(a) Schematic illustration of an ordinary flash memory cell. A floating gate surrounded by an insulating layer is located on the MOSFET. Electrons are injected/tunnel through the thin oxide layer and are stored in the floating gate. (b) Schematic diagram of the MEMS-based non-volatile memory device. The CNT is used for the source/drain channel and a MEM cantilever is added to transfer charges to the floating gate. (c) Field-emission scanning electron microscope image of the device. Colour-coded to show the main elements: source/drain (red), floating gate (yellow) and cantilever/actuating electrode (blue). Scale bar, 2 μm. (d) Diagrams demonstrating the programming (upper three panels)/erasing (lower three panels) processes. Blue/red colours indicate the polarities± of the applied voltages.
Several new generation memory devices have been developed to overcome the low performance of conventional silicon-based flash memory. In this study, we demonstrate a novel non-volatile memory design based on the electromechanical motion of a cantilever to provide fast charging and discharging of a floating-gate electrode. The operation is demonstrated by using an electromechanical metal cantilever to charge a floating gate that controls the charge transport through a carbon nanotube field-effect transistor. The set and reset currents are unchanged after more than 11 h constant operation. Over 500 repeated programming and erasing cycles were demonstrated under atmospheric conditions at room temperature without degradation. Multinary bit programming can be achieved by varying the voltage on the cantilever. The operation speed of the device is faster than a conventional flash memory and the power consumption is lower than other memory devices.
We have developed a memory device consisting of a CNT-FET combined with a metallic MEM cantilever with the following advantages. (1) Endurance rating: for the programming and erasing process in an ordinary flash cell, the thin oxide layer may be damaged by the high electric field needed to transfer electrons, which limits the endurance rating. However, in the device presented here, oxide durability is not an issue as the charges are transferred directly to the floating gate via the metallic cantilever. The endurance rating is determined by the number of switching cycles of the MEM cantilever, and it is known that RF MEMS switches can typically endure approximately three orders of magnitude more cycles than flash memories. (2) Multinary bit programming: the memory device shows well-controlled multi-level programmed states by applying different voltages on the cantilever, thus greatly increasing the density of data storage. (3) Energy consumption: the CNT-FET has low energy consumption, which is comparable to that of conventional MOSFETs and it shows good electrical properties such as high mobility and on/off ratio. Moreover, the MEMS switch is electrostatically activated and does not consume any current. Therefore, near-zero power is required for each switching operation. The detailed comparison of energy consumption for our device and conventional flash memories is described in Supplementary Discussion. As the presented memory device combines the advantages of both CNT-FET and the MEMS switch, the power consumption is much less than other memories. (4) Operation speed: the speed of the memory device is only limited by the speed of the cantilever switch, so that it is much faster than a flash memory. By replacing the MEMS switch with a NEMS switch, the speed may be further increased by at least an order of magnitude
The cantilever is anchored at one end and suspended above the actuating electrode and the floating gate. The beam appears to measure about 1-micron across and be more than 10 microns long. When a bias voltage is applied to the actuating electrode, the resultant electrostatic force pulls down the cantilever until it contacts the floating gate. The cantilever is composed of Cr/Al/Cr triple layer and the floating gate is made of Au above an 80-nm aluminum-oxide insulating layer
The charge on the floating gate controls the source-drain current in the p-type CNT semiconducting channel. The on-off switching ratio was 10^4 to 10^5. Data retention is good to 4,000 seconds and cycling endurance of 500 cycles was demonstrated, but the known switching endurance of MEMS switches is of the order of 100 million cycles. This is greatly superior to flash memory.
The memory has the capability for multiple bit storage and the operational speed of the memory device is only limited by the speed of the cantiliver switch which is also much faster than flash memory, the authors claimed.
The one aspect of the design that is not discussed extensively is size, density and scalability. Although the CNT transistor could in theory scale well the cantilever MEMS may not.
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