Patent Application
20160211447
The present application is a non-provisional patent application claiming priority to European Patent Application No. 15151265.4 filed Jan. 15, 2015, the contents of which are hereby incorporated by reference.
The present disclosure relates to Conductive Bridging Random Access Memory devices, also known as CBRAM devices. In particular, the present disclosure relates to a CBRAM device having a top electrode comprising Cu, Te, and (Ge or Si).
The Conductive Bridging Random Access Memory (CBRAM) device is considered as a valuable non-volatile storage technology. It offers fast switching, high endurance, low voltage and good scalability.
A CBRAM device contains an insulating layer sandwiched between an active electrode providing metal cations, e.g. Cu+ or Ag+, and an inert electrode. The operation of the CBRAM device relies on the voltage-induced redox-based formation and rupture of a Cu- or Ag-based conical shaped conductive filament (CF) in the insulating layer acting as a solid state electrolyte. When an electrical field is applied between both electrodes, Cu- or Ag cations provided by the active electrode will drift towards the opposite inert electrode through the insulating layer thereby forming the conductive filament in the electrolyte. As such, the CBRAM device can be reversibly switched between a high resistive (HRS) or reset state and a low resistive (LRS) or set state.
Some CBRAM devices, such as those having a Cu-containing active electrode, may have poor retention properties. Retention is the stability of respectively both the LRS and the FIRS state and may relate to the lifetime of the device at an elevated temperature. A typical specification is a lifetime of 10 years at, depending on the application, a temperature of 50° C., 85° C. or even up to 150° C.
One method to improve the retention of the LRS state only, i.e. the stability of the formed metallic CF, is to engineer an electrolyte element in which the Cu—CF will not dissolve. Examples of such electrolytes are oxides, such as GdO2 or Al2O3, resulting in a LRS retention of the CBRAM device higher than if chalcogenide electrolytes are used.
Another method to partly improve the retention of a CBRAM device is to engineer an appropriate active electrode that supplies Cu-ions. CBRAM devices having a Cu active electrode exhibit a poor FIRS retention. By adding Te to the Cu active electrode, the FIRS stability is increased, however at the expense of a reduced LRS stability. This reduced stability effect was attributed to the chemical affinity between Cu and Te, which favors the Cu back-diffusion from the CF towards the Cu—Te supply element. By incorporating Ge into the Cu—Te containing active electrode, a trade-off can be made between the thermal stability of the LRS and the FIRS states.
These prior art devices however showed a decreased performance when subjected to elevated temperatures.
Hence, there is a need for a CBRAM device that does not exhibit the shortcomings of the conventional CBRAM devices.
A Conductive Bridge Random Access Memory (CBRAM) device is disclosed comprising an insulating electrolyte element sandwiched between a cation supply metal electrode and a bottom electrode. The cation supply metal electrode (3) comprises a CuxZyTez alloy, with Z being Ge or Si and with 0<x, y, z<100 at. %, whereby y>15 at. %.
In an example embodiment including a CuxGeyTez alloy, the ratio Cu/(Cu+Te) of the alloy may range from 0.4 and 0.77, i.e. 0.4≦x/(x+z)≦0.77.
Within this range x, y, and z are determined to have a CuxGeyTez alloy having a mono-phase crystalline structure. In an example embodiment, this mono-phase crystalline structure is a CuxGeyTez alloy with a composition x=33 at. %, y=17 at. % and z=50 at. %.
Within this range, x, y, and z are determined to have a CuxGeyTez alloy with an amorphous structure. In an example embodiment, this amorphous structure is a CuxGeyTez alloy with a composition x=50 at. %, y=35 at. % and z=15 at. %. In another embodiment, this amorphous structure is a CuxGeyTez alloy with a composition x=48 at. %, y=20 at. % and z=32 at. %.
Within this range, x, y, and z are determined to have a CuxSiyTez alloy with an amorphous structure. In an example embodiment, this amorphous structure is a CuxSiyTez alloy with a composition x=48 at. %, y=20 at. % and z=32 at. %.
For the purpose of teaching, drawings are added. These drawings illustrate some aspects and embodiments of the disclosure. They are only schematic and non-limiting. The size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the disclosure. Like features are given the same reference number.
The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto. Furthermore, the terms first, second and the like in the description, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, under and the like in the description are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other orientations than described or illustrated herein.
The present disclosure relates to a CBRAM device (1) with improved thermal stability. The CBRAM device has an active electrode (3) containing Cu and Te, whereby this electrode (3) is further alloyed with Si or Ge.
In an example embodiment, a CBRAM device is disclosed that comprises an active electrode (3) comprising Cu, Ge and Te, whereby the active electrode (3) has an improved thermal stability.
Conventional Cu—Ge—Te active electrodes may include a microstructure of this (Cu,Ge,Te)-alloy. However, such active electrodes may be thermally unstable. At elevated temperatures, these conventional Cu—Ge—Te alloys suffer from, amongst other issues, phase separation or undesired crystallization. Such effects may lead to more roughened surfaces, which may reduce reproducibility and may cause the device to malfunction. Due to such thermal instability, the further manufacturing of the CBRAM device once the active electrode is formed, may be impacted. For example, the thermal budget allowed during the Back-End-Of-Line (BEOL) processing when establishing an electrical interconnect to the CBRAM device may depend, at least in part, on the thermal instability of the active electrode. This instability jeopardizes the programming properties of the finished CBRAM device. Additionally, thermal instability makes improving the retention of both the LRS and the HRS more difficult. For example, a non-uniformity might be introduced between CBRAM devices due to this thermal instability. The affinity between the conductive filament and the active electrode (supply layer) may vary locally if the microstructure of the active electrode is not uniform due to different bonding configuration of Cu electrode with the alloying elements Te and Ge. Hence the HRS and LRS stability may be non-uniform from cell to cell.
Such CBRAM device is illustrated in
In an example embodiment, the cation supply electrode (3) comprises Cu, Ge and Te, thereby supplying Cu+ cations during programming of the CBRAM device (1). The cation supply metal electrode (3) comprises a CuxGeyTez alloy, with 0<x, y, z<100 at. %, whereby y>15 at. %.
In a first embodiment, the cation supply electrode (3) consists of a Cu2Ge1Te3 alloy: Cu33Ge17Te50. Initially formed as an amorphous layer, its composition favors a single type of chemical binding between Cu and the alloying elements Te and Ge. After crystallization, typically at a temperature of about 200° C., this Cu—Ge—Te alloy exhibits a single stable phase up to the typical temperatures applied during the BEOL processing, i.e. temperatures between 350° C. and 450° C. Back-End-Of-Line (BEOL) processing refers the process steps establishing an electrical interconnect to the CBRAM device. These process steps include the formation of at least one metallic interconnect pattern separated by one or more dielectric layers, whereby openings in the dielectric layers allows electrically contacting the underlying CBRAM device and, if present, additional metallic interconnect patterns above. Alloys having another Cu—Ge—Te composition segregate into different crystal phases during such BEOL processing. Such segregations are not desired and may for example lead to non-uniform device properties. Retention properties were demonstrated for a CBRAM device having such Cu2GeTe3 alloy being at least comparable to conventional CBRAM devices having a Cu or Cu6Te4 cation supply electrode (3), but with an improved uniformity due, at least in part, to the homogenous alloy structure.
In an example embodiment, a 50 nm Pt/50 nm Cu-supply layer is deposited by sputter deposition through a 580 μm dot shadow mask onto a 3 nm Al2O3 layer. The Al2O3 layer in turn is deposited by an H2O based ALD process on n-doped Si.
In a second embodiment, the cation supply electrode (3) comprises a CuxGeyTez alloy whereby the composition is selected to yield an alloy that remains amorphous over a larger temperature window, such as up to the temperatures selected for the BEOL processing, which processing temperatures are typically below 500° C. or below 400° C.
In an example of this second embodiment, the ratio (Cu/Te)>1. Such an alloy is referred to as a Te-poor (Cu,Ge,Te)-alloy. In an example embodiment, this alloy has a composition of Cu50Ge35Te15. By reducing the amount of Te in the alloy, the crystallization temperature of the CuxGeyTez alloy may be increased, for example, to about 300° C. If the temperatures applied during the BEOL processing is below this crystallization temperature, i.e. below 300° C., the alloy will remain amorphous during the BEOL processing. This may prevent phase separation as the formation of different crystal phases will occur at higher temperatures. Good quality retention characteristics were demonstrated for compositions where ratio (Cu/Te)>1.
In an example embodiment, a 50 nm Pt/50 nm Cu-supply layer is deposited by sputter deposition through a 580 μm dot shadow mask onto a 3 nm Al2O3 layer, which in turn is deposited by an H2O based ALD process on n-doped Si.
In another example embodiment, the cation supply electrode (3) comprises a CuxGeyTez alloy, whereby the ratio (Cu/Te) ratio can be in the range of (50/35) to (50/40). In one example, this alloy has a composition Cu48Ge20Te32. By increasing the amount of Te in the alloy, the crystallization temperature of the CuxGeyTez alloy is reduced, for example, to about 200° C. If the temperatures applied during the BEOL processing are below this crystallization temperature, i.e. below 200° C., this alloy will remain again amorphous during the BEOL processing. This prevents phase separation because the formation of different crystal phases will generally take place at higher temperatures.
As discussed elsewhere herein, the CuTe cation supply electrode (3) may be alloyed to yield an alloy that remains amorphous over a larger temperature window, such as up to the temperatures selected for the BEOL, which processing temperatures are typically below 500° C. or below 400° C.
The inventors had found that using Si as alloying element to the CuTe cation supply electrode (3), in a device as illustrated in
In an example embodiment, such an amorphous electrode may be obtained by a CuxSiyTez alloy with a composition x=48 at. %, y=20 at. % and z=32 at. %. Similar to the embodiments illustrated by
In an example embodiment, the dielectric material of insulating electrolyte element (2) may be selected from the group of chalcogenides. This dielectric material can be selected from the group of mixed ionic-electronic conductors (MIEC), such as CeO, ZrO, Y2O3, Yttria Stabilized Zirconia (YSZ), characterized by their very high ionic conductivity. In an example embodiment, the dielectric material is selected from the group of alumina oxides, hafnium oxides, tantalum oxides, silicon oxides of silicon nitrides.
Optionally a metallic liner (not shown) may be formed. The metallic liner may separate the cation supply electrode (3) from the electrolyte element (2). This metallic liner may contain Ta or TiW, for example. The metallic liner may improve the adhesion between the Cu supply layer and the electrolyte. In addition, the metallic liner may reduce Cu migration into the electrolyte during the BEOL processing.
Typically the bottom electrode (4) comprises tungsten. In an example embodiment, the bottom electrode (4) is formed of tungsten.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15151265.4 | Jan 2015 | EP | regional |
