Patent Application
20140264247
The present disclosure relates to resistive memory cells, e.g., conductive bridging random access memory (CBRAM) or resistive random-access memory (ReRAM) cells, having an asymmetrical structure (e.g., including a ring-shaped bottom electrode) providing a reduced area for the formation of conductive paths (e.g., conductive filaments or vacancy chains).
Resistive memory cells, such as conductive bridging memory (CBRAM) and resistive RAM (ReRAM) cells are a new type of non-volatile memory cells that provide scaling and cost advantages over conventional Flash memory cells. A CBRAM is based on the physical re-location of ions within a solid electrolyte. A CBRAM memory call can be made of two solid metal electrodes, one relatively inert (e.g., tungsten) the other electrochemically active (e.g., silver or copper), with a thin film of the electrolyte between them. The fundamental idea of a CBRAM cell is to create programmable conducting filaments, formed by either single or very few nanometer-scale ions across a normally non-conducting film through the application of a bias voltage across the non-conducting film. The non-conducting film is referred to as the electrolyte since it creates the filament through an oxidation/reduction process much like in a battery. In a ReRAM cell the conduction is through creation of a vacancy chain in an insulator. The creation of the filament/vacancy-chain creates an on-state (high conduction between the electrodes), while the dissolution of the filament/vacancy-chain is by applying a similar polarity with Joule heating current or an opposite polarity but at smaller currents to revert the electrolyte/insulator back to its nonconductive off-state.
A wide range of materials have been demonstrated for possible use in resistive memory cells, both for the electrolyte and the electrodes. One example is the Cu/SiOx based cell in which the Cu is the active metal-source electrode and the SiOx is the electrolyte.
One common problem facing resistive memory cells is the on-state retention, i.e., the ability of the conductive path (filament or vacancy chain) to be stable, especially at the elevated temperatures that the memory parts would typically be qualified to (85 C/125 C).
As used herein, “conductive path” refers a conductive filament (e.g., in a CBRAM cell), vacancy chain (e.g., in an oxygen vacancy based ReRAM cell), or any other type of conductive path for connecting the bottom and top electrodes of a non-volatile memory cell (typically through an electrolyte layer or region arranged between the bottom and top electrodes). As used herein the “electrolyte layer” or “electrolyte region” refers to an electrolyte/insulator/memory layer or region between the bottom and top electrodes through which the conductive path propagates.
According to various embodiments, a non-volatile memory cell structure, and associated manufacturing process, provides a reduced area of contact between the bottom electrode and the electrolyte layer, thus restricting the area in which a conductive path can form, i.e., the “confinement zone,” and thereby create thicker, single conductive path root memory cells (e.g., CBRAM cells and ReRAM cells) having improved switching performance, retention performance, and/or reliability. For example, the confinement zone may be defined by a narrow ring having a width of less than 100 Å.
In one embodiment, a resistive memory cell includes a ring-shaped bottom electrode, a top electrode, and an electrolyte layer arranged between the bottom and top electrodes.
In another embodiment, a method for forming a resistive memory cell comprises forming a ring-shaped bottom electrode by a process including: forming a dielectric layer over a bottom electrode contact, etching a via in the dielectric layer to expose at least a portion of the bottom electrode contact, depositing a conductive via liner over the dielectric layer and into the via, the via liner deposited in the via forming a ring-shaped structure in the via and a contact portion in contact with the exposed bottom electrode contact, the ring-shaped structure defining a radially inward cavity of the ring-shaped structure, and filling the cavity with a dielectric fill material, such that the ring-shaped structure of the via liner forms the ring-shaped bottom electrode, depositing an electrolyte layer over the bottom electrode, and depositing a top electrode over the electrolyte layer.
Example embodiments are discussed below with reference to the drawings, in which:
FIGS. 4A-4B2 illustrate aspects of a conventional continuous bottom electrode structure;
FIGS. 5A-5B2 illustrate aspects of a ring-shaped bottom electrode structure according to an example embodiment of the present invention, to show one advantage of the ring-shaped bottom electrode structure as compared to a conventional continuous bottom electrode structure;
Each of the various component regions of structure 100 may be formed from any suitable material and formed in any suitable manner. For example, ring-shaped bottom electrode 102 may be formed from TiN or any other suitable bottom electrode material; top electrode 108 may be formed from Cu, e.g., a very thin Cu layer (e.g., 10-30 nm/5-15 nm) formed by PVD, or any other suitable top electrode material; electrolyte layer 106 may be formed from a thin layer (e.g., 30 Å-150 Å) of high quality SiO2 or SiO or any other suitable electrolyte material; and bit line(s) 110 may be formed from TaN or any other suitable bit line material.
An example filament, e.g., metal bridge, propagated from the ring-shaped bottom electrode 102 to the top electrode 108 through the electrolyte layer 106 is indicated at 120. The ring-shaped bottom electrode 102 provides a substantially reduced contact area between the bottom electrode 102 and overlying electrolyte layer 104 as compared with a solid bottom electrode structure, thus providing a reduced confinement zone. In this example, the ring-shaped bottom electrode 102 has a thickness (x) of less than 100 Å. Providing a bottom electrode thickness (x) less than a thickness (y) of the electrolyte layer (i.e., x/y<1) may provide a particularly reduced chance of multiple conductive path formation.
FIGS. 4A-4B2 and FIGS. 5A-5B2 illustrate aspects of a conventional continuous bottom electrode structure (FIGS. 4A-4B2) and a ring-shaped bottom electrode structure according to an embodiment of the present invention (FIGS. 5A-5B2), to show one advantage of the ring-shaped bottom electrode structure. In particular,
During SET (filament formation), a decreased number, and an increased thickness, of filament roots is preferred. In the conventional structure shown in FIGS. 4A-4B2, the volume of the electrolyte 106′ in which filaments 120 may form has a relatively large horizontal/vertical length ratio (e.g., x/y>5). In contrast, in the ring-shaped bottom electrode structure 100 disclosed herein, the volume of the electrolyte 106 in which filament(s) 120 may form has a relatively small horizontal/vertical length ratio (e.g., x/y<1). As shown, the ring-shaped bottom electrode structure disclosed herein may provide fewer, but thicker, filament roots, thus providing an advantage over the conventional structure.
As shown in
In some embodiments the resistive memory cell structure 200 can be formed using two masks. First, a via (or trench) open mask is used, into which a thin TiN layer is deposited, followed by a PECVD oxide fill and CMP process. This forms the bottom electrode 202. Following this, the electrolyte layer 206 (e.g. a thin SiOx layer) is deposited, followed by the top electrode 208 (e.g. Cu/TaN/W), and this stack is then etched with a second mask. Normally a thick Cu film cannot be etched in a plasma, hence a thin (50-300 A) PVD Cu layer may be formed, which can be plasma-etched with this second mask.
As discussed above, the disclosed concepts apply both the metallic filament type CBRAM cells and the vacancy type ReRAM cells. In the disclosed asymmetrical structure, one of electrodes in contact with the electrolyte/insulator is the source of these metallic ions/vacancies, while the other is typically inert.
Various embodiments may provide one or more advantages relative to conventional cell structures and/or formation techniques. For example, the asymmetric structure (e.g., incorporating a ring-shaped bottom electrode) may improve the functionality and reliability of Cu/SiOx based cells by reducing the bottom electrode area in contact with the electrolyte. Thus, the volume in which the number of roots of metallic filaments/vacancy-chain roots can form is greatly reduced over the conventional structures. This may provide various advantages. For example, the asymmetrical structure may provide improve switching characteristics and reliability because there is a far greater likelihood of creating a single, thick filament/vacancy-chain that is more stable for retention purposes. As another example, because the bottom electrode area is reduced, a much higher current density can be achieved for the same current flow. This may allow for a uni-polar operation in switching, i.e., both the set (filament formation) and reset (filament dissolution by joule-heating) can be done at the same voltage polarity. This has been demonstrated on the Cu/SiOx cells, but has needed a much higher current level under reset, the mechanism for dissolution being based on Joule heating rather than an electrolytic reduction of the metallic filament.
This application claims the benefit of U.S. Provisional Application No. 61/780,317 filed on Mar. 13, 2013, which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61780317 | Mar 2013 | US |
