thin films technology for rich detectors functionality production technologies performance presented...

Thin films technology for RICH detectors

Functionality

Production technologies

Performance

Presented at the CBM-RICH workshop 06-07 March

2006

André Braem, CERN, PH-DT2 department

A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006

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


Reflective coatings

Substrate(glass, Be, plastics…)

Adherence and barrier

layer

Metallic reflector

Protection and reflectance enhancement dielectric layers


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)

ABC

2 months at 100˚C :

Au

Cr + SiO

Al + MgF2

glass

Al + MgF2

Auglass

Al + MgF2

glass


Metallic reflective coatings

Reflectivity of metal coatings

0

20

40

60

80

100

150 200 250 300 350 400 450 500 550 600 650

W.L. [nm]

R [%

]

Aluminium

Silver

Aluminum is the best metallic reflector for a broad band reflectivity in the far UV.

220 < < 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…

Reflectance of Aluminium protected with MgF2

50

60

70

80

90

100

150 160 170 180 190 200 210 220 230 240 250

W.L. [nm]

R [

%]


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 indexhigh index

low index

high index inc.

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 > 220nm due to the lack of H-materials which are

transparent in VUV.


Metal multi-dielectrics reflective coatings

60

65

70

75

80

85

90

95

100

200 300 400 500 600

W.L. [nm]

R [%

]

Aluminium

Aluminium + MgF2

Al + 1 pair SiO2-HfO2

Al + 2 pairs SiO2-HfO2

Simulated reflectivity of aluminium + pairs of SiO2 – HfO2 layers optimized for = 300nm


Reflective coating optimized for LHCb RICH2

<HPD QE,(2-6 eV)> = 0.176 <HPD QE · R2 (2-6 eV)> = 0.149

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2.00 3.00 4.00 5.00 6.00

Energy [eV]

620 413 310 248 206

50

60

70

80

90

100

200 300 400 500 600

W.L. [nm]

R [

%]

Measure

Simulation

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, nc. = 30º

Absorbance in HfO2 film !

[nm]

Measurement

Simulation


Anti reflective single layer coatings

snnn 01

/ 4

ns

n1

n0

R

T

/ 4

ns

n1

n0

R

T

MgF2 is generally selected for single layer broad band AR coatings

Q uartz surf ace refl ectivity (simulation)

0

2

4

6

8

10

220 300 380 460 540

W.L. [nm]

R [

%]

Uncoated

Coated with MgF2

On quartz:

Optimum n1 = 1.22 !

n MgF2 (250nm) =1.412 !

The surface reflectivity is reduced by a factor 2.

Low refractive index (1.2 < n < 1.4) can be obtained with porous sol gel silica coatings.

Best performance if


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 !

Simulation of Hf O2-SiO2 AR coatings on quartz surf ace

0

1

2

3

4

5

6

7

8

9

10

220 300 380 460 540

W.L. [nm]

R [

%]

Uncoated

2 pairs

4 pairs


Coating technology

Thickness monitor

Substrate (rotation)

Metals and dielectrics are evaporated in a high vacuum deposition plant.

Aluminium is evaporated from a Tungsten filament

Dielectrics are evaporated from an electron gun source


Deposition parameters

Layer Rate [nm/s] Thickness [nm] Pressure [mb]

Cr 0.2 10 1x10-7

Al >20 85 2x10-7

MgF2 1.5 311

2x10-7

SiO2 0.2 382

2x10-5 (O2)

HfO2 0.2 282

2x 10-5 (O2)

1 Optimized for =160nm

2 Optimized for =275nm

- Substrate roughness

- Residual pressure

- Aluminium deposition rate

- Delay between Aluminium and MgF2 depositions

Well known technology available from most industrial partners

But for optimal reflectivity in VUV some critical parameters must be well under control:


Series production of mirrors for LHCb RICH2 @ CERN

Refl ectivity of 63 mirrors coated f or LHCb / RI CH2

60

65

70

75

80

85

90

95

100

200 300 400 500 600W.L. [nm]

R [

%]

90

91

92

93

94

95

96

97

98

0 5 10 15 20 25 30 35 40 45 50 55 60

Process number

Ave

rag

e r

efle

cta

nce

[%

]

0 5 10 15 20 25 30 35 40 45

90

91

92

93

94

95

96

97

98

Nb of mirrors

Average reflectivity (250-350 nm)


VUV reflectivity measurements

Essential for the production of VUV mirrors !

D2

lamp

VUVmonochromator

Rotating mirror

Meas. PMRef.

PM

Refl ectivity of diff erenc qualities of VUV coatings

40

50

60

70

80

90

100

5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50

Energy [eV]

R[%

]

250 225 207 190 183 175 155 nm


Photocathodes for RICH detectors

1). Alkali halide in gas photodetectors

- Large area CsI reflective photocathodes

- Sensitive in the 7.75 - 6.2 eV range

- Robust and transferable (in moisture free environment)

e-

h

2). Bi-alkali Antimonide in vacuum tubes

- Semitransparent encapsulated photocathodes

- Sensitive in the 2 – 4 eV range (K2CsSb + UV extended glass window)

h

e-


HPDs : PC and detector assembly inside vacuum chamber

CsI PC: coating under vacuum and detector assembly under gas

PCB production CsI deposition PC transfer & storage detector assembly & operation

• Need state of the art technologies (UHV, chemistry, thin film coating, vacuum sealing, encapsulated electronics)

Operational photon-detector

Focusing electrodes

Silicon sensor

FE electronics Vacuum seal

Photocathode processing

Photocathodes for RICH detectors


CsI Photocathodes production

Remote controlled

enclosure box

protective box

pcb substrate

Uniform deposition of 300 nm CsI

Deposition rate: ~1nm/sSubstrate temperature: 60˚CPressure ~6 x 10-7mbHeat post-treatment: 60˚C , 8-12hrs

CsI PC transferred in a protection box under Argon atmosphere after quality control

4 CsI sources+ shutters

Thickness monitor

PCB substrate

Vacuum evaporation of CsI powder from 4 sources.


CsI photo-current measurements

x [mm]

Inorm

y [mm]

stdev Ipc( )

mean Ipc( )0.01

mean Ipc( ) 3.71

QE obtained from test beam measurement on 6 CsI PCs

Photo-current scan on PC46:

Mean value <Inorm> = 3.71

min-max variation 6%

All CsI data from H.Hoedlmoser CERN ALICE/HMPID


Series production of CsI PCs

0

0,5

1

1,5

2

2,5

3

3,5

4

avera

ge n

orm

ali

zed

cu

rren

t

initial current level before enhancement

current level after enhancement phase

Initial current level before heat enhancement phase

Current level after enhancement phase


Development of HPD vacuum tubes at CERN

5” HPD

Indium joint

h

e-

Si sensor

Front end electronicsIn silicon:3.6eV 1 e/h pair20keV 5000 e/h

dV = 20kVEe = 20 keV

10” HPD

PET HPDSpherical HPD

Bi-alkali photocathode


Turbo Pump

The HPD development plant

“External” photocathode process

Ultra-high vacuum technology


Radial dependence of HPD (PC68) QE

for =230, 290 and 350 nm.

QE of a K2CsSb photocathode (HPD PC87)

Performance of bi-alkali photocathodes

200 300 400 500 600

lambda (nm)

0

4

8

12

16

20

24

28

32

Q.E

. (

%)

HPD PC87(produced Easter Sunday 2001)


Wavelength shifters coatings

A dedicated plant has been set-up for coatings on PMTs

Vacuum evaporation from a molybdenum crucible:

Pressure ~5x10-5 mbThickness >1 mRate > 10 nm/sVaporization temp. < 200˚C

Weak mechanical resistance

A protective layer of 30nm MgF2 is required !

P-Terphenyl: Absorbtion range 110-360 nm ; emission peak at 385 nm

Inhomogeneous coatings on glass surfaces

Alternative coating method:

Solvent spray with transparent binder


Performance of P-Terphenyl

Publication of G.J.Davis NIM B 117(1996) 421-427

Ext. QE: Nph emitted (4) / Nph incident

External QE of p-terphenyl at ~optimal thickness

P-terphenyl evaporated at pressure of 1-1.5 x 10-1 Torr

Deterioration in efficiency of p-terphenyl after a six month period (175 nm incident light)


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.


Spares


CsI quality control: VUV-scanner

Reference CsI PM

PC current reading

D2 light source

+100V

PMTranslation stage

_

_

CsI CsI noisenorm

PM PM noise

I II

I I

2d scan of photo-current across the whole photocathode

Relative measurement to a reference CsI photomultiplier


0 5 10 15 20 25

2,6

2,8

3,0

3,2

3,4

n

orm

aliz

ed

cu

rre

nt

time after CsI deposition [h]

• 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.

Heat post-treatment


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 !

thin films technology for rich detectors functionality production technologies performance presented...

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