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Photo-patternable GeO2-contained organic-inorganic hybrid films, which can be used for the low cost and mass production of integrated photonic circuits, were synthesized by combining a low-temperature sol-gel process with a spin-coating technique. Optical waveguide properties and photochemical activities of the hybrid sol-gel films were characterized and monitored by a prism coupling technique and a Fourier transform infrared spectroscopy. Advantages for fabrication of ridge structures based on the hybrid films were demonstrated by one-step spin-coating process followed by direct ultraviolet light irradiation. The results indicate that the as-prepared photo-patternable hybrid materials have great applicability for the fabrication of photonic components, and the fabrication process has the advantages of cost-effect and very short processing time over inorganic materials patterning methods.
©2008 Optical Society of America
1. Introduction
Recently, optically homogeneous and transparent organic-inorganic hybrid materials containing organic components have been widely studied and indicated that their optical properties can be largely enhanced as compared with organic polymer materials. [1,2] The introduction of organic groups into an inorganic network improves physical, chemical, and mechanical properties, thus organic-inorganic hybrids are anticipated as desirable materials for photonic applications. Especially, incorporation of photo-responsive organic groups has been used to produce a wide range of interesting materials, since there has been a great deal of interest in materials that can be photo-patterned for fabrication of micro-optical elements such as diffraction gratings and channel optical waveguides. [3–5] For example, the hybrid material containing a polymerizable organic group including unsaturated hydrocarbon or epoxide substituent has been successfully developed, which is patterned by ultraviolet (UV) light irradiation through an appropriate mask or laser writing followed by dissolving unpolymerized region. [6–8] Notably, this method enables us to fabricate ridge-shaped hybrids which can be used as channel waveguides. Therefore, photopatternable hybrid sol-gel materials have been extensively investigated for fabrication of micro-optical elements and optical waveguide devices in recent years, since the hybrid materials are not only favorable optical properties, but also simple and cost-effect materials for the fabrication of planar waveguide devices and micro-optical elements through UV photolithography. [9–11] So that SiO2-, ZrO2-, and SiO2/TiO2-based patternable hybrid materials have been reported and studied in recent years. [8,11] As one knows well, the GeO2-based glasses have a significant potential for photonic applications due to being made compatible with single-mode fibers as well as the ultraviolet photosensitivity of germanosilicate glasses, [12–13] but there have not been any reports of preparation of such photo-patternable GeO2-based organic-inorganic hybrid materials and low-cost fabrication of waveguide devices by one-step spin-coating process followed under direct UV light imprinting so far.
Here, we report what we believe to be the first study on the optical waveguide properties and fabrication of the patternable GeO2-contained organic-inorganic hybrid sol-gel materials for photonic applications. The polymerization of the hybrid materials was observed by a Fourier transform infrared spectroscopy (FTIR) and optical waveguide properties of the hybrid so-gel films were characterized by a prism coupling technique. In addition, to the best of our knowledge, the present study provides the first fabrication process of ridge structures based on the as-prepared photo-patternable hybrid materials by one-step spin-coating process followed by direct UV light irradiation.
2. Experimental study
The photo-patternable GeO2-contained organic-inorganic hybrid material sols were prepared by three solutions. one mole of methyltrimethoxysilane (MTES, CH3-Si(OC2H5)3) were mixed with 4 moles of ethanol and 4 moles de-ionized water, and after being stirred for about 30 minutes, 0.01 mole hydrochloric acid (37 wt.% in water) was added as solution I. For solution II, germanium isopropoxide (Ge(OCH(CH3)2)4), which is used as GeO2 source, was added to 2-methoxyethanol at a molar ratio of 1:4 under a nitrogen environment and the solution was agitated for homogenization. For Solution III, which was formed by a hydrolysis of 3-methacryloxypropyltrimethoxysilane (MEMO) in isopropanol and de-ionized water with a molar ratio of 1:3:3. Three solutions (solutions I, II, and III) were then mixed with a molar ratio of 0.35: 0.35: 30. It should be mentioned here that the refractive index of the final hybrid material can be tuned by changing GeO2 molar fraction. The final mixture solution was allowed to age for about two days at room temperature. Before the mixed solution being used, a photoinitiator in the form of IRGACURE 184 (CIBA) with about 4 wt. % was added into the final solution to enable the photo-polymerization of the carbon-carbon double bond from MEMO under UV light irradiation at room temperature. Silicon and microscope slide glass were used as substrates and they were ultrasonically cleaned in acetone and ethanol, respectively, rinsed with de-ionized water and dried with pure nitrogen. One layer of the sol-gel film was spun onto the substrate at 3000 rpm for 30 seconds. The coated-film samples were then baked at different temperatures for about 10 minutes in air to study their optical properties and observe the polymerization process of the hybrid materials.
The thickness, the refractive index, the propagation mode, and the propagation loss properties of the hybrid films were measured for transverse electric (TE) polarization by an m-line apparatus (Metricon 2010) based on a prism coupling technique at the wavelength of 633 nm. The FTIR spectra of the hybrid films deposited on silicon substrates were observed by a FTIR (Perkin Elmer 2000, with a resolution of ±1 cm-1) spectrometer in the range of 4000-400 cm-1 to monitor the photochemical activities of the hybrid films. The patterned structures and profiles, which were fabricated by one-step spin-coating process followed by direct UV light irradiation, were measured with a WYKO NT 2000 interferometer.
3. Results and discussion
The morphology and the roughness of the hybrid films were examined with a Digital Instruments Nanoscope IIIa AFM by means of a tapping mode. The results indicate that a dense and porous-free hybrid film can be easily obtained at a low heat treatment of 100°C. When organic groups are integrated into the inorganic system, the shrinkage of the hybrid film is low due to the organic components filling the pores between the inorganic oxide chains. Thus, the hybrid film can reach its final density at a low temperature. The value of the root mean square (RMS) surface roughness of the hybrid film was examined over a 5 µm×5 µm areas, and it was noted that the surface roughness of the hybrid film is related to the heat treatment temperature. With increase the heat treatment temperature, the volatilization and the thermal decomposition of the organic solvents, the RMS surface roughness reduces and the hybrid film becomes smooth. For example, the surface roughness of the hybrid films heated at 50 and 100°C is 5.6 and 2.8 nm, respectively.
Figure 1(a) shows the propagation modes of the hybrid planar waveguide film, which was deposited on a silica-on-silicon substrate by a single spin-coating process and heated at a temperature of 100°C, based on the prism coupling technique at the wavelength of 633 nm. It can be seen from the Fig. 1(a) that several mode dips in intensity can be observed, but only the two modes of TE0 and TE1 can be guided modes of the hybrid planar waveguide film since the substrate index is about 1.46 for the wavelength of 633 nm, and thus the other dips observed in the intensity spectrum are substrate modes due to their indices smaller than the value of the substrate index. The thickness and the refractive index of the hybrid films can be determined from the measured effective refractive indices values of the TE modes. The results indicate that as expected, with increase the heat treatment temperature, the refractive index of the hybrid film increases and the thickness drops. For example, a thick hybrid film with a thickness of 1.62 µm can be produced on silicon substrate by a single spin-coating process at room temperature, but the thickness reduces to 1.60 µm with increase the heat treatment temperature to 100°C. Correspondingly, the refractive index of the hybrid film increases from 1.486 to 1.489. It was also noted that the change in the film thickness and the refractive index is more substantial when the heat treatment temperature is beyond 300°C. These results indicate that the change in film thickness is only related to the evaporation of the organic solvent, and the organic compounds inside the hybrid film do not start to decompose and combust below the heat treatment temperature of 300°C. That is to say, this hybrid material has a better thermal stability as compared with organic polymer waveguide materials. Our experimental results also indicate that a crack-free and dense hybrid film with a thickness of several micrometers can be also obtained by a multiple spin-coating process and heated at 100°C. The optical propagation loss of the hybrid planar waveguide film is evaluated and typically is 1.46dB/cm. As known, for a guided wave propagating in a planar medium, except for the local refractive index fluctuations in the volume of the waveguide and the deviations from a perfectly plane geometry at the waveguide-cladding boundaries contribute to the losses, the nonuniform hydrolysis and condensation of the trinary alkoxides mixture undoubtedly result in the big scattering loss. The homogeneity of a sol-gel glass synthesized from a mixture of more than two alkoxide precursors can be affected by the relative rates of homocondensation and heterocondensation. That is to say, in mixtures of germanium and silicon alkoxides, it is possible that a heterogeneous network containing Ge-rich and Si-rich domains may be formed due to different hydrolysis rate of silicon and germanium alkoxides.
Figure 1(b) shows the FTIR spectra of the same films heated at different temperatures. The main band at about 1096 cm-1 is assigned to Si-O-R stretching vibrations of ethoxy groups directly bonded to silicon, [14] as seen that there is little change in intensity with increase the heat treatment temperature. The absorption peak at 1270 cm-1, which is present for all hybrid film samples heated at different temperatures, is due to symmetric deformation of Si-CH3 bond and decrease gradually in intensity with increase the heat treatment temperature. [15] The shoulder band at about 968 cm-1 is attributed to Ge-O-Ge anti-symmetric stretching. The band at 800 cm-1 is from symmetric stretching motions of oxygen atoms along the bisector of the Si-O-Si bridging angle. It should be mentioned that the peaks at 610 and 460 cm-1 observed for all the hybrid film samples are from the silicon substrate. Two peaks at about 1718 cm-1 and 1632 cm-1 can be clearly observed for the films heated below 300°C and they are attributed to the carbonyl ester groups and the vinyl group C=C stretching mode respectively. [16] Obviously, the intensity of these peaks weakens gradually with the increase of the heat treatment temperature, and even vanish when the heat treatment temperature was increased up to 400°C. These results indicate that a patternable organic-inorganic hybrid material can be obtained below a heat treatment temperature of 400°C and a polymerization of the hybrid material proceeds with the increase of the heat treatment temperature.
Fig. 1. (a). Modes of the hybrid planar waveguide film at the wavelength of 633 nm; (b). FTIR absorption spectra of the hybrid sol-gel films heated at different temperatures
In order to demonstrate the photo-patternable properties of the as-prepared hybrid sol-gel material under UV light irradiation, it is used to fabricate ridge-shaped structures by a waveguide mask. For a lithograph study, the hybrid film sample was prebaked on a hotplate at 90°C for 5 min to remove the excessive solvent and improve the adhesion of the hybrid sol-gel film to microscope slide glass. The UV exposure was done by using a Q 2001CT UV-mask contact aligner (Quintel Corporation) with a peak emission at a wavelength of 365 nm and an irradiance of 15 mW/cm2. After the exposure, the sample was developed in ethanol to remove the unexposed component and the patterns written can be clearly observed. Finally, the patterned sample was postbaked on hotplate at about 150°C for about 1 hour for further solidification of the fabricated structure. When the hybrid film is exposed under UV light irradiation, it produces a polymerization of the vinyl monomer in the photopolymerizable methacryloxy group, thus free radicals formed by photoinitiator lead to free-radical cross-linking polymerization of the unsaturated hydrocarbon bonds, that is to say, the response of the hybrid film to UV light exactly likes negative tone photoresist. Then, developing the exposed hybrid film in ethanol, and those unexposed components are solubilized away and the exposed components are remained.
Fig. 2. A ridge structure array on the hybrid sol-gel films fabricated by one-step spin-coating process followed by direct UV light (a) 2D image, (b) profile measured with the WYKO interferometer
Fig. 3. An enlargement of the image and profile of the patterned structure as shown in Fig. 2 (a) 3D image, (b) profile measured with the WYKO interferometer
Figure 2 shows 2-dimensional image and profile of the ridge structures fabricated on the hybrid sol-gel film, which was measured with the WYKO interferometer. It can be seen that the ridge structure is neat and clear. From the profile of the ridge structure as shown in Fig. 2(b), the period of the ridge structure is 122 µm. The patterned depth is about 400 nm. To observe better the patterned structure on the hybrid sol-gel film, Figure 3 shows an enlargement of 3-dimensional image and profile of the patterned structure as shown in Fig. 2. It can be seen that the width of the patterned structure is about 10 µm. Here, the original film thickness is approximate 1.60 µm, but the depth of the patterned structure is only 407 nm as seen in Fig. 3(b). The reason why the final depth is smaller than the original film thickness is related to the UV exposure time and development time. Notably, the UV exposure time will affect the final depth of the patterned structure. A shorter UV exposure time is probably not enough for the polymerization of the hybrid sol-gel film, so that it can lead to being washed away during development process, while a longer UV exposure time will also sacrifice the depth of the patterned structure due to over-exposure. For example, Fig. 4 presents a result of the patterned structure with a longer exposure time (40min) as compared to that of Fig. 3 (25 min). Clearly, there is a small dip to have been observed on the top of the patterned structure, but there is no reasonable explanation for this phenomenon. As can be seen in Fig. 4, the final depth of the patterned structure is only approximate 300 nm, which is shorter than that of Fig. 3. These results indicate that short or over-exposure will reduce the depth of the fabricated structure. It should be mentioned here that a maximum depth of about 0.85 µm can be achieved under present preparation conditions by controlling the UV exposure time carefully. Actually, the development process will also affect the depth of the patterned structure. The unexposed component of the hybrid film cannot be removed completely if the developing time is too short; however, the exposed component of the hybrid film is probably dissolved if the developing time is too long. Consequently, the depth of the patterned structure will be also reduced. The surface roughness of the patterned structure will be also affected during development process. It can be concluded that the optimization of the material composition and patterning parameters is necessary and important for improving the surface quality of the patterned structure.
Fig. 4. A patterned structure on the hybrid sol-gel film with longer UV-exposure time as compared to that of Fig. 3(a) 3D image, (b) profile measured with the WYKO interferometer
4. Conclusions
In summary, a new photo-patternable GeO2-contained organic-inorganic hybrid material for fabrication of the ridge structure devices has been prepared by the sol-gel process from an organic-inorganic hybrid system. The hybrid films have showed photo-patternable properties due to a polymerization of the vinyl monomer in the photopolymerizable methacryloxy group under UV light irradiation. The optical properties and polymerizable activities of the hybrid material have been investigated by the prism coupling technique and FTIR spectroscopy. A simple UV lithography fabrication technology for ridge-shaped structure on such hybrid film has been developed. It has been experimentally demonstrated that the ridge structures could easily be fabricated for the hybrid films by the single-step fabrication process without involving any etching process. Obviously, for multilevel diffractive optical element structures, this patternable hybrid material appears to be considerably useful since it does not involve the complicated etching process for different relief heights. The simplicity of the device fabrication process and the possibility of controlling accurately the device parameters suggest that the as-prepared hybrid materials have great applicability for the fabrication of microoptical elements for a simple, cost-effective and mass production fabrication method based on the self-development characteristic of the hybrid materials.
Acknowledgment
This work was supported by the National Natural Science Foundation of China under Grant No. 60477003.
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