1.  Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device comprising (a) a semiconductor substrate and (b) a wiring (made of a copper-based metal film) and a contact plug, both formed on the substrate (a). 
2.  Description of the Related Art
A conventional method for manufacturing a semiconductor device having a copper wiring is described below with reference to FIGS.  4 to 7. 
First, a device-isolating region 21 is formed on the principal surface of a silicon substrate 1, after which a gate electrode 3 and an impurity-diffused layer 2 are formed and, as an inter-layer insulating film, a silicon oxide film 4 is formed by plasma CVD [FIG.  4(a)]. 
Next, a contact hole reaching the impurity-diffused layer 2 is formed in the silicon oxide film 4, by dry etching.  Further, a Ti/TiN film 6 and a tungsten film 7 are formed in this order; the unnecessary portions of the Ti/TiN film 6 and the tungsten film 7, present outside the contact hole are removed by chemical mechanical polishing (CMP) to form a tungsten plug.  Then, on the whole principal surface of the resulting substrate is formed a silicon nitride film 8 by plasma CVD [FIG.  4(b)]. 
Thereon is formed a silicon oxide film 9 as an inter-layer insulating film by plasma CVD.  Then, dry etching is conducted in two stages in order to form a groove for wiring.  In the first stage, etching of the silicon oxide film 9 is conducted using a mixed gas containing C.sub.4 F.sub.8, Ar, O.sub.2 and CO; in the second stage, etching of the silicon nitride film 8 is conducted using a CHF.sub.3 -based gas [FIG.  4(c)]. 
After the completion of the etching, a Ti/TiN film 11 is formed by sputtering.  Thereon is formed, by sputtering, a seed metal (copper) film (not shown) in order to form thereon a copper plating.  The resulting substrate is immersed in an aqueous copper sulfate solution of about 25.degree.  C., and electrolytic plating is conducted to form a copper plating film 12 selectively on the seed metal film.  The thickness of the copper plating film 12 is controlled at about 1,000 nm at the flat portion.  In the electrolytic plating, copper-based metal contaminants 30 (e.g. Cu and CuO) adhere to the back surface of the semiconductor substrate, in a large amount.  This state is shown in FIG.  5(a). 
The copper-based metal contaminants 30 are removed using a mixed solution consisting f dilute hydrofluoric acid and an aqueous hydrogen peroxide solution [FIG.  5(b)].  Then, annealing is conducted at 400.degree.  C. for about 30 minutes, whereby the grains constituting the copper plating film grow and there appear reduced resistance and stabilization. 
Subsequently, the unnecessary portions of the Ti/TiN film 11 and the copper plating film 12, present outside the hole are removed by CMP to form a copper wiring.  In the CMP, copper-based metal contaminants 30 (e.g. Cu and CuO) adhere to the back surface of the semiconductor substrate in a large amount.  This state is shown in FIG.  6(a). 
The copper-based metal contaminants 30 are removed using a mixed solution consisting of dilute hydrofluoric acid and an aqueous hydrogen peroxide solution [FIG.  6(b)].  Then, on the principal surface of the substrate is formed an inter-layer insulating film, after which an upper wiring is formed to complete a semiconductor device (FIG.  7). 
In the above-mentioned conventional technique, metal contaminants (e.g. copper and copper compound) adhere to the back surface of a substrate during the formation of a copper-based metal film and the processing thereof (e.g. CMP and hole formation on copper wiring), as shown in FIG.  5(a) and FIG.  6(a).  These copper-based metal contaminants adhere in a large amount particularly in the formation of a copper-based metal film and the CMP thereof.  These metal contaminants diffuse into the substrate and reach the device region, in the subsequent heat treatment, deteriorating the properties of the device and causing current leakage.  The diffusion of metal contaminants is striking when the heat treatment is conducted at high temperatures, for example, 300.degree.  C. or higher. 
In order to avoid the above problem, it was necessary, in manufacturing a semiconductor device comprising a step of forming a copper-based metal film, to clean the back surface of a semiconductor substrate after the formation of a copper-based metal film and also after the processing of the copper-based metal.  In forming a copper wiring of, for example, 8 layers, it was necessary to conduct the above cleaning about 50 to 80 times, which made the steps complicated. 
Further, the copper-based metal contaminants, unlike tungsten, iron, nickel, aluminum, etc.,  are difficult to remove from the surface of silicon substrate and are impossible to completely remove by cleaning.  The reason is as follows.  The copper-based metal contaminants have a high redox potential; therefore, a redox reaction takes place between copper ion and silicon substrate; as a result, back adsorption of copper on silicon substrate takes place. 
Therefore, there were cases that copper-based metal contaminants remained unremoved even after considerable times of cleaning.  The remaining copper-based metal contaminants diffuse into the substrate, as shown in FIG.  7, and reach the impurity-diffused layer, the gate oxide film, the device-isolating oxide film, etc.,  deteriorating the properties of the device or causing current leakage. 
The above-mentioned problem occurs because copper diffuses into the silicon substrate at a very high velocity.  Therefore, the problem is peculiar to when a copper-based metal film is formed. 
The above-mentioned conventional technique includes a step of polishing tungsten by CMP.  In this step as well, metal contaminants adhere to the back surface of substrate.  With respect to these metal contaminants which are tungsten particles or iron particles (the latter particles are derived from the abrasive used in CMP), however, there hardly occur the above-mentioned problem of diffusion into silicon substrate and adverse effect on device region.  The tungsten particles and the iron particles diffuse into the silicon substrate at a very low velocity; their arrival at device region in heat treatment after CMP is rare; the tungsten particles, in particular, peel easily from the silicon substrate.  Further, these particles can be removed by polishing the back surface of substrate in the final step.