The first thing we did was measure the die size. This chip is 35 sq mm, compared to the previous generation 48-nm Samsung 1Gb DDR3 SDRAM, which is 28.6 sq mm. Clearly this 2 Gb die is much smaller than 2X the 48-nm 1 Gb die, so our assumption that we have a 3x nm part looks good so far.
|Die Photo of Samsung 3x DDR3 SDRAM|
Next we did a bevel-section of the part to take a look at the cell array. We were surprised with what we found. The capacitors are laid out in a square array instead of the more usual hexagonal pattern (see below), and the wordline (WL) and bitline (BL) pitches are both about 96 nm. The usual method of determining DRAM node is to take half the minimum WL or BL pitch. That places this DRAM at the 48-nm process node, the same as the previous Samsung generation of 48 nm. So why does the die size look like it should be a smaller technology? For this we need to look at cell size.
|Plan-View TEM image of Capacitors in Samsung 3x-nm SDRAM|
But before we get into that we should discuss the DRAM convention of describing the memory cell size in terms of the minimum feature size, F. Historically, DRAM cells have used an 8F2 architecture for many years. This allows for the use of a folded bitline architecture, which helps reduce noise. In order to decrease cell area, companies came out with the first 6F2 cells in 2007; this 6F2 architecture is now used by all major players in the DRAM market. The guys at ICInsights published the plot below in the latest McLean report which nicely illustrates the progress:
|DRAM Cell Size Reduction Through the Years|
ï»¿ï»¿We compared the poly and active layout under the array between the 48 nm SDRAM and this new one. The images are shown below. As can be seen, both have very similar layouts. The angle of the active silicon (diffusion) direction is about the same. The active areas are ovals. Each diffusion has two wordlines crossing it. There is a gap between all the active areas, such that a third WL does not cross active on this diagonal active direction.
|Samsung K4B1G0846F 48nm 1 Gb DDR3 SDRAM,
Poly and Active Area Image under Cell Array
|Samsung K4B2G0846D 2Gb DDR3 SDRAM,
Poly Remnants and Active Areas under Cell Array
So, do we have a 32 nm node, and a 6F2 architecture? Maybe. The only issue is that if we use 32 nm as F, then when we plug that into the 6F2 equation we get 0.0061 um2 as the cell size. However, the cell size is actually 0.0092 um2. If we use that number and use the equation to calculate F we find that F=39nm. Soâ??¦ do we call this a 32 nm or a 39 nm node? It depends how you calculate it – either way itâ€™s a 3x!
So, although it’s a little disappointing that I don’t think we can announce the worlds first 4F2 DRAM, we can announce the worlds smallest node, 32 or 39 nm, production 6F2 DRAM.