3D integration technologies for imaging applications
Radiation detection of both photons and particles has been evaluating during the last decades from fairly large (millimeter to centimeter) size single pixel detectors over one-dimensional arrays to full two-dimensional imaging systems with very small pixel sizes (down to a few micron), having large number of pixels (e.g. several Megapixels) and hence excellent spatial resolution. This evolution happened mainly thanks to the miniaturization capabilities of both advanced CMOS analog (and digital) readout electronics and sensor manufacturing technology. Another enabling technology is hybrid interconnection, e.g. using small solder bumps to connect the individual cells on the read-out chip with the individual pixels of the separately processed sensor array (consisting of e.g. non-Si materials). In line with the further miniaturization potential of the CMOS read-outs, the pitch at which this hybrid interconnection is realized will further decrease to less than 10 micron, thereby shifting to alternative materials and technologies for the interconnects.
A disruptive performance enhancement of these radiation imaging systems will be obtained when three-dimensional stacking of dies or embedding of thin dies is being used. Contrary to the current systems having lateral interconnects (e.g. bonding wires) to the read-out chips, 3D stacking of sensor, read-out chip and read-out system board will allow to tile individual 3D detectors and will enable large area detection combining both high resolution and very small non-sensitive area. Moreover, 3D integration of (e.g. multiple) sensor layers, analog read-out electronics, memory and digital signal processing chips will allow to build intelligent and yet compact imaging sensor nodes.
In order to manufacture these 3D integrated imaging devices, a number of different technologies have to be developed in a manufacturable and reliable way: through (thinned) Si via interconnects for bringing the electrical signals from one side of the chip to the other, and high density microbump interconnects to interconnect two or more different chips each of them having through Si via interconnects. Alternatively, sensor and read-out dies can be aggressively thinned and embedded using thin film technology, enabling very thin flexible, foldable systems. The required technology development and its status today will be discussed along with some imaging system application demonstrators such as backside illuminated active pixels sensors and tilable X-ray detector 3D stacks.
