Selective removal method and structure of silver in resistive switching device for a non-volatile memory device

Not Applicable

Not Applicable

The present invention is generally related to resistive switching devices. More particularly, embodiments according to the present invention provide a method to form an active metal material for resistive switching of a resistive switching device. The present invention can be applied to non-volatile memory devices but it should be recognized that the present invention can have a much broader range of applicability.

The success of semiconductor devices has been primarily driven by an intensive transistor down-scaling process. However, as field effect transistors (FET) approach sizes less than 100 nm, problems arise such as the short channel effect, that degrade device performance. Moreover, such sub 100 nm device sizes can lead to sub-threshold slope non-scaling and increase in power dissipation. It is generally believed that transistor-based memories such as those commonly known as Flash may end scaling within a decade.

Other non-volatile random access memory (RAM) devices such as ferroelectric RAM (Fe RAM), magneto-resistive RAM (MRAM), organic RAM (ORAM), and phase change RAM (PCRAM), among others, have been explored as next generation memory devices. These devices often require new materials and device structures that couple with silicon-based devices to form a memory cell, but they lack one or more key attributes. For example, Fe-RAM and MRAM devices have fast switching characteristics and good programming endurance, but their fabrication is not CMOS compatible they are usually large in size. Switching a PCRAM device requires a large amount of power. Organic RAM or ORAM devices are incompatible with large volume silicon-based fabrication and device reliability is usually poor.

From the above, a new semiconductor device structure and integration is desirable.

The present invention is generally related to resistive switching devices. More particularly, embodiments according to the present invention provide a method to form an active metal material for resistive switching of a resistive switching device. The present invention can be applied to non-volatile memory devices but it should be recognized that the present invention can have a much broader range of applicability.

In a specific embodiment, a method for forming an active material for a non-volatile memory device is provided. The method includes providing a substrate having a surface region. A first dielectric material is deposited overlying the surface region and a first wiring structure is formed overlying the first dielectric material. The method includes depositing a second dielectric material overlying the first wiring structure and forming a via opening in portions of the second dielectric material to expose a portion of the first wiring structure. A resistive switching material comprising a silicon material is deposited to fill a first portion of the via opening. In a specific embodiment, a metal material is deposited to fill a second portion of the via structure and overlying an exposed surface region of the second dielectric material. The metal material is configured to be in direct contact with the resistive switching material in a specific embodiment. The method deposits a diffusion barrier layer overlying the metal material and forms a masking layer overlying a first portion of the diffusion barrier layer while exposing a second portion of the diffusion barrier layer. In a specific embodiment the first portion overlies the via opening and the second portion overlies the second dielectric material. In a specific embodiment, the method includes selectively removing the metal material and the diffusion barrier layer from at least the surface region of the second dielectric material and maintaining the diffusion barrier material and the metal material in the second portion of the via opening and maintaining the metal material to be in physical and electrical contact with the resistive switching material.

Many benefits can be achieved by ways of the present invention. Certain materials such as noble metals have no suitable volatile species thus making reactive ion etching in fabrication not a viable option. Embodiments according to the present invention provide a method to eliminate a metal etching step for forming an active noble metal for resistive switching. The active noble material can be a part of a wiring stricture in certain implementation. The wiring structure is substantially free from defects resulting from etched particles from the noble materials and shorts are prevented. Additionally, the present method uses convention processing techniques without modification to the equipment. Depending on the embodiment, one or more of these benefits may be achieved. One skilled in the art would recognize other modifications, variations, and alternatives.

According to one aspect of the invention, a method for forming a non-volatile memory device configured with a resistive switching element, is disclosed. One technique includes providing a substrate having a surface region, depositing a first dielectric material overlying the surface region, and forming a first wiring structure overlying the first dielectric material. A process includes depositing a second dielectric material overlying the first wiring structure, and forming a via opening in the second dielectric material to expose a portion of the first wiring structure, while maintaining a portion of the second dielectric material. A method includes forming a resistive switching material comprising an amorphous silicon material within the via opening, and depositing a metal material comprising a silver material, wherein a first portion of the metal material overlies the resistive switching material, and wherein a second portion of the metal overlies at least the portion of the dielectric material, wherein the first portion of the metal material contacts the resistive switching material. An operation includes forming a diffusion barrier layer overlying the metal material, wherein a first portion of the diffusion barrier layer contacts the first portion of the metal material. and selectively removing the second portion of the metal material and a second portion of the diffusion barrier layer overlying the second portion of the metal material using a solution at a predetermined temperature while maintaining the portion of the diffusion barrier material and the first portion of the metal material within the via opening.

According to another aspect of the invention, a method for forming a silver active metal for a non-volatile memory device is disclosed. One technique includes forming a first wiring structure overlying a surface region of a substrate, depositing a first dielectric material overlying the first wiring structure, and forming a via opening in the first dielectric material to expose a portion of the first wiring structure, while maintaining a portion of the first dielectric material. A process includes forming a layer of resistive switching material within the via opening, wherein the resistive switching material comprises a silicon material, forming a silver material overlying the layer of resistive switching material and overlying the portion of the first dielectric material, and forming a diffusion barrier layer overlying the silver material to form a resulting structure. A method includes selectively removing a portion of the silver material and a portion of the diffusion barrier layer overlying the portion of the first dielectric material while maintaining a portion of the silver material and a portion of the diffusion barrier material overlying the layer of silicon material within the via opening.

In order to more fully understand the present invention, reference is made to the accompanying drawings. Understanding that these drawings are not to be considered limitations in the scope of the invention, the presently described embodiments and the presently understood best mode of the invention are described with additional detail through use of the accompanying drawings in which:

FIGS. 1-12 are simplified diagrams illustrating a method of fabricating a non-volatile memory device according to an embodiment of the present invention; and

FIG. 13 illustrates images of scanning electron microscope of a partially fabricated device according to an embodiment of the present invention.

The present invention is generally related to resistive switching devices. More particularly, embodiments according to the present invention provide a method to form an active metal material for resistive switching of a resistive switching device. The present invention can be applied to non-volatile memory devices but it should be recognized that the present invention can have a much broader range of applicability.

Resistive switching behavior has been observed and studied in micrometer-scale amorphous silicon (a-Si) devices since the 1980s. A typical device consists of a pair of metal electrodes sandwiching an amorphous-Si layer in a so-called Metal/a-Si/Metal (M/a-Si/M) structure, in which the voltage applied across the pair of metal electrodes causes changes in the resistance of the a-Si material. These conventional M/a-Si/M based switching devices can have the advantages of high Ion/Ioff ratios, and can be fabricated with a CMOS compatible fabrication process and materials.

To further decrease cost per bit, device shrinking, process simplification, as well as yield improvement, among others, are necessary. Particles from an etching process, especially from a metal etch, produce defects that cause electrical shorts and impacting device reliability and rendering device inoperable. Embodiments according to the present invention provide a method and a structure to form a non-volatile memory device using silver as the active material free from a silver etch step to improve desirable material characteristic and device reliability.

The terms “bottom” and “top” are for references only and not meant to be limiting.

FIG. 1 —area simplified diagrams illustrating a method for forming a resistive switching device for a non-volatile memory device according to an embodiment of the present invention. As shown in FIG. 1 , a semiconductor substrate 102 having a surface region 104 is provided. The semiconductor substrate can be a single crystal silicon wafer, a silicon germanium material, a silicon-on-insulator (commonly called SOI) depending on the embodiment. In certain embodiments, the semiconductor substrate can have one or more CMOS devices formed thereon. The one or more CMOS devices can be controlling circuitry for the resistive switching device in a specific embodiment. The one or more CMOS devices can include one or more control transistors for the non-volatile memory device, for a separate processor, logic device, logic array, or the like, in some embodiments.

As illustrate in FIG. 2 , the method includes depositing a first dielectric material 202 overlying the semiconductor substrate. The first dielectric material can be silicon oxide, silicon nitride, a dielectric stack of alternating layers of silicon oxide and silicon nitride (for example, an ONO stack), a low K dielectric, a high K dielectric, or a combination, and others, depending on the application. The first dielectric material can be deposited using techniques such as chemical vapor deposition, including low pressure chemical vapor deposition, plasma enhanced chemical vapor deposition, atomic layer deposition (ALD), physical vapor deposition, including any combination of these, and others

Referring to FIG. 3 , the method includes depositing a first wiring material overlying the first dielectric material. The first wiring material can be a suitable metal material including alloy materials, or a semiconductor material having a suitable conductivity characteristic. The metal material can be tungsten, aluminum, copper or silver, and others. These metal materials may be deposited using a physical vapor deposition process, chemical vapor deposition process, electroplating, or electroless deposition process, including any combinations of these, and others. The semiconductor material can be, for example, a suitably doped silicon material in certain embodiments. In certain embodiments, a first adhesion material 304 is first formed overlying the first dielectric material before deposition of the first wiring material to promote adhesion of the first wiring material to the first dielectric material. A diffusion barrier material 306 may also be formed overlying the metal material to prevent the metal material to contaminate other portions of the device in a specific embodiment.

The method subjects the first wiring material to a first pattern and etching process to form a first wiring structure 402 in a specific embodiment. As shown in FIG. 4 , the first wiring structure includes a plurality of elongated structures configured to extend in a first direction 404 in a specific embodiment. In a specific embodiment, the method deposits a second dielectric material 502 overlying the first wiring structure. The second dielectric material can be silicon oxide, silicon nitride, a dielectric stack of alternating layers of silicon oxide and silicon nitride (for example, an ONO stack), a low K dielectric, a high K dielectric, and others, depending on the application. The second dielectric material can be subjected to a planarizing process and to form a thickness 504 overlying the first wiring structure in a specific embodiment as shown in FIG. 5 .

As shown in FIG. 6 , the method includes subjecting the second dielectric material to a second pattern and etching process to form one or more via openings 602 to expose a top surface region 604 of the first wiring structure or the diffusion barrier material, if use. In a specific embodiment, a resistive switching material 702 is deposited to fill a first portion of each via opening and overlying the second dielectric material in FIG. 7 . The resistive switching material can be a silicon material in a specific embodiment. The silicon material can include amorphous silicon material, silicon germanium material, polycrystalline silicon, microcrystalline silicon, and others. The amorphous silicon material has essentially intrinsic semiconductor characteristic and is not intentionally doped in a specific embodiment. In various embodiments, the amorphous silicon is also referred to as non-crystalline silicon (nc-Si). nc-Si non-volatile resistive switching devices may be fabricated using existing CMOS technologies. In an exemplary process, a mixture of silane (SiH4) (45 sccm) and Helium (He) (500 sccm) is used to form an a-Si layer with a deposition rate of 80 nm per minute (T=260° C., P=600 mTorr) during PECVD. In another exemplary process, a mixture of silane (SiH4) (190 sccm) and Helium (He) (100 sccm) is used to form an a-Si layer with a deposition rate of 2.8 A per second (T=380° C., P=2.2 Torr) during PECVD. In another exemplary process, silane (SiH4 80 sccm) or disilane is used to form an a-Si layer with a deposition rate of 2.8 nm per minute (T=585° C., P=100 mTorr) during LPCVD. Portions of poly-silicon grains may form during the LPCVD process and result in an amorphous-poly silicon film. In various embodiments, no p-type, n-type, or metallic impurities are intentionally added to the deposition chamber while forming the amorphous silicon material. Accordingly, when deposited, the amorphous silicon material is substantially free of any p-type, n-type or metallic dopants, i.e. the amorphous silicon material is undoped.

In a specific embodiment, the resistive switching material is subjected to an etch back process to selectively remove the resistive switching material to maintain exposure of a second portion of the via and a substantially horizontal portion surrounding the via opening in a surface region of the second dielectric material while the resistive switching material is substantially maintained in the first portion of the via opening 802 as shown in FIG. 8 . The etch back process can be a chemical mechanical polishing process or a reactive ion etching process in a plasma environment. The etch back process can also be a wet etch process depending on the application.

Referring to FIG. 9 , the present method deposits an active material 902 overlying the resistive switching material in the second portion of the via opening and overlying the exposed horizontal surface region of the second dielectric material. For an amorphous silicon material as the resistive switching material, the active material can be a metal material such as silver, gold, platinum, palladium, nickel, or aluminum and others. The active material is preferably characterized by a suitable diffusivity in the resistive switching material under an applied electric field in a specific embodiment. In one embodiment, the metal material can be a silver material for amorphous silicon material as the resistive switching material. The silver material can be deposited using a physical vapor deposition process in a specific embodiment. Alternatively, the silver may be deposited using a chemical vapor deposition process, an electrochemical process including electroplating and electroless deposition, and any combinations of these, and others. The metal material is in physical and electric contact with the resistive switching material in a specific embodiment.

In some embodiments, the silver material is in direct contact with the amorphous silicon used as the resistive switching material in a specific embodiment. In other embodiments, a thin layer of material, e.g. oxide, nitride, is formed prior to the deposition of the silver material on top of the amorphous silicon used as the resistive switching material. This interposing thin layer of material may be naturally or specifically grown or formed. In some embodiments, one or more etch operations (e.g. HF etch, Argon etch) may help control the thickness of this layer. In some embodiments, the thickness of the material (e.g. oxide) prior to deposition of the silver material may range from about 20 angstroms to about 50 angstroms; in other embodiments, the thickness may range from about 30 angstroms to about 40 angstroms; or the like. In some embodiments, an additional layer of amorphous silicon may be disposed upon the top of the thin layer of (oxide, nitride, barrier) material, prior to deposition of the silver material. This additional layer of amorphous silicon (potentially not intentionally doped) may be used to help bind the silver material to the thin layer of material (e.g. oxide, nitride, barrier). In some examples, the thickness may be on the order of 20-50 angstroms. In one example, the order of layers may be: undoped amorphous silicon used as the resistive switching material, a thin layer of material (e.g. oxide, nitride, barrier), a thin layer of amorphous silicon, and the silver material.

In a specific embodiment, the method forms an adhesion material 904 overlying the active metal material (for example, silver material). The adhesion material can be titanium, titanium nitride, tantalum nitride, titanium tungsten, and any combinations of these, and others. The adhesion material may be formed using a physical vapor deposition process, a chemical vapor deposition process, atomic layer deposition process, or a combination, and others.

As shown in FIG. 10 , a masking layer 1002 is formed overlying a first portion of the metal material and the adhesion material while exposing a second portion of the metal material and the adhesion material. The first portion of the metal material and the adhesion material overlies the via opening and the second portion of the metal material and the adhesion material overlies the second dielectric material in a specific embodiment. In a specific embodiment, the method selectively removes the second portion of the adhesion material and the metal material from the surface region of the second dielectric material using an etching process. In other embodiments, the masking layer 1002 need not be performed, and a mask-less lift-off process is performed, as described below.

In various embodiments, the lift-off etching process can be performed with a wet etch process. The wet etch process includes using a solution comprising at least phosphoric acid, acetic acid, and nitric acid in a specific embodiment. In certain embodiment, the solution can be diluted using water, for example, de-ionized water. In a specific embodiment, the solution can have about 80% phosphoric acid, about 5% nitric acid, about 5% acetic acid, and about 10% de-ionized water. The wet etch process is further characterized by a removal rate or reaction rate. The reaction rate may be controlled by a proportion of acetic acid in the solution. An increase of acetic acid in the solution would slow the reaction rate in a specific embodiment. Depending on the embodiment, the wet etch process may be performed at room temperature or the solution may be heated, though the reaction may give out certain amount of heat. Reaction temperature can range from about room temperature (for example, about 25 Degree Celsius) to about 80 Degree Celsius or more preferably ranges from about 25 Degree Celsius to about 50 Degree Celsius. The solution is configured to weaken an adhesion between the metal material and the second dielectric material while maintaining the metal material to be in contact with the resistive switching material in the via opening in a specific embodiment. Etching by weakening the adhesion between two films is known as a lift-off process. As shown in FIG. 11 , the metal material together with the adhesion material delaminates from the second dielectric material surface while maintaining the metal material and a portion of the adhesion material 1102 in the via opening.

Referring to FIG. 12 , the method includes depositing a second wiring material overlying the second dielectric material, which is exposed and the adhesion material in the via opening. The second wiring material can be copper, aluminum, tungsten, and others. The second wiring material can be deposited using techniques such as physical vapor deposition, chemical vapor deposition, electroplating or electroless deposition, and others, depending on the application. In certain embodiment, an adhesion material is formed over at least the second dielectric material surface before deposition of the second wiring material. The second wiring material, including the adhesion material, is subjected to a pattern and etch process to form a second wiring structure 1202 in a specific embodiment. The second wiring structure includes one or more elongated structures configured to extend in a second direction orthogonal to the first direction in a specific embodiment.

Depending upon the embodiments, there can be other variations. For example, a contact material can be formed interposed between the first wiring material and the amorphous silicon material. The contact material can be a p+ polysilicon material or other silicon-based material having a suitable conductivity characteristic. The contact material controls a defect density in an interface region between the first wiring material and the amorphous silicon and allows for a desirable switching behavior in a specific embodiment. In other embodiments, the contact material may not be necessary.

Embodiments according to the present invention provide a method for forming an active metal material for resistive switching for a resistive switching device. The method selectively removes the active material from a surface region of a dielectric material to maintain the active material in a via structure for the resistive switching device in a specific embodiment. To illustrate the present invention, experiments have been performed. FIG. 13 illustrates scanning electron microscope images after removal of a silver material from a silicon oxide surface by a lift off process, as in FIG. 11 , according to an embodiment of the present invention. As shown in a top view image 1302 , via openings 1306 are clean and silicon oxide surface 1308 is substantially free from silver material. A cross sectional view 1304 of the via structure is also shown. Silver material 1310 is maintained in the via openings and is in contact with the amorphous switching material. A portion of the titanium/titanium nitride adhesion material forms a cap structure 1312 overlying the silver material, as shown.

In various embodiments, as the memory devices describe herein are small compared to standard memories, a processor, or the like, may include greater amounts of memory (cache) on the same semiconductor device. As such memories are relatively non-volatile, the states of such processors, or the like may be maintained while power is not supplied to the processors. To a user, such capability would greatly enhance the power-on power-off performance of devices including such processors. Additionally, such capability would greatly reduce the power consumption of devices including such processors. In particular, because such memories are non-volatile the processor need not draw power to refresh the memory states, as is common with CMOS type memories. Accordingly, embodiments of the present invention are directed towards processors or other logic incorporating memory devices, as described herein, devices (e.g. smart phones, network devices) incorporating processors or other logic incorporating such memory devices, and the like.

Further embodiments can be envisioned to one of ordinary skill in the art after reading this disclosure. In other embodiments, combinations or sub-combinations of the above disclosed invention can be advantageously made. The block diagrams of the architecture and flow charts are grouped for ease of understanding. However it should be understood that combinations of blocks, additions of new blocks, re-arrangement of blocks, and the like are contemplated in alternative embodiments of the present invention.

The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.