Short-Pulse Laser-Assisted Fabrication of a Si-SiO2 Microcooling Device

Abstract

Thermal management is one of the main challenges in the most demanding detector technologies and for the future of microelectronics. Microfluidic cooling has been proposed as a fully integrated solution to the heat dissipation problem in modern high-power microelectronics. Traditional manufacturing of silicon-based microfluidic devices involves advanced, mask-based lithography techniques for surface patterning. The limited availability of such facilities prevents widespread development and use. We demonstrate the relevance of maskless laser writing to advantageously replace lithographic steps and provide a more prototype-friendly process flow. We use a 20 W infrared laser with a pulse duration of 50 ps to engrave and drill a 525 μm-thick silicon wafer. Anodic bonding to a SiO₂ wafer is used to encapsulate the patterned surface. Mechanically clamped inlet/outlet connectors complete the fully operational microcooling device. The functionality of the device has been validated by thermofluidic measurements. Our approach constitutes a modular microfabrication solution that should facilitate prototyping studies of new concepts for co-designed electronics and microfluidics.

Keywords: laser materials processing; microcooling; microfluidic device.

Figure 1

Fabrication process flow for producing Si-SiO₂ microfluidic chips: (a) Surface patterning of Si by use of laser ablation, (b) Sample cleaning and anodic bonding to a borosilicate wafer for encapsulation, (c) Dicing of the individual chips and (d) Mechanical clamping of inlet/outlet connectors.

Figure 2

Laser processing experimental configuration for silicon engraving and drilling. The following acronyms are used, HWP: Half-wave plate, PBS: Polarizing beamsplitter cube.

Figure 3

Schematic of the inlet and outlet connectors (a) and picture of the microcooling device assembled with the inlet and outlet system (b).

Figure 4

Schematic of the cooling circuit used to test the functionality of the microcooling device.

Figure 5

Schematic of showing the temperature sensors positioning on the cooling-device.

Figure 6

Design pattern for 5 microfluidic chips. Top view shows a panoramic view of the test pattern on the sample. Side view corresponds to transverse cuts at positions marked by dashed lines.

Figure 7

(a) Confocal microscope image of a portion of a laser-fabricated microchannel on the silicon surface. The width is ~200 μm and the maximum depth is ~70 μm, (b) Zoom-in on the edge of the laser-ablated channel.

Figure 8

Microscope images of laser-drilled through holes. (a,c) correspond to the front surface and (b,d) to the rear surface. Different hole diameters are presented.

Figure 9

Results of laser-fabricated microchannels with through-holes at the extremities. (a) Photograph of the processed silicon wafer in its annular sample holder, (b) Confocal microscope image showing the morphology of a channel at its extremity, (c) Optical microscope image of the 10 microchannels with their through-holes at the bottom and (d) SEM picture of the hole and the channel.

Figure 10

SEM and optical images of the wafer surface after oxide removal (a,b) and after the polishing (c–f). The back dot on (b,d) is an artefact from the observation device.

Figure 11

Wafer stack after bonding (left) and dicing (right). Some bonding defects are visible at the wafer edge and one in the inner region, all far from the channels.

Figure 12

Measured temperature rises w.r.t. the input water temperature as a function of the power dissipated by the heater: ΔT2 (red) is the increase in the water temperature passing through the cooling device, ΔT5 (blue) is the difference between the temperature measured by the T5 sensor on the back of the device and the input water temperature. ΔT4 (orange) and ΔT3 (green) are the difference of temperature measured by the T4 and T3 sensors placed on the silicon side and the input water, respectively.