WO2005017908A1

WO2005017908A1 - Memory for storing information

Info

Publication number: WO2005017908A1
Application number: PCT/US2003/024550
Authority: WO
Inventors: George Radominski; Timothy L. Weber; Steven D. Leith
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2003-08-06
Filing date: 2003-08-06
Publication date: 2005-02-24
Also published as: AU2003254322A1

Abstract

A memory apparatus (10) has a plurality of first electrodes (12) and at least one second electrode (14) separated by an electrolyte solution (16). Information may be recorded by causing an electrical current to flow between a selected of the first electrodes (12) and the second electrode (14) to deposit an electrochemically active material (26) on one of the selected first (12) or the second electrodes (14). A method for recording and reading information has steps of writing the information by causing a current to flow between a first (12) and a second electrode (14) through an electrolyte solution (16) to cause a material (26) to electrodeposit, and reading the information by sensing the material (26) with a sensor (18).

Description

MEMORY FOR STORING INFORMATION

FIELD OF THE INVENTION

The invention is in the microelectronics and memory fields. The invention particularly concerns programmable memories for storing data, as well as methods for storing data.

BACKGROUND OF THE INVENTION

Memories such as programmable memories are used in the electronic and computer arts to store information in the form of binary data consisting of bits. Examples of memories include magnetic and optical media organized into discrete sections. Data is stored in these memory media by inducing a discernable change in a selected section, with the changed or unchanged state of the section indicative of a binary bit. As the computer and electronic arts have advanced and have evolved into the microelectronics field, demands on memory apparatuses have steadily increased. For example, the amount of data desired to be stored on devices has substantially increased. Also, it is desirable for memories to be small and lightweight for use in portable device applications. As a result, it is desirable to increase the storage density of memory apparatuses. SUMMARY OF THE INVENTION

An embodiment of a method of the invention is directed to causing an electrical current to flow through an electrolyte solution separating a first electrode from a second electrode to cause an electrochemical modification of the second electrode to thereby store the information, and then reading the information by detecting the electrochemical modification with a sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a portion of a preferred embodiment memory apparatus of the invention; FIG. 2 is a perspective of a portion of the preferred embodiment memory apparatus of FIG. 1 ; FIG. 3 is a top plan view of a portion of the preferred embodiment memory apparatus of FIG. 1 ; FIG. 4 is a cross section of a second exemplary preferred embodiment memory apparatus of the invention; and FIG. 5 is a flow chart of a preferred embodiment method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns a memory apparatus for storing information, and more preferably also for reading and erasing the stored information. Memory apparatus embodiments of the invention write, read, and erase information through electrochemical mechanisms. Information may be written by causing an electrical current to flow through an electrolyte solution between two electrodes to thereby electrochemically modify one of the electrodes. The information may be read by detecting the electrochemical modification with a sensor, and may be erased by reversing the electrochemical modification. Turning now to the drawings, FIG. 1 is a cross section of a portion of a preferred memory apparatus of the invention, shown generally at 10. The memory apparatus 10 has a plurality of first electrodes 12 that are spatially arranged with respect to one another and are individually addressable. A second electrode 14 is separated from the first electrodes 12 by a liquid electrolyte solution 16. Walls or other structure may be provided to create a chamber or other enclosure for containing the electrodes 12 and 14 and the electrolyte 16. An individually addressable and generally circular sensor electrode 18 surrounds each of the first electrodes 12, with the sensor electrode 18 supported on a dielectric material 20. The spatial configuration of a sensor electrode 18 and a first electrode 12 is further illustrated in the perspective view of FIG. 2 showing one generally circular sensor electrode 18 surrounding the tip of a first electrode 12. The first electrodes 12, second electrode 14, and sensor electrodes 18 all preferably comprise noble electrodes. The preferred memory apparatus of FIG. 1 further comprises a controller

22 linked by a control circuit 24 to each of the first electrodes 12, the second electrode 14, and the sensor electrodes 18. Each of the first electrodes 12 and the sensor electrodes 18 are individually addressable by the controller 22. Also, the controller 22 may facilitate electrically linking any one of the first electrodes 12 or the sensor electrodes 18 to the second electrode 14 in order to cause an electrical current to flow through the electrolyte 16 between a selected first electrode 12 or sensor electrode 18 and the second electrode 14. The controller 22 may comprise, for example, a chip or a microprocessor. In operation, the memory apparatus 10 may be used to store, read, and erase information. In particular, when the controller 22 causes an electric current to flow between one or more selected first electrodes 12 and the second electrode 14, the surface of the second electrode 14 is electrochemically modified. In particular, an electrochemically active material 26 is electrodeposited from the electrolyte on the second electrode 14. By causing the electrochemically active material 26 to be selectively deposited on the second electrode 14, information may be coded in a binary matter. That is, each first electrode 12 and corresponding location on the second electrode 14 may be thought of as a bit, and the presence or absence of an electrochemically active material 26 in that location corresponding to a high or low state for the bit (i.e., a 1 or O). The first electrodes 12 preferably have the general shape of a cone so as to more precisely direct the location of deposited solid on the second electrode 14. Also, the dielectric 20 supporting the sensor electrode 18 is provided to help minimize "cross talk" between the individual first electrodes 12. That is, the dielectric 20 helps to minimize influence that one first electrode 12 may have on a neighboring first electrode 12 when writing information. For this reason the preferred dielectric 20 has a height substantially equal or larger than the height of the first electrode 12. The electrochemically active material 26 preferably comprises a metal deposit. Those knowledgeable in the art will appreciate that the electrolyte solution may contain ions of one or more of many materials capable of being electrodeposited, with metal ions comprising preferred examples. By way of further example, metals such as Zn, Co, Ni, Fe, Cu, Al, Ag, Cr, Mn, V, Ti, Sn, In, Rh, Pd, Cd and/or Mo may be useful for practice of invention embodiments. Because switching speed is proportional to the square of the number of electrons transferred in the reaction, materials such as Ag that require the transfer of only one electron are preferred. The switching speed with a Ag solution is 4 times as fast as that of a solution using, for example, Al, Co, Cu, Fe, Ni, Cr or Zn (all of which require two electrons). Those skilled in the art will likewise appreciate that a desired concentration of ion in the electrolyte solution, the magnitude of the electrical current required to electrodeposit the solid, and other factors concerning the electrodeposition may be calculated using well known electrochemical relationships, such as, for example, Faraday's Law, Fick's Second Law of diffusion, and the Debye-Huckel or Guoy-Chapman model of double layer charging. By way of a particular exemplary electrolyte, an aqueous solution of Ag+ ions (as silver cyanide, AgCN) with a concentration of between about 0.1 and about 1.5 mol/l, and a temperature in the range of about 20 to about 75°C is believed to be useful with practice of the invention. The electrolyte may also contain additives such as potassium cyanide (KCN) in the concentration range of about 0.5 to about 2.5 mol/l, potassium carbonate (K₂CO₃) in the concentration range of about 0.1 to about 1.0 mol/l, potassium nitrate (KNO₃) in the concentration range of about 0.1 to about 1.0 mol/I and potassium hydroxide (KOH) in the concentration range of about 0.05 to about 0.5 mol/l. A number of organic brighteners in ppm concentrations may also be present. In addition to deposition of a metal, other invention embodiments may comprise deposits of other electrochemically active materials. For example, a magnetic material such as NiFe could be electrodeposited and detected through measurement of magnetic field. Further, some invention embodiments may not deposit a material at all, but instead may store information by causing an electrochemical modification on one of the selected first or second electrodes. For example, by causing a current to flow through the electrolyte, the oxidation state of the surface of one of the electrodes could be modified. The controller 22 may cause the memory apparatus 10 to read information by causing the sensor electrodes 18 to sense the electrochemical modification to the second electrode 14. In particular, the sensor electrode 18 may measure an electrical property such as resistance between it and the second electrode 14. Presence of the metal deposit 26 will result in a lower resistance between the sensing electrode 18 and the second electrode 14 since the metal deposit 26 has a higher conductivity than the electrolyte 16. The generally circular shape of the sensor electrode 18 is advantageous for focusing of an electric field proximate the location of the deposit 26. Those skilled in the art will appreciate that there are a number of manners in which to sense the presence of the metal deposit 26. For example, a sensor may measure a magnetic field strength to detect the presence of a deposited magnetic material. Presence of an electrochemically active material such as the metal deposit 26 in a particular region of the second electrode 14 may be interpreted as a change of state in a particular bit. That is, for each portion of the second electrode 14 that corresponds to one of the plurality of first electrodes 12, a binary "high" state may be indicated by the presence of a metal deposit 26, and a binary "low" state indicated by the absence of a metal deposit 26. Other invention embodiments may comprise storing more than one bit of information per metal deposit. That is, an electrochemically active deposit may store more than a 1 or 0 state, and may, for instance, store a 1 , 2, 3 or 4 state, a byte, or other amount of information. For example, x, y, and z dimensions of the deposit 26 could be further used to store additional bits of information. Also, different deposit materials in combination could be used to store additional bits of information. The controller 22 may also cause the memory apparatus 10 to erase information through change of a particular bit from a high to a low state. In particular, the controller 22 may cause an electric current to flow between one or more a selected of the sensing electrodes 18 and the second electrode 14 to cause a selected metal deposit 26 to go back into solution. For example, the controller 22 may cause one or more selected sensing electrodes 18 to function as anodes so as to ionize the metal deposit 26. Other means for erasing information could be provided. For example, erasing information could be accomplished by causing a current to flow between a first electrode 12 and the second electrode 14 (i.e., reverse the polarity of the writing current). It is preferred to use a separate electrode, such as a sensor electrode 18, however, so as to not foul the tip of the first electrode 12. It will be appreciated that a spatial arrangement of the plurality of first electrodes 12, sensor electrodes 18, and sections of the second electrode 14 may allow for additional coding of information. For example, the plurality of first electrodes 12 and sensor electrodes 18 may be spatially arranged in a plurality of rows and columns, as generally illustrated in the top plan view of FIG. 3 showing a plurality of first electrodes 12 and sensor electrodes 18. Also, it will be appreciated that the numbers of first electrodes 12 and sensor electrodes 18 shown in FIGS. 1 and 3 are for illustration purposes only, and that in practice it may be desirable to practice the invention using great multiplicities of electrodes to achieve high memory storage capacities. Additionally, it will be appreciated that the first electrodes 12, second electrode 14, and sensor electrodes 18 may be provided on a micro scale so as to accomplish a desirably high density of memory with a memory apparatus of the invention. By way of example preferred dimensions, generally inverted cone shaped first electrodes 12 having a base width of between about 250 and about 500 nm and a height of approximately the same dimensions could be used. Spacing between the first electrodes 12 and the second electrode 14 is preferably about 1 micron or less. Example preferred dimensions for the generally ring shaped sensor electrode 18 include an inside diameter about the same as or slightly larger than the base width of the first electrode 12, a height of about 100 nm or less, and a width of about 50 to about 100 nm. Using electrodes of this scale, a memory density of the order of about 4 bits per micron may be achieved. The operational speed of the memory apparatus 10 is believed to generally be limited by ionic diffusion in the electrolyte 16. That is, information can only be written, read and erased as fast as current moves (i.e. transport of ionic species) through the electrolyte 16. Regardless of the magnitude of current applied, the actual current realized (and accordingly the surface reaction rate at the electrode 14) depends on how readily the current is carried between the electrodes 12 and 14. The flow of current through the electrolyte 16, and thus information write/erase speeds, depends on a number of factors including chemical species, size, mobility, charge, concentration, electrolyte temperature, electrode 12 and 14 spacing, minimum deposit thickness, and the like. For example, the thickness of the metal deposit 26 will vary with the time of current flow. It is believed that a deposit thickness of between about 5 nm and about 100 nm will be useful with practice of the preferred apparatus of the invention. Taking a deposit thickness of about 100 nm and electrode 12 and 14 spacing of about 1 micron by way of example, the record and erase speeds may each be on the order of 10 millisecond for a typical nickel salt electrolyte at 50°C. The controlling variables could be manipulated to improve the speed, with a maximum record speed of the order of about 1 millisecond believed to be achievable. Sensing speeds are generally much faster than those of the write/erase functions. Sensing resistivity for example could be performed at speeds greater than 1 millisecond. Accordingly, it is estimated that a complete write/read/erase cycle for information could likely be achieved using the preferred apparatus of the invention on the order of between about 2 and about 5 milliseconds. Other exemplary memory apparatus embodiments may have a plurality of individually addressable second electrodes 14. A single second electrode 14 is preferred, however, for ease of control, cost of manufacture, and complexity considerations. Also, it will be appreciated that the memory apparatus 10 of the invention may be useful with the solids 26 deposited on either of the first electrodes 12 or second electrode(s) 14. Indeed, reversing the polarity between the respective electrodes 12 and 14 may reverse the direction of deposition. By way of still additional example, other apparatus embodiments may comprise a sensor that is separate from an erasing electrode. FIG. 4 illustrates an additional exemplary preferred memory apparatus 110 of the invention that is useful to further increase information storage density. Generally, the apparatus 110 is consistent in most respects with the apparatus 10. Similar element numbers have been used for elements of the apparatus 110 that are generally consistent with those of the apparatus 10 for convenience. Generally, a plurality of first electrodes 112 is separated from an opposing second electrode 114 by an electrolyte 116. To store information, an electric current flows across the electrolyte to cause an electrochemically active material 126 to deposit on the surface of the second electrode 114. A sensor electrode 118 may be used to detect the presence of the electrochemically active material 126 to thereby read the information, and also to erase the information by causing the electrochemically active material 126 to be ionized back into the electrolyte. A controller 122 and control circuit 124 controls the first electrodes 112 and second electrode 114. In addition to these and other aspects of the memory device apparatus 110 that are generally consistent with those of the apparatus 10, the apparatus 110 further comprises a mover 170 attached to the plurality of first electrodes for selectively moving the electrodes in relation to the second electrode 114. A mover control 172 linked to the mover 170 causes the electrodes 112 to move laterally as indicated by the arrow in predetermined distances. Those knowledgeable in the art will appreciate that movers 170 are generally known. By way of example, the mover 170 may be a micro-motion motor and carriage mechanism. By way of more particular example, description of a suitable mover may be found in the commonly assigned and pending U.S. patent application with serial no. 10/157,254 entitled "A Movable Micro- Electromechanical Device," filed on May 28, 2002. Also, it will be appreciated that the mover 170 could alternatively or additionally be connected to the second electrode 114. It will still further be appreciated that although FIG. 4 illustrates first electrodes along the X-axis only, they preferably also extend along a Y-axis to form an array of rows and columns, and that movement may likewise occur along the Y-axis. The mover control 172 is preferably connected to the controller 122 and controller circuit 124. Using the mover 170, any one of the plurality of first electrodes 112 may be used to electrochemically modify different discrete portions of the second electrode 114 surface. Preferably, the mover 170 is capable of accurately moving the electrodes 112 very small distances of the order of 100 nm or less. With movements of this order, the second electrode surface may have discrete modifications separated by about 100 nm or less. It is believed that with the mover 170 the apparatus 110 may accordingly be capable of storing information at a density of up to about 10 bits per micron. Those skilled in the art will further appreciate that a memory apparatus of the invention may be useful in a number of different applications. ^" In many of these applications, the memory apparatus of the invention may further comprise integrated circuitry. For example, a memory apparatus such as the that shown generally at 10 may be connected to integrated circuitry for use in a handheld electronic device such as a laptop or palmtop computer device, a communications device such as a cellular phone, or the like. Another embodiment of the invention is directed to a method for storing information. In considering methods of the invention, it will be appreciated that a memory apparatus of the invention such as that illustrated by FIGS. 1-3 may be useful in practice of method embodiments of the invention. FIG. 5 is a flowchart illustrating steps of a preferred embodiment method of the invention for writing, reading, and erasing information from a memory apparatus. In the invention method embodiment illustrated generally at 400, the dashed line block 402 includes steps for writing the information. At least one first electrode is selected from a plurality of first electrodes (block 404), and then moved using a mover to a desired position relative to a second electrode (block 405). A current is then caused to flow through an electrolyte solution that separates the selected first electrode from a second electrode to cause a solid to be electrodeposited onto the second electrode (block 406). The dashed line block 408 includes a step of reading the information. In particular, the presence of the solid is detected using a sensor (block 410). Preferably, a plurality of sensors is provided, with one each corresponding to one each of the first electrodes. The sensor preferably detects the presence of the solid by measuring an electrical property such as resistance or magnetic field. Most preferably, the sensor comprises an electrode that substantially surrounds the first electrode. The dashed line block 412 includes steps for erasing the information. The solid may be removed from the second electrode by causing a current to flow between that electrode and the sensor electrode to cause the metal to ionize back into solution (block 414). While a specific embodiment of the present invention has been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. Various features of the invention are set forth in the appended claims.

Claims

Claims:

1. A memory apparatus for storing information comprising: a plurality of spatially arranged first electrodes (12), each of said plurality of first electrodes (12) being individually addressable; at least one second electrode (14); and, an electrolyte solution (16) separating said plurality of first electrodes (12) from said at least one second electrode (14), said electrolyte solution (16) responsive to an electric current communicated therethrough between a selected of said plurality of first electrodes (12) and said at least one second electrode (14) to cause an electrochemical modification (26) to the surface of one of said second (14) or said selected first electrodes (12) to thereby store the information.

2. A memory apparatus for storing information as defined by claim 1 further comprising a plurality of sensors (18) for detecting said electrochemical modification (26) of said surface by measuring an electrical property between said at least one sensor and said one of said second or selected of said plurality of first electrodes.

3. A memory apparatus for storing information as defined by claim 2 wherein said electrochemically modified surface comprises an electrochemically active material (26) deposited on said surface, and wherein said plurality of sensors (18) are for erasing the information by causing said electrochemically active material (26) deposited on said surface to be ionized back into said electrolyte (16).

4. A memory apparatus for storing information as defined by claim 2, wherein said electrochemical modification comprises deposit of an electrochemically active material (26) having a thickness between about 5 nm and about 100 nm, wherein said electrolyte (16) is responsive to said electric current to cause said electrochemical deposit (26) in a time period of less than about 1 millisecond, wherein said plurality of first electrodes (12) and said second electrode (14) are separated by a distance of about 1 micron or less, and wherein said plurality of sensors (18) detects said electrochemical modification (26) in a time period of less than about 1 millisecond.

5. A memory apparatus for storing information as defined by claim 1 wherein each of said plurality of first electrodes (12) has a general cone shape with a height, and further comprising a dielectric (20) between each of said plurality of first electrodes (12), said dielectric (20) having a height at least as large as said first electrode (12) height.

6. A memory apparatus for storing information as defined by claim 1 further comprising a mover (170) connected to one of said second electrode (14) or to said plurality of first electrodes (12) for selectively moving said one of said second electrode (14) or said plurality of first electrodes (12) relative to the other.

7. A method for recording and reading information comprising the steps of: causing an electrical current to flow through an electrolyte solution

(16) separating a first electrode (12) from a second electrode (14) to cause an electrochemical modification (26) to said second electrode (14) to thereby store the information; and, reading said information by detecting said electrochemical modification with a sensor (18).

8. A method for recording and reading information as defined by claim 7 wherein the method further comprises the step of selecting said first electrode (12) from a plurality of individually addressable first electrodes (12), and selectively moving said first electrode (12) with a mover (170) to cause said electrochemical modification (26) at a desired position on said second electrode (14) surface.

9. A method for recording and reading information as defined by claim 7 wherein said electrochemical modification comprises an electrodeposited electrochemically active material, wherein the step of detecting said electrochemical modification (26) comprises measuring electrical resistance or magnetic field with said sensor (18), and further comprising the step of erasing the information by causing an electric current to flow between a third electrode (18) and said second electrode (14) to reverse said electrochemical modification (26).

10. A method for recording and reading information as defined by claim 7 wherein the step of recording the information is performed in a time period of about 1 millisecond and the step of reading the information is performed in a time period of less than about 1 millisecond.