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Posted on May 6th, 2010 by Barry Collins

Why are processor wafers round?

Otellini wafer

One question has bugged me for a long while: why are processor wafers round? After all, it doesn’t really make a lot of sense when those wafers are sliced and diced into square processor cores. As you can see from the photo above, it means the cores around the outer edges of the wafer are incomplete, creating waste.

Finally, at Intel’s European Research showcase this week, I got the answer.

Processor wafers are made out of silicon, or more precisely melted sand, which according to Intel has a “high percentages of silicon in the form of silicon dioxide”. The sand is melted in a huge vat and once it reaches the necessary temperature, a seed crystal is dropped into the melt and crystal growth begins around the seed. As the growth continues the seed is slowly rotated, gradually forming a solid, round ingot.

Sand to ingot

Each ingot weighs about 100kg and has a “silicon purity of 99.9999999%”, according to Intel. Those enormous silicon ingots are then sliced into individual wafers, each only 1mm thick.

Ingot to wafer

And that, in a nutshell, is why silicon wafers are circular. “Nature wants to build them round,” said Intel fellow, Jose Maiz. Although I still prefer the explanation offered by one of my colleagues. “They just want pretend they’re rock stars with gold discs”.

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17 Responses to “ Why are processor wafers round? ”

  1. John Gray Says:
    May 6th, 2010 at 10:28 am

    Silicon wafers are circular [because] “Nature wants to build them round”.

    And not because the vat from which the crystal is pulled is most easily made with a circular cross-section?

  2. Philip Says:
    May 6th, 2010 at 10:54 am

    I would imagine part of the reason why they are round is because if you had a square cross-section this would lead to corners which may cool at a different rate and cause cracking?

  3. Tomm174 Says:
    May 6th, 2010 at 11:44 am

    “Processor wafers are made out of silicon, or more precisely melted sand”

    What were you thinking.
    Sand (impure SiO2) is not Silicon (unbelievably pure Si)

  4. Tomm174 Says:
    May 6th, 2010 at 11:46 am

    As to why it is round. The ingot is rotated as it is pulled

  5. Tom Says:
    May 6th, 2010 at 11:48 am

    As to why it is round. The ingot is rotated as it is pulled

  6. Michael Says:
    May 6th, 2010 at 1:22 pm

    During the photolithography process the corners of a square wafer do not get exposed very well and there is fallout anyway

  7. Jamie Says:
    May 6th, 2010 at 2:33 pm

    Where do they get the sand from?

  8. Alexander Davidson Says:
    May 6th, 2010 at 5:15 pm

    “Where do they get the sand from?”

    The author’s brain. Incidentally, Czochralski, the inventor of the seeded crystal growth process [rotation is required, hence the circular cross section] was killed by Palestinian terrorists in the Lod airport massacre.

  9. NoHeWasnt Says:
    May 6th, 2010 at 7:25 pm

    Czochralski died in 1953. The Lod airport massacre happened in 1972. Not sure what you were trying to do there with that wild statement.

  10. Dave Ackroyd Says:
    May 7th, 2010 at 12:37 am

    I thought they were round because of Health and Safety – if they were square people might hurt themselves on the corners

  11. Alperian Says:
    May 7th, 2010 at 5:35 pm

    I saw a ‘Texas Instruments’ ingot being pulled out of the bath on ‘How it’s Made’ the other day. The pure ‘log’ of silicon is an awesomely beautiful object.

  12. Rich Says:
    May 7th, 2010 at 8:42 pm

    they been nicking the sand from britains beaches, hence the deminishing british coast

  13. Alpine White Teeth Whitening Says:
    May 9th, 2010 at 10:52 am

    I would guess it is because the crystal-growing process naturally produces a sausage-shaped (but much larger, nowadays) crystal, which is then sliced into wafers. Another factor might be that the processing chambers work better when they are round so there are no corners for the circulating gasses, etc.

  14. siliconSlave2000 Says:
    May 17th, 2010 at 11:34 am

    I have been growing them round for almost 30 years, the colder and faster you grow them the more out of round they become which leads to waste because the surface needs to be ground down afterwards to nice finished cylinder

  15. siliconSlave2000 Says:
    May 17th, 2010 at 11:43 am

    in a sense they are created from sand, the sand is cooked in reactors with different gases where the end product is a very pure product called “poly silicon”
    It then gets melted down inside a crystal growing furnace, not in a vat, in a quartz crucible (bowl)

  16. will Says:
    August 14th, 2011 at 2:52 pm

    why not use rotating square vat. and slightly bigger litographic printer.

  17. andrew Says:
    January 31st, 2012 at 1:37 pm

    turning the ingot into wafers is a multistage process, and the wafers are often spun rapidly to etch/clean them. Round wafers go better at 2000rpm + than square wafers


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