1. Thin films technology for RICH detectors
André Braem, CERN, PH-DT2 department
Functionality Production technologies Performance
Presented at the CBM-RICH workshop March 2006

2. Thin films in RICH detectors
The light yield is directly proportional to the performance of the coatings
Reflective coating (R~90%)
Anti-reflective coating (T 92%  ~98%)
Photocathode (QE ~25%)
Wavelength shifter

3. Adherence and barrier layer
Reflective coatings
Protection and reflectance enhancement dielectric layers
Metallic reflector
Adherence and barrier layer
Substrate (glass, Be, plastics…)

4. Adherence and barrier under-layer
A thin (~20nm) layer of Chromium or Nickel is generally used to promote adherence on most substrates.
A barrier layer (SiOx, CrOx…) is mandatory when the substrate material risks to react with the metallic reflective layer.
Inter-diffusion of aluminum (reflective layer) and gold (replicated substrate) :
Cr + SiO diffusion barrier (CF mirror of CERES inner RICH)
2 months at 100˚C : A B C
Al + MgF2
glass Au
Cr + SiO
Al + MgF2
glass
Al + MgF2 Au
glass

5. Metallic reflective coatings
Aluminum is the best metallic reflector for a broad band reflectivity in the far UV.
220 < l < 600 nm  R~90%
in VUV the reflectivity of aluminum is strongly dependent on:
- The production parameters such as vacuum quality, deposition rate etc..
- The substrate roughness (<1.5nm rms)
- The surface oxidation  a protective layer is required.
Magnesium fluoride is commonly used as single protective layer for VUV mirrors
“Standard” VUV coatings for Cerenkov detectors:
DELPHI, CERES, HADES, COMPASS…

6. Metal multi-dielectrics reflective coatings
Reflectivity enhancement at given wavelength by exploiting interferences
Aluminum over coated with n pairs of transparent films of high (H) and low (L) refractive index.
Al reflector
Cr adherence layer
n pairs LH
photons
substrate
low index
high index
qinc.
Dielectric films like SiO2, MgF2 (L-materials) or HfO2, Nb2O5, TiO2 (H- materials) are used.
Hard mirror surface can be achieved  good mechanical protection
Technology limited for l > 220nm due to the lack of H-materials which are
transparent in VUV.

7. Metal multi-dielectrics reflective coatings
Simulated reflectivity of aluminium + pairs of SiO2 – HfO2 layers optimized for l = 300nm

8. Reflective coating optimized for LHCb RICH2
Detection efficiency of an HPD detector (quartz window), with and without double reflection from the coated mirror, qinc. = 30º.
Reflectivity of Al + 1 pair SiO2/HfO2 on glass, qinc. = 30º
l [nm]
Measurement
Simulation
<HPD QE,(2-6 eV)> = 0.176
<HPD QE · R2 (2-6 eV)> = 0.149
Absorbance in HfO2 film !

9. Anti reflective single layer coatings
MgF2 is generally selected for single layer broad band AR coatings
On quartz:
Optimum n1 = 1.22 !
n MgF2 (250nm) =1.412 !
 The surface reflectivity is reduced by a factor 2.
Best performance if
Low refractive index (1.2 < n < 1.4) can be obtained with porous sol gel silica coatings.

10. Anti reflective multi-layers coatings
Pairs of low and high refractive index materials
Many solutions are available in coating industry for UV-VIS light.
 Low residual reflectivity but in a reduced band width !

11. Coating technology
Metals and dielectrics are evaporated in a high vacuum deposition plant.
Substrate (rotation)
Thickness monitor
Aluminium is evaporated from a Tungsten filament
Dielectrics are evaporated from an electron gun source

12. Deposition parameters
Layer Rate [nm/s] Thickness [nm] Pressure [mb]
Cr x10-7
Al > x10-7
MgF x10-7
SiO x10-5 (O2)
HfO x 10-5 (O2)
1 Optimized for l=160nm
2 Optimized for l=275nm
Well known technology available from most industrial partners
- Substrate roughness
Residual pressure
Aluminium deposition rate
Delay between Aluminium and MgF2 depositions
But for optimal reflectivity in VUV some critical parameters must be well under control:

13. Series production of mirrors for LHCb RICH2 @ CERN
Average reflectivity ( nm)

14. VUV reflectivity measurements
lnm
Essential for the production of VUV mirrors ! D2
lamp
VUV
monochromator
Rotating mirror
Meas. PM
Ref. PM

15. Photocathodes for RICH detectors
1). Alkali halide in gas photodetectors - Large area CsI reflective photocathodes - Sensitive in the eV range - Robust and transferable (in moisture free environment) h e-
2). Bi-alkali Antimonide in vacuum tubes - Semitransparent encapsulated photocathodes - Sensitive in the 2 – 4 eV range (K2CsSb + UV extended glass window)

16. Photocathodes for RICH detectors
CsI PC: coating under vacuum and detector assembly under gas
PCB production
CsI deposition
PC transfer & storage
detector assembly & operation
HPDs : PC and detector assembly inside vacuum chamber
Operational photon-detector
Focusing electrodes
Silicon sensor
FE electronics
Vacuum seal
Photocathode processing
Need state of the art technologies (UHV, chemistry, thin film coating, vacuum sealing, encapsulated electronics)

17. CsI Photocathodes production
Vacuum evaporation of CsI powder from 4 sources.
protective box
pcb substrate
Thickness
monitor
PCB substrate
4 CsI sources
+ shutters
Uniform deposition of 300 nm CsI
Deposition rate: ~1nm/s
Substrate temperature: 60˚C
Pressure ~6 x 10-7mb
Heat post-treatment: 60˚C , 8-12hrs
CsI PC transferred in a protection box under Argon atmosphere after quality control
Remote controlled
enclosure box

18. CsI photo-current measurements
Photo-current scan on PC46:
Mean value <Inorm> = 3.71
min-max variation 6%
QE obtained from test beam measurement on 6 CsI PCs
All CsI data from H.Hoedlmoser CERN ALICE/HMPID

19. Series production of CsI PCs
Initial current level before heat enhancement phase Current level after enhancement phase

20. Development of HPD vacuum tubes at CERN
Bi-alkali photocathode e-
dV = 20kV
Ee = 20 keV
Indium joint
Si sensor
In silicon:
3.6eV  1 e/h pair
20keV  5000 e/h
Front end electronics
Spherical HPD
PET HPD

21. The HPD development plant
Turbo Pump
“External” photocathode process
Ultra-high vacuum technology

22. Performance of bi-alkali photocathodes
QE of a K2CsSb photocathode (HPD PC87)
Radial dependence of HPD (PC68) QE
for =230, 290 and 350 nm.

23. Wavelength shifters coatings
P-Terphenyl: Absorbtion range nm ; emission peak at 385 nm
A dedicated plant has been set-up for coatings on PMTs
Vacuum evaporation from a molybdenum crucible:
Pressure ~5x10-5 mb
Thickness >1 mm
Rate > 10 nm/s
Vaporization temp. < 200˚C
Weak mechanical resistance
A protective layer of 30nm MgF2 is required !
Inhomogeneous coatings on glass surfaces
Alternative coating method:
 Solvent spray with transparent binder

24. Performance of P-Terphenyl
Publication of G.J.Davis NIM B 117(1996)
P-terphenyl evaporated at pressure of x 10-1 Torr
External QE of p-terphenyl at ~optimal thickness
Deterioration in efficiency of p-terphenyl after a six month period (175 nm incident light)
Ext. QE: Nph emitted (4p) / Nph incident

25. Summary and conclusions
Functional coatings play an important role at various places in a Cherenkov detector.
The light yield is directly proportional to the performance of the coatings.
Coatings exist for high reflectivity, Anti-reflection, photosensitivity and Wavelength shifting.
For detectors in the visible/near UV range standard industrial solutions are available.
For applications in the far UV / VUV technical challenge and cost increase drastically. Reliability and long term stability become issues.
The light yield is directly proportional to the performance of the coatings Coatings exist for high reflectivity, AR, photosensitivity, WLS For detectors in the visible/near UV range standard industrial solutions exist. For applications in the far UV / VUV technical challenge and cost increase drastically. Reliability and long term stability become issues.

26. Spares

27. CsI quality control: VUV-scanner
2d scan of photo-current across the whole photocathode Relative measurement to a reference CsI photomultiplier
Reference CsI PM
PC current reading
D2 light source
+100V PM
Translation stage

28. Heat post-treatment CsI deposition performed at 60°C
All PCs have similar initial response
Heat post treatment at 60°C for 24hrs
The PC response increases 20-50% during the first hours.

29. Heat post-treatment Decreasing response observed on some PCs !
The presence of residual water in the vacuum chamber is believed to strongly influence the photo-emission properties of the highly hygroscopic CsI film !