Why are silicon wafers round, instead of rectangular?

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?


23 thoughts on “Why are silicon wafers round, instead of rectangular?

  1. Gerry says:

    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.

  2. 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.

  3. Peter says:

    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.

  4. 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.

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  7. 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.

  8. 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! 🙂

  9. 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.

  10. Thomas says:

    I’m currently taking a grad course over Silicon fabrication, so according to the textbook…

    THE reason isn’t exactly what was discussed above, actually some answers weren’t exactly correct, but I am not trying to turn this into an insult-fest. The point of this discussion is to help out our fellow curious man. Ego-clashing should be done over a board game or game of basketball. Anyways

    The reason of the circular, cylindrical shape, is a by-product occurrence of the process with which Silicon wafers are grown.

    Industry regularly uses a Silicon Growth Method that is named “The Czochralski Growth Method.” This method grows Silicon crystal by solidifying a Crystal-Liquid into a solid.

    A Silicon molecule or more is attached to a mechanical device, which is then dipped into a pool of liquid Silicon-crystal Melt. To start the growth, the Mechanical Device rotates one direction in the pool of Crystal Liquid.

    The Liquid melt is rotated the OPPOSITE direction of the mechanical device. Little-by-little, the crystal starts to grow from this smaller piece. As the crystal grows, the two rotations keep happening, while the mechanical device is very slowly lifted out of the pool of Crystal Liquid.

    So, that’s why wafers are circular. When they grow Silicon crystal, it starts as a liquid, and rotates its way out as it becomes a solid.

  11. Kein says:

    I suspect the as-grown silicon ingots will have a rather lumpy uneven profile; not quite round. The as-grown ingot is ground and polished into a cylinder. Flats would be ground into one or more sides to identify the crystal orientation of the wafer (100) or (110).

    You could grind the ingots square if you wanted, but the square wafer would yield fewer dies than the original round wafer. Since dies could be fitted into the areas that would be lost during the square grinding. So more expense in grinding and fewer dies.

    Several manufacturing processes used to make IC’s are more easily done with round wafers: photoresists and spin-on-glass (SOG) come to mind. The material is dispensed by a syringe at the center of the wafer, which is spun at fairly high speeds. This spreads the resist or SOG over the surface of the wafer to a uniform thickness.

    A constant thickness of the photoresist is highly desirable, as one exposes the resist to an image of the metal interconnect layer (lithographic step) and you want a sharp exposure through the entire resist, everywhere on the wafer. A non-uniform resist thickness will make it almost impossible to achieve an in-focus lithographic step. This would hurt the die yield.

    You probably could not use a spin-on process for the photoresist with square wafers, as the resist piles up along the outer edges of the wafer. So the corner areas would have a different thickness than the middle. This is a minor problem with round wafers, and would be quite bad for square wafers. But cleverness might come up with a spray technique like car painting for square wafers.

    The answer probably lies in the economics. Which method gives the most good dies for the least cost. For now, the answer is round wafers.

  12. J says:

    The shape is not round because of the spinning, but because of the even distribution of heat around the crucible.

  13. kr says:

    Let me throw something else in the mix, a process called edge defined film fed growth has been able to product single crystal silicon in ribbons of defined thickness and width. The process is ingenious and was developed by Harry Labelle at Mobile/Tyco back in the 70’s. With this process, silicon wafers can be produced efficiently and in rectangular or square shapes.

  14. Mail says:

    This is because when you grow a silicon krystal you can only grow it as a cylinder. Then you cut the wafers off the cylinder (like cheese slices). And you end up with circular wafers.

  15. Rob says:

     The wafers are round because the silicon is “grown” on the silicon crystal pullers in a cylindrical shape.

  16. Angelabrogan3 says:

    The reason is to provide stability to the wafer. It is less prone to breaking with a smooth edge and also a nicely tapered edge will avoid edge beading of photoresist.

  17. Guest says:

    at least if they don’t want to use a square rotating vat to form it, they just can cut it in square just before litographic process..
    so the garbage can be melted again. And then just keep litographic printer 1cm to the edge.. ok better round xD

  18. Jamie says:

    It may also useful to have round wafers because of the tetrahedral bonding configuration of silicon. You can fit more of them on on a circular rather than rectangular wafer of the same surface area

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