200mm wafers were small enough for people to carry around and material handling robots were less advanced, so it made sense to make the entire fab into laminar-flow cleanroom and have people carry wafers around exposed to the air. This is obviously absurdly expensive, given the square footage of HEPA filters and sheer air-moving horsepower needed.
Now, a full lot of 300mm wafers is too heavy for a person to carry around all day, cleanliness standards are higher, and material handling robots are cheap enough to replace humans. The new fabs store all wafers in sealed plastic FOUPs which are robotically delivered to each process tool. Only the inside of the positively-pressurized process tools has to be truly clean. The big squirrel-cage VLF fans have been replaced by an array of axial fans covering the roof that can be individually tweaked and adjusted to optimize airflow and power.
Companies still pretend it's a cleanroom and force people to wear smocks out of habit, but most of them are only held to class 10,000 or so, which is cleaner than your living room but not clean enough to make wafers in, without the FOUPs.
Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/-Ce8sPDte6c/story01.htm
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