I don't think it's a TI part, because TI used aluminum wedge bonding, not gold ball bonding.
That sure is interesting.
It's not, though, because Noopy showed an old genuine TI 7400 with ball bonds.
Areas of transistors are clearly visible under reflective brightfield (if this is the correct term).
Nope, the bases aren't visible. Look at Noopy's 7400, the bases are red. You can clearly see one patch of base area with all the input emitters on it and a link to the pullup resistor, which is also made of a long and narrow "base". This is not visible on your chip and I don't think we have ever seen any classic bipolar chip from TI/ST/Motorola/Fairchild/Philips/etc which was like that. Smells like China to me.
BTW, there is a schematic on Noopy's 7400 page. Hopefully you can figure out the difference between 2 and 13 inputs yourself
The pinkish tinted image is an attempt at reflective brightfield by shining a led light into one eyepiece while looking through the other (it is an old Zeiss Letzlar microscope made in 1950s).
Yeah, I was wondering if it's like that. It's not too bad, but maybe not optimal yet because there is some unevenness. Are you placing the LED exactly in the exit pupil of the microscope?
If not sure what this means, set it up for transmitted observation, pull out one ocular to confirm that exit pupil of the objective is filled 100% with light (open the aperture diaphragm as necessary), reinstall the ocular, hang a paper above it and look where you can see a sharp image of the objective's exit pupil. This image determines the optimal location and diameter of your LED. Fairly obvious if you think about it.
When (ab)using biological objectives for reflected illumination, also make sure they have matte black backs, no shiny metal parts surrounding the pupil. Fairly easy to fix with matte electric tape if necessary.
BTW, for anyone curious. You don't need nitric acid for decapping. Concentrated sulphuric acid will do, but it has to be hot (over 120C roughly). It has to be done in a fume hood or outside because it gives off noxious gasses. Also, don't heat the acid and plop the chip in. Put it in cold and slowly heat in a beaker on a hot plate. After 20min let water run through the black goop that results until it clears. Then throw it out onto a filter paper and look for your prize. Finally clean with isopropyl alcohol and a small brush. Physical cleaning is required as no amount of stirring will clean specs of black goop from the die. Overall a much more approachable process than dealing with hot nitric acid (potentially fuming). If I couldn't do it with sulphuric I would give up. I don't know why online sources prefer nitric.
Because common 65% HNO₃ at 120°C produces same result in 5 minutes and without black goo. Bonding pads are completely etched away, though.
Because FNA at 70°C or so (never done it) produces again the same result, but without bonding pad corrosion at all. The part remains functional.
Online sources also say to clean with acetone if preservation of bonding pads is important. In my experience, H₂SO₄ damages them slightly by itself, even without water cleaning. I have seen claims that corrosion can be avoided with short bath in acid pre-heated to 200°C or more.
Also I wonder if any an help with reverse engineering the schematic. Could someone comment on the below:
These are clearly bipolar transistors. Right?
Yeah, these look like NPNs, unconnected of course. This silicon is likely capable of being several other ICs, depending on the final metal layer on top.
Also in future I might be interested in perhaps grinding the top surface down of this or similar chips slowly to reveal hidden layers. Anyone has an idea how thick those layers are on early pre-CMOS logic and how many there may be?
Nothing interesting in there. It's silicon with different dopants diffused into it (invisible without staining with nasty chemicals), some silicon oxide or nitride (transparent), some metal for connections, some more oxide or nitride. The oxide and metal layers can be cleanly removed chemically if desired.
The oxide layer itself contains most useful information, because its thickness depends on the number of processing steps received by given area and this determine the color of reflected light by thin film interference.
You may also want to take a look at this thread
https://www.eevblog.com/forum/projects/decapping-and-chip-documentation-howto/