Silicon chips have been getting smaller and smaller over the years, and this has made wiring them up more and more difficult. Most complicated chips are made on a large silicon wafer up to 18 inches across, then cut out of the wafer to produce individual chips. These are mostly wired up by making contacts on the surface and turning them over and putting them onto a connector. This is fine for most purposes but for camera chips this is a big problem, as the light then can't get to the sensors. This means that they have to cut the chips out of the large wafer, then connect wires to the edges which for a small sensor is difficult and fiddley especially for a very small chips and so expensive.
A group from the IZM Fraunhoffer Institute has come up with a solution to this, rather than connecting to the front of the silicon wafer, they have used a small wafer, effective drilled holes through the wafer to the back, and filled them with conductor creating what is called a via. This means that they can connect the wires onto the back of the wafer before it is cut up. It also means you can stick lenses on the other side.
Then you can just cut up the wafer to produce finished cameras. This has enabled them to make complete cameras that take up a volume of 1mm by 1mm by 1mm, with about 600 000 pixels, and they are made in a way that should be very cheap. They are suggesting using them in endoscopes and as they are so cheap they can be essentially disposable avoiding the difficult problem of disinfecting electronics before putting them in the next patient.