DRAM gets more exotic

If not already available when this article goes to press, it won't be long before Samsung's latest generation of DRAM enters the market. At the VLSI 2007 Symposium, Samsung promised many exciting new materials and structures for 56 nm. Samsung plans to make a number of changes to the basic transistor design--both for the cell access transistor in the memory array and the peripheral support transistors used to design the control logic.

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56-nm devices get a "lift" with raised source drains to improve transistor drive current. Micron was actually the first to produce this type of transistor on its 110-nm node DRAM. These earliest Micron raised source/drains were polysilicon. Samsung intends to create a raised source/drain of selective epitaxial growth (SEG) silicon with the elevated portions of the source/drain following the single-crystal silicon structure of the underlying substrate--another Micron "first," as its 78 nm already uses a SEG structure. That generation of Micron device also contained the famous recessed channel access transistor now used by all manufacturers of stacked capacitor DRAM. Samsung increased the channel length even more by adding a spherical region at the bottom of the transistor trenches. This 68-nm array transistor spawned the S-RCAT acronym, adding the "spherical" prefix.

Plan view TEM of wordline layout in 68nm Samsung DRAM Source: Semiconductor Insights

Samsung will further optimize transistor performance by introducing the dual poly gate process with n+ for NMOS and p+ for PMOS. This promises better effective electrical oxide thickness uniformity and better design margins. This is just one of the techniques, new to high-performance logic devices many nodes ago, finally making their way into commodity memory designs.

To improve both wordline resistance and peripheral FET performance, Samsung will add a titanium-nitride barrier between the polysilicon gate layer and the tungsten cap. This allows more control over the processing of the wordlines and should reduce variability in wordline resistance.