In the fabrication of semiconductor devices such as integrated circuits, memory cells, and the like, a series of manufacturing operations are performed to define features on semiconductor wafers (“wafers”). The wafers (or substrates) include integrated circuit devices in the form of multi-level structures defined on a silicon substrate. At a substrate level, transistor devices with diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define a desired integrated circuit device. Also, patterned conductive layers are insulated from other conductive layers by dielectric materials.
At present, ruthenium (Ru) is gaining attention as a base for fine copper wiring in semiconductors and as a material for use in dynamic random access memory (DRAM) capacitor electrodes. A sub-30 nanometer circuit line width has been achieved in the progressively miniaturized semiconductor market, and it is hoped that mass production of sub-20-nanometer next-generation semiconductors and eventually sub-10-nanometer mass production will be realized.
An aspect for realizing fine wiring utilizing sub-10-nanometer processes is improvement in the embedding of copper plating. One method for improving the embedding of copper platting entails deposition of a thin layer of ruthenium as a base for copper plating. Ruthenium is suitable as a base for copper due to its low resistance and excellent compatibility with copper. Various deposition technologies such as chemical vapor deposition CVD), atomic layer deposition (ALD), and electroless deposition, utilizing a variety of ruthenium precursors, may be employed to deposit ruthenium. However, challenges remain in implementing ruthenium deposition on a mass production scale.
It is in this context that embodiments of the invention arise.
Broadly speaking, the present invention fills these needs by providing a method for pretreatment of a ruthenium-containing liner/barrier, prior to metal deposition. Several inventive embodiments of the present invention are described below.
In one embodiment, a wet pretreatment method for preparing a ruthenium surface for metal deposition is provided. The method initiates with receiving a borohydride solution having a pH greater than about 12, and receiving deionized (DI) water. The borohydride solution is mixed with the DI water to form a pretreatment solution. The pretreatment solution is applied to the ruthenium surface.
In one embodiment, after applying the pretreatment solution, rinsing the ruthenium surface with DI water.
In one embodiment, the borohydride solution includes a base selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide, trimethylammonium hydroxide, triethylammonium hydroxide.
In one embodiment, the borohydride solution has a borohydride concentration approximately equal to a concentration of the base.
In one embodiment, the borohydride solution has a borohydride concentration of about 0.5 to 2.5 molar (M).
In one embodiment, the pretreatment solution has a borohydride concentration of about 50 to about 2500 millimolar (mM).
In one embodiment, the DI water is degassed DI water having an oxygen concentration of less than about 5 ppb.
In one embodiment, the method is used to perform at least one operation in the fabrication of an integrated circuit.
In another embodiment, a wet pretreatment method for preparing a ruthenium surface for metal deposition is provided. The method includes applying a stream of DI water onto a ruthenium surface. A borohydride solution is mixed into the stream of DI water, the borohydride solution having a pH greater than about 12 prior to the mixing. After a predefined period of time, the mixing of the borohydride solution into the stream of DI water is halted.
In one embodiment, the mixing of the borohydrde solution into the stream of DI water defines a pretreatment operation, and the halting of the mixing of the borohydride solution into the stream of DI water defines initiation of a rinse operation.
In one embodiment, the borohydride solution includes a base selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide, trimethylammonium hydroxide, triethylammonium hydroxide.
In one embodiment, the borohydride solution has a borohydride concentration approximately equal to a concentration of the base.
In one embodiment, the borohydride solution has a borohydride concentration of about 0.5 to 2.5 M.
In one embodiment, the mixing of the borohydride solution into the DI water stream defines a pretreatment solution having a borohydride concentration of about 50 to about 2500 mM.
In one embodiment, the DI water is degassed DI water having an oxygen concentration of less than about 5 ppb.
In one embodiment, after the halting of the mixing of the borohydride solution into the DI water stream, an electroless copper deposition solution is mixed into the DI water stream.
In one embodiment, the method is used to perform at least one operation in the fabrication of an integrated circuit.
In another embodiment, a system for preparing a ruthenium surface of a wafer is provided. The system includes a chamber configured to support the wafer. A DI water source is provided. A conduit is provided for delivering a DI water stream from the DI water source to the chamber for application onto the ruthenium surface of the wafer. A borohydride solution source contains a borohydride solution having a pH greater than 12. A mixer is provided for mixing the borohydride solution from the borohydride solution source into the DI water stream.
In one embodiment, the borohydride solution has a borohydride concentration of about 0.5 to 2.5 M.
In one embodiment, the mixing of the borohydride solution into the DI water stream defines a pretreatment solution having a borohydride concentration of about 50 to about 2500 mM.
In one embodiment, a controller is configured to control the mixer to initiate the mixing of the borohydride solution into the DI water stream and terminate the mixing after a predefined time period has elapsed.
In one embodiment, an electroless copper deposition solution source is provided, and a second mixer is provided for mixing electroless copper deposition solution from the electroless copper deposition solution source into the DI water stream.
In one embodiment, the system is configured to perform at least one operation in the fabrication of an integrated circuit.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.
The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements.
Several embodiments for the prevention of particle adders when contacting a liquid meniscus over a substrate are now described. It will be obvious, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.
In some embodiments, the barrier layer 14 can include tantalum nitride (TaN), titanium nitride (TiN), or other barrier materials which exhibit adequate adhesion to both the dielectric and to ruthenium. In some embodiments, the total thickness of the barrier layer 14 is built up through repeated deposition operations to deposit a series of sub-layers that together form the entire barrier layer 14. In some embodiments, as the sub-layers of the barrier layer are deposited, ruthenium is gradually mixed with the barrier layer material (e.g. TaN or TiN) in increasing relative amounts. This produces a gradient of ruthenium in the barrier layer 14, such that little or no ruthenium is present near the interface with the dielectric 12, whereas higher concentrations of ruthenium are present in portions of the barrier layer 14 situated away from the interface with the dielectric.
As with the barrier layer 14, the ruthenium layer 16 deposited over the barrier layer 14 can be deposited through a series of repeated deposition operations. This builds up the thickness of the ruthenium layer 16 through deposition of sub-layers of ruthenium. The ruthenium layer 16 adheres to the barrier layer 14, which in turn adheres to the dielectric 12. In this manner, though ruthenium does not directly adhere well to the dielectric 12, ruthenium can be deposited over the barrier layer 14 which serves as an intermediary enabling adhesion to the dielectric 12.
After deposition of the ruthenium layer 16, the ruthenium surface may become oxidized upon air and humidity exposure, such that ruthenium oxide 18 is present on the exposed surface of the ruthenium layer 16. It is important to remove this ruthenium oxide, as it prevents deposition of copper over the ruthenium. Therefore, it is desirable to reduce the ruthenium oxide to ruthenium by the application of a reducing agent 20.
After reduction of the surface ruthenium oxides to ruthenium, a copper layer 20 is deposited over the ruthenium layer 16 by any of various methods, including wet electroless deposition as well as dry vapor deposition methods.
As can be seen, reduction of the ruthenium surface to eliminate oxidation is important to enable subsequent deposition of copper over the ruthenium. A dry pretreatment reduction may employ a forming gas anneal at temperatures in the range of 250-300 degrees Celsius for about three to five minutes. However, the high temperatures employed also necessitate a subsequent cool-down period, during which reoxidation may occur. The length of time required for such a reduction process from start to finish is therefore not only prohibitive as it may reduce throughput, but also adversely impacts the reduction efficiency due to the possibility of reoxidation.
Several possible wet reduction pretreatments utilizing common reducing agents are also fraught with issues that make them unsatisfactory for production processes. For example one possible reducing agent that can be applied to reduce the ruthenium surface in a wet process is dimethylamineborane (DMAB). However, byproducts of the reduction process employing DMAB can attach to the Ru surface. Such byproducts will weaken the interface between the ruthenium layer and subsequently deposited copper. Additionally, DMAB solutions exhibit a high degree of instability, tending to spontaneously evolve hydrogen, which presents challenges when scaling to a production level process. Solution instability results in a low effective shelf life for DMAB solutions, which consequently require more frequent changing or replenishment. This necessitates additional oversight and causes increased process tool downtime, which ultimately reduces throughput and increases the cost of using DMAB as a reducing agent.
Another example of a reducing agent that can be utilized in a wet pretreatment reduction process is ammonia borane. However, as with DMAB, ammonia borane solutions also tend to exhibit a high degree of instability that results in low shelf life. Again, this drives up the cost of using ammonia borane as a reducing agent for reducing ruthenium surfaces in a production environment. In sum, commonly applied wet pretreatments for reduction purposes are generally not amenable to transport due to hydrogen evolution that results in low shelf life.
In view of these problems with commonly applied reducing agents, a method for reducing ruthenium oxide present on a ruthenium surface is herein described that provides a stable solution and long chemical shelf life. Broadly speaking, the method utilizes borohydride as a reducing agent. A concentrated borohydride solution is prepared with pH adjusted to be greater than about 12. The resulting concentrated solution is stable, exhibiting long shelf life, and can be diluted at the point of use just prior to application on the surface of a substrate.
Borohydrides have been utilized in fuel cell manufacturing to generate hydrogen. However, borohydrides are unstable in water, as they evolve hydrogen over time. It has been found that borohydrides can be stabilized by configuring the solution to be alkaline In an article entitled “An Ultrasafe Hydrogen Generator: Aqueous, Alkaline Borohydride Solutions and Ru Catalyst,” published in Journal of Power Sources, Volume 85, Issue 2, February 2000, pages 186-189, (the disclosure of which is incorporated by reference herein), Amendola et al. describe an alkaline borohydride solution that produces hydrogen when in the presence of a metal catalyst. When hydrogen gas is no longer required, the metal catalyst is removed from the solution and the hydrogen generation effectively stops. Additionally, Amendola et al. observed zero order kinetics for NaBH4 hydrolysis at NaBH4 concentrations as low as 0.1%.
In an article entitled “Stability of Aqueous-Alkaline Sodium Borohydride Formulations,” published in the Russian Journal of Applied Chemistry, Vol. 81, No. 3, 2000, pages 380-385, (the disclosure of which is incorporated by reference herein) Minkina et al. explore the stability of sodium borohydride in concentrated solutions, suspensions, and solids, including the effects of temperature, concentrations of sodium borohydride and alkali, and the nature of the alkali metal cation on the rate of sodium borohydride hydrolysis. Minkina et al. observed in systems containing sodium borohydride, alkali, and water, a rate of hydrolysis not exceeding 0.02% NaBH4 per hour at temperatures of up to 30 degrees Celsius, with increases in temperature significantly accelerating the rate of hydrolysis. Minkina et al. further state that for storage at temperatures above 30 degrees Celsius, it is necessary to add alkali in a concentration higher than 5 wt %.
As has been shown, borohydride solutions can be stabilized when adjusted to alkaline pH. In accordance with embodiments of the invention, this aspect of borohydride solutions can be leveraged to enable production-level semiconductor reduction processes. In one embodiment, a concentrated pretreatment solution includes about 0.5 to about 2.5 molar (M) borohydride in solution. The source of the borohydride can be any of various borohydride salts, including but not limited to, sodium borohydride, potassium borohydride, magnesium borohydride, calcium borohydride, lithium borohydride, tetramethylammonium borohydride, tetrabutylammonium borohydride, ammonium borohydride, etc. The pH of the concentrated pretreatment solution is adjusted to be greater than about 12 through the addition of an alkaline pH adjuster. The pH adjuster can be any of various bases, including sodium hydroxide (NaOH), potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), ammonium hydroxide (NH4OH), etc. In embodiments of the invention, the molar concentration of hydroxide is approximately equivalent to the molar concentration of borohydride. By utilizing equivalent molar amounts, metallic precipitates are generally avoided.
Prior to application onto a ruthenium surface of a substrate, the concentrated pretreatment solution is diluted with deoxygenated DI water to form a working pretreatment solution having a borohydride concentration of about 50 to about 2500 millimolar (mM). In another embodiment, the working pretreatment solution has a borohydride concentration of about 20 to about 2500 mM. In one embodiment, the working pretreatment solution has a borohydride concentration of approximately 80 to about 200 mM. While specific ranges have been provided, it should be appreciated that these are provided by way of example only, and that in various embodiments, the borohydride concentration may have any subrange defined therein. As noted above, zero order reaction kinetics are observed with respect to borohydride down to very low concentrations. Thus, only a relatively small concentration of borohydride is required for purposes of effective ruthenium surface reduction. The pretreatment solution can therefore be kept in concentrated form, stabilized by the high pH, until it is required for use, at which time the concentrated solution can be diluted at the point of use with deoxygenated DI water and applied to the substrate's ruthenium surface.
This configuration effectively addresses many of the problems inherent to the use of unstable reducing agents such as borohydrides in a manufacturing process. In large part, unstable reducing agents are difficult to utilize in production processes due to their short shelf life and sensitivity to factors such as temperature. These characteristics mean that reducing agents must be manufactured, shipped, handled and used at their destination under stringent control parameters and all within a limited time frame. This creates difficulties in terms of supply logistics, as manufacturing processes are highly dependent on frequent and precisely timed shipments of reducing agents. Flexibility in terms of production capacity is thereby reduced. Throughput is also adversely affected due to the frequent need to take tools offline to replace supplies of the reducing agents.
However, in accordance with embodiments of the invention described herein, a stable concentrated solution of borohydride is provided as a reducing agent source for a pretreatment operation for a ruthenium surface. The stable concentrated solution of borohydride can be shipped under a variety of conditions and exhibits a long shelf life that makes it better-suited to production manufacturing processes. The longer shelf life of the stabilized concentrated solution means that it can be used for a longer period of time before needing replacement. The result is increased throughput as the tool incurs less downtime from replacement of the concentrated solution.
A mixer 52 is provided for mixing various solutions with a DI water stream provided by a DI water source 64. The DI water and any solutions which have been mixed therewith are flowed into the chamber 40 and dispensed from a dispense head 46 onto the surface of the substrate 44. A heater 62 can be applied to heat the DI water to a predefined temperature. It will be appreciated that any of various types of solutions can be provided for use with the presently described wet process system. By way of example, in the illustrated embodiment, a concentrated reducing agent solution 54 is provided for pretreatment reduction of the substrate 44 prior to metal deposition. The concentrated reducing agent solution 54 can define a concentrated pretreatment solution that is diluted via mixing with the DI water stream to define a working pretreatment solution that is applied to reduce a ruthenium containing surface of a substrate.
In the illustrated embodiment, a copper deposition solution 56 and a cobalt deposition solution 58 are also shown. A solution 60 can be any of various other solutions useful for wet processing of a substrate. It will be appreciated that in various embodiments, any number of solutions may be configured to operate with the presently described wet processing system.
In one embodiment, the mixer 52 includes various valves 53A, 53B, 53C, and 53D for controlling the flow of the various solutions 54, 56, 58, and 60 into the DI water stream. For example, when all valves are closed, the DI water stream is supplied to the chamber 40 and flowed onto the substrate 44 without additives, acting as a DI water rinse. When, for example, the valve 53A is opened, then the concentrated pretreatment solution 54 is mixed with the DI water stream to define a working pretreatment solution that is then applied to the substrate 44. When the valve 53A is closed, then the DI water stream continues to flow without the addition of other solutions and is applied to the substrate, again acting as a DI water rinse. In a similar manner, when any of the valves 53B, 53C, or 53D is opened, its corresponding solutions is mixed with the DI water stream, effectively being diluted via the mixing to a working concentration level, with the working solution then being applied to the substrate 44. When the valve is closed, the flow of the concentrated solution is stopped, effectively returning the applied solution to a pure DI water state that then acts to rinse the substrate surface. Thus, the opening and closing of the valves of the mixer 52 can be controlled to define various process operations, by controlling periods of DI water application and working solution application onto the substrate surface.
It will be appreciated that the operation of the various components of the illustrated system can be controlled by one or more programmable controllers, which may be configured to enable execution of a sequence of processing operations utilizing the aforementioned system components, in accordance with principles of the invention as described herein.
At time 76, a plating initiation solution is mixed with the DI water stream, thereby defining an initiation step as the substrate surface is exposed to the mixed initiation solution and DI water. When the flow of the plating initiation solution is stopped, then at time 78, a DI water rinse step is effected. At time 80, a copper plating solution is mixed with the DI water stream so as to effect plating of copper onto the substrate surface, thus defining a copper plating step. At time 82, the flow of the copper plating solution has been stopped, resulting in a subsequent DI water rinse step.
The foregoing embodiment includes the introduction of an initiation step and subsequent DI water rinse. However, it should be noted that in some embodiments, these steps are not included. In such embodiments, the reduction step is followed by a DI water rinse and then copper plating.
In the presently described embodiment, the dilution of the concentrated pretreatment solution with DI water occurs at a fixed ratio for a specific duration over which the working concentration of borohydride is effective for achieving an acceptable reaction rate. In other words, the ratio of concentrated pretreatment solution to DI water remains constant, and the concentrated pretreatment solution is periodically replaced when its borohydride concentration falls to a level below which the working solution would no longer be suitably effective. In the foregoing embodiment, this level is the concentration C which occurs at approximately time N.
However, it will be appreciated that the borohydride concentration of the concentrated pretreatment solution does not fall to the concentration A until a later time P. Furthermore, because zero order kinetics are exhibited with respect to borohydride concentration above concentration A, there is little or no benefit to the rate of reaction at higher concentrations of borohydride in the working pretreatment solution. In view of these aspects, and in order to conserve the concentrated pretreatment solution and extend its usable lifetime, it may be desirable to vary its dilution ratio with respect to DI water.
Embodiments of the invention have generally been described with reference to pretreatment solutions having approximately equal molar amounts of borohydride and alkaline components. However, in other embodiments, the pretreatment solution can be configured to have different molar amounts of borohydride and alkaline components.
Embodiments of the invention have been described utilizing borohydride as a reducing agent. However, in other embodiments, boranes are utilized as reducing agents in a similar manner. For example, a concentrated pretreatment solution may have a borane concentration of approximately 0.75 to 1M borane, with a pH adjusted to be greater than about 12. The borane source can be DMAB, ammonia borane, etc., and the pH adjuster can be NaOH, KOH, TMAH, TEAH, NH4OH, etc.
While this invention has been described in terms of several preferred embodiments, it will be appreciated that those skilled in the art upon reading the preceding specifications and studying the drawings will realize various alterations, additions, permutations and equivalents thereof. It is therefore intended that the present invention includes all such alterations, additions, permutations, and equivalents as fall within the true spirit and scope of the invention.