Abstract:
Subject matter disclosed herein relates to a memory device, and more particularly to a single pulse algorithm for programming a phase change memory.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Subject matter disclosed herein relates to a memory device, and more particularly to a single pulse algorithm for programming a phase change memory. 
         [0003]    2. Information 
         [0004]    Phase change memory (PCM) may operate based, at least in part, on behavior or properties of one or more particular phase change materials, such as chalcogenide glass or germanium antimony telluride (GST), just to name a few examples. Electrical resistivities of crystalline or amorphous states of such materials may be different from one another, thus presenting a basis by which information may be stored. The amorphous, high resistance state may represent a stored first binary state and the crystalline, low resistance state may represent a stored second binary state. Of course, such a binary representation of stored information is merely an example: Phase change memory may also be used to store multiple memory states, represented by varying degrees of phase change material resistivity, for example. 
         [0005]    In a PCM cell, transitioning from an amorphous state to a crystalline state may involve a transition period that is short enough to provide a relatively fast PCM write operation while being long enough to allow a transition from an amorphous state to a crystalline state. Accordingly, a transition period that is too short may result in a PCM cell comprising a mixture of material in an amorphous and a crystalline state, leading to an undefined memory cell state resulting in a failure of the PCM cell, for example. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]    Non-limiting and non-exhaustive embodiments will be described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
           [0007]      FIG. 1  is a schematic diagram of a portion of an embodiment of phase change memory. 
           [0008]      FIG. 2  is a cross-section of a portion of phase change material in an embodiment of a memory system. 
           [0009]      FIG. 3  is a plot of characteristics of an electronic signal wave-form in an embodiment of a memory system. 
           [0010]      FIG. 4  is a plot showing a distribution of set rates for a population of memory cells in an embodiment of a PCM. 
           [0011]      FIG. 5  is a plot of characteristics of an electronic signal wave-form in an embodiment of a memory system. 
           [0012]      FIG. 6  is a schematic diagram of an electronic circuit to generate a set portion of an electronic signal in an embodiment of a memory system. 
           [0013]      FIG. 7  is a plot of characteristics of components of a set portion of an electronic signal in an embodiment of a memory system. 
           [0014]      FIG. 8  is a plot of characteristics of a set portion of an electronic signal in an embodiment of a memory system. 
           [0015]      FIG. 9  is a flow diagram of a process of applying a set portion of an electronic signal to a memory cell in an embodiment of a memory system. 
           [0016]      FIG. 10  is a schematic diagram illustrating an embodiment of a computing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of claimed subject matter. Thus, appearances of phrases such as “in one embodiment” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in one or more embodiments. 
         [0018]    A phase change memory (PCM) cell, if used to store a binary logic value, may be set or reset to one of two states. For example, an amorphous, high resistance state may represent a stored first binary state and a crystalline, low resistance state may represent a stored second binary state. A PCM cell may be reset to an amorphous state by applying a relatively high amplitude, relatively short duration electrical programming pulse or signal so as to melt and then relatively quickly quench phase change material of the PCM cell. In a reset state, an active region of phase change material may comprise an amorphous region that is substantially dome-shaped, disposed adjacent to a resistive joule-heater element in a PCM cell. Crystallized phase change material may surround an amorphous region of phase change material. In a reset state, a PCM cell may have a relatively high electrical resistance. In a subsequent process, a PCM cell may be set to a crystalline state by crystallizing a dome-shaped amorphous region so that a substantially entire region of phase change material may become crystalline. A process may involve decreasing a signal level value of an electronic signal applied to a PCM cell to crystallize phase change material. In a set state, a PCM cell may have a relatively low electrical resistance. A process of crystallizing a dome-shaped amorphous region may be performed relatively quickly to benefit operational performance (e.g., programming speed) of a PCM cell. However, performing crystallization too quickly may result in non-crystallized amorphous regions or imperfections in phase change material, resulting in a higher-than-desired electrical resistance. A desired crystallization process may involve decreasing an electronic signal so as to provide time to crystallize a dome-shaped amorphous region without adversely producing amorphous regions or imperfections. A process of selecting a rate of decrease of an electronic signal to set a PCM cell may involve a trade-off between PCM write speed and assuring that a relatively high percentage of phase change material is crystallized, for example. 
         [0019]    A memory device may include a plurality of PCM cells that may be arranged in an array. Due, at least in part, to variations in fabrication conditions from lot to lot or from region to region on a semiconductor wafer, for example, characteristics or physical parameters of PCM cells may vary among PCM cells. Physical parameters may include volume or dimensions of phase change material in a PCM cell, contact area between phase change material and a resistive heater, resistance of a resistive heater, just to name a few examples. Of course, variations may result from any of a number of situations or events. For another example, physical position of a PCM cell in a circuit may affect or modify physical parameters of the PCM cell. In particular, capacitance, magnetic or electric fields, or heat may contribute to variations. Thus, one portion of PCM cells in a PCM array may behave differently from another portion of PCM cells. For example, a rate at which a particular electronic signal affects some PCM cells may be different than for other PCM cells. Variations in physical or material parameters among PCM cells may be among a number of conditions leading to a variation in a rate at which a given electronic signal affects different PCM cells. 
         [0020]    In an embodiment, a method of operating a PCM cell may involve applying an electronic signal to melt phase change material in the PCM cell. For example, an electronic signal may be applied to a resistive heater of a PCM cell during write operations performed by a memory controller, though claimed subject matter is not so limited. An electronic signal may be applied at a first particular signal value level so as to melt phase change material. The signal value level of the electronic signal being applied may be subsequently decreased after the phase change material is melted, until a second particular non-zero signal value level is reached. The signal value level may be decreased in a manner so that a substantially vertical drop in signal value level does not occur between the first particular signal value level and the second particular signal value level. For example, a rate of decrease of a signal value level may be less than about 0.3 to about 12.0 micro-amps per nanosecond (e.g., a rate of crystallization), though claimed subject matter is not so limited. 
         [0021]    A method, such as that described above, may be applied to multiple PCM cells, which may be included in a PCM cell array, for example. In a case of multiple PCM cells, an electronic signal may be applied at a first particular signal value level to more than one resistive heater so as to melt phase change material of more than one PCM cell. Subsequent to melting phase change material of PCM cells, a signal value level may be decreased in a manner so that phase change material crystallizes from an outer periphery of the PCM cells inwardly, as explained in further detail below. In one implementation, a signal value level may be decreased according to an approximately linear ramp between a first particular signal value level corresponding to melted phase change material and a second particular signal value level. In one implementation, a second particular signal value level may correspond to a phase change material temperature that is at or below that of a PCM cell having a lowest crystallization temperature among a plurality of PCM cells. 
         [0022]    In an embodiment, methods of operating a PCM cell may involve a device comprising a resistive heater to program PCM cells in an array of PCM cells in a manner so that a substantially vertical drop in temperature does not occur during programming of the cells of the array. A resistive heater may program cells of a PCM cell array by reducing temperature from a reset temperature level to a set temperature level, for example. A reset temperature may comprise a temperature at or above a highest melt temperature of any cell of an array. On the other hand, a set temperature level may comprise a temperature at or below a lowest crystallization temperature of any cell of an array. 
         [0023]    A method of operating a PCM cell may involve applying an electronic signal comprising a single signal pulse to the PCM cell. A single signal pulse may be applied in a manner so as to reset and then set a plurality of PCM cells, some PCM cells reaching a crystallization temperature level at a different time than others. A single signal pulse may comprise a current pulse applied to heating elements of PCM cells. A single signal pulse may comprise a current pulse applied to PCM cells in a self heating cell architecture, wherein the PCM cells themselves include a heating element. A single signal pulse may be applied at an initial level so as to produce a temperature that corresponds to or exceeds a melt temperature for a PCM cell having a highest melt temperature of a plurality of PCM cells. A single signal pulse may be applied in a manner so that a plurality of PCM cells may substantially become fully crystallized. A single signal pulse may be applied in a manner so that a change in temperature results that is at or below a rate corresponding to the rate at which a slowest PCM cell among a plurality of PCM cells is able to substantially fully crystallize. A single signal pulse may produce a subsequent temperature that corresponds to or is below a temperature for a PCM cell having a lowest crystallization temperature among a plurality of PCM cells. 
         [0024]    A method of operating a PCM cell, in one implementation, may be performed using an integrated circuit including a memory array comprising a plurality of PCM memory cells. A plurality of PCM memory cells may be substantially fully melted and then substantially fully crystallized via a single pulse without state verification of any of the plurality of PCM memory cells during application of the single pulse. As a counter example, a process of programming PCM cells to a particular state may be performed using multiple pulses separated by processes of verifying whether the PCM cells, or portions thereof, have been programmed to the particular state. 
         [0025]      FIG. 1  is a diagram of a portion of an embodiment of phase change memory  100 . Such a portion is shown to include two memory cells, each memory cell being in a different memory state for illustrative purposes. A semiconductor substrate  150  may include N-doped regions  155 , though other configurations, including the use of P-doped regions for example, may be used. Phase change memory  100  may include word lines  105 , bit line  120 , or word line contact  110 . To represent one memory state, a heater  145  contacting a portion of phase change material  125  may heat to melt a portion  140  of phase change material  125 , which may then be cooled relatively quickly to comprise amorphous germanium antimony telluride (GST), for example. An amorphous material may be relatively highly resistive, resulting in a high-resistance connection to a bitline contact  120 . To represent another memory state, a heater  135  contacting a portion of phase change material  115  may heat to melt a portion of phase change material  115 , which may then be cooled relatively slowly to comprise a crystalline or polycrystalline, low-resistance material. Polycrystalline phase change material  115  may thus lead to a low-resistance connection to contact  120 . Of course, details of such a portion of a PCM are merely examples, and claimed subject matter is not so limited. 
         [0026]    As indicated above, polycrystalline phase change material  115  may develop one or more imperfections  130 , which may comprise non-crystallized amorphous regions, resulting from performing a crystallization process too quickly. Imperfections  130  may detrimentally increase resistance of a connection to bitline contact  120 , leading to malfunctioning of the memory cell. For example, a failed memory cell may fail to read the binary value represented by the low-resistance state. 
         [0027]      FIG. 2  is a cross-section of a portion of polycrystalline phase change material  210  in an embodiment of a memory system. Portion  210  of phase change material may be similar to phase change material  125  shown in  FIG. 1 , for example. Within a particular time, a dome-shaped amorphous region  220  may shrink to a smaller size portion  230  in response to a decrease in signal value level of an electronic signal to crystallize dome-shaped amorphous region  220 , as described above. As a signal value level of an electronic signal continues to decrease, smaller size portion  230  may continue to shrink until substantially an entire portion of amorphous region  220  becomes crystallized. A rate of such shrinkage or crystallization may be quantified in terms of a decrease in height  240  of dome-shaped amorphous region  220 , for example. For a particular rate of decrease of an electronic signal applied to a plurality of PCM cells, a shrinkage or crystallization rate may vary among PCM cells, as mentioned above. For example, dome-shaped amorphous region  220  included in one PCM cell may crystallize more quickly than that of another PCM cell, even though both PCM cells may experience an identical electronic signal rate. Of course, such details of crystallization of phase change material are merely examples, and claimed subject matter is not so limited. 
         [0028]      FIG. 3  is a plot of characteristics of a set pulse or electronic signal  300  including a set portion  310  which may comprise a decreasing ramp plotted against time in an embodiment of a memory system. Electronic signal  300  may comprise a time dependent single pulse to be applied to program a plurality of PCM cells, for example. Electronic signal  300  may include a connected, smooth signal path from a starting current to an ending current. For example, a parameter of electronic signal  300  may comprise a starting current that produces a temperature that corresponds to or exceeds a melt temperature for a PCM cell having a highest melt temperature of a plurality of PCM cells. Another parameter of electronic signal  300  may comprise an ending current that produces a temperature that corresponds to or is below a temperature for a PCM cell having a lowest crystallization temperature of a plurality of PCM cells. 
         [0029]    From time T 0  to time T 1 , electronic signal  300  applied to a PCM cell may be ramped-up to a melting level corresponding to a melting temperature of a phase change material during a process to reset the PCM cell, for example. A sufficiently high temperature may bring an active volume of phase change material of a PCM cell to a molten phase. From time T 1  to time T 2 , such an amorphous, resistive phase may comprise a steady state of the active volume of the phase change material while electronic signal  300  remains substantially constant, though claimed subject matter is not so limited. Subsequently, from time T 2  to time T 3 , a set process may include a ramping-down electronic signal  310  to crystallize the active volume of the phase change material. To achieve relatively fast operational performance, a set process may include applying an electronic signal having a relatively steep ramp-down slope  320  in order to crystallize phase change material at a relatively fast rate. However, as discussed above, if a ramp-down rate of an electronic signal is greater than a particular amount, such a rate may lead to imperfections or pocket regions of un-crystallized phase change material in a set state of a PCM cell. On the other hand, if a ramp-down rate of an electronic signal, such as ramp-down slope  330 , is less than a particular amount, such a rate may lead to decreased performance (e.g., reduced program speed) of a PCM cell. Thus, a ramp-down portion of electronic signal  310  may be selected to have a slope for a desired operational performance of a PCM cell while achieving substantially complete crystallization of phase change material for a set state of the PCM cell. For example, a desired operational performance may be selected after making trade-offs among programming speed, programming reliability, or programming yield. Also, electronic signal  310  may be applied to a plurality of PCM cells in a manner so that a change in temperature results that is at or below a rate corresponding to the rate at which the PCM cell that is the slowest of the plurality to respond to the change in temperature is able to substantially fully crystallize. 
         [0030]    As discussed above, due to variations in fabrication conditions, for example, characteristics or physical parameters of PCM cells in a PCM may vary. For example, a particular electronic signal may affect some PCM cells differently from other PCM cells. Accordingly, one portion of PCM cells in a PCM may behave differently from another portion of PCM cells in response to an applied electronic signal having a particular ramp-down slope. For example, a crystallization process using ramp-down slope  320  may be desired for one portion of PCM cells while being too fast (e.g., leading to imperfections in a phase change material) for another portion of PCM cells. Likewise, a crystallization process using ramp-down slope  330  may be desired for one portion of PCM cells while being slower than necessary (e.g., adversely affecting memory performance speed) for another portion of PCM cells. Accordingly, a medium ramp-down slope, such as that of ramp-down portion of electronic signal  310 , may lead to a rate of crystallization that is too fast for some PCM cells, too slow for other PCM cells, and desired for yet other PCM cells. Selection of a ramp-down slope that may take into consideration PCM cell variations will be discussed in detail below. 
         [0031]      FIG. 4  is a plot showing a distribution  405  of desirable set rates for a population  400  of memory cells in a PCM, according to an embodiment. “Desirable” is not intended to represent a precise condition. Accordingly, “desirable set rate” loosely refers to an electronic signal set rate that is slow enough to allow substantially complete crystallization of a phase change material in a particular PCM cell (or population of like PCM cells) while being fast enough to provide relatively high speed memory performance, for example. As discussed above, a PCM device may include PCM cells having variations in a set rate due to variations in fabrication conditions of the PCM cells, for example. For example, as described above, a rate at which a particular electronic signal affects some PCM cells may be different than for other PCM cells. To illustrate an example using electronic signal  300  in  FIG. 3 , a process of setting PCM cells  420  may involve ramping-down an electronic signal at a particular rate corresponding to ramp-down slope  320 ; a process of setting PCM cells  430  may involve ramping-down an electronic signal at a particular rate corresponding to ramp-down slope  330 , and a process of setting PCM cells  410  may involve ramping-down an electronic signal at a particular rate corresponding to ramp-down portion of electronic signal  310 . A particular population  435  of PCM cells may correspond to a relatively small or gradual slope of a ramping-down bias signal. Compared to other PCM cell populations, particular population  435  may have the slowest rate of crystallization. This may be the case, for example, for PCM cells having relatively large volumes of phase change material or relatively small contact area between heater and phase change material, among other reasons. Of course, such a distribution of PCM cells is merely an example, and claimed subject matter is not so limited. 
         [0032]      FIG. 5  is a schematic view of an electronic signal  500  that may be applied to a plurality of PCM cells, which may comprise the population represented in  FIG. 4 , for example. Electronic signal  500  may include a set portion  510  plotted against time. From time T 0  to time T 1 , electronic signal  500  may be ramped-up to a melting level  505  corresponding to a melting temperature of a phase change material. From time T 0  to time T 1 , electronic signal  500  may be applied to a plurality of PCM cells so as to produce a temperature that corresponds to or exceeds a melt temperature for a PCM cell having a highest melt temperature of the plurality. Subsequently, from time T 2  to time T 3 , a set process may include ramping-down electronic signal  500  to crystallize the active volume of phase change material. In a particular implementation, electronic signal  500  may comprise a substantially linear portion  510  having a slope that decreases electronic signal  500  between times T 2  and T 3 . Applying a ramping-down electronic signal comprising substantially linear portion  510  may provide benefits directed to improving the rate at which a population of PCM cells may be crystallized to a set state. In particular, crystallization may be carried-out substantially without generating imperfections or amorphous regions of phase change material in a final set structure. In one implementation, linear portion  510  of an electronic signal may be applied to a population of PCM cells having a distribution of desirable set rates, such as that shown in  FIG. 4 , for example. In an application, a slope of substantially linear portion  510  may be selected to correspond to particular population  435 , which may have the slowest rate of crystallization relative to PCM cells of other populations. For example, a slope of substantially linear portion  510  may be selected to correspond to a rate of temperature change of phase change material of PCM cells of particular population  435  substantially equal to a rate of crystallization of these PCM cells. Electronic signal  500  may be applied to PCM cells so that a rate of crystallization during a set process may be linear with respect to a height of a dome-shaped active region of phase change material in the PCM cell. 
         [0033]    Substantially linear portion  510  may extend to a crystallization level at time T 3 , upon or after which a slope  530  of electronic signal  500  may be increased to improve program speed. A crystallization level may correspond to a temperature at which phase change material of substantially all populations of PCM cells may be crystallized. In other words, a crystallization level may correspond to a temperature at which there may be a relatively high probability that phase change material of all PCM cells may be crystallized. A crystallization level of electronic signal  500  may be substantially equal to a read bias of substantially all populations of PCM cells. For example, for chalcogenide glass, read bias may comprise a value of about 1.3 volts, though claimed subject matter is not so limited. Electronic signal  500  may reach a non-zero signal value level  535  to conclude a process of setting PCM cells, though claimed subject matter is not so limited. 
         [0034]      FIG. 6  is a schematic of an electronic circuit  600  to generate a set portion of an electronic signal in an embodiment of a memory system. Circuit  600  comprises merely one of many techniques for generating or producing a set portion of an electronic signal having various characteristics, for example, and claimed subject matter is not limited to any particular technique. Circuit  600  may comprise sub-circuits  610 ,  620 , and  630  that include a transistor M 1 , M 2 , and M 3 , capacitor C 1 , C 2 , and C 3 , a current source I 1 , I 2 , and I 3 , two switches S 11 , S 12 , S 21 , S 22 , S 31 , and S 32 , and voltage V 1 , V 2 , and V 3 , respectively. Supply voltage Vsup may be provided to sub-circuits  610 ,  620 , and  630 . Circuit  600  may generate a voltage Vout to be applied to a PCM during a set process, for example. Vout may comprise a signal similar to electronic signal  500 . In such a case, Vout may include a substantially linear portion  510  that may be piece-wise continuous, comprising one or more slopes that may be averaged over time by a resistor-capacitor (RC) combination  650 . Though circuit  600  is herein described to comprise three sub-circuits  610 ,  620 , and  630 , a waveform generating circuit may comprise any number of sub-circuits. For example, circuit  600  may comprise additional sub-circuits. Accordingly, details of circuit  600  are merely examples, and claimed subject matter is not so limited. 
         [0035]      FIGS. 7 and 8  are plots of characteristics of components of a set portion of an electronic signal that may be generated by circuit  600 , for example. An electronic signal may be applied to a PCM cell during read or write operations by a memory controller, for example. In particular, waveforms  700  and  800  may comprise Vout plotted as a function of time, according to an embodiment. Returning to  FIG. 6 , initially, three switches, S 11 , S 21 , and S 31  may be closed while three switches S 12 , S 22 , and S 32  may be open. A configuration may initialize voltages Vg 1 , Vg 2 , and Vg 3  to voltages V 1 , V 2 , and V 3 , respectively. V 1  may be greater than V 2  and V 2  may be greater than V 3 . A situation may comprise an initial state  710  and  810  between t 0  and t 1 . Before t 1 , three switches, S 11 , S 21 , and S 31  may be opened and capacitors C 1 , C 2 , and C 3  may store their respective voltages. At t 1 , three switches S 12 , S 22 , and S 32  may close, connecting three independent current sources I 1 , I 2 , and I 3  to capacitors C 1 , C 2 , and C 3 . In an implementation, I 1  may be greater than I 2  and I 2  may be greater than I 3 . Capacitors C 1 , C 2 , and C 3  may begin to discharge at rates different from one another: Vg 1 , beginning at a highest voltage V 1  may discharge with a fastest slope  730 , Vg 2  at a slower rate  740 , and Vg 3  at a slowest rate  750 . M 1 , M 2 , and M 3  may comprise source-followers, so that Vout may follow approximately one of the gate voltages minus Vtn, wherein Vtn may comprise a threshold voltage of an N-channel MOSFET. The gate that Vout follows may comprise the gate at the highest potential in circuit  600 . For example, just after t 1  and before t 2 , since starting voltages V 2  and V 3  are less than V 1 , M 1  may determine Vout so that Vout may be approximately Vg 1 -Vtn. M 2  and M 3  may be initially off if their source voltages, Vout, is higher than Vg 2 -Vtn or Vg 3 -Vtn, for example. Eventually, at t 2 , gate voltage of M 1  may cross below that of M 2 . At this point, M 2  may turn on while M 1  transitions to a sub-threshold state and turns off. Accordingly, Vout may subsequently be determined by Vg 2 , with its slower ramp (smaller slope). At t 3 , as Vg 2  crosses below Vg 3 , M 2  may turn off, M 3  may turn on, and Vout may subsequently follow M 3  with a yet slower ramp rate. Of course, details of circuit  600  are merely examples, and claimed subject matter is not so limited. 
         [0036]      FIG. 8  shows a three-part piece-wise-linear output voltage Vout, approximating a decreasing portion of a set ramp, for example. A piece-wise-linear curve may result from sub-circuits  610 ,  620 , and  630  whose output voltage is individually shown in  FIG. 7 . A piece-wise-linear output voltage may be increasingly continuous (e.g., smooth) via additional sub-circuits in circuit  600  or using RC combination  650 , for example. A selection of a number of sub-circuits may be based, at least in part, on a desirable resolution of a resulting output voltage. Though electronic signal resolution may increase, added sub-circuits may occupy additional space leading to circuit  600  having an undesirably large size. 
         [0037]    Waveforms  700  or  800  may comprise a relatively quick voltage drop beginning at t 4 , as shown in  FIGS. 7 and 8 , for example. Though optional, applying a voltage having a quick transition to substantially zero to a PCM cell may provide benefits including improving operational performance of a PCM cell by ending a set process at a voltage below which there may be no further programming. In other words, rather than ramp all the way to zero volts, a set process may stop at approximately 1.3V, for example. 
         [0038]    Though waveforms  700  or  800  are herein described to comprise three discrete slopes  830 ,  840 , and  850 , set portions of an electronic signal waveform may comprise any number of discrete slope values or ranges of continuous slope values. Accordingly, details of waveforms  700  and  800  are merely examples, and claimed subject matter is not so limited. 
         [0039]      FIG. 9  is a flow diagram of a process  900  of applying an electronic signal to a plurality of PCM cells. Process  900  may involve a plurality of PCM cells that include populations of PCM cells having varying melt temperatures, crystallization rates, or crystallization temperature, among other possible characteristics. As discussed above, varying characteristics may result from variations in fabrication conditions of PCM cells, various placement of PCM cells in an electronic system, or any of a number of other possibilities, for example. Accordingly, one population of PCM cells may behave differently from another population of PCM cells. At block  910 , a signal may be applied at a first value to a plurality of PCM cells. A first value may correspond to a melt temperature of a particular population of PCM cells that are among those having highest melt temperatures of the combined populations of PCM cells. A first value of a signal may also be approximately the same as a magnitude of a reset pulse to transition the particular population of PCM cells to a RESET state, for example. 
         [0040]    Subsequently, at block  920 , a signal may be decreased at a rate corresponding to a crystallization rate of another particular population of PCM cells that are among those having the slowest rate of crystallization of the combined populations of PCM cells. For example, as mentioned above regarding  FIG. 4 , particular population  435  may have a slowest rate of crystallization among a combined population  400  of PCM cells. Next, at block  930 , a signal may be applied at a second value to a plurality of PCM cells. A second value may correspond to a crystallization temperature of yet another particular population of PCM cells that are among those having a lowest crystallization temperature of the combined populations of PCM cells. A second value of a signal may also be approximately the same as a magnitude of a read bias used to read PCM cells during a read operation, for example. Of course, such details of process  900  are merely examples, and claimed subject matter is not so limited. 
         [0041]      FIG. 10  is a schematic diagram illustrating an embodiment of a computing system  1000  including a memory device  1010 . For example, computing system  1000  may drive equipment capable of programming a plurality of PCM cells, and adjust parameters of a single pulse to be applied to program the plurality of PCM cells. Parameters may comprise a starting current, an ending current and a time dependent, connected, smooth signal path from a starting current to a ending current, for example. Computing system  1000  may include a capability to adjust parameters so as to make trade-offs between any of the following: programming speed, programming reliability, or programming yield. Computing system  1000  may include a capability to adjust parameters so as to reduce programming speed without significantly degrading nearly full crystallization of a plurality of PCM cells. 
         [0042]    A computing device may comprise one or more processors, for example, to execute an application or other code. For example, memory device  1010  may comprise a memory that includes PCM  100 , shown in  FIG. 1 . A computing device  1004  may be representative of any device, appliance, or machine that may be configurable to manage memory device  1010 . Memory device  1010  may include a memory controller  1015  and a memory  1022 . By way of example but not limitation, computing device  1004  may include: one or more computing devices or platforms, such as, e.g., a desktop computer, a laptop computer, a workstation, a server device, or the like; one or more personal computing or communication devices or appliances, such as, e.g., a personal digital assistant, mobile communication device, or the like; a computing system or associated service provider capability, such as, e.g., a database or data storage service provider/system; or any combination thereof. 
         [0043]    It is recognized that all or part of the various devices shown in system  1000 , and the processes and methods as further described herein, may be implemented using or otherwise including hardware, firmware, software, or any combination thereof. Thus, by way of example but not limitation, computing device  1004  may include at least one processing unit  1020  that is operatively coupled to memory  1022  through a bus  1040  and a host or memory controller  1015 . Processing unit  1020  is representative of one or more circuits configurable to perform at least a portion of a data computing procedure or process. By way of example but not limitation, processing unit  1020  may include one or more processors, controllers, microprocessors, microcontrollers, application specific integrated circuits, digital signal processors, programmable logic devices, field programmable gate arrays, and the like, or any combination thereof. Processing unit  1020  may include an operating system configured to communicate with memory controller  1015 . Such an operating system may, for example, generate commands to be sent to memory controller  1015  over bus  1040 . Such commands may comprise read or write commands. In response to a write command, for example, memory controller  1015  may provide an electronic signal, such as electronic signal  500 , described above. 
         [0044]    Memory  1022  is representative of any data storage mechanism. Memory  1022  may include, for example, a primary memory  1024  or a secondary memory  1026 . Primary memory  1024  may include, for example, a random access memory, read only memory, etc. While illustrated in this example as being separate from processing unit  1020 , it should be understood that all or part of primary memory  1024  may be provided within or otherwise co-located/coupled with processing unit  1020 . 
         [0045]    Secondary memory  1026  may include, for example, the same or similar type of memory as primary memory or one or more data storage devices or systems, such as, for example, a disk drive, an optical disc drive, a tape drive, a solid state memory drive, etc. In certain implementations, secondary memory  1026  may be operatively receptive of, or otherwise configurable to couple to, a computer-readable medium  1028 . Computer-readable medium  1028  may include, for example, any medium that can carry or make accessible data, code, or instructions for one or more of the devices in system  1000 . 
         [0046]    Computing device  1004  may include, for example, an input/output  1032 . Input/output  1032  is representative of one or more devices or features that may be configurable to accept or otherwise introduce human or machine inputs, or one or more devices or features that may be configurable to deliver or otherwise provide for human or machine outputs. By way of example but not limitation, input/output device  1032  may include an operatively configured display, speaker, keyboard, mouse, trackball, touch screen, data port, etc. 
         [0047]    It will, of course, be understood that, although particular embodiments have just been described, claimed subject matter is not limited in scope to a particular embodiment or implementation. For example, one embodiment may be in hardware, such as implemented on a device or combination of devices, for example. Likewise, although claimed subject matter is not limited in scope in this respect, one embodiment may comprise one or more articles, such as a storage medium or storage media that may have stored thereon instructions capable of being executed by a specific or special purpose system or apparatus, for example, to result in performance of an embodiment of a method in accordance with claimed subject matter, such as one of the embodiments previously described, for example. However, claimed subject matter is, of course, not limited to one of the embodiments described necessarily. Furthermore, a specific or special purpose computing platform may include one or more processing units or processors, one or more input/output devices, such as a display, a keyboard or a mouse, or one or more memories, such as static random access memory, dynamic random access memory, flash memory, or a hard drive, although, again, claimed subject matter is not limited in scope to this example. 
         [0048]    In the preceding description, various aspects of claimed subject matter have been described. For purposes of explanation, specific numbers, systems, or configurations may have been set forth to provide a thorough understanding of claimed subject matter. However, it should be apparent to one skilled in the art having the benefit of this disclosure that claimed subject matter may be practiced without those specific details. In other instances, features that would be understood by one of ordinary skill were omitted or simplified so as not to obscure claimed subject matter. While certain features have been illustrated or described herein, many modifications, substitutions, changes, or equivalents may now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all modifications or changes as fall within the true spirit of claimed subject matter.