Copper film forming method and manufacturing method of multi-layer wiring substrate

1. Field of the Invention

The present invention relates to a copper film forming method and manufacturing method of a multi-layer wiring substrate using the copper film formation method. More specifically, it pertains to a copper film forming method for forming a thin copper film on a surface of a substrate.

2. Description of Related Art

In order to form a certain pattern on one side of a substrate, it is the common practice to form a copper film on the entire surface on the one side of the substrate by electroless copper plating.

Such electroless copper plating inevitably uses a harmful substance such as formaldehyde. Therefore, a cumbersome treatment is necessary for the treatment of a waste liquid left after electroless copper plating.

Thus, process of thermally decomposing copper formate, thereby forming a copper film are proposed as a process capable of forming a copper film on one side of a substrate without using such electroless copper plating.

For example, Japanese Patent Unexamined JP-A-11-193461 proposes a process of thermally decomposing copper formate in a pressure-reduced atmosphere, thereby precipitating copper on one side of a polyimide film.

Further, Japanese Patent Unexamined Publication JP-A-2002-271000 proposes a process of forming a copper interconnect by applying and drying a copper formate solution on one side of a substrate and exposing the substrate to laser light to precipitate copper.

Still Further, Japanese Patent Unexamined Publication JP-A-06-93455 proposes a process of forming a copper film by thermally decomposing copper formate on the surface of a base material in the presence of palladium.

Furthermore, Japanese Patent Unexamined Publication JP-A-2005-35984 proposes a process of forming a copper film on the surface of a base material by placing a copper compound represented by the formula: [RCOO]_m[NH₃]_nCuX_p (wherein, m is from 1 to 3, n is from 1 to 3, and p is from 0 to 1) on the surface of a base material and heating the base material under a copper non-oxidizing atmosphere.

According to the above-described patent documents, a copper film can be formed on a surface of a substrate without employing electroless copper plating and therefore cumbersome waste treatment of an electroless copper plating solution can be omitted.

However, in the JP-A-11-193461 and JP-A-2002-271000, a copper film is formed by thermally decomposing powdery copper formate over a substrate. During thermal decomposition of copper formate, a copper film becomes porous due to emission of a thermal decomposition gas such as carbon dioxide gas. To provide a copper film with enough conductivity, the thickness of the film must be increased to about 2 μm.

Accordingly, the processes according to the JP-A-11-193461 and JP-A-2002-271000 have difficulty in forming a copper film as thin as 1 μm or less and at the same time, having enough conductivity.

Compared with these processes, the process disclosed in the JP-A-06-93455 uses expensive palladium, which raises a film formation cost. In addition, a dispersed state of palladium on the surface of the base material has an influence on the purity and the like of the copper film thus formed. It is therefore very hard to form a uniform copper film.

In the JP-A-2005-35984, the copper compound represented by the formula: [RCOO]_m[NH₃]_nCuX_p is not commercially available and must be synthesized so that a formation cost of a copper film becomes high. In addition, the copper compound thermally decomposes at a relatively low temperature even in the absence of a catalyst so that it is likely to be influenced by whether or not the atmospheric temperature during heating is uniform. A uniform copper film on the surface of a base material is therefore difficult.

An object of the present invention is therefore to overcome the problem of the above copper film formation method employing thermal decomposition of copper formate or a compound thereof, that is, difficulty in forming a thin and uniform film on the surface of a base material because it is likely to be influenced by whether or not the atmospheric heating temperature is uniform and provide a formation method of a thin and uniform copper film on the surface of a substrate by using copper formate which is stable and easy in handling.

The present inventors proceeded with an investigation with a view to overcoming the above-described problem. As a result, it was found that a thin and uniform copper film can be formed on the surface of a substrate by spraying an aqueous solution of copper formate to the surface of a substrate heated to a certain temperature in a nitrogen atmosphere and simultaneously carrying out, under the certain temperature, evaporation of water in the resulting mist and thermal decomposition of copper formate without using a catalyst.

It was however elucidated that an oxide film appeared on the surface of the copper film formed on the surface of the substrate or copper formate remained undecomposed on the surface of the substrate and they prevented stable formation of a copper film with good quality.

The present inventors have therefore investigated, thinking that if the oxide film which has appeared on the surface of the copper film formed on the surface of the substrate or the undecomposed residue of copper formate on the surface of the substrate can be reduced with a reducing agent, the copper film thus formed can be made as pure as possible, leading to the completion of the invention.

In view of the above, according to the invention, there is provided a copper film forming method including the steps of:

spraying a copper formate solution including a copper foramate and a solvent which is evaporated at a certain temperature on a surface of the substrate;