Laser direct writing using submicron-diameter fibers - OSA Publishing

Laser direct writing using submicron-diameter fibers Feng Tian, Guoguang Yang, Jian Bai, Jianfeng Xu, Changlun Hou,* Yiyong Liang, and Kaiwei Wang State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China [email protected]

Abstract: In this paper, a novel direct writing technique using submicrondiameter fibers is presented. The submicron-diameter fiber probe serves as a tightly confined point source and it adopts micro touch mode in the process of writing. The energy distribution of direct writing model is analyzed by Three-Dimension Finite-Difference Time-Domain method. Experiments demonstrate that submicron-diameter fiber direct writing has some advantages: simple process, 350-nm-resolution (lower than 442-nmwavelength), large writing area, and controllable width of lines. In addition, by altering writing direction of lines, complex submicron patterns can be fabricated. ©2009 Optical Society of America OCIS codes: (060.2340) fiber optics components; (140.3510) lasers, fiber; (230.4000) microstucture fabrication; (310.6628) subwavelength structures, nanostructures.

References and links 1.

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Received 3 Sep 2009; revised 29 Sep 2009; accepted 30 Sep 2009; published 19 Oct 2009

26 October 2009 / Vol. 17, No. 22 / OPTICS EXPRESS 19960

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1. Introduction Direct writing techniques allow maskless definition of surface micro- and nanostructures. They are adopted to fabricate various optical devices with micro- and nanostructures, such as wire-grid polarizers [1], photon crystals [2], fiber Bragg gratings [3], optical microelectromechanical systems [4], etc. The popular direct writing techniques include the laser direct writing [5–9] and electron beam direct writing [10]. The laser direct writing is a mature and reliable technique. However, due to the diffraction limit, it is very difficult for laser direct writing to reach the subwavelength-resolution. The electron beam direct writing can fabricate sub-100 nm structures, but its processes tend to be costly and inconvenient, thus restricting its applications in a narrow range. Optical submicron-diameter fibers have attracted much attention due to their advantages, such as tight optical confinement, low optical loss, high fraction of evanescent fields, strong field enhancement [11–13]. All these attractive properties make the submicron-diameter fiber suitable for maskless lithography. First, tight optical confinement can provide high exposure resolution. Second, low optical loss and strong field enhancement ensure exposure power. Third, high fraction of evanescent fields can be easily coupled into photoresist layer. Based on these properties, this paper presents a novel direct writing technique utilizing the submicrondiameter fibers. Herein, the fiber probes are similar to an inked pen, transferring a laser “ink” from the probe tip to a photoresist layer through direct micro touch. However, there are some differences between the direct writing fiber probes and the usual micro- and nanofibers. The usual micro- and nanofibers are used to fabricate a variety of micro- and nanophotonic components or devices, such as fiber gratings, Mach-Zehnder interferometers, filters, ring resonators, knot lasers, etc [14–18], where they act as waveguides. Contrary to the abovementioned uninterrupted micro- and nanofibers, the submicron-diameter fiber probes are interrupted and serve as submicron-dimension point sources. They are also different from usual fiber tapers, which have some applications, such as sensors, filters, optical tweezers, etc [19–22]. The fiber probes for direct writing gradually change its diameter to a submicron tip, so their cone angles are much smaller than those of usual fiber tapers. Moreover, the submicron-diameter fiber probes presented in this paper not only can be used for direct writing but also are helpful for possible applications in nanoprobe sensing, laser trapping and laser nanosurgery.

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2. Theoretical analyses

Fig. 1. (a) Schematic of the direct writing using submicron-diameter fiber. Micro touch exposure is adopted during submicron-diameter fiber scanning on the photoresist layer. (b) Optical microscope image of a fiber probe for direct writing fabricated by two-step process.

Figure 1(a) shows the scheme of laser direct writing using submicron-diameter fibers. A photoresist layer is coated onto a substrate, which is fixed on a three-dimensional stage. The submicron-diameter fibers probe keeps point-contact with the photoresist during writing, so the light is guided into the photoresist layer and exposes lines. In order to avoid the bending of fiber caused by Van der Waals force during micro touch writing, the fiber probe is tilted and keeps a small angle with the substrate plane. Figure 1(b) is optical microscope image of a fiber probe for direct writing, which is fabricated by a two-step process presented in section 3. In order to investigate intensity distribution, the structure of Fig. 1(a) is simulated by three-dimensional finite-difference time-domain (FDTD) method, which is performed by Lumerial FDTD Solutions, a commercial software. The simulation model is established with the same Cartesian coordinate directions as Fig. 1(a). Figure 2(a) shows the refractive index model in x = 0 plane. The simulation is performed with the following typical parameter: the diameter and the length of the silica probe is 300 nm and 4µm respectively, the thickness of the photoresist coated on the silicon substrate is 100 nm, and the tilted angle between the probe and the photoresist is 5°. Moreover, the refractive index of air is assumed to be 1.0, and the indices of silica, silicon and photoresist are 1.47, 4.76, and 1.69 at 442 nm wavelength respectively. In the simulation region, a non-uniform mesh is automatically generated by Lumerical FDTD Solutions. The mesh step is about λ0 10n , λ0 is light wavelength in air, and n is material index, such that areas with a high index will have a smaller mesh step size. Perfectly matched layers are selected as the boundary condition for the simulations. The fundamental mode source at 442 nm wavelength is injected into fiber probe, and the peak electric field amplitude is set to be 1 V/m.

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Fig. 2. (a) Index model of direct writing in x = 0 plane for FDTD simulation. Fiber diameter is 300 nm and wavelength is 442 nm. (b) Py in the model of Fig. 2(a) simulated by FDTD. (c) Pz in the model of Fig. 2(a) simulated by FDTD.

The energy flows from fiber probe into photoresist layer are firstly investigated. For the submicron-diameter fibers considered here, the average energy flow in radial direction is zero. Thus, for the model of Fig. 2(a), the energy flow in the X-direction is not considered. The Ycomponents (Py) and Z-components (Pz) of Poynting vectors in x = 0 plane are obtained by FDTD simulations, shown in Fig. 2(b) and 2(c) respectively. For the submicron-diameter fiber contacted with photoresist, there is not any significant amount of light energy propagating to the air, because the photoresist with the larger index than silica fiber can more easily couple the light from submicron-diameter fiber [11]. Since the submicron-diameter fiber is almost parallel with Y-axis, Py is much larger than Pz in the fiber. However, the coupling efficiency of Py into the photoresist layer is lower than Pz, because the lateral of submicron-diameter fiber is the contact part with the photoresist, which is more favorable for the coupling of Pz. The maximum Pz can be coupled into photoresist layer with very small loss, while the maximum Py in the photoresist layer is less than half of that in the fiber. Moreover, Fig. 2(c) shows that the Pz can be easily coupled into photoresist layer by evanescent fields in the noncontact part between fiber and photoresist, but in Fig. 2(b) this phenomena is not obvious.

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Fig. 3. (a) Py in the middle plane of the photoresist layer guided by 300-nm-diameter fiber. (b) Pz in the middle plane of the photoresist layer guided by 300-nm-diameter fiber. (c) Py in the middle plane of the photoresist layer guided by 450-nm-diameter fiber. (d) Pz in the middle plane of the photoresist layer guided by 450-nm-diameter fiber.

The power distribution in the photoresist layer is essential to determine the width of exposed lines. Figure 3 shows the simulated Py and Pz in the middle plane of the photoresist layer (z = −0.05 µm). Figure 3(a) and (b) are Py and Pz guided into photoresist layer by 300nm-diameter fiber. Figure 3(c) and (d) are the ones guided by 450-nm-diameter fiber. Standing wave patterns are clearly presented in the Fig. 3(b) and (d), which are caused by the interference between the forward light and the backward light reflected by the fiber endface. Compared with Py, Pz has more remarkable standing wave pattern. Py is the vector cross products of the magnetic field intensity in the Z-direction (Hz) and the electric field intensity in the X-direction (Ex). Py has an approximate vertical incidence on the fiber endface. After the reflection by endface, Hz has a half-wave loss, but Ex does not, and thus the standing wave patterns of Hz and Ex are staggered. Therefore, for the Py, standing wave patterns are weakened. Pz has an approximate parallel incidence on the fiber endface. Both magnetic field intensity in the Y-direction (Hy) and electric field intensity in the X-direction (Ex) do not have half-wave loss and thus their standing wave pattern coincide. Therefore, the standing wave patterns of Pz are clear. As Fig. 3 shown, the light spots guided by 450-nm-diameter fiber are wider than the ones guided by 300-nm-diameter fiber in X-direction. It is obvious that 300-nm-diameter fiber has tighter confinement to light than 450-nm-diameter fiber. In fact, the beam widths in the photoresist layer can be tuned by the fiber diameter, which ultimately determine the resolution of the direct writing. It can be seen that the light spots guided by 300-nm-diameter fiber are longer than the ones guided by 450-nm-diameter fiber in Y-direction. While a 450-nmdiameter fiber confines major power inside the fiber, a 300-nm-diameter fiber leaves a large amount of light guided outside as evanescent waves, which can be easily coupled into

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photoresist. The evanescent waves coupled into photoresist form the conical tails of light spots, so 300-nm-diameter fiber has a longer light spot than 450-nm-diameter fiber. In addition, the maximum energy guided by 300-nm-diameter fiber approximates to the one guided by 450-nm-diameter fiber, so the change of fiber diameter has not remarkable influence on exposure dose.

Fig. 4. Widths of light spots (measured in the middle plane of photoresist layer) versus fiber diameters. (a) Calculated results of Py. (b) Calculated results of Pz.

Based on the simulations, the widths of light spots with respect to the diameters of the fiber probes are investigated. The width of light spot is defined as the full width at half maximum (FWHM) of the Poynting vector. As Fig. 4 shown, when the fiber diameter is about 300 nm, both Py and Pz obtain minimum widths, which are 278 nm and 266 nm respectively. 3. Experimental setup

Fig. 5. Schematic of experiment setup for submicron-diameter fiber direct writing.

Figure 5 shows the schematic setup of direct writing using submicron-diameter fibers. A dilute photoresist (AZ MIR701) is spun onto a silicon substrate to obtain a 120-nm-thick photoresist layer. The substrate is then placed on XYZ translation stage driven by 3 highprecision motors with a resolution of 0.05 µm/pulse. The submicron-diameter fibers probe is fixed above the stage and keeps a small angle (~5°) with the substrate plane. He-Cd laser (442-nm-wavelength) is selected as the light source, after which an acoustooptic modulator (AOM) is placed immediately to control the light power. After passing through the AOM, the laser is coupled into single mode fiber (SMF) by a microscope objective and ultimately reaches the submicron-diameter fiber probe. Initially, the stage moves in the Z-direction to approach the submicron-diameter fiber probe until the probe tip touches the photoresist layer, which is monitored by the microscope cameras. After monitoring alignment, the stage starts #116534 - $15.00 USD

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moving along the Y-direction. Due to the micro touch mode during moving, the submicrondiameter fiber guides the light into photoresist layer and a narrow line is exposed. Once one line has been written, the substrate must be immediately separated from the probe tip. Then the stage moves a distance alone X-direction and repeats the above process to write another line. Compared with NSOM direct writing [23], the submicron-diameter fiber direct writing has three remarkable advantages. First, it does not need feedback control of the distance between probe and substrate during writing. Second, because light power guided into the photoresist is larger than near-field irradiation, the writing speed can be dramatically higher. Third, using motor for driving stage rather than piezoelectric ceramic can largely expand the writing area. The main challenge of this direct writing technique is fabrication of the submicrondiameter fiber probe. Usual silica micro- and nanofibers are fabricated by flamed-heated drawing of single-mode fibers [11]. Due to their length and flexibility, these micro- and nanofibers cannot support themselves in the air. In order to increase its stiffness, an ideal submicron-diameter fiber probe for direct writing must gradually change its diameter to a submicron tip. Here we report a two-step fabrication process of the submicron-diameter fiber probes. First, while heated by a flame, the silica fiber is draw to a taper with a diameter about 3 micrometers. Then, we use the Hydrofluoric (HF) acid solution to etch the taper until its diameter is reduced to submicron-dimension. 4. Experimental results and discussions

Fig. 6. (a) SEM image of the lines written by submicron-diameter fiber. The writing parameters of these lines are same: 16 µm/s writing speed and 40 nW probe output power. (b) Cross section profile of Fig. 6(a) measured by AFM. (c) SEM image of the lines written by submicron-diameter fiber at different speeds. The writing speeds of these lines from left to right are 20 µm/s, 12 µm/s, 12 µm/s, 9 µm/s respectively. Probe output power is 50 nW. (d) Cross section profile of Fig. 6(c) measured by AFM.

The direct writing samples using submicron-diameter fiber are characterized by scanning electron microscopy (SEM) and atom force microscopy (AFM). Figure 6(a) shows SEM image of lines written by the submicron-diameter fiber probe shown in Fig. 1(b). The writing parameters of all lines are same: writing speed is 16 µm/s and output power of probe is 40 nW. The probe output power is measured by power meter under the condition that probe tip contacts with detector surface. As Fig. 6(a) shown, the lines have uniform width, which is 510 nm shown in the inset. Figure 6(b) shows cross section profile of the lines measured by AFM. #116534 - $15.00 USD

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The depth of line marked by red triangle is 117 nm, which indicates that the whole depth of the photoresist layer is exposed. The 500-µm-length of these lines illustrates this method can be utilized to fabricate patterns in a large area. Figure 6(c) is SEM image of lines written by the same probe as Fig. 6(a) at different speeds. The output power of probe (50 nW) is invariable. The writing speeds of the lines from left to right are 20 µm/s, 12 µm/s, 12 µm/s, 9 µm/s respectively, and the widths of corresponding lines are 380 nm, 680 nm, 680 nm, 850 nm respectively. Faster writing speed provides lower exposure dose so that the two sides of the light fields with lower power (Fig. 3) cannot expose the photoresist, so the faster writing speed, the thinner line. The two middle lines illustrate that same writing speeds can keep the widths of different lines stable. Figure 6(d) shows cross section profile of these lines measured by AFM, in which the depth of line marked by the red triangle is 116 nm. This group of lines demonstrates that the widths of lines written by submicron-diameter fiber can be controlled. Moreover, the controllability of line widths makes the fabrication process of complex patterns simple and convenient.

Fig. 7. SEM image of lines with minimum width. The diameter of probe tip is about 300 nm and the writing parameters are 20 µm/s writing speed and 50 nW probe output power.

The spatial resolution of direct writing, which can be defined as minimum width of lines, is an important issue. The minimum width of lines is limited by the output pattern of submicron-diameter fiber, which has been numerically calculated by FDTD method in section 2. It shows that, the beam width in the photoresist layer can be tuned by the fiber diameter, and the 300-nm-diameter fiber emits light with the minimum width. Besides fiber diameter, the exposure dose can change the width of lines, but it does not mean that reducing exposure dose can constantly improve resolution. For the transfer of patterns from photoresist to substrate, exposure dose must be big enough to expose the whole depth of photoresist layer (120 nm). In the experiments, we fabricate a fiber probe with approximate 300-nm-diameter, with which a group of lines are exposed (Fig. 7). The writing speed and the probe output power are set to be 20 µm/s and 50 nW respectively, which are optimized for suitable exposure dose. As shown in Fig. 7, the widths of these lines are 350 nm, which are the minimum width we have obtained in the experiments. The results demonstrate that the resolution of submicron-diameter fiber direct writing can be less than exposure wavelength (442 nm). Thus, submicron-diameter fiber direct writing has the advantage of subwavelengthresolution over laser direct writing.

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Fig. 8. Optical microscope image of author’s name written by submicron-diameter fiber. Lines in different directions are aligned by two microscope cameras.

In addition, complex submicron patterns can be fabricated by altering writing direction of lines, which can be attributed to rotating the fiber probe around Z-axis. However, if the fiber probe is rotated, its tip will not coincide with the initial point. Here we adopt two microscope cameras to locate the probe tip, as Fig. 5 shown, one camera is fixed along Z-axis and the other keeps a small angle with X-axis. At first, the initial point of the probe tip is marked by the two microscope cameras. After rotation, the probe is continuously moved in threedimensional directions under the monitoring of cameras until its tip coincides with the initial point. The probe alignment precision is influenced by two factors: the imaging resolutions of microscope cameras and the displacement resolutions of translation stages. However, the size of light spot on the probe tip ultimately determines the alignment resolution. Using this method, the author’s Chinese name has been written (Fig. 8). For easily observation with microscope, the name has thicker strokes (~1.2 µm). Because the microscope cannot image the whole name in one field of view, the pictures of two Chinese characters are separately taken and then put together in Fig. 8. It can be seen that the strokes of these Chinese characters are well aligned. For the second character, there are little stroke protrusions in character’s top and right sides, the reason of which is that the end points of lines are slightly extended by the conical tails of light spots (Fig. 3). In addition, the intersection points of strokes are thicker than the other parts of them due to twice exposure. 5. Conclusions

In conclusion, the direct writing technique using submicron-diameter fibers is a novel application of interrupted micro- and nanofiber. Unlike traditional uninterrupted micro- and nanofiber, submicron-diameter fiber for direct writing serves as a tightly confined point source, which adopts micro touch mode to expose photoresist. Experiments demonstrate that submicron-diameter fiber direct writing has some advantages: simple process, high resolution, large writing area, and controllable width of lines. In addition, by altering writing direction of lines, complex submicron patterns can be fabricated. Moreover, the submicron-diameter fiber probes presented in this paper not only can be used for direct writing but also are helpful for potential applications in nanoprobe sensing, laser trapping and laser nanosurgery. Acknowledgments

This research was supported by two National Natural Science Foundations of China (No. 60778030 and No. 60678037).

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