1. Introduction

The demand for integration of optical components has increased significantly in many applications that use micro-optical elements to enhance performance, reduce size, and reduce cost. For high coupling efficiency in optical fiber applications, optical microlenses have been widely considered a promising replacement for conventional bulky lenses for coupling light from free space into fibers. A microlens can also be integrated with electro-optic devices, such as in a laser diode and detector on the same chip, to provide greater functionality at a potentially low cost. As application requirements move toward larger and larger arrays of channels, users are moving away from discrete optics toward microlenses that are manufactured in arrays and mounted on substrates. Aligning each lens to each fiber is time consuming and therefore expensive; prealigned arrays of microlenses, based on automated technology for mass production, are thus a more attractive solution for high-precision alignment. Consequently, the cost-effective mass fabrication of high-quality micro-optical components has become highly important.

There are a variety of lithographic techniques for fabrication of micro-optical components; for example, direct writing techniques (UV laser and electron beam), photolithographic mask projection, and contact printing techniques are widely used for manufacturing micro-optical diffractive–refractive elements. Generally, photoresist-based fabrication involves two steps: first, the microstructures on the photoresist are patterned by an exposure and development processes; second, the structure is transferred onto the substrate by etching. It is known that sol-gel materials have an advantage over materials used in conventional photoresist-based fabrication techniques because sol-gels lend themselves to a single-step process without etching. Here a high-energy beam–sensitive (HEBS) gray-scale mask is introduced into the process to provide a reliable and simple method for producing three-dimensional (3D) surface-relief refractive and diffractive microlenses with a single UV exposure. Using the sol-gel material with the HEBS gray-scale mask has considerable potential for low-cost mass production of high-optical-quality silica-based micro-optical components. With the requirements of low cost and mass production in mind, we explored the characterization of the sol-gel material for use with the HEBS mask for microlens fabrication in a single-step UV exposure. The surface profile depths of the sol-gel were measured as a function of a range of optical densities to help us understand the fabrication of microlenses on a sol-gel with the HEBS gray mask.

2. Fabrication and Characterization

The fabrication process is schematically shown in Figure 1, where a refractive microlens determined by the mask optical densities transfers the surface profile information into the sol-gel thin film. During fabrication, a thin sol-gel film is first spin coated onto a substrate; the photosensitive sol-gel film is then exposed through a HEBS gray-scale mask. Different optical density values on the mask will determine different UV doses that are absorbed in the sol-gel film. Our SiO₂:TiO₂ sol-gel is a negative-tone photosensitive material; after development the areas of highest exposure are preserved because of cross linking, and those with the lowest exposure are dissolved in the developer. Details about the synthesis of the sol-gel film can be found in our previous work [1] and in other references [2–7].

 

Fig. 1. Schematic diagram of microlens fabrication using the HEBS gray-scale mask.

Download Full Size | PDF

The hybrid sol-gel glass was synthesized from two solutions. Solution I was a silicon oxide network, which was formed by the hydrolysis of prolylmethacrylate-substituted trimethoxysilane and 3-(trimethoxysilyl) propyl methacrylate in isopropanol and acidified water. In this case the volume ratio was 30:12:1 ml. Solution II was a titanium oxide network, which was formed by adding titanium propoxide (Ti(OCH)₄) to acetylacetone at a molar ratio of 1:4 in a nitrogen environment; the solution was agitated until homogenization was reached. The two solutions were then mixed with a molar ratio of 4:1 (SiO₂:TiO₂). It should be mentioned that titanium was used to modify the refractive index of the silicon oxide network. The final mixture was allowed to age at room temperature for 30 h with vigorous stirring. This negative-tone silicon titanium material was made UV photosensitive by adding the photoinitiator IRGACURE 184 (CIBA) with 4% wt. to the sol-gel. Large particles in the mixed solution were removed by a 0.1-μm membrane filter attached to a syringe before the solution was spin-coating onto a precleaned BK7 microscope glass slide. A thin film layer of the sol-gel film was spun onto a glass substrate at 1200 rpm/min for 60 s, and it was possible to achieve a film thickness greater than 2.0 μm. Before UV exposure, the sample was baked on a hotplate at 90°C for 5 min to remove the excess solvent and to improve the adhesion of the sol-gel film to the glass slide. After the exposure, the sample was developed in ethanol for 30 s to remove the unexposed component, and a 3D structure was formed. Finally, the sample was again baked on hotplate at 160°C for 30 min for further solidification.

To fabricate arbitrarily shaped micro-optical components, it is necessary to tailor the sol-gel material with an HEBS calibration mask, which was especially designed for negative-tone sol-gel materials by Canyon Materials, Inc. [8]. This calibration helps customize the sol-gel material by with specific optical densities, allowing us to build a calibration curve for the depth of sol-gel glass to remain after development relative to the optical densities in the mask. The calibration plate has 200 gray levels, and each gray level consists of one test patch 100 μm × 100 μm in size. The 200 gray levels are determined by the optical density values, ranging from 0.126 to 1.5. Within this range the gray levels are ascribed optical density values according to the linear equation OD = 0.126 + 0.0069×i, where i = 0,1,2,…,199. The UV exposure was implemented on a Q 2001CT UV-mask contact aligner (Quintel Corporation) with a peak emission at 365 nm wavelength and an irradiance of 15 mW/cm².

Since the sol-gel is a negative-tone material, the thickness of the remaining film is directly determined by the optical density values of the HEBS calibration mask; higher optical densities correspond to lower UV irradiation, which leads to thinner areas of film on the substrate. In this paper, we establish a calibration curve for the sol-gel material over a dynamic range of optical densities. That is, for a given optical density range on the HEBS mask, the sol-gel material has a corresponding response in terms of its remaining height profile after development. Figure 2 presents a characterization of the sol-gel thickness versus with the optical density of the HEBS mask. The thicknesses of the test patch were measured with a Dektak³ surface profiler (Veeco Metrology). Based on the experimental observations, it was found that exposure time is also an important parameter in the fabrication, and different exposure times may result in a different calibration curve. In setting up a dynamic optical density response, we found that an exposure time between 35 and 37 min provided a stable and good dynamic response for optical density values between 0.12 and 1.2. It is seen in the figure that the sol-gel material exhibits a linearlike response over 156 gray-scale levels. This characteristic is convenient for generating any arbitrary surface profile. The sol-gel also shows little polymerization for optical density values beyond 1.2.

 

Fig. 2. Calibration of the sol-gel material with HEBS gray-scale mask for gray-scale values from optical density 0.12 to 1.2.

Download Full Size | PDF

The transmittance of the sol-gel glass for a wavelength range of 200–2000 nm, measured by a spectrophotometer, is presented in Figure 3. The solid curve, dotted curve, and plus-sign curve are the transmittances of the bare quartz substrate, the sol-gel film before UV exposure, and the sol-gel film after 35 min of UV exposure, respectively. It is seen in the figure that the transmittance of sol-gel film without UV exposure is greater than 90% from 415 to 2000 nm, whereas after UV exposure the dynamic spectral range with the same transmittance values has been changed to 454–2000 nm. The difference confirms that the sol-gel material is sensitive to UV and that it provides stable transmittance for visible and IR wavelengths. The hybrid sol-gel film is suitable for various applications, including optical waveguide and micro-optical components in optical communications. The refractive index and the thickness of the hybrid sol-gel thin film are 1.52 and 2.0 μm, respectively, measured by a prism coupler (Metricon Corporation). The measurement error of the refractive index and thickness are 0.004% and 0.627%, respectively.

 

Fig. 3. Transmittance of the hybrid sol-gel thin film on quartz substrate compared with transmittance of a bare quartz substrate.

Download Full Size | PDF

As an example, a refractive microlens was fabricated on the sol-gel glass. Figure 4 shows the two-dimensional surface profile of the lens measured by a laser interferometer (WYKO NT 2000). The single lens is a positive lens 50 μm in diameter. Optical density values on the calibration mask were set between 0.129 and 1.017 so that they decrease from the center to the outer edges of the lens. Thus the surface profile could be formed on the sol-gel material to conform to the corresponding optical density values. The microlens was formed under the same fabrication conditions stated above; the lens has a maximum height of ~1 μm at the lens center (see Figure 4). The HEBS gray-scale mask was a calibration mask provided by Canyon Materials, Inc. [8]. Detailed information about the optical density values for generating the refractive lens is not available with the calibration mask. Here we set up the calibration curve for the sol-gel material so that we can compare the actual profile with the desired one when the optical density values are generated for a specific design in future. For a specific microlens design, we need to generate concentric rings with different calibrated optical density values that will define the surface profile. Compared with techniques using a set of binary masks or halftone gray mask for generating gray-scale structures, the HEBS method provides better surface quality because the HEBS has a higher gray-scale resolution. It was reported that optical density levels in the range of 0.126–1.5 can be produced by electron-beam direct writing to a precision of ±0.001 [8]. In the fabrication under discussion here, the sol-gel thin film was exposed for 35 min followed by processes including development and postbaking.

 

Fig. 4. Measured surface profile of the sol-gel microlens.

Download Full Size | PDF

3. Conclusion

Fabrication of 3D gray-scale phase micro-optical elements in sol-gel film by a single, etching-free, lithographic process was demonstrated. A maximum sol-gel film thickness of 2 μm was obtained. For a visible light source, this enables us to realize a 2π phase shift in the design of many diffractive optical elements. It should be noted that in the fabrication of a refractive microlens the actual phase depth will be limited by the maximum thickness of the sol-gel thin film. In this paper the microlens is a very weak one, which has a phase depth of 2π for a wavelength of 500 nm. To fabricate thicker lenses, we can explore either thicker sol-gel films or a diffractive lens design. In the latter case the thickness obtained here should be adequate for a 2π phase change. The hybrid SiO₂:TiO₂ material was demonstrated to be a cost-effective material for production of micro-optical elements. Compared with the conventional photoresist-based microfabrication techniques, the prime advantage of our method is the single-step fabrication of surface-relief structures without involving etching steps. For 3D continuous-phase-level structures, the technique using sol-gel material and the HEBS grayscale mask has a high potential for low-cost production of optical-quality silica micro-optical elements without tedious alignment and complicated etching processes.