1. Introduction

Diffractive–refractive hybrid optical elements have received much attention as key components for optical pickup systems, integrated photonic sensor devices, and high-density storage. For example, hybrid lenses are expected to be used as multifocal lenses and achromatic lenses [1, 2]. Hybrid devices based on inorganic optical materials, such as SiO₂ and Si, show particular promise because of their high transparencies and physical and chemical stability. For fabrication of hybrid lenses from inorganic materials, Fresnel zone plates must be formed onto convex lenses. The semiconductor process, which is frequently used for the production of micro-optical elements, is effective for the fabrication of complex fine structures on planar substrates. However, it is rather difficult to extend the current semiconductor process to nonplanar substrates because of the difficulty in achieving uniform resist coating.

Radtke et al. reported the fabrication of diffractive lenses of resin on bulk convex lenses with small curvatures using an ultraviolet laser and specially modified stage systems [3]. A spray-coating technique was also proposed to obtain uniform resist thickness, even on nonplanar structures. However, the spatial resolution of this technique is not high (several tens of micrometers) [4,5]. These processes use linear optical process, and therefore require complex exposure systems.

Nonlinear optical phenomena such as two-photon absorption occur only near the focal volume when femtosecond laser pulses are focused tightly into transparent photosensitive resins. This phenomenon enables us to expose the internal region of the resin directly. Thereby, we can create actual three-dimensional (3-D) polymeric structures only by scanning the laser-spot inside the resin. To date, many reports have described 3-D micro-structures and nanostructures including bull, Venuses, tweezers, springs, photonic crystals using photopolymerizable resin SCR-500, photoresist SU-8, and hybrid materials [6-11].

In this study, we use femtosecond laser lithography-assisted micromachining (FLAM), a combined process of nonlinear lithography and plasma etching, for fabrication of nonplanar structures of inorganic materials. The fabrication of SiO₂-based diffractive–refractive hybrid microlenses was also demonstrated.

2. Experimental

For lithographic processing, we used a femtosecond fiber laser system. The wavelength, pulse duration, and repetition rate were, respectively, 780 nm, 68 fs, and 50 MHz. The laser beam was focused using an objective lens with a numerical aperture (NA) of 0.5. The focal spot diameter and focal depth of the optical setup were 2.6 µm and 14 µm, respectively, for our focusing conditions. SiO₂ glass plates of 1-mm thickness and SiO₂ microlens arrays were used. The diameter, height, curvature radius, and focal length of each individual lens were 240, 18.9, 380, and 830 µm, respectively. We used an epoxy based chemically amplified negative-tone photoresist KMPR-1050 (KAYAKU MicroChem. Co., LTD.). SiO₂ was etched by electron cyclotron resonance (ECR) plasma using CHF₃ gas.

 

Fig. 1. Schematic illustration of femtosecond laser lithography-assisted micromachining.

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3. Results and discussion

Figure 1 presents a schematic illustration of FLAM for microfabrication on nonplanar substrates. First, the negative-tone resist is spin-coated onto the substrates [Fig. 1(a)]. Then, patterns are written directly inside the resist using femtosecond laser-induced nonlinear optical processes [Fig. 1(b)]. These resist patterns are transferred to the underlying substrates (or thin films) by plasma after post-exposure-baking and development treatment [Figs. 1(c) and 1(d)]. For conventional laser/electron-beam lithography, linear absorption is used. Therefore, incident light or an electron beam is absorbed from resist surfaces in accordance with the Lambert–Beer law, which causes difficulty in the dose control and precise alignment of the exposure position. Compared to this, in FLAM, nonlinear optical absorption enables us to directly expose any region, even in a thick resist. Although 3-D microfabrication by use of single photon absorption was reported, compared to this, much higher spatial resolution below 100 nm can be achieved by using femtosecond laser nonlinear processes [6,12]. By combining this nonlinear lithography with plasma etching, various nonplanar structures of inorganic materials can be formed, including hybrid lenses and anti-reflection nanostructures. The resist patterns are eventually removed as sacrificed layers after the pattern transfer. Therefore, we used KMPR-1050 because of its lower adhesive strength than that of SU-8, which has been used frequently in femtosecond laser polymerization technique.

 

Fig. 2. SEM image of filamentary rod patterns of resist formed by femtosecond laser exposure. The z-position of the focal spot for writing each pattern is indicated. The laser was not translated along the z-axis during the writing process.

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It has been reported that fiber-like patterns are formed when a photopolymerizable resin such as SCR-500 is exposed to ultraviolet laser light under certain condition [13,14]. This channel propagation phenomenon enables us to expose a high-aspect-ratio region of materials. Figure 2 shows an SEM image of filamentary rod resist patterns of various heights, formed by femtosecond laser exposure. These rods were written by exposure from above along the z-axis (optical axis) in a 50-µm-thick layer of resist, as shown in the inset. The average laser power and exposure time were 27.4 mW and 1 s, respectively. The diameters of the rods were approximately 5 µm. The dotted line in the figure delineates the resist surface before development, indicating that the rods were formed directly inside the resist. It must be emphasized that we did not scan the laser spot along the z-axis during pattern writing of each rod. The figure indicates the focal position at which each rod was written. Here, the substrate surface is defined as the origin of the z-axis. In this experiment, the maximum rod height obtained was 47 µm, which was more than three times higher than the focal depth of our exposure setup. Such a height cannot be explained sufficiently by the elongation of the focal depth due to the refractive index difference between the resist and air. On the other hand, we could not find a clear threshold value of laser power for this channel propagation, suggesting that the formation of these rod patterns was not related to the optical Kerr effect. Kewitsch et al. reported the formation of high-aspect-ratio photopolymer fibers, and explained this phenomenon in terms of self-trapping based on photopolymerization [13]. The refractive index of photopolymerized regions increases through the Lorentz–Lorenz relation as a result of the cross-linking reaction of monomers. Thus, the photopolymerized regions act as a waveguide core, leading to the channel propagation of the subsequent laser pulses. Unlike the photopolymers used in their studies, in chemically amplified resists such as KMPR and SU-8, a cross-linking reaction that uses photogenerated acid as a catalyst occurs after post-exposure baking. That is, a large increase in the refractive index cannot be expected from exposure alone in this type of resist. However, Seet et al. reported the annealing effect of femtosecond laser exposure, and demonstrated the formation of SU-8 patterns by a laser-induced thermal cross-linking reaction without a post-exposure baking step [15]. The thermal cross-linking temperature of KMPR was measured to be approximately 170°C, which is close to that of SU- 8 (167°C). Therefore, in our experiments, similar thermal cross-linking reaction can occur by femtosecond laser exposure.

Although there have been studies on femtosecond laser polymerization of SU-8, only a few studies have reported the formation of high-aspect-ratio rod. The increase in refractive index of KMPR films, before and after post-exposure baking, was as low as approximately 2.0×10^-3 (in the case of an ultraviolet lamp source), which was measured by the prism coupling method, using an 832-nm-wavelength light source. Therefore, femtosecond laser pulses must be focused using a low-NA objective lens to achieve highly efficient coupling of the pulses into a guided mode of the polymerized waveguide (rod). Many researchers have used high-NA (>1.2) objective lenses to obtain high spatial resolution; however, this mismatch of NA is likely to impede the channel propagation of laser pulses. In fact, Matsuo et al. reported the formation of high-aspect-ratio SU-8 rods (>90-µm high) using a low-NA (0.30) objective lens [16]. The filamentary rods obtained in our study were most likely to be created by the femtosecond laser-induced thermal cross-linking reaction of KMPR and highly efficient coupling of the subsequent laser pulses into a guided mode of the polymerized waveguide using a low-NA objective lens.

Dependence of the z-position of the focal spot at which the patterns were written on the rod heights is presented in Fig. 3. The laser writing speed was 10–30 µm/s at the average power of 27.4 mW with a 22.5-µm-thick resist film. The heights increased linearly with the focal position at any writing speed except for the resist surface. Furthermore, the rod diameter did not depend on the depth-position of the focal spot in the resist as long as the writing speed and average power were equal. The absorption edge of KMPR is located around 370 nm wavelength, and there is almost no absorption at laser wavelength 780 nm, which mean that the resist patterns were not formed via linear optical process. In this experiment, the diameters of the central parts of the rods were approximately 2 µm.

 

Fig. 3. Dependence of the z-position of the focal spot on the filament height. The dotted line marks the resist surface before development.

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Using this nonlinear lithography, the binary micro-Fresnel patterns were written inside the 20-µm-thick resists on convex microlenses for SiO₂-based diffractive–refractive hybrid microlenses. The writing procedure for the Fresnel lens patterns is portrayed schematically in Fig. 4. Here, we scanned the laser-induced rod three-dimensionally inside the resist. The Fresnel lenses consist of a series of concentric circles of different radii and heights. The laser writing speed and average power were, respectively, 300 µm/s and 33 mW. The rods had sharp ends, as depicted in Fig. 2. For example, the diameters of the top and central parts of the rods were 0.5 and 2.0 µm, respectively, in the case of the writing speed of 100 µm/s and average power of 27.4 mW. Therefore, the separation between two adjacent circles was determined to be 300 nm, which was less than the diameter of the top part of the rod. The layer spacing was 3.0 µm, which was smaller than the rod height. The focal length of the hybrid lens can be designed by Eq. (1) [17]:

where f_h, f_F, and f_m are the primary focal lengths of a hybrid lens, a Fresnel lens, and a convex lens, respectively. From Eq. (1), the focal length of the Fresnel lens should be 2400 µm in order to obtain a hybrid lens with a focal length of 617 µm. The radius r_m of the m-th zone of the binary Fresnel lens can be expressed by Eq. (2):

where λ ₀ is the operating wavelength, 632.8 nm. Figures 5(a)–5(c) show SEM images of a SiO₂-based diffractive–refractive hybrid microlens after pattern transfer and a resist lens structure. The ECR power, bias voltage, and gas pressure were 100 W, 700 V, and 2.0×10^-2 Pa, respectively. The etching depth was approximately 1 µm. We obtained well-defined structures with smooth surfaces on curved substrates. When a 632.8-nm-wavelength He-Ne laser light was normally coupled to the hybrid lens, the primary focal length was found to be 614 µm. Due to the hybridization, the focal length changed by 216 µm. This amount of shift is close to theoretical value of 213 µm. FLAM uses a nonlinear optical process; therefore, it achieves much higher spatial resolution (below 100 nm), which is far beyond the diffraction limit [6]. Using high flexibility and resolution, nonplanar micro/nanodevices with height on the order of several hundred micrometers could be obtained by improving and optimizing the FLAM process.

 

Fig. 4. Schematic illustration of laser writing procedures for the formation of micro-Fresnel lens patterns inside the resist on convex lenses. The Fresnel lens consists of a series of concentric circles of different radii and heights. The patterns were written by scanning the laser-induced rod three-dimensionally inside the resist.

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Fig. 5. SEM images of (a) overview, (b) enlarged view of a SiO₂-based hybrid lens, and (c) resist structures before the pattern transfer.

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4. Conclusions

In conclusion, SiO₂-based diffractive-refractive hybrid microlenses were fabricated by femtosecond laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. Micro-Fresnel patterns were written directly inside the resist on 18.9-µm-thick convex lenses by using femtosecond laser pulses. Here, we scanned a high-aspect-ratio rod three-dimensionally. Then, the patterns were transferred to the underlying lenses by CHF3 plasma. We obtained nonplanar structures with smooth surfaces. When a 632.8-nm-wavelegth He-Ne laser light was coupled to the lens, the focal length of the hybrid lenses became shorter than that of the original ones by 216 µm, which agreed with theoretical value. More functional nonplanar photonic devices of inorganic materials can be realized by improvements in the FLAM process.