Similarly, Micron also announced in February that it would come to market with NAND flash devices with a memory capacity of 128 Gbit that also use TLC design. It was only last fall that Samsung introduced a 64 Gbit NAND flash using TLC and 21-nm process technology.

Clearly, the industry is moving toward TLC cell design even for demanding SSD applications. The concept of a multi-bit per cell technology was first introduced by Toshiba and for the last five years, all flash device makers have products using the 2-bit per cell design. (Download TechInsights’ flash NAND technology roadmap here).

A multi-bit cell device has a high density and a low cost per bit, but usually has a reduced endurance. In a conventional single bit flash device the number of electrons placed on the floating gate affects the threshold voltage (Vt) of the cell. This effect is used to set the state of the cell to either high or low.

In a multi-bit cell the threshold is set to several different values. The difference in voltage between these levels is small, which puts an additional constraint of placing the charge precisely on the floating gate and of sensing it correctly. In a 2 bit per cell memory, the cell is put in four states. In a 3-bit per cell there are eight states (states = 2n), which imposes a colossal task for flash device manufactures to have a tight cell threshold voltage distribution and a precise sensing of cell data.

According to several papers, the number of electrons stored on a floating gate for the 30-nm node class is slightly less than 100. So, in a 21-nm node with the TLC design, the circuitry for placing and sensing charge on the floating gate is dealing with only a few tens of electrons in each state.

Therefore, Samsung’s 21-nm, 64 Gbit TLC NAND flash can be considered an industry milestone. It is the first commercial SSD product using the TLC design in 21 nm node. TechInsights has done a structural analysis of the 21-nm, 64 Gbit TLC NAND flash and is also following up with a waveform analysis.

The K9CFGY8U5A-CCK0 21-nm flash memory  is one of the industry’s leading flash memory devices, packing 64G bits into a single 102.87 mm 2die, using conventional floating-gate flash memory technology. This NAND flash was found in Samsung’s latest SSD drive, the SSD840 (256GB). The basic teardown of the SSD drive is shown in figure 1.

Samsung’s 21 nm NAND flash device is fabricated using a triple metal, double poly, 21-nm CMOS process. The single transistor flash cell measures approximately 42 nm by 46 nm for a physical cell area of 0.0019 µm2.  Its  competitors  in  process technology include a 19-nm, 32-Gbit  MLC  NAND  flash from Toshiba and   a  20 nm 32 Gbit MLC NAND Flash from Intel-Micron.

Figure 1: Basic teardown, from SSD 840 drive to single NAND flash die.