| dc.description.abstract | Recently, laser interferometic lithography technique shows its potentials on the fabrication of photonic band gap structures in photoelectric industry, direct writing the interference patterns into photoresist, heat treatment of thin films and magnetization of the dot arrays on the magnetic materials. Therefore, laser interferometric lithography technique has been widely used and investigated. In this study, the principle of interference optics is utilized to build up a laser interferomeric lithography system. The advantages of this system are without using the mask when the exposure process is proceeded and the period of the interference patterns can be easily change by altering the interference angles. The two or three dimensional periodic structures can be constructed in photoresist by the assistance of the rotational stage to expose the photoresist for multi-times.
We utilize a modified interferometric exposure model, enhanced with Beer-Lambert law, to study how some process parameters influence the structural dimensions within the whole exposure area. An experimental apparatus is built to verify the accuracy of this model. The simulation results indicate that when the incident angle is larger than 15°, the effect of the beam deformation can not be neglected. One can not readily obtain periodic structures with the same dimension during static exposure because of the Gaussian distribution of the light intensity. The theoretical results match the experimental ones quite well. The variation of the Dill’s parameter A has a greater influence on the transmittance and the line width when A is decreasing. If a poor contrast fringe is exposed in the photoresist, it will not only cause greater non-uniformity of the structural dimensions, but also a decreased aspect ratio in the structure after the development process.
Besides, an interferometric lithographic technique and double exposure method are applied to theoretically and experimentally investigate several kinds of 2D periodic structures. The shape, lattice symmetries and lattice constants of the 2D structures, for different substrate rotational angles, are obtained from the simulated predictions. The shape of the 2D structures can be varied by controlling the rotational angle of the substrate and the development process, and they are validated experimentally. The variation of the lattice symmetry of the 2D structure with the substrate rotational angle is discussed in detail in relation to the axial angle and lattice constant. It is found that square, circular, rectangular and elliptical scatterers which are arranged in parallelogram, triangular and square lattices (with different lattice constants) can be obtained. The photonic band gaps for each condition are also investigated. When the substrate rotational angles are the same, the normalized frequency of photonic band gap structures with an equal filling factor are very similar regardless of the interference angle. Furthermore, the gap-midgap ratios and the forbidden frequencies vary with the substrate rotational angles are also discussed. The results are very helpful in designing the forbidden frequency when the lattice constant and scatterer shape can be controlled by the interferometric lithographic technique. | en_US |