Temperature adaptive ferro-electric memory access parameters

Briefly, one or more memory access parameters used to access a memory cell are adjusted based on a sensed operating temperature. In one embodiment, a pulse width of an access voltage is increased as the operating temperature decreases below a threshold. In another embodiment, a drive voltage is decreased as the operating temperature increases.

BACKGROUND

Description of the Related Art

In some memories, operation requirements, for example, a voltage level needed to access a cell, may change with temperature. In order to improve memory performance, system designers are continually searching for alternate ways to access memories under a wide range of temperatures.

DESCRIPTION OF THE EMBODIMENT(S)

In the following description and claims, the terms “include” and “comprise,” along with their derivatives, may be used, and are intended to be treated as synonyms for each other. In addition, in the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.

In the following description and claims, the term “data” may be used to refer to both data and instructions. In addition, the term “information” may be used to refer to data and instructions.

FIG. 1is a diagram illustrating a portion of one type of a ferroelectric memory cell10that may be used in embodiments of the present invention. Ferroelectric memory is a type of non-volatile memory that utilizes the ferroelectric behavior of certain materials to retain data in a memory device in the form of positive and negative polarization, even in the absence of electric power. A ferroelectric material16may contain domains of similarly oriented electric dipoles that retain their orientation unless disturbed by some externally imposed electric force. The polarization of the material characterizes the extent to which these domains are aligned. The polarization can be reversed by the application of an electric field of sufficient strength and polarity.

Ferroelectric material16may be a ferroelectric polymer polarizable material, and may also be referred to as a ferroelectric polarizable material or a dipole ferroelectric material. In various embodiments, the ferroelectric polymer material may comprise a polyvinyl fluoride, a polyethylene fluoride, a polyvinyl chloride, a polyethylene chloride, a polyacrylonitrile, a polyamide, copolymers thereof, or combinations thereof. Another example of a ferroelectric material may include a ferroelectric oxide material.

Ferroelectric material16having a polarization P may be located between a conductive word line (W/L)20and a conductive bit line (B/L)22. An electric field may be applied to the ferroelectric cell by applying an electric potential (voltage) between word line20and bit line22so as to effect changes in the polarization of ferroelectric material16.

FIG. 2shows a simplified hysteresis curve24that illustrates idealistically the polarization versus voltage properties of the ferroelectric cell ofFIG. 1. When a positive voltage (e.g., Vbit line−Vword line>0) of sufficiently large magnitude (shown here, for example, as Vs) is applied to the cell, all of the domains in the cell are forced to align, to the extent possible, in the positive direction, and the polarization P reaches the saturation polarization Psat at point25on the curve. A further increase in the voltage produces no further increase in the polarization because all of the domains are already aligned as far as possible in the direction of the electric field produced by the voltage between the word line and bit line. In one example, a positive voltage may be applied by applying Vs, for example, about 12 volts, to bit line22and applying about zero volts to word line20. In another example, two non-zero positive voltages may be applied to bit line22and word line20to generate a positive voltage across material16. Note that the voltages applied may very with specific implementations and the scope of the present invention is not limited in this respect.

If the voltage is then reduced to zero (following path32to arrive at point21), some of the domains switch their orientation (also referred to as rotating, flipping or reversing), but most of the domains retain their orientation. Thus, the ferroelectric material retains a remnant polarization Pr.

For purposes of data storage, ferroelectric cell10is considered to be in the logic “0” (zero) state when the polarization P is positive (preferably at Pr), and the logic “1” (one) state when the polarization is negative (preferably at −Pr). The assignment of a logic “1” or logic “0” to a positive or negative polarization is arbitrary, and in other embodiments, opposite conventions may be used.

If a negative voltage of sufficiently large magnitude (shown here, for example, as −Vs) is then applied to the word line20relative to bit line22(following path34to point27), all of the domains are forced to switch their orientation, and the polarization reaches the negative saturation level −Psat. Removing this negative voltage (following path36to point23) allows some of the domains to switch, and the cell polarization reaches the negative remnant polarization −Pr. If the positive voltage Vs is again applied to the cell (following path30to point25), the domains once again switch their orientation, and the cell takes on the positive saturation polarization Psat until the voltage is removed, and the polarization reaches the positive saturation level +Pr.

Ferroelectric materials also exhibit resilience, wherein a ferroelectric cell may return close to its remnant polarization despite a small disturbance. During an access cycle, bit lines and word lines of neighboring cells may be driven with voltages that provide quiescent level electric fields across the neighboring ferroelectric cells. Quiescent level voltages may be defined in accordance with the resilient qualities of the ferroelectric cell, wherein polarization disturbances of the cells are kept within a recovery range. For example, in accordance with one embodiment of the present invention, the quiescent level, also referred to as a disturb voltage threshold, may be set to a magnitude no greater than ⅓ the switching level voltage. For example, assuming a one state storage condition for a ferroelectric cell, as represented by remnant polarization position23of hysteresis curve24, a small voltage disturbance of Vs/3 may provide a small polarization shift40along path38. However, once the voltage is removed, a large portion of the domains of the ferroelectric cell may realign their orientations to that of the cell's overall orientation, as illustrated by return path39of hysteresis curve24. Note that the return path may not return to position23but to a point slightly above position23. Many small voltage disturbances may accumulate overtime, moving the storage condition along path38and affecting memory performance. Large voltage disturbances, for example, when the voltage is greater than Vs/3, may move the charge of the cell significantly closer to position25along path30of hysteresis curve24. Additionally, a large negative voltage disturbance, for example, a voltage of −2Vs/3, may move the charge of the cell significantly closer to position27along path34of hysteresis curve24. It is desirable to keep voltage disturbances to a minimum, and below Vs/3. Thus, there may be only a narrow operating window in which the drive voltage is sufficiently high and in which the disturb voltage of Vs/3 is sufficiently low.

FIG. 3is a block diagram illustrating a ferroelectric memory device300in accordance with an embodiment of the present invention. Memory device300includes a cross-point passive matrix memory array having word lines312that cross bit lines314. Ferroelectric material such as, for example, a ferroelectric polymer material, may be disposed between the word lines and bit lines to form ferroelectric cells at the intersections of word lines and bit lines. For example, a ferroelectric cell302is located at the crossing of the word line identified as306and the bit line identified as304inFIG. 3. In this example, cell302is referred to as “active” because it identifies a specific cell that has been selected to read or write. The word line coupled to the active cell is identified as an active word line (AWL), whereas the remaining word lines are passive word lines (PWL). Likewise, the bit line coupled to the active cell is identified as the active bit line (ABL), whereas the remaining bit lines are passive bit lines (PBL). Passive word lines and bit lines may also referred to as unselected word lines and bit lines. Active word lines and bit lines may also be referred to as selected, addressed, or target word lines and bit lines.

When accessing an active cell, an access or switching level voltage may be applied to the active bit line304. The access level voltage has a magnitude that is defined as (Vbitline−Vwordline), and is sufficient to effect a polarization reversal of the active cell302. As illustrated, the access level voltage has a magnitude of 9 Volts (V), obtained by applying 9 V to bit line304and 0 V to word line306. In order to keep disturb voltages below the disturb voltage threshold of Vs/3, voltages are applied to the passive bit and word lines. As illustrated, 3 V is applied to the passive bit lines and 6 V is applied to the passive word lines, resulting in voltages of 3 V, and −3 V being applied to neighboring cells. Thus, only the active cell is written, wherein application of the access level voltage may switch the cell's polarization state.

Temperature may also affect cell performance. The hysteresis curve24may shift along the voltage axis with temperature. For example, hysteresis curve24may contract along the voltage axis at higher temperatures. Thus, the drive voltage required to switch the cell to, for example, position25, is lower. Likewise, the disturb voltage threshold is lower. Additionally, hysteresis curve24may expand along the voltage axis at lower temperatures where the drive voltage required and the disturb voltage threshold is higher. Thus, the narrow operating window in which the drive voltage is sufficiently high, but in which the disturb voltage of Vdrive/3 is sufficiently low varies with operating temperature.

Control circuit technology may also affect cell performance. Certain technologies, for example, Complementary Metal Oxide Semiconductor (CMOS) technology, are limited in the amount of voltage that can be supplied before breakdown occurs. Further, the amount of voltage that can be supplied may vary with temperature. For example CMOS control circuitry may be able to supply, for example, 14 V at room temperature, but can only supply 12.5 V at zero degrees Celsius. Thus, a ferroelectric cell operating at very low temperatures requires a high amount of drive voltage at the same time that CMOS control circuitry is at its lowest voltage output capability.

According to an embodiment of the present invention, increasing the amount of time a voltage is applied to a cell has the same effect as applying a higher voltage for a shorter period of time. For example, in one embodiment of the present invention, with a pulse width of approximately 50 microseconds, doubling the pulse width may provide a similar polarization result as applying an additional 0.7 V. Thus, an 8× pulse width may provide the equivalent of an additional 2.1V of drive voltage. Note that the pulse width and effective voltage varies with specific implementation and the scope of the present invention is not limited in this respect.

According to an embodiment of the present invention, access parameters, for example, a drive voltage and/or a pulse width, is adjusted according to temperature, such that enough voltage is applied to the cell without creating a voltage greater than the disturb voltage threshold on neighboring cells.

FIG. 4illustrates multiple access diagrams across a range of temperatures according to embodiments of the present invention. At higher temperatures, for example, at position402on the temperature axis404, an access diagram406used to access a cell, for example, to read or write the cell, has a drive voltage of V1and a pulse width of P1. At an intermediate temperature, for example, at position412on temperature axis404, an access diagram416has a drive voltage of V2and a pulse width of P1, where V2is greater than V1. At a lower temperature, for example, at position422on temperature axis404, an access diagram426has a drive voltage of V2and a width of P2, where P2is greater than P1. Thus, at higher temperatures, a lower drive voltage may be used to access a memory cell. At lower temperatures, a wider pulse width may be used to access the memory cell. Either or both of these techniques or other techniques may be used according to embodiments of the present invention.

FIG. 5illustrates a memory system according to an embodiment of the present invention. Memory system500includes a ferroelectric memory array502. Memory array502may be any size, for example, 17,000 bit lines and 8,000 word lines. In some embodiments, memory array502may be partitioned or segmented into subarrays.

Memory system500also includes a temperature sensor504for sensing the operational temperature of memory array502. Temperature sensor504provides sensed temperature information to a drive voltage determination unit506and a pulse width determination unit508. Each determination unit may include a look up table or a calculation function. For example, voltage determination unit506may include a lookup table with multiple entries of drive voltages associated with a plurality of temperatures. Alternatively, drive voltage determination unit506may include a calculation unit to calculate an optimum drive voltage for a given temperature. Pulse width determination unit508may include a lookup table with multiple entries of pulse widths associated with a plurality of temperatures. Alternatively, pulse width determination unit508may include a calculation unit to calculate an optimum pulse width for a given temperature.

In an alternate embodiment, a single lookup table includes drive voltage and pulse width pairs that are selected according to a sensed operational temperature.

Drive voltage determination unit506provides the determined drive voltage information to a drive voltage generation unit510to generate the optimum drive voltage. The optimum drive voltage and optimum pulse width information is provided to an array drive voltage and pulse width selection unit512which provides access controls to memory array502.

Various embodiments may be optimized according to expected operational temperatures or other operational constraints. For example, in one embodiment, of memory system500, pulse width is fixed and only drive voltage varies according to temperature. In another embodiment of memory system500, drive voltage is fixed and only pulse width varies according to temperature. In yet another embodiment of memory system500, both drive voltage and pulse width vary according to temperature.

Turning toFIG. 6, shown is a block diagram of a computing system600in accordance with an embodiment of the present invention. As shown inFIG. 6, in one embodiment computing system600may include a processor610, a memory controller620, a cache memory630, and a mass storage640. Processor610may be a general-purpose or special-purpose processor such as a microprocessor, microcontroller, application specific integrated circuit (ASIC), a programmable gate array (PGA), or the like.

In one embodiment, cache memory630may be a relatively large non-volatile disk cache memory adapted to cache information for mass storage640. For example, cache memory630may be a ferroelectric polymer memory. Mass storage640may be a mass storage device such as, for example, a disk memory having a storage capacity of 512 Mbytes, although the scope of the present invention is not limited in this respect. Mass storage640may be an electromechanical hard disk memory or an optical disk memory, although the scope of the present invention is not limited in this respect. In one embodiment, cache memory630may have a storage capacity of at least about 500 megabytes and may include ferroelectric memory cells, wherein each cell includes a ferroelectric polymer material located between at least two conductive lines. The ferroelectric polymer material may be a ferroelectric polarizable material. In various embodiments, the ferroelectric polymer material may comprise a polyvinyl fluoride, a polyethylene fluoride, a polyvinyl chloride, a polyethylene chloride, a polyacrylonitrile, a polyamide, copolymers thereof, or combinations thereof.

In an alternate embodiment, cache memory630may be another type of plastic memory such as, for example, a resistive change polymer memory. In this embodiment, the plastic memory may include a thin film of polymer memory material sandwiched at the nodes of an address matrix. The resistance at any node may be altered from a few hundred ohms to several megohms by applying an electric potential across the polymer memory material to apply a positive or negative current through the polymer material to alter the resistance of the polymer material. Potentially different resistance levels may store several bits per cell and data density may be increased further by stacking layers.

The memory of the apparatus may be a ferroelectric polymer memory, a ferroelectric oxide memory, or any other ferroelectric memory, although the scope of the present invention is not limited in this respect. In alternate embodiments, the memory may also be a non-ferroelectric memory.