We present the fabrication of a refractive microlens in hybrid SiO2/TiO2 sol-gel glass by electron beam lithography. The hybrid sol-gel material has high transmittance (greater than 95%) in the wavelength range from 362nm to 2000nm. Under the electron beam exposure, the polymerized film thickness was as large as 4 µm. A 3D microlens profile can be formed by exposure of the sol-gel film under different electron dosages that leads to different polymerized film thickness after development. As an example, a microlens with a 250 µm diameter and a 2.05 µm sag height was fabricated.
© 2003 Optical Society of America
Micro-optical elements have been widely used in integrated optics. A microlens as a microoptical element can be used as a laser-to-fiber or waveguide-to-fiber coupler for high coupling efficiencies. To fabricate microlenses, different methods including high-energy beamsensitive (HEBS) gray scale mask [1–2], reflow of photoresist  or focused ion beam (FIB) direct writing  have been used. Different techniques have their own advantages and restrictions. The gray scale mask and reflow techniques could result in a smooth surface profile. However, since the mask is expensive it is not suitable for fabrication of prototype micro-optical elements for testing purposes in the laboratory. The reflow technique involves accurate temperature control around the glass point and the final surface relief shape of the elements could be limited by the nature of the temperature process. Direct write techniques are a relatively simple and feasible method for R&D purpose in the laboratory. Various direct writing techniques were used and reported for microfabrication of optical elements in past years [5–7].
We have reported a simple method based on a hybrid sol-gel material for fabrication of micro-optical elements using holographic, HEBS gray scale mask, laser direct writing and electron beam direct writing techniques in Refs. [8–11]. The advantages of this material lie in its good optical properties and single-step fabrication method. In Ref. , we have reported the fabrication of blazed gratings in sol-gel glass using the electron beam lithography (EBL) method. Since the sol-gel material is very sensitive to the electron beam exposure, it can be crosslinked in the exposed areas quickly even without the photoinitiator. Consequently, fabrication and profile control of gray scale micro-optical elements in the sol-gel by EBL is a tedious task. In Ref.  a grating height of only 1 µm was achieved. The problem is largely associated with the sol-gel sensitivity and the material synthesis. In this paper, with a new sol-gel recipe, the film thickness polymerized by the e-beam can be as large as 4 µm so that we can make refractive optical elements by EBL. In a previous paper , we have reported that such a new sol-gel recipe revealed a very good linear UV response to optical density of gray scale mask and could be used to produce diffractive optical elements with a 2π phase difference in the visible and near IR wavelength range.
2. Fabrication and characterization
The hybrid sol-gel glass is synthesized by hydrolysis of prolylmethacrylate-substituted trimethoxysilane, 3-(trimethoxysilyl) propyl methacrylate, in isopropanol and acidified water. The molar ratio was 0.04:0.048:0.035. At the same time, 0.01 mol of titanium propoxide (Ti (OCH) 4) was hydrolyzed in 0.04 mol of acetylacetone under nitrogen environment. After 1 hour, two solutions were mixed together for another 24 hours aging in room temperature. Before spinning, large particles in the mixed solution were removed by a 0.1 µm membrane filter attached to a syringe. The substrate was glass coated with a thin indium-tin-oxide (ITO) film to avoid charge-up of the sample surface during electron beam exposure. A 4 µm film was achieved by spun sol solution at 1500rpm for 60secs. The film thickness and refractive index (for 633nm wavelength) of the sol-gel material measured by using prism coupler (Metricon Corporation) is 4 µm and 1.51 respectively.
Figure 1 shows the transmittance spectra of hybrid SiO2/TiO2 sol-gel glass in a wavelength range between 200 nm and 2000 nm. It is seen that this material presents a high transmittance (greater than 95%) for wavelength greater than 362 nm, which means that the sol-gel material can be used as an optical material in the whole visible and near infrared range.
The electron beam exposure of the sol-gel film was implemented on an SEM (LEO982), which was upgraded and controlled by an ELPHY Quantum software package. The accelerating voltage of 25 keV and beam current of 80 pA were used. Calibration of the solgel film thickness versus e-beam dose was carried out by directly writing a binary grating with a period of 15 µm. After exposure, the sol-gel film was developed in isopropanol-2 (IPA) for 1 minute to remove the unexposed area. The film thickness was measured by Dektak Surface Profiler (Veeco Metrology).
Usually, the photoinitiator is added into the sol solution to make it photosensitive. When exposed to UV light, the free radicals formed by photoinitiator can cause a subsequent 3-D polymerization of sol-gel film, thus divides the sol-gel into dissolvable part in the unexposed areas and undissolvable part in the exposed areas. However, for electron beam exposure, since the high energy of the e-beam is strong enough to break chemical bonds such as double carbon bonds, the polymerization of sol-gel film could happen in the exposed regions even without photoinitiator, this phenomenon was reported in our previous work in .
Figure 2 shows the relationship between the sol-gel film thickness and electron beam dosage. It is noted that a good linear relationship exists between sol-gel film thickness and electron beam dosage, which is useful for parameters control in the fabrication. As we can see in the Fig that the film thickness after exposure can be as large as 4µm, it will be helpful for us to realize the refractive mcirooptical elements such as refractive microlens possible. The measured contrast value is 0.78, which can be determined by,
Where Dt is the dosage required for 100% crosslinking of the film and Ds is the dosage corresponding to a point when the crosslinking started. The sensitivity of the sol-gel material is 0.46 µC/cm2 defined by the dosage that leads to the remaining film thickness equal to 50% of its original thickness.
Based on the above calibration results, as an example, we fabricated one microlens with 250 µm diameter and 2.05 µm sag height. The solid line of the Fig. 3 shows the surface profile of the fabricated microlens measured by surface profiler. The theoretical surface profile represented by dot line is also shown so that we can make a comparison. As can be seen the resulted surface profile is very close to the designed one although the sag is a bit lower and the diameter is a bit smaller due to the shrinkage during postbaking. However, a bit rough surface profile advises us to optimize the process in future. Figure 4 shows the beam intensity distribution in the far filed through the fabricated microlens.
To enhance the surface corrugation quality of the lens, we should optimize the electron beam lithography process including using more gray levels (presently we use only 13 gray levels) to approach the spherical surface and improve the surface smoothness. Additionally, defocus the electron beam during exposure will also give a smoother surface profile. To lower the sensitivity of the sol-gel could be an alternative choice. For a lower sensitivity, a smaller step size in EBL point-to-point writing will result in a smoother surface profile. With the current high sensitivity, the smallest step size is about 0.5 µm. However, a lower sensitivity will require a longer exposure time.
We have demonstrated the fabrication of a microlens in a hybrid sol-gel glass by electron beam lithography. It is shown that the sol-gel material has a high transmittance (greater than 95%) in the wavelength range between 362 nm and 2000 nm. When exposed to an electron beam, the crosslinked film thickness can be as large as 4 µm, which is big enough for us to realize refractive 3D microoptical elements. Finally, as an example, a microlens with a diameter of 250µm and sag height of 2.05 µm was fabricated by the electron beam lithography method. The resulting surface profile was very close to that of the theoretical design. The surface corrugation quality can be improved by optimization of various parameters including sol-gel film sensitivity, electron beam dosage and the number of the EBL writing layers.
References and links
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