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Metal multi-dielectric mirror coatings for Cherenkov detectors
André Braem1), Claude David and Christian Joram
CERN, 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. Introduction
Cherenkov 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/dl = 1/l2), the wavelength/energy dependent sensitivity
of the photodetector eQ(E), and possible environmental constraints (temperature, humidity).
E2
n
R
 ( E )  e Q ( E )  T ( E ) dE
, where R describes the
E1
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.
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<l<300 nm and R>80%
for 210<l<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<l<650 nm for
incidence angles of about 70º. On average the photons undergo 6 reflections. The substrate is a 75
mm thick polyester film.
Pixel HPD
photodetectors
•
RC = 8.6m
• 40 segments
•
56 hexagonal
segments
• 41 x 38 cm2
each
circumscribed
in f=51cm
• <qinc.> = 48°
•
Iron matrix
‘Back shield’
<qinc.> = 21°
Al tube
Cherenkov light
from calorimeter
11
19°
71°
fibre bundle
(N fibres)
~ 40 cm
6 reflections on average
12m
II. Metal multi-dielectric mirror coatings
hn
high index
low index
high index
1)
R [%]
300
400
500
W.L. [nm]
Corresponding author: [email protected]
k
n
l [nm]
413
310
248
206
0.20
0.15
0.10
92
<HPD QE,(2-6 eV)> = 0.176
<HPD QE · R2 (2-6 eV)> = 0.149
0.05
91
0.00
2.00
90
0
5
10
15
20
Process number
25
30
0
5
10
15
20
25
3.00
Nb of mirrors
4.00
Energy [eV]
5.00
6.00
Detection efficiency of an HPD detector
(quartz window), with and without double
reflection from the coated mirror, qinc. = 30º.
Average reflectivity (250-350nm) of the first batch of LHCb
mirror coatings
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.
100
100
95
95
90
85
90
80
75
Aluminium
70
Aluminium + MgF2
Al + 1 pair SiO2-HfO2
60
200
600
Reflectivity of Al + 1 pair SiO2/HfO2 on
glass, qinc. = 30º
93
R [%]
R [%]
k
0.3
0.25
0.2
0.15
0.1
0.05
0
600
500
W.L. [nm]
94
85
20
40
60
80
Incidence angle [deg.]
400
0.25
90
Random polarisation
S
P
300
95
95
n
50
200
96
n pairs LH
85
80
400
500
75
600
W.L. [nm]
Simulated reflectivity of pure Aluminium and
Al coated with reflection enhancement
layers optimised at 300 nm.
70
350
65
2 pairs SiO2-TiO2 measurement
2 pairs SiO2-TiO2 simulation
1 pair SiO2-TiO2 simulation
75
300
80
70
Al + 2 pairs SiO2-HfO2
65
Complex refractive
index (n, k) of
Hafnium dioxide
HfO2
Measure
Simulation
60
97
100
3
2.5
2
1.5
1
0.5
0
200
70
98
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(l) , absorption coefficient k(l) ) 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.
0
80
0.30
A high reflectivity metal layer (Al, R~92%; Ag,
Al reflector
R~96%) is overcoated with n pairs of transparent Cr adherence layer
films of high (H) and low (L) refractive index. For
substrate
this purpose dielectric films like SiO2, MgF2 (Lmaterials) or HfO2, Nb2O5, TiO2 (H-materials) are
used. The optical thickness of the layers dopt = n·d·cosqinc. is usually chosen to be l/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.
Simulated angular
dependence of
reflectivity at 300nm .
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).
qinc.
low index
Al + 1 pair SiO2/HfO2
90
620
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.
100
90
80
70
60
50
40
30
V. Results
R [%]
10
100
back shield
PMT
The 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 substrate is a 75 mm
thick polyester film (Dupont
Melinex 400)
one of
1728 readout
channels
reflective
foil
CF4
IV. Industrial fabrication of reflective films at FEP
The chosen layer stack is
• Cr adhesion layer (10 nm)
• Al reflectance layer (70 nm)
• 2 pairs of SiO2 / Nb2O5
(66 / 53 nm)
2 spherical mirrors
2 flat mirrors:
Typical process parameters
Aluminium: 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 l = 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.
R [%]
(sectional view)
Process
Al 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.
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.
CMS
Hadron
Forward
Calorimeter
LHCb RICH 2
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.
R [%]
The quantity to be optimized can be expressed as
III. Fabrication and characterization of metal multidielectric coatings at CERN.
400
450
500
550
W.L. [nm]
600
55
650
Reflectivity of Aluminium + 1 and 2 pairs of
SiO2/TiO2 on glass, qinc. = 30º.
RICH 2004 Mexico, November 2004
Measurement
Simulation
60
50
350
400
450
500
550
W.L. [nm]
600
650
Reflectivity of Al + 2 pairs SiO2/Nb2O5 on
Melinex 400 Polyester film, qinc. = 30º.
André Braem, Claude David and Christian Joram
CERN PH / TA1 - SD