Longitudinal and transverse microholes are drilled in soda-lime glass by water-assisted ablation with femtosecond laser pulses. True three-dimensional microchannels consisting of longitudinal and transverse microholes are presented. At low incident pulse energy, only one transverse microhole is observed. At high incident pulse energy, multiple transverse microholes can be simultaneously drilled. Using a focusing lens with numerical aperture of 0.5, two, three and four transverse microholes are fabricated at 3.2, 4.9 and 9.3µJ/pulse, which is qualitatively explained by the multiple foci process.
© 2005 Optical Society of America
Femtosecond laser micromachining has attracted much attention due to advantages such as small heat-affected zones, the versatility to the materials and the ability of three-dimensional (3D) processing inside the transparent medium. When femtosecond laser pulses are focused into a transparent medium, ultrahigh fluence near the focus can result in nonlinear multiple photon absorption and subsequent avalanche ionization of the material. Dense electron plasma formation and expansion with shock wave induce micro-explosion. When the focus is located at the surface, material ablation and removal will take place. This is used to drill microholes for microfludic devices that are widely used in the areas of biological and chemical analysis, medical inspection and environmental monitoring.
Many researchers have studied techniques of drilling microchannels in transparent glass with femtosecond laser pulses [1–9]. Kondo et al. fabricated a Y-branched channel by use of femtosecond laser irradiation followed by heat treatment and dilute hydrofluoric acid etching . Adopting a similar approach, Masuda and Cheng et al. produced 3D microchannles in photosensitive glass [2,3]. Leaving out the baking process, Marcinkevičius et al. made an H-shaped 3D microchannel in silica glass . Without postfabrication such as heat treatment and wet etching, Li et al. drilled high aspect ratio and high quality 3D microchannels in silica glass in a single step by water-assisted ablation when the rear surface of the sample was in contact with water . During drilling, the inflow water dispersed ablated material and debris, which greatly reduced redeposition and blocking effect, and benefited the elongation of microholes. Straight holes were also drilled in soda-lime glass without or with inflow of water [6–8]. Hwang et al. have enhanced the liquid-assisted drilling by ultrasonic wave agitation, and achieved 3D bent and curved channels more efficiently .
In this paper we present our experimental results on simultaneous drilling of multiple transverse micoholes in soda-lime glass by water-assisted ablation with femtosecond laser pulses. Not only longitudinal holes in the laser beam propagation direction but also transverse holes perpendicular to laser beam could be drilled when the sample’s rear surface was in contact with water. A true 3D microchannel consisting of a longitudinal and two transverse microholes was fabricated. With higher pulse energy, it was easier to get a longer longitudinal hole with a larger diameter. And we could simultaneously drill multiple transverse microholes when the sample was moved perpendicularly to the drilled longitudinal hole starting from the rear surface. The dependence of the number and diameters of transverse microholes on the incident pulse energy was investigated. The underlying mechanism was discussed.
2. Experiments and results
A regeneratively amplified Ti:sapphire laser system (Spitfire, Spectra Physics) was used, which delivered pulses with a duration of 120fs (FWHM), a center wavelength at 800nm and a repetition rate of 1kHz. The laser beam propagated along +z direction. A long working distance objective lens with a numerical aperture of 0.50 (LMPLFL50X, Olympus) was applied to focus laser pulses. The energy of incident pulse could be continuously varied by rotating a half-wave plate before a Glan-prism. A mechanical shutter was used to select the number of ablation pulses. The soda-lime sample was cut from microscope glass slide that was widely used in the biological and chemical technology. It was mounted on a computercontrolled XYZ translation microstage with 0.1µm resolution. In order to in situ monitor the drilling process, the sample was optically polished on four sides. The thickness was 1 mm. A top view and a side view were obtained at the same time by two sets of transilluminated optical microscope system as shown in Fig. 1. The experiment was conducted at atmospheric pressure. During drilling, the rear surface was in contact with deionized water. When the pulse duration was shorter than 200 fs, we observed a brighter refractive-index-modulated filament in front of the dead-end of the dark longitudinal hole. The shorter the pulse duration was, the longer and clearer the filament . Therefore, we stretched the laser pulse by adjusting compressor gratings in the laser amplifier until the filament became invisible. The pulse duration was found to be about 380 fs .
To get a 3D microchannel, a longitudinal microhole should be first drilled for the inflow of water as demonstrated in Fig. 2. We first situated the focus at the rear surface of the sample and then moved the sample along the +z direction step by step. The translation step of the microstage was 1 µm and about 50 pulses irradiated on each pausing spot. The incident energy was 1.4 µJ/pulse. Accompanied by the inflow of water, the longitudinal microhole elongated continuously. The diameter of the microhole was ~3 µm. When the hole (AB) reached a length of ~35 µm, we changed the translation direction to -x and a ~20 µm transverse microhole (BC) was produced. After that, the translation direction was altered to -y, another ~20µm microhole (CD) was fabricated. As a result, a true 3D microchannel consisting of a longitudinal and two transverse holes was drilled. The three micoholes were perpendicular to each other and their diameters were approximately equal. The hollow of the hole was verified by the contrast change and the visible water flow when water entered or receded the hole . By controlling the translation direction according to the designed structure, complex 3D microchannels could be drilled.
Increasing the incident pulse energy, we could obtain a longer longitudinal microhole with a wider diameter. When we altered the translation direction to drill a transverse microhole step by step, we actually got multiple transverse microholes at the same time as shown in Fig. 3. At incident pulse energy of 3.2 µJ, two transverse microholes (A1 and A2) were obtained. When the pulse energies were further increased to 4.9 and 9.3 µJ, three (B1 to B3) and four (C1 to C4) transverse microholes were drilled simultaneously. In Fig. 3(c), the diameter of the longitudinal hole was about 10 µm, and those of four transverse micoholes were 10, 7, 5, 4 µm, respectively. Unlike the satellite crack damage line reported in Ref. , these transverse microholes were hollow and the number could be changed by varying the incident pulse energy.
The underlying mechanism might be the refocusing of an intense femtosecond laser pulse in a transparent material. When the pulse energy was high enough, multiple foci and a filament appeared due to the dynamics competition between self-focusing by Kerr effect and defocusing by the plasma [11–13]. In addition, the spherical aberration due to air-glass interface might elongate the focus along the propagation direction and also result in multiple foci . As the energy increased, besides the main focus, other foci had sufficient energy to ablate the material in the vicinities of these foci so that multiple transverse microholes could be simultaneously drilled. Because the number and diameters of microholes can be selected by adjusting the pulse energy, it is possible to drill more complicated 3D microchannels at high speed.
Diameters of the main transverse microholes (A1, B1 and C1) widened with the incident pulse energy as shown in Fig. 3(a) to 3(c). Furthermore, the succeeding microholes simultaneously drilled were smaller than the preceding ones because the energy of each focus descended from the preceding ones. For example, microholes C1 to C4 in Fig. 3(c) sequentially became slender. As the laser beam propagated toward the rear surface, part of it passed through the water rather than the glass. The succeeding focusing suffered from the deterioration of the laser beam so that the morphology and inner walls of succeeding microholes degraded. Since the refractive index of holes filled with water was lower than the glass, the succeeding microholes gradually tilted to the front surface. At very high energy, the intensity of the filament as well as the multiple foci was strong enough to modify the glass and affected areas between microholes were visible.
We could not drill single or multiple transverse microholes without inflow of water. Water played an important role in hole drilling. It prevented ablated material from blocking and redeposition. And vapor plume and shock wave could be partly released because the ablation always occurred at the interface between glass and water. When the laser pulses were confined inside the bulk of soda-lime glass, only damage track was observed. The difference between water-assisted interface ablation and internal modification or micro-explosion is shown in Fig. 4. We first drilled a longitudinal microhole with a length of about 90 µm at energy of 3.2 µJ/pulse, and simultaneously produced two 30µm long transverse microholes by altering the translation direction to -x. We observed that the ablation debris was ejected and dissolved into water, which indicated that the micoholes were really hollow. Then we moved the sample about 20 µm along -x without laser irradiation, leaving an unaffected spacing. After that, the laser pulses were launched as though to drill the transverse microholes, but only a filamentous damage track occurred. When we continued to move and irradiate the sample step by step, we obtained an modified area rather than multiple transverse microholes.
In conclusion, when the rear surface of the soda-lime glass was in contact with water, longitudinal and transverse microholes could be drilled. By combining a longitudinal microhole and two transverse microholes, we have obtained a true 3D microchannel in soda-lime glass. When we drilled at high pulse energy, we simultaneously fabricated multiple transverse microholes, which was qualitatively explained by the refocusing behavior of an intense femtosecond pulse propagating in a transparent material. This technique is expected to have potential in drilling and micromachining for microfludic devices.
This work was supported by the National Key Research Program of China under Grant No. TG1999075207, the National Natural Science Foundation of China under Grant No. 90101027, and the Scientific Research Foundation for Returned Overseas Chinese.
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