Thursday, December 31, 2009

Micron Technology Patent Details Nanocrystal Write Once Read Only Memory For Archival Storage- High Work Function Nanoparticles Enable High Density DRAM

Micron Technology, Inc. (Boise, ID) inventor Leonard Forbes discloses details of nanocrystal write once read only memory for archival storage in U.S. Patent 7,639,528.  

Forbes developed structures and methods for write once read only memory employing charge trapping. The write once read only memory cell includes a metal oxide semiconductor field effect transistor (MOSFET) in a substrate. The MOSFET has a first source/drain region, a second source/drain region, and a channel region between the first and the second source/drain regions. A gate insulator is formed opposing the channel region.  

The novel programmed MOSFETs conduct significantly less current than conventional MOSFETs, particularly at low drain voltages. Electrons will remain trapped in the number of high work function nanoparticles, or nanocrystals, within the gate oxide gate unless negative gate voltages are applied. The electrons will not be removed from the number of high work function nanoparticles, or nanocrystals, within a gate oxide when positive or zero gate voltages are applied. Erasure can be accomplished by applying negative gate voltages and/or increasing the temperature with negative gate bias applied to cause the trapped electrons to be re-emitted back into the silicon channel of the MOSFET. 

The gate insulator includes a number of high work function nanoparticles. A gate is formed on the gate insulator. A plug is coupled to the first source/drain region and couples the first source/drain region to an array plate. A transmission line is coupled to the second source/drain region. The MOSFET is a programmed MOSFET having a charge trapped in the number of high work function nanoparticles in the gate insulator adjacent to the first source/drain region.

Applications containing the novel memory cell include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.

Utilization of a modification of well established DRAM technology and arrays will serve to afford an inexpensive memory device. The high density of DRAM array structures will afford the storage of a large volume of digital data or images at a very low cost per bit. There are many applications where the data need only be written once for archival storage. The thicker gate insulators, lower operating voltages and larger work functions of the nanocrystals acting as floating gates will insure long retention and archival storage.

The MOSFETs can be programmed by operation in the reverse direction and utilizing avalanche hot electron injection to trap electrons in a number of high work function nanoparticles, or nanocrystals, within a gate oxide of the MOSFET. When the programmed MOSFET is subsequently operated in the forward direction the electrons, trapped in the number of high work function nanoparticles, or nanocrystals, within the gate oxide, are near the source and cause the channel to have two different threshold voltage regions.

The key high work function nanoparticles include nanocrystals selected from the group of p-type nanocrystals of silicon germanium for gates, p-type nanocrystals gates of other semiconductors as silicon carbide, silicon oxycarbide, gallium nitride (GaN), and aluminum gallium nitride (AlGaN). The nanocrystals are isolated from one another and not in conductive contact. In still other embodiments, the number of high work function nanoparticles include heavily doped p-type polysilicon floating and isolated nanocrystals with a vacuum work function of 5.3 eV. 


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