SILICON MICROFLUIDIC
SCINTILLATION DETECTORS
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
Physics Department, European Organization for Nuclear Research (CERN), Switzerland
2 Microsystems Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
3 Sezione di Roma 1, Istituto Nazionale di Fisica Nucleare (INFN), Italy
1
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
1
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
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
2
OPERATING PRINCIPLE
Microchannel
Scintillation
• 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)
Photodetector array
10.10.2013 // pietro.maoddi@cern.ch
 microfluidics
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
3
FIRST PROTOTYPE
MAPMT
First prototype ( first shown at IPRD08 )
• Microchannels made by SU-8 photolithography
• Gold reflective coating
DAQ
system
𝑁𝑝𝑒 = 1.6
20 mm
(200 µm deep channel)
𝑁𝑝𝑒
𝑡
Photo: J. Daguin
10.10.2013 // pietro.maoddi@cern.ch
~8 mm-1
15 mm
A. Mapelli PhD thesis (2011)
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
4
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
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
5
ONLINE BEAM MONITORING
• Hadron therapy
• Cancer treatment using hadron beams
• Microfluidic detectors
Particle beam
• Very thin detectors can be made with
microfabrication techniques
• Very good radiation resistance expected
Real-time monitoring of the beam
during patient irradiation
Thin microfluidic
detector
• Safer treatment
• Optimized beam time use
• Treatment cost reduction
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
Patient under treatment
6
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 with
semiconductor devices (photodetectors, electronics, …)
All microfabrication activities performed at the
EPFL Center for Micronanotechnology cleanroom
Photo: V. Floraud
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
7
DRY ETCHING AND SMOOTHING
2 µm
•
•
•
•
5 µm
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
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
8
DRY ETCHING OF MICROCHANNELS
• Starting substrate: silicon wafer
• Etching of microchannels via DRIE process
1. Patterning of silicon oxide as
etching mask
Microchannels etched in silicon
200 µm
2. Deep Reactive Ion Etching
(alternated etching and passivation steps)
3. Smoothing by thick SiO2 growth
and removal
 surfaces with suitable optical quality
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
9
OPTICAL COATING AND BONDING
• Deposition of reflective aluminum layer
• Wafer-level bonding of metallized glass top
0.5 mm
0.5 mm
4. Reflective coating by aluminum
sputtering
5. Preparation of top cover
(aluminum patterning on glass wafer)
Bonded channels section
Two devices superimposed and staggered
Pyrex 100 µm
6. Anodic bonding
Total thickness
~0.96 mm
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
10
OPTICAL AND FLUIDIC PACKAGING
Microchannels cut open
15 mm
7. Dicing
Channel ends are cut open
Finished device
8. «Packaging»
Thin glass window and fluidic
connectors glued on chip
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
11
Event count
CHARACTERIZATION WITH PMTS
Radioactive
source (90Sr )
𝑁𝑝𝑒 = 1.4
(180µm deep channel)
𝑁𝑝𝑒
𝑡
~7.8 mm-1
Charge signal
PMT
Photoelectron spectrum fitted with:
QDC
Scintillating fiber
(trigger)
PMT
Signal
𝑆 =𝑃+ 𝒫∗𝒩
Pedestal
β-
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
Gaussians convoluted
with Poisson distribution
(PMT response)
12
LIGHT YIELD
• Expected light yield consistent with PMT measurements
𝑁𝑝𝑒 = 𝑁𝑠𝑐𝑖𝑛𝑡 ∙ 𝜀𝑡 ∙ 𝜀𝑖 ∙ 𝜀𝑞 = 2.07
(measured: 1.42)
PMT quantum efficiency (~0.25)
Interface optical efficiency (~0.9)
Light transport efficiency (~0.03)
Needs improvement!
Average scintillation photons (~307)
50 µm
Effects of surface roughness and defects
at the liquid/glue/glass interface
(lowering 𝑁𝑝𝑒 ) not calculated!
10.10.2013 // pietro.maoddi@cern.ch
Possible solution:
low refractive index
dielectric cladding
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
13
BEAM MONITORING
• High flux of relatively high energy particles
• High light output expected
• No need for high sensitivity photodetectors
Energy distribution
x
x
10.10.2013 // pietro.maoddi@cern.ch
Hamamatsu S8866-128-02 photodiode array
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
14
EXPERIMENTS WITH PHOTODIODES
Microchannels
0.8
x
(mm)
90Sr
source
(2.4 MBq)
Readout system developed in
collaboration with INFN Rome
0.7
0.18
...
Microchannel section
Microchannels window
on photodiodes
Hamamatsu S8866-128-02
photodiode array
(connected to DAQ board)
Long integration time used (1 sec)
Integrated light signal
β-
Photodiode (pixel)
0.8
Plastic support
Pixel number
(0 … 127)
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
15
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 application
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
16
DOUBLE L AYER MICROCHANNELS
Patent filed in 2012, PCT/EP2012001980
y
• Adding an orthogonal microchannel layer
 XY position resolution
• Technological solution: patterning both sides of the
silicon substrate
X side
x
Y side
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
17
WET ETCHING OF XY MICROCHANNELS
• Starting substrate: silicon wafer
• Etching of microchannels on both sides
at the same time
1. Patterning of etching mask on both
sides of the wafer
2. Etching of both sides of the wafer
3. Reflective coating by aluminum
Sputtering on both sides
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
18
PACKAGING OF DOUBLE L AYER CHIPS
• One-step bonding of 3 wafers stack
• Dry etching for inter-layer connection
Si
Bonding interface
Al
200 nm
Silicon or pyrex cover wafers
Si
4. Bonding of 3 wafers stack by
aluminum thermocompression
Fluid inlet
Top layer
5. Channel cutting and gluing of
two glass windows and fluidic
connectors as before
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
Bottom layer
80 µm inter-layer Si
19
EXPERIMENTS WITH XY DEVICES
(preliminary)
Radioactive
source (90Sr )
𝑁𝑝𝑒 = 1.0
X PMT
Y PMT
X layer (150 µm)
Data acquisition from both
layers at the same time
Trigger PMT
Trigger fiber
(preliminary)
β-
Y layer (150 µm)
𝑁𝑝𝑒 = 0.9
𝑁𝑝𝑒
𝑡
~6 mm-1
(Glass windows and tubing not shown)
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
20
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
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
21
OTHER TECHNOLOGIES
... aside from silicon, research on polymeric microchannels also ongoing!
200 µm
~0.03% X0
110 µm total thickness
(30 + 50 + 30)
20 x 20 mm
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
22
THANK YOU
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
23
STAGGERING
• Staggered channels for improved geometrical coverage
100 µm staggering
Pyrex grinded
to 100 µm
Total thickness
~0.96 mm
10.10.2013 // pietro.maoddi@cern.ch
(focal plane in the middle)
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
24
MATERIAL BUDGET
X0 (mm) Single layer thickness (mm)
Silicon 94
Double layer thickness (mm)
0.2 (0.21% X0)
0.58 (0.62% X0)
0.5 (0.4% X0)
0.5 (0.21% X0)
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
Pyrex 126
EJ-305 ~500
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
0.5 mm
min
10.10.2013 // pietro.maoddi@cern.ch
max
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
25
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
~7s
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
needed
depth
Channel section
width
10.10.2013 // pietro.maoddi@cern.ch
13th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2013
26