Galvanic Processes on Silicon Surfaces in Cu(II) Alkaline Fluoride-Free Solutions

Abstract

Galvanic processes on silicon surfaces in alkaline fluoride-free solutions containing Cu(II) ions were investigated in this work. Deposition of copper (I) oxide (Cu 2 O) and copper metal at pH 14 onto silicon surfaces at room or elevated temperatures has been demonstrated. This deposition does not require the presence of a reducing agent in the solution. The results clearly show that Cu(II) ions can be reduced to Cu(I) or Cu (0) Metallization of silicon with copper, silver and gold can be realized using electrochemical methods such as electrodeposition or electroless deposition from aqueous solutions. These methods are very attractive since they are simple and relatively easy to scale up. Practical difficulties arise due to the fact that at the surface of silicon wafers, oxide films of SiO 2 are present. SiO 2 films are non-conductive, and consequently, they must be removed prior to direct metallization. Although the metallization of silicon substrates can be achieved via autocatalytic deposition through surface activation with SnCl 2 /PdCl 2 catalysts, 1 this process would not remove the SiO 2 film leading to unacceptable electronic device performance. In order to remove the SiO 2 film and to deposit metals such as copper or silver from aqueous solutions directly onto silicon, pre-treatment steps are required during processing. These pre-treatment steps may involve fluoride-containing solutions, 2 which have deleterious effects on the environment. The direct deposition of copper, silver or gold from aqueous fluoride-containing solutions onto silicon substrates via the galvanic displacement reaction can be quite successful.3-6 According to recent investigations, silver can be successfully deposited onto silicon substrates from fluoride-free aqueous solutions.7 Reports on the deposition of copper onto silicon substrates via the galvanic displacement reaction from fluoride-free aqueous solutions have not been found in the literature. In an attempt to further explore the behavior of silicon in fluoride free solutions without a presence of reducing agents in the aqueous phase, the present communication describes preliminary observations on galvanic reactions on silicon substrates using Cu(II) containing alkaline fluoride-free aqueous solutions.Experimental p-(B doped) and n-(P doped) single crystal (100) silicon wafers were used as substrates in the present work. As received, silicon substrates were very carefully washed with 3:1 (H 2 SO 4 -H 2 O 2 ) solution and then with water and ethanol. After the cleaning procedure, silicon substrates were immersed into respective proprietary fluoridefree solutions containing Cu(II) ions. Immersions were performed at pH 14 using a CuC 4 H 4 O 6 (copper tartrate) complexed solution. The pH of this solution was adjusted with NaOH. The concentration of Cu(II) ions in alkaline solutions was 3.54 g/L. The immersion solutions in the present work were used as prepared. Consequently it should be assumed that O 2 was present in the solutions during the immersion.The specific immersion ...

Communication—Galvanic Deposition of Gold on Silicon from Au(I) Alkaline Fluoride-Free Solutions

“…As in previous work, 5,6 silicon substrates were mechanically scratched with tweezers immediately prior to immersion into alkaline Au(I) solutions. The SEM results are shown in Figure 2 for both n-and p-silicon substrates.…”

“…The existence of SiO 2 at the surface of the silicon substrate was described and attributed as a difficulty for the galvanic displacement deposition before, [1][2][3][4] and notably, in the recent publications. 5,6 The solution used in the present work did not contain any Au(I) ion reducing agents. Also, fluorides were purposely avoided and highly alkaline solutions (pH 14) of Au (I) were used.…”

“…[1][2][3][4] However, the fluorides represent a health hazard and as such are environmentally deleterious. Recent reports showed that silver 5 or copper 6 can be deposited from alkaline fluoride-free solutions directly onto silicon via the galvanic displacement reaction. It was clearly shown that the native SiO 2 films can be removed from the surface of a silicon wafer upon immersion into alkaline fluoride-free solutions.…”

Electroless Metallization of the Elements: Survey and Progress

“…In recent years, however, health and environmental hazards associated with the use of fluorides have prompted development of alternative fluoride-free baths, such as those based on alkali, as potential replacements. For example, Djokić et al have recently described a simple immersion Cu bath comprising a pH 14 Cu II -Rochelle’s salt aqueous solution for use with Si substrates. The OH – in their high pH Cu bath dissolves SiO 2 per eq and oxidizes Si 0 per eq to provide the electrons necessary to reduce Cu II : Si IV normalO 2 + 2 OH − → Si IV normalO 3 2 − + normalH 2 normalO Si 0 + 4 OH − → Si IV normalO 3 2 − + normalH 2 normalO + normalH 2 + 2e − In strongly alkaline solution, however, Cu II can be reduced to both Cu I and Cu 0 per eqs and , respectively, and H 2 O can be reduced to H 2 per eq : 2Cu 2 + + 2 OH − + 2e − → Cu 2 normalI normalO + nor...…”

“…In recent years, however, health and environmental hazards associated with the use of fluorides have prompted development of alternative fluoride-free baths, such as those based on alkali, as potential replacements. For example, Djokić et al have recently described a simple immersion Cu bath comprising a pH 14 Cu II -Rochelle’s salt aqueous solution for use with Si substrates. The OH – in their high pH Cu bath dissolves SiO 2 per eq and oxidizes Si 0 per eq to provide the electrons necessary to reduce Cu II : In strongly alkaline solution, however, Cu II can be reduced to both Cu I and Cu 0 per eqs and , respectively, and H 2 O can be reduced to H 2 per eq : The net reaction occurring involves codeposition of both Cu metal and Cu 2 O as shown in eq : Although the Cu 2 O deposits as a powder and can be removed from the adherent Cu metal film atop the Si, its presence is not suitable for electronics manufacture.…”