Design and status of the OSN Coating Plant
M.C. Cárdenas (IAA), J. Sánchez del Río (IAA), M. A. Sánchez Carrasco (IAA) & I. Bustamante (IAA).
Abstract
VACA (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.
Status
Introduction
The 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.
The 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.
Electrical design
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.
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.
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.
Lateral flanges
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.
Vacuum chamber model: external view
Vacuum chamber and opening system at the IAA mechanics laboratory (Granada)
Design criteria
Design criteria for the high vacuum generation system:
• Clean high vacuum,
• Pumping speed,
• Reliability,
• and low cost.
for 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.
Diagram of the electrical design done for the filaments electrical feeding
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.
46
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.
60
1500
72
Vacuum gauge
controller
Glow discharge cleaning device:
94
1000
Intensidad (A)
Electronic frequency converter
for the turbomolecular pump
Pressure
gauges
Resistance heated sources:
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.
500
Mirror substrate support structure:
0
14
120
Upper flat support structure and the bottom adapted to the vessel curvature.
0
0
20
40
60
80
100
120
Tiempo (s)
High vacuum
flange
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.)
High vacuum
gate valve
Glow discharge cleaning device:
Rotary vane
forevacuum pump
This subsystem is used for clean the substrate surface before the aluminium
evaporation process in order to achieve a higher adherence of the coating.
High vacuum
turbomolecular pump
High vacuum generation system from atmosphere (approx. 10+3 mbar) to 10-6 mbar
made for vacuum-tight evaluation tests of the chamber.
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
Mirror
substrate
The final high vacuum generation system shares the design criteria of the
preliminary one.
0 Vacuum vessel
1 Pressure gauges:
Pirani vacuum gauge 5·10-4-10-3 mbar
Ionization vacuum gauge 10-10-10-1 mbar
2 Flexible tubing
3 Manual valve
4 Electromagnetic valve
5 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)
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.
Configuration of the preliminary high vacuum generation system
1000
Vacuum chamber model: inside view
Ultimate pressure: pend = 2,12·10-4 mbar
Leak rate: QL = 0,080 mbar·l/s
Pump-down time: 6 h
100
Filaments: perimetral ring geometry
Project plan
10
Presión (mbar)
Substrate support
structure
• Manufacturing of the subsystems as well as the evaporation system control
1
0
100
200
300
400
0.1
0.01
Primera carga
Segunda carga
0.001
Tercera carga
500
600
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.
Glow discharge assembly
0.0001
Liquid nitrogen trap
(Meissner trap)
Tiempo (min.)
Results of the final pumping tests
Details of the three main subsystems and the supporting framework
We invite you to e-mail us for further information.
Conchi Cárdenas ([email protected]) and Justo Sánchez ([email protected])
VACA