Patent Application
20100295560
Embodiments of the present invention are directed to mapping local impedance in a dielectric film and, more particularly, to techniques to detect conductive filaments in a dielectric film such as may be used in a resistive change random access memory (RRAM).
Conventional solid state memories employ microelectronic circuit elements for each memory bit. Since one or more electronic circuit elements are required for each memory bit (e.g., one to four transistors per bit), these devices can consume considerable chip “real estate” to store a bit of information, which limits the density of a memory chip. The primary memory element in these devices is typically a floating gate field effect transistor device that holds a charge on the gate of field effect transistor to store each memory bit. Typical memory applications include dynamic random access memory (DRAM), static random access memory (SRAM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM).
A different type of solid state memory commonly known as a phase-change memory uses a phase-change material as the data storage mechanism and offers significant advantages in both cost and performance over conventional memories based on charge storage. Phase change memories use phase change materials—in other words, materials that can be electrically switched between two or more phases having different electrical characteristics such as resistance. One type of memory element, for example, uses a phase change material that can be electrically switched between a generally amorphous phase and a generally crystalline local order, or between different detectable phases of local order across the entire spectrum between completely amorphous and completely crystalline phases.
The phase-change memory can be written to, and read from, by applying current pulses that have the appropriate magnitude and duration and that cause the needed voltages across and current through the volume of phase change material. A selected cell in a phase-change memory can be programmed into a selected state by raising a cell voltage and a cell current for the selected cell to programming threshold levels that are characteristic of the phase-change material. The voltage and current are then typically lowered to quiescent levels (e.g. essentially zero voltage and current) that are below the programming threshold levels of the phase-change material. This process can be performed by the application of, for example, a reset pulse and a set pulse which can program the cell into two different logic states. In both of these pulses, the cell voltage and cell current are caused to rise at least as high as certain threshold voltage and current levels needed to program the cell.
Next, to read the programmed cell, a read pulse can be applied to measure the relative resistance of the cell material, without changing its phase. Thus, the read pulse typically provides a much smaller magnitude of cell current and cell voltage than either the reset pulse or the set pulse.
These electrical memory devices typically do not use field effect transistor devices, but comprise, in the electrical context, a monolithic body of thin film material. As a result, very little chip real estate is required to store a bit of information, thereby providing for inherently high density memory chips. The phase change materials are also truly non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous phase representing a resistance value, that value is retained until reset as that value represents a physical phase of the material (e.g., crystalline or amorphous).
The foregoing and a better understanding of the present invention may become apparent from the following detailed description of arrangements and example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing arrangements and example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
A type of phase change memory is the so-called seek-scan probe (SSP) memories which are a type of memory that uses non-volatile storage media as the data storage mechanism and offers significant advantages in both cost and performance over conventional charge-storage memories. Typical SSP memories include storage media made of materials that can be electrically switched between two or more states having different electrical characteristics such as resistance, polarization dipole direction, or some other characteristic.
In the case of a resistive change random access memory (RRAM), the media layer or phase change layer may be a dielectric material such as PZT, for example. The dielectric material, which is normally insulating, can be made to conduct through a conduction path, sometimes called a “conductive filament” formed by application of a sufficiently high voltage. This filament may be detected or read, for example, by a cantilevered probe and may represent a bit of data depending on its current state of being high resistance or low resistance. It is believed that the conduction filament formation may be formed by different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be broken, resulting in high resistance, or re-formed, resulting in lower resistance, over and over again by an appropriately applied voltage.
Embodiments of the invention describe a technique to map out local impedance in a dielectric film sample. This may be used to detect conductive filaments in a dielectric film, to characterize semiconductor interfaces, and to be used a reading scheme for resistive change memory such as RRAM.
Referring now to
The configuration shown in
Where, the current signal is i, IC0 is the current of the capacitive coupling between the sample 202 and the cantilever arm 204, and IC is the capacitive current through the film 202 between the cantilever tip 204 and the electrode 200 through the film 202. Since the probe or tip is not located at a conductive filament IC may be negligible.
Where, φ is the angle on the vector diagram between IC0 and IC0R and w is the phase, and where the resistive current and the filament resistance are calculated based on the current signal vector diagram using:
Thus, if the current signal is of this nature, it can be determined that that area of the film 202 comprises a conductive filament.
Referring to
Embodiments may also be used to for a reading scheme for a resistance change memory by detecting conductive filaments. The same apparatus and scanning methods described above may be used; however the DC amplitude may be stepped incrementally while resistive current is recorded. Switching is detected as resistive current increases non-linearly. This is demonstrated in
In
Advantageously, the described scanning impedance microscopy described herein is relatively simple while provides nanometer resolution. Because the described Scanning Impedance Microscopy (SIM) technique maps out local impedance in a dielectric film with nanometer resolution, it can be used to detect conductive filaments in a PZT film, which provides valuable information for film development and retention study. Further, in additional to the high resolution provided by the invention, the scanning speed is much faster compared to the state of the art tunneling current (Atomic Force Microscopy) AFM methods with good signal to noise ratio and nanometer resolution. As a result, it is an attractive reading scheme for resistive change memory such as RRAM.
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
