How is wafer dicing economical (for small ICs)?

Question

Is my understanding that, for a given technology, the cost of making a silicon wafer is pretty much fixed, in the sense that it will not change no matter how much you fill the space (how many ICs you can squeeze in a single wafer.)
Once the wafer is finished, it will come the time to dice the wafer to separate all the ICs. My understanding here is that a saw comes in and cuts between the dies separating them.

This saw will have a finite thickness, even though I wasn't able to find a figure on the internet. For the sake of the argument let's say that a reasonable thickness is 0.5mm.
There are some very small ICs out there. Once again, I wasn't able to find a precise figure, but let's say that my IC is 1x1mm.
Let's now take a look at one cell.

It appears that, for every square millimeter of useful product, we are wasting 1.25 square millimeters by sawing them off, getting a yeld even smaller than 50%.
This won't be an issue for big dies as the percentual drop in yield would be far less, but how did the industry got arround this problem for small die ICs? Sorry if the numbers aren't accurate, this is just an example, I don't know how realistic it is.

Martin Stiko · Answer

There is also Stealth laser dicing, which has "zero kerf". The laser creates a tiny stress fracture inside the silicon, and the laser focal point is passed along the dicing channel multiple times at different heights within the silicon. Then the wafer is stretched, and the stress-fractured planes break. No silicon material is lost.
The kerf is not actually zero - the dicing street must be as wide as the accuracy of the laser positioning system, which is about +/- 5um (so the street is 10um wide).

GNA · Answer

In general: Yes. You're losing area through the dicing street as the way the saw runs through is called.
However, your assumption of the thickness is wrong. The saw is more like a thin foil. Usually around 20 micrometers thick (factor 25 thinner than you assumed) and I've seen very specialized ones that were even thinner around < 8 micrometers. As a comparision: Typical bond wire pads (where the wires are connected to the Chip) are around 30-50 micrometers big. So your saw is thinner than the outer pad ring of the chip.
If you have a dicing saw in your hand it's kind of wobbely and does not very much look like a "saw". It is only able to cut the wafer because it is spinning at very very high speeds, which  stabilizes the blade. The saws also have a very limited lifetime becuase of their small thickness. Usually they can only cut a few thousand meters before they need replacement.

How is wafer dicing economical (for small ICs)?

2 Answers

Add your own answers!

Ask a Question