Narrowband Filter With Adjustable Coherence Length Of Quasi-monochromatic Light Interference Study

With the development of semiconductor materials and devices, micro-optical disc technology, micro-machinery, computer and information industries, growing attention has be paid on the testing of the surface quality of the semiconductor and optical elements, including the surface error testing as an important item. Laser phase-shifting interferometry is one widely used method to test the surface of the semiconductor and optical elements, but when we use this method to test the surface of the daul-polishing transparent elements with parallel surfaces, there will be three-surface interference, leading to greater measurement error. To address this issue, this paper presents a method, which applies no pretreatment to the surfaces and can achieve low cost, high accuracy measurement, that replacing laser light source by white light source and different bandwidth band-pass filters. We can adjust the bandwidth of the quasi-monochromatic light source by changing filters, accordingly adjust its coherent length, and then achieve the surface testing of the daul-polishing transparent elements with parallel surfaces with different thickness.The commonly used design methods of the band-pass filter films are introduced in this paper, especially the Thelen design method of the Fabry - Perot filter with several cavities, correspondingly a mathematical model is established. Using the model we work out the initial film stacks of the 5 nm and 10 nm bandwidth filters, and then use film design software to do the simulation and the further optimization. Before the fabrication of the films, we firstly choose TiO₂, SiO₂ as the best coating materials, secondly study their best coating process parameters. On the basis of the studies above, we fabricate bandwidth 50 nm, 10nm, and 5 nm filters respectively, and then measure their transmittance spectrum and coherence length. Finally, we eliminate the three-surface interference with the help of the filters, and obtain the interference fringes we want.