rich 2004 mexico, november 2004 andré braem, claude david and christian joram cern ph / ta1 - sd v....
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RICH 2004 Mexico, November 2004 André Braem, Claude David and Christian Joram CERN PH / TA1 - SD
V. Results
Metal multi-dielectric mirror coatings for Cherenkov detectors
André Braem1), Claude David and Christian JoramCERN, PH Department, Geneva, Switzerland
Abstract
Application specific reflective coatings have been developed and are being implemented in LHC experiments currently under construction. The broad-band reflective coating consists of an aluminum film combined with one or two pairs of low and high index dielectric layers. The layer stacks are designed and optimized using a commercial thin film software and verified on small mirror samples. The wavelength of peak reflectivity is tuned to maximize the light yield, taking into account the emission spectrum (e.g. Cherenkov) and the photosensor characteristics. We report about the coatings of mylar foil based light guides for the Hadronic Forward calorimeter of CMS and spherical mirrors for the RICH2 counter of LHCb.
I. IntroductionCherenkov radiation is a weak light source. Good detector performance depends on optimized light transport and collection. High reflectivity mirrors are key elements in many Cherenkov detectors, both in imaging and non-imaging designs. The importance of the mirror reflectivity grows with the number of reflections the Cherenkov photons undergo from source to detection.
Reflectivity enhancing coatings on top of a high quality metallic film provide the possibility to tailor the mirror characteristics to the specific needs of the experiment, taking into account the form of the Cherenkov emission spectrum (dN/dE = const., dN/d= 1/2), the wavelength/energy dependent sensitivity of the photodetector Q(), and possible environmental constraints (temperature, humidity).
The quantity to be optimized can be expressed as , where R describes the
mirror reflectivity, n the number of reflections and T accounts for any other energy dependent transport efficiencies (e.g. transparency of a radiator window). For the choice of the limits E1 and E2 parameters like
the chromaticity of the radiator medium or radiation driven aging of components may play a role.
We report about two developments of multilayer dielectric coatings on Aluminum films to produce high reflectivity mirrors.
II. Metal multi-dielectric mirror coatings
III. Fabrication and characterization of metal multi- dielectric coatings at CERN.
IV. Industrial fabrication of reflective films at FEPThe CMS HF application required to produce ~100 m2 reflective film. Based on positive experience in a previous project, the optimization of layer stack and substrate choice was done in collaboration with Fraunhofer Institute for Electron Beam and Plasma Technology (FEP), Dresden, Germany (contact person: Dr. M. Fahland). FEP carried out also the final production.
The FOSA 600 roll coater at FEP can coat polymer webs of 600 mm width with 6 different materials at a speed of 0.3–3 m/min. The applied technique is dual magnetron sputtering.LHCb RICH 2
(sectional view)CMS Hadron Forward Calorimeter
2
1
)()()(E
E
Qn dEETEER
1. The flat and spherical mirrors of the RICH2 detector of LHCb require optimized reflectivity in a broad band centered around 275 nm for incidence angles of 21º (spherical mirror) and 48º (flat mirror). The substrate is a 6 mm borosilicate glass. The goal is to achieve R>94% for 250<<300 nm and R>80% for 210<<500 nm.
2. In the CMS Forward Hadron Calorimeter (HF) Cherenkov light produced in quartz fibers needs to be transported via a 40 cm long air core light guide (2.5 cm Ø) from the fiber bundle to the PMTs. The inner surface of the light guide has to provide maximum reflectivity in the range 400<<650 nm for incidence angles of about 70º. On average the photons undergo 6 reflections. The substrate is a 75 m thick polyester film.
The layer stack developed for the mirrors of LHCb RICH2 consists of Al + 1 pair SiO2/HfO2. The Hafnium Oxide layer
which terminates the stack represents a hard abrasion resistant finish. The measured reflectivity reaches 96% at 275 nm, the wavelength for which it was optimized, and agrees well with the FilmStar simulation. The wavelength dependence of the reflectivity is well matched to the quantum efficiency of the LHCb Pixel HPD photodetectors. The reflection from the two mirrors leads to a reduction of the number of photons of only 15%. So far 32 (of ~100) mirrors have been coated with an average reflectivity of 95.5 ± 0.9 % (250-350 nm, see plot below).
The chosen layer stack is• Cr adhesion layer (10 nm)• Al reflectance layer (70 nm)• 2 pairs of SiO2 / Nb2O5 (66 / 53 nm)
The substrate is a 75 m thick polyester film (Dupont Melinex 400)
Al reflector
Cr adherence layer
n pairs LH
h
substrate
low index
high indexlow index
high index inc.
The basic principle consists in enhancing (or decreasing) the reflectivity of a metal film at a given wavelength band by exploiting interference effects in thin films.
A high reflectivity metal layer (Al, R~92%; Ag, R~96%) is overcoated with n pairs of transparent films of high (H) and low (L) refractive index. For this purpose dielectric films like SiO2, MgF2 (L-materials) or HfO2, Nb2O5, TiO2 (H-materials) are used. The optical thickness of the layers dopt = n·d·cosinc is usually chosen to be /4. The dielectric coatings lead therefore to a wavelength and angle dependent modulation of the reflectivity. The larger the ratio of the refractive index of the LH pair, the higher is the peak reflectivity and width of the enhanced region. Adding LH pairs, optimized for the same wavelength, will increase the reflectivity but narrow down the useful range.
The mathematical treatment of multi-layer coatings relies on elaborated matrix methods (see e.g. the text books ‘Thin Film Optical Filters’ by H. A. MacLeod, and ‘Optical Properties of Thin Solid Films’ by O.S. Heavens. Simulation codes like TFG FilmStar (used by us) and SCI Film Wizard allow to predict the behavior of multilayer films with an accuracy which depends on the knowledge of the relevant optical properties (refractive index n() , absorption coefficient k() ) of the deposited layers. Those are generally dependent on the deposition method and process parameters. The simulation is therefore useful to define a concept and a promising starting point. The optimum result can however only be achieved by systematically varying the experimental parameters, where the simulation will again be helpful in their interpretation.
Reflectivity of Aluminium + 1 and 2 pairs of SiO2/TiO2 on glass, inc. = 30º.
Reflectivity of Al + 2 pairs SiO2/Nb2O5 on Melinex 400 Polyester film, inc. = 30º.
Detection efficiency of an HPD detector (quartz window), with and without double reflection from the coated mirror, inc. = 30º.
Reflectivity of Al + 1 pair SiO2/HfO2 on glass, inc. = 30º
Average reflectivity (250-350nm) of the first batch of LHCb mirror coatings
Simulated reflectivity of pure Aluminium and Al coated with reflection enhancement layers optimised at 300 nm.
Al + 1 pair SiO2/HfO2
Simulated angular dependence of reflectivity at 300nm .
Complex refractive index (n, k) of Hafnium dioxide HfO2
A dedicated plant has been installed at CERN for the production of coatings on substrates up to 0.9 m diameter. It consist of a high vacuum evaporation chamber of 1 m diameter evacuated with a cryopump (3500 l/s). The end pressure is about 1x10-5 Pa. Uniform layer thickness (±5%) is achieved by rotating the substrate. The layer thickness is controlled with a calibrated quartz crystal balance. The typical production rate is 2 full process cycles per day.
ProcessAl of 99.999% purity is evaporated from a Tungsten spiral filament on a Chromium adherence undelayer (20 nm) with the substrate kept at room temperature. The dielectrics are evaporated from an electron gun source in an oxygen atmosphere (pO2 ~ 10-3 Pa) to compensate for partial dissociation and oxygen loss during the evaporation process. The purity of the use materials is: SiO2 99.99% and HfO2 99.9%, from tablets. TiO2 is obtained from Ti3O5 99.5% tablets.
Typical process parametersAluminium: pressure 2x10-5 Pa, deposition rate 5 nm/s, thickness 85 nm.Dielectrics: deposition rate 0.2 nm/s, thickness: SiO2 38 nm,HfO2 28 nm. These values are derived from the quartz crystal balance and are optimized for peak reflectivity at = 275 nm. The reflectivity can be measured only after the process is completed. A spectrophotometer (Perkin Elmer Lambda 15) allows to determine the absolute reflectivity at an angle of incidence of 30º in the wavelength range 200 – 800 nm.
For CMS, prototype films with one and two pairs SiO2/TiO2, evaporated on glass at CERN, gave excellent reflectivity values in the desired 400 – 600 nm range. For the final production at FEP, a 2 pair stack of SiO2/Nb2O5 was sputtered in the roll coater onto the polyester film. Nb2O5 replaced TiO2 for its higher sputtering yield. This minimizes the heat load and therefore reduces the stress in the layer stack. The resulting film has good adherence on the polyester and can easily be rolled to a light guide of 2.5 cm diameter.
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Incidence angle [deg.]
R [%
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Random polarisationSP
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W.L. [nm]
R [%
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Aluminium + MgF2
Al + 1 pair SiO2-HfO2
Al + 2 pairs SiO2-HfO2
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W.L. [nm]
R [%
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2 pairs SiO2-TiO2 measurement
2 pairs SiO2-TiO2 simulation
1 pair SiO2-TiO2 simulation
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W.L. [nm]
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Energy [eV]
<HPD QE,(2-6 eV)> = 0.176 <HPD QE · R2 (2-6 eV)> = 0.149
620 413 310 248 206
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R [
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Process number
R [%
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Nb of mirrors
one of 1728 readout channels
back shield
Al tube
fibre bundle (N fibres)
19°
71°
Iron matrix ‘Back shield’
PMT
~ 40 cm6 reflections on average
Cherenkov lightfrom calorimeter
reflective foil
Pixel HPD photodetectors
2 spherical mirrors
• RC = 8.6m
• 56 hexagonal segments
circumscribed in =51cm
• <inc.> = 21°
2 flat mirrors:
• 40 segments
• 41 x 38 cm2 each
• <inc.> = 48°
CF4
10 11 12m
1) Corresponding author: [email protected]