> Hope to see some examples of the difficult spots and the correct answers for them,
> these could be helpful for the future typings. Like,
> http://cs.sipr0n.org/static/sega_315-5678_xpol/sega_315-5678_xpol_23_27.png - row 8,
> column 3 - is it 0 or 1? Too bright to be 0, but no bright "circle" in the middle to
> be 1.
I'd like to know more about what causes these difficult spots so that they can be better mitigated, if at all possible.
If I recall correctly, the steps for decapping a chip are:
1. A small amount of the package is ground off in order to provide a divot for the fuming nitric acid.
2. A small amount (just a few drops, possibly even less) of fuming nitric acid is placed in the divot.
3. After a rather short amount of time, the fuming nitric acid is neutralized with acetone or some other alkaline material.
4. Return to step 2, until the die is sufficiently exposed for imaging.
I seem to recall reading somewhere that fuming nitric acid doesn't eat away silicon or precious metals like gold, but regular nitric acid can and does, and moreover, fuming nitric acid turns into regular nitric acid upon reacting with water in the air. Thus if the chip is allowed to process too long, it could damage the silicon die itself, and if it's not allowed to process long enough, there could still be flecks of the package left on the die.
Is that the source of these problem spots in this case, or is there some additional potential source of defects in the photos?
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