For teaching a course I needed to take a closer look at a CPU. I asked around and got my hands on an old P-III Coppermine that was about to get thrown out. I’ll start with a disclaimer: I know virtually nothing about CPUs, so if I claim something to be true, it probably isn’t.
The first challenge is to get the actual silicon processor chip off of the plastic bonding board. In the picture below, the blue thing you see is the back side of the processor chip. When the processor is finished, it is turned upside down and bonded to the green circuit board. This allows the metal pads on the silicon chip and the pads on the circuit board to join, creating a connection (this is one of those claims...). I believe that the CPU at that stage is heated up in order to melt the joints and thereby solder them together.
Click images for full-size. Especially the scanning electron microscope images below could be interesting to view in all its splendor.
I figured I should be able to remove the chip by heating it. I first tried using a heat gun, but that just made some bad smelling fumes. I instead turned to brute force and used a power-saw to cut out the part containing the actual chip. Using pliers I managed to get a few pieces off of the board (cracking the chip in the process, but I was planning to do that anyway) and got the rest off by using a scalpel.
Below you can see the result. On the bottom side of the piece that came off you can see all the connector pads that were previously connected to the pins on the backside of the circuit board.
Now the interesting part begins. I looked at the piece above in an optical microscope. The picture below shows an enhanced version of the little dots in the above picture.
Furthermore, by changing the focus of the microscope, we can see multiple layers within each hole
In a CPU there are multiple layers of sandwiched metal leads going down to the transistors at the bottom (at the surface of the silicon wafer). I believe what we're seeing is simply those different layers.
Since optical microscopy doesn't show very much detail, I decided to load the chip into a scanning electron microscope (SEM).
What I did was to cleave the chip into smaller pieces. This way I can peek from the side of the chip and get some cross-sectional images. Below is a series of images that shows a zoom-in on the surface of the Si chip. For some reason I had a lot of trouble getting a good focus. I'm not sure what kind of Si is used for these CPUs, but if it's a non-conducting silicon, the electron beam in the SEM can charge the material making it difficult to focus. Another possibility is that all the plastic encapsulation material caused charging effects.
I believe that in order to see the actual transistors at the bottom (at the surface of the silicon wafer) I will need to remove all the layers. I'll let you know if I figure out a good way of doing this.