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We demonstrate a new 3D fabrication method to achieve the same results as those obtained by the two-photon excitation technique, by using a simple one-photon elaboration method in a very low absorption regime. Desirable 2D and 3D submicrometric structures, such as spiral, chiral, and woodpile architectures, with feature size as small as 190 nm have been fabricated, by using just a few milliwatts of a continuous-wave laser at 532 nm and a commercial SU8 photoresist. Different aspects of the direct laser writing based on ultralow one-photon absorption (LOPA) technique are investigated and compared with the TPA technique, showing several advantages, such as simplicity and low cost.
© 2013 Optical Society of America
1. Introduction
Optical microscopy and nanoscopy play a major role in modern science and technology, with numerous applications including optical imaging, confocal scanning microscopy, optical data storage, micro- and nano-fabrication, etc [1]. Conventional high-aperture microscopy (up to NA = 1.4 with oil immersion) is nowadays close to reach its maximum with large wide aplanetic (= diffraction limited) fields available in the best objectives lenses (OLs). A focusing spot with an effective volume of less than 1 μm3 is then achieved. Such high NA OLs allow addressing or elaborating an object with a submicrometer resolution through a large volume. For this purpose, the focused laser beam is usually raster-scanned across the target sample in all dimensions. Furthermore, for imaging applications, the use of a confocal laser scanning microscope (CLSM) provides depth discrimination and improves the longitudinal resolution within the focal plane [2].
Two excitation ways have been implemented up to now for photo-induced fabrication of submicrometric structures, namely one-photon and two-photon absorption (OPA, TPA), involving different excitation mechanisms and aiming at specific applications. Indeed, when using a thin film, the OPA excitation method is a very convenient technique, based on a simple and low cost laser source operating at a wavelength located within the absorption band of the thin film material. However, due to a strong linear absorption, in the 104 – 105 cm−1 range, this method is limited to the surface of the sample, i. e. to a two-dimensional (2D) scanning. High spatial resolution three-dimensional (3D) imaging or fabrication is therefore impossible with the OPA method applied to strongly absorbing samples. In contrast, TPA excitation provides intrinsic 3D addressing, thanks to a quasi absence of linear absorption and a local nonlinear absorption [3], which occurs only for the very high local intensity provided by tight focusing in a CLSM combined with a femtosecond laser operating at a wavelength around 800 nm for which the material is kept fully transparent [4]. This TPA nowadays allows many potential applications in different domains, in particular, for 3D imaging and 3D fabrication of submicrometric structures [4–10], but requires expensive laser sources and delicate specialized optics.
In this work, we propose a new way to benefit from the advantages of both excitation techniques, by using a simple, low cost one-photon elaboration method operating in an ultralow absorption regime. This technique employs a continuous laser only, as in the case of OPA, while allowing 3D addressing as in the case of TPA
2. Working principle of LOPA microscopy
In the optical confocal microscope system, the propagation of light towards the focusing region is formally described by a nonlinear wave equation for the optical electric field. The intensity increases nonlinearly and reaches a maximum value at the focal plane. For a high NA OL with a cm-size pupil, the light intensity at the focusing plane can be about 108 times higher than that at the input beam. Inside a material, the excitation efficiency and the resulting effect, e.g. fluorescence or photopolymerization, is strongly dominant at the focusing region. In some cases, the photo-induced effect is only observed at the limited size focusing spot where the intensity is high enough to induce physical or mechanical or chemical changes. A good example is the use of an infrared pulsed laser to induce TPA in the focusing region. In an overwhelming majority of cases, only such TPA techniques have been applied to fluorescence 3D microscopes [3, 10], to 3D data storage [9], and to 3D optical lithography [4–8]. Until now, it was not possible to use OPA to achieve the same results as those obtained by TPA technique. In fact, in the case of OPA, the excitation wavelength is often chosen at high absorption regions of the studied material, to minimize exposure and time requirements. As a consequence, most incoming photons are absorbed beyond a propagation length of a few μm. The intensity then exponentially decreases along the propagation direction of the light and may even vanish at the focusing region. The OPA has been therefore used only to deal with a thin material layer, which is repositioned in the focal plane of the optical system [11]. However, we may notice that in this OPA technique, the material presents a strong linear absorption at the excitation laser wavelength. Therefore, optically induced physical or chemical effects are observed even if the light intensity is weak, not only at the focusing spot but within the whole irradiation region.
We propose to use the specific advantage of OPA technique, i. e., a simple and low cost laser, to achieve 3D addressing by a combination of an ultralow absorption effect and a tightly focusing spot. Indeed, the absorption of photosensitive materials has marked spectral features. At the red edge of the absorption band, the absorption is very weak, or close to zero. If a laser beam, whose wavelength is positioned in this range, is applied to CLSM, the light intensity distribution remains almost the same as in the absence of material, i. e., the intensity is increased by a factor of 109 at the focusing spot. Although the absorption is ultra weak, the effective photo-induced effect in focusing region is therefore comparable to that obtained by a laser beam with a high absorption effect. Of course, nothing happens at other out-of-focus areas due to ultralow absorption effect. Figure 1(a) illustrates a typical CLSM, in which a simple continuous and low power laser is used. The laser beam is tightly focused into a submicrometer spot by using a high NA OL (NA = 1.3, oil immersion). Figure 1(b) shows the intensity distribution at the focusing region, simulated by using the vector Debye method [12]. In this calculation, we have included the absorption of the material to be used (σ, absorption coefficient) and investigated the effect of this parameter on the intensity distribution. The result shown in Fig. 1(b) was obtained with a very low absorption coefficient (σ = 723 m−1), corresponding to the value measured in our experiment. In the presence of a low absorption coefficient, the focal spot shape remains almost the same within a 300 μm depth inside the material. The typical size of the focal spot is 0.4λ (transverse) and 1.33λ (longitudinal), where λ is the wavelength of the excitation laser. Due to the ultra-low absorption of the material at this wavelength, the excitation intensity (accordingly the laser power) must be high. However, thanks to the use of a high NA OL, just a few milliwatts are enough to achieve a huge light intensity at the focusing area, and to induce photochemical effects in that region. Furthermore, since we are dealing with a linear absorption effect, only a simple continuous laser is needed, and the results obtained by this low OPA (called hereafter LOPA) will be same as those obtained by TPA method, in which an expensive femtosecond laser and accompanying optical components are required.
Fig. 1 (a) Sketch of the experimental setup used to fabricate desired sub-micrometer 3D structures. PZT: piezoelectric translator; OL: oil immersion microscope objective (×100, NA = 1.3); DM: dichroic mirror; M: mirror; QWP: quarter-wave plate; S: electronic shutter; L1-L3: lenses; PH: pinhole; F: 580 nm long-pass filter; APD: silicon avalanche photodiode. (b) Simulation of intensity distribution at the focusing region using vector Debye method, including the absorption of the material (σ = 723 m−1). The two curves represent the intensities distributions along transverse (x- or y-axis) and longitudinal (z-axis) directions, respectively, at a 10λ depth.
3. Demonstration of LOPA based microfabrication
This LOPA technique can obviously be used for all 3D applications, including 3D imaging and 3D fabrication. In this paper, we present the use of this technique for the fabrication of some relevant 2D and 3D submicrometer structures, as a proof of principle.
For such a demonstration, two conditions are required: a photoresist that presents an ultralow absorption at the wavelength of the excitation laser, and a high focusing CLSM. For the first condition, we have found that SU8, a commercial photoresist, is an ideal candidate, thanks to its ultralow absorption in the visible range, in particular at 532 nm, which is the wavelength of a very popular and low cost solid state frequency-doubled neodymium laser. Figure 2(a) shows the absorption spectra of SU8, on a logarithmic scale, highlighting the very low absorption coefficient, σ = 723 m−1, at λ = 532 nm. Note that the absorption of commercial SU8 photoresist has been remeasured [13,14] but little or not attention was paid to its ultralow absorption in the visible range. The second condition is fulfilled by using a CLSM presented in Fig. 1(a). In such setup, an avalanche photodiode is used to detect very low numbers of photons, which can be a reflection or emission light from the focusing spot.
Fig. 2 (a) Absorption spectrum of SU8 photoresist, plotted on a logarithmic scale. (b) Fluorescence photon number versus excitation intensity. Square dots are experimental data and continuous line is a linear fit.
We first demonstrated, by analyzing fluorescence emission, that the SU8 photoresist linearly absorbs the excitation laser at 532 nm wavelength. Figure 2(b) shows the fluorescence intensity of the SU8 photoresist as a function of the excitation intensity, obtained by using the CLSM shown in Fig. 1(a). The fluorescence spectrum has been also measured, but not shown here. The same measurement is realized on a glass substrate in order to verify the fluorescence emission of this photoresist. It is clear that the fluorescence emission linearly depends on the excitation at 532 nm, confirming thus an OPA effect. We notice that the intensity is on the order of 107 W/cm2 at the focusing region, and is only 10−2W/cm2 at the input of the optical system. The LOPA CLSM therefore allows the excitation and fluorescence detection of the focal spot volume only. The use of an avalanche photodiode (APD) is also a key point in fluorescence measurements in which the emission is quite low and cannot be detected by common p-i-n photodiodes.
4. Submicrometer 3D structures fabrication enabled by LOPA direct laser writing
By using a standard fabrication process, we then demonstrated that polymerization is achieved only at the focusing spot of the microscope objective, where the excitation intensity is largely sufficient to compensate the low linear absorption of the resist. Note that, in the case of TPA fabrication, there exists a first threshold related to light intensity, above which two photons are simultaneously absorbed, and a second threshold related to dose, above which complete photopolymerization is achieved. However, it should be borne in mind that resist exposure is also per se a highly nonlinear process. Indeed, there exists a dose threshold above which the polymerization process can be fully completed. An example of the complete polymerization condition is the use of the interference technique with a low power continuous-wave laser to fabricate 2D and 3D structures [15–17]. Therefore, in the case of LOPA, there exists only one threshold related to dose. Furthermore, thanks to a high intensity of the focusing spot, the complete photopolymerization is only achieved in this region. Figure 3 shows an example of experimental results, imaged by a scanning electron microscope (SEM). For each exposure, a solid structure, called “voxel”, corresponding to a focusing spot, is obtained. By changing either the excitation power or the exposure time, i. e. the dose, the voxel size and shape can be adjusted, as shown in Figs. 3(a) and 3(b). Note that, in order to obtain and to evaluate these small voxels on a glass substrate, we fabricated the same voxel array at different z-positions and with different exposure times. Only 2.5 mW of a continuous green laser is needed to create these structures, for an exposure time of about 1 second per voxel. Figure 3(b) shows, for example, a voxel array realized at z4-position, with three different exposure times. The dose dependence can be explained theoretically by calculating the iso-intensity of the focusing spot at different levels, as shown in Fig. 3(c). Three kinds of voxels obtained with t1, t2 and t3 as shown in Fig. 3(b) correspond to three different iso-intensities, namely 0.94, 0.63 and 0.44, respectively. The evolution of voxel size and shape observed experimentally fully confirms this operation in the OPA regime. Indeed, in the case of TPA, the creation of bone-like voxel shape, corresponding to that obtained with t3, for example, requires very high excitation intensity and could not be easily realized due to the TPA intensity threshold, as already mentioned above. In the case of LOPA, all these voxels shapes are obtained by simply adjusting the exposure time while keeping a low laser power. To obtain small voxels as shown in Fig. 3(d), shorter exposure times, t1, for example, should be applied. Of course, the exposure time required to create submicrometer structures varies as a function of the laser power.
Fig. 3 Fabrication of voxels at different z-positions and with different exposure times. (a) Identical voxels array realized at different z-positions. (b) Example of a voxels array at z4 showing the effect of exposure time, i.e., dose. The yellow color curves indicate the shape of the voxel as a function of the exposure time (t1, t2 and t3, etc.). (c) Theoretical calculation of the contour plot of light intensity at the focusing region of the used OL (NA= 1.3, n= 1.515, λ = 532 nm). This explains the evolution of the voxel shape as a function of the exposure dose as shown in (b). (d) An array of touching voxels fabricated with low dose (short exposure time, t1), showing an ellipsoidal form similar to results obtained by the TPA method.
Figure 4 shows the voxels size as a function of exposure time for three values of laser power. The exposure time is changed finely around time t1 as shown in Fig. 3(d). A voxel with a size as small as 190 nm is obtained with, for example, a power of 2.5 mW and an exposure time of 0.5 second, as shown in Fig. 4(b). The fabrication of smaller voxels is possible, but in this first demonstration this performance is limited, due to the quality of the instruments used (piezoelectric translation system, SEM, etc.) and also due to the poor adhesion of the small polymerized voxel on a glass substrate. The size of individual voxels is quite small when considering the wavelength (532 nm) used for the writing process. As for the linear dependence with intensity (OPA vs. TPA) [18, 19], the diameter-dose relationship agreement confirms this behaviors, as we have used the intensity I0 for our fit, instead of in the case of TPA.
Fig. 4 (a) SEM image of a voxels array fabricated at different exposure times and with P = 2.5 mW. (b) Exposure time dependence of voxel size, with different laser power values, P = 2.5 mW; 5 mW; 7.5 mW, respectively. The continuous curves are obtained by a tentative fit using the diameter-dose relationship for one-photon absorption [18, 19]. Insert shows a SEM image of a small voxel obtained with an exposure time of 0.5 second at a laser power of 2.5 mW.
With the above evidence that the LOPA polymerization is only realized at the focusing spot where the excitation intensity is very high, we scanned this focal spot in 3D to fabricate different desired structures. Figures 5 and 6 show, as examples, two kinds of submicrometer 3D structures fabricated by this LOPA direct laser writing (DLW) method. Clearly, 3D chiral or spiral structures are well created, similar to the results obtained by induced TPA polymerization. The structures features are well separated, layer by layer, in horizontal and in vertical directions. The feature sizes are about 300 nm (horizontal) and 650 nm (vertical), as shown in Fig. 5(c). For the fabrication of these structures, the focusing spot was scanned continuously with a scanning speed of about 1.34 μm/s. This LOPA technique is easily applicable to any desired 3D structure with sufficient void fraction. The limit in terms of void fraction is, likely, rather high, as we can guess for instance, from the absence, in the present attempt, of any difficulty related to the excess exposure of the regions above the main exposed regions, as apparent also in the voxel array (Fig. 4(a)). Furthermore, the use of a shorter laser wavelength (532 nm versus 800 nm) allows a reduction of the voxel size, which is related to the diffraction limit of the optical system.
Fig. 5 SEM image of a chiral structure fabricated with the following parameters: distance between rods = 2 μm; distance between layers = 0.75 μm; number of layers = 20; laser power = 2.8 mW. (a) View of the whole structure, (b) Zoom in on the top surface of the structure, and (c) Side view of the structure.
Fig. 6 SEM image of a spiral structure fabricated with the following parameters: diameter of a spiral = 2 μm; period of spiral in z direction = 2 μm; distance between centers of two close spirals = 2.5 μm; spiral height = 15 μm; laser power = 2.6 mW. (a) View of the whole structure, (b) Zoom in the top surface of the structure, and (c) Side view of the structure.
We note that Maruo and Ikuta [20] have employed a continuous-wave UV laser to fabricate 3D structures, but the mechanism is not understood in their work and the structure size is very large. Recently, two teams reported on the fabrication of SU8 structures, in which the authors claimed a TPA polymerization of SU8 photoresist by using a continuous-wave laser at 532 nm [21, 22]. However, as evidenced by our theoretical calculations and experimental results, the fabrication of SU8 structure with a continuous laser is actually due to the LOPA effect. Indeed, such a continuous laser emitting only a few milliwatts (≈ 107 W/cm2) cannot induce a decent third-order nonlinear effect, i. e. TPA, which usually requires a high peak intensity such as that from a femtosecond laser (≈ 1012 W/cm2). Of course, TPA on SU8 may occur at this 532 nm wavelength by using a femto or pico-second laser [22], but its efficiency is however very weak as compared with that of the linear absorption even if the OPA coefficient is very low.
5. Energy accumulation effect in LOPA direct laser writing
We have further investigated the fabrication of submicrometer structures when two voxels or two lines structures are created closely. In fact, the intensity distribution at the focusing region of a high NA OL in the focal plane, for example, is represented by an Airy spot surrounded by diffraction rings of weaker intensity. When the separation between two focal spots is reduced, approaching to the second- or first-order diffraction ring of the Airy spot (focal spot), typically 1 μm, a proximity effect is observed. Indeed, in the case of OPA, photons could be absorbed anywhere if they are present, with an efficiency depending on the linear absorption cross-section of the irradiated material. The absorbed energy is gradually accumulated as a function of exposure time. If a single exposure is made, a single voxel could be created corresponding to a volume in which the accumulated energy is larger than the polymerization threshold [15–17]. The area where the accumulated energy is lower than this polymerization threshold will be washed out during the development process. When multiple exposures are applied, the energy of this exposure will contribute to others, if they are very close, and the effective voxel size consequently increases. Figure 7 shows the theoretical calculations and experimental results of the distance dependence of the voxels size. A clear accumulation effect is observed when two voxels are separated by a distance shorter that 1 μm, resulting in a voxel of larger size. According to the same model as that used above, the full width at half maximum of each voxel is increased from 190 nm to 300 nm when the separation changes from 2 μm to 0.5 μm. Moreover, for a short separation, the voxels array is not uniform from its center to the edge part, as can be seen in case of 0.5 μm. Such a consequence is well-known for the proximity effects in electron-beam lithography for instance and was recently considered for deep UV used for photonic chips in a microelectronic multichip-project foundry [23]. Small voxels separated by less than 0.5 μm-separation are possible. But in this case, a proximity correction, i.e., a control of exposure time or power and a compensation of the dose between different voxels, should be applied.
Fig. 7 Dependence of voxel sizes on separation distance between voxels, showing the influence of energy accumulation from this focusing spot to others. Insets show simulated images (blue background) and SEM images (black background) of 2D submicrometer voxels array fabricated with different distances between two voxels: 2 μm; 1 μm; 0.5 μm.
Finally, as mentioned above, this LOPA technique can be applied for any 3D optical addressing. The fabrication of arbitrary 3D structures by this simple technique is just a particular but also very important application. Indeed, the LOPA CLSM could be used to image submicrometer structures in 3D by using a very modest laser power, which allows avoiding the destruction of studied materials, in particular in biology. An other application could be also envisioned is 3D data storage, for which LOPA allows minimize the optical system by using a very simple diode laser at suitable wavelengths.
6. Conclusion
As a conclusion, we have developed a simple and low cost 3D laser writing process based on OPA phenomena in a weakly absorbing photoresist material. This novel technique enables the fabrication of objects as small as 190 nm, and of submicrometer 2D and 3D arbitrary structures, similar to those obtained by TPA technique. The demonstration of this LOPA direct laser writing technique is realized by a continuous-wave laser at 532 nm with only a few milliwatts and a commercial SU8 photoresist. Our work also brings a clear understanding about the fabrication of 3D structures using a continuous laser. We could notably evaluate that the proximity effects arise only in relatively “full” structures, such effect being not observed in “sparse” structures. The idea of using LOPA also opens a new and inexpensive way to address optically 3D structures, namely 3D fluorescence imaging.
Acknowledgments
This work has been supported by the “ Laboratoire d’Excellence NanoSaclay”, in the framework of the project “ONE-FAB-3D”, and by the “ Triangle de la Physique”, in the framework of the project “PUTTON”. Special thanks are devoted to J.-F. Roch for various equipment helps. M. T. Do and T. T. N. Nguyen acknowledge the fellowship from the Vietnam International Education Development “322 program” and Q. Li acknowledges the fellowship from the China Scholarship Council.
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