Patent Document

This application is a divisional of application Ser. No. 09/894,922, filed Jun. 28, 2001, entitled “Electrically Programmable Resistance Cross Point Memory,” invented by Sheng Teng Hsu, and Wei-Wei Zhuang, now U.S. Pat. No. 6,531,371. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to nonvolatile memory, and more particularly to a cross point structure utilizing electric pulse induced resistance change effects in magnetoresistive films. 
     Materials having a perovskite structure, among them colossal magnetoresistance (CMR) materials and high temperature superconductivity (HTSC) materials are materials that have electrical resistance characteristics that can be changed by external influences. 
     For instance, the properties of materials having perovskite structures, especially for CMR and HTSC materials, can be modified by applying one or more short electrical pulses to a thin film or bulk material. The electric field strength or electric current density from the pulse, or pulses, is sufficient to switch the physical state of the materials so as to modify the properties of the material. The pulse is of low enough energy so as not to destroy, or significantly damage, the material. Multiple pulses may be applied to the material to produce incremental changes in properties of the material. One of the properties that can be changed is the resistance of the material. The change may be at least partially reversible using pulses of opposite polarity from those used to induce the initial change. 
     SUMMARY OF THE INVENTION 
     Accordingly, a memory structure is provided, which comprises a substrate, a plurality of bottom electrodes overlying the substrate, a plurality of top electrodes overlying the bottom electrodes, and a continuous active layer interposed between the plurality of top electrodes and the plurality of bottom electrodes. The plurality of top electrodes and the plurality of bottom electrodes form a cross point memory structures. A region of the active layer located at each cross point acts as a variable resistor. Each region serves as a bit within the memory structure. 
     The resistivity of a bit within the memory structure can be changed by a method comprising the following steps. Providing the bit formed at the cross point of a word line and a bit line with a perovskite active layer interposed between the word line and the bit line. The bit line is connected through a bit pass transistor having a bit gate, to a load transistor, having a load gate, connected to ground. By applying a programming voltage to the word line, and a first on voltage to the bit gate, current is allowed to flow through the bit pass transistor. By applying another on voltage to the load gate, current is allowed to flow through the load transistor. This allows the current to flow through the active layer to change the resistivity of the bit. Depending on the polarity of the programming voltage, the resistivity of the bit can be increased or decreased. The on voltages applied to the bit gate and the load gate will be different for different polarities of programming voltage. 
     The value of a bit can be determined by providing the bit formed at the cross point of a word line and a bit line with a perovskite active layer interposed between the word line and the bit line. The bit line is connected through a bit pass transistor, having a bit gate, to an inverter, with a load transistor, having a load gate, connected between the inverter and ground. Applying a load voltage to the load gate sets a threshold, whereby current above a saturation current of the load transistor is allowed to flow through the load transistor, and current below the saturation current will not flow through the load transistor. Applying an on voltage to the bit gate selects the bit pass transistor, which determines the bit to be read. Applying a readout voltage to the word line causes current to flow through the bit corresponding to the cross point of the word line and the bit line, which has been selected by applying an on voltage at the bit gate. The current flows through the bit pass transistor. If the current exceeds the saturation current of the load transistor, current will pass through the load transistor and the inverter will produce an output voltage of approximately zero volts. If the current is less than the saturation current, the current will not flow through the load transistor and the output voltage will be equal to an operation voltage of the inverter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a cross point memory array area. 
         FIG. 2  is a schematic view of a memory readout circuit connected to a cross point memory array area. 
         FIG. 3  is a schematic view of a cross point memory device with readout circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a cross point memory array area  10 . The memory array area  10  comprises a substrate  12  with a plurality of bottom electrodes  14  formed thereon. An active layer  16  has been deposited overlying the plurality of bottom electrodes  14 . A plurality of top electrodes  18  overly the active layer  16 , such that the active layer  16  is interposed between the bottom electrodes  14  and the top electrodes  18 . 
     The top electrodes  18  and the bottom electrodes  14  are each preferably substantially parallel rows. The top electrodes  18  and the bottom electrodes  14  are arranged in a cross point arrangement such that they cross each other in a regular pattern. A cross point refers to each position where a top electrode crosses a bottom electrode. As shown, the top electrodes and the bottom electrodes are arranged at substantially 90 degrees with respect to each other. The top electrodes and the bottom electrodes can each function as either word lines or bit lines as part of a cross point memory array. 
       FIG. 1  shows just the memory array area. It should be clear that in an actual device, the substrate  12 , the bottom electrodes  14  and the top electrodes  18  may extend well beyond the memory array area, which is defined by the active layer  16 . The active layer is substantially continuous, such that the active layer extends across more than one cross point. 
     The substrate  12  is any suitable substrate material, whether amorphous, polycrystalline or crystalline, such as LaAlO 3 , Si, TiN or other material. 
     The bottom electrodes  14  are made of conductive oxide or other conductive material. In a preferred embodiment, the conductive material is a material, such as YBa 2 Cu 3 O 7  (YBCO), that allows the epitaxial growth of an overlying perovskite material. In another preferred embodiment, the conductive material is platinum. The bottom electrodes are a thickness in the range of between about 5 nm and about 500 nm. In a preferred embodiment, the bottom electrodes  14  are formed by forming a trench, depositing the conductive material and polishing the conductive material until level with the substrate. The polishing can be accomplished using chemical mechanical polishing (CMP) or other suitable means. Alternatively, the bottom electrodes may be deposited and patterned without first forming a trench and without polishing. 
     The active layer  16  is a material capable of having its resistivity changed in response to an electrical signal. The active material is preferably a perovskite material, such as a colossal magnetoresistive (CMR) material or a high temperature superconducting (HTSC) material, for example Pr 0.7 Ca 0.3 MnO 3  (PCMO). Another example of a suitable material is Gd 0.7 Ca 0.3 BaCo 2 O 5+5 . The active layer is preferably between about 5 nm and 500 nm thick. The active layer  16  can be deposited using any suitable deposition technique including pulsed laser deposition, rf-sputtering, e-beam evaporation, thermal evaporation, metal organic deposition, sol gel deposition, and metal organic chemical vapor deposition. The active layer is removed from outside the memory array area by ion milling or other suitable process. It is also possible to form a large recessed area to deposit perovskite material over and then use chemical mechanical polishing (CMP) to form an active layer  16 . 
     The top electrodes  18  comprise a conductive material, preferably platinum, copper, silver, or gold. 
     Referring now to  FIG. 2 , a memory device  20  comprising the memory array area  10  connected to a memory circuit  22  is shown. The memory circuit  22  comprises at least one bit pass transistor  24  connected to at least one load transistor  26  and at least one inverter  28 . These structures are shown schematically, as the formation of the individual semiconductor elements are well known. 
     In a preferred embodiment of a method of making the memory device  20 , one, or more, of transistor structures, interconnects or other components of the memory circuit  22  may be formed prior to the formation of the memory array area  10 . By forming components of the memory circuit  22  prior to the memory array area  10 , possible degradation of the active layer due to subsequent processing is reduced, or eliminated. 
     Referring again to  FIG. 1 , the active layer is shown with a region  40  (shown by a dashed circle) to illustrate the region as transparent for illustration purposes. A bit region  42  is shown. The bit region  42  is a portion of the active layer  16  interposed between the bottom electrodes  14  and the top electrodes  18  such that an electrical signal passing between the top and bottom electrodes passes primarily through the bit region. Each bit region corresponds to a cross point. Under normal operation, the bit region  42  is formed in the active layer by having its resistivity changed in response to an electrical signal. A bulk region  44  of the active layer  16  is contiguous with the bit region  42 . That portion of the active layer  16  that is not changed by an electrical signal during normal operation forms the bulk region  44 . The bit region  42  acts as a variable resistor that can be changed between at least two resistivity values. Changes to the resistivity of the bit region  42  are preferably reversible. The reversibility of the resistivity change may incorporate some hysteresis. For some applications, such as write once read many (WORM) the resistivity change need not be reversible at all. 
     For example, if the bit region  42  has a cross sectional area of one micrometer by one micrometer and the active layer is YBCO deposited to a thickness of 60 nm, the high resistance state is approximately 170 MΩ and the low resistance state is approximately 10 MΩ. For a low voltage memory device, if the bit region  42  is biased to 1 volt, the current through the bit will be approximately 6 nA for the high resistance state and approximately 100 nA for the low resistance state. This example has been provided for illustration purposes only. The resistance values will change depending upon the active layer thickness and material, as well as the cross sectional area of the bit itself. The voltage applied across the bit will further affect the current through the bit. 
       FIG. 3  shows a schematic diagram of a 16 bit, 4×4-memory array, memory block  20 . The memory block  20  comprises the memory array area  10  connected to the memory circuit  22 . In this schematic view the active layer is shown as being an array of resistors connected between the lower electrodes  14 , which are also designated as bit lines B 1  through B 4 , and the upper electrodes  18 , which are also designated as word lines W 1  through W 4 . Alternatively, the lower electrodes could be the word lines and the upper electrodes could be the bit lines. The bit lines are connected to the memory circuit  22 . As shown, the lower electrodes are bit lines, so the lower electrodes are connected to the memory circuit  22 . 
     Looking at the memory array area  10 , each bit  50  can be treated as comprising primarily a bit resistor  52  with an accompanying bulk resistor  54  in parallel. This array does not require a gated transistor for each bit. There is also no need for a separate capacitor as any data value is stored using a changing resistance of each bit resistor  52 . The total resistance of each bit is going to be controlled primarily by the bit resistor  52 , which acts as a variable resistor. The bit resistor  52  has a resistance that can be changed between at least two values in response to an electrical signal, including a high resistance state and a low resistance state. Preferably, the bulk resistor  54  will have a higher resistance than the bit resistor  52 , especially when the bit resistor is in a low resistance state. 
     Referring now to the memory circuit  22 , each bit line is connected to the bit pass transistor  24 . The bit pass transistor  24  has a bit pass gate  64 . The bit pass gate  64  contributes to determining which bit is being programmed or read out. The bit pass transistor is connected to the load transistor  26 , which has a load gate  66 , and the inverter  28 . The load transistor is used to determine which memory block is being programmed or read out. The inverter is used in combination with the load transistor to set the output between two voltage levels, so that a binary state can be read out. 
     Referring again to the memory array area, the active layer will preferably have a higher resistivity than the resistivity of the low resistance state of the bit region, which corresponds to the bit resistor  52 . If necessary, the resistivity of the active layer can be increase by applying one or more electrical pulses to the active layer during manufacturing. 
     Once a device is completed and in operation, it can be programmed and read. It may also be desirable to set all of the bit resistors  52 , especially those along a single word line, to the same resistance level either high resistance or low resistance. This may be used to produce a word erase or a block erase. For example, if n-channel transistors are used for the pass transistor and the load transistor, applying a negative voltage, or a plurality of negative voltage pulses, to a word line (e.g. W 1 ) and grounding the bit pass gate  64  and the load transistor gate  66  of the memory block  20 , sets all bit resistors  52  at the cross point of the word line to the same resistance state, either high resistance or low resistance. It would also be possible to use positive voltages at the word line, provided the bit pass gate and the load gate are properly biased to allow current to flow through the bit. 
     In another embodiment, p-channel transistors may be used for the bit pass transistor and the load transistor. In which case a positive voltage could be applied to the word line while grounding the bit pass gate and the load gate. A negative voltage pulse may be used provided that a sufficiently negative voltage is applied to the bit pass gate and the load gate to allow current to flow through the bit. 
     The applied voltage, or the plurality of voltage pulses, is preferably at a level that will not damage the active layer material. Preferably, all bit resistors  52  at the cross point of the word line will be set to the high resistance level. If a single pulse is not sufficient to change the resistivity of the bit region, multiple voltage pulses, at a level lower than the level at which the active layer would be damaged, can be used to affect the change without damaging the active layer. By repeating the process with the remaining word lines, the entire memory block can be set to the same state. 
     The bit  50  can be programmed by applying an on voltage to the bit pass gate  64 , applying a second on voltage to the load gate  66 , and applying at least one programming voltage pulse to the word line. The voltage pulse applied to the word line is the opposite polarity to the polarity used for the word, or block, erase, such that the resistivity of the bit resistor  52  is changed to the opposite resistivity state. If n-channel transistors are used as described above in one embodiment, the programming pulse will be positive and the resistance of the bit resistor  52  will preferably change from a high resistance state to a low resistance state. 
     The bit pass gate  64  of any unselected bits and the load transistor gate  66  of any unselected memory blocks  20  are connected to ground. Any voltage at the cross point of the word line and bit line will be very small, such that no significant change in resistance will occur at unselected bits. 
     As discussed above, the polarity and the voltage applied at the word line, the bit pass gate, and the load gate can be selected depending on whether n-channel or p-channel transistors are used to obtain the desired behavior of the memory circuit. 
     The bit  50  can be read. A load voltage is applied to the load gate  66 . The load voltage is smaller than the threshold voltage of the load transistor  26 . In addition, at this load voltage the saturation current of the load transistor  26  is larger than the current flow through the bit  50  when it is at a high resistance level. But, at this load voltage the saturation current of the load transistor  26  is lower than the current flow through the bit  50  when it is at a low resistance level. The bit pass gate  64  is held at a voltage sufficient to allow current to flow through the bit pass transistor  24 , for example V cc . A readout voltage is applied to the word line. The voltage applied to the word line is preferably a pulse with a voltage lower than the critical voltage necessary to change the resistivity of the bit resistor  52 , and correspondingly the resistivity of the bit  50 . 
     If the bit resistor  52  is at a high resistance state, the current flow through the bit  50  is smaller than the saturation current of the load transistor  26 . The bit line voltage is then lower than the threshold voltage of an n-channel transistor at an input of the inverter  28 . The output voltage of the inverter is then equal to approximately its power supply voltage. 
     If the bit resistor  52  is at a low resistance state, such that the bit  50  is at a low resistance state, a large current tends to flow through the bit  50 . This large current is larger than the saturation current of the load transistor. The bit line voltage is larger than the threshold voltage of an n-channel transistor at an input of the inverter  28 . The output voltage of the inverter is then equal to approximately zero volts, which corresponds to ground. 
     Using the example discussed above, the current through the bit is expected to be between 6 nA and 100 nA. The bias voltage applied at the load gate of the load transistor should be selected so that the saturation current of the load transistor is between 6 nA and 100 nA, for example 50 nA. If the resistance of the bit is high enough that the current through it is less than 50 nA current will not flow through the load transistor and the output of the inverter will go to the operation voltage, for example Vcc. If the resistance of the bit is low, so that more than 50 nA flow through it, the current will flow through the load transistor and the output of the inverter will go to approximately 0 volts, or ground. If it is desired to have the bit at high resistance correspond to 0 volts, and the bit at low resistance correspond to the operation voltage, an additional inverter can be added at the output of the inverter. 
     Although a preferred embodiment, and other embodiments have been discussed above, the coverage is not limited to these specific embodiments. Rather, the claims shall determine the scope of the invention.

Technology Category: g