast laser welding, a very close proximity between the parts to be joined is usually thought to be necessary. In order to obtain sample gap with a few microns, samples are pressed. This would result in adding residual stress in the welding interface and causing complication to the welding process and could cause cracking. For practical applications, it is important to simplify the welding processes. For large area welding/joining, it is difficult to obtain close contact in large area. In this study, we demonstrate direct joining of PMMA with an air gap of 14 μm using an ultrashort laser (250 fs) with 1064 nm wavelength and 1 MHz repetition rate. Ultrashort laser pulses are focused at the interface of two PMMA substrates with a gap of more than 14 μm. Melted materials in laser-irradiated region spread within a gap of the substrates and dendrite morphology of molten PMMA was spreading outside the laser irradiated area of a square spiral contour and facilitated stronger joining. Morphology of the seam after laser joining was imaged by the confocal laser and an optical transmission microscopies. Dendrite morphology of the melted PMMA was extending outside the laser irradiated area and increased the joint area. Joining strength was measured.
2. Experimental methods
2.1 Experimental setup
An ultrashort fiber laser system (FP1060S-PP-D; Fianium Ltd.) generated 1064-nm laser pulses at a 1 MHz repetition rate. The maximum input energy was 1.2 μJ/pulse (1200 mW) in front of the objective lens. The pulse energy was controlled by rotating a half-wave plate in front of a Glan-laser polarizer. The fixture with a sample was mounted on a two-dimensional translation stage (XPS100; Newport Corp.). PMMA substrates (acrylite, #001; Mitsubishi Rayon Co. Ltd.) were used. The laser pulses were focused inside the PMMA substrates with a 20 × magnification objective lens (LMPlan 20 × IR; Olympus Corp.) with a numerical aperture (NA) of 0.40. Transmission images of the jointed PMMA substrates were observed using optical microscopies. The morphology of the cleaved surfaces of the two PMMA substrates after laser welding was observed with a confocal laser scanning microscope (VK-X200; Keyence Co.).
2.2 Laser processing parameters in PMMA by ultrashort fiber laser
In order to explore parameters for welding condition between PMMA substrates, the laser parameters for structural modification in PMMA were investigated. Among several factors influencing structural modification in PMMA, we investigated the incident pulse energy and the scan speed. The laser pulses were focused at the depth of 1 mm below sample’s surface inside PMMA of 2 mm thickness. To inscribe continuous lines inside PMMA, the samples were translated perpendicular to the optical axis with respect to the laser focus. The resulting structural modifications (refractive index change, scattering damage, and void) were imaged by transmission optical microscopy. Figures 1(a), 1(b), and 1(c), respectively, present top views of structural modifications in PMMA at speeds of 0.1 mm/s, 1 mm/s, and 5 mm/s and at energies between 0.7 and 1.0 μJ/pulse. When the refractive index change was induced, optical transmission microscopy revealed a smooth modification, with no light scattering damage in the region of the refractive index change, as presented in Fig. 1 at energy of 0.7 μJ/pulse. The refractive index change was approximately 10 μm wide. However, when scattering damage occurred, the transmitted energies were decreased. The inscribed lines were found to be opaque by observation with a transmission optical microscope. The black spot denotes a void formed at the energy of 0.8 μJ/pulse.
Figure 2 presents a summary of the thresholds of structural modifications. Structural modifications were examined discretely at speeds of 0.1 mm/s and at 0.5 mm/s – 5 mm/s with steps of 0.5 mm/s. The energy was changed from 0.4 to 1.0 μJ/pulse. The laser energies between 0.4 and 1.0 μJ/pulse correspond to laser fluence between 4.4 and 11.1 J/cm2 for a 1.6 μm radius. No visible change was observed at energy of 0.4 μJ/pulse and a speed of more than 3 mm/s. Refractive index change was induced at energies of 0.4–0.7 μJ/pulse. Scattering damage occurred when 0.8 μJ/pulse was exceeded.
When laser pulses are focused and translated in bulk PMMA, structural modifications are dependent on writing parameters with high-repetition rate ultrashort laser pulses (>1 MHz). Homogeneous modifications, frayed edges distortion, periodic disruption, and non-periodic disruption were demonstrated [5,6]. Inhomogeneous modification lines were produced by heat accumulation [19]. In our experiments, refractive index change, scattering damage, voids, spallation cracks, and thermally-melted modification were all observed at different irradiation energies and doses as in the previous reports [5,6,19,20].
2.3 Ultrafast laser welding of PMMA
A schematic showing laser welding of two PMMA substrates with an air gap is depicted in Fig. 3(a). Two PMMA substrates were stacked one on another. The gap of two substrates was measured to be 14 μm.
Fig. 3 (a) Schematic diagram of laser micro-joining of two substrates with an air gap. (b) Translation method of stages with respect to the focal point. (c) Top view of welding volumes fabricated at a speed of 1 mm/s. (d) Two jointed PMMA substrates. The top PMMA substrate was picked up using tweezers.
To create a welding volume, we translated the focal points with a square spiral contour. In the square spiral contour, the focal point was translated two-dimensionally from the center. From repeated displacements by adding 10 μm along the x-axis and y-axis, welding was conducted as portrayed in Fig. 3(b). The laser translation area was 1 × 1 mm2. Figure 3(c) shows part of the top view of the welding volumes with a transmission microscope. Figure 3(d) shows a picture of jointed PMMA substrates after laser welding when the top PMMA substrate was picked up by tweezers. To investigate the effect of the pitch in square spiral contour, we performed joining at energies of 0.6–1.0 μJ/pulse at a speed of 1 mm/s. Above the energy of 0.6 μJ/pulse in the square spiral contour, joining was successful at pitch of 10 μm.
2.4 Dendritic morphology of cleaved surface after joining of PMMA substrates
We investigated the morphology of the cleaved surfaces of the two PMMA substrates after laser joining. PMMA substrates were jointed by square-spiral contour joining with 10 μm pitch at an energy of 0.8 μJ/pulse and at a speed of 1 mm/s. The laser translation area was 1 × 1 mm2. After first separating the samples after laser welding, we observed each cleaved surface using a confocal laser scanning microscope (VK-X200; Keyence Co.).
Figures 4(a) and 4(b) show optical transmission images of two cleaved substrates. The welded seam morphology can be observed where laser pulses were not irradiated. The morphology was dendritic structures and developed from the laser-irradiated area to outside where laser pulses were not irradiated. Figures 4(c) and 4(d) show height distributions of a cleaved surface with a confocal laser scanning microscope. The width of dendritic structures was changing from a few microns to 100 μm and their height was approximately 7 μm on the both cleaved surfaces. At laser irradiation area, damage structures or voids were observed (blue part in Fig. 4(c) and 4(d)). PMMA was melted around focal volume and melted materials moved outside in contact with both substrates. Then, melted materials solidified in the form of dendrite structures. The results indicated that the liquid pool formed by ultrashort laser pulses spread and filled in the 14 μm-gap outside the laser-irradiated area in the form of dendrite structures. Figure 4(e) and 4(f) shows perspective view of dendrite morphology of the cleaved surfaces of the two PMMA substrates after laser joining.
Fig. 4 (a), (b) Optical transmission image of a cleaved PMMA surface after joining of PMMA substrates. (c), (d) Height distribution of a cleaved PMMA surfaces shown in (a) and (b) obtained with a confocal laser scanning microscope. Dendrite morphology was observed outside laser irradiated area. Inset in the top shows a schematic for observation of cleaved surface. (e), (f) Perspective view of dendrite morphology of cleaved surface in (c) and (d). Inset shows pseudo-color height distribution.
Figure 5 shows optical images of a cleaved PMMA surface after joining of PMMA substrates. In the outside circumference of the laser translation area, dendrite morphology was observed outside the directly laser irradiated area.
Fig. 5 Optical images of a cleaved PMMA surface after welding of PMMA substrates. Dendrite morphology was observed outside laser irradiated area, which was calculated using image processing. Inset shows the schematic area for laser irradiated area and dendrite morphology from side.
2.5 Discussion
The dendrite is found to be prevalent morphology of diffusion controlled crystal growth in metals [21], glass [22], and polymers [23]. The dendritic morphology was finger-like or tree-like structures. Dendritic surface structure on polyethylene-terephthalate (PET) was observed after excimer laser irradiation when the irradiation is carried out in ambient pressures [24–26]. The lateral dimensions of dendritic structures are in the micrometer range and their height was less than 100 nm and dendritic structures were observed only in irradiation area [24–26]. The qualitative mechanism of formation of dendritic structures is a surface tension driven dewetting of the molten PMMA. The thinner segments of molten PMMA thermally quenches faster and pulls on a molten region. Due to narrow 14 μm gap between PMMA workpieces a buildup ablation pressure was additional factor which facilitated a wide spread of molten material and helped to increase a footprint of the joint area. From optical imaging in Fig. 5, it was possible to estimate that the surface area of dendritic pattern was approximately 50% on the footprint which was 2.6 times larger than the area enclosed in the outside circumference of the laser path.
In the welding of PMMA substrates, heat accumulation at high repetition rate of laser pulses during translation of the focal point causes localized melting of PMMA in the focal volume. Melted materials diffused in contact with both substrates with an air gap by capillary effect and filled the gap of the substrates outside laser irradiated area. The temperature gradient between melted materials and PMMA substrates with an air gap caused final re-solidification of melted materials. In a square spiral contour welding, focal spot was moved from the center. Around center, translation length was short and speed was slow around the corner of square. Therefore, heat accumulation occurs and material melts after successive pulses arrived. Melted materials were pushed away and dendrite morphology was apparent extending beyond the area of direct irradiation.
In previous reports on the welding of PMMA [19], two samples were clamped in order to achieve an air gap of a few micrometers range between two substrates. After welding, the width of the welding seam was 60 μm. In the welding of cycloolefin copolymers, spacing between two substrates was confirmed by the appearance of Newton ring upon white light illumination and the width of welding seams increased with higher laser powers up to 137 μm [20]. Ultrashort laser welding between transparent materials has features of high space-selectivity with micron-dimension welding by minimizing the air gap of substrates. In our experiments, the gap of two PMMA substrates was 14 μm. Melted materials fill the gap of substrate in the form of dendrite structures. Space selectivity in the welding was reduced, however, PMMA substrates with an air gap were achieved with dendrite pattern. In PMMA welding with a gap by ultrashort laser pulses, melted materials formed by ultrashort laser pulses spread and filled in the gap outside the laser-irradiated area. The morphology of dendrite will be dependent on surface properties, roughness of PMMA substrate, temperature of PMMA substrates, the distance of gap between PMMA substrates. The axial extent of Gaussian beam can be estimated as double the Rayleigh length 2zr = πwo2/λ = 7.8 μm, where the waist (radius) of the beam focused with NA = 0.4 lens is wo = 0.61λ/NA = 1.6 μm. If we consider M square of Gaussian beam, M square of 1.2 makes waist and the Rayleigh length by 20% larger. Focusing through the entire thickness of the first PMMA workpiece caused strong spherical aberration which caused additional elongation of the focal region. Moreover, at the used focusing conditions there was a presence of strong self-focusing. All this was helpful to affect both sides of the PMMA workpieces.
A mechanical strength of welding was estimated as approximately 11.3 MPa for joint strength and 21.4 MPa for the shear stress. To estimate the joint strength, we applied a simple tensile test after welding the substrates (AG-1-10kN; Shimadzu Corp.). Two samples were joined to a string with adhesive. Then the load was increased by adding weights until the jointed sample was cleaved into two substrates. When the sample was cleaved, we ascertained the joint strength by dividing the load by the welding areas. The maximum applied force was 29.5 N. If we assume that the welded zone is 2.62 mm2 (1 mm2 for laser translation area plus dendrite welded zone 1.62 mm2), then the joint strength was 11.3 MPa. We also measured shear stress. The shear stress was 21.4 MPa at the maximum. Conventional laser welding of polymer materials is based on linear absorption, and inserting an intermediate layer or absorbing materials is usually necessary [27,28]. For example, because a thulium laser at 2 μm wavelength has an absorption in PMMA, the welding of PMMA substrates can be performed by linear absorption [27]. The strength achieved by Thulium laser bonding was 13 MPa for PMMA [27]. The welding of PMMA substrates can be performed using a light-absorbing intermediate layer such as titanium between the substrates [28]. Tensile bonding strength was 6.1 MPa for PMMA [28]. In comparison, the strength of the femtosecond laser joining of PMMA with a gap achieved higher strength.
3. Summary
We performed joining of PMMA substrates using an ultrashort fiber laser with 1064 nm wavelength and a 1 MHz repetition rate. First, we demonstrated the parameters for structural modifications in PMMA. Structural modifications were induced by scanning the sample up to 5 mm/s. Joining of PMMA substrates with 14-μm air gap was performed at translation velocities up to 1 mm/s. The observation of dendrite morphology reveals that melted materials spread and fill the gap of the substrates outside the laser irradiated area. This ultrashort laser welding technique of polymer materials with dendrite structures introduces possibilities for direct welding of polymer substrates with an air gap. The joint strength was 11.3 MPa for tensile and 21.4 MPa for the shear stress. This dendrite joining technique with a gap will offer versatility of large area joining and decrease of the residual stress.
Funding
Amada Foundation (AF2013206).
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