1. Field of the Invention
The present invention relates to a plating apparatus and a plating method for a substrate, and more particularly to a plating apparatus and a plating method for a substrate for filling a metal such as copper (Cu) or the like in fine interconnection patterns (recesses) formed on a semiconductor substrate.
The present invention also relates to an electrolytic treatment method for applying electrolytic treatment, such as plating or etching, to the surface of a substrate to be treated, and an apparatus therefor.
The present invention further relates to an electrolytic treatment apparatus for applying, for example, plating or etching to the surface of a member to be treated, especially an electrolytic treatment apparatus and a method for controlling the state of its electric field.
2. Description of the Related Art
Aluminum or aluminum alloy has generally been used as a material for forming interconnection circuits on semiconductor substrates. As the integrated density increases, there is a demand for the usage of a material having a higher conductivity as an interconnection material. A method has been proposed to plate a substrate to fill an interconnection pattern formed thereon with copper or its alloy.
There are various processes known including CVD (chemical vapor deposition), sputtering, etc. to fill the interconnection pattern with copper or its alloy. However, if the material of the metal layers is copper or its alloy, i.e., for forming copper interconnects, the CVD process is costly, and the sputtering process fails to embed copper or its alloy in interconnection patterns having a high aspect ratio, i.e., a high ratio of depth to width. The plating process is most effective to deposit a metal layer of copper or its alloy.
Various processes are available for plating semiconductor substrates with copper. They include a process of immersing a substrate in a plating liquid held at all times in a plating tank, referred to as a cup-type or dipping-type process, a process of holding a plating liquid in a plating tank only when a substrate to be plated is supplied to the plating tank, an electrolytic plating process of plating a substrate with a potential difference, and an electroless plating process for plating a substrate with no potential difference.
Conventionally, a plating apparatus for performing this type of copper plating was equipped with a horizontal arrangement of a plurality of units, such as a unit for performing a pretreatment step incidental to plating, a unit for performing a cleaning/drying step after plating, and a unit for performing a plating step, and a transfer robot for transferring the substrate between these units. The substrate was subjected to a predetermined treatment in each unit while being transferred between the units, and was sequentially transported to a subsequent step after plating treatment.
In the conventional plating apparatus, however, separate units were provided for respective steps, such as plating treatment and pretreatment, and the substrate was transferred to the respective units and treated thereby. Thus, there were problems that the apparatus was considerably complicated and difficult to control, occupied a great area, and involved a considerably high manufacturing cost.
With electroplating, moreover, if air bubbles are present in a plating liquid filled between a surface to be plated of a substrate (cathode) and an anode, the air bubbles, as insulators, function as if they were anode masks. As a result, the film thickness of a plating formed at positions corresponding to these portions may decrease, or a complete lack of plating may occur. To obtain a uniform, high quality plated film, therefore, it is necessary to leave no air bubbles in the plating liquid between the surface to be plated of the substrate and the anode.
Furthermore, electrolytic treatment, especially electroplating, is widely used as a method for forming a metal film. In recent years, copper electroplating for multilayer interconnects of copper, and gold electroplating for bump formation, for example, have attracted attention because of their effectiveness (inexpensiveness, hole filling characteristics, etc.), and have found increased use, for instance, in the semiconductor industry.
FIG. 71 shows a conventional general constitution of a plating apparatus for applying electroplating onto the surface of a substrate to be treated (hereinafter referred to as a substrate), such as a semiconductor wafer, by the use of a so-called face-down method. This plating apparatus includes a cylindrical plating tank 602 opening upward and holding a plating liquid 600 therein and a substrate holder 604 for detachably holding a substrate W face-down and at such a position that the substrate W covers the top opening of the plating tank 602. Inside the plating tank 602, a flat sheet type anode plate 606, immersed in the plating liquid 600 to constitute an anodic electrode, is placed horizontally. On the other hand, a conductive layer S is formed on the lower surface (plating surface) of the substrate W, and this conductive layer S has, at its peripheral edge portion, contact with cathodic electrodes.
A plating liquid jet pipe 608 for forming an upwardly directed jet of the plating liquid is connected to the center of the bottom of the plating tank 602, and a plating liquid receiver 610 is placed on an upper external portion of the plating tank 602.
With the above structure, the substrate W held by the substrate holder 604 is placed facedown above the plating tank 602. The plating liquid 600 is gushed upward from the bottom of the plating tank 602 to strike a jet of the plating liquid 600 on the lower surface (plating surface) of the substrate W. Simultaneously, a predetermined voltage is applied between the anode plate 606 (anodic electrode) and the conductive layer S (cathodic electrode) of the substrate W from a plating power source 612 to form a plated film on the lower surface of the substrate W. At this time, the plating liquid 600 which has overflowed the plating tank 602 is collected from the plating liquid receiver 610.
Wafers and liquid crystal substrates for LSI's tend to increase in area year by year. In line with this tendency, variations in the film thickness of a plated film formed on the surface of the substrate are posing problems. In detail, to supply a cathode potential to the substrate, contacts with the electrode are provided in a peripheral edge portion of the conductive layer formed beforehand on the substrate. As the area of the substrate increases, the electric resistance of the conductive layer ranging from the contact on the periphery of the substrate to the center of the substrate also increases. As a result, a potential difference is produced in the surface of the substrate, causing a difference in the plating speed, thereby leading to variations in the film thickness of the resulting plated film.
That is, to apply electroplating onto the surface of the substrate to be treated, a common practice is to form a conductive layer on the surface of the substrate to be treated (hereinafter referred to simply as “substrate”), bring contacts for supplying a cathode potential into contact with a site on the conductive layer in proximity to the outer periphery of the substrate W, install an anode at a position facing the substrate W, fill a plating liquid between the anode and the substrate W, and apply an electric current between the anode and the contacts with a direct current power source to perform plating on the conductive layer of the substrate W. In the case of a large-area substrate, however, the electric resistance of the conductive layer ranging from the contact close to the outer periphery of the substrate to the center of the substrate W becomes so high that a potential difference arises in the surface of the substrate W, causing differences in the plating speed among respective portions.
FIG. 72 is a view showing the film thickness distribution of copper plated films over the surface of the substrate when copper electroplating was performed, using a conventional general plating apparatus as shown in FIG. 71, on a silicon substrate of 200 mm in diameter having a conductive layer (a copper thin film) with a film thickness of 30 nm, 80 nm and 150 nm formed thereon. FIG. 73 is a view showing the film thickness distribution of copper plated films over the surface of the substrate when copper electroplating was similarly performed on each of silicon substrates of 100 mm, 200 mm and 300 mm in diameter having a conductive layer (a copper thin film) with a film thickness of 100 nm formed thereon. As shown in FIGS. 72 and 73, when the conductive layer is thin, or the diameter of the substrate is large, there are great variations in the distribution of the film thickness of the copper plated film formed by electroplating. In extreme cases, no copper film may be formed in the vicinity of the center of the substrate.
This phenomenon will be explained electrochemically as follows:
FIG. 74 shows an electrical equivalent circuit diagram of the conventional general electroplating apparatus shown in FIG. 71. When a predetermined voltage is applied by a plating power source 612 between the anode plate 606 (anodic electrode) submerged in the plating liquid 600 and the conductive layer S (cathodic electrode) of the substrate W to form a plated film on the surface of the conductive layer S, the following resistance components exist in this circuit:
R1: Power source wire resistance between power source and anode, and various contact resistances
R2: Polarization resistance at anode
R3: Plating liquid resistance
R4: Polarization resistance at cathode (plated surface)
R5: Resistance of conductive layer
R6: Power source wire resistance between cathode potential lead-in contact and power source, and various contact resistances.
As shown in FIG. 74, when the resistance R5 of the conductive layer S becomes higher than the other electric resistances R1 to R4 and R6, the potential difference produced between both ends of this resistance R5 of the conductive layer S increases, and accordingly, a difference occurs in the plating current. Thus, the plated film growth rate lowers at a position distant from the cathode lead-in contact. If the film thickness of the conductive layer S is small, the resistance R5 further increases, and this phenomenon appears conspicuously. Furthermore, this fact means that the current density differs over the surface of the substrate, and the characteristics of plating themselves (resistivity, purity, filling characteristics, etc. of the plated film) are not uniform over the surface of the substrate.
Even in electrolytic etching, in which the substrate is an anode, the same problems occur, merely with the direction of electric current being reversed. In a manufacturing process for a large-diameter wafer, for example, the etching rate at the center of the wafer slows compared with the peripheral edge portion.
As a method for avoiding these problems, it is conceivable to increase the thickness of the conductive layer or decrease the electric conductivity. However, the substrate is subject to various restrictions, even in manufacturing steps other than plating. Furthermore, for example, when a thick conductive layer is formed on a fine pattern by sputtering, voids easily form inside the pattern. Thus, it is impossible to easily increase the thickness of the conductive layer or change the film type of the conductive layer.
Placement of the cathode potential lead-in contacts on the entire surface of the substrate makes it possible to make the potential difference over the surface of the substrate small. However, this placement is unrealistic because the site used as the electrical contacts cannot be used as LSI. Furthermore, increasing the resistance value of the plating liquid (resistance R3, R2 or R4 in FIG. 74) is also effective. However, changing the electrolyte of the plating liquid means changing all of the plating characteristics. Lowering the concentration of metal ions to be plated, for example, brings about the restriction that the plating speed cannot be made sufficiently high.
As described above, in the step of performing electroplating by providing contacts in a peripheral portion of the substrate and using the conductive layer on the surface of the substrate, the problem arises that as the size of the substrate increases, the plated film thickness greatly varies over the surface of the substrate. This problem, in particular, is a major restriction in the semiconductor industry, which places emphasis on the uniformity of the film thickness over the surface of the substrate to be treated, and the uniformity of the process.