Preparation of functionally graded optical filter

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Dielectric multilayres are used for bandpass filters in optical communication and for antireflection coatings in solar cells. In these cases.optical losses should be lowered by smoothing interfaces and reducing defects in films. In a helicon sputtering method, high density plasma can be maintained under a low gas pressure. Sputtering ions keep their kinetic energy due to a long mean-free-pass, which enables one to elongate a target-substrate distance. Therefore less plasma-damaged films having a smooth surface can be obtained.In the present work, silica/titania multilayres or Rugert filter were prepared by helicon sputtering. The structure and optical properties of films were measured. Effect of helicon wave power was studied with a plasma-emission spectrometer.The plasma emission intensity increased with increasing radio frequency (RF) power, but plasma density was almost saturated at more than 100W.Thus the RF power was fixed at 100W.Refractive index of titania film decreased from 2.42 to 2.35 with decreasing total gas pressure, but that of silica film was constant (1.48). Both silica and titania films were amorphous. A silica-titania multilayr whose reflection peak is 800nm was designed. It was confirmed that the multilayr had almost the same transmittance spectrum as that of calculation. A transmission electron micrograph showed that the multilayred films had smooth interface.