1. Microchannel cooling - Update
Outline
Test of 1st full-scale prototype
Hydraulic behaviour
Thermal behaviour
Layout optimization for the prototype
Structural analysis and Manifold Pressure tests
Next steps
07 Sep 2010
G. Nüßle

2. 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
Outlet
07 Sep 2010
G. Nüßle

3. First tests on a full-scale prototype
Simulated vs. experimental pressure drop
07 Sep 2010
G. Nüßle

4. 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
07 Sep 2010
G. Nüßle

5. 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 3 2
07 Sep 2010
G. Nüßle

6. Wedged manifold, depth 150mm, 280mm and 400mm
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 are then performed with 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
07 Sep 2010
G. Nüßle

7. Layout optimization Effect of inlet manifold geometry on DP
Wedged manifold, 1.6 mm Max width, 150 mm deep, opposed inlet & outlet
Wedged manifold, 1.6 mm Max width, 280 mm deep, opposed inlet & outlet
Rectangular manifold, 1 mm wide, 100 mm thick, central inlet & outlet
Wedged manifold, 1.6 mm Max width, 400 mm deep, opposed inlet & outlet
07 Sep 2010
G. Nüßle

8. Layout optimization
07 Sep 2010
G. Nüßle

9. Structural analysis Experiments
“Sacrificial” samples with different manifold widths 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 3
0.05
0.025
varying width
07 Sep 2010
G. Nüßle

10. Structural analysis 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]
07 Sep 2010
G. Nüßle

11. Structural analysis
Extrapolations
07 Sep 2010
G. Nüßle

12. Next steps Immediate future
Perform full-scale thermal tests in cold (vacuum vessel)
Define the details and properties of the Si-Si fusion bonding process and verify with a new series of tests
Advance in the integration study of the GTK module
07 Sep 2010
G. Nüßle

13. Back Up
07 Sep 2010
G. Nüßle

14. Thermo-fluid dynamic basic calculations
The cooling fluid circulating in the micro-channels is the perfluorocarbon C6F14, which is 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
-25°C
Density r [kg/m3]
1805
Viscosity n [10-7 m2/s]
8.2
Heat capacity cp [J/(kg K)]
975
Thermal conductivity l [10-2 W/(m K)]
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
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
07 Sep 2010
G. Nüßle

15. Layout optimization
Summary table
two inlets
07 Sep 2010
G. Nüßle

16. 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
07 Sep 2010
G. Nüßle

17. 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.
07 Sep 2010
G. Nüßle

18. 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.
Fabrication technique studies
Possible layouts
Common specs
Thermo-fluid dynamic simulations
Optimal layout
Numerical structural simulations
Pressure limits
Experimental tests
07 Sep 2010
G. Nüßle

19. m-fabrication process
START: Czochralski silicon wafer polished on both sides (4′′ diameter, 380 μm thick, 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 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
07 Sep 2010
G. Nüßle

20. 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.
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
....
30 mm
Scanning Electron Microscope image of the cross-section of 50 x 50 mm 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.
07 Sep 2010
G. Nüßle