Sandisk 3D LLC (Milpitas, CA) earned U.S. Patent 7,667,999 for its method to form a rewriteable nonvolatile memory comprised of a steering element in series with a carbon nanotube fabric. The steering element is preferably a diode, but may also be a transistor. The carbon nanotube fabric reversibly changes resistivity when subjected to an appropriate electrical pulse. The different resistivity states of the carbon nanotube fabric can be sensed, and can correspond to distinct data states of the memory cell. A first memory level of such memory cells can be monolithically formed above a substrate, a second memory level monolithically formed above the first, and so on, forming a highly dense monolithic three dimensional memory array of stacked memory levels.
Carbon nanotube memories are believed to operate by flexing of individual carbon nanotubes or carbon nanotube ribbons in an electric field. This flexing mechanism requires space within which the carbon nanotubes can flex. In nanotechnologies, forming and maintaining such an empty space is extremely difficult.
The Sandisk 3D method is used to form a memory array in which memory cells are comprised of carbon nanotube fabric and a steering element, such as a diode or a transistor, arranged electrically in series.
Sandisk 3D inventors S. Brad Herner and Roy E. Scheuerlein also developed a method for programming a carbon nanotube memory cell, in which the memory cell includes a first conductor, a steering element, a carbon nanotube fabric, and a second conductor, wherein the steering element and the carbon nanotube fabric are arranged electrically in series between the first conductor and the second conductor. Also the entire carbon nanotube memory cell is formed above a substrate and the carbon nanotube fabric has a first resistivity. The programming method includes: applying a first electrical set pulse between the first conductor and the second conductor, wherein, after application of the first electrical set pulse, the carbon nanotube fabric has a second resistivity, the second resistivity is less than the first resistivity.