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 20101P. Petagna & A. Mapelli

Outline of the talk 30 Sep 20102P. Petagna & A. Mapelli 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

Why  -channel cooling? 30 Sep 20103P. Petagna & A. Mapelli Radiation length (X 0 ): mean distance over which the energy of a high-energy electron is reduced to 1/e (0.37) by bremsstrahlung 1 – Minimization of material budget (Dahl, PDG) More readily usable quantity: X 0 = X 0 /  [cm] Cu: cm Steel: ~1.7 cm Al alloy: ~8.9 cm Ti: 3.56 cm Si: 9.37 cm C 6 F -20  C : cm K13D2U 70% vf: 23 cm CO  C: cm Minimize material budget Minimize x[cm] / X 0 [cm] (i.e. use material with high X 0 and minimize thickness)  -channel cooling naturally addresses this issue through the use of Si cooling plate and tiny (PEEK?) pipes in extremely reduced thickness Additional advantage: no CTE mismatch

Why  -channel cooling? 30 Sep 20104P. Petagna & A. Mapelli Material budget of the CMS Si-strip tracker (10 layers)Material budget of the CMS Si-Pixel tracker (2 layers) Present LHC large Si trackers (ATLAS and CMS) ~ 2% X 0 per layer SLHC “phase II” upgrade: “significant” reduction needed Future trackers at ILC ~ 0.1 ÷ 0.2% X 0 per layer

Why  -channel cooling? 30 Sep 20105P. Petagna & A. Mapelli 2 – Cooling power enhancement Newton’s law for convective heat flux: Heat transfer coefficient for m-channel system: Hydraulic diameter ~ m or less Nusselt number = 3.66 ÷ 4.36 for fully developed laminar flow Fluid thermal conductivity = 0.05 ÷ 0.11 W/mK for low temperature fluids ~10 3 W/m 2 K  -channel cooling: very high heat transfer coefficients (very small D h possible) and very high heat flux (large S available)

Why  -channel cooling? 30 Sep 20106P. Petagna & A. Mapelli 3 – Reduction of  T between heat source and heat sink With a standard cooling approach, the  T between the module and the fluid ranges between 10 and 20  C (small contact surface + long chain of thermal resistances) With an integrated  -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  T between the fluid and the element to be cooled Lower temperatures are envisaged for the future Si-trackers at SHLC. This has non-negligible technical impacts on the cooling plants

An example of future potential use 30 Sep 20107P. Petagna & A. Mapelli Concept of module for a “level-0 trigger” SHLC (courtesy of A. Marchioro) Sensor RO chips  -channel cooling plate Manifolds Interconnect

A first application: the NA62 GTK 30 Sep 20108P. Petagna & A. Mapelli

A first application: the NA62 GTK 30 Sep 20109P. Petagna & A. Mapelli

A first application: the NA62 GTK 30 Sep P. Petagna & A. Mapelli Vacuum tank Mag2Mag3 Mag4Mag1 GTK1 GTK3 GTK2 Cedar selects particles with 75 GeV/c sees kaons only Achromat 250 m beam: hadrons, only 6% kaons-> only 20% decay in the vacuum tank into a pion and 2 neutrinos -> out of which only decays are of interest straw chambersRICH hit correlation via matching of arrival times – 100 ps RICH identifies pions straw chambers measure position GTK sees all particles

A first application: the NA62 GTK 30 Sep P. Petagna & A. Mapelli Sensor & bonds: 0.24% X 0 (~200 µm Silicon) RO chip: 0.11% X 0 (~100 µm Silicon) Passive or active cooling plate Final target: 0.10 – 0.15 % X 0 Priority: minimize X 0 Acceptable DT over sensing area ~ 5 °C Dimension of sensing area: ~ 60 x 40 mm Max heat dissipation: ~ 2 W/cm 2 Target T on Si sensor ~ -10 °C Support structure outside acceptance region: ~ FREE Pixels / station (300 x 300  m, 200  m thick) 10 ASICS chips bump-bonded to the sensor

Proposed solution 30 Sep P. Petagna & A. Mapelli Schematic of the layout of the proposed  -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

Approach to the problem 30 Sep P. Petagna & A. Mapelli Take advantage of recent results obtained in two different fields of development:  -channel cooling devices have started to be actively studied for future applications for high power computing chips or 3D architectures. Thin and light  -fluidic devices in silicon are largely in development for bio- chemical 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.

Approach to the problem 30 Sep P. Petagna & A. Mapelli 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. Fabrication technique studies Thermo-fluid dynamic simulations Numerical structural simulations Experimental tests Common specs Possible layouts Optimal layout Pressure limits

 -fabrication process 30 Sep P. Petagna & A. Mapelli START: Czochralski silicon wafer polished on both sides (4′′ diameter, 380 μm thick, ohm-cm p-type). (a)A layer of 1 µm of oxide (SiO 2 ) 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)At present, the processed Si wafer and an unprocessed Pyrex wafer (4” diameter and 525 µm thick) are then cleaned in a Piranha bath (H 2 SO 4 + H 2 O 2 ) at 100°C and anodic bonding is performed to close the channels with the Pyrex wafer

 -fabrication process 30 Sep P. Petagna & A. Mapelli Scanning Electron Microscope image of the cross-section of 50 x 50  m channels etched in silicon bonded to a Pyrex wafer 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. 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. In the final production both the processed and the unprocessed wafers will be in 525 µm thick silicon. 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 1 mm mm

Structural analysis 30 Sep P. Petagna & A. Mapelli “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 varying width 1 – Experiments

Structural analysis 30 Sep P. Petagna & A. Mapelli 2 – Numerical simulations vs. tests Yield stress ~25 MPa [ICES 2009] 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

Structural analysis 30 Sep P. Petagna & A. Mapelli 3 – Extrapolations

Thermo-fluid dynamics simulations 30 Sep P. Petagna & A. Mapelli The choice of the cooling fluid circulating in the micro-channels has naturally been oriented towards perfluorocarbon fluids (C n F 2n+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 C 6 F 14 is liquid at room temperature and is used as single phase cooling fluid in the inner tracking detectors of CMS. Properties C 6 F -25°C Density r [kg/m 3 ]1805 Viscosity n [10 -7 m 2 /s]8.2 Heat capacity c p [J/(kg K)]975 Thermal conductivity l [10 -2 W/(m K)]6.275 Based on the properties of C 6 F 14, 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 The results from the analytical calculations performed indicate that the suited range of the micro channel geometry is the following: Width: between 100  m and 150  m Height: between 80  m and 120  m Fin width: between 25  m and 75  m Between 300 and 500 channels to cover the area Flow rate attained with 2 bar  p vs. channel width for a fixed height of 90  m

First tests on a full-scale prototype 30 Sep P. Petagna & A. Mapelli 1mm Inlet Outlet manifold depth 100  m Channel cross section 100  m x 100  m Power density 1 W/cm 2 (50% nominal) Mass flow 3,66 x kg/s (50% nominal) Inlet temperature 18  C Outlet pressure 1bar Laminar flow Channel cross section 100  m x 100  m Power density 1 W/cm 2 (50% nominal) Mass flow 3,66 x kg/s (50% nominal) Inlet temperature 18  C Outlet pressure 1bar Laminar flow Test sample and numerical model

First tests on a full-scale prototype 30 Sep P. Petagna & A. Mapelli Simulated vs. experimental pressure drop

First tests on a full-scale prototype 30 Sep P. Petagna & A. Mapelli Thermal visualization IN OUT Thermograph before injection Thermograph at injection IN OUT Thermograph after few seconds of coolant circulation Heat load simulated by a Kapton heater of suited resistance and geometrical dimension

First tests on a full-scale prototype 30 Sep P. Petagna & A. Mapelli Steady state  T between inlet and surface probes

Layout optimization 30 Sep P. Petagna & A. Mapelli Outlet Wedged manifold, depth 150  m, 280  m and 400  m Optimized geometry for uniform and minimal  P 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

Layout optimization 30 Sep 2010P. Petagna & A. Mapelli26 Effect of inlet manifold geometry on  P Rectangular manifold, 1 mm wide, 100  m thick, central inlet & outlet Wedged manifold, 1.6 mm Max width, 150  m thick, opposed inlet & outlet Wedged manifold, 1.6 mm Max width, 280  m thick, opposed inlet & outlet Wedged manifold, 1.6 mm Max width, 400  m thick, opposed inlet & outlet

Layout optimization 30 Sep P. Petagna & A. Mapelli

Layout optimization 30 Sep P. Petagna & A. Mapelli two inlets Summary table

Next steps and beyond 30 Sep P. Petagna & A. Mapelli 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

1.Study  -channels in combination with CO 2 evaporative cooling 2.Challenge the system aspects for larger and more complex detectors (e.g. ATLAS IBL? CMS PIX? LHCb VeLo?) Next steps and beyond 30 Sep P. Petagna & A. Mapelli Next year Two-phase CO 2 vs. single phase C 6 F 14 :  P and  T in a 50 x 50  m channel Two-phase flows comparison:  P and  T in a 50 x 50  m channel plate under the same heat and mass flow for CO 2, C 3 F 8 and C 2 F 6

Next steps and beyond 30 Sep P. Petagna & A. Mapelli Sensor Chips Embedded  -channels! A long-term dream?