Investigation Aspect just one of my attempt to fully grasp the transition from 2D to 3D NAND begun out by striving to fully grasp how 2D NAND is created, so that its progress in direction of 3D NAND can be comprehended.
To recap just a minor, a important part of 2D or planar NAND is that it is created up of bitline strings – particular person cells related in collection and laid out in rows throughout a flash die, with phrase traces functioning at ideal angles to the bit traces but not touching them.
We are going to use Toshiba’s BiCS (Little bit Charge Scalable) 3D NAND to illustrate what comes about.
The very first 3D NAND notion to grasp is that planar NAND cells are flipped vertically, so that earlier horizontal cells turn into perpendicular ones.
Toshiba BiCS notion with upright pillars of NAND cells. The purple traces are the bitlines. The inexperienced layers characterize phrase line sheets with pick gate sheets at the best and base
Secondly, in Toshiba’s BiCS plan the strings of cells are lengthened in the center, to variety a room there, and then shaped into a U shape with the room at the base and the two sides standing upright.
Toshiba’s bitline string folding notion (Click image to enlarge)
This is a conceptual diagram it’s not how the stuff is created.
Let us use an additional diagram to relate this bit line string folding to Toshiba’s BiCS notion earlier mentioned.
Toshiba BiCS diagram demonstrating folded bitline strings
What comes about to the phrase traces and bitlines?
In 2D NAND, the phrase traces are horizontal strips of polysilicon. In 3D NAND they are horizontal sheets of the stuff. How are the alternating layers created in a semiconductor system?
Jim Handy writes:
Then an array of circular holes will have to be slice – by etching – into this flash chemical ingredient layer cake down to the substrate, and loaded with alternating components to construct the flash cell construction.
Handy tells us, “The holes in 3D NAND only go by way of the layers that are only about 2.5-3µm or 1/15th-1/20th the peak of a TSV. These holes are made use of to construct the NAND strings.”
We can visualise this with yet an additional diagram:
6-phase system from depositing layers to building loaded holes (Click image to enlarge)
Move 2 is the etching of a gap. Step 3 is coating the inside of of the gap with the dielectric oxide to variety an insulating layer amongst the command gate and the floating gate.
Move 4 results in the floating gate(s) by depositing a Silicon Nitride demand entice lining on to the now narrower hole’s walls. Oxide is made use of to line that in step 5 and so insulate it from the Polysilicon filling shaped in step 6.
An unlabelled SanDisk image illustrates the finish end result:
The far more layers there are, the far more specific and correct the gap etching has to be the part ratio requirements to be managed to an amazing diploma. Also the far more challenging it is to get a uniform deposition layer on the inside of of these etched holes.
Up coming the “up” bitline string so constructed requirements to have its command gates separated from the equivalent “down” string’s command gates. Extensive slits are slice into the length of the array amongst adjacent related columns to accomplish this.
Shrinking 3D NAND’s cell geometry is diverse from shrinking planar NAND cell size. Handy suggests: “Shrinking the cell size entails thinning the layers. The issue … is that shrinking the diameter of a gap would worsen the part ratio. This is taboo.”
We need to have to fully grasp that there are diverse kinds of holes in 3D NAND chips.
Vias are holes that operate partly by way of a silicon die.
Jim Handy tells us, “TSVs do go all the way by way of the die/wafer. On the other hand, the vertical connections that are created in practically all chips are basically named ‘vias.’ These vias usually penetrate a single thickness of silicon dioxide, a little something like 10nm. Sometimes they will go by way of a selection of these layers, maybe 5. Continue to, that is 1/1,000th the 50µm depth of a TSV.”
Vias need to have to have their depth managed precisely so that they end at the ideal layer in the ideal substance.
Micron and Intel and Samsung and SK Hynix have their personal 3D NAND producing processes that will vary from Toshiba’s in detail. But we can use the being familiar with of the Toshiba system to get an perception into the fiendishly challenging and complicated producing system essential to make 3D NAND chips. ®
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