Low material budget microfabricated cooling devices for particle detectors

1. 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) P. Petagna & A. Mapelli

2. 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 P. Petagna & A. Mapelli

3. 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 (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 More readily usable quantity: X0 = X0/r [cm] Additional advantage: no CTE mismatch Minimize material budget 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 P. Petagna & A. Mapelli

4. 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 P. Petagna & A. Mapelli

5. Why m-channel cooling? 2 – Cooling power enhancement 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: Hydraulic diameter ~ 10-4 m or less ~103 W/m2K m-channel cooling: very high heat transfer coefficients (very small Dh possible) and very high heat flux (large S available) P. Petagna & A. Mapelli

6. 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 P. Petagna & A. Mapelli

7. 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 Manifolds P. Petagna & A. Mapelli

8. A first application: the NA62 GTK P. Petagna & A. Mapelli

9. A first application: the NA62 GTK P. Petagna & A. Mapelli

10. 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 seesall particles straw chambers measure position RICH identifies pions selects particleswith 75 GeV/c Mag2 Mag3 seeskaons only straw chambers RICH GTK3 GTK1 GTK2 Cedar Achromat Mag1 Mag4 250 m Vacuum tank P. Petagna & A. Mapelli

11. 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 • 3 INDEPENDENT STATIONS: “SIMPLE” SYSTEM • 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 P. Petagna & A. Mapelli

12. 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 P. Petagna & A. Mapelli

13. 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 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. P. Petagna & A. Mapelli

14. 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. Possible layouts Fabrication technique studies Optimal layout Common specs Thermo-fluid dynamic simulations Pressure limits Numerical structural simulations Experimental tests P. Petagna & A. Mapelli

15. 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 layer of 1 µm of oxide (SiO2) is grown on both sides of the wafer 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 Dry etching of the top layer oxide is used to transfer the micro-channels pattern 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. Deep Reactive Ion Etching (DRIE) is used to partially etch the access holes down to 280 µm The photoresist is stripped in Microposit Remover 1165 at 70°C and DRIE is used to anisotropically etch 100 μm deep channels separated by 25 µm wide structures in silicon Subsequently the oxide layers are removed by wet etching in BHF 7:1 for 20 min at 20°C 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 P. Petagna & A. Mapelli

16. m-fabrication process 30 mm 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 .... 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. 1 mm Scanning Electron Microscope image of the cross-section of 50 x 50 mm channels etched in silicon bonded to a Pyrex wafer P. Petagna & A. Mapelli

17. Structural analysis 1 – Experiments “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 3 0.05 0.025 60 P. Petagna & A. Mapelli

18. 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 Yield stress ~25 MPa[ICES 2009] P. Petagna & A. Mapelli

19. Structural analysis 3 – Extrapolations P. Petagna & A. Mapelli

20. 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. 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 • 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 Flow rate attained with 2 bar Dp vs. channel width for a fixed height of 90 mm P. Petagna & A. Mapelli

21. First tests on a full-scale prototype Test sample and numerical model manifolddepth 100mm Inlet 1mm • Channel crosssection 100mm x 100mm • Power density 1 W/cm2 (50% nominal) • Massflow 3,66 x 10-3 kg/s (50% nominal) • Inlettemperature 18 C • Outletpressure 1bar • Laminar flow Outlet P. Petagna & A. Mapelli

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

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

24. First tests on a full-scale prototype Steady state DT between inlet and surface probes 3 2 1 3 2 1 5 4 6 5 4 6 P. Petagna & A. Mapelli

25. 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 Wedgedmanifold, depth150mm, 280mm and 400mm Outlet P. Petagna & A. Mapelli

26. 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 Wedged manifold, 1.6 mm Max width, 400 mm thick, opposed inlet & outlet P. Petagna & A. Mapelli

27. Layout optimization P. Petagna & A. Mapelli

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

29. Next steps and beyond Immediate future Perform full-scale thermal tests in cold (vacuum vessel) 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 Complete the detailed study of the integration in the GTK module P. Petagna & A. Mapelli

30. Next steps and beyond Next year Study m-channels in combination with CO2 evaporative cooling Challenge the system aspects for larger and more complex detectors (e.g. ATLAS IBL? CMS PIX? LHCbVeLo?) 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 Two-phase CO2vs. single phase C6F14: DP and DT in a 50 x 50 mm channel P. Petagna & A. Mapelli

31. Next steps and beyond A long-term dream? Embedded m-channels! REMOVE Sensor Chips P. Petagna & A. Mapelli