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Holographic interference is widely used for fabrication of sinusoidal gratings in photosensitive optical material. We present variable surface profile gratings fabricated in photosensitive hybrid sol-gel glass by use of holographic interference. It is found by experiment that different surface profiles of gratings including sinusoidal, binary, and two-peak sinusoidal can be obtained by precise control of exposure time. The experimental results are helpful for researchers fabricating gratings by holographic interference.
©2003 Optical Society of America
1. Introduction
Many methods including contact imprinting in a mask aligner, laser direct writing, electron-beam lithography, and holographic interference have been used for fabrication of gratings. In these methods, contact imprinting and laser direct writing are suitable for fabrication of gratings with a pitch of several micrometers, whereas electron-beam lithography and holographic interference can produce a grating with a submicrometer period. Electron-beam lithography is a relatively expensive technology and is not suitable for fabrication of gratings in a large field. Holography, however, can produce gratings with a fine pitch in a large area. In addition, this technique is simple and is easy to control.
Photosensitive hybrid sol-gel glass has been used for fabrication of microoptical elements for a long time. The advantages of this material are its favorable optical properties and single-step fabrication method. Much research has been done on fabrication of micro-optical elements in photosensitive hybrid sol-gel glass [1–9]. Optical gratings as a key optical element have been used frequently in many optical systems, and the fabrication of gratings in photosensitive sol-gel glass can be further improved for industry and commercial use on account of its fast single-step fabrication process. Thus it is useful to explore the parameters that affect the grating profile in the holographic interference method.
In this paper we present a thorough investigation of how the grating surface profile is controlled by adjustment of its fabrication parameters.
2. Fabrication and discussion
In our experiment a photosensitive hybrid SiO2/TiO2 sol-gel glass was used for fabrication of gratins by the holographic interference method. The sol solution is synthesized by hydrolysis of 3-(trimethoxysilyl) propyl methacrylate (MPTMS) in isopropanol and acidified water with a molar ratio of 0.04:0.048:0.035 and titanium propoxide in acetylacetone under a nitrogen environment with a molar ratio of 0.01:0.04. Here the TiO2 network is doped into the SiO2 network so that the refractive index can be tailored. To make the sol-gel film photosensitive, 4% (weight) of photoinitiator (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) is added. The sol solution was spun onto a fused silica glass substrate that was cleaned in an acetone bath. A 500-nm-thick film was coated onto the substrate in a single step with a 3500-rpm spinning speed, and the film thickness was measured by a prism coupler system. When sol-gel film is irradiated with a UV laser, the photoinitiator can be decomposed into free radicals that induce the polymerization of MPTMS monomers, which makes the sol-gel film a negative tone like resist, and the grating patterns can be formed in this way. After exposure, the film was developed in ethanol for 15 s to reveal the grating patterns. In this experiment, development time always remained the same.
The holographic grating fabrication system is shown in Fig. 1, where a He-Cd laser with a wavelength of 325 nm is used as a UV light source. This laser provides a maximum optical power of 200 mW. The laser beam irradiating from the laser is first focused by a microscope and then filtered and expanded into a spherical wave with a pinhole of a several micrometers in diameter. A rotation stage that has a resolution of 0.1 deg is used for mounting the reference mirror and the samples. The reference mirror is used to deflect the laser beam to produce a reference wave onto the samples. The rotation stage should be placed at a distance such that the spherical wave acts as quasi-plane wave.
When this system is used for fabrication of gratings, the grating period is defined by the following equation,
T=λ/(2sinθ),
where λ is the laser wavelength, 325 nm, and θ is the optical wave incident angle onto the sol-gel samples. The rotation stage is used to control the optical wave’s incident angle to produce a different period of interference wave field. This simple apparatus is tunable, and a large pattern area can be obtained.
Figure 2 shows a grating with a period of 1 µm fabricated with this system in sol-gel glass. It is seen in the figure that the grating profile is a perfect sinusoid, which is obtained by precise control of the exposure time. The optimum exposure time for obtaining a perfect sinusoidal grating in sol-gel glass is found to be 2 min.
Different grating surface profiles can be obtained if one adjusts the exposure time. Figures 3 and 4 show 1-µm pitch gratings fabricated with exposure times of 1 and 3 min, respectively. Clearly, for the grating in Fig. 3, the exposure time is not sufficient. The optical intensity of less than roughly half its peak cannot lead the photosensitive sol-gel film to polymerize, which means that the sol-gel film is not polymerized completely. As a result, a flat groove appears in the grating valley rather than the expected sinusoidal groove, whereas for the grating in Fig. 4 there is an overexposure time in the experiment, since we are fabricating a fine pitch grating of 1-µm scale. The proximity effect between the neighboring periods is obvious. We believe that the subpeaks between the main peaks are induced by the proximity effect that results from a longer exposure time. However, if one reduces the incident angle to produce a bigger period grating, see 2 µm. The same exposure time, 3 min, will not induce subpeaks, but a nearly binary grating. Figure 5 shows a quiz-binary grating with a period of 2 µm fabricated with the holographic interference method. It is seen in Fig. 5 that the peaks of the grating remain a small sinusoidal profile.
Thus the different exposure time will result in a different grating profile in the holographic interference fabrication method. One can control the exposure time to produce a specific grating surface profile.
In addition, the pattern area will also affect the grating profile. We note in the center area that the grating lines remain straight. However, if one scans the grating with an atomic force microscope in the edge of the pattern area, a tile of the grating line can be observed. Figure 6 shows a 1-µm pitch grating with tilted grating lines. This is because the optical wave is far from a plane wave in the edge of the pattern area. A larger distance between the interference wave and the spherical wave source can overcome this divergence. The maximum size of the grating obtainable without tilting at the edge relies on the holography interference setup. In this experiment, the maximum size can be up to 5 cm2.
3. Conclusion
We have demonstrated the fabrication of the different grating profiles in photosensitive sol-gel glass by using the holographic interference method. It is found that the exposure time will significantly affect the final surface profile of the gratings. Thus, if one expects a perfect sinusoidal grating, exposure time should be controlled accurately. In addition, we note that the pattern area will also affect the resulting grating profile. The grating lines remain straight in the center area, but they tilt in the edge of the pattern area.
References and links
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