We report on an optical interference method for transferring periodic microstructures of metal film from a supporting substrate to a receiving substrate by means of five-beam interference of femtosecond laser pulses. Scanning electron microscopy and optical microscopy revealed microstructures with micrometer-order were transferred to the receiving substrate. In the meanwhile, a negative copy of the transferred structures was induced in the metal film on the supporting substrate. The diffraction characteristics of the transferred structures were also evaluated. The present technique allows one-step realization of functional optoelectronic devices.
© 2005 Optical Society of America
Recently, the femtosecond laser interference (FLI) technique has been paid great attention to [1–9] for the fabrication of periodic microstructures, because FLI can produce a periodically modulated light intensity distribution with a period in the order of its wavelength. A periodically modulated microstructure can be obtained if the periodically modulated light intensity distribution is transferred to a photoreactive material. The interference of two beams creates a one-dimensional periodic pattern. By increasing the number of beams, in principle, two-dimensional and three-dimensional periodic patterns can be obtained. By this way, Kawamura et al. , Si et al. [2,3] and Li et al.  fabricated surface or volume micrograting in various materials. Kondo et al. [5,6] demonstrated the fabrication of photonic crystal in photorsist and resin. Nakata et al.  showed the fabrication of dot array and bump array on metal film. There are two kinds of beam delivery approaches, which have been used in the aforementioned researches. The first is mainly composed of a beam splitter and an optical delay  in order to achieve the spatial and temporal interference for femtosecond laser pulses. The second is mainly composed of a diffraction beam splitter (DBS) and a confocal imaging system , in which no optical delay is needed. For the first approach, precise adjustments of optical delay for each optical path are needed by observing the second-harmonic generation or third-harmonic generation  to obtain the temporal overlap of femtosecond pulses. In addition, when multiple beams (more than two beams) interference is needed, a complicated optical setup is required and it’s precise adjustment is very difficult. For the second approach, the optical setup is quite simple and temporal overlap is achieved without any adjustment.
On the other hand, laser-induced forward transfer (LIFT) technique has been extensively studied for microprinting diffractive optical structures and computer-generated holograms [10,11], writing active and passive mesoscopic circuit elements  and arranging pad array in microelectronic packaging , etc. The LIFT technique utilizes pulsed lasers to remove thin film material from a transparent supporting substrate and deposit it onto a suitable receiving substrate. The LIFT process was first shown by Bohandy et al.  to produce direct writing of Cu lines by using single ns excimer laser pulses under high vacuum.
In this letter, we combined the FLI technique and the LIFT technique to transfer periodic microstructures from a metal aluminum (Al) film deposited on a supporting glass substrate to a receiving glass substrate. By five-beam interference of near-IR 800 nm femtosecond laser pulses, periodic microstructures with micrometer-order were successfully transferred to the receiving substrate. The characteristics of the transferred structures as diffractive optics were evaluated. Morphology of the structures was investigated by optical microscope and scanning electron microscope (SEM).
2. Experiments and results
The experimental setup is similar to previous research . A commercial regeneratively amplified 800 nm Ti: Sapphire laser that emits 150 fs, 1 kHz, mode-locked pulses, was split into five beams by a DBS (MEMS Optical, Inc.). The split five beams were made parallel by a lens with focal length of 100 mm, and then were gathered to the sample by a 20×optical objective lens (Nikon 20MI, NA (numerical aperture)=0.45), where the pulses’ temporal and spatial overlap of the five beams was perfectly achieved. The four outer beams with equal intensity were symmetrically placed around the central beam, and made an angle (θ) of 12° with it. The typical input laser power (incident DBS) and exposure time were set as 200 mW and ~10 s, respectively. The materials used in the transfer experiments were thin Al film deposited on transparent quartz plate. Al film of 200 nm thickness was prepared by sputtering and e-beam evaporation. Quartz glass was used as receiving substrate. All experiments were carried out at room temperature in ambient atmosphere.
The experimental details are shown in Fig. 1. The metal film was contacted with the receiving substrate. Two kinds of methods can be applied for focusing the interfered laser beams onto the film surface. The first way, being applicable to transparent receiving substrate, is focusing interfered beams through the receiving substrate onto the film surface, which is termed front-side transfer. The second way, being applicable to transparent supporting substrate, is focusing interfered beams through the supporting substrate onto the film surface, which is termed rear-side transfer. In our case, both front-side transfer and rear-side transfer can be employed, as the receiving and supporting substrates are all transparent to the laser wavelength. Here, we employed the front-side transfer scheme. After interfering femtosecond laser irradiation, the supporting substrate and receiving substrate were separated. Then the induced structures on supporting substrate and the transferred structures on receiving substrate were observed by a 10× optical objective, as shown in the inset of Fig. 1. As can be seen, the diameters of the induced spot on the supporting substrate and the transferred spot on the receiving substrate are about 300 µm. We can see there are concentric circles distributions in the spots, which were caused by unequal intensity of the five-beam lasers. We measured the intensity of central beam to be about 20 times larger than that of the other beams. Using a filter can reduce the intensity of central beam and make the distribution even. The details of the structures in the spots are shown in Fig. 2.
Figure 2 shows the optical microscope and SEM images of the microstructures on the supporting substrate (a and c) and on the receiving substrate (b and d). As can be seen, a four-fold symmetric structure was transferred from the supporting substrate to the receiving substrate and a similar structure also was induced on the supporting substrate. The period of structures on both receiving substrate and supporting substrate was about 2.5 µm. This is in agreement with the calculated value of λ/(√2sinθ . The SEM image of the microstructures on supporting substrate is clear. Some debris can be observed around every ablated hole. Although the SEM image of the structures on receiving substrate illustrates the array structures, the structures look illdefined when observed by the same magnification with Fig. 2(c). We suggest that there are two reasons responsible for the irregularity for the formed patterns. First, the intensity of central beam is larger than that of the other beams, making the intensity modulation uneven. Using a filter can reduce the intensity of central beam, making the distribution even. Second, as we employed the front-side transfer scheme, it is possible to cause re-ablation of the metal deposited on the receiving substrate by the following fs laser pulses. Using rear-side transfer scheme can avoid the re-ablation. In addition, by adjusting laser focusing parameters and the thickness of the film on supporting substrate, the quality of the structures on receiving substrate are desired to be improved.
The structures in Fig. 2 work as diffractive beam splitter, as shown in Fig. 3. To investigate the diffraction characteristics of the structures, we coupled a diode-pumped all solid state laser with wavelength of 532 nm by a 10×optical objective into the structures on both receiving substrate and supporting substrate. The diffraction patterns were recorded by a digital camera. The experimental scheme is shown in Fig. 3(c). Figures 3(a) and 3(b) shows the diffraction patterns of the periodic structures on both receiving substrate and supporting substrate. Four-order diffraction spots and two-order diffraction spots were observed for the periodic structures on supporting substrate and on receiving substrate, respectively. Here, we define the first-order diffraction efficiency as the ratio of the averaged power of the first-order diffraction to the power of the incident light. The measured diffraction efficiencies of first order diffraction spot were 3.73% and 1.93% for the periodic structures on supporting substrate and on receiving substrate, respectively. From the diffractive optics of the structures, we can conclude that the structures on supporting substrate should be periodic holes and the structures on receiving substrate should be periodic pads. The diffraction patterns can be changed by interval and shape of holes and pads, which are related with laser energy, angle of the beams, focal length and focusing of the lens, irradiation time, laser wavelength, etc.
Adrian et al.  have investigated the mechanism of metal deposition by the LIFT process. They proposed that the LIFT process involves vapor-driven propulsion of metal from the film onto the target. In our case, the five-beam interference of femtosecond laser pulses produces a periodically modulated light field with the distribution of enhanced and weakened intensities. The enhanced optical intensity drives the metal vapor onto the receiving substrate to form periodic pad structures, leaving periodic hole structures on the supporting substrate. The dynamic mechanisms for the process are under investigation.
In summary, we have shown a method for transferring periodic microstructures of metal film from a supporting substrate to a receiving sunstrate by multibeam interference of femtosecond laser pulses. This method makes it possible to transfer periodic microstructures with large dimension immediately (only a few seconds). This technique may be applicable to fabricate diffractive optical elements, arrange pad array in microelectronic packaging and fabricate sensor elements.
The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No: 50125208) and Shanghai Science and Technology Committee (Grant No: 04dz05112), and one of the authors (Q. Z. Zhao) gratefully acknowledges the support of China Postdoctoral Science Foundation, and K. C. Wong Education Foundation, Hong Kong.
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