Bidirectional threshold switch, cross-point memory device and manufacturing method thereof

Abstract

The present invention discloses a bidirectional threshold switch, a cross-point memory device and a method for manufacturing the same, wherein the bidirectional threshold switch is composed of Si, Ge, Se, As and a specific content of a chalcogen, wherein the chalcogen effectively passivates oxidation of the composition in the presence of water vapor, wherein the chalcogen includes but is not limited to Te, S or a combination thereof. In one or more embodiments, the chalcogen is S. In one or more embodiments, the chalcogen is Te. In one or more embodiments, the effective content of the chalcogen is greater than 1% by atomic percentage. In one or more embodiments, the effective content of the chalcogen is less than 10% by atomic percentage. In one or more embodiments, the bidirectional threshold switch includes 10% Si, 15% Ge, 40% Se, 30% As and 5% chalcogen by atomic percentage.

Description

Technical Field

[0001] The present invention relates to the electrical field, the electronic field and the computer field, and more specifically to a bidirectional threshold switch, a cross-point memory device and a manufacturing method thereof. Background Art

[0002] Phase change memory has been demonstrated to be a good candidate for storage class memory and has been commercialized. For cross point memory, phase change material should be incorporated into selector material (e.g., chalcogenide) as a single memory cell. Phase change memory material has an amorphous phase and a crystalline phase, and the phase change is accomplished by a supply current flowing through a resistive element (e.g., TiN) to heat the material briefly and quickly (to switch to the amorphous phase) or slowly and for a longer period of time (to switch to the crystalline phase). In the amorphous phase, the phase change material has a very high resistance; however, in the crystalline phase, the phase change material has a low resistance. Cross point memory also incorporates ovonic threshold switches (OTS), which use chalcogenide materials that conduct electricity between an off state (highly resistive) or an on state (highly conductive) by applying a voltage. The material of the ovonic threshold switch is critical to preventing sneak currents in the cross point memory array. One requirement is that during a read operation on a cross-point memory array, the off current of a single bidirectional threshold switch device should be sufficiently low (the off current is usually measured at half the threshold voltage (Vt) of the bidirectional threshold switch material). For example, if the read current of the cross-point array is 1 micro-amp in a 1000×1000 element array, the off current at half the threshold voltage in a single bidirectional threshold switch device should be less than 1 nano-amp. Summary of the invention

[0003] The principles of the present invention provide a technique for inhibiting oxidation of silicon selenide arsenide materials. In one embodiment, an exemplary material composition consists essentially of Si, Ge, Se, As, and a chalcogen in an amount effective to passivate oxidation of the composition in the presence of water vapor, wherein the chalcogen is selected from, but not limited to, Te, S, or a combination thereof. In one or more embodiments, the chalcogen is S. In one or more embodiments, the chalcogen is Te. In one or more embodiments, the effective content of the chalcogen is greater than 1% by atomic percentage. In one or more embodiments, the effective content of the chalcogen is less than 10% by atomic percentage. In one or more embodiments, the material composition includes, by atomic percentage, 10% Si, 15% Ge, 40% Se, 30% As, and 5% chalcogen.

[0004] According to another embodiment, an exemplary cross-point memory device includes: a row line; a top electrode adjacent to the row line and connected in electrical communication with the row line; a bidirectional threshold switch (OTS) adjacent to the top electrode and connected in electrical communication with the top electrode; a middle electrode adjacent to the bidirectional threshold switch and connected in electrical communication with the bidirectional threshold switch; a phase change memory (PCM) adjacent to the middle electrode and connected in electrical communication with the middle electrode; a bottom electrode adjacent to the PCM and connected in electrical communication with the PCM; and a column line adjacent to the bottom electrode and connected in electrical communication with the bottom electrode. The bidirectional threshold switch includes a thin film composed of the composition of matter of the present invention.

[0005] According to another embodiment, an exemplary method includes: forming a bottom electrode; forming a phase change memory (PCM) adjacent to the bottom electrode and connected in electrical communication with the bottom electrode; forming an intermediate electrode adjacent to the PCM and connected in electrical communication with the PCM; forming a bidirectional threshold switch (OTS) adjacent to the intermediate electrode and connected in electrical communication with the intermediate electrode; and forming a top electrode adjacent to the bidirectional threshold switch and connected in electrical communication with the bidirectional threshold switch. The bidirectional threshold switch includes a thin film composed of the composition of matter of the present invention.

[0006] In view of the foregoing, the technology of the present invention can provide substantially beneficial technical effects. For example, one or more embodiments provide one or more of the following:

[0007] In the final stable state, SiGeSeAs materials doped with S or Te can effectively prevent the release of arsenic gas.

[0008] Advantages and other features of the present invention will become apparent from the detailed description of embodiments of the present invention and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a phase change memory (PCM) cell according to an exemplary embodiment.

[0010] Figure 2 A cross-point memory array including a plurality of phase-change memory cells according to an exemplary embodiment.

[0011] Figure 3A It is the relationship between the outgassing from the SiSe(As) composition and time.

[0012] Figure 3B is the relationship between exhaust gas and time from a composition according to an exemplary embodiment.

[0013]

Explanation of symbols

[0014] 100: Phase change memory cell

[0015] 102: Memory stack

[0016] 104: Metal

[0017] 106: Column metal

[0018] 108: Top electrode

[0019] 110: Bidirectional threshold switch

[0020] 112: Intermediate electrode

[0021] 114: Phase Change Memory

[0022] 116: Bottom electrode

[0023] 200: Cross-point memory array DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0025] Among the materials for bidirectional threshold switches, GeAsSe substrate material is a good candidate because it can provide low leakage current in the off state. However, the thermal stability of GeAsSe does not meet the process requirements. The thermal stability can be improved by adding certain dopants, but the dopants may make the doped material chemically unstable (for example, the material reacts with moisture in the air and releases gas, causing the electrical characteristics of the bidirectional threshold switch to be changed).

[0026] Figure 1 A phase change memory (PCM) cell 100 is depicted according to an exemplary embodiment. The phase change memory cell 100 includes a memory stack 102 electrically connected between a row metal 104 and a column metal 106. The memory stack 102 includes a top electrode 108, a bidirectional threshold switch 110, an intermediate electrode 112, a phase change memory 114, and a bottom electrode 116. In operation of the phase change memory cell 100, a row voltage is supplied to the row metal 104 and a column voltage is supplied to the column metal 106. When the difference between the column voltage and the row voltage exceeds the threshold voltage of the bidirectional threshold switch 110, current flows through the memory stack 102.

[0027] For a write operation, the column and row voltages are set so that the flowing current exceeds the threshold current of the phase change memory 114, and the phase change memory is heated above its phase change temperature. Once the phase change memory 114 exceeds its phase change temperature, there are two options: 1) remove the column and row voltages, in which case the current stops flowing and the phase change memory quickly cools to the amorphous (high resistance) phase; or 2) maintain the column and row voltages for a certain period of time, in which case the current continues to flow and the phase change memory stabilizes in the crystalline (lower resistance) phase.

[0028] For a read operation, the column voltage and the row voltage are set so that the flowing current does not exceed the threshold current; the current is then read as the value of the memory cell according to the change in resistance of the phase change memory 114. Since the resistance value of the phase change memory 114 has the characteristics of an analog signal (it changes according to how long the phase change memory is kept above the phase change temperature), the phase change memory cell can have a higher density than other types of memory.

[0029] Figure 2 Depicted is a cross-point memory array 200 including a plurality of phase change memory cells 100, row lines 104, and column lines 106 according to an exemplary embodiment. In particular, given that each phase change memory cell can store non-binary (e.g., ternary) values, a dense array (e.g., 4,096×4,096) of phase change memory cells 100 is capable of storing megabits of data.

[0030] We have discovered that GeAsSe material (which has not been used previously in phase change memory cells) has substantially lower leakage current than GeAsTe in bidirectional threshold switch applications. However, GeAsSe has not met the thermal stability requirements needed in processing, but the electrical properties are strongly dependent on the material's operating temperature. We have also discovered that adding small amounts of Si dopants can improve the electrical properties of SiGeAsSe. We have also discovered that SiSe(As) substrate materials are susceptible to oxidation and release AsH3 or SeH2 when the material is exposed to moisture in the air after film deposition or processing. Exhausting either gas will change the electrical properties of the substrate material. For example, when a SiSe(As) film is exposed to moisture, any of the following reactions will occur:

[0031] SiSe+H2O→SiO2+SeH2

[0032] SiAs+H2O→SiO2+AsH3

[0033] If the material is oxidized or unstable, the device becomes very leaky and produces higher off-state current.Addition of any highly reactive elements (eg, Al, Si, or possibly Ti) to the GeSeAs material is also expected to enhance thermal stability, but also produce outgassing.

[0034] Therefore, SiGeAsSe is not thermally stable but chemically (humidity) stable. For thin films, oxidation can be prevented by a thick protective layer; however, if this protective layer is not deposited on the material during sample preparation, the electrical properties of the material may degrade before they can be measured.

[0035] From other studies, we know that AlAs (like AsSeGeSi) is very susceptible to oxidation to release AsH3 when exposed to a water vapor environment, and we know that the oxidation rate can be reduced by adding Ga (up to about 10%) to AlAs. Therefore, we experimentally discovered whether dopants could be added to the AsSeGeSi system to chemically stabilize the electrical characteristics for cross-point memory operation.

[0036] We have found that Te dopants or S dopants at an effective level (ie, 1 to 10 atomic percent) can reduce oxidation and outgassing from SiSe(As) compositions (and more specifically, AsGeSeSi compositions) and stabilize electrical properties.

[0037] For example, by adding an effective amount of Te to SiGeSeAs substrate material, as in Figure 3A and Figure 3B As shown in the comparison, arsenic emissions can be reduced. Figure 3A The time curve of arsenic emission rate (in parts per billion (ppb)) from AsGeSeSi composition under given test conditions in the atmosphere is shown. In contrast, Figure 3B The time curve of arsenic emission (in ppb) from an AsGeSeSiTe composition under the same given test conditions according to an embodiment of the present invention is shown. Specifically, Figure 3B The time curve of arsenic emission is shown for a composition consisting essentially of, in atomic percent: about 30% As, about 15% Ge, about 40% Se, about 10% Si, and about 5% Te.

[0038] The test conditions for both curves include: sputtering the material onto a silicon wafer substrate in a processing chamber and transferring it from the processing chamber to a wafer loadlock, opening the gate of the wafer loadlock, introducing air into the wafer loadlock, and sampling the gas in the chamber after a period of time until the exhaust is determined to be continuous or stopped. It is worth noting that the AsGeSeSiTe composition ( Figure 3B ) stops exhausting in about six minutes, while the composition without Te dopant ( Figure 3A) was degassing for more than seven hours. Thus, the Te dopant passivates the composition under oxidizing conditions. We expect similar beneficial results to be obtained for compositions including Te-doped As, Ge, Se and Si and other combination compositions.

[0039] Similarly, by doping an effective amount of S (1% to 10% by atomic percentage) into the SiGeSeAs substrate material, oxidation can be passivated and arsenic gas emissions can be reduced, thereby achieving the effect of stabilizing electrical characteristics.

[0040] 1% dopant is the minimum effective content for chemical stability, whereas a dopant percentage greater than 10% will unduly increase leakage current and / or change the phase change temperature of a phase change memory or bidirectional threshold switch formed from the composition.

[0041] In view of the discussion thus far, in general, exemplary compositions of matter according to embodiments of the present invention consist essentially of Si, Ge, Se, As, and a specific amount of a chalcogen that is effective to passivate oxidation of the composition in the presence of water vapor, wherein the chalcogen includes, but is not limited to, Te, S, or a combination thereof. In one or more embodiments, the chalcogen is S. In one or more embodiments, the chalcogen is Te. In one or more embodiments, the effective amount of the chalcogen is greater than 1% by atomic percentage. In one or more embodiments, the effective amount of the chalcogen is less than 10% by atomic percentage. In one or more embodiments, the composition of matter includes, by atomic percentage, 10% Si, 15% Ge, 40% Se, 30% As, and 5% chalcogen.

[0042] According to another embodiment, an exemplary cross-point memory device includes: a row line; a top electrode adjacent to the row line and connected in electrical communication with the row line; a bidirectional threshold switch (OTS) adjacent to the top electrode and connected in electrical communication with the top electrode; a middle electrode adjacent to the bidirectional threshold switch and connected in electrical communication with the bidirectional threshold switch; a phase change memory (PCM) adjacent to the middle electrode and connected in electrical communication with the middle electrode; a bottom electrode adjacent to the PCM and connected in electrical communication with the PCM; and a column line adjacent to the bottom electrode and connected in electrical communication with the bottom electrode. The bidirectional threshold switch includes a thin film composed of the composition of matter of the present invention.

[0043] According to another embodiment, an exemplary method includes: forming a bottom electrode; forming a phase change memory (PCM) adjacent to the bottom electrode and connected in electrical communication with the bottom electrode; forming an intermediate electrode adjacent to the PCM and connected in electrical communication with the PCM; forming a bidirectional threshold switch (OTS) adjacent to the intermediate electrode and connected in electrical communication with the intermediate electrode; and forming a top electrode adjacent to the bidirectional threshold switch and connected in electrical communication with the bidirectional threshold switch. The bidirectional threshold switch includes a thin film composed of the composition of matter of the present invention.

[0044] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A bidirectional threshold switch, wherein: The bidirectional threshold switch is composed of 10% Si, 15% Ge, 40% Se, 30% As and 5% chalcogen, in terms of atomic percentage, and the chalcogen effectively passivates the oxidation of the bidirectional threshold switch in the presence of water vapor, wherein the chalcogen includes Te, S or a combination thereof.

2. A cross-point memory device comprising: Line a top electrode adjacent to the row line and connected in electrical communication with the row line; a bidirectional threshold switch adjacent to the top electrode and connected in electrical communication with the top electrode; an intermediate electrode, adjacent to the bidirectional threshold switch and electrically connected to the bidirectional threshold switch; A phase change memory, adjacent to the middle electrode and connected to the middle electrode in an electrically connected manner; a bottom electrode adjacent to the phase change memory and connected to the phase change memory in an electrically connected manner; as well as a column line adjacent to the bottom electrode and connected in electrical communication with the bottom electrode; Wherein, the bidirectional threshold switch is composed of 10% Si, 15% Ge, 40% Se, 30% As and 5% chalcogens by atomic percentage, and in the presence of water vapor, the chalcogens effectively passivate the oxidation of the bidirectional threshold switch, wherein the chalcogens include Te, S or a combination thereof.

3. A method for manufacturing a cross-point memory device, comprising: forming a bottom electrode; forming a phase change memory, the phase change memory being adjacent to the bottom electrode and being connected in electrical communication with the bottom electrode; forming an intermediate electrode, the intermediate electrode being adjacent to the phase change memory and being electrically connected to the phase change memory; forming a bidirectional threshold switch adjacent to and in electrical communication with the middle electrode; as well as forming a top electrode adjacent to and in electrical communication with the bidirectional threshold switch, Wherein, the bidirectional threshold switch is composed of 10% Si, 15% Ge, 40% Se, 30% As and 5% chalcogens by atomic percentage, and in the presence of water vapor, the chalcogens effectively passivate the oxidation of the bidirectional threshold switch, wherein the chalcogens include Te, S or a combination thereof. 4 . The method for manufacturing a cross-point memory device according to claim 3 , wherein the chalcogen element is Te.

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