21 ms·
So first, to put a term to what the grand parent is describing, they are generally referred to as 'queue times' referencing the wafers waiting in a queue. Effe
by avs733 8y ago
So first, to put a term to what the grand parent is describing, they are generally referred to as 'queue times' referencing the wafers waiting in a queue.
Effectively it is an atmospheric interaction, most commonly with oxygen but also light, hydrogen sulfide (from agricultural production - land is cheaper in the countryside), and other sources of contamination.
Most wafer processing - especially depositions of materials like metals - are done in a vacuum. if you deposit a metal (lets assume aluminum) in a vacuum you effectively get a pure aluminum layer. Pure aluminum is highly reactive with oxygen
As soon as that layer is exposed air it oxidizes - same thing as rust. You know this as the dull surface finish that raw/bare aluminum gets if left out or when you purchase it. It's what happens very quickly after you machine aluminum - the shiny surface goes away. That is the aluminum oxidizing.
Aluminum oxide has different electrical properties than pure aluminum, different enough to fundamentally affect the function of the transistor devices being constructed. If I have a 2" thick piece of aluminum that layer, maybe only a few microns (assume 5) thick after 24 hours, is basically inconsequential. It represents 0.01% of the thickness of the piece. It is error but not much more in the electrical properties of the piece. However, if semiconductor layer is much thinner... maybe 10microns thick...the effect of the oxidation on the electrical properties is much much higher and pushes it outside the tolerances the device can have and still function.
And because the deposition layers aren't just surfaces, but penetrate to fill trenches in the devices they are basically impossible to remove at most steps.
- mirimir 8y agoSo why don't they hold the queue under vacuum? Or under some inert gas?
- avs733 8y agoexpense 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 agoOK. But maybe plastic bags and tanks of helium?
- froindt 8y agoI 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 agoWell, 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%.
- ori_b 8y agoBecause it costs money and doesn't solve a problem in a functional production line.
- oh_sigh 8y agoPresumably if these kind of multi-day offline issues become the norm, we actually will see these investments being made?