4 ms·

>Interesting. I had assumed that the difference between state of the art fabs and older fabs was that state of the art fabs can make smaller things, and perhaps

>Interesting. I had assumed that the difference between state of the art fabs and older fabs was that state of the art fabs can make smaller things, and perhaps more geometrically complicated things, but that they don't have to. If they wanted to make a design that only uses bigger things and simpler geometry they could. >Where does this assumption break down? Why would they want to? There is heavy and overbooked demand for the 'smaller and complicated things' that make more money, and add the extra cost of the design to make the 'bigger and smaller' things, and it's a no brainer.

tzs 5y ago
They wouldn't want to. What I'm curious about is if they can without retooling or purchasing equipment. Say due to chip shortages affecting the US military the government invoked the Defense Production Act to require some company with only state of the art fabs to make an older chip. From what I've read, the way a layer on a chip is made is a coating is deposited on the surface that is sensitive to light at certain wavelengths. They then use light of wavelengths that the coating is sensitive to in order to transfer a pattern from a photomask to the surface, making some change to the coating at those places where the light is not blocked by the photomask. Then they can treat the surface to remove the coating at those places were it was changed by the light. They then can deposit thing onto or remove things from those parts of the surface not still protected by the coating. Then they can use something that can remove the coating. The result is a layer on the chip with things deposited or removed right where they wanted them to, as determined by the pattern on the photomask. Repeat for more layers. The major improvements from generation to generation are the ability to use shorter and shorter wavelengths of light for the photolithography which allows for transferring patterns that have smaller features, and maybe the ability to make thinner layers so you can make chips with more layers. I don't see anything in the photolithography process that requires using small feature size. If for some reason you wanted to build an older chip that only used much larger features, I'd expect that it would just be a matter of using the photomasks for that older chip, and setting the layer thickness to what the old design called for.
- showerst 5y ago
  I think the deal breaker is "without retooling". For one thing there's a lot of steps involved in photomask -> repeat. If your masks are all built for a smaller wafer size, or a different focal length setup or physical process diameter, then they literally won't fit in the die, or won't focus correctly. You'll run into similar issues all up and down the chain. If your doping and prep all assumes a smaller wafer or different geometry, you're out of luck either physically because things don't fit, or because your chemistry needs to be redone. Your various deposition, bake and cure steps will necessarily also assume a process geometry, because some of the processes diffuse out from the center, or at different angles relative to some source that's bombarding them. It's sort of akin to saying "well if we can build a vw bug they can build a model-T". Generations of equipment changes make things incompatible.
  - lief79 5y ago
    As someone with no experience in this area, I think you're ignoring the option of making the equivalent parts on the newer equipment. The design costs would be higher (based on what I read, the new methods are more complex and costly), but if you're legally required to do so ... this would possibly be faster and cheaper then redesigning the factory. Larger wafers, larger yields, etc ... once working they could probably meet the required output quickly(month long delay?), provided the demand / legal punishments are great enough to justify the costs.
- darksofa 5y ago
  Only the most critical layers use the most advanced litho tools. The other layers use n-1 and n-2 litho tools and print larger geometries. You are correct that it is not a problem to print larger geometries. However, there is more to it than just litho. Many thin films, implants, etching, polishing. A product is designed to a specific fabrication process flow. The real showstopper is that the process flow guarantees the electrical parameters of the transistors and passive devices will be with a certain range -- the product circuits are designed based on these parameters' values. A different process flow has all different electrical characteristics and the product won't work without redesign.

tzs 5y ago

They wouldn't want to. What I'm curious about is if they can without retooling or purchasing equipment. Say due to chip shortages affecting the US military the government invoked the Defense Production Act to require some company with only state of the art fabs to make an older chip. From what I've read, the way a layer on a chip is made is a coating is deposited on the surface that is sensitive to light at certain wavelengths. They then use light of wavelengths that the coating is sensitive to in order to transfer a pattern from a photomask to the surface, making some change to the coating at those places where the light is not blocked by the photomask. Then they can treat the surface to remove the coating at those places were it was changed by the light. They then can deposit thing onto or remove things from those parts of the surface not still protected by the coating. Then they can use something that can remove the coating. The result is a layer on the chip with things deposited or removed right where they wanted them to, as determined by the pattern on the photomask. Repeat for more layers. The major improvements from generation to generation are the ability to use shorter and shorter wavelengths of light for the photolithography which allows for transferring patterns that have smaller features, and maybe the ability to make thinner layers so you can make chips with more layers. I don't see anything in the photolithography process that requires using small feature size. If for some reason you wanted to build an older chip that only used much larger features, I'd expect that it would just be a matter of using the photomasks for that older chip, and setting the layer thickness to what the old design called for.

showerst 5y ago
I think the deal breaker is "without retooling". For one thing there's a lot of steps involved in photomask -> repeat. If your masks are all built for a smaller wafer size, or a different focal length setup or physical process diameter, then they literally won't fit in the die, or won't focus correctly. You'll run into similar issues all up and down the chain. If your doping and prep all assumes a smaller wafer or different geometry, you're out of luck either physically because things don't fit, or because your chemistry needs to be redone. Your various deposition, bake and cure steps will necessarily also assume a process geometry, because some of the processes diffuse out from the center, or at different angles relative to some source that's bombarding them. It's sort of akin to saying "well if we can build a vw bug they can build a model-T". Generations of equipment changes make things incompatible.
- lief79 5y ago
  As someone with no experience in this area, I think you're ignoring the option of making the equivalent parts on the newer equipment. The design costs would be higher (based on what I read, the new methods are more complex and costly), but if you're legally required to do so ... this would possibly be faster and cheaper then redesigning the factory. Larger wafers, larger yields, etc ... once working they could probably meet the required output quickly(month long delay?), provided the demand / legal punishments are great enough to justify the costs.
darksofa 5y ago
Only the most critical layers use the most advanced litho tools. The other layers use n-1 and n-2 litho tools and print larger geometries. You are correct that it is not a problem to print larger geometries. However, there is more to it than just litho. Many thin films, implants, etching, polishing. A product is designed to a specific fabrication process flow. The real showstopper is that the process flow guarantees the electrical parameters of the transistors and passive devices will be with a certain range -- the product circuits are designed based on these parameters' values. A different process flow has all different electrical characteristics and the product won't work without redesign.