Chip Decapping Adventure #5: Removing the top metal layer

Having successfully removed the top glass layer of the die with [Armour Etch], which [Amazon] says is NOT RECOMMENDED FOR CLEANING GLASSES! I proceeded to remove the top metal layer. First, a shot of the die with the glass still intact:

Following a recipe I found in the slides of Lecture 6 [PDF] from RPI's [Hardware Reverse Engineering] course for removing copper metal, I put the chip in a teflon beaker (it was small, so I used it) and added 6 mL of 3% hydrogen peroxide and 1 mL of 32% hydrochloric acid [DudaDiesel]. I could see bubbling coming mainly from the leftover metal of the leadframe, so I figured the solution was working. I left it for 5 minutes, then removed the chip, sprayed with diluted water, and sprayed with acetone to dry.

The result:

Success! You can see that the top metal layer is gone, leaving the glass underneath it. The bottom metal layer was not touched. Maybe because it is aluminum?

Here's an interesting feature, showing where a via from top metal to bottom metal was removed, and there is some glass around where it was:

And here is something I noticed while I was taking images after I thought the chip was dry:

That is probably acetone that leaked under the glass and is slowly being pulled out by drying in atmosphere.

Here is something that I'm struggling to make sense of:

On the right is the die before the top glass and top metal was removed. On the left is the die after. We can see that in two cases, the inner rectangles have expanded, and in one case it looks like an over-etch of glass. In the expanded cases, maybe this is just an optical effect from the glass.

Anyway, the chip is quite dirty. I did not clean it in an ultrasonic bath. Also, I think to get a better picture, I should try another round of Armour Etch to get rid of the remaining glass so that the whole image is clear. Hopefully this will not damage the bottom doped layer.