Flexible monolithic active pixel sensors embedded in ultra thin polymer film

A CMOS Monolithic Active Pixel Sensor (MAPS) integrates on the same silicon substrate the radiation sensor element (thin epitaxial layer) with processing electronics (based on CMOS transistors) on top. Total thickness of active volume of this stack is usually very small: typically 15 m for silicon and 5 m for interconnections (several metal-insulator layers). Excellent minimum ionizing particle tracking performance has been demonstrated with these devices and recent availability of high-resistivity epitaxy allows them to reach radiation hardness comparable to standard, fully depleted silicon sensors. MAPS can be thinned down to less than 30 m, without loosing their tracking performance. This allows not only very small material budget but also construction of non-planar detector layer: such thin silicon is flexible enough and can be bended to form a cylinder with a radius of the order of centimeter. However the structure is fragile, difficult to handle and to connect using standard methods (wire bonding). We propose a new method of packaging of such thin devices based on embedding in thin polymer film, with an application of modern silicon wafer processing technologies. In this solution, the sensor is protected from both sides by plastic material (10-20 m thick, insulator); metal paths deposited on top and directly connected to bonding pads through integrated vias in the polymer provide electrical connection. High-precision sensor Mimosa-18 (10 m pitch, 512x512 pixel array) has been chosen for the embedding process demonstrator. In this work, we present details of embedding process together with study of tracking performance of the sensor (charge collection properties, ENC, S/N ratio for MIPs) as a function of mechanical stress (bending). Possible extension of embedding process to larger area (tens of cm), multi-sensor structures and more than one metal layer interconnections will be also proposed and discussed.