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Exclusion Zone on A Sapphire Wafer

What’s called the exclusion zone on a wafer?During epitaxy, LED material is not properly formed in this area, meaning these chips shouldn’t be counted because they will not result in good LEDs. For our LED chip calculator, we are using an industry standard 3-mm exclusion zone, which is shown as red chips in Fig. 2. Note that the chips on the extreme edge of the wafer – that are actually hanging off the wafer if they were full rectangles – are not going to be counted at all for our simulation.

One important characteristic of the exclusion zone is that it is 3-mm from the edge regardless of wafer diameter. This fact means that the large diameter wafers have larger exclusion zone areas. However, as a percentage of the total wafer surface area, the large wafers have a smaller proportion of their area in exclusion zones.
So you can see how a 6-in wafer that has 9× more gross surface area actually has more than 9× more net area (gross area minus exclusion zone). The advantage results in 6-in wafers having 10.3× more net area, and 8-in wafers having 18.8× more, both as compared to a 2-in wafer.

We also have to account for the rectangular footprint of LEDs. They don’t perfectly fit in the round shape of the wafer – some LEDs will be lost by partially crossing into the exclusion zone. In a similar way to the exclusion zone, these losses are a higher percentage of the total for the smaller wafers. The final advantage is shown in the chart in Fig. 3 that is based on 45x45-mil (thousandths of an inch) LEDs, including the spaces between chips. The result is slightly higher gains in chip count compared with area – 10.9× for the 6-in and 19.8× for the 8-in wafers.

At this point, we’ve seen that a 6-in-diameter wafer actually holds slightly more than the often-quoted 9× more LED chips compared to a 2-in wafer. But now we have to consider that LEDs are grown in groups of wafers in an MOCVD reactor.

The LED epitaxy process is one of the most expensive and time consuming of all the steps that go into the final delivery of an SSL product. The input is a group of wafers, and the output is thousands of LEDs on those wafers. What we are seeking to answer is how switching to large diameter will change that LED count after the epitaxy process. Of course, yield – a measure of chips that function correctly – matters too, but we will look at that later.

We’ve already said that you shouldn’t expect the chip count you get after epitaxy to jump by a factor of nine, and now we’ll see why. The primary reason is the fact that so many more small-diameter wafers can fit in the reactor chamber. In a typical MOCVD reactor configuration, 56 2-in wafers can be loaded. In the same reactor only eight 6-in wafers will fit. That’s a ratio of 7:1 in favor of small diameter.

So to simply break even in the final count, each 6-in wafer would need to hold 7× more LED chips than a single 2-in wafer. However, we’ve already seen that a 6-in wafer has almost 11× more LED chips. Put in other terms, the 6-in configuration results in 55% more LED chips (1.55×). This is the final true advantage we’ve been looking for. While this is much less than the 9× (900%) figure that we started with, it is still a very significant improvement in the number of LED chips you get for the same cost of time and money for an MOCVD run. You can compare a typical MOCVD layout for small and large diameter wafers and their respective chip counts.


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