WO2023235099A1

WO2023235099A1 - Removal of metal salt precipitates in an electroplating tool

Info

Publication number: WO2023235099A1
Application number: PCT/US2023/020862
Authority: WO
Inventors: Daniel James MARTIN; Boon Kang Ong; Nirmal Shankar SIGAMANI; Frederick Dean Wilmot; Lee CHUA
Original assignee: Lam Research Corporation
Priority date: 2022-06-02
Filing date: 2023-05-03
Publication date: 2023-12-07
Also published as: TW202407168A

Abstract

Examples are disclosed that relate to the removal of metal salt precipitates from within a circulating loop of an electroplating tool. In one example method, during a processing phase of operation, a metal salt solution is flowed through a circulating loop at a process temperature to deposit a metal on a substrate. The metal salt solution comprises at least a metal cation and a counter ion. In a precipitate removal phase of operation, the metal salt solution is heated to a temperature higher than the process temperature. The heated metal salt solution is then flowed through the circulating loop for a duration to dissolve metal salt precipitates of the metal salt solution within the circulating loop. The metal salt solution is then cooled to the process temperature.

Description

REMOVAL OF METAL SALT PRECIPITATES IN AN ELECTROPLATING

TOOL

BACKGROUND

[0001] Electroplating can be used in integrated circuit manufacturing processes to deposit electrically conductive films onto substrates. Electroplating involves the electrochemical reduction of dissolved ions of a selected metal to an elemental state on a substrate to form a film of the selected metal. Electroplating systems comprise a cathode chamber through which a catholyte solution circulates, and an anode chamber through which an anolyte solution circulates. A cation exchange membrane is positioned between the catholyte chamber and anolyte chamber. The cation exchange membrane allows protons and ions of the selected metal to pass from the anode to the cathode while preventing the passage of anions and organic additives.

SUMMARY

[0002] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.

[0003] Examples are disclosed that relate to the removal of metal salt precipitates from within a circulating loop of an electroplating tool. In one example method, during a processing phase of operation, a metal salt solution is flowed through a circulating loop at a process temperature to deposit a metal on a substrate. The metal salt solution comprises at least a metal cation and a counter ion. In a precipitate removal phase of operation, the metal salt solution is heated to a temperature higher than the process temperature. The heated metal salt solution is then flowed through the circulating loop for a duration to dissolve metal salt precipitates within the circulating loop. The metal salt solution is then cooled to the process temperature.

[0004] In some such examples, the method further comprises receiving an indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop, and initiating the precipitate removal phase of operation in response to the received indication. [0005] In some such examples, the indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop alternatively or additionally comprises an indication of passivation of a cation exchange membrane.

[0006] In some such examples, the indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop alternatively or additionally comprises an observed non-uniform current at a cathode.

[0007] In some such examples, initiating the precipitate removal phase of operation further alternatively or additionally comprises monitoring a total amount of coulombs of electroplating performed by the electroplating tool following a previous precipitate removal phase of operation.

[0008] In some such examples, the method alternatively or additionally further comprises, responsive to the total amount of accumulated coulombs being below a threshold when accumulation of metal salt precipitates is indicated, initiating the precipitate removal phase of operation at a next idle period.

[0009] In some such examples, the method alternatively or additionally further comprises, responsive to the total amount of accumulated coulombs being above the threshold when the accumulation of metal salt precipitates is indicated, initiating the precipitate removal phase of operation before a next idle period.

[0010] In some such examples, the metal salt solution alternatively or additionally comprises a catholyte or anolyte.

[0011] In some such examples, the metal cation alternatively or additionally comprises Cu²⁺, and the counter ion alternatively or additionally comprises SC ²'.

[0012] Another example provides an electroplating tool. The electroplating tool comprises a substrate, a circulating loop for a metal salt solution, the circulating loop comprising a heater, a logic machine, and a storage machine storing instructions executable by the logic machine. The instructions are executable to, in a process phase of operation, control a flow of the metal salt solution through the circulating loop at a process temperature to deposit a metal on the substrate, and in a precipitate removal phase of operation, control the heater to heat the metal salt solution to a temperature higher than the process temperature, control a flow of the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates within the circulating loop, and control the heater to cool the metal salt solution to the process temperature. [0013] In some such examples, the circulating loop alternatively or additionally comprises a cathode or anode chamber.

[0014] In some such examples, the cathode chamber alternatively or additionally is separated from an anode chamber by a cation exchange membrane.

[0015] In some such examples, the anode chamber alternatively or additionally comprises a consumable anode.

[0016] In some such examples, the consumable anode alternatively or additionally comprises copper metal.

[0017] In some such examples, the metal cation alternatively or additionally comprises Cu²⁺, and the counter anion alternatively or additionally comprises SC ²'.

[0018] In some such examples, the precipitate removal phase of operation alternatively or additionally is initiated in response to an indication of metal salt precipitates accumulated within the circulating loop.

[0019] Another example provides a storage machine storing instructions executable by a logic machine to, at an electroplating tool, in a processing phase of operation, control a flow of an metal salt solution through a circulating loop at an process temperature to deposit a metal on a substrate, and in a precipitate removal phase of operation, control a heater heat the metal salt solution to a temperature higher than the process temperature, control a flow of the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates of the metal salt solution within the circulating loop, and control the heater to cool the metal salt solution to the process temperature.

[0020] In some such examples, the storage machine alternatively or additionally further stores instructions executable by the logic machine to receive an indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop, and initiate the precipitate removal phase of operation in response to the received indication.

[0021] In some such examples, the storage machine alternatively or additionally further stores instructions executable by the logic machine to, when the total amount of accumulated coulombs is below a threshold when the accumulation of metal salt precipitates is indicated, initiate the precipitate removal phase of operation at a next idle period.

[0022] In some such examples, the storage machine alternatively or additionally further stores instructions executable by the logic machine to, when the total amount of accumulated coulombs is above the threshold when the accumulation of metal salt precipitates is indicated, initiate the precipitate removal phase of operation before a next idle period.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows a block diagram of an example electroplating tool.

[0024] FIG. 2 schematically shows an example electroplating cell of an electroplating tool.

[0025] FIGS. 3A and 3B schematically show example cation current flow within the electroplating tool of FIG. 2 respectively without and with metal salt precipitates.

[0026] FIG. 4 shows a flow diagram depicting an example method for operating an electroplating tool.

[0027] FIG. 5 shows an example plot of metal salt solution temperature as a function of time for an example implementation of the method of FIG. 4.

[0028] FIG. 6 schematically shows example states of the electroplating tool of FIG. 2 over time while performing an example implementation of the method of FIG. 4. [0029] FIG. 7 schematically shows an example computing system.

DETAILED DESCRIPTION

[0030] The term “anode” may generally represent an electrically conductive structure at which electrochemical oxidation occurs during an electroplating process.

[0031] The term “anode chamber” may generally represent a physical structure configured to hold at least an anode and anolyte and that provides selective separation from a cathode chamber.

[0032] The term “anolyte” may generally represent a solution used in an anode chamber during an electroplating process.

[0033] The term “anolyte bath” may generally represent a liquid environment within an anode chamber.

[0034] The term “cathode” may generally represent a conductive layer on a substrate that is grown during electroplating by the electrochemical reduction of ions.

[0035] The term “cathode chamber” may generally represent a physical structure configured to hold at least a cathode and catholyte and that provides selective separation from an anode chamber. [0036] The term “catholyte” may generally represent a solution used in a cathode chamber during an electroplating process.

[0037] The term “catholyte bath” may generally represent a liquid environment of a cathode chamber.

[0038] The term “circulating loop” may generally represent a path along which a liquid is recirculated over time. The term circulating loop may generally represent a catholyte circulating loop and also may generally represent an anolyte circulating loop. [0039] The term “consumable anode” may generally represent an electrode material that is electrochemically oxidized during an electroplating process.

[0040] The term “coulombs of electroplating” may generally represent a quantification of electric charge, measured in coulombs, used to electrochemically reduce a metal onto one or more substrates over time.

[0041] The term “counter anion” may generally represent an anion that provides charge balance to a cation in a solution.

[0042] The terms “electroplating”, “plating”, “deposition”, and variants thereof may generally represent a process in which dissolved ions of one or more metals are reduced on a substrate surface to form a film of the one or more metals.

[0043] The term “electroplating tool” may generally represent a machine configured to perform electroplating.

[0044] The term “high-resistance virtual anode” (HRVA) may generally represent an ionically resistive structure positioned between a substrate holder and an anode of an electroplating tool through which ions flow from the anode to a cathode during electroplating. A HRVA approximates a suitably constant and uniform current source in proximity to the cathode.

[0045] The term “idle period” may generally represent a period of time during which an electroplating tool is not scheduled to perform electroplating on substrates.

[0046] The term “cation exchange membrane” may generally represent a membrane that selectively passes one or more cationic species while blocking the transport of other species, such as anionic species and organic species.

[0047] The term “metal salt solution” may generally represent a solution comprising dissolved metal ions and counter anions.

[0048] The term “metal cation” may generally represent a metal atom with a positive oxidation state. [0049] The term “metal salt precipitates” may generally represent solid phase metal salt deposits that have formed from solution.

[0050] The term “non-uniform current” may generally represent an ionic or electric current that comprises different magnitudes across an area that intersects a direction of current flow.

[0051] The term “passivation”, when used to refer to a cation exchange membrane, may generally refer to a coating of metal salt precipitates that reduces or inactivates transport through the cation exchange membrane.

[0052] The term “precipitate removal phase” may generally refer to an operational period of an electroplating tool during which metal salt precipitates are dissolved into a metal salt solution and substrates are not being processed.

[0053] The term “processing phase” may generally refer to operational segment of an electroplating tool when substrates are being processed.

[0054] The term “process temperature” may generally refer to a temperature of a metal salt solution during an electroplating operation.

[0055] The term “substrate” represents any object on which a film can be deposited.

[0056] In the semiconductor processing industry, there is increasing demand to electrodeposit metals onto substrates rapidly while maintaining high uniformity. To meet this demand, electroplating technologies are tending toward higher plating currents, and chemistries that can support increased plating rates. For example, catholyte and anolyte solutions may comprise near- saturation concentrations of metal salts. However, such conditions may lead to deposits of metal salt precipitates forming in an electroplating tool over time.

[0057] As a more specific example, to achieve relatively higher copper electroplating growth rates, plating currents may exceed 10 Amperes. Further, catholytes and anolytes may contain nearly-saturated solutions of copper sulfate (CuSC ), sulfuric acid (H2SO4), and potentially other supporting additives. Copper electroplating utilizes a cation exchange membrane to separate anolyte and catholyte chambers to prevent the oxidation of organic additives at the anode. Example cation exchange membranes may comprise sulfonated tetrafluoroethylenebased fluoropolymer-copolymer. Metal ions passing through the cation exchange membranes from an anolyte to a catholyte add to the concentration of metal ions in the catholyte adjacent to the cation exchange membrane. This localized increase in concentration may lead to super-saturation of the catholyte solution at the cation exchange membrane under some conditions. This may cause metal salt precipitates to form on the cation exchange membrane and/or adjacent structures. Low convection of the catholyte at this membrane also may lead to precipitate formation and crystal growth. Over time, precipitate buildup may passivate portions of the cation exchange membrane. This may negatively impact a uniformity of an electroplated film on a substrate.

[0058] Currently, a primary strategy for recovering the target substrate deposition profile for tools that suffer from precipitate accumulation is manual intervention. Manual intervention may involve removing/replacing the affected membranes and parts. Such intervention may be time consuming and relatively expensive due at least to the cost of parts and impact to tool uptime. Other strategies, such as reducing metal ion concentrations and/or electric currents used for electroplating, may reduce tool utility.

[0059] Accordingly, examples are disclosed that relate to removing metal salt precipitates from a circulating loop in an electroplating tool using an in-situ heat treatment cycle. The disclosed examples are non-invasive, in-situ processes that use potentially modest increases in the temperature of a circulating metal salt solution to dissolve metal salt precipitate buildup from plating tool components. The disclosed examples may help to avoid part replacement due to precipitate buildup, and may decrease the overall impact of precipitate removal on tool uptime. While described primarily in the context of copper electroplating tools and copper sulfate crystal buildup at a cation exchange membrane, the methodology may be used with any suitable chemistry on any suitable electroplating tool. Further, the disclosed examples may allow for the remediation of even severe crystal buildup. Also, as the disclosed processes are non-invasive, the processes may be performed without breaking the plane of an electroplating tool.

[0060] Prior to discussing these examples in more detail, FIG. 1 schematically shows a block diagram of an example electroplating tool 100. Electroplating tool 100 comprises an electroplating cell 102 comprising an anode chamber 104 and a cathode chamber 106. Electroplating tool 100 further comprises a cation exchange membrane 108 separating the anode chamber 104 and the cathode chamber 106, and a HRVA 109 within cathode chamber 106. Anode chamber 104 comprises an anode 110. Anode chamber 104 further comprises an anolyte. Cathode chamber 106 comprises a catholyte. The catholyte comprises an ionic species to be deposited on a cathode layer of a substrate 111 as a metal by electrochemical reduction. Anode 110 may comprise a consumable anode formed from the metal being deposited, or may comprise an inert anode. Where anode 110 comprises the metal being deposited, electrochemical oxidation of anode 110 at least partially replenishes the ionic species consumed by the electroplating process. Bulk anolyte and/or catholyte solutions may be added at times to replenish the ionic species.

[0061] Cation exchange membrane 108 prevents organic species and anionic species from crossing between cathode chamber 106 and anode chamber 104, while allowing metal ions to cross from anode chamber 104 to cathode chamber 106. As mentioned above, HRVA 109 comprises an ionically resistive element that approximates a suitably constant and uniform current source in proximity to a substrate cathode.

[0062] Substrate holder 112 is coupled to a substrate holder movement system 113 comprising a lift 114 that is configured to adjust a spacing between substrate holder 112 and HRVA 109. For example, lift 114 may lower substrate holder 112 to position substrate 111 within the catholyte for electroplating. Lift 114 further may raise substrate holder 112 from the catholyte after electroplating. Substrate holder movement system 113 further may comprise components to control the opening and closing of substrate holder 112.

[0063] The catholyte may be circulated between cathode chamber 106 and a catholyte reservoir 120 via a combination of gravity and one or more pumps 122. Likewise, the anolyte may be circulated through anolyte reservoir 124 and anode chamber 104 via a combination of gravity and one or more pumps 126. In-line heaters 127 and 128, e.g., heater/chillers, may be included to control the temperatures of the catholyte and the anolyte, respectively.

[0064] In some electroplating tools, plating operations may be performed in parallel on multiple substrates using multiple plating cells. In some such examples, central catholyte and/or anolyte reservoirs may supply multiple plating cells with catholyte and/or anolyte. In other such examples, separate catholyte and/or anolyte reservoirs may be used to supply multiple plating cells. In yet other examples, an electroplating tool may comprise a single plating cell. Where an electroplating tool comprises multiple plating cells, a single lift may be configured to lift two or more substrate holders for two or more different plating cells. [0065] Substrate holder 112 is lowered by lift 114 toward HRVA 109 after substrate I l l is loaded into substrate holder 112. Substrate 111 faces a surface of the HRVA 109, and is spaced from HRVA 109 by a plating gap during electroplating. An electric field is established between anode 110 and substrate 111. This electric field drives dissolved metal cations from anode chamber 104 into cathode chamber 106. At the substrate 111, the metal cations are electrochemically reduced to deposit on substrate 111. An anodic potential is applied to anode 110 via charge plate 115 and a cathodic potential is provided to the cathode of substrate 111 via a cathode electrical connection 116 to form a circuit. In some examples, substrate holder 112 may be rotated via a rotational motor 117 during electroplating.

[0066] Electroplating tool 100 further comprises a computing system 130, aspects of which are described in more detail below with regard to FIG. 7. Computing system 130 may comprise instructions executable to control any suitable functions of electroplating tool 100. Example functions include electroplating processes, substrate loading/unloading processes, and precipitate dissolution processes. Example precipitate dissolution processes are described in more detail below. In some examples, computing system 130 may be configured to communicate with a remote computing system 140 via a suitable computer network. Remote computing system 140 may comprise any suitable computing system. Examples include a networked workstation computer, an enterprise computing system, and/or a cloud computing system. It will be understood that remote computing system 140 may be in communication with and control a plurality of electroplating tools in some examples.

[0067] FIG. 2 schematically shows an example electroplating cell 202 of an electroplating tool. Electroplating cell 202 is an example of electroplating cell 102 of FIG. 1. Electroplating cell 202 comprises an anode chamber 204 and a cathode chamber 206. Electroplating cell 202 further comprises a cation exchange membrane 208 separating anode chamber 204 and cathode chamber 206. Electroplating cell 202 further comprises a HRVA 209 disposed within cathode chamber 206.

[0068] Anode chamber 204 comprises an anolyte bath 210 in which anode 212 is disposed. In this example, anode 212 comprises a copper metal anode. In other examples, anode 212 may comprise an inert anode. Cathode chamber 206 comprises a catholyte bath 214 that comprises ionic copper (Cu²⁺) to be deposited onto a substrate 216 that acts as a cathode. Catholyte bath 214 is located within a catholyte circulating loop 220. Catholyte enters cathode chamber 206 at inlet 222 and exits at outlet 224. Catholyte circulating loop 220 comprises a comprise a heater 226 (e.g. a heater/chiller) configured to adjust and/or maintain a temperature of catholyte flowing through catholyte circulating loop 220.

[0069] Similarly, anolyte bath 210 is located within an anolyte circulating loop 230. Anolyte enters anode chamber 204 at inlet 232 and exits at outlet 234. Anolyte circulating loop 230 comprises a heater 236 (e.g. a heater/chiller) configured to adjust and/or maintain a temperature of anolyte flowing through anolyte circulating loop 220. A voltage source 240 applies a voltage across substrate 216 and anode 212 to drive flow of copper ions for deposition on substrate 216.

[0070] For copper electroplating, in some examples, anode 212 may comprise copper pieces (e.g. balls) or a copper slab, as examples. As mentioned above, in other examples, anode 212 may comprise an inert anode. In the depicted example, the applied voltage causes oxidation of copper metal to Cu²⁺ at anode 212. Cation exchange membrane 208 passes Cu²⁺ ions from anolyte bath 210 to catholyte bath 214. The Cu²⁺ ions crossing cation exchange membrane 208 replace at least some copper ions in catholyte bath 214 that are reduced onto substrate 216.

[0071] Heater 226 may be controlled to maintain catholyte bath 214 at a predetermined process temperature during an electroplating process. Heater 236 similarly may be controlled maintain anolyte bath 210 at a predetermined process temperature during an electroplating process. The process temperature may comprise a relatively low temperature in some processes. For example, a process temperature in a range of 22-26 °C may be used in some copper deposition processes.

[0072] As mentioned above, some electroplating conditions may increase a concentration of dissolved copper ions in a catholyte near the cation exchange membrane surface. The resulting concentration may exceed the solubility limit of the catholyte solvent (e.g. water) at the process temperature. This may result in the formation of crystals at the cation exchange membrane. In the example of FIG. 2, Cu²⁺ ions in anolyte bath 210 pass through cation exchange membrane 208 and into catholyte bath 214. Thus, the catholyte just above cation exchange membrane 208 contains ions both from the catholyte in catholyte bath 214 and the additional Cu²⁺ from anolyte bath 210. The additional Cu²⁺ concentration increases as plating current increases. Without adequate convection of catholyte to remove excess Cu²⁺ ions, the solution above the membrane may precipitate solid copper salts onto the membrane. CuSC may be a primary component of the copper salt precipitates. Copper-containing catholytes also may comprise chloride and sulfonate anions, which also may form copper precipitates. Metal salt precipitates also may occur in other electroplating chemistries. Examples include tin and tin-silver alloys. For tin-silver, example counter anions comprise methylsulfonic acid and organic acids. Precipitates also may occur for other electroplating metals, including cobalt, indium, and nickel.

[0073] The presence of metal salt precipitates on cation exchange membrane 208 may block fluid transport and electrical current distribution. This condition may be referred to as passivation. If areas of the membrane are partially or fully passivated, non-uniform current distribution may result. This may lead to nonuniform plating of metal onto a substrate.

[0074] In some examples, one or more sensors may be positioned in and around electroplating cell 202 to monitor conditions and to provide indications of nonuniformity that may be indicative of metal salt precipitate accumulation. For example, electroplating cell 202 may include a cathode current sensor array 242 and/or an optical sensor 246. Cathode current sensor array 242 may monitor electrical current at different locations at substrate 216. Optical sensor 246 may be positioned to observe a surface of cation exchange membrane 208 to provide visual monitoring of metal salt precipitate accumulation. Optical sensor 246 may comprise a photodiode, a camera, and/or any other suitable optical sensing device. Additional optical sensors, and/or other sensors, other than those shown may be positioned in and around electroplating cell 202.

[0075] FIGS. 3A and 3B schematically show example current flow within electroplating cell 202. FIG. 3 A shows current flow without metal salt precipitates on cation exchange membrane 208. FIG. 3B shows current flow with metal salt precipitates on cation exchange membrane 208. Referring first to FIG. 3 A, ionic current 305 from anode 212 is uniform across anode 212, through cation exchange membrane 208, and through HRVA 209. This yields uniform current 310 and deposition at substrate 216.

[0076] Referring next to FIG. 3B, cation exchange membrane 208 is partially passivated by metal salt precipitates 325. Regions 322 of cation exchange membrane 208 with the higher degree of buildup of metal salt precipitates 325 correspond with portions 327 of the substrate that experience slower metal deposition rates compared to portions 330. [0077] Returning to FIG. 2, metal salt precipitates may deposit at other locations within catholyte circulation loop 220 and/or anolyte circulation loop 230. For example, electroplating tools may utilize a water management and removal system in one or more of catholyte circulation loop 220 or anolyte circulation loop 230 for removing excess water. As the solvent is being removed from the baths, the ionic concentration is effectively increased. This can lead to metal salt precipitates in the water management and removal system. Precipitates also may occur at other locations near high evaporation areas.

[0078] In some situations, metal salt precipitates may develop in anolyte circulation loop 230. For example, some electroplating processes may utilize a similarly high concentration of copper in anolyte bath 210 as in a catholyte bath 214. Where low circulation conditions exist in an anode chamber, metal salt precipitates may form. Metal salt precipitates in the anode chamber may result in similar performance issues as cation exchange membrane passivation. For example, when metal salt precipitates form on regions of the anode, copper oxidation may be blocked in those regions. This may cause nonuniformity in the ionic current between anode 212 and substrate 216.

[0079] Metal salt precipitates may also form between parts of electroplating cell 202. For example, metal salt precipitates may occur on seals between HRVA 209 and the membrane frame as (as shown at 340). If these seals are not cleaned thoroughly, the deposits can result in current leakage.

[0080] As mentioned above, manual intervention to clean metal salt precipitates from an electroplating tool may be time-consuming and expensive. Thus, FIG. 4 shows a flow diagram depicting an example method 400 for operating an electroplating tool in a manner that may remove metal salt precipitates without manual intervention. Method 400 may be controlled by a computing system of an electroplating tool, such as computing system 130. For example, method 400 may be encoded as instructions stored on a storage device of the computing system that are executable by a logic device of the computing system. Method 400 also may be controlled manually by an operator of an electroplating tool.

[0081] At 410, method 400 comprises, in a processing phase of operation, flowing a metal salt solution through a circulating loop comprising a cathode chamber during an electroplating process. The metal salt solution is flowed through the circulating loop at a process temperature. The metal salt solution comprises at least a metal cation and a counter anion. The process temperature may be predetermined based on conditions of the processing phase.

[0082] In some examples, the deposited metal comprises copper and the metal cation comprises Cu²⁺. In such examples, example process temperatures for the metal salt solution may comprise temperatures within a range of 22-26 °C. In other examples, the deposited metal may comprise tin, silver, indium, cobalt, nickel, or other conductive metal. In such examples, the metal cation may be a soluble cation of the respective metal. In some examples the deposited metal may be an alloy or solder material comprising two or more metals, such as tin-silver alloy. In such examples, the metal salt solution may comprise two or more metal cations, such as Sn²⁺ and Ag⁺. Any appropriate counter anion or combination of counter anions for the selected metal cation or cations may be used. Examples include SCU²', Cl", hydroxide, sulfonate, etc. In some examples, the metal salt solution comprises a catholyte. In other examples, the metal salt solution may additionally or alternatively comprise an anolyte.

[0083] In some examples, visual inspection of the electroplating tool and its components may be performed periodically to detect metal salt precipitates. Likewise, substrate analyses may be performed to sense metal film properties (e.g. nonuniform growth) that may indicate a presence of unsuitable amounts of metal salt precipitates.

[0084] Optionally, in other examples, automated sensing may be used to sense conditions that may indicate the accumulation of metal salt precipitates. As such, method 400 may comprise, at 420, receiving an indication of accumulated metal salt precipitates within the circulating loop, and initiating the precipitate removal phase of operation in response to the received indication. For example, an indication of passivation of the cation exchange membrane may be received. As one example, current sensors configured to sense cathode currents (e.g., cathode current sensor array 242) may sense non-uniform current flow at different locations on a substrate. As another example, optical sensing (e.g., optical sensor 246) may be used to detect precipitates on structures prone to precipitate accumulation. As a further example, a camera may be used to image an electroplating cell, and a trained machine learning classifier may be used to detect a presence of precipitates in image data from the camera. Such a machine learning function may be trained via a supervised learning process using labeled image data that show a range of precipitate conditions. Example machine learning functions include suitable neural networks, such as residual neural networks. Such a machine learning function may be trained using backpropagation and a suitable cost function. Example cost functions include quadratic cost functions, crossentropy cost functions, and exponential cost functions, among others.

[0085] As another example, charge sensing may be used to detect an accumulated amount of electrochemical metal reduction performed by an electroplating tool since a last precipitate removal phase of operation. Such charge sensing may be performed using a current sensor that monitors a cathode electrical current. In some examples, an accumulated coulombs total exceeding a threshold value may be used as a trigger to perform a precipitate removal process.

[0086] In other examples, when process conditions are detected that may indicate the presence of unsuitable amounts of precipitates, the accumulated coulombs total may be used to determine an action to take. As one example, at 430, method 400 optionally comprises monitoring a total amount of coulombs of electroplating performed by the electroplating tool following a previous precipitate removal phase of operation. At 432, process conditions are detected that indicate possible precipitate risk, as described above. In response, at 434, method 400 determines whether the accumulated coulombs total is less than or meets a threshold. When the accumulated coulombs is less than the threshold, then a precipitate removal phase of operation may be performed at a next idle period, as indicated at 436. On the other hand, at 438, when the accumulated coulombs meets the threshold, the precipitate removal phase of operation may be initiated before a next idle period (e.g. upon completion of any substrates currently awaiting processing at the tool). After performing the precipitate removal phase, the accumulated coulombs total may be reset.

[0087] An accumulated coulombs threshold may be predetermined, or may vary based on operating conditions. For example, the accumulated coulombs threshold may be automatically varied based on changes in conditions such as metal salt solution compositions and concentrations, metal salt solution flow rates, plating gap distance, and/or electroplating current, over time. In some examples, a user interface may be configured to alert a user as to when a precipitate removal phase of operation will be performed (e.g. at a next idle period, or at a specified point before a next idle period, such as when all wafers of a current front-opening unified pod (FOUP) or other substrate carrier have been processed).

[0088] Continuing with FIG. 2, at 430, method 400 comprises, in a precipitate removal phase of operation, heating the metal salt solution to a temperature higher than the process temperature. The precipitate removal phase of operation further comprises flowing the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates of the metal salt solution within the circulating loop. The precipitate removal phase of operation also comprises, after flowing the metal salt solution through the circulating loop for a duration, cooling the metal salt solution to the process temperature. As an example, if the process temperature is 25 °C, the temperature of the metal salt solution may be raised to a temperature within a range of 34 °C to 37 °C during the precipitate removal phase. The temperature of the metal salt solution then may be cooled back to the process temperature after the precipitate removal phase. The redissolved metal salt precipitates may have only a modest impact on the total amount of metal cations and counter ions in the metal salt solution when redissolved. Thus the metal salt solution concentration may not need adjustment following the precipitate removal phase.

[0089] In some examples, other operating conditions may be adjusted during the precipitate removal phase. For example, a concentration of metal cation may be reduced by diluting the metal salt solution temporarily. The concentration then may be returned to a concentration used for electroplating onto substrates. Metal salt solution flow rates also may be adjusted.

[0090] A temperature and time duration for precipitate removal may be determined based on various factors. Example factors include process temperatures, metal salt solution flow rates, metal salt solution concentrations and/or compositions, electrical currents used for electroplating, measured nonuniformities in cathode currents at different substrate locations, and/or a length of time since a previous precipitate removal phase operation.

[0091] FIG. 5 shows a plot 500 illustrating an example metal salt solution temperature over time for an example implementation of method 400. At time to, the electroplating tool is operating in a processing phase of operation, and metal salt solution is being circulated through a circulating loop at a process temperature (Tl). At time ti, an indication of metal salt precipitates of the metal salt solution within the circulating loop is received. At time t2, a precipitate removal phase is initiated, and the temperature of the metal salt solution is raised until it reaches a selected temperature higher than the process temperature (T2) at time t3.

[0092] The metal salt solution is then circulated through the circulating loop at the increased temperature for a duration (e.g., from time t3 to time to). This allows precipitates to redissolve. This also allows passive irrigation/convection to remove any resulting concentrated solution from the membrane or other locations of buildup. For example, the flow of catholyte across the cathode chamber may create a solution exchange across the HRVA, causing catholyte to flow through the HRVA into the volume between the HRVA and the ion exchange membrane. The flow of catholyte through the HRVA also causes convection of the catholyte within the volume between the HRVA and the cation exchange membrane. The duration may be on the order of 2- 4 hours in some examples. In other examples, the duration may be shorter or longer, depending on operating conditions.

[0093] At time t4, the precipitate removal phase ends, and the temperature of the metal salt solution is reduced to process temperature Tl. Some temperature oscillation may occur around Tl as the solution temperature stabilizes. At time ts, the metal salt solution temperature has stabilized, and the electroplating tool may be returned to a processing phase.

[0094] FIG. 6 schematically shows example states of the electroplating tool of FIG. 2 over time while performing an example implementation of the method of FIG. 4. At 600, electroplating cell 202 is operating in a processing phase of operation, and catholyte is being circulated through circulating loop 220 at a process temperature of 25 °C, passively irrigating HRVA 209, as shown at 602. As shown, cation exchange membrane 208 is passivated with metal salt precipitates 604, and an impact is observed at substrate 216.

[0095] At 610, electroplating cell 202 moves to the precipitate removal phase of operation. The catholyte is heated to an elevated temperature (e.g. 35 °C) via heater 226. As the temperature is increased, dissolution of metal salt precipitates progresses, as shown at 612. The solid precipitates readily dissolve under these conditions. The resulting concentrated metal salt solution is removed by passive irrigation (e.g., 70 liters per minute catholyte flow rate). At 620, the elevated temperature is maintained as all metal salt precipitates are dissolved and irrigated from the cathode chamber 206. At 630, the plating chemistry is returned to the initial temperature to resume electroplating operation.

[0096] The solubility limit of CuSC in aqueous solutions increases exponentially with temperature. Under conditions where crystals have formed (e.g. low temperature plating solutions), the concentration of dissolved copper sulfate is saturated at the cation exchange membrane interface. By temporarily raising the temperature of the plating chemistry, even by a relatively small amount (e.g. 10-12 °C), the capacity of the solution to dissolve additional CuSC increases significantly.

[0097] Thus, the disclosed examples may allow metal salt precipitates to be removed from within a catholyte circulation loop and/or an anolyte circulation loop of an electroplating tool without manual intervention. The disclosed examples further may help to restore a passivated cation exchange membrane. This may help to reduce a frequency at which the cation exchange membrane is replaced. Thus, precipitates may be removed with less tool downtime and expense than manual intervention methods.

[0098] In some embodiments, the methods and processes described herein may be tied to a computing system of one or more computing devices. In particular, such methods and processes may be implemented as a computer-application program or service, an application-programming interface (API), a library, and/or other computerprogram product.

[0099] FIG. 7 schematically shows a non-limiting embodiment of a computing system 700 that can enact one or more of the methods and processes described above. Computing system 700 is shown in simplified form. Computing system 700 may take the form of one or more personal computers, workstations, computers integrated with wafer processing tools, and/or network accessible server computers.

[00100] Computing system 700 comprises a logic machine 710 and a storage machine 720. Computing system 700 may optionally comprise a display subsystem 730, input subsystem 740, communication subsystem 750, and/or other components not shown in FIG. 7. Computing system 130 is an example of computing system 700.

[00101] Logic machine 710 comprises one or more physical devices configured to execute instructions. For example, the logic machine may be configured to execute instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs. Such instructions may be implemented to perform a task, implement a data type, transform the state of one or more components, achieve a technical effect, or otherwise arrive at a desired result.

[00102] The logic machine may comprise one or more processors configured to execute software instructions. Additionally or alternatively, the logic machine may comprise one or more hardware or firmware logic machines configured to execute hardware or firmware instructions. Processors of the logic machine may be single-core or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. Individual components of the logic machine optionally may be distributed among two or more separate devices, which may be remotely located and/or configured for coordinated processing. Aspects of the logic machine may be virtualized and executed by remotely accessible, networked computing devices configured in a cloud-computing configuration.

[00103] Storage machine 720 comprises one or more physical devices configured to hold instructions 755 executable by the logic machine to implement the methods and processes described herein. When such methods and processes are implemented, the state of storage machine 720 may be transformed — e.g., to hold different data.

[00104] Storage machine 720 may comprise removable and/or built-in devices. Storage machine 720 may comprise optical memory (e.g., CD, DVD, HD-DVD, Blu- Ray Disc, etc.), semiconductor memory (e.g., RAM, EPROM, EEPROM, etc.), and/or magnetic memory (e.g., hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), among others. Storage machine 720 may comprise volatile, nonvolatile, dynamic, static, read/write, read-only, random-access, sequential-access, location-addressable, file-addressable, and/or content-addressable devices.

[00105] It will be appreciated that storage machine 720 comprises one or more physical devices. However, aspects of the instructions described herein alternatively may be propagated by a communication medium (e.g., an electromagnetic signal, an optical signal, etc.) that is not held by a physical device for a finite duration.

[00106] Aspects of logic machine 710 and storage machine 720 may be integrated together into one or more hardware-logic components. Such hardware-logic components may comprise field-programmable gate arrays (FPGAs), program- and application-specific integrated circuits (PASIC / ASICs), program- and applicationspecific standard products (PSSP / ASSPs), system-on-a-chip (SOC), and complex programmable logic devices (CPLDs), for example.

[00107] When included, display subsystem 730 may be used to present a visual representation of data held by storage machine 720. This visual representation may take the form of a graphical user interface (GUI). As the herein described methods and processes change the data held by the storage machine, and thus transform the state of the storage machine, the state of display subsystem 730 may likewise be transformed to visually represent changes in the underlying data. Display subsystem 730 may comprise one or more display devices utilizing virtually any type of technology. Such display devices may be combined with logic machine 710 and/or storage machine 720 in a shared enclosure, or such display devices may be peripheral display devices.

[00108] When included, input subsystem 740 may comprise or interface with one or more user-input devices such as a keyboard, mouse, or touch screen. In some embodiments, the input subsystem may comprise or interface with selected natural user input (NUI) componentry. Such componentry may be integrated or peripheral, and the transduction and/or processing of input actions may be handled on- or off- board. Example NUI componentry may comprise a microphone for speech and/or voice recognition, and an infrared, color, stereoscopic, and/or depth camera for machine vision and/or gesture recognition.

[00109] When included, communication subsystem 750 may be configured to communicatively couple computing system 700 with one or more other computing devices. Communication subsystem 750 may comprise wired and/or wireless communication devices compatible with one or more different communication protocols. As non-limiting examples, the communication subsystem may be configured for communication via a wireless telephone network, or a wired or wireless local- or wide-area network. In some embodiments, the communication subsystem may allow communication system 750 to send and/or receive messages to and/or from other devices via a network such as the Internet.

[00110] It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and/or described may be performed in the sequence illustrated and/or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed.

[00111] The subject matter of the present disclosure includes all novel and non- obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.

Claims

CLAIMS:

1. A method for operating an electroplating tool, the method comprising: in a processing phase of operation, flowing a metal salt solution through a circulating loop at a process temperature to deposit a metal on a substrate, the metal salt solution comprising at least a metal cation and a counter anion; and in a precipitate removal phase of operation, heating the metal salt solution to a temperature higher than the process temperature, flowing the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates of the metal salt solution within the circulating loop, and cooling the metal salt solution to the process temperature.

2. The method of claim 1, further comprising receiving an indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop, and initiating the precipitate removal phase of operation in response to the received indication.

3. The method of claim 2, wherein the indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop comprises an indication of passivation of a cation exchange membrane.

4. The method of claim 2, wherein the indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop comprises an observed non-uniform current at a cathode.

5. The method of claim 2, wherein initiating the precipitate removal phase of operation further comprises monitoring a total amount of coulombs of electroplating performed by the electroplating tool following a previous precipitate removal phase of operation.

6. The method of claim 5, further comprising, responsive to the total amount of accumulated coulombs being below a threshold when the accumulation of metal salt precipitates is indicated, initiating the precipitate removal phase of operation at a next idle period.

7. The method of claim 6, further comprising, responsive to the total amount of accumulated coulombs being above the threshold when the accumulation of metal salt precipitates is indicated, initiating the precipitate removal phase of operation before the next idle period.

8. The method of claim 2, wherein the metal salt solution comprises a catholyte.

9. The method of claim 1, wherein the metal cation comprises Cu²⁺, and wherein the counter anion comprises SC ²'.

10. An electroplating tool, comprising: a substrate; a circulating loop for a metal salt solution, the circulating loop comprising a heater; a logic machine; and a storage machine storing instructions executable by the logic machine to: in a process phase of operation, control a flow of the metal salt solution through the circulating loop at a process temperature to deposit a metal on the substrate; and in a precipitate removal phase of operation, control the heater to heat the metal salt solution to a temperature higher than the process temperature, control the flow of the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates within the circulating loop, and control the heater to cool the metal salt solution to the process temperature.

11. The electroplating tool of claim 10, wherein the circulating loop comprises a cathode chamber.

12. The electroplating tool of claim 11, wherein the cathode chamber is separated from an anode chamber by a cation exchange membrane.

13. The electroplating tool of claim 12, wherein the anode chamber comprises a consumable anode.

14. The electroplating tool of claim 13, wherein the consumable anode comprises copper metal.

15. The electroplating tool of claim 14, wherein the metal salt solution comprises at least Cu²⁺ and SC ²'.

16. The electroplating tool of claim 10, wherein the precipitate removal phase of operation is initiated in response to an indication of metal salt precipitates accumulated within the circulating loop.

17. A storage machine storing instructions executable by a logic machine to: at an electroplating tool, in a processing phase of operation, control a flow of a metal salt solution through a circulating loop at a process temperature to deposit a metal on a substrate; and in a precipitate removal phase of operation, control a heater heat the metal salt solution to a temperature higher than the process temperature, control the flow of the metal salt solution through the circulating loop for a duration to dissolve metal salt precipitates of the metal salt solution within the circulating loop, and control the heater to cool the metal salt solution to the process temperature.

18. The storage machine of claim 17, further storing instructions executable by the logic machine to: receive an indication of accumulated metal salt precipitates of the metal salt solution within the circulating loop and initiate the precipitate removal phase of operation in response to the received indication.

19. The storage machine of claim 18, further storing instructions executable by the logic machine to: when a total amount of accumulated coulombs is below a threshold when the accumulation of metal salt precipitates is indicated, initiate the precipitate removal phase of operation at a next idle period.

20. The storage machine of claim 19, further storing instructions executable by the logic machine to: when the total amount of accumulated coulombs is above the threshold when the accumulation of metal salt precipitates is indicated, initiate the precipitate removal phase of operation before the next idle period.