Coating machine

The coatings were fabricated with a SYRUSpro 1110 optical coating production system, by Leybold Optics. It is a plasma ion assisted e-beam evaporation system especially designed for industrial production of optical thin films. A photo of the Syrus’ deposition chamber is shown in figure 12. The substrates are held by a spherical, rotating calotte near the ceiling of the deposition chamber. A diagram of the calotte is shown in figure 13. There are two electron beam guns (EBGs) and two rotating crucible holders, one for each electron gun. EBG generates an electron beam which is deflected by magnetic field to a crucible holder. The evaporant material in the crucible melts and evaporates under the e-beam. The crucible acts as a point source for the evaporated molecules. The evaporation can be controlled using shutters to block the vaporized molecules. Resistive evaporation is also possible from a single container called a boat, which is mainly used for evaporating metals like aluminium and silver.

SYRUSpro 1110 employs an advanced plasma source (APS) which is located in the middle of the deposition chamber floor. APS generates dense argon plasma that is led into the deposition chamber as an expanding beam. It offers many of the same benefits as the early ion bombardment mechanisms, but also has few advantages. APS guarantees nearly amorphous and especially durable films, and it can be applied to wide substrate areas. In addition to argon the APS also has an optional gas inlet for oxygen. [10, 38, 39] Additionally the plasma bombardment may be used for pre-deposition substrate surface etching. The energetic ions remove contaminants from the surface and can enhance adhesion even further. The central location of the APS ensures even distribution of plasma over the entire calotte. The long distance from the APS to the calotte minimizes the effects of thermal radiation on the substrates.

The deposition machine has both a QCM unit and an intermittent monochromatic transmittance monitoring system installed, and they can both be used during a deposition process. The optical monitoring system is a model OMS 5100 by Leybold Optics. For the intermittent measurement one of the calotte sector plates has to be replaced with an unique sector plate that contains a hole for reference light measurement and a slot for the monitoring glass (also called test glass or ’TG’ from

OMS sector plate (on the calotte) Deposition mask

Heater Evaporant crucible

and e-beam gun

Boat for resistive evaporation Advanced plasma

source (APS)

Figure 12. A photo of the coating machine’s deposition chamber. Shutters for the crucibles, the APS and the boat have been removed.

here on). A halogen lamp acts as the broadband light source. It is protected from the evaporated material by a leaf-shaped deposition mask. The lamp casts a light beam on the chamber ceiling where the OMS’s collimator is located. The calotte rotates between the lamp and the collimator. This set-up is described in figure 14 along the principle of intermittent measurement.

The OMS sector plate has five rows and two columns for substrate slots. The OMS test glass is located on the 3rd row of the plate. The substrates on the third row receive the same amount of deposited material in the same incidence angle as the test glass. The coatings on the other rows differ slightly from that of the test glass due to a slightly different angle of incidence and distance from the EBGs. For this reason the 3rd row usually yields the best results, as their actual coating is almost exactly identical to that of the test glass.

The deposition distribution vertically across the calotte can be optimized by using a deposition mask. The mask shadows a section of the calotte obstructing the deposition proportionally to its size. Because thermal evaporation has no corner conformation abilities, altering the shape of the mask alters the deposition distribution vertically across the rotating calotte. When optimizing the deposition distribution test glasses are loaded into the chamber on every available calotte row. After a

1 2 3 4 5

Ref TG

OMS sector plate

Figure 13. A diagram of the calotte found in the deposition chamber. The calotte is divided into four segments in which the sector plates are placed, supported by the cross-shaped metal frame. The calotte rotates around its central axis. The sector plates contain holes where the substrates are loaded.

Sector plates can have differing substrate layouts, but only the sector plate used for intermittent transmittance monitoring is shown here. ’TG’ is the slot for the monitoring glass, abbreviated from ’test glass’. ’Ref’ is a hole through which the 100 % transmittance signal is measured. Numbered slots are where the substrates were loaded during experiments.

deposition process has been performed either the reflectance or transmittance profile is measured for all the test glasses. In an optimal scenario all the glasses have received an equivalent amount of coating material and therefore they should have identical spectra. If the deposition distribution over the calotte is not optimized the measured spectra will have shifted along the wavelength axis. A shift towards longer wavelengths indicates that the layers are too thick, and a shift towards shorter wavelengths indicates that the layers are too thin. In order to fix thin layers the mask should be narrowed to increase the deposition rate. Respectively by widening the mask thick layers can be corrected. The horizontal deposition distribution is only affected by the sector plate geometry, and therefore should not be of concern unless the plates have become deformed. When depositing sensitive coatings it should

be noted that the deposition distribution is also affected by the APS anode tube geometry.

For evaporant materials common optical dielectrics were used. For low refractive index material SiO2 was used (Umicore, 1.5−3.5 mm granulates of 99.99 % purity).

For high refractive index materials Ti3O5 (Umicore, 0.7−5.0 mm granulates of 99.5 % purity) and HfO2 (COTEC, 1.0−2.8 mm white granulates) were used. Ti3O5

has been studied and shown to create robust TiO2 thin film layers with consistent refractive index when vacuum evaporated [40, 41]. HfO2 does not have quite as high refractive index as TiO2 but is better suited for UV-region filters than TiO2

due to its low absorptivity. Silver (Balzers, 0.7−1.5 mm granulates of 99.99 % purity) was also used in the absorption type filter. Substrate materials used for coatings presented in this thesis were either BK7 or B270, which are common optical glasses with refractive index n = 1.52. The B270 substrates (thickness 1 mm) were purchased from SCHOTT. The BK7 raw material was also purchased from SCHOTT, and polished at Millog (thickness 2.5 mm). They have similar optical properties at visible and infrared wavelength region.