Floating Body RAM Becomes an Industrial Reality

Toshiba has successfully developed a high-performance, high-density, low-cost 128Mb FBRAM.

FBRAM is Random Access Memory (RAM) with a Floating Body Cell (FBC). It is a capacitor-less DRAM cell consisting of a MOSFET on an SOI wafer. Data “1” and Data “0” are distinguished by the hole density in the floating body of the MOSFET.

The conventional DRAM cell consists of a capacitor and a transistor, whereas an FBC, consisting of only a transistor, provides three kinds of advantages:

• Scalability: there is no need for 3-dimensional capacitor structures (like a stack and a trench capacitor, which are both approaching the “red brick wall”).

• High performance: there is no parasitic resistance (as with a poly-Si plug in a stack capacitor).

• Low cost: unlike a stack or trench capacitor, there are no additional processes. FBC can be fabricated alongside conventional logic devices.

Fabrication on UT-BOX SOI

To verify the FBC technology, a 128Mb FBRAM has been designed and fabricated based on 90nm-SOI technology. Figure 1 shows a cross-sectional picture of the memory cell-array along the Bit Line (BL) direction. The thicknesses of silicon and buried oxide are 55nm and 25nm, respectively.

An ultra thin BOX has been used in order to stabilize the body potential, which has led to enhancement of the storage signal. The gate electrode of a MOSFET is used as a Word Line (WL). Two layers of copper wiring are used. The 1st Cu wiring is used as a Source Line (SL), which is connected to the source of MOSFET. The 2nd Cu wiring is used as a Bit Line (BL), which is connected to the drain of MOSFET.

The features of the fabricated 128Mb FBRAM device shown in Figure 2 are as follows:

• Design Rule : 90nm-node technology.
• Power supply : 3.3V.
• Chip size : 64.6 mm2 (7.6 mm × 8.5 mm).
• Random Access : 18.5 nsec.
• Random Cycle : 25 nsec (read), 32 nsec (write).
• Cell size : 0.17 µm2 (0.33 µm × 0.515 µm).
• Wiring : Cu 6 layers.

A 128Mb FBRAM with FBC has been successfully fabricated. We have already obtained good die, good yield, and good retention characteristics. FBC is a promising embedded memory structure for system-on-a-chip (SoC) on SOI.

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