A negative-tone sensitive SiO2/TiO2 organic-inorganic hybrid sol-gel material was synthesized and characterized for fabrication of multilevel micro-optical elements by direct electron-beam lithography. The exposure was carried out by an in-house modified electron-beam writing system using LEO SEM-982 with Elphy Quantum exposure beam-blanking control system at 25keV. The hybrid Sol-Gel material demonstrated a superb sensitivity for doses between 0.22μC/cm2 and 0.33μC/cm2.
©2002 Optical Society of America
In recent years, hybrid Sol-Gel technology has been intensively studied as a potential alternative material for realization of high performance micro-optical elements [1–3]. The primarily push for this advancement is mainly due to the intrinsic material properties of sol-gel glass as a simple and cost-effective material with good optical properties. The major application of the sol-gel material is focused on the fabrication of planar waveguide devices and micro-optical elements by UV photolithography [4–6]. A minimum features size for production of binary structures for micro-optical elements is restricted with approximately 1-2 μm because of the resolution limitation of the conventional lithographic technology such as direct UV laser writing and proximity mask photolithography. A strong demand to increase the efficiency and quality of micro-optical elements imposes new challenges to the existing lithographic techniques, particularly in views of the immature production technology for multi-level and 3 dimensional continuous surface relief micro-optical elements. Electron-beam (e-beam) lithography has been widely used as a laboratory tool for the fabrication of deep-submicron micro-optical element features in conventional e-beam resist. This approach, however, might instituted complex technological constrains in subsequence processes especially during the pattern transfer and ion etching of 3 dimensional microstructures
Recently e-beam lithography technology was used for direct structures formation in the hybrid silicon-zicronium sol-gel material [7, 8]. It should be noted that relatively higher dose was required for this material exposure, whereby the applied dose ranges from 10μC/cm2 to 2000μC/cm2. In this paper, we present a hybrid Sol-Gel material for e-beam exposure with doses, 3-order magnitude lower, from 0.05μC/cm2 to 1.0μC/cm2. In this range, the Sol-Gel exhibits a linear response of post-development film thickness on exposure. This property is very important for the fabrication of multi-level structures or continuous surface relief structures by a gray-scale exposure technique whereby precise structure height control as well as fabrication reproducibility of optical elements are very critical.
2. Sample Preparation
The hybrid organosilicate glass was formulated by the hydrolysis and subsequent copolymerization of silicon-oxide [SiO2] and a titanium-oxide [TiO2] network solution. Proplyl-methacrylate-substituted tri-methoxysilane [3-(tri-methoxysilyl) proplyl-methacrylate] was hydrolyzed with Isopropanol-2 [IPA] and acidified water, by a volumetric ratio of 30:12:1, produced the required silicon-oxide network solution. Similarly, the formulation of titanium-oxide network solution was achieved by adding titanium propoxide [Ti(OCH)4] in acetylacetone at a molar ratio of 4:1 under nitrogen environment, followed by homogenization by agitation. The addition of titanium was primarily used to modify the refractive index and to increase the mechanical strength of final material. The two separately prepared solutions were mixed subsequently with molar ratio of 4:1 between SiO2 and TiO2. The final mixture was allowed to age at ambient temperature for 30hrs under vigorous stirring. Usually, to initiate photo-polymerization during UV exposure, a photo-initiator IRGACURE 184 (CIBA) with 4% wt was added to this final mixture. Upon absorption of UV radiation, the photo-initiator would raise to an electronically excited state whereby it generates radical fragment that adds to the initialization of polymerization of unsaturated monomer. Finally, a 0.1 μm membrane was used to filter the final mixed solution before it was spin-coated on a pre-cleaned BK7 microscope glass slide. The newly prepared hybrid sol-gel was spun onto the glass slide at 2000rpm for 60sec. The sample was subjected to a pre-bake of 95°C hotplate for 5mins to remove excessive solvent and to improve film adhesion to glass slides. Film areas exposed to UV radiation are crosslinked and remain on the substrate, and whereas, the unexposed film areas would be washed away or dissolved by the developer. Typically, the sample was developed in alcohol or acetone for ~30sec. The percentage of crosslinked material and its post-exposure film thickness is proportional to its level of exposure. This provides a possible means in controlling structure height by exposure dosage variation, i.e., 3 dimensional lithography with gray-scale exposure.
3. Sol-Gel Film Characterization as E-beam Radiation Sensitive Material.
Experiments on e-beam exposure of Sol-Gel film were carried out with a SEM LEO982 converted to lithography tool with ELPY Quantum. A 25kV accelerating voltage, 20pA beam current, and 0.5μm step-size, to provide very small exposures, were used. A test pattern consisting array of blocks with a dose of 0.05μC/cm2 to 10.0μC/cm2, with ~12.8% dose increment, was exposed by our e-beam lithographic tool. After sol-gel film exposure, development and hard-baking dependence of film thickness on e-beam exposure were measured. The sol-gel post-exposure film thickness vs. exposure with 60sec development with isopropanol-2 [IPA] and 30sec with acetone is given in Figure 1. For both developers, the sol-gel film demonstrates an extremely high sensitivity of 0.33μC/cm2 and 0.22μC/cm2 respectively (it is by definition, exposure that provides thickness of the remaining film equal to 50% its original thickness). Note that this value of sensitivity is about two orders magnitude higher than those for standard positive E-beam resists, for example poly-methyl-methacrylate [PMMA] and three orders of magnitude higher than for silicon–zirconium sol-gel material. The gradient of the dose characteristic plots as shown in the Figure 1 gives another important lithographic parameters, i.e., the film contrast. It is defined as for negative tone resists (g = [Log (D1/D0)]-1, where D1 and D0 are doses corresponding to 0 and 100% remaining film thickness). In this case, the film has a contrast of 0.9 and 0.6 for development with IPA and acetone correspondently. This value of contrast is very low in comparison with conventional PMMA resist (whereby g = 3 to 4) and CAR (whereby g > 5). This is a very essential property for fabrication of multi-level and continuous surface relief structures by gray-scale exposure technique as a reasonably big variation of exposure (12 to 18%) would consequence a well controllable and relatively small (10%) variation in structure height.
Similar height on dose characteristics were observed for this sol-gel material with photosensitive initiator component concentration of 0%, 2%, 4% and 6%, as shown in Figure 2. These samples were developed for 60sec in IPA and hard baked at 160°C for 30mins. The photo-initiator is used to increase the materials sensitivity to UV radiation. However, our experimental data has indicated that the photo-initiator in the hybrid sol-gel has negligible effects when it was subjected to the e-beam radiation. The material sensitivity remains approximately 0.3 μC/cm2 with no sign of drastic changes with the variation of photosensitive initiator concentration. It is possible that under high-energy electron irradiation, a different methodology in free radicals production occurs whereby polymer molecules could be broken in many different places as the electron energy is significantly higher than bonding energy. It is possible that this free radicals production mechanism by high-energy electron is more efficient than those by UV photons with photo-initiator.
As shown in both figures mentioned above, a full polymerization or cross-linking of the film occurred at about 1μC/cm2 with or without photo-initiator. The exclusion of photo-initiator in sol-gel material during e-beam exposure would further simplify its preparation processes, and hence, resulting its production cost.
With the intrinsic properties of low contrast and high sensitivity under the direct e-beam radiation, the hybrid sol-gel film could be used for realization of 3D gray-scale micro-optical elements, especially with its excellent optical properties, i.e. high refractive index, good thermal resistance, high mechanical hardness, etc. Owing to the high sensitivity and low contrast of the material, undesired material polymerization in unexposed zones caused by proximity effect would inevitably affect feature edge sharpness, hence, reducing features resolution and registration.
4. Blazed Grating Fabrication
The main technological problem arising during small feature size structure formation in the Sol-Gel material is connected with its extremely high sensitivity. For high quality blazed gratings formation by the gray-scale exposure distribution, it is necessary to vary the exposure step by step across each line from zero to the desired sensitivity of material. Structure profile with the continuous film height variation will be optically smooth if the number of steps will be of the order of 10. However, this would cause the exposure for the first smallest step has to be approximately10 times smaller than the material sensitivity. In our case, the smallest pixel size we can set for such short exposure is 0.25 μm, hence resulting a blazed grating pitch of the order of at least 2.5μm. Of course, gratings with bigger pitch could be done with smoother surface using the same hardware. It is should be mentioned that with similar limitation, the pitch of the blazed grating made with the gray-scale photolithography may be with the value of the order of 20 μm.
A blazed grating of 4μm pitch shown in Figure 3, with a reasonably smooth surface profile was produced in the Sol-Gel material with 25kV e-beam exposure and 0.25μm exposure step size. Exposure and development of the sol-gel material is followed by the final ½ hour hard-baking at 160°C. This procedure would converted the softly crosslinked hybrid sol-gel material into a glass-like hard film. Therefore, in this case, we have demonstration the fabrication of a 3 dimensional glass-line continuous surface relief structures primarily by lithographic means using single-step e-beam gray-scale exposure with no ion or plasma-chemical etching process.
We have reported a novel in-house formulated SiO2/TiO2 hybrid Sol-Gel material that exhibits a negative-tone resist property under e-beam radiation. The material has demonstrated extremely high sensitivity, between 0.22μC/cm2 to 0.33μC/cm2, and a wide dynamic range for the residual film height on exposure dependence i.e. low contrast of 0.6 to 0.9 when it was subjected to different development procedures. For continuous surface relief structures, such as a multi-phase level diffractive element, low contrast film property is absolutely obligatory. The reported hybrid sol-gel material offers the advantages of single-step fabrication of continuous surface relief structures by gray-scale e-beam exposure technique without involving etching process.
E-beam radiation was found to be able to polymerize the organically modified hybrid sol-gel material with subsequent converting it into sol-gel glass with the post-development hard-baking process. The formulation of the three-dimensional material network due to molecular cross-linking by the free radical mechanism under e-beam irradiation yields negative-tone film properties. It is observed for this sol-gel, the methacrylate-based polymers could be cross-linked by free radical polymerization after e-beam radiation even without the addition of the photo-initiator. This makes the material more stable and low-cost. It is believed that the energy of electrons used for exposure is sufficient to break the molecular bonds, hence producing free radical, within the organic polymer itself.
References and links
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