|Publication number||US20050124155 A1|
|Application number||US 10/725,500|
|Publication date||Jun 9, 2005|
|Filing date||Dec 3, 2003|
|Priority date||Dec 3, 2003|
|Also published as||US7691683, US20060148248|
|Publication number||10725500, 725500, US 2005/0124155 A1, US 2005/124155 A1, US 20050124155 A1, US 20050124155A1, US 2005124155 A1, US 2005124155A1, US-A1-20050124155, US-A1-2005124155, US2005/0124155A1, US2005/124155A1, US20050124155 A1, US20050124155A1, US2005124155 A1, US2005124155A1|
|Inventors||Joseph Brooks, John Moore|
|Original Assignee||Brooks Joseph F., Moore John T.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (7), Classifications (13), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to the field of random access memory (RAM) devices formed using a resistance variable material, and in particular to electrodes and method of forming electrodes used in resistance variable memory devices.
Recently, resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements and molecular memory elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical PCRAM device is disclosed in U.S. Pat. No. 6,348,365 assigned to Micron Technology, Inc., and hereby incorporated by reference. Typical molecular memory devices are described in U.S. Patent Application Publication No. 2002/0163831 to Krieger et al, which is hereby incorporated by reference. In typical PCRAM devices, conductive material, such as silver, is incorporated into a chalcogenide material, which is positioned between two electrodes. The unprogrammed PCRAM device is normally in a high resistance “on” state. A write operation programs the PCRAM device to a lower resistance “off” state via application of a threshold voltage potential between the two electrodes. Molecular memory devices function similarly, but utilize a molecular memory element such as a polymer, rather than a chalcogenide glass.
The programmed lower resistance state can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The PCRAM device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the element to the lower resistance state. Again, the higher resistance state is maintained in a semi-volatile manner once the voltage potential is removed. In this way, such a device can function as a resistance variable memory element having two resistance states, which can be used as two logic states.
Formation of electrode structures for resistance variable memory cells often requires introducing adhesion and barrier layers between layers of conductive materials. Adhesion layers are useful for preventing two or more layers of conductive material from separating during processing of the memory device, while barrier layers can prevent the unwanted migration of metal ions through the memory cell during operation.
In accordance with various exemplary embodiments of the present invention, electrode structures, variable resistance memory devices, and methods of making the same, which minimize electrode work function variation, are provided. In the various embodiments, an electrode having a minimal work function variation is provided by eliminating concentric circles of electrode materials having different work functions.
In accordance with a first embodiment of the invention, an electrode structure is made by forming a dielectric layer over a first conductive layer, forming an opening in the dielectric so as to expose a portion of the first conductive layer, forming an adhesion layer over the dielectric layer and the exposed portion of the first conductive layer, forming a second conductive layer over the adhesion layer, recessing the second conductive layer and the adhesion layer within the opening in the dielectric layer, and forming a third conductive layer over the dielectric layer and over the second conductive layer and the adhesion layer within the opening. This third conductive layer is planarized and recessed in the opening, thus presenting an electrode contact surface having a uniform work function.
In accordance with a second embodiment of the invention, an electrode structure is fabricated by forming a dielectric layer over a first conductive layer, forming an opening in the dielectric so as to expose a portion of the first conductive layer, forming an adhesion layer over the dielectric layer and the exposed portion of the first conductive layer, forming a second conductive layer over the adhesion layer, recessing the second conductive layer and the adhesion layer within the opening in the dielectric layer and forming a third conductive layer over the dielectric layer and the second conductive layer and the adhesion layer within the opening. The third conductive layer is subsequently patterned and etched.
In accordance with a third embodiment of the invention, the electrode structures of the first and second embodiments are coupled with a resistance variable memory element and another electrode to form a resistance variable memory device.
In accordance with a fourth embodiment of the present invention, a memory system is provided, wherein memory devices utilizing the electrode structures of the first and second embodiments are arranged in rows and columns such that each memory device can be selectively read, written, or erased.
These and other features and advantages of the invention will be better understood from the following detailed description, which is provided in connection with the accompanying drawings.
FIGS. 1A-D show an electrode structure in various stages of processing in accordance with the prior art.
In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention.
The present invention relates to electrode structures, variable resistance memory devices, and methods of making the same that minimize electrode workfunction variation. The workfunction of an electrode is a measure of the difficulty of escape of an electron from the electrode and is typically measured in electron volts. The lower the work function of the electrode, the greater the number of electrons that will be capable of escaping from the cathode when an electrical potential is applied thereto.
In the examples given below, the present invention is described as being used for electrode structures in resistance variable memory devices. It should be appreciated, however, that the teachings of the present invention can be used in conjunction with any device employing electrode structures.
Resistance variable memory devices typically employ an ionic conductor disposed between at least two electrodes. Ionic conductors include chalcogenide glasses having metal ions dissolved therein and molecular conductors, such as conductive polymers. The top and bottom electrodes may be formed from any conductive material, for example, tungsten, nickel, tantalum, aluminum, platinum, conductive nitrides, and other materials. Furthermore, the top and bottom electrodes may be formed from the same material or different materials and thus, the electrode work functions of the two electrodes may also be the same or different. It is critical to the performance of the memory device, however, that each of the top and bottom electrodes exhibit a uniform work function as described below.
The second layer 112 of conductive material is planarized such that a top surface of said conductive layer 112 is substantially level with a top surface of a said dielectric layer 104, as shown in
At this stage of processing, the electrode structure 100 may be used in conjunction with other elements to form various devices. For example, as shown in
For example, the materials used for the second conductive layer 112 and the barrier/adhesion layer 110 may be different metal materials, each exhibiting different structural characteristics, such as grain size. Differences in grain size, for example, can lead to a variation in localized electric fields at the interface of the layers 110, 112 during device operation. Such variation in localized electric fields affects the magnitude of the threshold voltage required to switch the device and is not necessarily uniform from device to device. Thus, an array of PCRAM devices, for example, employing the electrode structure described above, may exhibit non-uniform threshold voltages from memory device to memory device. Such non-uniform threshold voltages seriously compromise the ability to reliably read, write, and erase any give memory device in the array.
Accordingly, the present invention provides electrode structures and their methods of formation, which eliminate the problem of having concentric circles of materials with different work functions.
In accordance with a first exemplary embodiment of the invention, an electrode structure and method of forming an electrode structure is provided, wherein the electrode structure has a second conductive layer having a uniform work function.
Referring now to
In accordance with a second exemplary embodiment of the present invention, an electrode structure and a method of forming an electrode structure is provided in which a patterned bottom electrode structure is formed such that it presents a contact surface having a uniform electrode work function.
Referring again to
In accordance with a third exemplary embodiment of the present invention, a resistance variable memory device and a method of forming a resistance variable memory device are provided.
As described above, the electrode structures 250, 300 of the first and second exemplary embodiments may be used in conjunction with a resistance variable memory element 200 in order to form a resistance variable memory device 400 as shown, for example, in
In accordance with a fourth exemplary embodiment of the present invention, a memory system 500 is provided, including a plurality of memory devices 502 utilizing electrodes with uniform work functions as shown in
In the case of a computer system, the processor system may include peripheral devices such as a floppy disk drive 650 and a compact disc (CD) ROM drive 660, which also communicate with CPU 620 over the bus 610. Memory 640 is preferably constructed as an integrated circuit, which includes one or more resistance variable memory devices 502 as shown in
The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.
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|U.S. Classification||438/654, 257/E45.002, 257/E21.585|
|International Classification||H01L21/44, H01L21/768, H01L45/00|
|Cooperative Classification||H01L21/76877, H01L45/1253, H01L45/16, H01L45/085, H01L45/1233|
|European Classification||H01L45/04, H01L21/768C4|
|Dec 3, 2003||AS||Assignment|
Owner name: MICRON TECHNOLOGY, INC., IDAHO
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BROOKS, JOSEPH F.;MOORE, JOHN T.;REEL/FRAME:014761/0668
Effective date: 20031121