Patent Application
20090026437
This relates generally to phase change memories that use chalcogenide materials.
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
Referring to
In accordance with some embodiments of the present invention, the chalcogenide that forms the memory element may be isolated from the copper used for interconnects. Namely, copper may be utilized for the metal connections such as the row and column lines in some embodiments.
Referring to
For example, in one embodiment, the first layer 22 of metal may be U-shaped and may be formed of tantalum. The second layer 24 of metal, on top of the first layer 22, may be U-shaped as well and may be formed of tantalum nitride. The center 26 of the plug may be formed of titanium silicon nitride in one embodiment. The effect of the plug and its multiple layers is to block copper migration from the copper line 16 into the overlying chalcogenide.
The pore may be formed within a layer of dielectric which may be formed of one or more materials. For example, in one embodiment, the dielectric may include a nitride layer 18 covered by an oxide layer 20. However, other dielectric materials may be utilized.
At the next level, the memory element may be formed using a heater 38. In one embodiment, the heater 38 may be formed of titanium silicon nitride. The heater 38 may be formed in a pore having a pair of spacers 32 and 34. However, in some embodiments, a single spacer may be utilized and, in other embodiments, more or less spacers may be used.
The pore may be defined by dielectric materials which may include one or more layers. For example, a nitride layer 28 may be covered by an oxide layer 30 in one embodiment. Over the heater 38 and within the pore defined by the spacers 32 and 34 may be the chalcogenide material 36. In one embodiment, the chalcogenide material may be GeSbTe (GST).
The next level may include the first upper electrode 40 which may be aligned with the chalcogenide material 36.
Next, one or more additional memory elements may be built on top of the memory element already defined. Specifically, a second memory element, in series with the first memory element, may include a dielectric layer 42, a sidewall spacer 48, and a chalcogenide material 46 covered by a second upper electrode 44.
A pore may be formed in the dielectric layer 42, the pore may be filled with a sidewall spacer material which is then anisotropically etched to form the sidewall spacers 48. Thereafter, the chalcogenide material 46 may be deposited and planarized. Finally, the second upper electrode 44 may be formed by deposition. Of course, other fabrication techniques may also be utilized.
In some embodiments, the formation of two or more memory elements in series configuration enable the threshold voltage of the overall cell to be adjusted. While an embodiment is illustrated in which stacked memory elements are utilized, memory elements may also be formed in the same plane and then coupled by electrical routing.
Depending on the number of memory elements in series, different combined threshold levels may be demonstrated, allowing adjustment of the threshold voltage of the cell. In some embodiments, phase change memories may have threshold voltages which are too low for some applications. By combining memory elements in series, the threshold voltages may be raised. For example, a stack of two memory elements may have a threshold voltage which is roughly equal to twice the threshold voltage of one of said memory elements.
The second upper electrode 44 may be patterned with a lithographic etch step and covered with a nitride encapsulation layer. Subsequently, oxide may be deposited and polished. Then trenches are patterned into which a seed layer and copper is subsequently deposited.
In some embodiments, a threshold device may be formed on top of the memory elements just described. The threshold device, in one embodiment, may be an ovonic threshold switch.
In accordance with one embodiment, a fabrication sequence may begin, as shown in
The next level may include a dielectric material which is formed over the layer 12. In one embodiment, it may include a nitride layer 18, covered by an oxide layer 20 of greater thickness as shown in
Referring to
In one embodiment, the plug may be made up of three different materials, including a first layer 22 of tantalum, a second layer 24 of tantalum nitride, and a third layer 26 of titanium silicon nitride. The tantalum nitride and tantalum is preferably not included within the upper portion that contacts the subsequently deposited chalcogenide.
Following the formation of the plug, as shown in
After the plug polish, nitride 28 and oxide 30 layers may be deposited and patterned.
Then, referring to
Into the resulting pore may be deposited a heater 38, as shown in
Then the chalcogenide material 36 is polished back in a chemical mechanical planarization step. Thereafter, the first upper electrode 40 is deposited, as shown in
In accordance with another embodiment of the present invention, after planarizing the layers 22, 24, and 26, those layers may be etched back partially from the pore, creating an opening at the upper end of that pore. Then, a heater and a chalcogenide material may be formed thereover, still within the same pore as the plug made up of the layers 22, 24, and 26. As a result, the pore that includes the heater and the chalcogenide memory is self-aligned to the plug. In other embodiments, the heater may be totally omitted.
Programming of the chalcogenide material 36 to alter the state or phase of the material may be accomplished by applying voltage potentials to the line 16, formed on substrate 12, and top electrode 40, thereby generating a voltage potential across the memory element. When the voltage potential is greater than the threshold voltage of memory element, then an electrical current may flow through the chalcogenide material 36 in response to the applied voltage potentials, and may result in heating of the chalcogenide material 36 by the heater 38.
This heating may alter the memory state or phase of the chalcogenide material 36. Altering the phase or state of the chalcogenide material 36 may alter the electrical characteristic of memory material, e.g., the resistance of the material may be altered by altering the phase of the memory material. Memory material may also be referred to as a programmable resistive material.
In the “reset” state, memory material may be in an amorphous or semi-amorphous state and in the “set” state, memory material may be in a crystalline or semi-crystalline state. Both “reset” and “set” states can exist without any energy (electrical, optical, mechanical) applied to bistable chalcogenide. The resistance of memory material in the amorphous or semi-amorphous state may be greater than the resistance of memory material in the crystalline or semi-crystalline state. It is to be appreciated that the association of reset and set with amorphous and crystalline states, respectively, is a convention and that at least an opposite convention may be adopted.
Using electrical current, memory material may be heated to a relatively higher temperature to amorphosize memory material and “reset” memory material (e.g., program memory material to a logic “0” value). Heating the volume of memory material to a relatively lower crystallization temperature may crystallize memory material and “set” memory material (e.g., program memory material to a logic “1” value). Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material.
Turning to
System 500 may include a controller 510, an input/output (I/O) device 520 (e.g. a keypad, display), static random access memory (SRAM) 560, a memory 530, and a wireless interface 540 coupled to each other via a bus 550. A battery 580 may be used in some embodiments. It should be noted that the scope of the present invention is not limited to embodiments having any or all of these components.
Controller 510 may comprise, for example, one or more microprocessors, digital signal processors, microcontrollers, or the like. Memory 530 may be used to store messages transmitted to or by system 500. Memory 530 may also optionally be used to store instructions that are executed by controller 510 during the operation of system 500, and may be used to store user data. Memory 530 may be provided by one or more different types of memory. For example, memory 530 may comprise any type of random access memory, a volatile memory, a non-volatile memory such as a flash memory and/or a memory such as memory discussed herein.
I/O device 520 may be used by a user to generate a message. System 500 may use wireless interface 540 to transmit and receive messages to and from a wireless communication network with a radio frequency (RF) signal. Examples of wireless interface 540 may include an antenna or a wireless transceiver, although the scope of the present invention is not limited in this respect.
In accordance with some embodiments of the present invention, the process may be completely compatible with conventional copper interconnect lines, thereby allowing stackable phase change memory array layers. Also, the problems associated with dot patterning of the chalcogenide material may be reduced.
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
