Memory device and method for thermoelectric heat confinement

Abstract

A memory device for thermoelectric heat confinement and method for producing same. The memory device includes a plurality of phase-change memory cells, wherein each of the phase-change memory cells has a first electrode, a second electrode and a phase-change material. The first electrode and the phase-change material are arranged such that a surface normal of a dominating interface for a current flow between the first electrode and the phase-change material points on one side to the phase-change material of the phase-change memory cell and on an opposite side to a phase-change material of a neighboring phase-change memory cell. A method for producing a memory device for thermoelectric heat confinement is also provided.

Claims

1. A memory device for thermoelectric heat confinement, the memory device having a plurality of phase-change memory cells, wherein each of the phase-change memory cells comprises: a first electrode; a second electrode; and a phase-change material, wherein the first electrode and the phase-change material are arranged such that a surface normal of an interface for a current flow between the first electrode and the phase-change material points on one side to the phase-change material of the phase-change memory cell and on an opposite side to a phase-change material of a neighboring phase-change memory cell, wherein the first electrode is a cup-shaped electrode having a too region and a side region, wherein the top region of the cup-shaped electrode interfaces a top region of the phase-change material and the side region of the cup-shaped electrode interfaces a side region of the phase-change material, and wherein each of the plurality of phase-change memory cells has a geometry fulfilling the following condition, pcm.Math.A.sub.EL>el.Math.A.sub.PCM; where A.sub.EL designates a cross-section of the side region of the cup-shaped top electrode for a vertical current flow through the side region; where A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material; where pcm designates an electrical resistivity of the phase-change material; and where el designates an electrical resistivity of the cup-shaped top electrode.

2. The memory device according to claim 1, wherein the interface is configured for heating the phase-change memory cell being programmed while cooling the neighboring phase-change memory cell.

3. The memory device according to claim 1, wherein the first electrode and the phase-change material are arranged such that the surface normal of the interface for a current flow between the first electrode and the phase-change material points on the one-side to the phase-change material of the phase-change memory cell and on the opposite side to the phase-change material of a plurality of neighboring phase-change memory cells.

4. The memory device according to claim 1, wherein the first electrode is a top electrode and the second electrode is a bottom electrode, wherein the phase-change material is arranged between the first electrode and the second electrode.

5. The memory device according to claim 1, wherein each of the plurality of phase-change memory cells has a geometry fulfilling the following condition: H.Math.A.sub.INT>L.Math.A.sub.PCM; where A.sub.INT designates a cross-section of an interface for a lateral current flow between the side region of the cup-shaped top electrode and the phase-change material; where A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material; where H designates a height of the phase-change material; and where L designates a minimum distance between the cup-shaped top electrode and the bottom electrode.

6. The memory device according to claim 1, wherein the first electrode includes a sidewall-electrode connected to a word line and the second electrode includes a heater electrode connected to a bit line.

7. The memory device according to claim 6, further comprising: a first interface for a current flow between the sidewall-electrode and the phase-change material; a second interface for a current flow between the heater electrode and the phase change material; and wherein one of the first and second interfaces is a lateral interface and the other one of the first and second interfaces is a vertical interface.

8. The memory device according to claim 7, wherein the first interface is the lateral interface and the second interface is the vertical interface.

9. The memory device according to claim 7, wherein the first interface is the vertical interface and the second interface is the lateral interface.

10. The memory device according to claim 8, wherein the first interface is the vertical interface and the second interface is the lateral interface.

11. The memory according to claim 1, wherein the material parameters of each of the plurality of phase-change memory cells are chosen such that a Seebeck coefficient of the phase-change material is maximized.

12. The memory device according to claim 11, wherein the temperature-dependence of the Seebeck coefficient of the phase-change material is maximized and/or that the difference between the Seebeck coefficient of the phase-change material and a Seebeck coefficient of the first electrode is maximized.

13. The memory device according to claim 1, further comprising: a read/write apparatus for reading and writing data in the phase-change memory cells.

14. The memory device according to claim 12, further comprising: a read/write apparatus for reading and writing data in the phase-change memory cells.

15. A method for producing a memory device for thermoelectric heat confinement, the memory device including a plurality of phase-change memory cells, each having a first electrode, a second electrode and a phase-change material, the method comprising: arranging the first electrode and the phase-change material such that a surface normal of an interface for a current flow between the first electrode and the phase-change material points on one side to the phase-change material of the phase-change memory cell and on an opposite side to a phase-change material of a neighboring phase-change memory cell, wherein the first electrode is a cup-shaped electrode having a top region and a side region, wherein the top region of the cup-shaped electrode interfaces a to region of the phase-change material and the side region of the cup-shaped electrode interfaces a side region of the phase-change material, and wherein each of the plurality of phase-change memory cells has a geometry fulfilling the following condition, pcm.Math.A.sub.EL>el.Math.A.sub.PCM; where A.sub.EL designates a cross-section of the side region of the cup-shaped top electrode for a vertical current flow through the side region; where A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material; where pcm designates an electrical resistivity of the phase-change material; and where el designates an electrical resistivity of the cup-shaped top electrode.

16. The method according to claim 15, wherein the interface is configured for heating the phase-change memory cell being programmed while cooling the neighboring phase-change memory cell.

17. The method according to claim 15, wherein the first electrode and the phase-change material are arranged such that the surface normal of the interface for a current flow between the first electrode and the phase-change material points on the one-side to the phase-change material of the phase-change memory cell and on the opposite side to the phase-change material of a plurality of neighboring phase-change memory cells.

18. The method according to claim 15, wherein the first electrode is a top electrode and the second electrode is a bottom electrode, wherein the phase-change material is arranged between the first electrode and the second electrode.

19. The method according to claim 15, wherein each of the plurality of phase-change memory cells has a geometry fulfilling the following condition: H.Math.A.sub.INT>L.Math.A.sub.PCM; where A.sub.INT designates a cross-section of an interface for a lateral current flow between the side region of the cup-shaped top electrode and the phase-change material; where A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material; where H designates a height of the phase-change material; and where L designates a minimum distance between the cup-shaped top electrode and the bottom electrode.

20. The method according to claim 15, wherein the first electrode includes a sidewall-electrode connected to a word line and the second electrode includes a heater electrode connected to a bit line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a first embodiment of a memory device for thermoelectric heat confinement;

(2) FIG. 2 shows an embodiment of a PCM cell of the memory device of FIG. 1;

(3) FIG. 3 shows an equivalent circuit of the PCM cell of FIG. 2;

(4) FIG. 4 shows a second embodiment of a memory device for thermoelectric heat confinement;

(5) FIG. 5 shows a third embodiment of a memory device for thermoelectric heat confinement;

(6) FIG. 6 shows an embodiment of a memory system;

(7) FIG. 7 shows an embodiment of a sequence of method steps for producing a memory device for thermoelectric heat confinement;

(8) FIG. 8 shows V-R programming curves for a cell with a projected top electrode and a cell with a flat top electrode;

(9) FIG. 9 shows I-R programming curves for a cell with a projected top electrode and a cell with a flat top electrode;

(10) FIG. 10 shows P-R programming curves for a cell with a projected top electrode and a cell with a flat top electrode;

(11) FIG. 11 shows a temperature map illustrating a hotspot location in a cell with a projected top electrode;

(12) FIG. 12 shows V-R programming curves for a cell with a projected top electrode and a cell with a flat top electrode;

(13) FIG. 13 shows a temperature map illustrating a hotspot location in a cell with a flat top electrode;

(14) FIG. 14 shows a schematic diagram illustrating the current flow and the current density in a cell with a projected top electrode;

(15) FIG. 15 shows a schematic diagram illustrating the current flow and the current density in a cell with a projected flat electrode;

(16) FIG. 16 shows a temperature difference map for two neighboring PCM cells;

(17) FIG. 17 shows a diagram illustrating a thermal disturb temperature from a PCM cell to a neighboring PCM cell; and

(18) FIG. 18 shows a diagram illustrating a temperature gradient in the neighboring PCM cell of FIG. 17.

(19) Similar or functionally similar elements in the figures have been allocated the same reference signs if not otherwise indicated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(20) FIG. 1 shows a first embodiment of a memory device 100 for thermoelectric heat confinement. The memory device 100 includes a plurality s of PCM cells 10, 20 (s2). Without loss of generality, s=2 in FIG. 1.

(21) Each of the PCM cells 10, 20 has a first electrode 11, 21, a second electrode 12, 22 and a phase-change material 13, 23 (or phase-change layer).

(22) For example, the PCM cell 10 has the first electrode 11, the second electrode 12 and the phase-change material 13. The phase-change material 13 is arranged between the first electrode 11 and the second electrode 12.

(23) In the following, the thermoelectric heat confinement for the PCM cell 10 is described. The PCM cell 20 has an identical structure as the PCM cell 10. Thus, the same description can be also applied to memory cell 20.

(24) In the PCM cell 10, the first electrode 11 and the phase-change material 13 are arranged such that the surface normal N of a dominating interface 15 for a current flow between the first electrode 11 and the phase-change material 13 points on one side (identified by arrow A1) to the phase-change material 13 of the PCM cell 10 and on an opposite side (identified by arrow A2) to the phase-change material 23 of the neighboring PCM cell 20. To meet this arrangement, the first electrode 11 of FIG. 1 has a cup shape. The first electrode 11 can also have a pen shape or a pot shape. The dominating interface 15 is configured for heating the PCM cell 10 being programmed while cooling the neighboring PCM cell 20. In FIG. 1, the reference sign HA indicates the heating area or hotspot while the memory cell 10, 20 is programmed.

(25) As shown in FIG. 1, the first electrode 11, 21 of each of the memory cells 10, 20 can be a top electrode, wherein the second electrode 12, 22 of each of the PCM cells 10, 20 can be a bottom electrode. The phase-change material 13, 23 of each of the PCM cells 10, 20 is arranged between the first electrode 11, 21 and the second electrode 12, 22, respectively.

(26) In FIG. 2, the embodiment of the PCM cell 10 of FIG. 1 is shown in more detail. FIG. 2 shows that the PCM cell 10 has a first electrode 11, a second electrode 12 and a phase-change material 13 arranged between the first and second electrodes 11, 12.

(27) As indicated above, the first electrode 11 is a cup-shaped top electrode 11. The cup-shaped top electrode 11 has a top region 16 and a side region 17. The top region 16 of the cup-shaped electrode 11 interfaces a top region 18 of the phase-change material 13. Moreover, the side region 17 of the cup-shaped electrode 11 interfaces a side region 19 of the phase-change material 13. The dominating interface 15 for the current flow between the first electrode 11 and the phase-change material 13 is a lateral interface for the current flow. As a result, the lateral interface 15 is configured to heat the PCM cell 10 at the heating area HA very precisely during the memory cell 10 is programmed, while cooling the neighboring PCM cell 20 (see FIG. 1).

(28) In FIG. 2, the following parameters describe the cell design of the PCM cell 10: A.sub.EL designates a cross-section of the side region of the cup-shaped top electrode 11 for a vertical current flow through the side region 17. A.sub.PCM designates a cross-section of the phase-change material 13 for vertical current flow between the cup-shaped top electrode 11 and the phase-change material 13. ppcm designates an electrical resistivity of the phase-change material 13. A.sub.INT designates a cross-section of the interface 15 for a lateral current flow between the side region 17 of the cup-shaped top electrode 11 and the phase-change material 13. H designates a height of the phase-change material 13. L designates a distance between the cup-shaped top electrode and the bottom electrode. el designates an electrical resistivity of the cup-shaped top electrode.

(29) Further, FIG. 3 shows an equivalent circuit EL of the PCM cell 10 of FIG. 2.

(30) The equivalent circuit EL of FIG. 2 is based on the following formulas:
R2=R2a+R2b; R1=pcm H/A.sub.PCM; R2a=el H/A.sub.EL; and R2b=pcm L/A.sub.INT.

(31) In the cup-shaped cell design of FIG. 2, the relevant interface 15 between the first electrode 11 and the phase-change material 13 of which the surface normal N points into the favorite directions A1, A2 (see FIG. 1) can be made dominant (i.e. more or higher current flow through interface 15 than through the path of R1) by designing the resistances R1 and R2 such that R1>R2.

(32) To maximize or increase the heating and cooling effects, one can maximize or increase R1/R2a and/or R1/R2b. Maximizing or increasing R1/R2a means maximizing (_pcm.Math.A_el)/(_el.Math.A_pcm). Further, maximizing or increasing R1/R2b means to maximize or increase (H.Math.A_int)/(L.Math.A_pcm).

(33) FIG. 4 shows a second embodiment of a memory device 100 for thermoelectric heat confinement. The memory device 100 of FIG. 4 is based on a sidewall-electrode cell design.

(34) The memory device 100 of FIG. 4 shows two PCM cells 10, 20, wherein both share one common bit line BL. The PCM cell 10 of FIG. 4 has a first electrode 11, a second electrode 12 and a phase-change material 13. The first electrode 11 is a sidewall-electrode connected to a word line WL1 for the PCM cell 10. The second electrode 12 is a heater electrode connected to the common bit line BL. The phase change material 13 is arranged between the sidewall-electrode 11 and the heater electrode 12.

(35) The PCM memory cell 20 shown in the right of FIG. 4 has a first electrode 21 embodied as a sidewall-electrode, a second electrode 22 and a phase-change material 23. As it can be seen in FIG. 4, the second electrode 22 of the PCM cell 20 corresponds to the second electrode 12 of the PCM cell 10. That means that the PCM cells 10, 20 also share the second electrodes 12, 22 as a heater electrode. Moreover, in FIG. 4, reference signs 14, 24 show insulators.

(36) Each of the PCM cells 10, 20 of FIG. 4 has a first interface 31, 33 for a current flow between its sidewall-electrodes 11, 21 in its phase-change material 13, 23. Further, each of the PCM cells 10, 20 of FIG. 4 has a second interface 32, 34 for a current flow between its heater electrode 12, 22 and its phase-change material 13, 23. In the example of FIG. 4, the first interface 31, 33 is a vertical interface, wherein the second interface 32, 34 is a lateral interface. The embodiment of FIG. 5 differs from that of FIG. 4 therein that in FIG. 5 the first interface 31, 33 is the lateral interface and the second interface 32, 34 is the vertical interface. Recapitulating both embodiments of FIGS. 4 and 5, one of the first and second interfaces 31, 33; 32, 34 is a lateral interface and the other one of the first and second interfaces 31, 33; 32, 34 is a vertical interface.

(37) FIG. 6 is a simplified schematic of a memory system 1. The memory system 1 includes a PCM 2 for storing data in at least one memory devices 100 (see FIG. 1). The memory devices 100 of the memory 2 can be provided as integrated arrays of multilevel PCM cells. Though shown as a single block in FIG. 6, in general the PCM 2 can include any desired configuration of memory devices 100 e.g. of FIG. 1, for example, from a single chip or die to a plurality of storage banks each containing multiple packages of storage chips. Reading and writing of data to memory 2 is performed by read/write apparatus 3. Apparatus 3 includes datawrite and read-measurement circuitry 4 for writing data to the PCM cells and for making cell measurements allowing determination of cell state and hence read back of stored data.

(38) Circuitry 4 can address individual PCM cells for write and read purposes by applying appropriate voltage signals to an array of word and bit lines in memory ensemble 2.

(39) This process is performed in generally known manner except as detailed hereinafter. A read/write controller 5 controls operation of apparatus 3. In general, the functionality of controller 5 can be implemented in hardware or software or a combination thereof, though use of hardwired logic circuits is generally preferred for reasons of operating speed. Suitable implementations will be apparent to those skilled in the art from the description herein. As indicated by block 6 in FIG. 6, user data input to device 1 is typically subjected to some form of write-processing, such as coding for error-correction purposes, before being supplied as write data to read/write apparatus 3. Similarly, read back data output by apparatus 3 is generally processed by a read-processing module 7, e.g. performing codeword detection and error correction operations, to recover the original input user data. Such processing by modules 6 and 7 is independent of the cell-state measurement system to be described and need not be discussed further here.

(40) Each of the multilevel cells in memory 2 can be set to one of s resistance levels, where s>2, corresponding to different amorphous/crystalline states of the cell. To write data to memory cells, circuitry 4 applies programming pulses (via cell bit-lines or word-lines depending on whether voltage-mode or current-mode programming is used) of appropriate amplitude to set cells to states representative of the write data. Subsequent reading of a memory cell involves determining the state of the cell, i.e. detecting which of the possible levels that cell is set to. In a read operation of memory device 1, circuitry 4 performs cell measurements from which cell-states can be determined and the stored data recovered.

(41) FIG. 7 shows an embodiment of a sequence of method steps for producing a memory device 100 for thermoelectric heat confinement, as shown in FIG. 1, for example. The method of FIG. 7 has the following steps S1 and S2.

(42) In step S1, a plurality of PCM cells is provided, each having a first electrode, a second electrode and a phase-change material.

(43) In step S2, the first electrode and the phase-change material are arranged such that a surface normal of an interface, in particular, a dominating interface for a current flow between the first electrode and the phase-change material points on one side to the phase-change material of the PCM cell and on an opposite side to a phase-change material of a neighboring PCM cell.

(44) The effects and advantages of the present cell design for thermal electric heat confinement can be evident by FIGS. 8-18.

(45) In this regard, FIGS. 8-10 show programming curves for a cell with a projected top electrode (PTE) compared to a cell with a flat top electrode (FTE). In detail, FIG. 8 shows the programming voltage V, FIG. 9 the programming current I and FIG. 10 the programming power P over the resistance R. FIGS. 8 to 10 illustrate that in the PTE cell, lateral heat confinement due to the thermoelectric effect (TE model) at the top electrode and the phase-change interface helps in improving the efficiency of the cell. Further, the SET resistance of the PTE cell is lower than that of the FTE cell. The reason is that the volume of phase-change material that needs to be amorphized is smaller in case of the PTE cell. As indicated by arrow A3 in FIG. 10, there is a reduction of about 16.7% in programming power for the PTE cell compared to the FTE cell.

(46) Furthermore, FIGS. 11 and 13 show temperature maps for a hotspot location for the PTE cell and the FTE cell, respectively, for programming voltages that just achieve plugging of the heater electrode (resulting in the first sharp increase in read out resistance). To give a reference, FIG. 12 shows the programming voltages for the PTE cell and the FTE cell (see FIG. 8). Comparing FIGS. 11 and 13, it is apparent that the hotspot of 900 K is smaller in the PTE cell than in the FTE cell. Also, the hotspot area of 450 K is smaller in the PTE cell of FIG. 11 compared to the FTE cell of FIG. 13.

(47) FIGS. 14 and 15 show the current density and the direction of current flow in the PTE cell and the FTE cell, respectively. In FIG. 14, ellipse E1 shows a higher current density in the projected top electrode of the first electrode 11. In contrast, ellipse E2 in FIG. 15 shows a uniform current density inside the phase-change material 13 of the FTE cell. Further, ellipse E3 in FIG. 14 shows the lateral direction of the current flow at the interface 15 between the projected top electrode 11 and the phase-change material 13 of the PTE cell.

(48) FIG. 16 shows a temperature difference map for two neighboring PCM cells, PCM cell 1 and PCM cell 2. The temperature difference map of FIG. 16 clearly shows the additional heating and cooling provided by the projected top electrode. For a full RESET scenario, the projected top electrode clearly has greater thermal confinement compared to an FTE cell, for example. Thus, the thermal disturb of the present cell design is much smaller compared to a conventional FTE cell design.

(49) Moreover, FIG. 17 shows a diagram illustrating the thermal disturb temperature from a first cell, e.g. PCM cell 1, to a second cell, e.g. PCM cell 2. In detail, FIG. 17 shows two temperature curves in dependence on distance. A first curve with a dashed line shows the temperature curve for a flat top electrode in the TE model, wherein a second curve with a full line shows a cup-shaped top electrode in the TE model. Further, FIG. 18 illustrates details for the temperature gradient in the PCM cell 2, also with a full line for the cup-shaped top electrode and a dashed line for the flat top electrode.

(50) With respect to FIG. 17, because of the thermal confinement in the cup-shaped top electrode and the Peltier cooling at the top electrode interface, the surrounding temperature is reduced greatly. For conventional cell dimensions, one can clearly see around 8% to 10% reduction in the disturb temperature. Also, the 2D thermal profile gives a clear picture of how this projected top electrode helps in reducing the surrounding temperature, while attaining higher temperatures at the hotspot.

(51) The interface Peltier effect can assist the heating in the memory cell being programmed while cooling the surrounding. The interface for the current flow between the first electrode and the phase-change material points on the one side to the phase-change material of the PCM cell and on the opposite side to the phase-change material of at least one of neighboring PCM cells. The heating of the memory cell being programmed helps to improve the efficiency inside the memory cell being programmed, while the cooling of the surrounding helps to reduce the thermal disturbed temperature at surrounding memory cells.

(52) In particular, because of the different Seebeck coefficients of the first electrode and the phase-change material, thermoelectric heating and cooling can take place within the PCM cell. Depending on the direction of the current density, cooling and heating with respect to Joule heating without thermoelectric effect can happen.

(53) According to some implementations, the electrodes, in particular the first electrode, are designed such that the current density is directed suitably, the heating can occur within the written memory cell while cooling can occur at the neighboring cell(s). This reduces the thermal disturb, while at the same time increasing programming efficiency.

(54) According to an embodiment, the interface for the current flow between the first electrode and the phase-change material can be also called dominating interface, because the main amount of the current between the first electrode and the phase-change material that causes the interface Peltier effect flows over that interface.

(55) The PCM cell can be understood as a non-linear resistive device. In particular, the PCM cells form a memory array. In an embodiment, the dominating interface is configured for heating the PCM cell being programmed while cooling the neighboring PCM cell.

(56) In a further embodiment, the first electrode has a cup shape. The first electrode can also have a pot shape or a pan shape. The cup-shaped electrode, the pan-shaped electrode or the pot-shaped electrode can be also called extended electrode. By using that extended electrode, an interface is generated where the current flows laterally inwards the interface resulting in heating on one side while cooling at the other side at the surrounding.

(57) Further, a higher current density is provided in the side region of the extended electrode compared to the top region of the phase-change material. In case of a cup-shaped first electrode, the interface or dominating interface for the current flow between the first electrode and the phase-change material is at the connection of the side region of the cup-shaped electrode and the side region of the phase change material, particularly at the maximum distance to the top region of the phase-change material.

(58) In particular, the interface or dominating interface is a lateral interface ensuring a lateral current flow between the first electrode and the phase-change material such that the phase-change material of the programmed cell is heated while the phase-change material of the neighboring PCM cells is cooled.

(59) In a further embodiment, the first electrode and the phase-change material are arranged such that the surface normal of the dominating interface for a current flow between the first electrode and the phase-change material points on the one side to the phase-change material of the PCM cell and on the opposite side to the phase-change material of a plurality of the neighboring PCM cells.

(60) In a further embodiment, the first electrode is a top electrode and the second electrode is a bottom electrode, wherein the phase-change material is arranged between the first electrode and the second electrode.

(61) In a further embodiment, the first electrode is a cup-shaped electrode having a top region and a side region, wherein the top region of the cup-shaped electrode interfaces a top region of the phase-change material and the side region of the cup-shaped electrode interfaces a side region of the phase-change material.

(62) In a further embodiment, the top electrode is a cup-shaped top electrode having a top region and a side region, wherein the top region of the cup-shaped electrode interfaces a top region of the phase-change material and the side region of the cup-shaped electrode interfaces a side region of the phase-change material.

(63) In a further embodiment, each of the plurality of PCM cells has a geometry fulfilling the following condition: pcm.Math.A.sub.EL>el.Math.A.sub.PCM. A.sub.EL designates a cross-section of the side region of the cup-shaped top electrode for a vertical current flow through the side region. A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material. pcm designates an electrical resistivity of the phase-change material and

(64) el designates an electrical resistivity of the cup-shaped top electrode.

(65) In a further embodiment, each of the plurality of PCM cells has a geometry in which the following quotient is maximized: pcm.Math.A.sub.EL/el.Math.A.sub.PCM. By maximizing the above-mentioned quotient, it is ensured that the essential amount of the current flows over the side region of the cup-shaped top electrode to the phase-change material, but not over its top region to the phase change material.

(66) In a further embodiment, each of the plurality of PCM cells has a geometry fulfilling the following condition: H.Math.A.sub.INT>L.Math.A.sub.PCM, where A.sub.INT designates a cross-section of an interface for a lateral current flow between the side region of the cup-shaped top electrode and the phase-change material. A.sub.PCM designates a cross-section of the phase-change material for vertical current flow between the cup-shaped top electrode and the phase-change material. H designates a height of the phase-change material and L designates a minimum distance between the cup-shaped top electrode and the bottom electrode.

(67) In a further embodiment, each of the plurality of PCM cells has a geometry in which the following quotient is maximized: H.Math.A.sub.INT/L.Math.A.sub.PCM. By maximizing the above-mentioned quotients, it is ensured that the essential amount of the current flows over the side region of the cup-shaped top electrode to the phase-change material, but not over its top region to the phase change material.

(68) In a further embodiment, the first electrode includes a sidewall-electrode connected to a word line and the second electrode includes a heater electrode connected to a bit line.

(69) Thus, the design of the present embodiment is a sidewall-electrode cell design. In particular, the heater electrode or the sidewall-electrode is penetrated into the phase-change material (PCM layer) to generate a lateral current flow through the interface.

(70) In a further embodiment, the memory device includes a first interface for a current flow between the sidewall-electrode and the phase-change material, and a second interface for a current flow between the heater electrode and the phase change material. One of the first and second interfaces is a lateral interface and the other one of the first and second interfaces is a vertical interface.

(71) In a further embodiment, the first interface is the lateral interface and the second interface is the vertical interface.

(72) In a further embodiment, the first interface is the vertical interface and the second interface is the lateral interface.

(73) In a further embodiment, the material parameters of each of the plurality of PCM cells are chosen such that a Seebeck coefficient of the phase change material is maximized.

(74) In a further embodiment, the material parameters of each of the plurality of PCM cells are chosen such that a temperature-dependence of the Seebeck coefficient of the phase-change material is maximized.

(75) In a further embodiment, the material parameters of each of the plurality of PCM cells are chosen such that the difference between the Seebeck coefficient of the phase-change material and a Seebeck coefficient of the first electrode is maximized.

(76) The memory system includes a memory device of above mentioned in the embodiment thereof including a plurality of PCM cells each having a plurality of programmable levels, and a read/write apparatus for reading and writing data in the PCM cells.

(77) While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted without departing from the scope of the present invention. In addition, many modifications can be made to adapt a particular situation to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.