How do die defects on a wafer work?

[TemjinGold]

So I've been curious: When making cpu wafers, is each whole wafer an exact copy of each other whole wafer made? Like if wafer 1 has a perfect 1800x in the center and one with a busted core a few chips down, does that mean every wafer made will have this issue (i.e. are the wafers carbon copies of each other) or are defects random occurrences (i.e. the center cpu could be busted on some and perfect on others)? If it's random, then how does sampling rooted out problems since you only test a small percent of what you actually produce?

 

[Arachnotronic]

Wafers are not carbon copies of each other. And what makes you think that only a small percentage of the dies that are produced are tested? They're all tested.

 

[imported_jjj]

start with this https://en.wikipedia.org/wiki/Substrate_mapping

 

[VirtualLarry]

Yeah, every die is tested. They may use a high-res optical scanner, to eliminate obvious defects right off, or perhaps that's how they eliminate all functionally-defective dies? I don't know.

 

[sm625]

Wafers are supposed to be carbon copies of one another, but each wafer has its own defects in generally random locations. There are probably some instances where every wafer has a flaw in a specific location, but those are likely to be fixed early on in the process' development.

 

[Sheep221]

Carbon wafers? That's new because they are made from silicon.
And as such even on mature fabrication, they lose quite lot of chips per wafer, that's why we have celerons, pentiums or i5s, respectively.
Only i7 and i3 nodes are produced and all the stuff inbetween are just chips that are functional but defective to a degree that won't allow them to be stable at flagship's configuration, so they just downclock it and cut-off unstable parts, such as computational or graphic cores or blocks of cache memory, disable HT and so on and sell it for lower price proportional to value they have left. Variety of these defects is really huge and can be seen by how many SKUs of few CPU types Intel does sell.
Ofc every chip is thoroughly tested for long term functionality and only those which do pass the testing are then sold, rest is discarded.
Nothing is really 100% or intact, you can't make anything completely clean in physical and chemical sense, the wafers are defective and that's why top tier CPUs are so expensive, compared to low end ones.

 

[Tuna-Fish]

Just to expand on the testing: Every single CPU die produced by either Intel or AMD runs a complex test suite designed to test every individual single possible logic path through the CPU, done at different clock speeds. This is how binning works -- chips are binned based on the highest speed at which they have no errors, and based on their power consumption during the test.

 

[Doom2pro]

Each transistor is different from the one next to it, even if the pattern is perfect the resulted etch is wavy and rounded, with random variations... Imagine billions of these and you can see how each die is an average of these variations.

I believe the center of the wafer has the best yields, that is where they pour the developers and spin them outward.

Check out this link that explains how they get around the problem of the source wavelength limitations --> https://en.wikipedia.org/wiki/Optical_proximity_correction

It's not perfect but it gives you an idea of why there is so much variation between devices and dies as a whole.

 

[LTC8K6]

Carbon wafers? That's new because they are made from silicon.
And as such even on mature fabrication, they lose quite lot of chips per wafer, that's why we have celerons, pentiums or i5s, respectively.
Only i7 and i3 nodes are produced and all the stuff inbetween are just chips that are functional but defective to a degree that won't allow them to be stable at flagship's configuration, so they just downclock it and cut-off unstable parts, such as computational or graphic cores or blocks of cache memory, disable HT and so on and sell it for lower price proportional to value they have left. Variety of these defects is really huge and can be seen by how many SKUs of few CPU types Intel does sell.
Ofc every chip is thoroughly tested for long term functionality and only those which do pass the testing are then sold, rest is discarded.
Nothing is really 100% or intact, you can't make anything completely clean in physical and chemical sense, the wafers are defective and that's why top tier CPUs are so expensive, compared to low end ones.

Carbon copy, not carbon wafer...
Too young to remember carbon paper and carbon copies?

 

[Sheep221]

Carbon copy, not carbon wafer...
Too young to remember carbon paper and carbon copies?

Nah, just I can't wrap this analogy around wafer fabrication, but yes last time I seen one was when I was kid, they had been phased out fast.

 

[Doom2pro]

This is a very good series of videos of design flow, I believe there is binning/defect stuff in there too: https://www.youtube.com/watch?v=sTfGzfyMrVc

 

[TemjinGold]

Lots of good stuff, thanks. Didn't realize they test every single cpu. That's a lot of testing!

 

[Rayniac]

Lots of good stuff, thanks. Didn't realize they test every single cpu. That's a lot of testing!

Well it's easy if you have special machines for that

 

[Sheep221]

Lots of good stuff, thanks. Didn't realize they test every single cpu. That's a lot of testing!

If they would test one piece at a time, then yes, they would still be shipping Pentium IV today. They put them in arrays in size of several tens to hundreds units for simultaneous testing. There are dozens of videos on the web explaining fabrication, assembly and real footage from fabs operating, I guess it is worth to see.