More particularly, the invention relates to programmable microelectronic structures, having a metal oxide ion conductor, and to devices and circuits including the structures.

BACKGROUND OF THE INVENTION Memory devices are often used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.

For example, random access memory ("RAM"), which may be found in personal computers, is typically volatile semiconductor memory; in other words, the stored data is lost if the power source is disconnected or removed. Dynamic RAM ("DRAM") is particularly volatile in that it must be"refreshed" (i. e. , recharged) every few hundred milliseconds in order to maintain the stored data. Static RAM ("SRAM") will hold the data after one writing so long as the power source is maintained; once the power source is disconnected, however, the data is lost. Thus, in these volatile memory configurations, information is only retained so long as the power to the system is not turned off. In general, these RAM devices can take up significant chip area and therefore may be expensive to manufacture and consume relatively large amounts of energy for data storage. Accordingly, improved memory devices suitable for use in personal computers and the like are desirable.

Other storage devices such as magnetic storage devices (e. g., floppy disks, hard disks and magnetic tape) as well as other systems, such as optical disks, CD-RW and DVD-RW are non-volatile, have extremely high capacity, and can be rewritten many times.

Unfortunately, these memory devices are physically large, are shock/vibration-sensitive, require expensive mechanical drives, and may consume relatively large amounts of power. These negative aspects make such memory devices non-ideal for low power portable applications such as lap-top and palm-top computers, personal digital assistants ("PDAs"), and the like.

Due, at least in part, to a rapidly growing numbers of compact, low-power portable computer systems and hand-held appliances in which stored information changes regularly,

low energy read/write semiconductor memories have become increasingly desirable and widespread. Furthermore, because these portable systems often require data storage when the power is turned off, non-volatile storage devices are desired for use in such systems.

One type of programmable semiconductor non-volatile memory device suitable for use in such systems is a programmable read-only memory ("PROM") device. One type of PROM, a write-once read-many ("WORM") device, uses an array of fusible links. Once programmed, the WORM device cannot be reprogrammed.

Other forms of PROM devices include erasable PROM ("EPROM") and electrically erasable PROM (EEPROM) devices, which are alterable after an initial programming.

EPROM devices generally require an erase step involving exposure to ultra violet light prior to programming the device. Thus, such devices are generally not well suited for use in portable electronic devices. EEPROM devices are generally easier to program, but suffer from other deficiencies. In particular, EEPROM devices are relatively complex, are relatively difficult to manufacture, and are relatively large. Furthermore, a circuit including EEPROM devices must withstand the high voltages necessary to program the device.

Consequently, EEPROM cost per bit of memory capacity is extremely high compared with other means of data storage. Another disadvantage of EEPROM devices is that, although they can retain data without having the power source connected, they require relatively large amounts of power to program. This power drain can be considerable in a compact portable system powered by a battery.

Various hand-held appliances such as PDAs, portable phones, and the like as well as other electronic systems generally include a memory device coupled to a microprocessor and/or microcontroller formed on a separate substrate. For example, portable computing systems include a microprocessor and one or more memory chips coupled to a printed circuit board.

Forming memory devices and the microprocessor on separate substrates may be undesirable for several reasons. For example, forming various types of memory on separate substrate may be relatively expensive, may require relatively long transmission paths to communicate between the memory devices and any associated electronic device, and may require a relatively large amount of room within a system. Accordingly, memory structures that may be formed on the same substrate as another electronic device and methods of forming the same are desired. Furthermore, this memory technology desirably operates at a

relatively low voltage while providing high speed memory with high storage density and a low manufacturing cost.

SUMMARY OF THE INVENTION The present invention provides improved microelectronic programmable devices, structures, and systems and methods of forming the same. More particularly, the invention provides programmable structures that include a metal oxide ion conductor. Such structures can replace both traditional nonvolatile and volatile forms of memory and can be formed on the same substrate as and/or overlying another microelectronic device.

The ways in which the present invention addresses various drawbacks of now-known programmable devices are discussed in greater detail below. However, in general, the present invention provides a programmable device that is relatively easy and inexpensive to manufacture and which is relatively easy to program.

In accordance with one exemplary embodiment of the present invention, a programmable structure includes a metal oxide ion conductor and at least two electrodes.

The structure is configured such that when a bias is applied across two electrodes, one or more electrical properties of the structure change. In accordance with one aspect of this embodiment, a resistance across the structure changes when a bias is applied across the electrodes. In accordance with other aspects of this embodiment, a capacitance or other electrical property of the structure changes upon application of a bias across the electrodes. In accordance with a further aspect of this embodiment, an amount of change in the programmable property is manipulated by altering (e. g., thermally or electrically) an amount of energy used to program the device. One or more of these electrical changes and/or the amount of change may suitably be detected. Thus, stored information may be retrieved from a circuit including the structure.

In accordance with another exemplary embodiment of the invention, a programmable structure includes a metal oxide ion conductor, at least two electrodes, and a barrier interposed between at least a portion of one of the electrodes and the ion conductor. In accordance with one aspect of this embodiment, the barrier material includes a material configured to reduce diffusion of ions between the ion conductor and at least one electrode.

In accordance with another aspect, the barrier material includes an insulating or high- resistance material. In accordance with yet another aspect of this embodiment, the barrier

includes material that conducts ions, but which is relatively resistant to the conduction of electrons.

In accordance with another exemplary embodiment of the invention, a programmable microelectronic structure is formed on a surface of a substrate by forming a first electrode on the substrate, forming a layer of ion conductor material over the first electrode, and depositing conductive material onto the ion conductor material. In accordance with one aspect of this embodiment, the ion conductor layer is formed by oxidizing a portion of the first electrode material. In accordance with another aspect of this embodiment, a solid solution including the ion conductor and excess conductive material is formed by dissolving (e. g., via thermal and/or photodissolution) a portion of the conductive material in the ion conductor. In accordance with a further aspect, only a portion of the conductive material is dissolved, such that a portion of the conductive material remains on a surface of the ion conductor to form an electrode on a surface of the ion conductor material.

In accordance with another embodiment of the invention, a lateral programmable structure is formed by forming an ion conductor layer overlying a substrate and forming two or more electrode structures in contact with the ion conductor layer.

In accordance with another embodiment of the invention, a programmable device may be formed on a surface of a substrate. In accordance with one aspect of this embodiment, the substrate includes a microelectronic circuit. In accordance with a further aspect of this embodiment, the memory device is formed overlying the microelectronic circuit and conductive lines between the microelectronic circuit and the memory are formed using conductive wiring schemes within the substrate and the memory device. This configuration allows transmission of more bits of information per bus line.

In accordance with yet another embodiment of the invention, a volatility of a memory cell in accordance with the present invention is manipulated by altering an amount of energy used during a write process for the memory. In accordance with this embodiment of the invention, higher energy is used to form nonvolatile memory, while lower energy is used to form volatile memory. Thus, a single memory device, formed on a single substrate, may include both nonvolatile and volatile portions. In accordance with a further aspect of this embodiment, the relative volatility of one or more portions of the memory may be altered at any time by changing an amount of energy supplied to a portion of the memory during a write process.

In accordance with yet another embodiment of the invention, pulse mode programming is used to read and write information. In this case, information can be retrieved from the device using a destructive read or a destructive write process.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention may be derived by referring to the detailed description and claims, considered in connection with the figures, wherein like reference numbers refer to similar elements throughout the figures, and: Figure 1 illustrates a programmable structure formed overlying a substrate in accordance with an exemplary embodiment of the present invention; Figure 2 illustrates a programmable structure formed overlying a substrate in accordance with another exemplary embodiment of the present invention; and Figure 3 illustrates a current-voltage diagram for a programmable structure in accordance with one embodiment of the invention.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.

DETAILED DESCRIPTION The present invention generally relates to programmable microelectronic structures and devices and to methods of forming and using the devices.

Figure 1 illustrates a portion 100 of a programmable microelectronic circuit, which includes structures 102 and 104 formed overlying a substrate 106, in accordance with an exemplary embodiment of the present invention. Structures 102 includes electrodes 108, 112 and metal ion conductor 114. Similarly, structure 104 includes electrodes 110,112 and ion conductor 116. Structures 102,104 may additionally include one or more buffer layers interposed between the ion conductor and one or both of the electrodes.

Generally, structures 102 and 104 are configured such that when a bias greater than a threshold voltage (VT), discussed in more detail below, is applied across two electrodes (e. g., electrodes 108 and 112), the electrical properties of a structure change. For example, in accordance with one embodiment of the invention, as a voltage V > VT is applied across electrodes 108 and 112, conductive ions within ion conductor 114 begin to migrate and form

a region 118 having an increased conductivity compared to the bulk ion conductor at or near the more negative of electrodes 108 and 112. As region 118 forms, the resistance between electrodes 108 and 112 decreases, and other electrical properties may also change.

In the absence of any barriers, which are discussed in more detail below, the threshold voltage required to grow region 118 from one electrode toward the other and thereby significantly reduce the resistance of the device is approximately a few hundred millivolts. If the same voltage is applied in reverse, region 118 will dissolve back into the ion conductor and the device will return to a high resistance state. In a similar fashion, an effective barrier height of a diode that forms between an ion conductor and an electrode can be reduced by growing region 118; thus current flow may be increased through the structure, even if the resistance of the structure is substantially the same.

Structures 102 and 104 may be used to store information and thus may be used in memory circuits. For example, structure 102 or other programmable structures in accordance with the present invention may suitably be used in memory devices to replace DRAM, SRAM, PROM, EPROM, EEPROM devices, or any combination of such memory.

In addition, programmable structures of the present invention may be used for other applications where programming or changing of electrical properties of a portion of an electrical circuit are desired.

In accordance with various embodiments of the invention, the volatility of a programmable structure (e. g., cell 102 or 104) can be manipulated by altering an amount of energy (e. g., altering time, current, voltage, thermal energy, and/or the like) applied during a write process. In the case where region 118 forms during a write process, the greater the amount of energy (having a voltage greater than the threshold voltage for the write process) applied during the write process, the greater the growth of region 118 and hence the less volatile the memory. Conversely, relatively volatile, easily erased memory can be formed by supplying relatively little energy to the cell. Thus, relatively volatile memory can be formed using the same or similar structures used to form nonvolatile memory, and less energy can be used to form the volatile/easily erased memory. Use of less energy is particularly desirable in portable electronic devices that depend on stored energy for operation. The volatile and nonvolatile memory may be formed on the same substrate and partitioned or separated from each other such that each partition is dedicated to either volatile or nonvolatile memory; or, an array of memory cells may be configured as volatile or nonvolatile memory using programming techniques, such that the configuration (i. e. ,

volatile or nonvolatile) of the memory can be altered by changing an amount of energy supplied during programming the respective portions of the memory array.

Referring again to Figure 1, substrate 106 may include any suitable material. For example, substrate 106 may include semiconductive, conductive, semiinsulating, insulating material, or any combination of such materials. In accordance with one embodiment of the invention, substrate 106 is formed of a semiconductor material (e. g., silicon) and a portion of the substrate is used to form a microelectronic device.

In accordance with various embodiments of the invention, portion 100 includes an insulating layer 120 to facilitate isolation of devices formed in substrate 106 from structures 102,104. Layer 120 and substrate 106 may be separated by additional layers (not shown) such as, for example, layers typically used to form integrated circuits. Because the programmable structures can be formed over insulating or other materials, the programmable structures of the present invention are particularly well suited for applications where substrate (e. g., semiconductor material) space is a premium. In addition, forming a memory cell overlying a microelectronic device may be advantageous because such a configuration allows greater data transfer between memory cells and the microelectronic device formed in substrate 106 using, for example, conductive plugs formed within layer 120.

Electrodes 108,110, 112 may be formed of various suitable conductive materials. In accordance with exemplary embodiments of the invention, electrodes 108,110 are formed of metal or metal compounds typically used in the manufacture of semiconductor devices, such as, for example, tungsten, nickel, molybdenum, platinum, metal silicides, doped polysilicon or silicon.

In accordance with various additional exemplary embodiments of the invention, electrode 112 is formed of a material including a metal that dissolves in ion conductor 114, 116 when a sufficient bias (V > VT) is applied across the electrodes (an oxidizable electrode) and the other electrode (e. g. , electrode 108) is relatively inert and does not dissolve during operation of the programmable device (an indifferent electrode). For example, electrode 112 may be an anode during a write process and be comprised of a material including silver and/or copper that dissolves in ion conductor 114 and electrode 108 may be a cathode during the write process and be comprised of an inert material such as tungsten, nickel, molybdenum, platinum, metal silicides, doped polysilicon or the like. Having at least one electrode formed of a material including a metal which dissolves in ion conductor 114

facilitates maintaining a desired dissolved metal concentration within ion conductor 114, which in turn facilitates rapid and stable region 118 formation within ion conductor 114 or other electrical property change during use of structure 102. Furthermore, use of an inert material for the other electrode (cathode during a write operation) facilitates electrodissolution of any region 118 that may have formed and/or return of the programmable device to an erased state after application of a sufficient voltage. Various configurations of electrodes 108,110, and 112 and materials suitable for the electrodes are discussed in greater detail in Application Serial No. 10/390,268, entitled PROGRAMMABLE STRUCTURE, AN ARRAY INCLUDING THE STRUCTURE, AND METHODS OF FORMING THE SAME, filed March 17,2003, the contents of which are hereby incorporated herein by reference.

In accordance with one embodiment of the invention, at least one electrode 108 and 110 is formed of material suitable for use as an interconnect metal. This allows formation of the electrode on a conductive plug (e. g., plug 122) that can be used to electrically connect various other sections of portion 100. Exemplary materials suitable for both interconnect and electrode 108,110 material include metals and compounds such as tungsten, nickel, molybdenum, platinum, metal silicides, and the like.

As noted above, programmable structures of the present invention may include one or more barrier or buffer layers 124,126 interposed between at least a portion of ion conductor 114 and one of the electrodes 108,112. Layers 124,126 may include ion conductors such as AgXO, AgXS, AgxSe, AgxTe, where x > 2, AgyI, where x > 1, CuI2, CuO, CuS, CuSe, CuTe, GeO2, GezSl z, GezSel z, GezTel z, AszSl z, AszSel z, AszTel z, where z # about 0.1, SiOx, W03, and combinations of these materials interposed between ion conductor 114 and a metal layer such as silver.

Buffer layers can also be used to increase the off resistance and"write voltage"by placing a high-resistance buffer layer (e. g., GeO2, SiOx, air, a vacuum, or the like) between the ion conductor and the indifferent electrode. In this case, the high-resistance buffer material allows metal such as silver to diffuse through or plate across the buffer and take part in the electrochemical reaction. When the barrier layer between the indifferent electrode and the ion conductor includes a high resistance material, the barrier may include ions that contribute to electrodeposit growth or the barrier may be devoid of ions. In either case, the barrier must be able to transmit electrons, by conduction or tunneling, such that the redox reaction occurs, allowing for region 118 growth. Additional materials suitable for

layers 124 and/or 126 are discussed in Application Serial No. 10/390,268, entitled PROGRAMMABLE STRUCTURE, AN ARRAY INCLUDING THE STRUCTURE, AND METHODS OF FORMING THE SAME, filed March 17,2003.

Ion conductor 114 is formed of material that conducts ions upon application of a sufficient voltage. Suitable materials for ion conductor 114 include porous thin films that contain significant amounts of defects and nano-voids. In general, ion conductors in accordance with the present invention can conduct ions without requiring a phase change, can conduct ions at a relatively low temperature (e. g., below 125 °C), can conduct ions at relatively low electrical currents, have a relatively high transport number, and exhibit relatively high ion conductivity. Although the thickness of ion conductor 114 may vary from application to application, in accordance with one aspect of the invention, ion conductor 114 is about 10 to 100 nanometers and preferably about 50 nanometers thick.

In accordance with various exemplary embodiments of the invention, ion conductor 114 is formed of a metal oxide, such as tungsten oxide, which can be easily formed on a surface of a tungsten plug or the like, using typical semiconductor manufacturing techniques.

Ion conductor 114 also includes dissolved conductive material within the metal oxide ion conductor. For example, conductor 114 includes metal and/or metal ions dissolved in metal oxide ion conductor. Conductor 114 may also include network modifiers and/or fillers that affect mobility of ions through conductor 114.

Referring again to Figure 1, in accordance with various exemplary embodiments of the invention, at least a portion of structure 102 is formed within a via of an insulating material 128. Forming a portion of structure 102 within a via of insulating material 128 may be desirable because, among other reasons, such formation allows relatively small structures, e. g., on the order of 10 nanometers, to be formed. In addition, insulating material 128 facilitates isolating various structures 102,104 from each other and from other electrical components.

Insulating material 128 suitably includes material that prevents undesired diffusion of electrons and/or ions from structure 102. In accordance with one embodiment of the invention, material 128 includes silicon nitride, silicon oxynitride, silicon dioxide, fluorinated silicate glasses, polymeric materials such as polyimide or parylene, Poly- methylmethacrylate, or any combination thereof.

Contacts 130,132 may suitably be electrically coupled to one or more electrodes 108,110 to facilitate forming electrical contact to the respective electrode. Contacts 130, 132 may be formed of any conductive material and are preferably formed of a metal, alloy, or composition including aluminum, tungsten, or copper.

Although illustrated as formed at least partially within a via in an insulating layer, structures can suitably be formed as lateral cells. Exemplary lateral cells are illustrated in Application Serial No. 10/390,268, entitled PROGRAMMABLE STRUCTURE, AN ARRAY INCLUDING THE STRUCTURE, AND METHODS OF FORMING THE SAME, filed March 17,2003.

As described below, programmable structures may be formed using manufacturing processes typically used in the manufacture of semiconductor devices. The method described below illustrates one exemplary method of forming the structures and circuits of the present invention.

A method of forming structures begins with providing a substrate 106, which may include microelectronic devices formed therein. Insulating layer 120 is then formed overlying substrate 106, using for example, a traditional oxide or nitride deposition process.

In accordance with one aspect of this exemplary embodiment, about 200 nanometers of Si02 are deposited onto substrate 106. A via and corresponding plug 134 may similarly be formed, using traditional etch and deposition and/or damascene processing, to form an electrical connection to a device 136 formed within or using substrate 106.

Contacts 130,132, and other conductive elements 138 are then formed on a surface of insulating layer 120 by depositing about 450 namometers of conductive material such as tungsten, patterning the a desired structure with photoresist, and etching the material using a typical wet or dry-etch process. About 100 nanometers of insulating material 128 is then deposited onto material 120 and contacts 130,132, and element 138. Vias are then formed within material layer 128, using, for example, a photoresist/etch process, and the vias are filled with a conductive material such as tungsten.

Ion conductor regions 114 and 116 are then formed by exposing a portion of metal material to an oxidizing atmosphere. By way of one particular example, a tungsten oxide ion conductor is formed by exposing the surface of tungsten to a low-temperature (less than 400°C, e. g., 300°C) plasma-enhanced N20 environment. Another method of forming a tungsten oxide ion conductor includes exposing a surface of a tungsten plug to a wet chemical oxidant, such as hydrogen peroxide. This process may be optionally enhanced

using an ultraviolet light. During the formation of ion conductors 114,116, other sections of portion 100 may be covered with an appropriate mask material, such as silicon nitride to prevent oxidation of conductive plugs 122 and the like. Using a portion of the via plugs to form the ion conductor is advantageous because the resultant ion conductor self-aligns with the underlying plug, which forms one of the electrodes.

Alternatively, metal doped oxides may be deposited from a synthesized source which contains all the necessary elements in the correct proportions (e. g., tungsten oxide (W03 with Ag or Cu, nickel oxide with Ag or Cu, molybdenum oxide with Ag or Cu. AgXO (x>2), CuxO (x>2), Ag/Cu-Ge02, Ag/Cu-As203, or Ag/Cu-Si02) or the silver or copper may be introduced into the binary oxide film (tungsten oxide (W03), nickel oxide, molybdenum oxide. Ag20, Cu ,, 2) 0, Ge02, As203, or Si02) by thermal-or photo-dissolution from a thin surface layer of the metal. Alternatively, a base layer of W, Ni, Mo, Ag, Cu, Ge, As, or Si may be deposited first and then reacted with oxygen to form the appropriate oxide and then diffused with Ag or Cu as discussed above. The oxygen reaction could be purely thermal or plasma-assisted, the latter producing a more porous oxide.

In accordance with alternative embodiments of the invention, solid solutions containing dissolved metals may be directly deposited and the electrode then formed overlying the ion conductor. For example, a source including both a metal oxide and conductive material can be used to form ion conductor 114 using physical vapor deposition or similar techniques.

Top electrode 112 and conductive element 140 are then formed overlying layer 128, ion conductors 114,116, and conductive plug 122 using a traditional deposition, pattern, and etch process, such as a process employed to form a metal two layer in a semiconductor device.

Metal from electrode 112 is then introduced into ion conductor regions 114,116 using photo and/or thermal dissolution processing. In accordance with one exemplary embodiment of the invention, silver is introduced into ion conductor regions 114,116 by depositing silver to form electrode 112 and exposing the silver film to light having energy greater than the optical gap of the tungsten oxide,--e. g., light having a wavelength of less than about 500 nanometers.

Any barrier layers 124 and/or 126 may similarly be formed using any suitable oxidation or deposition and etch processes. For example, barrier layer 124 may be formed by additional oxidation of region 114.

Although the process above is described in connection with various deposition and etch processes, those skilled in the art will appreciate that other processes such as damascene and dual damascene processing can be used to form the various elements illustrated in Figure 1.

Figure 2 illustrates a portion 200 of a microelectronic circuit according to another embodiment of the invention. Portion 200 is similar to portion 100, except that portion 200 includes structures 202,204, which include ion conductors 214,216, which are formed by exposing underlying contacts 230,232 to an oxidizing process, such as the process described above in connection with forming regions 114,116. After ion conductor regions 214,216 are formed, insulating material 128 is deposited, vias are formed within the insulating material, and conductive material for electrode 112 and element 140 are deposited overlying layer 128 and into the vias to form top electrode 112, conductive element 140, and plug 122.

As will be appreciated by those skilled in the art, plugs 122,240, 243, electrode 112 and element 140 may alternatively be formed using a damascene or dual damascene process.

Information may be stored using programmable structures of the present invention by manipulating one or more electrical properties of the structures. For example, a resistance of a structure may be changed from a"0"or off state to a"1"or on state during a suitable write operation. Similarly, the device may be changed from a"1"state to a"0" state during an erase operation. In addition, as discussed in more detail below, the structure may have multiple programmable states such that multiple bits of information are stored in a single structure.

WRITE OPERATION Figure 3 illustrates current-voltage characteristics of a programmable structure (e. g. structure 102) in accordance with the present invention. As illustrated, current through structure 102 in an off state (curve 302) begins to rise upon application of a bias of over about one half volt; however, once a write step has been performed (i. e. , conductive region has formed), the resistance through conductor 114 drops significantly (i. e. , to about 700 ohms), illustrated by curve 304. As noted above, when electrode 112 is coupled to a more negative end of a voltage supply, compared to electrode 108, a conductive region 118 begins to form near electrode 112 and grow toward electrode 108.

As noted above, the relative volatility of the memory structures of the present invention may be altered by applying different amounts of energy to the structures during a

write process. For example, a relatively high current pulse of a few hundred microamperes for a period of about several hundred nanoseconds may be applied to the structures illustrated in Figures 1 and 2 to form a relatively nonvolatile memory cell. Alternatively, a smaller current of a few microamperes or less may be supplied to the same or similar memory structure for a shorter amount of time, e. g., several nanoseconds to form a relatively volatile memory structure. In either case, the memory of the present invention can be programmed at relatively high speeds and even the"volatile"memory is relatively nonvolatile compared to traditional DRAM. For example, the volatile memory may operate at speed comparable to DRAM and only require refreshing every several hours.

READ OPERATION A state of a memory cell (e. g., 1 or 0) may be read, without significantly disturbing the state, by, for example, applying a forward or reverse bias of magnitude less than a threshold voltage (about 0.7 V for a structure illustrated in Figure 3) for electrodeposition or by using a current limit which is less than or equal to the minimum programming current (the current which will produce the highest of the on resistance values) which is 500 microamperes in the case of Figure 3. Another way of performing a non-disturb read operation is to apply a pulse, with a relatively short duration, which may have a voltage higher than the electrochemical deposition threshold voltage such that no appreciable Faradaic current flows, i. e., nearly all the current goes to polarizing/charging the device and not into the electrodeposition process.

ERASE OPERATION A programmable structure (e. g., structure 102) may be erased by reversing a bias applied during a write operation, wherein a magnitude of the applied bias is equal to or greater than the threshold voltage for electrodeposition in the reverse direction. Curve 304 illustrates an initiation of the erase process. In accordance with an exemplary embodiment of the invention, a sufficient erase voltage (V > VT) is applied to structure 102 for a period of time, which depends on energy supplied during the write operation, but is typically less than about 1 microsecond to return structure 102 to its"off'state having a resistance well in excess of a million ohms, as illustrated by curve 306. In cases where the programmable structure does not include a barrier between conductor 114 and electrode 108, a threshold voltage for erasing the structure is much lower than a threshold voltage for writing the

structure because, unlike the write operation, the erase operation does not require electron tunneling through a barrier or barrier breakdown. Figure 3 demonstrates erasure at approximately-0.4 volts.

PULSE MODE READ/WRITE In accordance with an alternate embodiment of the invention, pulse mode programming is used to write to and read from a programmable structure. In this case, similar to the process described above, region 118 forms during a write process; however, unlike the process described above, at least a portion region 118 is removed or dissolved during a read operation. During an erase/read process, the magnitude of the current pulse is detected to determine the state (1 or 0) of the device. If the device had not previously been written to or has previously been erased, no ion current pulse will be detected at or above the reduction/oxidation potential of the structure. But, if the device is in a written state, an elevated current will be detected during the destructive read/erase step. Because this is a destructive read operation, information must be written to each structure after each read process-similar to DRAM read/write operations. However, unlike DRAM devices, the structures of the present invention are stable enough to allow a range of values to be stored (e. g., various sizes of region 118). Thus, a partially destructive read that decreases, but does not completely eliminate region 118, can be used. In accordance with an alternate aspect of this embodiment, a destructive write process rather than a destructive erase process can be used. In this case, if the cell is in an"off'state, a write pulse will produce an ion current spike as region 118 forms, whereas a device that already includes a region 118 will not produce the ion current spike if the process has been limited by a lack of oxidizable material.

CONTROL OF OPERATIONAL PARAMETERS The concentration of conductive material in the ion conductor can be controlled by applying a bias across the programmable device. For example, metal such as silver may be taken out of solution by applying a negative voltage in excess of the reduction potential of the conductive material. Conversely, conductive material may be added to the ion conductor (from one of the electrodes) by applying a bias in excess of the oxidation potential of the material. Thus, for example, if the conductive material concentration is above that desired for a particular device application, the concentration can be reduced by reverse biasing the device to reduce the concentration of the conductive material. Similarly, metal may be

added to the solution from the oxidizable electrode by applying a sufficient forward bias.

Additionally, it is possible to remove excess metal build up at the indifferent electrode by applying a reverse bias for an extended time or an extended bias over that required to erase the device under normal operating conditions. Control of the conductive material may be accomplished automatically using a suitable microprocessor.

The threshold voltage of programmable devices may be manipulated in accordance with various embodiments of the present invention. Manipulation of the threshold voltage allows configuration of the programmable devices for desired read and write voltages. In general, as noted above, the threshold voltage depends on, among other things, an amount of conductive material present in the ion conductor and/or any barrier.

One way to manipulate the electrodeposition threshold voltage is to manipulate the conductive material dispersed within the ion conductor material. Another technique for manipulating the threshold voltage is to alter an amount of oxidizable material at or near the indifferent electrode. In this case, the oxidizable metal at the cathode can be altered by first forming an electrodeposit at or near the indifferent electrode and then applying a reverse bias sufficient to dissolve a portion of the electrodeposit. The threshold voltage generally goes down as the amount of oxidizable metal at the cathode goes up. This electrochemical control of the threshold voltage can be used to heal or regenerate an electrodeposit that has been thermally or electrochemically damaged or redistributed.

As noted above, in accordance with yet another embodiment of the invention, multiple bits of data may be stored within a single programmable structure by controlling a size of region 118 which is formed during a write process. A size of region 118 that forms during a write process depends on a number of coulombs or charge supplied to the structure during the write process, and may be controlled by using a current limit power source. In this case, a resistance of a programmable structure is governed by Equation 1, where Ron is the"on"state resistance, VT is the threshold voltage for electrodeposition, and ILIM is the maximum current allowed to flow during the write operation. <BR> <BR> <BR> <BR> <BR> <BR> <P>VT<BR> <BR> <BR> ILIUM Equation 1

In practice, the limitation to the amount of information stored in each cell will depend on how stable each of the resistance states is with time. For example, if a structure with a programmed resistance range of about 3.5 kQ and a resistance drift over a specified time for each state is about 250 Q, about 7 equally sized bands of resistance (7 states) could be formed, allowing 3 bits of data to be stored within a single structure. In the limit, for near zero drift in resistance in a specified time limit, information could be stored as a continuum of states, i. e. , in analog form.

A programmable structure in accordance with the present invention may be used in many applications which would otherwise utilize traditional technologies such as EEPROM, FLASH or DRAM. Advantages provided by the present invention over present memory techniques include, among other things, lower production cost and the ability to use flexible fabrication techniques which are easily adaptable to a variety of applications. The programmable structures of the present invention are especially advantageous in applications where cost is the primary concern, such as smart cards and electronic inventory tags. Also, an ability to form the memory directly on a plastic card is a major advantage in these applications as this is generally not possible with other forms of semiconductor memories.

Further, in accordance with the programmable structures of the present invention, memory elements may be scaled to less than a few square microns in size, the active portion of the device being less than one square micron. This provides a significant advantage over traditional semiconductor technologies in which each device and its associated interconnect can take up several tens of square microns.

Programmable structures and devices and system including the programmable structures described herein are advantageous because the programmable structures require relatively little internal voltage to perform write and erase functions, require relatively little current to perform the write and erase functions, are relatively fast (both write and read operations), require little to no refresh (even for"volatile"memory applications), can be formed in high-density arrays, are relatively inexpensive to manufacture, are robust and shock resistant, and do not require a monocrystalline starting material and can therefore be added to other electronic circuitry.

Although the present invention is set forth herein in the context of the appended drawing figures, it should be appreciated that the invention is not limited to the specific form shown. For example, while the programmable structure is conveniently described above in

connection with programmable memory devices, the invention is not so limited; the structure of the present invention may additionally or alternatively be employed as programmable active or passive devices within a microelectronic circuit. Various other modifications, variations, and enhancements in the design and arrangement of the method and apparatus set forth herein, may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.