RF plasma sources
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Plasma ion assisted deposition with radio frequency powered plasma sources H. Hagedorn, M. Klosch, H. Reus, A. Zoeller Leybold Optics GmbH, Siemensstrasse 88, 63755 Alzenau, Germany ABSTRACT The deposition of oxide coatings with excellent optical and mechanical properties requires powerful plasma or ion sources. We investigated the layer performance of oxide coatings using a large aperture radio frequency powered plasma source for plasma ion assisted application and related the achieved coating performance to beam parameter of the source. The coatings exhibit low compressive stress values, high refractive indices and low absorption values. Using the new type of source in combination with direct optical monitoring on the dome for the production of challenging interference filters shows a huge potential in terms of stability, running costs and easy maintenance. Examples of application in the visible spectral region will be given. Keywords: SiO2, TiO2, PIAD, thin films 1. INTRODUCTION For the production of interference filters in the visible spectral region SiO2 and TiO2 as coating materials are widely used. These materials are inexpensive, delivering a high refractive index contrast and high resistive coatings with excellent optical properties when deposited with the appropriate technology. Box coaters with ebeam evaporation are the commonly used deposition technique for producing interference filters using these materials. Combining the thermal evaporation with plasma or an ion assisted deposition process allows the production of dense-shift-free coatings. In combination with direct optical monitoring challenging interference filters can be produced. However, to deposit these materials with high rates and to achieve the combination of excellent optical and mechanical properties is still a challenge. Dense-shift-free optical coatings can be produced with high bombardment or by sputtering methods. But these coating often show high amounts of compressive stress and/or having higher absorption values than acceptable. We therefore investigated the potential of a large radio frequency plasma source in the production of interference filters for the visible spectral region in respect to achievable optical and mechanical properties. 2. EXPERIMENTAL Figure 1 shows the large radio frequency source installed in the chamber door the SYRUSpro 1510. Details of the working principle and the chamber set up are described in [1, 2]. The distance between source and dome center was 1300mm. For the evaporation of the material we used two large HPE 12 electron beam evaporators. The system was equipped with a direct optical monitoring system OMS 5000 to control the film thickness of the evaporated layers and layer stacks on the dome. Details of the monitoring set up are described in reference [3, 4]. Spectral measurements were performed in vacuum immediately after deposition. By comparison with subsequent ex-situ measurements we were able to verify dense-shift-free coatings. As substrates we used B270 float glass and 3” silicon wafer for evaluating the intrinsic stress values and the surface roughness of the layers. Transmittance and reflection were characterized by spectro-photometrical measurements with a PE l 900. The integral haze of the coatings measured with BYK-Gardner haze meter plus. Stress values were evaluated by means of the bending of the silicon wafer prior and past the deposition of the layers. For characterization of the ion energy distribution and the achievable ion current density we used a faraday cup measurement tool with retarding grids. For measuring of beam profiles it could be mounted at any high above the source and translated horizontal via a linear feed trough while the source was working. For measuring of the ion current densities and ion energy distribution on the dome, the cup could be mounted in any substrate position.

3. RESULTS 3.1 Source characterization The radio frequency plasma source can be operated with inert gases like Argon, reactive gases like oxygen or nitrogen or mixtures of these. The permanent magnetic field created by to two induction coils allows the use of the electron wave resonance principle [5]. Thereby large amounts of radio frequency power can be coupled into the plasma. We were able to use powers up to 5kW, which was the limit of the available power supply. The power can be either coupled inductively via an induction coil or capacitivly through a coupling capacitor into the...

3.2 Single layer Single layers of SiO2 and TiO2 were investigated with different parameter sets of the plasma source, using ion energies between 400eV and 900eV. At deposition rates of 0,6nm/s we were able to achieve dense-shift-free silica layer with moderate compressive stress values of -245MPa and excellent refractive index uniformity over the whole dome as determined by the reflection curves over the 6 radial position of the 1400mm dome (fig.5). The coatings were very smooth. We measured by scanning atomic force microscope a surface rms roughness of 0,345 nm at a 430 nm thick layer,...

Table 1 Thickness Refractive index Intrinsic Stress Surface roughness @ 10ìm X 10ìm nm n@l MPa Rms (nm) Ra (nm) SiO2 @ 0,6nm/s 430 1,475 @512nm -245 0,345 0,263 TiO2 @ 0,6nm/s 253 2,54 @468nm -54 0,360 0,192 UV-IR-Cut @ 0,5nm/s 5415 - -153 0,487 0,363 3.3 Multilayer As an example of a challenging multilayer we deposited a 42 layer UV-IR-Cut filter with edge wavelengths of 400 and 690 nm and a broad blocking range up to 1200nm. The thickness of the layers was controlled by direct monitoring on the dome with the OMS 5000. The dispersion data necessary for an optimum use of the multiwavelength...

90 92 94 96 98 100 400 420 440 460 480 500 520 540 560 580 600 620 640 660 Wavelength / nm R + T / % 42 Layer UV-IR-Cut with LION XXL: R2_P2 R+T calculated 0 10 20 30 40 50 60 70 80 90 100 350 450 550 650 750 850 950 1050 1150 Wavelength / nm R + T / % R2_P2: Measurement date: 31.01.2008 R2_P2: Measurement date: 05.08.2008 No wavelength shift during 6 month Fig.8 Spectral performance and stability of a 42-layer UV-IR-Cut filter without backside AR Fig.7 R+T plot of a 42-layer UV-IR-Cut filter produced by plasma ion assisted deposition in a SYRUSpro box coater and a comparison with...