Method and apparatus providing high density data storage

A data storage device and methods for storing and reading data are provided. The data storage device includes a data storage medium and second device. The data storage medium has an insulating layer, a first electrode layer over the insulating layer and at least one layer of resistance variable material over the first electrode layer. The second device includes a substrate and at least one conductive point configured to electrically contact the data storage medium.

FIELD OF THE INVENTION

The invention relates to the field of data storage devices, particularly devices formed using a resistance variable material.

BACKGROUND OF THE INVENTION

The maximum data density that can be achieved by magnetic storage media is limited to 60-100 Gb/inch2by the superparamagnetic limit. Alternatives to magnetic storage media are needed to further increase data density.

One such alternative developed by IBM Research is the “millipede” high-density data storage system. The millipede system is based on micromechanical structures taken from atomic force microscopy (AFM). Data is written as depressions in a polymer medium by a thermomechanical AFM probe. The data is also read and erased by the same probe. The millipede system includes an array of probes that operate in a highly parallel manner, so that each individual probe capable reads, writes and erases data in a small area. See, Vettiger et al., “The ‘Millipede’—More than one thousand tips for future AFM data storage,” IBM J. RES. DEVELOP., vol. 44, no. 3, pp. 323-339 (May 2000), which is incorporated herein by reference, for additional details regarding this technology.

This technology, however, has a number of drawbacks. Since the technology uses an indentation in the polymer medium to record data and a thermal conduction sensing scheme to read the data, it requires very good temperature control of the array and polymer medium between read and write cycles. Specifically, the temperature of the probe array chip must be maintained at 350° C. As a result, large energy consumption is expected due to heat loss. Also, the technology requires critical material selection with matching thermal expansion coefficients. Additionally, data bit size is limited to 40 nm by the size of the AFM probes as well as the indentation profile the probes create in the polymer media. Accordingly, data density, while increased over magnetic storage media, is limited to 500 Gb/inch2. Furthermore, read and write processes are slow, limited by the maximum resonant frequency of the cantilever probes, which are only operable on a microsecond scale.

Accordingly, a data storage device and system having increased data density is needed.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention.

The term “substrate” used in the following description may include any supporting structure including, but not limited to, a semiconductor substrate that has an exposed substrate surface. A semiconductor substrate should be understood to include silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. When reference is made to a semiconductor substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation. The substrate need not be semiconductor-based, but may be any support structure suitable for supporting an integrated circuit, including, but not limited to, metals, alloys, glasses, polymers, ceramics, and any other supportive materials as is known in the art. Additionally, for purposes of this specification, a substrate can include layers and structures over a semiconductor substrate, wafer, or other material, such as conductive lines and/or insulating layers.

The invention is now explained with reference to the figures, which illustrate exemplary embodiments and throughout which like reference numbers indicate like features.FIG. 1illustrates a high density resistance variable data storage device100according to an exemplary embodiment of the invention. The device100includes a programming/sensing chip130and a data medium131. The programming/sensing chip130is connected to first and second multiplexer circuits140,141(described below in more detail). The data storage medium131is connected to a scanner device170(described below in more detail).

The data storage medium131includes an insulating layer101, a first electrode layer102and a resistance variable material portion103. This first electrode layer102is preferably tungsten (W), but could be any other suitable conductive material, such as silver (Ag).

FIGS. 2A and 3Aare cross-sectional views of the device100along the line A-A′ and according to exemplary embodiments of the invention. In both of theFIG. 2AandFIG. 3Aembodiments, the resistance variable portion103includes one or more layers of resistance variable material and may include layers of additional materials. In the illustrated embodiments, the resistance variable portion103includes a chalcogenide material layer103a, a metal layer103b, a metal-chalcogenide layer103c, and a chalcogenide material layer103d. The chalcogenide material layers103a,103dcan be, for example, germanium selenide (GexSe100-x) glass. The germanium selenide may be within a stoichiometric range of about Ge33Se67to about Ge60Se40. Chalcogenide layers103a,103dcan be a same material or different materials. Further, each layer103a,103dneed not be a single layer, but may also be comprised of multiple chalcogenide sub-layers having the same or different stoichiometries.

Below the chalcogenide material layer103dis a layer of metal-chalcogenide103c, such as tin-chalcogenide (e.g., tin selenide) or a silver chalcogenide (e.g., silver selenide). It is also possible that other chalcogenide materials may be substituted for selenium, such as sulfur, oxygen, or tellurium. The thickness of layer103cdepends, in part, on the thickness of the chalcogenide material layer103d. The ratio of the thickness of the metal-chalcogenide layer103cto that of the chalcogenide material layer103dis preferably between about 5:1 and about 1:3.

During operation, data can be written to specific locations on the resistance variable portion103by applying a voltage to a desired location on the resistance variable material to form a conductive pathway (e.g., a conduction channel or path)120, which has a lower resistance than other areas of the resistance variable portion103. Data is read by applying a voltage pulse of a lesser magnitude than required to program a conductive pathway120; the resistance across the location of the resistance variable portion103is then sensed as higher or lower to define two logic states.

The programmed low resistance conductive pathway120can remain intact for an indefinite period, typically years or longer, after the voltage potentials are removed; however, some refreshing may be useful. The conductive pathway120can be erased and the specific location of the resistance variable portion103can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the location to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. Alternatively, the memory portion can be configured to be programmable one time only. In such a case, the conductive pathway120will remain indefinitely, but can not be erased.

In this way, the resistance variable portion103provides data storage for storing data bits at the locations, each location able to exhibit at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.

The programming/sensing chip130is spaced a distance160above the data storage medium131and includes an array of cantilevers110(FIGS. 2A-3B). The cantilevers are affixed to a substrate150. In the exemplified embodiment, the cantilevers110are moveably affixed to the substrate150to allow for movement along the z direction. Alternatively, the cantilevers can be stationary. The substrate150includes circuitry151for operating the individual cantilevers110in response to signals from multiplexers140,141as described in more detail below.

Each cantilever110is connected to circuitry151of the programming/sensing chip130for enabling the cantilever110, and corresponding conductive point(s)111,112to perform the desired read, write and erase functions. In turn, the programming/sensing chip130is connected to multiplexing circuitry (MUX)140,141(FIG. 1), for addressing and actuating particular cantilevers110and conductive points111,112. Once a first set of data is written to a first location (as described above), the conductive points111,112of the cantilevers110are then raster-scanned to the next location on the resistance variable portion103, which is, e.g., about 20 nm to about 50 nm away from the first location, to write a second set of data. The raster scan can be achieved, for example, as in the millipede system, by relative movement of the data storage medium131in the x and y directions. Accordingly, in one exemplary embodiment the data storage medium131can be connected to a scanner device170for providing movement of the data storage medium131in the x and y directions. Alternatively, data storage medium131can be fixed while the programming/sensing chip130is configured for movement in the x and y directions.

As the dimensions in the x and y direction of a conductive pathway (e.g., a conduction channel or path)120are very small, the device100can achieve a data density of about 2 Tb/inch2. Additionally, the device100does not require elevated operation temperatures. Further, the read and programming operations can be fast as it does not involve producing mechanical indentations, a process that is limited by the cantilever's resonant frequency. It has been shown that a 1 ns pulse can cause the formation of a conductive pathway120.

When a cantilever110is actuated, each moves along the z direction such that the conductive point(s)111,112are place in or are removed from electrical contact with the resistance variable portion103of the data storage medium131. Alternatively, the cantilevers110can be stationary and the spacing161between the data storage medium131and the programming/sensing chip130can be such that the conductive points111,112of the cantilevers110are in constant electrical contact with the resistance variable portion103. In such a case, when the chips130,131are moved relative to one another, the conductive points111,112can move along the surface of the resistance variable portion103. Also, when no movement of the conductive points111,112in the z direction is needed, the conductive points need not be included on a cantilever110, but can instead, for example, be included on a structure connected to the substrate150at more than one end or to a structure protruding from the substrate150.

When addressed, conductive point(s)111,112provide a voltage across a desired location of the resistance variable portion103to read, write or erase the location. In this way, each conductive point111,112serves as a second electrode to the first electrode102.

In theFIG. 2Aembodiment, the chip130includes an array of cantilevers110, which are individually addressable by multiplexers140,141through circuitry151. As shown inFIG. 2Beach cantilever includes a single conductive point, which can be, for example, a sharp metal tip111(e.g., tungsten or silicon) or a conductive nanotube112(e.g., a carbon nanotube) or nanorod (not shown) (e.g., a metal nanorod, such as, a silicon nanorod). Although both sharp metal tips111and conductive nanotubes112are shown inFIG. 2A, it should be understood that the device100could include only a single type of conductive point. The conductive point111,112is connected to the circuitry151. When addressed, the circuitry151causes the cantilever110to move such that the conductive point111,112is in electrical contact with the resistance variable portion130. Additionally, the circuitry151provides a voltage to the conductive point111,112, which in turn provides a voltage to a location on the resistance variable portion130.

In theFIG. 3Aembodiment, each cantilever110includes a plurality of individually addressable conductive tips, e.g., nanotubes112, as shown inFIG. 3B. Each conductive point112is connected to the circuitry151. When addressed, the circuitry151causes the cantilever110to move such that the conductive points112are in electrical contact with the resistance variable portion130. Additionally, the circuitry151provides a voltage to the conductive points112, which in turn provide a voltage to locations on the resistance variable portion130.

FIG. 4illustrates a processor system400which includes a data storage circuit448, including a data storage device100constructed according to the invention. The processor system400, which can be, for example, a computer system, generally comprises a central processing unit (CPU)444, such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device446over a bus452. The system400may also include a memory unit454in communication with the CPU over the bus452. The data storage circuit448communicates with the CPU444over bus452typically through a controller.

In the case of a computer system, the processor system400may include peripheral devices such as a compact disc (CD) ROM drive456, which also communicate with CPU444over the bus452.

The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the present invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.