Antireflective MgF 2 Coating on Polycarbonate
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Antireflective MgF2 Coating on Polycarbonate Takanobu Hori1, Isao Tokomoto1, Kazuo Uetani1, Masashi Fukinbara2, Akira Kato3 and Hiroshi Kajiyama2, 3 1 R&D Center, ShinMaywa Industries, Ltd., Hyogo 663-8001, Japan. 2 Graduate School of Engineering, Ibaraki University, Ibaraki 316-8511, Japan. 3 Hitachi Research Laboratory, Hitachi, Ltd., Ibaraki 319-1292, Japan.
ABSTRACT We deposited MgF2 thin films using the ion-plating method that features RF and DC biasing to the substrate holder. We investigated the effects of RF powers, substrate temperatures (Ts) and DC bias voltages on optical absorption and abrasion resistance of the films. The optical absorption deteriorated by increasing RF power and Ts. In particular, the deterioration was quite serious when the Ts were above 423 K. The abrasion resistance increased by increasing the Ts. We found that the abrasion resistance could be improved also by increasing a DC bias voltage. The same abrasion resistance as the film deposited by a conventional heat resistance deposition at 573 K was obtained by applying the DC bias voltage of -500 V and the Ts of 300 K.
INTRODUCTION Antireflective (AR) coating is indispensable to optical devices for reducing optical losses due to the reflection at interfaces [1]. AR coating is done by stacking optical thin films with low and high refractive index (n) material. For example, the SiO2 (n = 1.46) and TiO2 (n = 2.30) layers have been alternately stacked to the each optimal optical thickness. If the SiO2 layer is replaced by a material with a lower n, it becomes possible to expand optical ranges to the shorter wavelengths. This is also crucial for reducing the repetition of layered structures. Among the candidates, magnesium fluoride (MgF2) is the most desirable replacement because of its low refractive index (1.38) and high abrasion resistance. MgF2 thin films have been extensively deposited by a heat resistance method [2, 3]. However, its deposition temperature should be higher than 573 K to satisfy both optical absorption and abrasion resistance. Sputtering depositions have also been tried on low temperature substrate [4-8]. So far MgF2 deposition at the Ts below 373 K has been unsuccessful. The purpose of our research is to develop the synthesis method of optical MgF2 films at the temperatures applicable to polycarbonate substrate. In this paper, we deposited the MgF2 film using the advanced ion-plating (AIP) apparatus we have developed [9]. We investigated the effects of RF powers, substrate temperatures and DC bias voltages on the absorption characteristics and the abrasion resistance of the films.
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EXPERIMENTAL The films with a thickness of 300 nm were deposited on silica glass plates (BK-7) and polycarbonate at the Ts below 573 K using the AIP apparatus [9]. Figure 1 shows a schematic diagram of
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