1. Field of the Invention
The present invention relates to an electroless plating apparatus and an electroless plating method. More particularly, the invention relates to an electroless plating apparatus and an electroless plating method for use in selectively forming a protective film of a magnetic material on exposed surfaces of embedded interconnects that are produced by embedding an interconnect material (conductive material) such as copper, silver, or the like in interconnect recesses that are provided in a surface of a substrate such as a semiconductor wafer or the like.
2. Description of the Related Art
In efforts to produce high-speed large-scale integrated (LSI) circuits in recent years, interconnects (copper interconnects) made of copper rather than an aluminum alloy have begun to be used in the art. Copper interconnects are generally produced by the so-called damascene method by forming interconnect recesses such via holes, trenches, or the like in an insulating film (interlevel dielectric film) on a substrate, depositing a thin barrier layer of tantalum or tantalum nitride (TaN) on an entire surface of the substrate including the interconnect recesses for the purposes of preventing copper from being diffused and improving the adhesiveness of copper, thereafter depositing a copper film on the barrier layer such that the copper film is embedded in interconnect recesses, and removing the copper film and the barrier layer except those buried in interconnect recesses by chemical mechanical polishing (CMP).
On a polished surface of the substrate, surfaces of interconnects (copper interconnects) made of copper embedded in the insulating film are directly exposed. If a multilevel interconnect of copper is to be formed, then another insulating film needs to be formed on exposed surfaces of the copper interconnects. Silicon oxide (SiO2), which is generally used as an insulating film material, and many other materials are of poor adhesive power with respect to copper, and allow copper to be diffused quickly therein. Therefore, materials including SiO2 are not generally used to form an insulating film covering exposed interconnect surfaces.
At present, only silicon nitride (SiN) and silicon carbide (SiC) are available as insulating film materials which have adhesive power with respect to copper interconnects exposed on the surface of the substrate and which are capable of preventing copper from being diffused therein. However, even these materials are not sufficiently capable of preventing copper from being diffused therein and do not have sufficient adhesive power with respect to copper. In addition, since these materials have a high dielectric constant, they tend to increase electrostatic capacitance between copper interconnects and hence to present an obstacle to attempts to reduce a delay of interconnect signals.
Recently, the use of a material of low dielectric constant, i.e., a low-K material, for producing an insulating film (interlevel dielectric film) in which to form interconnects, has been examined for reducing electrostatic capacitance between copper interconnects. Such a material of low dielectric constant generally has a low density and allows copper to be diffused at a greater rate than SiO2. Therefore, a multilevel copper interconnect that is formed in an interlevel dielectric film made of a material of low dielectric constant is liable to suffer low long-term reliability.
Specifically, according to a conventional process, the exposed surfaces of copper interconnects that are formed in an insulating film (interlevel dielectric film) are covered with an insulating film that is made of a silicon compound or the like. The insulating film of a silicon compound or the like is responsible as a limitation on improved interconnects characteristics and makes it difficult to keep interconnect reliable for a long period of time.
One solution to the above problems is to selectively cover the exposed surfaces of copper interconnects with a protective film (cap material) made of an alloy of cobalt and tungsten (CoW alloy) or the like. The alloy of cobalt and tungsten (CoW alloy) or the like is produced by electroless plating, for example.
For example, as shown in FIG. 1, fine interconnect recesses (trenches) 4 are formed in an insulating film (interlevel dielectric film) 2 made of SiO2, low-K material or the like, which has been deposited on a surface of a substrate W such as a semiconductor wafer. A barrier layer 6 of TaN or the like is formed on a surface of the insulating film 2, and then, for example, copper plating is carried out to deposit a copper film on the surface of the substrate W so as to embed copper in the interconnect recesses 4. Thereafter, CMP (chemical mechanical polishing) is carried out on the surface of the substrate W to planarize the surface of the substrate W, thereby forming interconnects 8 made of copper in the insulating film 2. A protective film (cap material) 9 of a CoWP alloy, which is obtained, for example, by electroless plating, is formed selectively on surfaces of the interconnects (copper) 8 so as to protect the interconnects 8.
There will be described a process of forming a protective film (cap material) 9 of a CoWP alloy selectively on surfaces of interconnects 8 by using a general electroless plating method. First, the substrate W after a CMP process is immersed, for example, in dilute sulfuric acid having an ordinary temperature for about one minute to remove an oxide film on surfaces of interconnects 8, CMP residues, such as copper, remaining on a surface of an insulating film 2 and the like. After the surface of the substrate W is cleaned (rinsed) with a cleaning liquid such as pure water, the substrate W is immersed, for example, in a PdCl2/HCl mixed solution having an ordinary temperature for about one minute to adhere Pd as a catalyst to the surfaces of the interconnects 8 so as to activate exposed surfaces of the interconnects 8.
After the surface of the substrate W is cleaned (rinsed) with pure water or the like, the substrate W is immersed, for example, in a CoWP plating solution at 80° C. for about 120 seconds to carry out electroless plating (electroless CoWP plating) selectively on surfaces of the activated interconnects 8. Thereafter, the surface of the substrate W is cleaned with a cleaning liquid such as pure water. Thus, a protective film 9 made of a CoWP alloy is formed selectively on the exposed surfaces of interconnects 8 so as to protect the interconnects 8.
When the protective film 9 is selectively formed on the exposed surfaces of the interconnects 8, as shown in FIG. 1, abnormal deposits 10, each having a size of several tens nm, are formed on the surface of the insulating film 2 other than the interconnects 8 due to metal particles and foreign matter attached to the surface of the insulating film 2 by the CMP process and trapped into the electroless plating solution, and catalytic metal released into the electroless plating solution. The abnormal deposits 10 may be removed by post-CMP cleaning, pre-electroless plating cleaning, or modifying the insulating film. Improving the selectivity by post-CMP cleaning or pre-electroless plating cleaning depends on a chemical process using a chemical solution of acid or alkali. However, the chemical process is not sufficiently effective to remove contaminants of those types that are not predicted by the chemical solution. Therefore, it is difficult to prevent contaminants that have not been removed by the chemical solution from being trapped into the electroless plating solution, and attached to the insulating film to produce abnormal deposits on the insulating film.
When abnormal deposits are produced on the insulating film other than the interconnects, the ability of the protective film covering the surfaces of the interconnects to prevent copper from being diffused is lowered, and the insulating film positioned between the interconnects is unable to provide a highly reliable insulation between the interconnects. Furthermore, the contaminants, which have not been removed by the chemical solution and which have been trapped into the electroless plating solution, and the catalytic metal released into the electroless plating solution change the properties of the electroless plating solution, tending to make plating reactions unstable.