Comparison of Anodic and Au-Au Thermocompression Si-Wafer Bonding Methods for High-Pressure Microcooling Devices.

Silicon-based microchannel technology offers unmatched performance in the cooling of silicon pixel detectors in high-energy physics. Although Si-Si direct bonding, used for the fabrication of cooling plates, also meets the stringent requirements of this application (its high-pressure resistance of ~200 bar, in particular), its use is reported to be a challenging and expensive process. In this study, we evaluated two alternative bonding methods, aiming toward a more cost-effective fabrication process: Si-Glass-Si anodic bonding (AB) with a thin-film glass, and Au-Au thermocompression (TC). The bonding strengths of the two methods were evaluated with destructive pressure burst tests (0-690 bar) on test structures, each made of a 1 × 2 cm2 silicon die etched with a tank and an inlet channel and sealed with a plain silicon die using either the AB or TC bonding. The pressure resistance of the structures was measured to be higher for the TC-sealed samples (max. 690 bar) than for the AB samples (max. 530 bar), but less homogeneous. The failure analysis indicated that the AB structure resistance was limited by the adhesion force of the deposited layers. Nevertheless, both the TC and AB methods provided sufficient bond quality to hold the high pressure required for application in high-energy physics pixel detector cooling.

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

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.