Method for selectively removing surface oxides on a metal film

Abstract

The present disclosure provides a novel process and method for pre-treating a metal surface in the back-end-of-line (BEOL) manufacturing of an integrated circuit (IC). More specifically, the present disclosure provides a selective and self-limiting process and method for removing a natural oxide surface layer that may form on an exposed metal surface during the processing of an IC. The processes and methods disclosed herein utilize the basic concept of metal complex formation to provide a novel solution that enables the selective removal of the natural oxide surface layer from the exposed metal film in a self-limiting manner. In particular, the disclosed processes and methods use a complexing agent (e.g., a ligand) to selectively dissolve the natural oxide surface layer without significantly etching or removing the underlying metal film.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims priority to and the benefit of the filing date of U.S. Non - Provisional Patent Application No. 17 / 838,440, filed on June 13, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] This disclosure relates to semiconductor process technologies for the manufacture of integrated circuits. More specifically, the present invention relates to the treatment of metals, metal surfaces, and metal - semiconductor interfaces during the manufacture of integrated circuits.

Background Art

[0003] In the back - end - of - line (BEOL) manufacturing of integrated circuits (ICs), metals are commonly used to electronically connect various active components within the circuit. Conventionally, copper (Cu) has been the predominantly selected metal due to a set of desirable electronic properties and processability. In some integrated circuits, one layer of copper wiring can be coated with an ultrathin metal cap layer before proceeding to the next layer. The metal cap layer can be used, for example, to mitigate reliability concerns (such as electro - migration of Cu or drift into the surrounding dielectrics) that can cause a reduction in conductivity and short - circuits in subsequent formed devices.

[0004] The metal cap layer often includes an ultrathin metal film of cobalt (Co) or ruthenium (Ru) (preferably with a thickness of only a few nanometers (nm)) and is uniformly coated on the underlying copper metal lines. In the case of a cobalt metal cap on copper, the thickness of the cobalt cap layer is typically about 2 nm. Cobalt, like most metals, tends to oxidize when exposed to air or moisture, so the treatment of such thin films is difficult. In practice, a natural oxide surface layer is always formed on the surface of the cobalt metal film. Since the natural oxide surface layer is not conductive, it degrades the interfacial properties of the copper wiring and ions (e.g., Co 2+ / 3+There is a risk of incorporating, etc. into the cap layer, which further increases the risk of electromigration and defect formation. Therefore, it is desirable to use cobalt surface pretreatment to remove the natural oxide surface layer without etching the underlying metal cobalt cap layer before proceeding to subsequent processing steps (e.g., before forming the second metal layer).

[0005] In some applications such as area-selective dielectric-on-dielectric (DoD) deposition, cobalt-capped copper metal lines can be coated with a self-assembled monolayer (SAM) to inactivate the metal pattern and prevent the metal pattern from being coated with a dielectric material. For effective SAM formation, cobalt surface pretreatment is required to remove all or part of the natural oxide that is essentially formed on the cobalt metal surface during processing. In these applications, a selective process that enables removal of the natural surface oxide without removing the underlying metal cobalt cap layer is desired.

[0006] Considering the thickness of the metal cap layer (e.g., a few nanometers), an ideal cobalt surface pretreatment process is self-limiting, i.e., a process that stops once all the surface oxide has been removed. However, current methods for pretreating the cobalt surface use acidic solutions (e.g., solutions containing dilute hydrofluoric acid and citric acid), and the selectivity of cobalt metal to natural cobalt oxide is insufficient. As a result, these pretreatment methods often result in pits and non-uniform thinning of the cobalt cap layer, and the purpose is not achieved. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] Therefore, a new method for pretreating metal surfaces in the BEOL manufacturing of integrated circuits is required. MEANS FOR SOLVING THE PROBLEMS

[0008] The present disclosure provides a novel process and method for pretreating metal surfaces in the back-end-of-line (BEOL) manufacturing of integrated circuits (ICs). More specifically, the present disclosure provides a selective and self-limiting process for stripping a native oxide surface layer that may form on an exposed metal surface during the processing of an IC. The processes disclosed herein utilize the basic concept of metal complexation to provide a novel solution that enables selective removal of the native oxide surface layer from an exposed metal film in a self-limiting manner. In particular, as described in more detail below, the disclosed processes use a complexing agent (e.g., a ligand) to selectively dissolve the native oxide surface layer without significantly etching or removing the underlying metal film.

[0009] According to one embodiment of the present disclosure, a novel wet process for selectively removing surface oxides on a metal film in one step is provided. In the present disclosure, a substrate having a native oxide surface layer formed thereon is exposed to a dissolution solution containing a complexing agent (e.g., a ligand) dissolved in a non-aqueous solvent. The ligand reacts with and binds to the native oxide surface layer to form a ligand-metal complex that is soluble in the non-aqueous solvent and is thereby selectively dissolved. When the ligand-metal complex is selectively dissolved, the substrate can be rinsed to remove excess reactants and soluble species from the surface of the substrate. Without further oxidation (e.g., unintentional exposure to residual water, oxygen, or air), the wet processes disclosed herein selectively remove the native oxide surface layer without etching the underlying metal film, thereby leaving a metal surface (or a ligand-bound passivating monolayer) on the metal film.

[0010] The processes and methods described herein for stripping or removing a native oxide surface layer offer various advantages compared to conventional methods used for removing surface oxides. For example, the processes and methods described herein are carried out at low temperatures (e.g., above room temperature) using mild chemicals (e.g., ligands dissolved in a non-aqueous solvent), thus minimizing the risk of damage to the components of the device. The processes and methods disclosed herein are also inherently self-limiting by using a reactive dissolution chemical that contains a ligand in a non-aqueous solution, and the ligand selectively binds to metal oxides rather than metals. Additionally, the disclosed processes and methods are fast and low-cost and do not require expensive vacuum tools. Instead, the disclosed processes and methods are simple and can be implemented in existing wet processing tools and integrated with the existing processing infrastructure in BEOL IC manufacturing.

[0011] The techniques described herein can be used to strip or remove a native oxide surface layer from a variety of metal films. In some embodiments, for example, the techniques described herein can be used to strip a transition metal oxide formed on a transition metal film. Examples of transition metal films commonly used in IC manufacturing include, but are not limited to, cobalt (Co), copper (Cu), ruthenium (Ru), tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), platinum (Pt), nickel (Ni), rhodium (Rh), iridium (Ir), and the like. The disclosed techniques can also be used to strip metal oxides from other metal films and semiconductor films such as aluminum (Al), gold (Au), silicon (Si), germanium (Ge), and the like.

[0012] As described above and further described herein, the present disclosure provides various embodiments of a method for removing a native oxide surface layer from a metal film using a wet process. The methods described herein may include various process steps. Of course, the order of the description of the various steps described herein is presented for clarity. Generally, these steps may be performed in any suitable order. Additionally, although each of the various features, techniques, configurations, etc. described herein may be described in separate places in the present disclosure, each concept is to be construed as being capable of being implemented independently of or in combination with one another. Accordingly, the present invention can be embodied and contemplated in many different ways.

[0013] According to a first embodiment, a method for removing a native oxide formed on the surface of a metal film is provided. This method may generally begin with the step of receiving a substrate having a metal film exposed on the surface of the substrate, where the native oxide is formed on the surface of the metal film. This method may further include the step of exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent, where the ligand forms a complex with the native oxide and is soluble in the non-aqueous solvent and thereby forms a ligand-metal complex that is selectively dissolved. This method may further include the step of rinsing the substrate to remove the dissolution solution and the dissolved ligand-metal complex from the surface of the substrate, thereby leaving a metal surface or a ligand-bonded passivation monolayer on the metal film.

[0014] Depending on the native oxide to be removed, various ligands may be included in the dissolution solution. For example, the ligand included in the dissolution solution may be selected from the group consisting of β-diketonates, carboxylates, aminopolycarboxylates, oximes, and amines. In some embodiments, the dissolution solution may further include a base that activates the ligand and promotes complex formation between the ligand and the native oxide. Regardless of the specific ligand used, the ligand included in the dissolution solution can react with and bind to the native oxide but does not bind to the metal film, thereby forming a self-limiting ligand-metal complex on the metal film.

[0015] A variety of non-aqueous solvents may also be included in the dissolution solution, including but not limited to polar organic solvents such as alcohols (e.g., methanol, isopropanol, amyl alcohol), ketones (e.g., acetone, methyl ethyl ketone), acetates (e.g., ethyl acetate, amyl acetate), acetonitrile, dimethyl sulfoxide, and n-methylpyrrolidone. Regardless of the specific solvent used, the non-aqueous solvent included in the dissolution solution can be used to selectively dissolve the ligand-metal complex while preventing re-oxidation of the metal surface after the ligand-metal complex has been removed.

[0016] The method disclosed in the first embodiment can generally be used to remove the native oxide formed on the exposed surface of the metal film during processing or by exposure to the ambient environment. In some embodiments, the method disclosed in the first embodiment can be used to strip the native oxide from a relatively thin metal film. For example, the thickness of the metal film can be about 10 nm or less (optionally 2 nm or less), and this method can be used to strip the native oxide with a thickness of 5 nm or less (optionally 1 nm or less) from the surface of the metal film. In some embodiments, the steps of exposing the surface of the substrate to the dissolution solution and rinsing the substrate can remove the native oxide formed on the surface of the metal film without removing or significantly etching the metal film.

[0017] The method disclosed in the first embodiment can be used to remove native oxides from a variety of metal films. In some embodiments, for example, this method can be used to remove native oxides formed on a variety of transition metal films such as metal films of cobalt (Co), copper (Cu), ruthenium (Ru), tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), platinum (Pt), nickel (Ni), rhodium (Rh), and iridium (Ir). This method can also be used to remove metal oxides from other metal films and semiconductor films such as aluminum (Al), gold (Au), silicon (Si), germanium (Ge), etc.

[0018] In some embodiments, the method disclosed in the first embodiment can be performed at a low temperature. For example, the step of exposing the surface of the substrate to the dissolution solution and the step of rinsing the substrate can be performed at a temperature in the range of 20°C to 55°C.

[0019] According to a second embodiment, another method for removing a native oxide formed on the surface of a metal film is provided herein. This method can generally begin with a step of receiving a substrate having a metal film exposed on the surface of the substrate, where the native oxide is formed on the surface of the metal film. Next, the method can include a step of exposing the surface of the substrate to an oxidizing agent to further oxidize the native oxide and form a metal oxide surface layer or a metal hydroxide surface layer, followed by a step of exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent. The ligand contained in the dissolution solution can react with and bind to the metal oxide surface layer or the metal hydroxide surface layer to form a ligand-metal complex that is soluble in the non-aqueous solvent and is thereby selectively dissolved. Next, the method can include a step of rinsing the substrate to remove the dissolution solution and the ligand-metal complex from the surface of the substrate. As described above, by exposing the surface of the substrate to an oxidizing agent, exposing the surface of the substrate to a dissolution solution, and rinsing the substrate, this method can selectively remove the native oxide formed on the surface of the metal film without removing the metal film.

[0020] The surface of the substrate can be exposed to various oxidizing agents. In some embodiments, for example, the oxidizing agent can be selected from the group consisting of deionized water, air, hydrogen peroxide, and ammonium hydroxide. In one embodiment, the surface of the substrate can be exposed to deionized water to further oxidize the native oxide and form an upper metal hydroxide surface layer on the surface of the metal film. In such an embodiment, the metal hydroxide surface layer can be more easily dissolved in the non-aqueous solvent than the native oxide.

[0021] The surface of the substrate can also be exposed to various dissolution solutions. In some embodiments, exposure to the dissolution solution can form a self-limiting ligand-metal complex on the metal film. To form a self-limiting ligand-metal complex, the ligand contained in the dissolution solution needs to react with and bind to the native oxide. However, the ligand contained in the dissolution solution does not react with or bind to the metal film after the ligand-metal complex is dissolved in the non-aqueous solvent.

[0022] In some embodiments, the step of exposing the surface of the substrate to the dissolution solution and the step of rinsing the substrate can leave a metal surface or a ligand-bound passivating monolayer on the metal film and can prevent further oxidation of the metal film. In some embodiments, the step of rinsing the substrate can include exposing the substrate to a second non-aqueous solvent, which can be the same as (or different from) the non-aqueous solvent used in the dissolution solution. Since the non-aqueous solvents used in the dissolution and rinsing steps do not contain oxidizing agents, they prevent the metal surface remaining on the metal film from being intentionally oxidized after the native oxide is selectively removed.

[0023] According to a third embodiment, a method for stripping a native oxide from a cobalt metal film is provided herein. This method can generally begin with the step of receiving a substrate having a cobalt metal film exposed on the surface of the substrate, where the native oxide is formed on the cobalt metal film. This method can also include the step of exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent, where the ligand reacts with and binds to the native oxide to form a self-limiting and selectively soluble ligand-metal complex in the non-aqueous solvent. This method can further include the step of rinsing the substrate to remove the dissolution solution and the ligand-metal complex from the surface of the substrate. As described above, by exposing the surface of the substrate to the dissolution solution and rinsing the substrate, this method can strip the native oxide from the cobalt metal film without removing the cobalt metal film.

[0024] In some embodiments, the method disclosed in the third embodiment may further include exposing the surface of the substrate to an oxidizing agent to convert the native oxide into a cobalt oxide surface layer or a cobalt hydroxide surface layer before exposing the surface of the substrate to the dissolution solution. In such embodiments, the ligand contained in the dissolution solution can react with and bind to the cobalt oxide surface layer or the cobalt hydroxide surface layer to form a ligand-metal complex. The surface of the substrate can be exposed to various oxidizing agents. In one embodiment, the surface of the substrate can be exposed to deionized water to further oxidize the native oxide and form a cobalt hydroxide surface layer on the surface of the metal film. In some embodiments, the cobalt hydroxide surface layer can be more easily dissolved in a non-aqueous solvent than the native oxide.

[0025] Various ligands and non-aqueous solvents can be included in the dissolution solution to selectively dissolve the native oxide (or optionally a cobalt oxide surface layer or a cobalt hydroxide surface layer). In one embodiment, the dissolution solution can include acetylacetonate (i.e., ligand) dissolved in methanol (i.e., non-aqueous solvent). In some embodiments, the dissolution solution can further include a base that deprotonates the ligand to facilitate the formation of a complex between the ligand and the native oxide. In such embodiments, the base can be selected from the group consisting of quaternary ammonium hydroxides (e.g., tetrabutylammonium hydroxide (TBAH), tetramethylammonium hydroxide (TMAH)), nitrogen bases (e.g., trimethylamine, piperidine), and phosphines (e.g., triphenylphosphine). In one embodiment, the base can be tetrabutylammonium hydroxide (TBAH).

[0026] In another embodiment, the dissolution solution can include glacial acetic acid (GAC) (i.e., ligand) dissolved in isopropyl alcohol (IPA) (i.e., non-aqueous solvent). In some embodiments, the concentration of GAC in IPA can range from 0 to 100%. In one embodiment, the concentration of GAC in IPA can be about 1%.

[0027] The present invention and its advantages will be more fully understood by reference to the following description in conjunction with the accompanying drawings, in which like reference numerals refer to like features. However, it should be noted that the accompanying drawings merely illustrate a plurality of exemplary embodiments of the disclosed concepts, and the disclosed concepts may encompass other equally valid embodiments as well, and thus do not limit the scope of the present invention.

Brief Description of the Drawings

[0028]

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Best Mode for Carrying Out the Invention

[0029] The present disclosure provides a novel process and method for pretreating metal surfaces in the back end of line (BEOL) manufacturing of integrated circuits (ICs). More specifically, the present disclosure provides a selective and self-limiting process for stripping a native oxide surface layer that may form on an exposed metal surface during the processing of an IC. The processes disclosed herein utilize the basic concept of metal complex formation to provide a novel solution that enables the native oxide surface layer to be selectively removed from the exposed metal film in a self-limiting manner. In particular, as described in more detail below, the disclosed processes use a complexing agent (e.g., a ligand) to selectively dissolve the native oxide surface layer without significantly etching or removing the underlying metal film.

[0030] According to one embodiment of the present disclosure, a novel wet method is provided for selectively removing surface oxides on a metal film in one step. In the present disclosure, a substrate having a native oxide surface layer formed on a metal film is exposed to a dissolution solution containing a complexing agent (e.g., a ligand) dissolved in a non-aqueous solvent. The ligand reacts with and binds to the native oxide surface layer to form a ligand-metal complex that is soluble in the non-aqueous solvent and thereby selectively dissolved. When the ligand-metal complex is selectively dissolved, the substrate can be rinsed to remove excess reactants and soluble species from the surface of the substrate. In the absence of further oxidation (e.g., unintentional exposure to residual water, oxygen, or air), the wet method disclosed herein selectively removes the native oxide surface layer without etching the underlying metal film, thereby leaving a metal surface or a ligand-bound passivation monolayer on the metal film.

[0031] FIG. 1 shows one embodiment of a wet process that can be used to remove the native oxide surface layer from a metal film in accordance with the present disclosure. The wet process shown in FIG. 1 can generally begin in process step 100 by receiving a substrate having a metal film 102 surrounded by a dielectric material 104. In some embodiments, the metal film 102 can be a metal cap layer formed on underlying metal wiring or a metal liner formed on underlying metal lines. For example, the metal cap layer or metal liner can include a relatively thin (e.g., 10 nm or less) metal film of cobalt (Co), ruthenium (Ru), or another transition metal that is uniformly coated on the underlying metal wiring or metal lines. Optionally, the thin layer or liner can be 2 nm or less.

[0032] As shown in FIG. 1, a native oxide 106 is formed on the metal film 102. The native oxide 106 can be formed on the metal film 102 during a previous process step or by exposure to the air or moisture in the ambient environment. Since the native oxide 106 layer is not conductive, it degrades the interfacial properties of the metal wiring or metal lines and there is a risk of incorporating ions (e.g., Co 2+ / 3+ ) into the metal cap layer, which further increases the risk of electromigration and defect formation. For these reasons, it is desirable to remove the native oxide 106 without etching the underlying metal film 102.

[0033] In process step 110, the surface of the substrate is exposed to a dissolution solution containing a complexing agent (e.g., ligand) 114 dissolved in a first non-aqueous solvent 112. When the native oxide 106 contacts the dissolution solution, the complexing agent (ligand) 114 reacts with and binds to the native oxide 106 to form a ligand-metal complex 116 that is soluble in and thereby selectively dissolved by the first non-aqueous solvent 112. In some embodiments, the dissolution solution can further include a base that deprotonates (i.e., activates) the ligand to facilitate complex formation between the ligand and the native oxide 106.

[0034] In process step 120, the substrate is rinsed with a second non-aqueous solvent 122 to remove the dissolution solution and the ligand-metal complex 116 from the surface of the substrate, whereby a metal surface 132 (or a ligand-binding inactivated monolayer) remains on the metal film 102 in process step 130. The second non-aqueous solvent 122 used in process step 120 can be the same as (or different from) the first non-aqueous solvent 112 used in process step 110.

[0035] In the wet process shown in FIG. 1, the reactive dissolution method is used to selectively remove the native oxide 106 without removing (or significantly etching) the underlying metal film 102 or the dielectric material 104 surrounding the metal film 102. In the wet process shown in FIG. 1, the native oxide 106 is selectively removed by utilizing a ligand that reacts with and binds to the native oxide 106 but does not bind to the metal film 102. Thereby, a self-limiting ligand-metal complex 116 can be formed on the metal film 102, i.e., when the ligand-metal complex 116 is selectively removed from the surface of the metal film 102 and the metal surface 132 is exposed, the complex formation reaction stops. A variety of ligands can be used in the dissolution solution, which will be described in more detail below. In addition to removing the ligand-metal complex 116 from the metal film 102, the non-aqueous solvents (112 and 122) used within process steps 110 and 120 prevent further oxidation of the metal surface 132 of the metal film 102 by avoiding the use of oxidants. As will be described in more detail below, a variety of non-aqueous solvents can be used in the dissolution solution and the rinse solution.

[0036] Figure 2 shows another embodiment of a wet process that can be used to remove the native oxide surface layer from a metal film in accordance with the present disclosure. The process shown in Figure 2 is similar to the process shown in Figure 1. As described above, in addition to process steps 100, 110, 120, and 130, the wet process shown in Figure 2 includes an additional process step 105 after the substrate is received in process step 100 and before the substrate is exposed to the dissolution solution in process step 110. It should be noted that process step 105 shown in Figure 2 is optional and may or may not be used to remove the native oxide 106 from the metal film 102.

[0037] In process step 105, the surface of the substrate is exposed to an oxidizing agent 108 to further oxidize the native oxide 106 and form a metal oxide surface layer (or metal hydroxide surface layer) 109 on the metal film 102, and then the surface of the substrate is exposed to the dissolution solution in process step 110. In process step 105, various oxidizing agents 108 can be used to convert the native oxide 106 into a metal oxide surface layer (or metal hydroxide surface layer) 109. Examples of the oxidizing agent 108 that can be used in process step 105 will be described in more detail below. Subsequently, when the surface of the substrate is exposed to the dissolution solution in process step 110, the complexing agent (ligand) 114 reacts with and binds to the metal oxide surface layer (or metal hydroxide surface layer) 109 to form a ligand-metal complex 116 that is soluble in and thereby selectively dissolved by the first non-aqueous solvent 112. In some embodiments, the metal oxide surface layer (or metal hydroxide surface layer) 109 can be more easily removed from the metal film 102 than the native oxide 106.

[0038] Unlike the wet process shown in FIG. 1, the wet process shown in FIG. 2 uses oxidation (in process step 105) to prepare the native oxide 106 layer for removal in a subsequent dissolution step (process step 110). In some embodiments, the metal oxide surface layer (or metal hydroxide surface layer) 109 formed during process step 105 can be more readily dissolved in the non-aqueous solvent 112 than the native oxide 106 that was originally formed on the surface of the metal film 102. In one exemplary embodiment, process step 105 can add hydroxide atoms to the native oxide 106 layer, and these hydroxide atoms are more readily removed during process step 110 because they are soluble in the dissolution solution.

[0039] The wet processes shown in FIGS. 1-2 can be used to strip native oxide surface layers from a variety of metal films. In some embodiments, the processes shown in FIGS. 1-2 can be used to strip native oxides formed on a variety of transition metal films, including but not limited to metal films of cobalt (Co), copper (Cu), ruthenium (Ru), tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), platinum (Pt), nickel (Ni), rhodium (Rh), and iridium (Ir). In addition to transition metals, the wet processes disclosed herein can also be used to strip native oxides from other metal films and semiconductor films such as aluminum (Al), gold (Au), silicon (Si), germanium (Ge), etc. Exemplary etching chemistries for selectively removing native oxides from cobalt metal films using the techniques disclosed herein are described in more detail below.

[0040] As described above, the wet process shown in FIGS. 1-2 uses a dissolution solution containing a complexing agent (e.g., ligand) 114 dissolved in a first non-aqueous solvent 112 to selectively remove the native oxide 106 without significantly etching the entire metal film 102, leaving a metal surface 132 (or ligand-bound passivating monolayer) on the metal film 102. The complexing agent 114 promotes the dissolution of the native oxide 106 by substituting the oxo ligand or hydroxo ligand of the native oxide 106. In the absence of oxidation, metal atoms do not form complexes with the complexing agent 114 used in the dissolution solution, so the wet process stops at the metal surface 132. The selection of the ligand and the solvent is important to achieve etching of the native oxide 106 and selectivity for the native oxide 106. For example, the ligand-metal complex 116 needs to be soluble in the first non-aqueous solvent 112 to cause dissolution. On the other hand, the ligand needs to react only with the native oxide 106 and not with the metal atoms of the metal film 102. Examples of ligands that can be used in the wet process disclosed herein include, but are not limited to, β-diketonates, carboxylates, aminopolycarboxylates, oximes, and amines. The specific ligand selected for use in the dissolution solution can generally vary depending on the native oxide 106 to be removed.

[0041] In some embodiments, the dissolution solution may further include a base that deprotonates (i.e., activates) the ligand to facilitate complex formation between the ligand and the native oxide 106. The dissolution solution can include a variety of bases, including, but not limited to, quaternary ammonium hydroxides (e.g., tetrabutylammonium hydroxide (TBAH), tetramethylammonium hydroxide (TMAH)), nitrogen bases (e.g., trimethylamine, piperidine), phosphines (e.g., triphenylphosphine), and the like. In one embodiment, TBAH can be used in the dissolution solution to activate the ligand.

[0042] In addition to dissolving and removing the ligand-metal complex 116, the non-aqueous solvents (112 and 122) used in the dissolution solution and the rinse solution prevent re-oxidation of the metal surface 132 after the ligand-metal complex 116 has been selectively removed. Examples of non-aqueous solvents 112 that can be used for the reactive dissolution of the natural oxide and the surface layer of metal oxide / hydroxide include, but are not limited to, polar organic solvents such as alcohols (e.g., methanol, isopropanol, amyl alcohol), ketones (e.g., acetone, methyl ethyl ketone), acetates (e.g., ethyl acetate, amyl acetate), acetonitrile, dimethyl sulfoxide, and n-methylpyrrolidone. In addition, some ligands are liquids under typical reaction conditions and can serve both the dual roles of the ligand and the solvent. This is the case for acetic acid and acetylacetone. Examples of non-aqueous solvents 122 that can be used for rinsing the substrate include, but are not limited to, all the solvents listed in the reactive dissolution step.

[0043] In some embodiments, as described above with reference to FIG. 2, prior to exposing the surface of the substrate to the dissolution solution, an oxidizing agent 108 can be used to convert the natural oxide 106 to a metal oxide surface layer (or metal hydroxide surface layer) 109. A variety of oxidizing agents 108 can be used, including but not limited to deionized water, air, hydrogen peroxide (H2O2), ammonium hydroxide (NH4OH), etc. In one embodiment, the surface of the substrate can be exposed to deionized water in process step 105 to further oxidize the natural oxide 106 and form a metal hydroxide surface layer 109 on the surface of the metal film 102. As described above, the metal hydroxide surface layer 109 can be more easily dissolved in the first non-aqueous solvent 112 than the natural oxide 106.

[0044] In some embodiments, the wet process shown in FIGS. 1-2 can be used to highly selectively and self-limitingly remove the native oxide formed on a cobalt (Co) metal film. However, it should be understood that the wet process disclosed herein is not strictly limited to cobalt and can be used to highly selectively and self-limitingly remove the native oxide formed on other metal films and semiconductor films. For cobalt (Co), two solvent / ligand systems are described in the following embodiments along with the relevant experimental results.

[0045] According to the first embodiment, the wet process shown in FIGS. 1-2 can be used to selectively remove the native oxide formed on a cobalt metal film by exposing the surface of the substrate to a dissolution solution containing a β-diketonate ligand dissolved in an alcohol in process step 110. Examples of β-diketonate ligands include, but are not limited to, acetylacetonate (ACAC) and hexafluoroacetylacetone (HFAC) that form strong bonds with metal ions such as 2+ / 3+ Co.

[0046] In one example of the first embodiment, the dissolution solution used in process step 110 can contain acetylacetonate (ACAC) dissolved in methanol. In this exemplary embodiment, the acetylacetonate (ACAC) ligand dissolved in methanol promotes the self-limiting removal of the native oxide formed on the cobalt metal film at low temperatures (e.g., near room temperature and above). Once the native oxide is removed, a new metal surface is exposed and becomes inert to the acetylacetonate ligand without further oxidation. In some embodiments, a base such as TBAH can be used to deprotonate (i.e., activate) the acetylacetonate ligand to accelerate the complex formation between the native oxide and the acetylacetonate ligand.

[0047] The above wet removal method provides a robust process for selectively and self-limitingly removing the native oxide formed on the cobalt metal film. In a typical process according to the first embodiment, a dissolution solution containing 1-25% acetylacetonate (ACAC) and 50 mM TBAH dissolved in methanol can be used, and then rinsed with methanol or IPA. Higher concentrations of acetylacetonate (ACAC) can be used, but may not always be necessary. In such a process, the native oxide can be selectively removed from the surface of the cobalt metal film by immersing and / or rinsing the surface of the substrate in the dissolution solution and the rinse solution. The typical processing temperature can be in the range of 20 °C to 55 °C.

[0048] Etching experiments were performed on coupons cut from 300 mm silicon wafers with physical vapor deposition (PVD) cobalt of various thicknesses deposited on one side. By exposing to the ambient, native oxide was formed on the cobalt metal film. After an etching recipe including immersion in 25% ACAC and 50 mM TBAH dissolved in methanol for 30 seconds, rinsing with methanol and drying by blowing with compressed air, the native oxide was selectively removed from the surface of the cobalt metal film. This process was carried out at 55 °C.

[0049] Using the above etching recipe, two etching experiments were conducted to evaluate the selectivity between native oxide and cobalt metal etching. The results of this experiment are shown in graph 300 of FIG. 3. In the first experiment (circles), the cobalt metal film with native oxide formed was exposed to the above dissolution solution for a long time (e.g., 5 minutes) without intentional oxidation. In this experiment, only the native oxide was removed, and no significant change was seen in the thickness of the cobalt metal film.

[0050] In the second experiment (triangle), the selectivity of acetylacetonate (ACAC) for the etching of native oxides was evaluated using a cyclic process. In this experiment, an intentional oxidation step was incorporated into the etching recipe after the dissolution step, enabling the replication of native oxides before repeating the process steps. Oxidation was performed by immersing the substrate containing the cobalt metal film in deionized water at 55 °C for several seconds (e.g., 30 seconds). In this experiment, the cobalt metal film was etched during each cycle, and the etching amount per cycle (i.e., the etching rate) was obtained. The experiment demonstrated that the dissolution solution containing 25% ACAC and 50 mM TBAH dissolved in methanol did not etch the cobalt metal film in the absence of oxidation during the experiment. Comparing the etching rates of cobalt with and without intentional oxidation indicated that any achieved etching was associated with the etching of native oxides, indirectly confirming the self-limiting removal behavior of native oxides. Furthermore, the experimental results showed that the dissolution solution containing 25% ACAC in methanol was at least 70 times more selective for the etching of native oxides than for the cobalt metal (i.e., the ratio of the etching rate of native oxides to the etching rate of cobalt metal). Theoretically, higher selectivity can be achieved by controlling unwanted oxidation.

[0051] According to the second embodiment, the wet process shown in FIGS. 1-2 can be used to selectively remove the native oxide formed on the cobalt metal film by exposing the surface of the substrate to a dissolution solution containing carboxylate (such as acetic acid) dissolved in alcohol (such as isopropyl alcohol, IPA) in process step 110. In one example of the second embodiment, the dissolution solution used in process step 110 was prepared by diluting glacial acetic acid (GAC). High-concentration GAC continuously etches cobalt at a high etching rate, presumably due to the presence of moisture and dissolved oxygen in the GAC solution. To avoid the spontaneous etching of cobalt, various concentrations of GAC were diluted with IPA, thereby reducing the etching rate of the very dilute GAC solution to almost zero.

[0052] Graph 400 shown in FIG. 4A shows various etching amounts (represented in nm) that can be achieved when etching cobalt using various concentrations of acetic acid (e.g., 1%, 5%, 25%, 50%, and >99%) in isopropyl alcohol (IPA) at 35°C. Graph 450 shown in FIG. 4B shows the dependence of the etching rate (represented in nm / min) on the acetic acid concentration. As shown in FIGS. 4A and 4B, the etching rate of cobalt increases when the GAC concentration is 25% or more. However, with 5% GAC dissolved in IPA, little cobalt etching occurs. With less than 1% GAC dissolved in IPA, the spontaneous etching rate of cobalt is negligible. At this concentration, the acetic acid ligand selectively dissolves the native oxide from the cobalt metal film without significantly etching the cobalt metal film, similar to the first embodiment using the ACAC ligand dissolved in methanol.

[0053] The above wet removal method provides another robust process for selectively and self-limitingly removing the native oxide formed on the cobalt metal film. In a typical process according to the second embodiment, a dissolution solution containing 0 - 5% GAC dissolved in IPA can be used, followed by rinsing with IPA. Similar to the previous process, the native oxide can be selectively removed from the surface of the cobalt metal film by immersing and / or rinsing the surface of the substrate in the dissolution solution and the rinse solution. The processing temperature of the dissolution solution and the rinse solution can generally depend on the solvent used therein. For example, the processing temperature can be in the range of -89°C to 82°C when using IPA, and in the range of 19°C to 190°C when using DMSO. However, in some embodiments, processing close to room temperature may be desirable. In such embodiments, the typical processing temperature can be in the range of 20°C to 55°C.

[0054] The etching experiments were carried out on coupons cut from 300 mm silicon wafers with physical vapor deposition (PVD) cobalt of various thicknesses deposited on one side. By exposing to the ambient, a native oxide was formed on the cobalt metal film. A 1% GAC dissolved in IPA was used to selectively remove the native oxide from the surface of the cobalt metal film using an etching recipe that included dipping for 30 seconds in 1% GAC dissolved in IPA, then rinsing with IPA and blow-drying with nitrogen (N2). This process was carried out at 35 °C.

[0055] Three etching experiments were conducted to evaluate the selectivity between native oxide and cobalt metal etching using the above etching recipe. The results of such experiments are shown in graph 500 in Figure 5. In the first experiment (circles), the cobalt metal film with the native oxide formed was exposed to the above dissolution solution for a long time (e.g., 5 minutes) without intentional oxidation. In the first experiment, only the native oxide was removed and no significant change was seen in the thickness of the cobalt metal film.

[0056] In the second experiment (diamonds), an intentional oxidation step was incorporated into the etching recipe to enable replication of the native oxide by exposing the cobalt metal film to air after the dissolution step. In this experiment, by exposing to air, the formation of the native oxide on the cobalt surface was repeated, thus improving the etching rate of the cobalt metal film.

[0057] In the third experiment (triangles), a circulation process was used to evaluate the selectivity of glacial acetic acid (GAC) for native oxide etching. In this experiment, an intentional oxidation step was incorporated into the etching recipe after the dissolution step so that the native oxide could be replicated before repeating the process steps. In the third experiment, oxidation was carried out by immersing the substrate containing the cobalt metal film in deionized water at 35 °C for several seconds (e.g., 30 seconds). Similar to the previous experiment, since the native oxide reformed on the cobalt surface, the etching rate of the cobalt metal film increased when exposed to water. In the case of the coupons treated with water, the dissolution rate of the native oxide can be accelerated by surface hydroxylation.

[0058] From the above experiments, it was proven that the dissolution solution containing 1% GAC dissolved in IPA does not etch the cobalt metal film in the absence of oxidation (air or water) during the experimental period. Comparing the etching rates of cobalt with and without intentional oxidation, any etching achieved was associated with the etching of the native oxide, and an indirectly self-limiting native oxide removal behavior was confirmed. Furthermore, from the experimental results, it is shown that the dissolution solution containing 1% GAC in IPA is at least 10 times and 25 times more selective for the etching of native oxide than cobalt metal by air oxidation and deionized water oxidation, respectively. Theoretically, higher selectivity can be achieved by controlling unwanted oxidation.

[0059] Figures 6 to 8 show exemplary methods using the processing techniques described herein. It will be understood that the embodiments of Figures 6 to 8 are merely illustrative and that additional methods may use the techniques described herein. Further, since the described processing steps are not intended to be exclusive, additional steps may be added to the methods shown in Figures 6 to 8. Further, the order of the steps is not limited to the order shown in the drawings since different orders may occur and / or various steps may be combined or performed simultaneously.

[0060] FIG. 6 shows one embodiment of a method 600 for removing a native oxide formed on the surface of a metal film. The method 600 shown in FIG. 6 can generally start with receiving a substrate having a metal film exposed on the surface of the substrate, and the native oxide is formed on the surface of the metal film (in step 610). The method 600 can further include exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent (in step 620). When the native oxide is exposed to the dissolution solution in step 620, the ligand forms a complex with the native oxide to form a ligand-metal complex that is soluble in the non-aqueous solvent and thereby selectively dissolved. When the ligand-metal complex is dissolved, the method 600 can include rinsing the substrate to remove the dissolution solution and the dissolved ligand-metal complex from the surface of the substrate, thereby leaving a metal surface or a ligand-bound passivation monolayer on the metal film (in step 630).

[0061] The method 600 shown in FIG. 6 can be used to remove a native oxide formed on the exposed surface of the metal film due to processing or exposure to the ambient environment. In some embodiments, the method 600 shown in FIG. 6 can be used to strip the native oxide from an ultrathin metal film. For example, the thickness of the metal film can be about 10 nm or less, optionally further 2 nm or less, and the method 600 can be used to strip a native oxide with a thickness of 5 nm or less, optionally 1 nm or less, from the surface of the metal film. In some embodiments, the step of exposing the surface of the substrate to the dissolution solution (in step 620) and the step of rinsing the substrate (in step 630) can remove the native oxide formed on the surface of the metal film without removing or significantly etching the metal film.

[0062] The method 600 shown in FIG. 6 can generally be performed at a low temperature. In some embodiments, the step of exposing the surface of the substrate to the dissolution solution (in step 620) and the step of rinsing the substrate (in step 630) can be performed at a temperature in the range of 20°C to 55°C.

[0063] The method 600 shown in FIG. 6 can be used to remove native oxides from various metal films. In some embodiments, for example, the method 600 shown in FIG. 6 can be used to remove native oxides formed on various transition metal films such as, but not limited to, metal films of cobalt (Co), copper (Cu), ruthenium (Ru), tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), platinum (Pt), nickel (Ni), rhodium (Rh), and iridium (Ir). The method 600 shown in FIG. 6 can also be used to remove metal oxides from other metal films and semiconductor films such as aluminum (Al), gold (Au), silicon (Si), germanium (Ge), etc.

[0064] Depending on the native oxide to be removed, various ligands can be included in the dissolution solution used in step 620. Generally, the ligand included in the dissolution solution can be selected from the group consisting of β-diketonates, carboxylates, aminopolycarboxylates, oximes, and amines. In some embodiments, the dissolution solution can further include a base that activates the ligand to facilitate complex formation between the ligand and the native oxide. Regardless of the specific ligand used, the ligand included in the dissolution solution can react with and bind to the native oxide, but does not bind to the metal film, thereby forming a self-limiting ligand-metal complex on the metal film.

[0065] Various non-aqueous solvents including, but not limited to, polar organic solvents such as alcohols (e.g., methanol, isopropanol, amyl alcohol), ketones (e.g., acetone, methyl ethyl ketone), acetates (e.g., ethyl acetate, amyl acetate), acetonitrile, dimethyl sulfoxide, and n-methylpyrrolidone can be included in the dissolution solution used in step 620. Regardless of the specific solvent used, the non-aqueous solvent included in the dissolution solution can be used to selectively dissolve the ligand-metal complex while preventing re-oxidation of the metal surface after the ligand-metal complex is removed.

[0066] FIG. 7 shows another embodiment of a method 700 for removing a native oxide formed on the surface of a metal film. The method 700 shown in FIG. 7 can generally begin by receiving a substrate having a metal film exposed on the surface of the substrate, where the native oxide is formed on the surface of the metal film (in step 710). Next, the method 700 can include exposing the surface of the substrate to an oxidizing agent to further oxidize the native oxide and form a metal oxide surface layer or a metal hydroxide surface layer (in step 720), followed by exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent (in step 730). When the native oxide is exposed to the dissolution solution in step 730, the ligand contained in the dissolution solution reacts with and binds to the metal oxide surface layer or the metal hydroxide surface layer to form a ligand-metal complex that is soluble in the non-aqueous solvent and thereby selectively dissolved. When the ligand-metal complex is dissolved, the method 700 can include rinsing the substrate to remove the dissolution solution and the ligand-metal complex from the surface of the substrate (in step 740). The method 700 shown in FIG. 7 selectively removes the native oxide formed on the surface of the metal film without removing the metal film by exposing the surface of the substrate to an oxidizing agent (in step 720), exposing the surface of the substrate to the dissolution solution (in step 730), and rinsing the substrate (in step 740).

[0067] The surface of the substrate can be exposed to various oxidizing agents in step 720. In some embodiments, for example, the oxidizing agent can be selected from the group consisting of deionized water, air, hydrogen peroxide, and ammonium hydroxide. In one embodiment, the surface of the substrate can be exposed to deionized water (in step 720) to further oxidize the native oxide and form a metal hydroxide surface layer on the surface of the metal film. As described above, the metal hydroxide surface layer can be more readily dissolved in the non-aqueous solvent than the native oxide.

[0068] In operation 730, the surface of the substrate can also be exposed to various dissolution solutions. In some embodiments, upon exposure to the dissolution solution, a self-limiting ligand-metal complex can be formed on the metal film in operation 730. To form a self-limiting ligand-metal complex, the ligand contained in the dissolution solution needs to react with and bind to the native oxide. However, the ligand contained in the dissolution solution does not react with or bind to the metal film after the ligand-metal complex is dissolved in the non-aqueous solvent.

[0069] In some embodiments, the operation of exposing the surface of the substrate to the dissolution solution (in operation 730) and the operation of rinsing the substrate (in operation 740) can leave a metal surface (or a ligand-bound passivating monolayer) on the metal film and prevent further oxidation of the metal film. In some embodiments, for example, the operation of rinsing the substrate (in operation 740) can include exposing the substrate to a second non-aqueous solvent, which can be the same as (or different from) the non-aqueous solvent used in the dissolution solution. Since the non-aqueous solvents used in operations 730 and 740 do not contain oxidizing agents, it prevents the metal surface remaining on the metal film from being intentionally oxidized after the native oxide is selectively removed.

[0070] FIG. 8 shows one embodiment of a method 800 for stripping a native oxide from a cobalt metal film. The method 800 shown in FIG. 8 may generally begin by receiving a substrate having a cobalt metal film exposed on a surface of the substrate, where the native oxide is formed on the cobalt metal film (in step 810). The method 800 may further include exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent (in step 830). When the native oxide is exposed to the dissolution solution in step 830, the ligand reacts with and binds to the native oxide to form a ligand-metal complex that is self-limiting and selectively soluble in the non-aqueous solvent. The method 800 may further include rinsing the substrate to remove the dissolution solution and the ligand-metal complex from the surface of the substrate (in step 840). By exposing the surface of the substrate to the dissolution solution (in step 830) and rinsing the substrate (in step 840), the method 800 shown in FIG. 8 can strip the native oxide from the cobalt metal film without removing the cobalt metal film.

[0071] In some embodiments, the method 800 shown in FIG. 8 may further include exposing the surface of the substrate to an oxidizing agent to convert the native oxide to a cobalt oxide surface layer or a cobalt hydroxide surface layer (in step 820), and then exposing the surface of the substrate to a dissolution solution (in step 830). In such embodiments, the ligand contained in the dissolution solution can react with and bind to the cobalt oxide surface layer or the cobalt hydroxide surface layer to form a ligand-metal complex. The surface of the substrate can be exposed to various oxidizing agents in step 820. In one embodiment, the surface of the substrate can be exposed to deionized water (in step 820) to further oxidize the native oxide and form a cobalt hydroxide surface layer on the surface of the metal film. As described above, the cobalt hydroxide surface layer can be more readily soluble in the non-aqueous solvent than the native oxide.

[0072] For selectively dissolving the native oxide (or optionally a cobalt oxide surface layer or a cobalt hydroxide surface layer) in Project 830, various ligands and non-aqueous solvents can be included in the dissolution solution. In one embodiment, the dissolution solution can include acetylacetonate (i.e., a ligand) dissolved in methanol (i.e., a non-aqueous solvent). In some embodiments, the dissolution solution can further include a base that deprotonates the ligand to facilitate complex formation between the ligand and the native oxide. In such embodiments, the base can be selected from the group consisting of quaternary ammonium hydroxides (e.g., tetrabutylammonium hydroxide (TBAH), tetramethylammonium hydroxide (TMAH)), nitrogen bases (e.g., trimethylamine, piperidine), and phosphines (e.g., triphenylphosphine). In one embodiment, the base can be tetrabutylammonium hydroxide (TBAH).

[0073] In another embodiment, the dissolution solution can include glacial acetic acid (GAC) (i.e., a ligand) dissolved in isopropyl alcohol (IPA) (i.e., a non-aqueous solvent). In some embodiments, the concentration of GAC in IPA can range from 0 to 100%. In one exemplary embodiment, the concentration of GAC in IPA can be about 1%.

[0074] The processes and methods described herein for stripping the native oxide surface layer from a metal film have several advantages compared to conventional methods used to remove native oxides. For example, the processes and methods described herein are carried out at low temperatures (e.g., 20°C to 55°C) using mild chemicals (e.g., ligands dissolved in a non-aqueous solvent), thus minimizing the risk of damage to the components of the device. The processes and methods disclosed herein are inherently self-limiting because they use reactive dissolution chemicals that include ligands in a non-aqueous solution, where the ligand is selective for metal oxides but not for metals. In addition, the disclosed processes and methods are fast and low-cost and do not require expensive vacuum equipment. Instead, the disclosed processes and methods are simple and can be implemented on existing wet processing equipment and integrated with the existing processing infrastructure in BEOL IC manufacturing.

[0075] The processes and methods described herein can be used to integrate cobalt (and other metals affected by native oxide formation) into BEOL. For example, the processes and methods disclosed herein can be used to pretreat ultra-thin cobalt metal films (e.g., cobalt metal caps, liners, etc.) by selectively removing the native oxides that typically form on these films during processing or exposure to the ambient. In one practical implementation, the processes and methods disclosed herein can ensure the reliability of fully self-aligned vias (FSAV) in BEOL by removing the native oxide from a cobalt cap layer formed on copper wiring. In another practical implementation, the processes and methods disclosed herein can be used as a cobalt pretreatment method prior to SAM formation in area-selective dielectric-on-dielectric (DoD) deposition in BEOL. Selective native oxide stripping is becoming increasingly important as the critical dimension (CD) continues to shrink in BEOL. The processes and methods described herein can be used to strip other metal oxides from other metal films.

[0076] The processes and methods described herein for removing native oxides from metal films can be achieved using a variety of techniques. For example, the processes and methods disclosed above can be performed by immersing a substrate having metal features formed thereon into a beaker containing a dissolution solution. In this case, the dissolution solution can be removed from the surface of the substrate by rinsing or immersing the substrate in a suitable solvent bath. The processes and methods disclosed above can also be performed within a variety of semiconductor processing systems. The disclosed processes can be achieved using many different process chambers, tools, and apparatuses, but the processing apparatus used to perform the disclosed processes is preferably operable at low temperatures (e.g., near room temperature and above).

[0077] In one exemplary implementation, the wet processes described herein can be performed within a spin chamber. When a spin chamber is used, the etching solution is dispensed from a nozzle disposed above the substrate and dispersed by the rotational movement of the spin chuck on which the substrate is disposed. After a set exposure time, the nozzle begins to dispense the next solution in the etching recipe. For high-volume manufacturing, the dispensing of the etching solution and rinse can be performed using conventional tools such as wet etching tools and rinse tools.

[0078] Throughout this specification, references to "one embodiment" or "an embodiment" mean that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention, but do not necessarily mean that they are present in all embodiments. It should be noted that, accordingly, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the present invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. In other embodiments, various additional layers and / or structures may be included and / or the features described may be omitted.

[0079] Systems and methods for processing a substrate are described in various embodiments. The substrate can include any material portion or structure of a device, particularly a semiconductor or other electronic device, and can be, for example, a basic substrate structure such as a semiconductor substrate, or a layer on or covering a basic substrate structure such as a thin film. Thus, the substrate is not intended to be limited to any particular basic structure, underlying layer, or upper layer that is patterned or unpatterned, but rather is intended to include any such layer or basic structure, as well as any combination of layers and / or basic structures.

[0080] As used herein, the term "substrate" means and includes the basic material or structure on which materials are formed. It will be understood that the substrate can include a single material, multiple layers of different materials, or one or more layers having regions of different materials or different structures therein. These materials can include semiconductors, insulators, conductors, or combinations thereof. For example, the substrate can be a semiconductor substrate, a basic semiconductor layer on a support structure, a metal electrode, or a semiconductor substrate having one or more layers, structures, or regions formed thereon. The substrate can be a conventional silicon substrate or other bulk substrate that includes a layer of a semiconductive material. As used herein, the term "bulk substrate" means and includes not only silicon wafers, but also silicon-on-insulator (SOI) substrates such as silicon-on-sapphire ("SOS") substrates and silicon-on-glass ("SOG") substrates, epitaxial layers of silicon on a basic semiconductor substrate, as well as other semiconductors or optoelectronic materials such as silicon germanium, germanium, gallium arsenide, gallium nitride, and indium phosphide. The substrate can be doped or undoped.

[0081] Those skilled in the art will appreciate that the various embodiments can be practiced without one or more of the specific details, or with other alternative and / or additional methods, materials, or components. In other instances, well-known structures, materials, or operations are not illustrated or described in detail to avoid obscuring aspects of the various embodiments of the present invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are described to provide a thorough understanding of the present invention. Nevertheless, the present invention can be practiced without specific details. Further, it should be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

[0082] Upon review of this specification, further modifications and alternative embodiments of the described systems and methods will be apparent to those skilled in the art. Accordingly, it will be understood that the described systems and methods are not limited by these exemplary configurations. It should be understood that the forms of the systems and methods illustrated and described herein are to be construed as exemplary embodiments. Various modifications can be made in the implementation. Thus, while the technology has been described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present disclosure. Accordingly, this specification and the drawings are to be regarded in an illustrative rather than a limiting sense, and such modifications are intended to be included within the scope of the present disclosure. Further, no solution to any benefit, advantage, or problem described herein with respect to a particular embodiment is intended to be construed as a critical, required, or essential feature or element of any or all of the claims.

Claims

1. 1. A method for removing native oxide formed on a surface of a metal film, comprising: receiving a substrate having the metal film exposed on a surface thereof, the native oxide being formed on the surface of the metal film; exposing the surface of the substrate to a dissolution solution comprising a ligand dissolved in a non-aqueous solvent, the ligand complexing with the native oxide to form a ligand-metal complex, the ligand-metal complex being soluble in and selectively dissolved by the non-aqueous solvent; rinsing the substrate with a second non-aqueous solvent to remove the dissolution solution and the ligand-metal complex from the surface of the substrate, thereby leaving a metal surface or ligand-bound passivating monolayer on the metal film; A method comprising:

2. The method of claim 1 , wherein the metal film has a thickness of less than 10 nm and the native oxide has a thickness of 5 nm or less.

3. 10. The method of claim 1, wherein the ligand reacts with and bonds to the native oxide but does not bond to the metal film, thereby forming a self-limiting ligand-metal complex on the metal film.

4. 2. The method of claim 1, wherein the ligand is selected from the group consisting of β-diketonates, carboxylates, aminopolycarboxylates, oximes, and amines.

5. The non-aqueous solvent used in the dissolution solution is selected from the group consisting of alcohols, ketones, acetates, acetonitrile, dimethyl sulfoxide, and n-methylpyrrolidone; 10. The method of claim 1, wherein the second non-aqueous solvent used to remove the dissolution solution and the ligand-metal complex from the surface of the substrate is the same as or different from the non-aqueous solvent used for the dissolution solution.

6. 10. The method of claim 1, wherein the dissolution solution further comprises a base to activate the ligand and promote complexation of the ligand with the native oxide.

7. The method of claim 1 , wherein exposing the surface of the substrate to the dissolving solution and rinsing the substrate does not etch the metal film.

8. 10. The method of claim 1, wherein exposing the surface of the substrate to the dissolving solution and rinsing the substrate are carried out at a temperature ranging between 20°C and 55°C.

9. 1. A method for removing native oxide formed on a surface of a metal film, comprising: receiving a substrate having the metal film exposed on a surface thereof, the native oxide being formed on the surface of the metal film; exposing the surface of the substrate to an oxidizing agent to further oxidize the native oxide and form a metal oxide or metal hydroxide surface layer; exposing the surface of the substrate to a dissolution solution containing a ligand dissolved in a non-aqueous solvent, wherein the ligand reacts with and bonds to the metal oxide or metal hydroxide surface layer to form a ligand-metal complex, which is soluble in and selectively dissolved by the non-aqueous solvent; rinsing the substrate with a second non-aqueous solvent to remove the dissolution solution and the ligand-metal complex from the surface of the substrate; and The steps of exposing the surface of the substrate to the oxidizing agent, exposing the surface of the substrate to the dissolving solution, and rinsing the substrate selectively remove the native oxide formed on the surface of the metal film without removing the metal film.

10. 10. The method of claim 9, wherein the oxidizing agent is selected from the group consisting of deionized water, air, hydrogen peroxide, and ammonium hydroxide.

11. 10. The method of claim 9, wherein exposing the surface of the substrate to the oxidizing agent comprises exposing the surface of the substrate to deionized water to oxidize the native oxide and form the metal hydroxide surface layer.

12. 10. The method of claim 9, wherein the ligand does not react with or bond to the metal film after the ligand-metal complex is dissolved in the non-aqueous solvent.

13. 10. The method of claim 9, wherein exposing the surface of the substrate to the dissolution solution and rinsing the substrate leaves a metal surface or ligand-bound passivating monolayer on the metal film, preventing further oxidation of the metal film.

14. The method described in claim 9, wherein the second non-aqueous solvent used to remove the dissolution solution and the ligand-metal complex from the surface of the substrate is the same as or different from the non-aqueous solvent used for the dissolution solution.

15. 1. A method for stripping native oxide from a cobalt metal film, comprising: receiving a substrate having the cobalt metal film exposed on a surface thereof, the native oxide being formed on the cobalt metal film; exposing the surface of the substrate to a dissolution solution comprising a ligand dissolved in a non-aqueous solvent, the ligand reacting and bonding with the native oxide to form a ligand-metal complex, the ligand-metal complex being self-limiting and selectively dissolved in the non-aqueous solvent; rinsing the substrate with a second non-aqueous solvent to remove the dissolution solution and the ligand-metal complex from the surface of the substrate; and The method of claim 1, wherein exposing the surface of the substrate to the dissolving solution and rinsing the substrate strips the native oxide from the cobalt metal film without removing the cobalt metal film.

16. The method further comprises exposing the surface of the substrate to an oxidizing agent to convert the native oxide into a cobalt oxide surface layer or a cobalt hydroxide surface layer prior to exposing the surface of the substrate to the dissolution solution; 16. The method of claim 15, wherein the ligand contained in the dissolution solution reacts with and bonds to the cobalt oxide or cobalt hydroxide surface layer to form the ligand-metal complex.

17. 17. The method of claim 16, wherein exposing the surface of the substrate to the oxidizing agent comprises exposing the surface of the substrate to deionized water to oxidize the native oxide and form the cobalt hydroxide surface layer.

18. 16. The method of claim 15, wherein the ligand comprises acetylacetonate and the non-aqueous solvent comprises methanol.

19. The lysis solution further comprises a base; 20. The method of claim 18, wherein the base deprotonates the ligand, facilitating complexation of the ligand to the native oxide.

20. 20. The method of claim 19, wherein the base is selected from the group consisting of quaternary ammonium hydroxides, nitrogen bases, and phosphines.

21. 16. The method of claim 15, wherein the ligand comprises glacial acetic acid (GAC), the non-aqueous solvent comprises isopropyl alcohol (IPA), and the concentration of the GAC in the IPA is 0-100%.

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