Low material budget microfabricated
cooling devices for particle detectors
P. PETAGNA and A. MAPELLI
On behalf of:
• CERN PH/DT
• The NA62 Collaboration
• EPFL – LMIS4
• EPFL – LTCM
• UCL – ELEC/DICE (SOI & MEMS)
30 Sep 2010
P. Petagna & A. Mapelli
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Outline of the talk
• Why micro-channel cooling for HEP?
• A first application: local cooling for the NA62 GTK
• Proposed solution and approach to the problem
• Micro-fabrication process
• Structural analysis
• Thermo-fluid dynamics simulations
• First tests on a full-scale prototype
• Layout optimization
• Next steps and beyond
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Why m-channel cooling?
1 – Minimization of material budget
Radiation length (X0): mean distance over which the energy of a
high-energy electron is reduced to 1/e (0.37) by bremsstrahlung
More readily usable quantity: X0 = X0/r [cm]
Minimize material budget
(Dahl, PDG)
Cu: 1.436 cm
Steel: ~1.7 cm
Al alloy: ~8.9 cm
Ti: 3.56 cm
Si: 9.37 cm
C6F14 @ -20 C : 19.31 cm
K13D2U 70% vf: 23 cm
CO2(liquid) @ -20 C: 35.84 cm
Minimize x[cm] / X0[cm]
(i.e. use material with high X0 and minimize thickness)
m-channel cooling naturally addresses this issue through the use of Si
cooling plate and tiny (PEEK?) pipes in extremely reduced thickness
30 Sep 2010
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Why m-channel cooling?
Material budget of the CMS Si-Pixel tracker (2 layers)
Material budget of the CMS Si-strip tracker (10 layers)
Present LHC large Si trackers (ATLAS and CMS) ~ 2% X0 per layer
SLHC “phase II” upgrade: “significant” reduction needed
Future trackers at ILC ~ 0.1 ÷ 0.2% X0 per layer
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Why m-channel cooling?
2 – Cooling power enhancement
Q  hS(Tw  T f )
Newton’s law for convective heat flux:
Nusselt number = 3.66
÷ 4.36 for fully
developed laminar flow
Fluid thermal conductivity
= 0.05 ÷ 0.11 W/mK for
low temperature fluids
Heat transfer coefficient for m-channel system:
h
~103 W/m2K
K f  Nu
Dh
Hydraulic diameter
~ 10-4 m or less
m-channel cooling: very high heat transfer coefficients (very
small Dh possible) and very high heat flux (large S available)
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Why m-channel cooling?
3 – Reduction of DT between heat source and heat sink
Lower temperatures are envisaged for the future Si-trackers at SHLC. This
has non-negligible technical impacts on the cooling plants
With a standard cooling
approach, the DT between the
module and the fluid ranges
between 10 and 20 C (small
contact surface + long chain
of thermal resistances)
With an integrated m-channel cooling approach, the large surface available
for the heat exchange (cold plate vs. cold pipe) and the natural minimization
of the thermal resistance between the source and the sink effectively
address the issue of the DT between the fluid and the element to be cooled
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An example of future potential use
Concept of module for a “level-0 trigger” layer @ SHLC (courtesy of A. Marchioro)
Sensor
RO chips
Interconnect
m-channel cooling plate
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Manifolds
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A first application: the NA62 GTK
30 Sep 2010
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A first application: the NA62 GTK
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A first application: the NA62 GTK
beam: hadrons, only 6% kaons-> only 20% decay in the vacuum tank into a pion and 2
neutrinos -> out of which only 10-11 decays are of interest
hit correlation via matching of arrival times – 100 ps
GTK sees
all particles
selects particles
with 75 GeV/c
sees
kaons only
Mag2
Mag3
straw chambers
measure position
RICH
identifies pions
straw chambers
GTK3
GTK1
Cedar
RICH
GTK2
Achromat
Mag1
30 Sep 2010
250 m
Mag4
P. Petagna & A. Mapelli
Vacuum tank
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A first application: the NA62 GTK
• Priority: minimize X0
• Acceptable DT over sensing area ~ 5 °C
• Dimension of sensing area: ~ 60 x 40 mm
• Max heat dissipation: ~ 2 W/cm2
• Target T on Si sensor ~ -10 °C
• RO chip: 0.11% X0
(~100 µm Silicon)
• Sensor & bonds: 0.24% X0
(~200 µm Silicon)
• Passive or active cooling plate
Final target: 0.10 – 0.15 % X0
• Support structure outside acceptance region: ~ FREE
• 18 000 Pixels / station (300 x 300 mm, 200 mm thick)
• 10 ASICS chips bump-bonded to the sensor
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Proposed solution
Schematic of the layout of the proposed m-channel
cooling plate the coolant will enter and exit the
straight channels via manifolds positioned on top
and bottom. The channels, distribution manifold and
openings for the inlet and outlet connectors are
etched into a silicon wafer, which is then coupled to
a second wafer closing the hydraulic circuit.
The final goal is to have both wafers in silicon bonded together
by fusion bonding to produce a monolithic cooling element
An alternative design, in case of technical difficulties with the fusion bonding process, relies on a flat Pyrex
cover 50 µm thick anodic-bonded to the silicon wafer carrying the hydraulic circuit. On top of this flat plate,
an additional silicon frame (surrounding the beam area) will be again anodic-bonded. In this way the global
structure of the cooling wafer will be symmetric, the effects of coefficient of thermal expansion (CTE)
mismatching between silicon and Pyrex will be minimized and the same resistance to pressure and
manipulation as in the baseline case will be attained
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Approach to the problem
Take advantage of recent results obtained in two different fields of development:
• m-channel cooling devices have started to be actively studied for future
•
applications for high power computing chips or 3D architectures.
Thin and light m-fluidic devices in silicon are largely in development for biochemical applications.
Anyway for the first case, where the power densities are extreme, the mass of
the device (hence its material budget) is an irrelevant parameter. In the second
case the typical values of the flow rate and pressure are much lower.
Furthermore, the presence of a low temperature fluid and possibly of a high
radiation level is unique to the HEP detector case.
dedicated R&D is nevertheless unavoidable
for the specific application under study.
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Approach to the problem
The procedure followed to tackle the different challenges and to converge in a limited time
on a single device satisfying all the requirements is to move in parallel along different lines
of R&D in a “matrix” approach, where the intermediate results of one line are used to steer
the parallel developments.
Common
specs
Fabrication
technique studies
Possible
layouts
Thermo-fluid
dynamic
simulations
Optimal
layout
Numerical
structural
simulations
Pressure
limits
Experimental tests
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m-fabrication process
START: Czochralski silicon wafer polished on both sides (4′′ diameter, 380 μm
thick, 0.1-0.5 ohm-cm p-type).
(a) A layer of 1 µm of oxide (SiO2) is grown on both sides of the wafer
(b) Clariant AZ-1512HS photoresist is spin coated on one side of the wafer at
2000 rpm and lithography is performed to obtain an image of the channels
in the photoresist
(c) Dry etching of the top layer oxide is used to transfer the micro-channels
pattern
(d) A second lithography is performed with frontside alignment to image two
fluid transfer holes, 1.4 mm diameter, for fluid injection and collection from
the two manifolds.
(e) Deep Reactive Ion Etching (DRIE) is used to partially etch the access holes
down to 280 µm
(f) The photoresist is stripped in Microposit Remover 1165 at 70°C
(g) and DRIE is used to anisotropically etch 100 μm deep channels separated
by 25 µm wide structures in silicon
(h) Subsequently the oxide layers are removed by wet etching in BHF 7:1 for
20 min at 20°C
(i)
30 Sep 2010
At present, the processed Si wafer and an unprocessed Pyrex wafer (4”
diameter and 525 µm thick) are then cleaned in a Piranha bath (H2SO4 +
H2O2) at 100°C and anodic bonding is performed to close the channels with
the Pyrex wafer
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m-fabrication process
The anodic bonding is performed at ambient
pressure and T is raised to 350°C then lowered to
320°C. At this stage a constant voltage of 800 V is
applied between the Si and Pyrex wafer.
30 mm
In the final production both the processed and the
unprocessed wafers will be in 525 µm thick silicon.
1 mm
....
The bonded wafer undergoes a further processing:
this includes a final local etching to obtain a thinner
region in the beam acceptance area
The resulting wafer is diced according to alignment
marks previously etched in Si to obtain a cooling
plate with precise external references for integration
into the electromechanical assembly
Scanning Electron Microscope image of the
cross-section of 50 x 50 mm channels etched
in silicon bonded to a Pyrex wafer
30 Sep 2010
Finally, PEEK connectors (NanoPort®
assemblies from Upchurch Scientific) are
aligned, together with a gasket and a
preformed adhesive ring to the inlet and
outlet on the silicon and clamped. They
undergo a thermal treatment at 180°C for 2
hours to develop a complete bond between
the connectors and the silicon substrate.
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Structural analysis
1 – Experiments
varying width
3
0.05
0.025
“Sacrificial” samples with different manifold width
are produced and brought to collapse by gradually
increasing pressure under a high speed camera in
order to determine the limit pressure and the exact
breaking mechanics.
60
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Structural analysis
2 – Numerical simulations vs. tests
A simplified ANSYS 2D parametric model has been
developed and calculations are checked against
experimental results in order to validate the model for
further forecasts, including the effect of wall thinning or
of geometrical variations
80
Pyrex rupture
Connector detachment
ANSYS model
Yield stress ~25 MPa [ICES 2009]
Pint (bar)
60
40
20
0
0.0
0.5
1.0
1.5
2.0
Manifold width (mm)
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Structural analysis
3 – Extrapolations
Rupture Pressure for Silicon Cover (165 Mpa)
Rupture Pressure for Pyrex Cover (25 Mpa)
450
60
tp=50µ
tp=50µ
400
tp=200µ
350
tp=525µ
300
tp=200µ
tp=350µ
Pint (bar)
Pint (Bar)
40
tp=350µ
tp=525µ
250
200
150
20
100
50
0
0
0.0
0.5
1.0
1.5
2.0
0
Manifold width (mm)
30 Sep 2010
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0.5
1
Manifold Width (mm)
1.5
2
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Thermo-fluid dynamics simulations
The choice of the cooling fluid circulating in the micro-channels has naturally been oriented towards
perfluorocarbon fluids (CnF2n+2), which are widely used as coolant medium in LHC detectors. They exhibit
interesting properties for cooling applications in high radiation environment such as thermal and chemical
stability, non-flammability and good dielectric behaviour. In particular C6F14 is liquid at room temperature
and is used as single phase cooling fluid in the inner tracking detectors of CMS.
Properties
Density r [kg/m3]
Viscosity n [10-7 m2/s]
Heat capacity cp [J/(kg K)]
Thermal conductivity l [10-2 W/(m K)]
C6F14 @ -25°C
1805
8.2
975
6.275
Based on the properties of C6F14, a mass flow of
7.325*10-3 kg/s is required to extract the heat dissipated
by the readout chips (~32 W) with a temperature
difference of 5K between the inlet and outlet temperature
of the coolant
0.014
0.012
0.01
m pt ( 90mm b )
Flow rate
attained with 2
bar Dp vs.
channel width
for a fixed
height of 90 mm
3
810
3
610
3
410
3
210
4
110
4
4
210
310
The results from the analytical calculations performed
indicate that the suited range of the micro channel
geometry is the following:
• Width: between 100 mm and 150 mm
• Height: between 80 mm and 120 mm
• Fin width: between 25 mm and 75 mm
• Between 300 and 500 channels to cover the area
4
410
b
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First tests on a full-scale prototype
Test sample and numerical model
manifold depth 100mm
Inlet
1mm
•
•
•
•
•
•
Channel cross section 100mm x 100mm
Power density 1 W/cm2 (50% nominal)
Mass flow 3,66 x 10-3 kg/s (50% nominal)
Inlet temperature 18 C
Outlet pressure 1bar
Laminar flow
30 Sep 2010
Outlet
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First tests on a full-scale prototype
Simulated vs. experimental pressure drop
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First tests on a full-scale prototype
Thermal visualization
IN
OUT
Thermograph before
injection
IN
OUT
Heat load
simulated by a
Kapton heater of
suited resistance
and geometrical
dimension
Thermograph at
injection
Thermograph after few
seconds of coolant circulation
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First tests on a full-scale prototype
Steady state DT between inlet and surface probes
1
2
3
4
5
6
4
6
5
1
2
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Layout optimization
Optimized geometry for uniform and minimal DP
CFD models of the geometry presently under tests have been successfully validated. Further
optimization of the manifold geometry and of the channel cross section can then be performed
through CFD analysis in order to reduce the amount of samples to be produced for testing purposes
Inlet
Wedged manifold, depth
150mm, 280mm and 400mm
Outlet
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Layout optimization
Effect of inlet manifold geometry on DP
Wedged manifold, 1.6 mm
Max width, 150 mm thick,
opposed inlet & outlet
Wedged manifold, 1.6 mm
Max width, 280 mm thick,
opposed inlet & outlet
Rectangular manifold, 1 mm wide,
100 mm thick, central inlet & outlet
30 Sep 2010
Wedged manifold, 1.6 mm
Max width, 400 mm thick,
opposed inlet & outlet
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Layout optimization
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Layout optimization
Summary table
two inlets
two inlets
two inlets
two inlets
two inlets
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Next steps and beyond
Immediate future
1. Perform full-scale thermal tests in cold (vacuum vessel)
2. Define the details and properties of the Si-Si fusion bonding
process (industrial partnership), fix the final thickness and verify
with a new series of tests
3. Complete the detailed study of the integration in the GTK module
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Next steps and beyond
Next year
1. Study m-channels in combination with CO2 evaporative cooling
Two-phase CO2 vs. single phase C6F14: DP and
DT in a 50 x 50 mm channel
Two-phase flows comparison: DP and DT in a 50
x 50 mm channel plate under the same heat and
mass flow for CO2 , C3F8 and C2F6
2. Challenge the system aspects for larger and more complex
detectors (e.g. ATLAS IBL? CMS PIX? LHCb VeLo?)
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Next steps and beyond
A long-term dream?
Embedded m-channels!
Sensor
Chips
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