Someone emailed me an Intel-related question - why are silicon wafers, used in chip production, round instead of squared or rectangular? The reasoning being, since the die pieces that are being cut out of the wafer are square, isn’t there waste at the edges? Couldn’t this be avoided with squared/rectangular wafers?
At one point in my Intel career, I knew the answer to this question. It was talked about during a fab tour soon after I was hired. But I’m a carpet-dweller, and far removed from the manufacturing process, so I’ve forgotten.
I’m sure this is a brain dead simple question for anyone who works in the fabs. So let’s have it? Why ARE wafers round?
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Seriously, though, sorry if I caused you any heartburn…
See http://www.ee.byu.edu/cleanroom/EW_formation.phtml for details on how silicon wafers are made. They start with a seed that is rotated along with the quartz heater; the rotation naturally produces a round shape — going with a rectangle would require cutting off more of the crystalline structure.
Round wafers, in addition to the process by which they are made, provide the most usable chips per area of wafer cross-section. Sure you lose some around the edges, but you would lose more with a rectangular or other shape. It’s pretty elementary.
Thank you Ben, this was the answer where I was looking for…
And thanks to Josh to post my question on your blog…
>Paul
Actually they aren’t completely round. They have a bit cut off on one side so that they can be aligned with the crystals in the proper direction. Look close and you can see it.
Apart from the already mentioned fact that they are round from the way they are produced, there are also practical problems of making some of the process steps on a square wafer. Even/homogenoues growth of films etc is a major problem as heat is not evenly spread on a wafer (cooler at the edges) and that there might be differences in strain. These problems can partly be dealt with, but using a square wafer would make an even distribution of temperature and stress across the wafer impossible.
A key process step that uses the round form of the wafer is photoresist application - this is done by spinning the wafer rapidly and dripping the liquid resist at the center; centrifugal force takes care of spreading it evenly.
Great question, thanks for posting it to your blog for answers!
However, I think a few commenters have missed the point. Espicially Gerry who says “It’s pretty elementary”.
The only thing that is elementary, Garry, is your way of thinking. You and Ben appear to assume that to get a rectangle shape that one would cut a square out of the cirlce that is produced (Ben says: “going with a rectangle would require cutting off more of the crystalline structure”). Now THAT is elementary.
The question that really should be asked is “why is it inefficient to produce a square waffer rather than a circular waffer?”
The only reasons given above are:
1. spinning the waffer (rectangles spin, too)
2. even heating (okay, good point but I am sure a solution could be found even though @Pete calls it “impossible”)
So before we call something elementary, lets try thinking outside of the box.
…and Josh, no disrespect meant to you, the tone comments just frustate me. I admire the fact that instead of trying to answer yourself, you put it out there for your audience.
Jeff, as I understand it, here’s how they’re made, and why they’re round. I could be way off on this.
Imagine you start with an infinitely small thread of silicon. Like the wick of a candle. The process for creating the larger “column” of silicon involves rotating that “thread” while silicon is added to it, sort of like the process of dipping a candle. That shape is naturally round, and the wafers are then cut from that solid “column” of silicon.
You could make the “column” squared, or the wafers square, by cutting off the rounded parts, but why would you want to waste perfectly usable surface area?
My brother in law actually works for one of the companies that supplies Intel with it’s silicon wafers. I should call him up and run all this by him, to see if I’m full of crap or not. I hope I’m kind of sort of close!
Okay, we are getting closer! The thread/wick example is a good one and makes better sense. I would also love to hear what your BIL has to say….the devil is in the details as the saying goes!
I just like to ask why (like an annoying 3 year old kid). I am never satisfied with the status quo. One day, maybe 100 years from now (if we are still using silicon) we’ll find a way to make square waffers efficiently and I’ll be happy
Thanks for the discussion, it’s always good over here on TinyScreenfuls.