Design and status of the OSN Coating PlantM.C. Cárdenas (IAA), J. Sánchez del Río (IAA), M. A. Sánchez Carrasco (IAA) & I. Bustamante (IAA).

AbstractVACA (Vacuum Aluminizer Chamber for Astronomical mirrors) is a project to develop an aluminizing system for the astronomical mirrors of the Sierra Nevada Observatory (OSN). After the successful pass of the vacuum tests the chamber has been accepted in order to proceed with the project. We present the current status of the project. The design of the aluminization system is based on an Evaporative Coating System. A perimetral ring source geometry has been proposed. Evaporation source positions have been studied to optimize the number of sources and the sources spacing to achieve a uniform coating thickness over the diameter of the mirrors. A HV system based on Turbomolecular pumps are considered for reasons of economy, quality of vacuum, pumping speed and reliability. The interaction of the Glow discharge and the Meissner cooling with the cleaning - pumping -deposition cycle has been designed to achieve an optimum aluminium coating.

Electrical design

Resistance heated sources:· coil evaporation filaments made of twisted tungsten with 3 wires,· 12 V @ 25 A, 300 W.

Our system design consist of 48 filaments divided in four electrical sectors that are interconnected inside of the chamber. Every sector has three current terminals and it is fed with a triphase transformer 380/12 V, 150 A/phase, that implies 3100 W/sector or 25 A/filament.

The four triphase transformers are electrical fed with a autotransformer motorized 400/0-400 V, which allows to rise, hold and fall the total system wattage in a programmable way.

Mechanical design

Chamber size: 2.2 m height, 2.5 m diameter.Internal volume: 11 m3 .Weight: 2.5 TM approx. Material: Cast iron.

The chamber has three parts.The lower part has two lateral flanges (DN 160) used to pump the chamber.The upper part has four lateral flanges (DN 160) for installing the process components:

• a viewing window,• a vent valve for the glow discharge cleaning process,• the liquid nitrogen feedthrough,• the high current feedthroughs,• the film thickness monitor feedthrough.

IntroductionThe coating unit is designed to deposit high reflective, homogeneous aluminium coatings. For this process the Evaporative Coating technology was proposed.

The first step of the project was the vacuum-tight evaluation of the chamber in the High Vacuum (HV) region (10-3-10-7 mbar).With this aim we:•Designed and made a preliminary high vacuum generation system.•Done mechanical improvements in the chamber.•Done pressure rise tests and analyzed the results.

The results of the final tests (done until end of January03) were satisfactory so the vacuum vessel was accepted and we continue with the whole design of the coating unit.

Project plan

• Manufacturing of the subsystems as well as the evaporation system control have started and their integration will coming soon.• The final high vacuum generation system is in process of design and construction.• The vacuum system control via LabView under PC program.• The performance of the evaporation coating system must be verified and qualified: the film thickness, uniformity and reflectance, as well as the film purity and durability.

High vacuum generation system from atmosphere (approx. 10+3 mbar) to 10-6 mbarmade for vacuum-tight evaluation tests of the chamber.

Vacuum gauge controller

Electronic frequency converter for the turbomolecular pump

Rotary vaneforevacuum pump

High vacuumturbomolecular pump

High vacuumgate valve

High vacuum flange

Pressuregauges

We invite you to e-mail us for further information.Conchi Cárdenas (conchi@iaa.es) and Justo Sánchez (justo@iaa.es)

Vacuum chamber and opening system at the IAA mechanics laboratory (Granada)

Design criteria for the high vacuum generation system: • Clean high vacuum,• Pumping speed, • Reliability,• and low cost.

The way in which the vacuum is generated has a significant impact on the quality of the coating. By pumping the vacuum chamber down to pressures in the range of 10-6 mbar interfering gas and water molecules are removed from the processing chamber. In the case of evaporation coating the turbomolecular pump meets all requirements of the coating system as to a hydrocarbon-free vacuum.

Results of the final pumping tests

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Ultimate pressure: pend = 2,12·10-4 mbarLeak rate: QL = 0,080 mbar·l/sPump-down time: 6 h

StatusThe design of the subsystems for the coating unit is completed (done until end of April04). That covers the following parts:• the thin film coating equipment based on resistance heated sources, • the glow discharge cleaning device, • the liquid nitrogen trap (Meissner trap),• the film thickness monitor,• the mirror substrate support structure,• the supporting framework and the auxiliary equipment,• the vacuum system control and the evaporation system control.

Configuration of the preliminary high vacuum generation system

0 Vacuum vessel1 Pressure gauges:

Pirani vacuum gauge 5·10-4-10-3 mbarIonization vacuum gauge 10-10-10-1 mbar

2 Flexible tubing3 Manual valve4 Electromagnetic valve5 Rotary vane forevacuum pump 30 m3/h(from atmosphere to 10-2 mbar)

6 Turbomolecular pump 400 l/s(from 10-1 to 10-6 mbar)

VACAVACA

Diagram of the electrical design done for the filaments electrical feeding

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Electrical feeding process proposed for the tungsten filaments with Al.Note: To prepare the filaments for the aluminium evaporation is necessary to do a previous burning process without Al in high vacuum. In this process every filament has to stand 500 W during 10 seconds (approx.)

Design criteriafor the subsystems, the supporting framework and the auxiliary equipment:

• modular systems for easy assembly,• internal structures of stainless steel, aluminium and cooper,• ceramic thermoelectrical isolatings, • the biggest telescope mirror to introduce is a 1.5 m diameter.

Resistance heated sources:Evaporation source positions have a perimetral ring geometry. The number of sources, the sources spacing, the ring diameter and the ring height have been optimized according with the electrical requirements, and in order to achieve a uniform coating thickness over the surface of the mirror substrate.

Glow discharge cleaning device:The device is based on a ring design of a conductive material centred on the middle of the substrate. The ring has a diameter equal to the half mirror diameter. And it is placed at the same height of the filaments to achieve an optimum surface cleaning.

Mirror substrate support structure:Upper flat support structure and the bottom adapted to the vessel curvature.

Details of the three main subsystems and the supporting framework

Liquid nitrogen trap (Meissner trap)Glow discharge assembly

Filaments: perimetral ring geometry

Vacuum chamber model: external view

Lateral flanges

Glow discharge cleaning device:

This subsystem is used for clean the substrate surface before the aluminium evaporation process in order to achieve a higher adherence of the coating.

To produce the plasma our device is fed with a high voltage power supply DC unit from 0 to 3.000 V/500 mA (maximum ripple of 5% approx.).

High vacuum generation system design

The final high vacuum generation system shares the design criteria of the preliminary one.

Basically the system design is similar to the preliminary one but with the following improvements:1. In order to decrease the pump-down time we are evaluating two possibilities to increase the total conductance values. The first one is to increase the nominal width of the pumping flange. And the second one is to pump simultaneously for two flanges.2. A Meissner trap in the inside of the chamber will allow to reach the ultimate pressure (in the range of 10-6 mbar) needed to start the aluminium evaporation process. Filling the cold trap with liquid nitrogen will cause the pressure to drop abruptly, by one power of ten or more. If the container is contaminated since the vapours will freeze out in the trap.

Mirror substrate

Substrate support structure

Vacuum chamber model: inside view