Storage capacitor, DRAM

B. DRAM (Dynamic RAM) stores information in capacitors. This storage method is much more efficient and, therefore, has gained widespread popularity over SRAM. And, as the name dynamic implies, it requires a constant recharge to maintain the information. 

Thin films of BST have been the most widely studied dielectric for ferroelectric DRAMs (FRAMs or FeRAMs). The highest capacitance reported for a BST dielectric is 145fF/ j,m, which was achieved with a 20 nm film of a material having k = 325. Prototype BST capacitor DRAMs were first reported in 1995, but have not been widely used commercially because of the advances in other storage technologies. 

Capacitors are charge storage devices that are essential in many circuit families, including dynamic random access memory, DRAM, and RF chips. For example, in RF chips, capacitors occupy a large fraction (at present about 50 %) of the area of the... 

Before the metal-organic CVD (MOCVD) process of the BST film is discussed, the reasons for using the CVD process, despite its difficulties should be described. The storage node size of the capacitors in the current state-of-the-art DRAMs is about 0.15 x 0.35 x 1.0 pm with minimum spacing of about 0.15 pm between the nodes when the dielectric layer is a SiO/SijN bi-layer or Ta O, and the electrode materials are poly-Si. Here 1 pm is the height of the nodes. Even smaller lateral dimensions are expected when DRAMs use a BST thin film as the capacitor dielectric layer. Even though the BST films have a much smaller t value, which... 

Figure 1.64 Cross-section of a Best -cell DRAM (BuriEd STrap) showing the MOSFET access transistor and the storage trenches (deep trench-capacitor) 0.25 (Xm feature size (256 Mbit DRAM).

The dynamic random access memory (DRAM) device, a two-element circuit, was invented by Dennard in 1967. The DRAM cell contains one MOSFET and one charge-storage capacitor. The MOSFET functions as a switch to charge or discharge the capacitor. Although a DRAM is volatile and consumes relatively high power, it is expected that DRAMs will continue to be the semiconductor memory of choice for nonportable electronic systems in the foreseeable future. ... 

The SrTi03 films have even larger dielectric constants than Ta20s (up to 270 [280]) and are also of interest for use as storage capacitors in DRAM and for silicon VLSI applications. The SrTiOs directly deposited on a Si substrate reveals a much lower effective dielectric constant dne to the formation of a Si02... 

DRAM Gate capacitor/4-T/planar/ stacked/trench cells Wide range of compatibility High-density, high bit rate storage... 

Because ferroelectrics can retain their polarization indefinitely imtil their polarity is switched by the application of an electric field, they can serve as nonvolatile memory elements or ferroelectric RAMs (FRAMs). The storage capacitors in dynamic RAMs (DRAMs) can be replaced by ferroelectrics and eliminate the need to continually refresh the memory. 

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