Another common definition for technology node is the minimum lithographically implemented feature size. In the early nodes, as seen in the table below, the shallow trench isolation (STI) pitch, BL pitch, and the WL pitch are all similar. Half of the WL pitch represents the minimum feature size. After the 70-nm node, the STI pitch shrink did not keep pace with the WL pitch shrink until the recessed-channel-array-transistor (RCAT) was introduced by Samsung and later by others. RCAT shrunk the STI pitch in the array. When Samsung introduced its 30-nm class SDRAM K4B2G0846D 2Gbit DDR3, it used STI pitch to define process node.
The Hynix 30-nm-class SDRAM followed the same trend. Micron is the only major SDRAM manufacturer that still uses the historic definition of half WL pitch to define the technology node. Micron’s 30-nm class SDRAM MT41K512M8RH-125 has a half WL pitch of 31 nm and half STI pitch of 45 nm. Irrespective of what parameter the manufacturer has taken as the minimum feature size, one parameter is common: every new technology node has a smaller SDRAM cell area than the previous generation.
Table 1: Summary of 6 generations of Hynix SDRAM products.
(*STI pitch is measured perpendicular to the active array and its half pitch is used to define the technology node for Hynix’s 30-nm class SDRAM).
The table above indicates that Hynix 3X SDRAM is different compared to previous generations. It is the only memory device that uses half-STI pitch to define the technology node while utilizing a new cell layout. Hynix was the only manufacturer still using 8F2 layout for sub-70nm nodes. The 8F2 layout has two major advantages:
1.The noise immunity is higher and the process complexity is lower due to larger cell size compared to the 6F2 layout.
2. Conversely the 6F2 scheme provides a significant (25 percent) cell area reduction with the same design rule. Micron was the first company to switch to 6F2 with its 95-nm node SDRAM and Samsung introduced 6F2 layout in its 80-nm node SDRAM. Hynix continued using 8F2 layout until the introduction of this device.