Making a phone sensor is quite different from a camera sensor. It mostly comes down to size - the camera sensor is much larger. A fab plant capable of making a camera sensor can make a phone sensor (provided it can make the circuitry small enough for the required specifications - newer technology is often introduced in phones/small sensors first, because it's cheaper than introducing it in a large-sensor plant) but a fab plant built for making small sensors can't possibly make a larger sensor. Fab plants making large sensors are few and far between.
No, it is the same sensor plant. This is CMOS technology after all, it is the same base technology to build microchips, like processors and so on. So, basically this works like this:
- They have this big rods of pure Si - Silicon that are cut into thing wafers, typical sizes now a days are 300 mm in diameter. Purity of the Si is very important, as we will see later.
- Over each one of these buffers, in really clean rooms, they start to apply several chemical and physical process. This is basically growing the different transistors, circuits, and sensor parts. In each one of this wafers they build several sensor chips. If they are little ones for smartphones, they can make more, if they are bigger ones for medium format they can make less.
- After everything is finish, they wafer are cut into the sensors and testing starts.
Here is where physical size of the sensor plays a big role. So, if they Si wafer has an impurity in any of the parts of the sensor, that sensor is not going to work. If any of the transistors, sensors, circuits does not work, that sensor needs to be discarded.
So, if you are making like 10000 sensors per wafer (invented number... I didn't do the math of the area of a mobile phone chip vs the area of the wafer), you are getting a high ratio of good sensors, even if you have some impurities or errors in the manufacturing, you still getting high number of good sensors per wafer that you can sell.
If you are making like 100 sensors per wafer (for example, full frame), you are probably getting worst ratio between good vs bad sensors. This is one of the main reasons bigger chips are more expensive. Apart that you are getting less good chips, the rest of the process to build the chips over the wafer is the same, so you have to divide the total costs between all the sensors coming out of the wafer, if you are getting more sensors, less they will have to pay.
Trying new technologies, like for example BSI, it is better to do this first in smaller ones. Because the technology is new and needs a tuning in the manufacturing process, the firsts wafers will have a very low yield, manufactures try to build first smaller sensors since it is easy for then to still get a good number of sensors that work (or the most simply ones, for example, memory chips are typically the ones used by Intel when then try a new physical/chemical process, they are the simplest chips they build, so easy to see how to fine tune the process). When that process is fine tune, you can take the risk of building bigger sensors with it and still get a good yield performance, so you are not risking too much money in the process.
So, big or small, all sensors are made in the same buildings. Take a look at the earthquakes that affected the manufacturing plants of Sony, all sensors lines were affected. Probably smartphone ones were the first one to recover because of what I said before, after rebuilding everything, having to calibrate again all machinery, doing it with the sensor size that has the highest probability of getting highest rate of good sensors make more sense. After those lines were starting to get good yield rates, Sony started to getting back to building bigger sensors.
Here:
http://blog.teledynedalsa.com/2012/02/the-art-of-making-large-image-sensors-or-quilting-with-silicon/ and here:
http://www.siliconimaging.com/cmos_fundamentals.htm you have images of wafers before being cut. Here you have a nice schema about how many full-frame sensors, APS-C sensors and 1" sensors are built per wafer:
http://photo.stackexchange.com/questions/53826/why-does-increasing-sensor-size-necessarily-lead-to-lower-silicon-wafer-utilizatRegards,
David
