The present patent application claims the priority of the French patent application FR19/04900 that shall be considered as forming part of the present disclosure.
The present disclosure relates generally to electronic devices and, more particularly, to resistive memories and their selectors. More particularly, the present disclosure applies to a composition of a given selector.
Resistive random access memories integrated in “1T1R” type arrays are known. These arrays consist of structures comprising one transistor and one resistive cell. In such a structure, the transistor acts as a selector. This selector makes it possible to access the resistive memory cell, for read- or write- or erase- or programming operations (programming=write and erase) in a given memory cell, while limiting undesirable leakage currents in the rest of the memory array.
The selectors currently used in memory arrays have large dimensions as compared to those of the resistive cell. As a result, this impairs the density of memory arrays.
There is a need to optimize current selector elements.
One embodiment addresses all or some of the drawbacks of known selector elements.
One embodiment provides a selector for a memory cell, intended to change from a resistive state to a conductive state so as to prohibit or authorize access to the memory cell, respectively, characterized in that it is made of a GS-AT alloy consisting of germanium, selenium, arsenic and tellurium.
According to one embodiment, said GS-AT alloy is Ge3Se7As2Te3.
According to one embodiment, said GS-AT alloy has an arsenic and tellurium AT compound content of between 20% and 80%.
According to one embodiment, said GS-AT alloy has an arsenic and tellurium compound content AT of 40%.
According to one embodiment, said GS-AT alloy is obtained by physical vapor deposition.
According to one embodiment, said selector is an ovonic threshold switch.
One embodiment provides a memory point comprising:
According to one embodiment, the proportions of arsenic, tellurium, germanium and selenium are such that a threshold voltage of said selector is greater than or equal to a programming voltage of said resistive memory element.
According to one embodiment, the arsenic and tellurium content of the compound AT and the germanium and selenium content of the compound GS are such that a threshold voltage of said selector is greater than or equal to a programming voltage of said resistive memory element.
According to one embodiment, the arsenic and tellurium content of the compound AT and the germanium and selenium content of the compound GS are such that a threshold voltage of said selector is less than or equal to a state switching current of said resistive memory element.
One embodiment provides a memory having a plurality of memory points as described.
According to one embodiment, the memory is a resistive oxide memory or a conductive link random access memory.
According to one embodiment, each memory point comprises, in series:
According to one embodiment, the memory points are organized as an array.
According to one embodiment, each memory point is interposed between a first conductor and a second conductor.
One embodiment provides for such a memory comprising a three-dimensional stack of memory points, as described, separated by conductors.
One embodiment provides a method for manufacturing a memory comprising the following steps:
The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:
Like features have been designated by like references in the various Figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties.
For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the memory points may include elements not described, such as electrical connections.
Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements.
In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the Figures, as orientated during normal use.
Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%.
According to this embodiment, the memory 1 comprises memory points or cells 3 that make it possible to store all or part of a data item. The memory 1 also has electrical conductors 50, 51, 52, 53, 54 and 55. These conductors 50, 51, 52, 53, 54 and 55 are made of an electrically conductive metallic material such as copper.
The conductors 50, 51, 52, 53, 54 and 55 in
In
In
According to one embodiment, the memory 1 has a three-dimensional structure consisting of a stack of memory point layers separated by conductors. In the example shown in
According to a preferred embodiment, each memory point 3 comprises a stack, defining an electrical connection in series:
In
The conductors 50, 51, 52, 53, 54 and 55 make it possible to address the memory points 3 of the memory 1. Each memory point 3 is in fact connected to a conductor pair of its own, consisting of a first and a second conductor.
To read or write in the resistive memory element 31 of a memory point 3 of the memory 1, the memory point 3 considered is first selected. The selection is made by applying a difference in potential between the two conductors forming the pair of conductors specific to the memory point 3 considered, for example. This difference in potential is of sufficient value to make it possible to modify a state of the selector element 33 so that an electric current can flow through the memory point 3 considered. This electric current makes it possible to read or write in the memory point 3, based on its intensity.
Once the read or write operation is completed, a difference in potential is no longer applied between the two conductors forming the conductor pair specific to the memory point 3 considered. This has the effect of returning the selector element 33 to an initial or resting state.
During a read or write operation in a memory point 3, a leakage current, noted Ileak, parasitizes with other memory points in contact with one of the two conductors of the pair specific to the memory point 3 considered. This leakage current Ileak is due to the fact that, in practice, the selector elements 33 at rest do not have infinite resistance.
In an oxide-based resistive memory (OxRAM) 1, the resistive memory element 31 preferably consists of hafnium dioxide (HfO2) and titanium (Ti). The hafnium dioxide (HfO2) and titanium (Ti) form a bilayer, for example. In the stacked structure previously described, the titanium layer forming the resistive memory element 31 is optionally located in contact with the metal layer 35 or the second conductor 51, 53 or 55 of the memory point 3.
The metal layer 35, interposed between the memory element 31 and the selector element 33, is made of titanium nitride (TiN), for example.
In a variant, the memory 1 is a conductive bridging random access memory (CBRAM).
The selector element 33 is an ovonic threshold switch (OTS). The selector element 33 is made from amorphous chalcogenide materials, for example. One property of amorphous chalcogenide materials is that when a voltage greater than a threshold voltage (Vth) is applied to them, they go from a highly resistive level to a highly conductive level. This highly conductive level has a lower electrical resistance than the highly resistive level. Conventionally, the highly resistive (or low conductive) level is said to correspond to a blocking state, preventing access to the memory element 31, while the highly conductive (or low resistive) level corresponds to an ‘on’ or conducting state, allowing access to the memory element 31.
The ‘on’ state is called volatile (or temporary, or non-permanent), because this ‘on’ state is maintained as long as an electric current flowing through the selector element 33 remains higher than a holding current, noted Ih. When the electric current flowing through the selector element 33 becomes lower than the holding current Ih, the selector element 33 is then in back the blocking state.
It is useful to take advantage of this property of amorphous chalcogenide materials to make selector elements 33 of the memory 1. The ‘on’ state makes it possible to have an electric current flow, in order to read or program in a memory element 31. The blocking state, which is highly resistive, prevents access to the memory element 31 and limits the leakage current Ileak in the non-selected or non-addressed memory points 3 of the memory 1.
According to the embodiment of
Each memory point 3 of the array 5 has a structure similar to that of the memory points 3 shown in relation to
A specific memory point is considered in the example of
Assuming that one wishes to read or write in the memory point 3 of the array 5, a voltage, noted V, is applied between the two conductors 52 and 53 connected on either side of the memory point 3. This voltage V is obtained by bringing the conductor 52 to an electric potential of value −V/2 and the conductor 53 to a higher electric potential, of value V/2, for example. All other conductors 50, 51, 54 and 55 are maintained at a potential of approximately zero volts (0 V). Thus, a difference in potential is imposed between the points 533 and 523, where the memory point 3 contacts the conductors 53 and 52 respectively, which is approximately equal to the voltage V.
When this voltage V reaches a value greater than or equal to the threshold voltage Vth of the selector element 33, this selector element 33 then becomes conductive and a current, noted I, flows (dotted arrows) in the memory point 3 and in part of the conductors 52 and 53 under the effect of the voltage applied to the memory point 3. The memory element 31 of the memory point 3 is then said to be selected by the selector element 33. The value of the current I is adapted, based on the desired read or write operation.
It is advisable to ensure that the selector element 33 has as low a leakage current Ileak as possible in the blocking state. This Ileak leakage current is detrimental to the operating performance of the array 5.
It is also desirable for the selector element 33 to be non-linear, i.e. to have the greatest possible difference in conductivity between the conducting state and the blocking state. This prevents neighboring memory points of a selected memory point (e.g. the memory points to the left and right of the memory point detailed in
In
It is assumed that the resistive memory element 31 of the memory point 3 stores a binary value. Conventionally, a high state of this binary value is designated “ON” and a low state of this same binary value is designated “OFF”. In this example, the ON state is considered to correspond to a low-resistance state (LRS) and the OFF state is considered to correspond to a high-resistance state (HRS). The low-resistance state, ON, is of a lower electrical resistance than the high-resistance state, OFF.
The left side of
The resistive memory element 31 is changed over from the OFF state to the ON state by causing a current Ito flow through the memory point 3 at an intensity greater than a switching threshold, noted IHRS. In
For a read operation in the memory point 3, a voltage noted VREAD is applied between its terminals 533 and 523 (
The value of the current flowing in the memory point 3 is then measured. If the resistive memory element 31 is in the low state, OFF (curve 24), a current of value noted IOFF is measured by applying the voltage VREAD. On the other hand, if the resistive memory element 31 is in the high state, ON (curve 26), a current of value noted ION greater than the value IOFF is measured by applying the voltage VREAD. Measurement of the current flowing through the memory point 3, when a voltage VREAD within the range of voltages ΔVth is applied to its terminals, thus provides the binary value stored or recorded by the resistive memory element 31.
The resistive memory element 31 is initially in the ON state. The resistive memory element 31 is switched from the ON state to the OFF state by applying a reset voltage, noted VRESET (not shown in
In
The selector element 33 of the memory point 3 consists of a germanium (Ge), selenium (Se), arsenic (As) and tellurium (Te) based alloy. Again, according to this embodiment, the selector element 33 is an ovonic threshold switch. Alloys or compounds of germanium and selenium enriched in selenium, noted GS, have good thermal stability. This makes it possible to facilitate the implementation of manufacturing steps of memory points 3 of a memory such as the memory 1 (
However, the GS alloys used in the selector elements 33 are characterized by poor switching properties, in particular due to too high a threshold voltage Vth. The selector elements 33 based on GS alloys also exhibit poor switching endurance when successive transition cycles between their highly resistive level (blocking state) and their highly conductive level (conducting state), are applied to them.
According to the described embodiments, advantage is taken of the fact that the addition of materials called dopants improves the switching properties of selector elements made from GS alloys. In particular, adding an arsenic and tellurium-based material or compound, denoted AT, to a GS alloy of a selector element 33 makes it possible to reduce the threshold voltage Vth of the selector element 33 while improving its switching endurance. By adjusting the AT content (or AT proportion) in a GS alloy of which the selector element 33 is made, it is thus possible to obtain a balance between thermal stability of the alloy and switching properties of the selector element 33.
According to one preferred embodiment, the selector element 33 is manufactured from an alloy formed from a GS/AT pseudo-binary system, more preferably Ge3Se7As2Te3. The selector element 33 is manufactured by physical vapor deposition (PVD), for example. The Ge3Se7As2Te3 alloy thus consists of:
In
The curve 4 has six points, with each corresponding to a different AT alloy content in the GS-AT alloy:
It is of interest to seek to adjust the threshold voltage Vth, the threshold current Ith and/or the leakage current Ileak of the selector element 33, in particular to associate this selector element 33 with a resistive memory element 31, to make them compatible. In this example, the programming voltage VSET of the resistive memory element 31 is assumed to be approximately equal to 1.5 V (point 41, VSET×IHRS). Again, in this example, the reset voltage VRESET of the resistive memory element 31 is assumed to be approximately equal to 2 V (point 43, VRESET×ILRS).
As discussed in connection with the preceding Figures, it is then sought to obtain a selector element 33 having a threshold voltage Vth greater than or equal to the programming voltage VSET of the resistive memory element 31. This makes it possible to ensure that the selector element 33 is compatible with the resistive memory element 31.
Additionally, it may also be sought to make the threshold current Ith of the selector element 33 less than or equal to the changeover threshold current IHRS of the resistive memory element 31. The closer the threshold current Ith of the selector element 33 is to the changeover threshold current IHRS of the resistive memory element 31, the larger the programming window. The ideal case would therefore be that Ith is equal to IHRS.
In addition, it may also be sought to ensure that the leakage current Ileak is as low as possible. The lower the Ileak leakage current, the larger the size of the envisaged memory array can be.
Thus, according to the example characteristics (41, 43) of the resistive memory element 31 shown in
Thus, for any memory type (OxRAM, CBRAM, or PCM), the selector consisting of a GS-AT alloy has proportions of either of its constituent GS or AT compounds such that it has the following features, in order of priority, taken alone or in combination:
If one wishes to manufacture a selector element 33 for a memory element 31 with known characteristics (VSET, IHRS), one can thus adjust the GS-AT alloy composition by means of the curve 4 in
A memory 1 (
In
As discussed in connection with
In the example shown in
In order to make a selector element 33 that offers the largest possible programming window with the resistive memory element 31, a GS-AT alloy with an AT content is thus selected that makes it possible to obtain a threshold current value Ith that is as close as possible to the changeover threshold current IHRS of the memory element 31, while limiting the leakage currents Ileak, depending on the desired array size. Specifically, this
As discussed in connection with
In view A, the variation of the threshold voltage Vth of the selector element 33 (y-axis, in volts) based on its AT alloy content or proportion (x-axis, in atomic percent) in the GS-AT mixture is of interest. It can be seen that the threshold voltage Vth decreases as the AT alloy content (in As2Te3, in this example) increases. Noting the As2Te3 content or concentration as CAT and a thickness of the selector element 33 as tOTS, advantage is taken of the fact that the value of the threshold voltage Vth of the selector element 33 is linked to the atomic percentage of the arsenic and tellurium compound in the GS-AT alloy, in particular in Ge3Se7As2Te3, by a relationship of the type:
Vth=(−1.5×10−3×CAT+0.13)×tOTS+(−4×10−4×CAT+0.7) [Math 1]
In view B, the variation of the leakage current Ileak of the selector element 33 (y-axis, in amperes) based on its AT alloy content (x-axis, in atomic percent) in the GS-AT alloy, namely in Ge3Se7As2Te3, is of interest. It can be seen that the Ileak leakage current increases as the AT alloy content (in As2Te3, in this example) increases. Still noting the As2Te3 content or concentration as CAT and a thickness of the selector element 33 as tOTS, and noting the exponential function as exp, advantage is taken of the fact that the value of the leakage current Ileak is linked to the atomic percentage of the arsenic and tellurium compound in the GS-AT alloy, in particular in Ge3Se7As2Te3, by a relationship of the type:
Ileak=exp(0.09×CAT)×exp((75.8/tOTS)−27) [Math 2]
Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. In particular, the content CAT composed of arsenic and tellurium in the GS-AT alloy of the selector element 33, may be adjusted according to other characteristic properties of the resistive memory element 31.
Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional description provided hereinabove.
Number | Date | Country | Kind |
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1904900 | May 2019 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2020/050741 | 5/4/2020 | WO | 00 |