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expense of retrofitting. there is a standardized 'FOUP' that wafers are stored in and making a vacuum capable FOUP and redesigning the systems to support it for

by avs733 8y ago

expense of retrofitting. there is a standardized 'FOUP' that wafers are stored in and making a vacuum capable FOUP and redesigning the systems to support it for just a couple areas of the FAB would be hard and expensive Edit to add a few more details: A vacuum capable FOUP would be much MUCH heavier, requiring retorfitting of the overhead vehicles that transport wafers aroudn the FAB. further, each FOUP carries 25 wafers. A FAB can easily put out 30k wafers a week...it's a lot of FOUPS. even if you just used them at critical steps it would get heavy and expensive. I believe there was some talk abotu inert gas purges with the move to 450mm wafers but, like 450mm wafers, that never happened.

mirimir 8y ago
OK. But maybe plastic bags and tanks of helium?
- froindt 8y ago
  I spent some time working in a fab. Nothing is cheap, competition is tough, yield is everything. 99.8% yield on each step of a 500 step process is good for 38% overall recovery. These events have been shown to be very rare, and the costs would greatly outweigh the benefits (for the time being). The cheapest piece of equipment was the stand the FOUPS come down and rest on. From memory, that was easily 5 figures, perhaps low 6. Every other piece of processing equipment was insanely more expensive. Queuing theory is an active field of research with great applications in how wafer processing. The total process has to be looked at. Wafer priority, wafer value, planning preventative maintenance on the machines, etc. could all be taken into consideration when scheduling when wafers will run. Retrofitting every "robot" that grabs FOUPS for a much heavier load, upgrading the tracks if needed, replacing every single FOUP, adjusting factory flow and inventory holding areas, doing qualifying trials on the FOUPS (to make sure they're functioning correctly and not leaking), etc. are considerable expense. Prioritization of wafers and timing the preceding step so the required machine is certain to be ready in the window would yield better results from a strict business ROI perspective.
  - toast_coder 8y ago
    >99.8% yield on each step of a 500 step process is good for 38% overall recovery. What does that mean? Is 'yield' not the end usable product from the batch?
    - mirimir 8y ago
      Well, Calc tells me that 0.998^500 is 0.3675 :) It's a familiar concept from organic chemistry. Edit: If yield per step averages 99.8%, and there are 500 steps, overall yield is 36.8%.
      - oh_sigh 8y ago
        'yield' is a really impossible measure for this kind of process, because it presupposes full end to end knowledge of fabrication processes.
        froindt 8y ago
        Not sure I follow. The 99.8% is just an example number. If one machine messed up really bad and had just 50% yield, all other machines would need to do far better in terms of overall quality to achieve 36% yield. Of course every single machine isn't at the same quality level. At the same time, when wafers get scrapped, they do justify why they were scrapped. There are metrology processes and tests performed as you go to ensure you don't run ruined wafers through additional manufacturing steps.
    - froindt 8y ago
      Yes it is. The 99.8% hypothetical number is on a per operation basis. Depending on the layers and complexity of the chip, the ballpark number of operations might be 300-1500 (lots of hand waiving here). The gist of that part is meant to show how important and impressive the entire process is. In "normal" manufacturing, 99.8% good parts is a pretty darn good process. Many of the easy wins are already implemented. Most normal things are manufactured in far fewer steps, so even if you're only making 95% good parts, it doesn't absolutely kill your total recovery (start to finish).

by avs733 8y ago

mirimir 8y ago

OK. But maybe plastic bags and tanks of helium?

froindt 8y ago
I spent some time working in a fab. Nothing is cheap, competition is tough, yield is everything. 99.8% yield on each step of a 500 step process is good for 38% overall recovery. These events have been shown to be very rare, and the costs would greatly outweigh the benefits (for the time being). The cheapest piece of equipment was the stand the FOUPS come down and rest on. From memory, that was easily 5 figures, perhaps low 6. Every other piece of processing equipment was insanely more expensive. Queuing theory is an active field of research with great applications in how wafer processing. The total process has to be looked at. Wafer priority, wafer value, planning preventative maintenance on the machines, etc. could all be taken into consideration when scheduling when wafers will run. Retrofitting every "robot" that grabs FOUPS for a much heavier load, upgrading the tracks if needed, replacing every single FOUP, adjusting factory flow and inventory holding areas, doing qualifying trials on the FOUPS (to make sure they're functioning correctly and not leaking), etc. are considerable expense. Prioritization of wafers and timing the preceding step so the required machine is certain to be ready in the window would yield better results from a strict business ROI perspective.
- toast_coder 8y ago
  >99.8% yield on each step of a 500 step process is good for 38% overall recovery. What does that mean? Is 'yield' not the end usable product from the batch?
  - mirimir 8y ago
    Well, Calc tells me that 0.998^500 is 0.3675 :) It's a familiar concept from organic chemistry. Edit: If yield per step averages 99.8%, and there are 500 steps, overall yield is 36.8%.
    - oh_sigh 8y ago
      'yield' is a really impossible measure for this kind of process, because it presupposes full end to end knowledge of fabrication processes.
      - froindt 8y ago
        Not sure I follow. The 99.8% is just an example number. If one machine messed up really bad and had just 50% yield, all other machines would need to do far better in terms of overall quality to achieve 36% yield. Of course every single machine isn't at the same quality level. At the same time, when wafers get scrapped, they do justify why they were scrapped. There are metrology processes and tests performed as you go to ensure you don't run ruined wafers through additional manufacturing steps.
  - froindt 8y ago
    Yes it is. The 99.8% hypothetical number is on a per operation basis. Depending on the layers and complexity of the chip, the ballpark number of operations might be 300-1500 (lots of hand waiving here). The gist of that part is meant to show how important and impressive the entire process is. In "normal" manufacturing, 99.8% good parts is a pretty darn good process. Many of the easy wins are already implemented. Most normal things are manufactured in far fewer steps, so even if you're only making 95% good parts, it doesn't absolutely kill your total recovery (start to finish).