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Design of a real time interceptive beam monitor
based on a thin semiconductor sensor for hadron therapy
Proposal submitted to the FIRB Futuro in Ricerca 2012
Massimo Manghisoni
Università di Bergamo
Bologna - 14 giugno 2012
Motivation
Hadrontherapy with protons and carbon
ions is a fast developing methodology in
radiation oncology, since it is an effective
treatment against cancers located in areas
which are inaccessible to the surgeon’s
instruments or which are hard to treat by
radiotherapy (brain tumours, in areas close
to the spinal cord, or inside the eye)
The facilities that treated patients with protons and ions are nowadays about 27 but several
hospital based facilities will be operational in the next years (about 16)
Medaustron (Austria) http://www.medaustron.at Etoile (France) http://centre-etoile.org
Cyclotrons are employed, together with synchrotrons, for proton therapy while for carbon ion
therapy synchrotrons have been till now the only option
The accelerated particle beam is then extracted by High-Energy-Beam-Transport (HEBT)
lines and driven to the treatment room where the patient has to be cured
Patient safety, accelerator operation, and optimum dose delivery would all benefit if the beam
intensity and profile along the HEBT lines could be continuously monitored during treatment,
rather than just during the set-up
This is not possible with existing interceptive monitors which interfere with the beam,
causing a non-negligible beam blow up or a beam disruption for therapeutic kinetic energies
(60 to 250 MeV for protons and 120 to 400 MeV/nucleon for carbon ions).
While non-interceptive instrumentation is not sensitive enough to detect average beam
intensities from few pA to few nA, with spill durations of 1 s
1 / 19
General features of the project
Project aim: design of a novel real time interceptive beam monitor to be used in the HEBT
lines of hadron therapy accelerators for cancer treatment.
Purpose of the monitor: measurement of both intensity and profile of the beam during
patient treatment, rather than just during the set-up, as for the existing interceptive monitors.
This feature can be exploited in particular in the use of the active scanning technique for
cancer treatment. In the implementation of this technique, which is of primary interest in the
new hadron-therapy centers, the uniformity of the spill is of utmost importance as the better
the spill quality the faster and more accurate the scan becomes.
Duration: 3 years
Budget request: 1.2 Meuro
Participating Units:
Università degli Studi di Bergamo (u1)
Massimo Manghisoni, Gianluca Traversi, Michele Caldara, Lodovico Ratti
Università degli Studi di Trento (u2)
Lucio Pancheri, Gian Franco Dalla Betta
Istituto Nazionale di Fisica Nucleare (u3)
Nicola Neri, Mauro Citterio, Luigi Gaioni, Simone Coelli, Gianluca Alimonti
Università degli Studi di Bologna (u4)
Filippo Giorgi, Alessandro Gabrielli, Davide Falchieri
ERC (European Research Council) research fields
PE Physical Sciences and Engineering
PE7 Systems and communication engineering PE7 2 Electrical and electronic engineering
LS Life Sciences
LS7 Diagnostic tools, therapies and public health LS7 2 Diagnostic tools (e.g. imaging)
2 / 19
The CNAO facility in Pavia
80 m long synchrotron1
Two types of ions:
Protons
(60÷250MeV/u)
Carbon ions
(120÷400MeV/u).
Ions energy settable with
0.02 MeV steps
The beams are sent to the
treatment rooms by the
HEBT lines
3 horizontal + 1 vertical
treatment lines
1 experimental line
(in future)
Active scanning
1
www.cnao.it
3 / 19
HEBT beam main parameters
3 treatment rooms
4 extraction lines
3 horizontal (TUZ)
1 vertical (V)
1 Qualification
Monitor
Profile Monitor
Intensity Monitor
Scintillating Fibers
Harp (SFH) monitors
The beam extracted into the HEBT lines is nominally a continuous beam, but it is subjected to
strong modulations.
Ions accelerated in the synchrotron are extracted during a period settable from 1 to 10 seconds,
according to treatment planning requirements.
Beam nominal intensities: 1·1010 protons per spill and 4·108 Carbon ions per spill,
corresponding to 1.6 nA and 0.38 nA, respectively (assuming 1 second long spill)
4 / 19
Qualification Monitor at CNAO Facility
As the beam enters the extraction line, after a quadrupoles triplet, it meets three dipoles and
four Chopper magnets.
Chopper magnets are usually off and the not-bumped beam is stopped against a dump.
Only if the beam is allowed to go downstream, Chopper magnets are turned on and the beam is
kicked so to avoid the dump and go to a treatment room
Just in front of the Dump, there is the so called “Beam Qualification Monitor” which measures
relevant beam quantities before sending it to the patient
The Beam Qualification Monitor is comprised of:
Profile Monitor (QPM): Orthogonal harps of scintillating fibers
Intensity Monitor (QIM): Scintillating plate coupled with a photomultiplier placed in air side
5 / 19
Beam monitors in extraction lines: state of the art
Due to:
the slow extraction (which can last up to ten seconds)
the number of extracted particles and their energies
the ion species used in hadron-therapy
Beam diagnostics instrumentation along the HEBT lines must be interceptive
Typically the monitors installed at the existing hadron therapy facilities are based on
Gas ionization principle
Scintillating materials with camera readout
Secondary Emission Monitor (SEM) grids
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Multi Wire Proportional Chamber (MWPC)
Consists in two grids of metallic wires inserted in a gas mixture
the voltage potential of some kV between the grids accelerates secondary electrons created
by the interaction of the beam with the wires
such electrons, accelerated by the electric field between the grids travel in the gas creating
some avalanche
the electronics typically consists in an analog multiplexer, programmable integrators channels,
sample and holds and finally analog to digital conversion
Typical performances:
0.5 mm spatial resolution (due to the wire minimum spacing ≈1 mm)
750 Hz maximum profile rate (minimum settable integration time in order to obtain a reliable
beam transverse profile)
around few 106 particles/sec rate resolution.
Main limitations:
quite complex, expensive and big instruments
need high voltage electronics and gas control systems
The total thickness results to be of one centimeter, thus not permitting simultaneous
measurements in different HEBT line locations
Nevertheless they are the most popular solution, adopted at Loma Linda Proton Treatment Center
(US), HIMAC (Chiba, JP), HIT (Heidelberg, DE), since they have been studied for more than 30
years
7 / 19
Scintillating plates in association with a CCD camera
Provides the best sensitivity in beam profiles measuring (among all the possible instruments)
with a spatial resolution similar to that of MWPCs, but the profile rate is limited by the camera
readout speed (typically less than 50 frames/s). Scintillating plates are used to measure also the
beam intensity, guiding the light to a photomultiplier followed by an amplifier
The method adopted at CNAO to measure extracted
beam profile and, to a certain extent, beam intensity,
consists in using aluminized scintillating fibers arranged
in two planes grids
All the fibers are adjacent, so that the dead area is
minimized. Light generated inside each fiber is guided
into a rectangular matrix at a viewport, where a CCD
camera, in air side, collects images
Such a solution allows very high sensitivity and reliable beam profiles with a very simple
readout, just applying a software mask at the acquired image
Main limitations:
Frame rate limited to the camera maximum rate, which is less than 43 Hz for the chosen camera
Spatial resolution limited by the minimum fiber dimension (0.5 mm)
Material: the total thickness of a two planes detector (1 mm) produces a large emittance
increase of the beam, which is visible at the downstream detectors and that doesn’t permit
measurements during treatment
8 / 19
SLIM (Secondary emission monitor for Low Interception Monitoring)
SLIM is the study of a less interceptive detector2
A very thin Aluminum foil (0.2 µm
thickness) acts as a source of secondary
electrons when the beam crosses the
target, set at a certain angle with the
beam
The emitted secondary electrons are
guided by a focusing system to a position
sensitive readout system, placed outside
the beam path
A prototype has been developed obtaining a detector with 1 mm spatial resolution; its longitudinal
space occupancy flange to flange is 460 mm and readout was studied in two versions:
The first makes use of a multi-channel plate (MCP), a phosphor P43 screen and a CCD camera
the second solution was to develop an application-specific integrated circuit (the MIMOTERA
chip developed by the SUCIMA collaboration)
Main limitations:
Needs high voltages for the focusing system
SLIM secondary electrons resulted susceptible to the focusing system voltages, that must be
carefully set, and to the residual magnetic fields of the nearby magnets.
2
P.N. Gibson et al, ”SLIM (Secondary emission monitor for Low Interception Monitoring) an innovative
non-destructive beam monitor for the extraction lines of hadrontherapy centre”, Jacow database ( www.jacow.org )
9 / 19
Final objectives of the project
This research project aims at the commissioning of a
relatively large (in the square centimeter range), ultra-thin, pixellated silicon
detector based on high density microelectronic processes (nanoscale) and on
sparse (non sequential) readout architectures
The device to be developed is meant to be the elementary brick of a monitor for
real-time measurement of profile and intensity of a proton or carbon ion beam
Typical applications for such an instrument are in the field of beam monitoring in
HEBT lines of hadron therapy accelerators
Use of advanced processing (aggressive silicon wafer thinning and packaging) and
design techniques for real-time, fast pixel detectors has the potential for
significantly improve the uniformity of the spill extracted for tumor scanning
The uniformity of the spill is the most important aspect of all as the better the
spill quality the faster and more accurate the scan becomes
The outcomes of this research activity are expected to be beneficial also for other
fields involving radiation instrumentation, such as nuclear and particle physics and
medical imaging for disease diagnosis and monitoring
The research program also aims at advancing the knowledge in the use of
emerging technologies (aggressive thinning and packaging techniques), whose
development is mainly driven by the consumer application market, to exploit their
potential in the fabrication of scientific instrumentation
10 / 19
Main detector requirements
Overall sensitive area. Beam position in the HEBT lines moves along the V and H plane in a range
of some tens of mm ⇒ quite large sensitive silicon area required (≈64×64 mm2 ).
Typical reticule size of standard microelectronic processes is of the order of a few cm2 ⇒ suitable
tiling or mosaic techniques are required to cover the full area
Image spatial granularity. According to the needs of the end users, the pixel pitch should be of the
order of 100 µm ⇒ not a problem for the monolithic approach where the state-of-the-art pixel
pitch is below 10 µm
Particle fluences. Very large particle fluxes are expected for the considered application (107 to 1010
particles per spill) ⇒ issues related to the large amount of charge generated in each pixel as well
as radiation damage (of both ionizing and displacement type) should be carefully considered
Frame rate. The extracted beam spill is strongly modulated ⇒ without a fast measurement and
control system this would result in locally over- or under-dosing of the tumor. A frame rate of the
order of 10 kHz is essential to guarantee the required dose uniformity. Taking into account the
pixel pitch, the overall sensitive area and the beam spot diameter (1÷10 mm) this rate can be
achieved with the use of a sparse (non sequential) readout architecture
Material budget. Since the beam in the HEBT line will traverse continuously the detector, its
thickness value should not cause an unacceptable beam emittance increase
aggressive back-thinning of the silicon detector substrate down to a thickness of few microns
other materials used for the system packaging (flex cables and support) must contribute to
the overall budget with a negligible amount of material
Radiation hardness. Yielding particle fluences of ≈3·1013 particles/cm2 per year, bulk radiation
damage is believed to represent a serious problem for the proposed detector
Heat dissipation. Due to the small thickness of the final module and the need for operation in
vacuum for the considered application, a thorough investigation on heat dissipation issues has to
be conducted to verify the consistency of the module design from the thermal point of view
11 / 19
Front-end electronics for ultra-thin detectors
Monolithic CMOS MAPS have been chosen as the sensor technology for the development of the
proposed monitor mainly due to the specification on the sensor thickness (≈20 µm).
Many options are available:
DNW MAPS. The deep n-well process creates a large n-well which acts as the charge collecting
diode and has an embedded p-well housing the nMOS transistors while the pMOS transistors are
in an n-well which is geometrically smaller and less deep than the charge collecting deep n-well.
INMAPS quadruple well. This 180 nm process employs, beside a deep n-well, a deep p-well
placed underneath the n-well containing the p-channel devices thus preventing it from acting as
a charge drain. Process features providing a higher resistivity epitaxial layer for faster and more
efficient charge collection are also available. The high resistivity substrate improves the
radiation tolerance to bulk damage.
HV technology. The AMS 350 nm HV technology employs high-voltage n-wells in a p-substrate
to create a monolithic pixel sensor which features full charge collection by drift. 100% fill factor
without charge loss is obtained by embedding the entire structure (nMOS and pMOS in n-wells)
in a deep n-well which also is the collecting diode.
SOI technology. The SOI technology promises full CMOS circuitry in the active area without
bump bonding with high sensitivity and full charge collection. The main technical issues of the
SOI technology are or have been a reliable fabrication process, the question of how to avoid the
backgate effect, the radiation hardness due to hole trapping inside the BOX, and the attainable
resistivity of the substrate material.
The first part of the monitor development activity will be devoted to the identification of a suitable
CMOS technology compatible with MAPS fabrication to be used within the project.
12 / 19
Sensor thinning and packaging
Nowadays applications of integrated circuits shrink in
size, due to the demand for portable communications
devices (memory cards, smart cards, cellular telephones
and portable computing)
To cope with this requirements integrated circuit
packaged devices must be reduced both in footprint and
thickness
One of many crucial aspects in developing ultra-thin
packages is die thickness. Thinning of the whole wafer,
or of the single die, at the back end (after the complete
device processing on the front side) is the most effective
way for preparing ultra-thin chips
Wafer thinning is also of utmost importance in the realization of 3D devices, a technology
breakthrough which allows the fabrication of semiconductor devices with multiple tiers of
copper connected active devices
Thinning techniques are also increasingly used in CMOS image sensors, exploiting backside
illumination to improve the pixel fill factor
As chip thickness is reduced, new processes for temporary bonding and packaging will be
required
The market and the available technology options are expected to grow fast in the next years3
The proposed project aims at take advantage from improved thinning and packaging
techniques in the realization of an extremely thin semiconductor based detection system
3
Eric Mounier, “Thin Wafer Manufacturing”, Technology & MarKet report - June 2011, www.i-micronews.com
13 / 19
Available techniques for aggressive wafer thinning
Time- and cost-efficient processes are today available with good yields using standard
back-grinding techniques through commercial providers, with thickness tolerance < µm even at
final wafer thickness of 20 µm (preliminary contacts have been taken with Aptek Industries, San
Jose, CA, USA, that can ensure thinning to about 25 µm) [http://aptekindustries.com ]
Other quite striking approaches promise even thinner devices with substrate thicknesses lower
than 1 µm [ http://www.monolithic3d.com ]
Alternative chemical thinning strategies must be pursued in order to reduce the die thickness to
the micrometer range. A thorough evaluation of the available thinning techniques will be carried
out by the Trento unit, also in connection with the MT Lab. at FBK
Depending on the final thickness requirements and considering also cost, yield and reliability
constraints, the most suitable thinning technology will be used
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Work-packages
WP1: test chip for a small matrix and building blocks validation
After the choice of the optimal detector technology a MAPS for imaging, consisting of a matrix of 32×32 or
64×64 elements, will be designed, fabricated and tested
1.1
1.2
1.3
1.4
1.5
sensor simulation and design (u2)
analog front-end design and simulation (u1)
digital front-end design and simulation (u4)
building blocks characterization (u1, u2, u4)
small matrix test (u3)
WP2: design of the full matrix chip
A large scale detector (including no less than 128×320 elements), with an area of the order of a few square
centimeter, will be designed and fabricated
2.1
2.2
2.3
2.4
sensor simulation, design and optimization (u2)
analog front-end simulation, design and optimization (u1)
digital front-end and readout architecture design and simulation (u4)
full matrix characterization (u3)
WP3: cm-scale multichip module integration
Fabrication of a large-scale module (a few cm2 ) which aims at demonstrating the feasibility of the final monitor
3.1 system integration (u2, u3)
3.2 module characterization and field tests (u1, u2, u3, u4)
WP4: hardware tools for sensor assembly and test
Ancillary activities aiming at developing hardware tools for sensor assembly and test
4.1
4.2
4.3
4.4
investigation of available techniques for aggressive wafer thinning (u2)
development of a real time data acquisition system for sensor testing (u4)
flex hybrid and printed circuit board (PCB) design (u1)
investigation of heat spreader and thermal conductivity aspects (u3)
15 / 19
In each of the first 3 WPs, an integrated detector prototype is expected to be delivered
small scale (32×32 or 64×64 elements) 100 µm pitch
MAPS in CMOS technology
Periphery
or
64 x 64
128 x 320
128 x 320
Periphery
Deliverable 1 (in the frame of WP1):
32 x 32
128 x 320
128 x 320
128 x 320
centimeter scale (including 128×320 elements) 3-side
buttable matrix (100 µm pitch) featuring a selective
(non sequential) digital readout architecture
128 x 320
Periphery
128 x 320
Periphery
128 x 320
128 x 320
128 x 320
Periphery
128 x 320
Periphery
128 x 320
Periphery
128 x 320
128 x 320
Periphery
128 x 320
Periphery
Periphery
128 x 320
Periphery
Periphery
128 x 320
Periphery
128 x 320
Deliverable 2 (in the frame of WP2):
Periphery
Periphery
128 x 320
128 x 320
Periphery
Periphery
Periphery
or
64 x 64
Periphery
32 x 32
Periphery
20
Final objectives of the project
Deliverable 3 (in the frame of WP3):
demonstrator composed of a 2×3 matrix of 3-side
buttable chips
16 / 19
Monitor assembly
Periphery
128 x 320
128 x 320
Periphery
128 x 320
128 x 320
Periphery
Periphery
128 x 320
128 x 320
Periphery
128 x 320
128 x 320
Periphery
Periphery
128 x 320
Periphery
Periphery
128 x 320
Periphery
The monitor is obtained by the side by side connection of the 3-side buttable chips
An overall area of 2W×nH (W being the width of the sensitive layer in the single chip, H its
height and n the number of chips in a column) can be covered without any inefficient region
L0w mass
Low Mass
Flex Cable
to
Service Board
to Service Board
L0w mass
Foam Support
to Service Board
L0w mass
Support
oard
17 / 19
Time Schedule
18 / 19
Collaboration with national and international Institutions
National Centre for Oncological Hadrontherapy - Pavia
Provide support in defining the specifications of the new detector
Field test of the developed system directly with a proton and heavy ion
particles beam
Institut Pluridisciplinaire Hubert Curien - Strasbourg
Common interest for the development of large area, thin pixel sensor modules
For the development of new devices for high-performance beam monitoring,
there is an agreement to have technical discussions with the PICSEL group
Fondazione Bruno Kessler (FBK) - Trento
Device characterization phase
Technological issues (wafer thinning and packaging)
The Santa Cruz Institute for Particle Physics (SCIPP)
Sensor design and characterization phase
Benchmarking the final system performance
The Jozef Stefan Institute - Ljubljiana
Radiation damage tests by using the JSI neutron reactor facility
19 / 19