When we last left our bumbling idiot back in Adventure #3, I had successfully decapped a plastic-encased die with hot sulfuric acid.
Just a note on this. Prior to sticking the entire chip in the beaker, I tried using a Dremel to gently shave away at the bottom of the chip to locate the die, then to cut the chip apart to remove much of the bulk plastic, leaving a "chiplet". I tried this four times on the same type of chip (same manufacturer and date code), and each time I ended up with a cracked die.
Each time I tried to change a variable: heat, ultrasonic, and so forth, but nothing worked. It was only until I put the entire chip in without Dremeling that I obtained a successful, intact die. Now, the wires on this die didn't seem thin and round. They seemed flat. So my theory is that the vibration of the Dremel was transmitted strongly to the die, causing it to crack while still in the plastic.
This is why I'll now always put the entire chip in rather than risk a cracked die.
Our brave individual now stumbled on to the next leg on the journey, The Removing of the Top Glass Layer!
Here is a stitched image of a 54LS244 from a ceramic package, after it has been cleaned in the ultrasonic bath. Rather than use a test tube in the bath, I used a plastic bag so that the chip wouldn't bounce around and get damaged. When I do this with a die from a plastic package, this is also the method I'll use.
You can see that there is a thick metal layer on top that obscures a lot of the detail. For example, this image is a closeup of the middle section:
We need to remove this metal layer. But first, we need to remove the clear glass layer that is on top of it. This is traditionally done with [hydrofluoric acid], which is also known as The Devil's Piss, and not in a ha-ha this is a fun alcoholic drink sense, but in the oh shit this stuff just got on me and now I'm going to lose my finger sense. It is incredibly dangerous: if you get it on your skin, you will not notice it because it is painless. It will eat through your skin without you knowing it and settle in your bone, and will then proceed to dissolve the bone from inside.
There is a product, [Eagle One Chrome Wheel Cleaner], whose [MSDS sheet] shows that it contains 5-10% HF. Apparently this is dilute enough not to cause instant regret. Of course, it has a lot of other crap in it, too.
I was talking with Ken Shirriff, and he mentioned just using glass etchant from any old art supply store. So I found [Armour Etch], which must be spelled with the UK ou spelling. The [MSDS] shows it contains, among other things, sodium bifluorite, ammonium hydrogen difluoride and sulfuric acid. Ammonium hydrogen difluoride attacks glass, and apparently sodium bifluorite plus an acid generates hydrofluoric acid, which also attacks glass.
I think with MSDS sheets, the company lists everything that might possibly go in the product. So for example, perhaps Armour Etch sometimes contains sodium bifluorite plus sulfuric acid, and sometimes just the ammonium hydrogen difluoride, depending on what costs less at time of production.
Anyway, I donned some nitrile gloves to make me feel good, and first dipped a cotton swab into the cream. I dabbed it onto the die and then rubbed it around for 30 seconds. The cream didn't last long. I did this again three more times, then sprayed acetone on it and viewed it under the microscope.
Ugh. Clearly the etch had some effect, but it was pretty random and sparse.
Next, I dabbed some cream on the die and left it for three minutes. Then I used a cotton swab to rotate the cream a bit to maybe replace the attacked glass with fresh etchant. Then I left it there for another three minutes, and repeated once more. Then I sprayed distilled water on the die, and then sprayed acetone to dry it. Finally, I examined the result under the microscope.
This was much better! On the top metal layer you can see some scuff marks, probably from the first attempt when I was rubbing with the cotton swab. There's also a piece of dirt because I didn't ultrasonic clean this before popping it in the microscope.
There may even be a small bit of over-etching, as evidenced slightly to the left of the middle of the image where there's some discoloration.
This is all looking very good. The next step will be to try some hydrochloric acid to remove the top metal layer. The hope is that the bottom metal layer is still protected by glass. If not, it is probably not a big deal, since we can see the outline of the bottom metal layer through the top metal layer anyway. It's the doping areas that really need to be uncovered.
But that will have to wait for the next step of the journey, Adventure #5.