1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element and method for fabricating the same.
2. Description of the Related Art
Phase-change memory technology requires high reliability, fast speeds, low current, and low operating voltage, in order to function as a viable alternative to current memory technologies such as flash and DRAM. A phase-change memory cell must therefore provide low programming current, low operating voltage, a smaller cell size, a fast phase transformation speed, and a low cost. These requirements are difficult to meet given the current state of the art.
Current phase-change memory technology makes use of heating at the interface between a metal electrode contact and the phase-change material. More effective heating requires a smaller contact area, or equivalently a smaller heating area. A benefit of this strategy is simultaneous reduction of cell size. However, reducing the area results in higher cell resistance, which increases the required driving voltage. This is clearly not desirable. Reducing heating area does not necessarily improve other performance features. There is a large temperature gradient that exists between the contact and the bulk of the phase-change material. Phase transformation speed requires good thermal uniformity within the active region of the cell. The rate of phase-change is extremely sensitive to temperature. Non-uniform heating results in a loss of reliability due to accumulation of incomplete phase-change in the programming volume.
United States Patent 20070012905 utilizes a single edge contact to the lower electrode, while the upper electrode uses a conventional planar contact. In addition, U.S. Pat. No. 6,881,603 also minimizes only the lower electrode contact area while the upper electrode contact is planar. Meanwhile, U.S. Pat. No. 6,864,503 makes use of a phase-change material spacer with top and bottom edge contacts, however, the heating area is proportional to the electrode radius, so it is relatively large, and the upper and lower electrodes are effective heat sinks.
Therefore, it is desirable to devise a phase-change memory cell structure that improves thermal uniformity as well as heating efficiency while allowing for a smaller heating area.
An exemplary embodiment a phase-change memory element comprises a bottom electrode. A first dielectric layer is formed on the bottom electrode. A first electrical contact is formed on the first dielectric layer and electrically connected to the bottom electrode. A second dielectric layer is formed on and covers the first electrical contact. A second electrical contact is formed on the second dielectric layer, wherein the second electrical contact comprises an outstanding terminal. An opening passes through the second electrical contact, the second dielectric layer, and lands on the first electrical contact, wherein the bottom of the opening is separated from the bottom electrode by the first dielectric layer. A phase-change material occupies at least one portion of the opening, wherein the first and second electrical contacts interface the phase-change material at the side-walls of the phase-change material. A third dielectric layer is formed on and covers the second electrical contact and exposes a top surface of the outstanding terminal. A top electrode is formed on the third dielectric layer and directly contacts the top surface of the outstanding terminal of the second electrical contact.
According to another embodiment of the invention, a method for fabricating a phase-change memory element is provided, comprising the following steps: providing a bottom electrode; forming a first dielectric layer on the bottom electrode, exposing the periphery of the top surface of the bottom electrode; conformally forming a first electrical contact on the first dielectric layer electrically connecting to the bottom electrode; forming a second dielectric layer with a trench to cover the first electrical contact; conformally forming a second electrical contact on the second dielectric layer; forming a third dielectric layer on the second electrical conduct; planarizing the third dielectric layer and the second electrical conduct to expose a top surface of an outstanding terminal of the second electrical conduct; forming an opening passing through the second electrical contact, the second dielectric layer, landing on the first electrical contact and separated from the bottom electrode by the first dielectric layer; filling a phase-change material into a part of the opening, forcing the first and second electrical contacts to interface the phase-change material at the side-walls of the phase-change material; filling a fourth dielectric layer into the opening, leaving coplanar top surfaces of the fourth dielectric layer and the outstanding terminal; and forming a top electrode formed on the third dielectric layer directly contacting the top surface of the outstanding terminal.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
a-1n are cross sections of a method for fabricating a phase-change memory element according to an embodiment of the invention.
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
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According to another embodiment of the invention, the profile of the phase-change material block 28a can be square (referring to
By forming side-wall contacts to both the top and bottom electrodes of the phase-change memory cell, heating is confined at the side-walls of the block of phase-change material, which is also the location of greatest cooling. This allows for thermal uniformity to be improved compared to devices which are heated near the center of the phase-change material. Furthermore, the voltage required can be minimized by reducing the distance between the edge contacts. Also, the heating area is reduced, hence heating efficiency improved by reducing the side-wall contact thickness as well as by reducing the width of the phase-change material.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.