[jmpe]

Same as regular chips: in a grid on a wafer, then cut and pick&placed.

Don't forget: yield goes up when the die shrinks because the defects are typically small spots. The smaller the die the better you pixelate the defects.

The wafers are only 2 inch diameter to avoid yield loss due to edge effects: at the edge of the wafer you have lowest quality components (optical ring effects)

[tedsanders]

I doubt that they 2-inch wafers to avoid yield loss from edge effects. The ratio of area to perimeter rises as you increase the wafer size. Perhaps the reasons they use 2-inch wafers are for better uniformity, better flexibility, or lower capital investment.

[jmpe]

Probably uniformity & capital. The profit margins on LED aren't what they used to be.

[chrisBob]

Your reasoning for the 2" wafer surprises me because most semiconductor work scales well, and the cost to buy and process larger wafers is only slightly more expensive while the surface area is much larger. A 6" wafer is 9 times as big as a 2" one.

[jmpe]

A 6" wafer will also have lots of dies on the edge that don't pass automatic testing. Don't forget that most devices are located near the edge.

Secondly, upgrading your Fab to switch your process to larger wafer size has a 10-figure $ price tag. It's not trivial for digital, let alone opto electronics. LEDs are special in that they deviate severely from the standard mos process. When Monsanto first produced them they almost dumped the idea for LEDs because of all the issues to deal with the exotic materials.

[tzs]

> A 6" wafer will also have lots of dies on the edge that don't pass automatic testing. Don't forget that most devices are located near the edge.

I don't understand this. Given a fixed size die, the number of dies near the edge would go up linearly as the wafer size goes up. The number of interior dies would go up quadratically.

[tedsanders]

Yeah, what he/she is saying doesn't really make sense. Perhaps he/she means that there are problems with uniformity and the dies on the edge suffer most, not because they are next to an edge, but just because they are further from the center. In that case 2-inch wafers may make more sense than 6-inch wafers.