silicon microfluidic scintillation detectors

13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013

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SILICON MICROFLUIDIC SCINTILLATION

DETECTORS

1 Physics Department, European Organization for Nuclear Research (CERN), Switzerland2 Microsystems Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

3 Sezione di Roma 1, Istituto Nazionale di Fisica Nucleare (INFN), Italy

10.10.2013 // [email protected]

P. Maoddi1,2, A. Mapelli1, P. Bagiacchi3, B. Gorini1, M. Haguenauer1, G. Lehmann Miotto1, R. Murillo Garcia1, F. Safai Tehrani3, L. Serex1, S.

Veneziano3, P. Renaud2


2

OUTLINE

• Introduction• Microfluidic scintillation detectors: concept and previous work• Advantages and applications

• Single layer devices• Fabrication technology• Experiments

• Double layer devices• Fabrication technology• Experiments

• Conclusions and outlook



3

OPERATING PRINCIPLE


• Microfluidic channel filled with liquid scintillator defining an array of waveguides• Photodetector pixel coupled to each channel end• Scintillation light guided along microchannel and detected

For fine spatial resolution:small channels (10 µm – 1 mm)

microfluidicsPhotodetector array

Microchannel

Scintillation


4

FIRST PROTOTYPE


DAQ syste

m

A. Mapelli PhD thesis (2011)Photo: J. Daguin

20 mm

15 mm

First prototype ( first shown at IPRD08 )

• Microchannels made by SU-8 photolithography

• Gold reflective coating

𝑁 𝑝𝑒=1.6(200 µm deep channel)

MAPMT

~8 mm-1


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ADVANTAGES AND APPLICATIONS

Advantages• Increased radiation resistance

(liquid scintillator can be easily circulated in microchannels)• Microfabrication technology allows to make very thin

detectors

Potential applications individuated• Tracking/calorimetry in high energy physics• Beam monitoring in hadron therapy



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Thin microfluidic detector

Particle beam

Patient under treatment

ONLINE BEAM MONITORING• Hadron therapy

• Cancer treatment using hadron beams

• Microfluidic detectors• Very thin detectors can be made with

microfabrication techniques• Very good radiation resistance expected


Real-time monitoring of the beamduring patient irradiation

• Safer treatment• Optimized beam time use• Treatment cost reduction


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WHY SILICON?

SU-8 photosensitive polymer• Easy micropatterning (one-step photolithography)

• Good radiation resistance (comparable to Kapton)

• Main challenge: incompatible with high temperature processing(required for other materials in the device, e.g. metal bonding)

Silicon• Many reliable microfabrication techniques available

• Better thermal and mechanical resistance

• Possibility of tight integration of microchannels withsemiconductor devices (photodetectors, electronics, …)

All microfabrication activities performed at theEPFL Center for Micronanotechnology cleanroom


Photo: V. Floraud


8

DRY ETCHING AND SMOOTHING

• RF plasma reactor alternating SF6 (etching) and C4F8 (polymer coating) plasmas

• Vertical etching profile but resulting in «scalloping»

• Wet oxidation SiO2 has larger volume than Si surface features loss

• SiO2 removal with hydrofluoric acid smooth silicon


2 µm 5 µm


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DRY ETCHING OF MICROCHANNELS

• Starting substrate: silicon wafer• Etching of microchannels via DRIE process


2. Deep Reactive Ion Etching(alternated etching and passivation steps)

3. Smoothing by thick SiO2 growthand removal surfaces with suitable optical quality

1. Patterning of silicon oxide asetching mask 200 µm

Microchannels etched in silicon


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OPTICAL COATING AND BONDING

• Deposition of reflective aluminum layer• Wafer-level bonding of metallized glass top


5. Preparation of top cover(aluminum patterning on glass wafer)

6. Anodic bonding

4. Reflective coating by aluminumsputtering

0.5 mm

0.5 mm

Bonded channels section

Pyrex 100 µm

Total thickness~0.96 mm

Two devices superimposed and staggered


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20 m

m15 mm

OPTICAL AND FLUIDIC PACKAGING


7. DicingChannel ends are cut open

8. «Packaging»Thin glass window and fluidicconnectors glued on chip

Finished device

Microchannels cut open


12

CHARACTERIZATION WITH PMTS


PMT

PMT

QDC

β-

Radioactivesource (90Sr )

Scintillating fiber(trigger)

Photoelectron spectrum fitted with:

𝑆=𝑃+𝒫∗𝒩Signal Pedestal Gaussians

convoluted with Poisson distribution(PMT response)

𝑁 𝑝𝑒=1.4

Charge signal

Event

count

(180µm deep channel)

~7.8 mm-1


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LIGHT YIELD

• Expected light yield consistent with PMT measurements


2.07 (measured: 1.42)

Average scintillation photons (~307)

Light transport efficiency (~0.03)

Interface optical efficiency (~0.9)

PMT quantum efficiency (~0.25)

Needs improvement!

Possible solution:low refractive indexdielectric cladding

50 µm

Effects of surface roughness and defects at the liquid/glue/glass interface (lowering ) not calculated!


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BEAM MONITORING


Particle beam

x

x

Energy distribution

• High flux of relatively high energy particles

• High light output expected

• No need for high sensitivity photodetectors

Hamamatsu S8866-128-02 photodiode array


15

EXPERIMENTS WITH PHOTODIODES


Plastic support

Microchannels

Microchannels windowon photodiodes

Hamamatsu S8866-128-02photodiode array(connected to DAQ board)

90Sr source(2.4 MBq)

β-

Readout system developed incollaboration with INFN Rome

x

0.8

0.7

0.8

(mm)

. . . 0.18

Microchannel section

Photodiode (pixel)

Pixel number(0 … 127)

Inte

gra

ted lig

ht

signal

Long integration time used (1 sec)


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EXPERIMENTS WITH PHOTODIODES

Problem: flux from radioactive source too low for scintillation photons to «sum-up» in the photodiodes• Test setup 90Sr source: ~104 e-/sec @ ~2 MeV/e-

• For comparison, proton therapy: ~1011 p+/sec @ ~100 MeV/p+

Conclusions:• Test setup with 90Sr source not suitable for readout with

photodiodes• Test with actual beam envisioned to validate this kind of

application10.10.2013 // [email protected]


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DOUBLE LAYER MICROCHANNELS


x

y

• Adding an orthogonal microchannel layer XY position resolution

• Technological solution: patterning both sides of the silicon substrate

X side

Y side

Patent filed in 2012, PCT/EP2012001980


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WET ETCHING OF XY MICROCHANNELS


• Starting substrate: silicon wafer• Etching of microchannels on both sides

at the same time

2. Etching of both sides of the wafer

3. Reflective coating by aluminumSputtering on both sides

1. Patterning of etching mask on bothsides of the wafer


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PACKAGING OF DOUBLE LAYER CHIPS

• One-step bonding of 3 wafers stack• Dry etching for inter-layer connection


4. Bonding of 3 wafers stack byaluminum thermocompression

Si

Si

Al

Bonding interface

Fluid inlet

5. Channel cutting and gluing of two glass windows and fluidic connectors as before

200 nm

Top layer

Bottom layer 80 µm inter-layer Si

Silicon or pyrex cover wafers


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EXPERIMENTS WITH XY DEVICES


β-

Trigger PMT

Trigger fiber

Y PMT X PMT

Radioactivesource (90Sr )

X layer (150 µm)𝑁 𝑝𝑒=1.0

Y layer (150 µm)𝑁 𝑝𝑒=0.9

Data acquisition from both layers at the same time

(Glass windows and tubing not shown)

~6 mm-1

(preliminary)

(preliminary)


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CONCLUSIONS AND OUTLOOK

Conclusions• Different processes for microchannel patterning on silicon

developed• Single and double layer devices demonstrated with PMT

readout• Issues on tests with photodiode array readout

Perspectives• Beam tests with photodiode readout• Integration of on-chip a-Si:H photodiodes • Readout system based on SiPMs



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OTHER TECHNOLOGIES


110 µm total thickness(30 + 50 + 30)

200 µm

20 x 20 mm

... aside from silicon, research on polymeric microchannels also ongoing!

~0.03% X0


23

THANK YOU



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STAGGERING

• Staggered channels for improved geometrical coverage


Pyrex grindedto 100 µm

(focal plane in the middle)Total thickness~0.96 mm

100 µm staggering


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MATERIAL BUDGETX0

(mm)Single layer thickness (mm)

Double layer thickness (mm)

Silicon 94 0.2 (0.21% X0) 0.58 (0.62% X0)

Pyrex 126 0.5 (0.4% X0) 0.5 (0.21% X0)

EJ-305 ~500 0.18 (0.04% X0) 0.3 (0.06% X0)

Aluminum 89 0.0002 (negligible) 0.0004 (negligible)

Total 0.65% to 0.8% X0 0.89% to 1.2% X0


0.5 mm Excess material can be ground down to 100 – 50 µm Single layer: 0.12% to 0.28% X0

Double layer: 0.24% to 0.5% X0

maxmin


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FLUIDIC OPERATION

Detector area

(mm2)Depth(mm)

Width(mm)

N channels

Internal volume

(µL)

Hydraulic Resistance (bar s

µL-1)

Refill time @ ΔP = 1

bar

12.8 x 12.8 0.18 0.7 16 25.8 0.0042 ~ 100 ms

12.8 x 12.8 0.18 0.1 64 14.7 0.5 ~ 7 s

204.8 x 204.8

0.18 0.7 256 6450 1.1 ~ 2h


24h operation at ΔP = 1 bar:less than 80 mL of scintillator neededChannel section

width

depth

silicon microfluidic scintillation detectors

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