After seeing a really neat video by Ben Krasnow on YouTube about decapping integrated circuits to reveal the tiny silicon chips inside, I was intrigued. By dissolving away the black epoxy surrounding the chip innards, Ben Krasnow uncovered the silicon wafer square that actually holds all the circuitry for the IC. I thought this was pretty cool, and since I had nitric acid, I decided to give it a shot.
I started by sanding down the metal pins and most of the epoxy. This made it so the nitric acid would have less material to dissolve, so I wouldn't need as much acid. Once I had my chips prepared, I put them in a glass beaker on my hotplate. With the chips on medium heat, I slowly dripped nitric acid onto the black epoxy. It is absolutely critical to add the nitric acid extremely slowly! If it is added too quickly, there will be billowing clouds of nitrogen dioxide, which has an awful odor. Additionally, adding the acid too quickly can cause thermal shock on the hot beaker, which may make it crack (personal experience).
The experiment used substantially more nitric acid than I expected, but after a while of slowly dripping the acid onto the epoxy and regulating the heat to prevent excess boiling, I saw what looked like a silicon chip. The epoxy hadn't actually dissolved, though. It had disintegrated into a thick black paste, which made finding the extremely small silicon chips difficult. I let the beaker cool and then poured everything into water to dilute any remaining acid. From two ICs, I got three silicon chips. I cleaned them with acetone and then put them on a slide for inspection.
I was shocked by how much detail fit onto a chip only a few millimeters square. All these pictures were taken by simply lining my Nikon J1 up with my microscope eyepiece. The results were pretty impressive (the text is even readable at the top of the left picture):