After spending too much time at the zeptobars site, I decided to decap some ICs on my own, to see what can be achieved with reasonable but not professional equipment. In my home lab I have good laboratory glassware, quality reactives and a decent biological microscope with a camera. The question is if that equipment can be used to solve some doubts about ICs, like general manufacture and authenticity.
For decapping, I tried to follow
zeptobars' method: in a closed Erlenmeyer flask I put the chips and 98% sulfuric acid. That flask was connected to an empty buffer flask, and that one to another flask containing a saturated NaOH solution, which degassed outside. The buffer flask was there to avoid the alkaline solution from flowing into the acid flask when cooling, due to the pressure differential. The acid flask was very gently heated until the acid started to bubble, then I let it react for half an hour. A fine mist (I assume SOx) formed in the flask-- the bubbling was not violent at all and the degassing scheme worked fairly well. The whole proccess took about 45 minutes, before I stopped the heat and let the flask to slowly cool to ambient temperature. Then I transferred the tarry acid to another vessel, leaving the solid parts in the flask, then neutralized the remains and filtered them. The yield was a lot of carbonized particles, metal remains from the legs of the components, and three silicon dies. Two ICs, a TO220 and a DIP8, were not corroded thorougly and will need another cook-- zeptobars mentions that. The SOT666 and TO92 packages were either completely corroded, or corroded enough, in just one cook.
Of course, the whole proccess is dangerous, and must be made with great care and adequate equipment and protection. Dispossing safely of the remaining acid is a nontrivial part of the matter.
From the filtered remains, the dies were easy to spot. It's a bit like looking for gold: when the light hits just right, the dies shine very clearly. I was not prepared for how tiny the dies are: even a sot package is huge compared to a moderate size IC, never mind a discrete transistor.
I chose chips that were interesting for some reason or another:
A
pmp2401V matched pair npn bjt from Mouser. The question was if the transistors were in one die or two dies,
like the model analyzed at zeptobars. Two images, one using the back illumination by the microscope, the other using an LED, frontally.
The only fragment I recovered from this IC is a single die BJT with the same structure than the zeptobars one. So there is one die for each transistor. Wouldn't matching improve if they made them in the same die? Is there a cost/performance hit to the single die solution?
Then a 2SC3355 UHF bjt I got from ebay. Is it a real UHF transistor, or rather a lesser bjt marked as such. For this transistor the decapping was incomplete, and some carbon remained. Even so, the image reveals more or less the typical interdigitated structure of a fast transistor, so I assume it is legit.
Finally, a noname TL431 from ebay. There are many images of TL431 dies around, so I used this as a control IC. The chip was completely decapped and rescued from the carbon dust. Since there is only silicon, I used the backlight of the microscope, though frontal illumination with LED also works.
The die is similar but not exactly equal to any other I've seen. Since the silicon is translucid, I could go for more amplification than with the other pieces, and get finer details. I think there are several lateral PNPs in this capture:
With this level of resolution, it is clearly possible to discern if a chip is genuine: ICs usually have marks from the manufacturer that are difficult to fake.
So even with a modest microscope, and using lab equipment with care, ICs can be decapped and analyzed, yielding quite interesting information. My microscope technique needs a lot of improving. The pictures I got are to zeptobars ones like amateur astronomy is to the Hubble telescope, but I'm surprised my humble protozoan peeking microscope worked so well.