The earliest way to view chips was to "cheat" and use old chips that were more readily accessible by soldering their physical caps. However, modern chips generally don't do this. The ones that do do similar things include chips like Xilinx Virtex or Intel x86 chips that run very hot. These can probably be decapped simply by throwing into some moderate acid to eat away the metal.
There seem to be three major techniques for decapping, but only two are used in the hobbyist space. From what I can tell, professional shops use plasma etch machines (http://www.stockly.com/forums/archive/index.php?t-16.html). These are slow, but accurate, and the machines can be had for only a few thousand dollars for a small one. I've even seen presumably working industrial sized ones for under a thousand on eBay. While a few thousand dollars is nothing to the core of a busines, its a bit much for a hobbyist.
The first technique I was introduce to was from Karsten Nohl's talk at the 25thCCC (http://events.ccc.de/congress/2008/Fahrplan/events/2896.en.html). He advocates the use of sanding techniques. Since I didn't need a particular chip to start with, I started with a wide DIP I could mount in a ZIF socket.
This is after I just started doing a thin amount of Dremeling. I then used a Dremel to go down until I could see the wire bonding arcs. Wikipedia has a good picture to help visualize this (http://en.wikipedia.org/wiki/File:DIP_Cross-section.svg):
On some chips I could see this with naked eye, but usually had to be made wet with something first. It was easiest to just use a 10X lens to do periodic inspection when you think you are about there. In practice, this might look something like this:
You can't see the wires very well in this one, but just note how we are almost at the leads. Here is a (slightly) better picture with some zoom:
They are barely noticeable as gold dots. Much more visible if you put some fluid on there. Basically, I could see them easily by getting it wet or by inspection under a 10X lens.
The rest will need to be carefully sanded and polished. After a brief bit of sanding, the bonding wires become much more visible:
Some sanding later...
We can now begin to see the die. I didn't sand it very evenly as you can see one corner much sooner than the rest of the die. This takes a bit of practice to learn how to sand evenly. Basically though, once you begin to see this, tilt your peice slighly so as to try to get the other parts to catch up.
I decided this uneveness was probably just as well anyway since it would give me a gradient slice of the die. After sanding away until we have a good gradient:
You can see resin still in the upper right hand corner. It turns out you can see through this more than you might expect:
The top section still has some resin and the bottom section doesn't It gets a bit fuzzy, but not too much more so, even on the deeper sections. Here we can see some damage from the sanding:
Two types of damage occur: planar depth going too low and particulates causing die scratches. Both need to absolutely be avoided for a decent die image. In any case, after polishing up one of the better sections a bit with some mystery paste substance:
Although we have a lot of damage, we are getting a smoother image. Future work will be around getting better images. A key factor will probably be higher quality polishing pastes instead of the mystery brand dried out paste I tried to use. In any case, I still can't see multiple layers and even sanded down very finly until I saw raw silicon.
And to top it off, I sanded it down really far just to get a feel for it.
Basically, you'll notice some cement used to hold the die in place under the silicon and how the leads go through the package. I'm curious what those two extra leads are for...someone suggested to me something related to power.
Well, that was what came out of round 2. Since I did this, I bought some more polishing compounds including a bottle of actual Dremel brand.
In conclusion, sanding seems like a viable option to get to the die, but I've had some bad luck actually getting multiple layers out of it. As you will find out in later posts, I had decent results with chemical etch of the die, but bad resulsts with chemical etch of the resin. Here, I had good resulsts with sanding off the resin, but bad results sanding off the die. So, I will probably be focusing on this hybrind technique in the future.