Testing Samsung 850 Pro Endurance & Measuring V-NAND Die Sizeby Kristian Vättö on July 7, 2014 1:20 PM EST
V-NAND Die Size: Bigger Than What We Initially Thought
In our 850 Pro review, I did some calculations to estimate the die size of the second generation V-NAND based on the first generation V-NAND. As some of our clever readers pointed out, the die size does not scale perfectly with the die capacity because the peripheral circuitry cannot be scaled as aggressively as the capacity can be. As a result, my initial estimation ended up being way too small because I did not take the scaling of the peripheral circuitry into account.
Unfortunately, Samsung would not disclose the die size during the Q&A at the SSD Global Summit so I had to seek for an alternative way to figure out the correct die size. In the showroom, Samsung had the second generation 32-layer V-NAND wafer, which actually gives us everything we need to calculate a fairly accurate estimation of the die size.
The picture on top shows what the wafer looks like. It does not look any different from the typical 2D NAND wafer, which should not be a surprise given that the change happened deep inside. And no, the V-NAND wafer is not thicker than any normal wafer (at least not by eye) because the V-NAND stack is still in the order of microns.
As for the die size, I calculated 44 dies in the vertical direction, which means that with a 300mm wafer the depth of a single die is approximately 6.8mm.
Courtesy of PC Perspective
To figure out the width of the die, you can either count the dies in the horizontal direction or go with the lazy route and just calculate the proportion of the depth and width. Allyn Malventano from PC Perspective was able to grab this awesome photo of the die itself, so I used the ruler tool in Photoshop to measure the size of the die along both X and Y axes and it appears that the width of the die is about 2.05 times its depth (note that it is a two-plane design). In other words, the die size is around 6.8mm x 14.0mm, which works out to be 95.4mm2.
|NAND Die Comparison|
|Samsung 1st Gen V-NAND||Samsung 2nd Gen V-NAND||Micron 16nm NAND||Toshiba A19nm NAND|
Here is an updated bit density graph. Even with the updated, larger die size, V-NAND is noticeably denser than 2D NAND, although the first generation appears to be more dense. I am guessing the smaller die size is better for yields (larger chips have higher probability of manufacturing errors), which makes the second generation more cost efficient despite the slightly lower density.