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1. WO2007145710 - REFRESHING A PHASE CHANGE MEMORY

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[ EN ]

Refreshing A Phase Change Memory
Background
This invention relates generally to processor-based systems.
Processor-based systems may include any device with a specialized or general purpose processor. Examples of such systems include personal computers, laptop computers, personal digital assistants, cell phones, cameras, web tablets, electronic games, and media devices, such as digital versatile disk players, to mention a few examples.
Conventionally, such devices use either semiconductor memory, hard disk drives, or some combination of the two as storage. One common semiconductor memory is a dynamic random access memory (DRAM). A DRAM is a volatile memory. Without refreshing, it does not maintain the information stored thereon after power is removed. Thus, DRAMs may be utilized as relatively fast storage that operates with microprocessors. One typical application of DRAM is in connection with system memory.
Conventionally, a processor-based system included a variety of different memories or storages. Examples of such systems include hard disk drives, static random access memory, and dynamic random access memory. The more memories that must be plugged into the processor-based system, the more space that is required. Moreover, the more memories that are required, the more overhead that is associated with maintaining those various memories.

In many processor-based systems, especially in embedded applications, it is desirable to implement the systems as cost effectively as possible. Moreover, in a variety of applications, including embedded applications, it may be desirable to implement the systems in the smallest possible size that is possible.
Thus, there is a need for improved processor-based systems.

Brief Description of the Drawings
Figure 1 is a schematic depiction of a portion of an array in one embodiment of the present invention;
Figure 2 is a schematic and cross-sectional view of a cell in accordance with one embodiment of the present invention; and
Figure 3 is a system depiction of one embodiment of the present invention.

Detailed Description
Referring to Figure 1, in one embodiment, a memory 100 may include an array of memory cells MC arranged in rows WL and columns BL in accordance with one embodiment of the present invention. While a relatively small array is illustrated, the present invention is in no way limited to any particular size of an array. While the terms "rows," "word lines," "bit lines," and "columns" are used herein, they are merely meant to be illustrative and are not limiting with respect to the type and style of the sensed array.
The memory device 100 includes a plurality of memory cells MC typically arranged in a matrix 105. The memory cells MC in the matrix 105 may be arranged in m rows and n columns with a word line WLl-WLm associated with each matrix row, and a bit line BLl-BLn associated with each matrix column.
The memory device 100, in one embodiment, may also include a number of auxiliary lines including a supply voltage line Vdd, distributing a supply voltage Vdd through a chip including the memory device 100, that, depending on the specific memory device
embodiment, may be, typically, from 1 to 3 V, for example 1.8 V5 and a ground voltage line GND distributing a ground voltage. A high voltage supply line Va may provide a relatively high voltage, generated by devices (e.g. charge-pump voltage boosters not shown in the drawing) integrated on the same chip, or externally supplied to the memory device 100. For example, the high voltage Va may be 4.5-5 V in one embodiment.
The cell MC may be any memory cell including a phase change memory cell.
Examples of phase change memory cells include those using chalcogenide memory element 18a and an access, select, or threshold device 18b coupled in series to the device 18a. The threshold device 18b may be an ovonic threshold switch that can be made of a chalcogenide alloy that does not exhibit an amorphous to crystalline phase change and which undergoes a rapid, electric field initiated change in electrical conductivity that persists only so long as a holding voltage is present.
A memory cell MC in the matrix 105 is connected to a respective one of the word lines WLl-WLm and a respective one of the bit lines BLl-BLn. In particular, the storage element 18a may have a first terminal connected to the respective bit line BLl-BLn and a second terminal connected to a first terminal of the associated device 18b. The device 18b may have a second terminal connected to a word line WLl-WLm. Alternatively, the storage element 18a may be connected to the respective word line WLl-WLm and the device 18b, associated with the storage element 18a, may be connected to the respective bit line BLl-BLn.
A memory cell MC within the matrix 105 is accessed by selecting the corresponding row and column pair, i.e. by selecting the corresponding word line and bit line pair. Word line selector circuits 110 and bit line selector circuits 115 may perform the selection of the word lines and of the bit lines on the basis of a row address binary code RADD and a column address binary code CADD, respectively, part of a memory address binary code ADD, for example received by the memory device 100 from a device external to the memory (e.g., a microprocessor). The word line selector circuits 110 may decode the row address code RADD and select a corresponding one of the word lines WLl-WLm, identified by the specific row address code RADD received. The bit line selector circuits 115 may decode the column address code CADD and select a corresponding bit line or, more generally, a corresponding bit line packet of the bit lines BLl-BLn. For example, the number of selected bit lines depending on the number of data words that can be read during a burst reading operation on the memory device 100. A bit line BLl-BLn may be identified by the received specific column address code CADD.
The bit line selector circuits 115 interface with read/write circuits 120. The read/write circuits 120 enable the writing of desired logic values into the selected memory cells MC, and reading of the logic values currently stored therein. For example, the read/write circuits 120 include sense amplifiers together with comparators, reference current/voltage generators, and current pulse generators for reading the logic values stored in the memory cells MC.
In one embodiment, in a stand-by operating condition, as well as before any read or write access to the memory device 100, the word line selection circuits 110 may keep the word lines WLl-WLm at a relatively high de-selection voltage Vdes (e.g., a voltage roughly equal to half the high voltage Va (Va/2)). At the same time, the bit line selection circuits 115 may keep the bit lines BLl-BLn disconnected, and thus isolated, from the read/write circuits 120 or, alternatively, at the de-selection voltage Vdes. In this way, none of the memory cells MC is accessed, since the bit lines BLl-BLn are floating or a voltage approximately equal to zero is dropped across the access elements 18b. Spare (redundant) rows and columns may be provided and used with a selection means to replace bad rows, bits, and columns by techniques familiar to those reasonably skilled in the art.
During a reading or a writing operation, the word line selection circuits 110 may lower (or raise if an MOS transistor select device is used) the voltage of the selected one of the word lines WLl-WLm to a word line selection voltage VWL (for example, having a value equal to OV - the ground potential if a bipolar diode or chalcogenide cell, such as an ovonic threshold switch, select device is used), while the remaining word lines may be kept at the word line de-selection voltage Vdes in one embodiment. Similarly, the bit line selection circuits 115 may couple a selected one of the bit lines BLl-BLn (more typically, a selected bit line packet) to the read/write circuits 120, while the remaining, non-selected bit lines may be left floating or held at the de-selection voltage, Vdes. Typically, when the memory device 100 is accessed, the read/write circuits 120 force a suitable current pulse into each selected bit line BLl-BLn. The pulse amplitude depends on the reading or writing operations to be performed.
In particular, during a reading operation a relatively high read current pulse is applied to each selected bit line in one embodiment. The read current pulse may have a suitable amplitude and a suitable time duration. The read current causes the charging of stray capacitances CBLI— CBLΠ (typically, of about 1 pF), intrinsically associated with the parasitic bit lines BLl-BLn and column drive circuitry, and, accordingly, a corresponding transient of a bit line voltage VBL at each selected bit line BLl-BLn. When the read current is forced into each selected bit line BLl-BLn, the. respective bit line voltage raises towards a corresponding steady-state value, depending on the resistance of the storage element 18a, i.e., on the logic value stored in the selected memory cell MC. The duration of the transient depends on the state of the storage element 18a. If the storage element 18a is in the crystalline state and the threshold device 18b is switched on, a cell current flowing through the selected memory cell MC when the column is forced to a voltage that has an amplitude greater than the amplitude in the case where the storage element 18a is in the higher resistivity or reset state, and the resulting voltage on the column line when a constant current is forced is lower for a set state relative to reset state.

The logic value stored in the memory cell MC may, in one embodiment, be evaluated by means of a comparison of the bit line voltage (or another voltage related to the bit line voltage) at, or close to, the steady state thereof with a suitable reference voltage, for example, obtained exploiting a service reference memory cell in an intermediate state or its equivalent. The reference voltage can, for example, be chosen to be an intermediate value between the bit line voltage when a logic value "0" is stored and the bit line voltage when a logic value "1" is stored.
In order to avoid spurious reading of the memory cells MC, the bit line stray capacitances CBLI— CB may be discharged before performing a read operation. To this purpose, bit line discharge circuits 125j— 125n are provided, associated with the bit lines BLl-BLn. The bit line discharge circuits 125!-12Sn may be enabled in a bit line discharge phase of the memory device operation, preceding and after any operation, for discharging the bit line stray capacitances CBLI— CBLΠ, in one embodiment.
The bit line discharge circuits 125i— 125n may be implemented by means of transistors, particularly N-chaπnel MOSFETs having a drain terminal connected to the corresponding bit line BLl-BLn, a source terminal connected to a de-selection voltage supply line Vdes providing the de-selection voltage Vdes and a gate terminal controlled by a discharge enable signal DIS_EN in one embodiment. Before starting a writing or a reading operation, the discharge enable signal DIS_EN may be temporarily asserted to a sufficiently high positive voltage, so that all the discharge MOSFETs turn on and connect the bit lines BLl-BLn to the de-selection voltage supply line Vdes. The discharge currents that flow through the discharge transistors cause the discharge of the bit line stray capacitances CBLI— CB for reaching the de-selection voltage Vdes. Then, before selecting the desired word line WLl-WLm, the discharge enable signal DIS_EN is de-asserted and the discharge MOSFETs turned off. Similarly, the selected row and column lines may be respectively pre-charged to an appropriate safe starting voltage for selection and read or write operation.
A control 32 may be a programmable device to control reading and writing of cells. The control 32 may include a refresh circuit 12. In some embodiments, the circuit 12 may also be programmable. The refresh cycle may be implemented automatically on timed intervals or event detection, to mention two embodiments.

Referring to Figure 2, a cell MC in the array 105 may be formed over a substrate 36. The substrate 36, in one embodiment, may include the conductive word line 52 coupled to a selection device 18b. The selection device 18b, in one embodiment, may be formed in the substrate 36 and may, for example, be a diode, transistor, or a non-programmable
chalcogenide selection device formed as a thin film alloy above the substrate.
The selection device 18b may be formed of a non-programmable chalcogenide material including a top electrode 71, a chalcogenide material 72, and a bottom electrode 70. The selection device 18b may be permanently in the reset state in one embodiment. While an embodiment is illustrated in which the selection device 18b is positioned over the phase change memory element 18a, the opposite orientation may be used as well.
Conversely, the phase change memory element 18a may be capable of assuming either a set or reset state, explained in more detail hereinafter. The phase change memory element 18a may include an insulator 62, a phase change memory material 64, a top electrode 66, and a barrier film 68, in one embodiment of the present invention. A lower electrode 60 may be defined within the insulator 62 in one embodiment of the present invention.
In one embodiment, the phase change material 64 may be a phase change material suitable for non-volatile memory data storage. A phase change material may be a material having electrical properties (e.g., resistance) that may be changed through the application of energy such as, for example, heat, light, voltage potential, or electrical current.
Examples of phase change materials may include a chalcogenide material or an ovonic material. An ovonic material may be a material that undergoes electronic or structural changes and acts as a semiconductor once subjected to application of a voltage potential, electrical current, light, heat, etc. A chalcogenide material may be a material that includes at least one element from column VI of the periodic table or may be a material that includes one or more of the chalcogen elements, e.g., any of the elements of tellurium, sulfur, or selenium. Ovonic and chalcogenide materials may be non-volatile memory materials that may be used to store information.
In one embodiment, the memory material 64 may be chalcogenide element composition from the class of tellurium-germanium-antϊmony (TexGeySbz) material or a GeSbTe alloy, although the scope of the present invention is not limited to just these materials.

In some embodiments of the present invention, a chalcogenide alloy that rapidly crystallizes may be utilized as the memory material 64. Relatively fast set and reset operations permit replacement of dynamic random access memory by a phase change memory. Data retention lifetime or the stability of the reset state at room temperature can be sacrificed in order to achieve rapid programming of the set state since the dynamic random access memory replacement device is not intended to be a non-volatile memory cell.
In some embodiments of the present invention, the memory material 64 may have a crystallization speed sufficiently high to enable the programming of the set state in 10 nanoseconds or less. The data retention may be sacrificed to achieve these speeds.
In some embodiments, the data retention issue may be corrected by using a refresh, as is typically done in dynamic random access memories. At periodic intervals, the program state may be refreshed by reapplying programming signals. For example., the refresh may be done at time intervals, such as once an hour, or on event detection such as the booting of a processor-based system that includes the memory material 64.
A number of suitable chalcogenide materials may be obtained from different regions of the GST ternary phase diagram. The first region is on the far side of the GeTe — SB2Te3 pseudo-binary tie-line at or near the Sb2Te3 point. A second region of rapidly crystallizing phase change memory materials is near the SbTe eutectic point at Sb69Te31 and may include addition of other dopant elements such as Ag, In, Ge, or Sn. A third region is along the GeSb line. Examples of potential suitable alloys include the following:

Calculated
60 nm Archival
125 n m Laser Spot OUM Life
Composition Erase Time SET Time @ 50C
(ns) (ns)
In5Sb71Te24 49 24
In5Sb74Te21 36 17
In5Sb77Tel8 25 12
In5Sb80Tel5 18 9
Ag8Sb72Te20 42 20 100 days
Ge8Sb72Te20 33 16 —
In8Sb72Te20 29 14 Ih
Ga8Sb72Te20 20 10 2 h
Sn8Sb72Te20 17 8 ~ 1 day
Gel2Sb88 13 6 —
Gel5Sb85 15 7 lE6 yr
Ge22Sb78 23 11 > lE14 yr - S -

See L. van Pietersen et al. (J. Appl. Phys. Vol. 9 (2005) 083520).
These alloys may be unsuitable for conventional phase change memories because of thermal instability in the amorphous state. Cells that have been programmed to the reset state are periodically refreshed to reverse the thermal crystallization, that occurs spontaneously during device operation using alloys such as those described above with poor data retention.

In one embodiment, if the memory material 64 is a non- volatile, phase change material, the memory material may be programmed into one of at least two memory states by applying an electrical signal to the memory material. An electrical signal may alter the phase of the memory material between a substantially crystalline state and a substantially amorphous state, wherein the electrical resistance of the memory material 64 in the substantially amorphous state is greater than the resistance of the memory material in the substantially crystalline state. Accordingly, in this embodiment, the memory material 64 may be adapted to be altered to a particular one of a number of resistance values within a range of resistance values to provide digital or analog storage of information.
Programming of the memory material to alter the state or phase of the material may be accomplished by applying voltage potentials to the lines 52 and 54 or forcing a current of adequate amplitude to melt the material, thereby generating a voltage potential across the memory material 64. An electrical current may flow through a portion of the memory material 64 in response to the applied voltage potentials or current forced, and may result in heating of the memory material 64.
This heating and subsequent cooling may alter the memory state or phase of the memory material 64. Altering the phase or state of the memory material 64 may alter an electrical characteristic of the memory material 64. For example, resistance of the material 64 may be altered by altering the phase of the memory material 64. The memory material 64 may also be referred to as a programmable resistive material or simply a programmable resistance material.
In one embodiment, a voltage potential difference of about .5 to 1.5 volts maybe applied across a portion of the memory material by applying about 0 volts to a line 52 and about 0.5 to 1.5 volts to an upper line 54. A current flowing through the memory material 64 in response to the applied voltage potentials may result in heating of the memory material. This heating and subsequent cooling may alter the memory state or phase of the material.

In a "reset" state, the memory material may be in an amorphous or semi-amorphous state and in a "set" state, the memory material may be in a crystalline or semi-crystalline state. The resistance of the memory material in the amorphous or semi-amorphous state may be greater than the resistance of the material in the crystalline or semi-crystalline state. The association of reset and set with amorphous and crystalline states, respectively, is a convention. Other conventions may be adopted.
Due to electrical current, the memory material 64 may be heated to a relatively higher temperature to amorphisize memory material and "reset" memory material. Heating the volume or memory material to a relatively lower crystallization temperature may crystallize memory material and "set" memory material. Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material, or by tailoring the edge rate of the trailing edge of the programming current or voltage pulse, such as by using a trailing edge rate of less than 100 nsec to reset the bit or a trailing edge greater than 500 nsec to set the bit.
The information stored in memory material 64 may be read by measuring the resistance of the memory material. As an example, a read current may be provided to the memory material using opposed lines 54, 52 and a resulting read voltage across the memory material may be compared against a reference voltage using, for example, the sense amplifier 20. The read voltage may be proportional to the resistance exhibited by the memory storage element.
In order to select a cell MC on column 54 and row 52, the selection device 18b for the selected cell MC at that location may be operated. The selection device 18b activation allows current to flow through the memory element 18a in one embodiment of the present invention.

In a low voltage or low field regime A, the device 18b is off and may exhibit very high resistance in some embodiments. The off resistance can, for example, range from 100,000 ohms to greater than 10 gigaohms at a bias of half the threshold voltage, such as about 0.4V. The device 18b may remain in its off state until a threshold voltage Vτ or threshold current Iτ switches the device 18b to a highly conductive, low resistance on state. The voltage across the device 58 after turn on drops to a slightly lower voltage relative to Vthreshold, called the holding voltage VH and remains very close to the threshold voltage. In one embodiment of the present invention, as an example, the threshold voltage may be on the order of 1.1 volts and the holding voltage may be on the order of .9 volts.

Afler passing through the snapback region, in the on state, the device 18b voltage drop remains close to the holding voltage as the current passing through the device is increased up to a certain, relatively high, current level. Above that current level the device remains on but displays a finite differential resistance with the voltage drop increasing with increasing current. The device 18b may remain on until the current through the device 18b is dropped below a characteristic holding current value that is dependent on the size and the material utilized to form the device 18b.
In some embodiments of the present invention, the selection device 18b does not change phase. It remains permanently amorphous and its current- voltage characteristics may remain the same throughout its operating life.
As an example, for a .5 micrometer diameter device 18b formed of TeAsGeSSe having respective atomic percents of 16/13/15/1/55, the holding current may be on the order of 0.1 to 100 micro-ohms in one embodiment. Below this holding current, the device 18b turns off and returns to the high resistance regime at low voltage, low field. The threshold current for the device 18b may generally be of the same order as the holding current. The holding current may be altered by changing process variables, such as the top and bottom electrode material and the chalcogenide material. The device 18b may provide high "on current" for a given area of device compared to conventional access.devices such as metal oxide semiconductor field effect transistors or bipolar junction transistors.
In some embodiments, the higher current density of the device 18b in the on state allows for higher programming current available to the memory element 18a. Where the memory element 18a is a phase change memory, this enables the use of larger programming current phase change memory devices, reducing the need for sub-lithographic feature structures and the commensurate process complexity, cost, process variation, and device parameter variation.
One technique for addressing the array 12 uses a voltage V applied to the selected column and a zero voltage applied to the selected row. For the case where the device 56 is a phase change memory, the voltage V is chosen to be greater than the device 18b maximum threshold voltage plus the memory element 18a reset maximum threshold voltage, but less than two times the device 18b minimum threshold voltage. In other words, the maximum threshold voltage of the device 18b plus the maximum reset threshold voltage of the device 18a may be less than V and V may be less than two times the minimum threshold voltage of - li the device 18b in some embodiments. All of the unselected rows and columns may be biased at V/2.
With this approach, there is no bias voltage between the unselected rows and unselected columns. This reduces background leakage current.
After biasing the array in this manner, the memory elements 18a may be programmed and read by whatever means is needed for the particular memory technology involved. A memory element 18a that uses a phase change material may be programmed by forcing the current needed for memory element phase change or the memory array can be read by forcing a lower current to determine the device 18a resistance.
For the case of a phase change memory element 18a, programming a given selected bit in the array 105 can be as follows. Unselected rows and columns may be biased as described for addressing. Zero volts is applied to the selected row. A current is forced on the selected column with a compliance that is greater than the maximum threshold voltage of the device 18b plus the maximum threshold voltage of the device 18a. The current amplitude, duration, and pulse shape may be selected to place the memory element 18a in the desired phase and thus, the desired memory state.
Reading a phase change memory element 18a can be performed as follows.
Unselected rows and columns may be biased as described previously. Zero volts is applied to the selected row. A voltage is forced at a value greater than the maximum threshold voltage of the device 18b, but less than the minimum threshold voltage of the device 18b plus the minimum threshold voltage of the element 18a on the selected column. The current compliance of this forced voltage is less than the current that could program or disturb the present phase of the memory element 18a. If the phase change memory element 18a is set, the access device 18b switches on and presents a low voltage, high current condition to a sense amplifier. If the device 18a is reset, a larger voltage, lower current condition may be presented to the sense amplifier. The sense amplifier can either compare the resulting column voltage to a reference voltage or compare the resulting column current to a reference current.
The above-described reading and programming protocols are merely examples of techniques that may be utilized. Other techniques may be utilized by those skilled in the art.

To avoid disturbing a set bit of memory element 18a that is a phase change memory, the peak current may equal the threshold voltage of the device 18b minus the holding voltage of the device 18b that quantity divided by the total series resistance including the resistance of the device 18b, external resistance of device 18a, plus the set resistance of device 18a. This value may be less than the maximum programming current that will begin to reset a set bit for a short duration pulse.
Turning to Figure 3, a portion of a system 500 in accordance with an embodiment of the present invention is described. System 500 may be used in wireless devices such as, for example, a cellular telephone, personal digital assistant (PDA), a laptop or portable computer with wireless capability, a web tablet, a wireless telephone, a pager, an instant messaging device, a digital music player, a digital camera, or other devices that may be adapted to transmit and/or receive information wirelessly. System 500 may be used in any of the following systems: a wireless local area network (WLAN) system, a wireless personal area network (WPAN) system, or a cellular network, although the scope of the present invention is not limited in this respect.
System 500 may include a controller 510, an input/output (LO) device 520 (e.g. a keypad, display), a memory 530, and a wireless interface 540, coupled to each other via a bus 550. A battery 580 may supply power to the system 500 in one embodiment. It should be noted that the scope of the present invention is not limited to embodiments having any or all of these components.
Controller 510 may comprise, for example, one or more microprocessors, digital signal processors, micro-controllers, or the like. Memory 530 maybe used to store messages transmitted to or by system 500. Memory 530 may also optionally be used to store instructions that are executed by controller 510 during the operation of system 500, and may be used to store user data. The instructions may be stored as digital information and the user data, as disclosed herein, may be stored in one section of the memory as digital data and in another section as analog memory. As another example, a given section at one time may be labeled as such and store digital information, and then later may be relabeled and
reconfigured to store analog information. Memory 530 may be provided by one or more different types of memory. For example, memory 530 may comprise a volatile memory (any type of random access memory), a non- volatile memory such as a flash memory, and/or phase change memory that includes a memory element 18a such as, for example, memory 100 illustrated in Figure 1.

The I/O device 520 may be used to generate a message. The system 500 may use the wireless interface 540 to transmit and receive messages to and from a wireless
communication network with a radio frequency (RF) signal. Examples of the wireless interface 540 may include an antenna, or a wireless transceiver, such as a dipole antenna, although the scope of the present invention is not limited in this respect. Also, the FO device 520 may deliver a voltage reflecting what is stored as either a digital output (if digital information was stored), or it may be analog information (if analog information was stored).

While an example in a wireless application is provided above, embodiments of the present invention may also be used in non- wireless applications as well.
m some embodiments, the phase change memory may be more effectively embedded with other circuits, such as logic, because phase change memories may have fewer layers. Dynamic random access memory, for one, requires the addition of layers that are not needed by logic. In some cases, dynamic random access memory may require 10 to 15
semiconductor layers. These layers may double the number of layers actually needed by other memories, such as phase change memories. AU the layers must be provided throughout the chip, even if they are only utilized by 10 to 15 percent of the chip. Thus, many advantages may be achieved by providing a plug-in replacement for a dynamic random access memory via a phase change memory.
In some embodiments of the present invention, the system 500 may notice no difference from the use of the phase change memory instead of a DRAM. In other words, the system 500 may have been designed to use dynamic random access memory, but a phase change memory may be effectively utilized in its stead. This may achieve a variety of advantages as described above and other advantages not set forth herein.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
What is claimed is: