Room-temperature intermixing for adhesion enhancement of Cu/SiO₂ interface by adopting SiO₂ surface dope and noble metal catalyzation

In order to confirm the incorporation of Zn into the SiO₂ surface, the interfaces of ZnO/SiO₂ were examined before removing the ZnO layer.

Figure 1 shows TEM images and Zn, Si, and O maps of the interfaces of (a, b) as-deposited MOCVD-, (c, d) as-deposited MBE-, and (e, f) annealed MBE-ZnO/SiO₂ stacks. In any stack, a sound interface without defects such as voids and inclusions was formed. An approximately 5-nm-thick interfacial layer having an intermediate contrast between ZnO and SiO₂ was clearly observed at the as-deposited MOCVD- and the annealed MBE-ZnO/SiO₂ interfaces. The interfacial layer was formed on the SiO₂ side of the interface. In the as-deposited MBE-ZnO/SiO₂ stack, equi-axed grains were formed on the ZnO side of the interface and their contrast was similar to that of ZnO.

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Strong Zn and Si signals were detected over the entire ZnO and SiO₂ regions, respectively. The Zn and Si signal intensities decreased slightly in the interfacial layer regions of the as-deposited MOCVD- and the annealed MBE-ZnO/SiO₂ stacks [Figs. 1(b) and 1(f)], whereas the as-deposited MBE-ZnO/SiO₂ stack showed very sharp transition between the Zn and Si intensities [Fig. 1(d)]. These results indicate that Zn was diffused approximately 5 nm into the SiO₂ when the samples were processed at high temperatures; i.e., the SiO₂ surface was doped with Zn by heat treatment.

In order to identify the phases of the interfacial layer, high-resolution TEM (HRTEM) observations and TEM–EDX analyses were carried out. Figure 2 shows a HRTEM image of the interfacial layer formed at the ZnO/SiO₂ interface. A typical amorphous structure having no fringe contrast can be observed in the SiO₂ region. The lattice image is clearly observed in the interfacial layer regions as in the ZnO region, obviously proving that the interfacial layer is crystalline. Figure 3(a) shows a high-angle annular dark field (HAADF) image of the interfacial layer. The interfacial layer consists of high-contrast regions [A in Fig. 3(b)] and low-contrast regions [B in Fig. 3(b)], indicating that the interfacial layer is not a single phase.

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Table I shows EDX point analysis results taken from the A- and B-regions. Zn, Si, and O were detected in both the regions but the atomic ratio among Zn, Si, and O was different. The A-region had a high Zn concentration, whereas the atomic percentage of Si was almost half of that of O. We suppose that the A-region is Zn-containing SiO₂ [SiO₂(Zn)] (discussed in Sect. 3.4). The atomic ratio among Zn, Si, and O in the B-region was $\text{2}:\text{1}:\text{4}$ which fairly agrees with the stoichiometry of the equilibrium Zn₂SiO₄ phase.

Figure 4 shows (a) a cross-sectional TEM image and (b) Zn, Si, and O maps of the SiO₂ surface taken after removing the ZnO layer. The TEM image exhibits the presence of an extremely thin surface layer different from the original SiO₂ substrate. These EDX maps indicate that the surface layer consists of Zn, Si, and O, and this composition distribution was similar to that of the interfacial layer formed at the ZnO/SiO₂ interface by heat treatment [Figs. 1(b) and 1(f)]. This means that the interfacial layer remained at the SiO₂ surface as a Zn-doped layer even after the original thick ZnO layer being removed.

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Noble metal particles (Pt in this case) and Cu films were deposited on the Zn-surface-doped SiO₂ and non-doped SiO₂. The adhesion strengths of these stacks were shown in Table II. When the non-doped SiO₂ was used, the average adhesion strength was approximately 25 mN. This value was higher than that of the Cu/SiO₂ stack (5.2 mN²⁹⁾) but was not remarkably increased. On the other hand, when the Zn-surface-doped SiO₂ was employed, no delamination occurred even when the applied load was increased to 500 mN, which is identical to the fracture value of the SiO₂ substrate. This indicates that the doping of SiO₂ surface with Zn was effective for improving the adhesion at Cu/SiO₂ interface.

Table III shows adhesion strengths of Cu sputtered on the Zn-doped SiO₂. When no noble metals were used (Cu/Zn-doped SiO₂), the average adhesion strength was approximately 50 mN. This value was ten times higher than the adhesion strength of a Cu/SiO₂ stack.²⁹⁾ This means that the SiO₂ surface doping with Zn is also effective for improving the adhesion between Cu and SiO₂; however, this adhesion strength of 50 mN is not high enough. In contrast, no film delamination occurred in any stacks when the noble metals were inserted between the Cu and the Zn-doped SiO₂. This proves that Pt, Pd, and Pt–Pd particles are effective catalysts, and, moreover, the combination of the Zn dope and the noble metal catalyzation enhances the adhesion between Cu and SiO₂.

A cross-sectional TEM image and EDX maps of the interface of the Cu/Pt/Zn-doped SiO₂ stack are shown in Fig. 5. An extremely thin (less than 10 nm) layer was clearly formed at the interface. Strong Cu signals were detected throughout the Cu film region and a slight decrease in their intensity was observed in the interfacial layer region. Pt signal intensity was high in the interfacial layer and become weaker on the Cu side of the Cu/interfacial layer interface. Zn signals were observed either in the interfacial layer or in the Cu film. Both Si and O signals have a constant intensity in the SiO₂ region and become weaker in the interfacial layer region. Quantitative analysis using EDX point probing indicates that the composition of this interfacial layer is Cu₄₂Pt₁₈Zn_0.8Si₁₅O₂₂, meaning that intermixing of all the elements existing in the interface occurred. From these observations, it is safely said that the formation of this intermixing layer results in the adhesion improvement. In the previous study, the formation of such an intermixing layer was observed when we used Pd as a catalyst, which shows the intermixing acceleration.¹⁶⁾

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As mentioned above, it was found that the interfacial intermixing occurs at room-temperature by adopting Zn doping and noble metal catalyzation. In addition, the formation of this intermixing layer resulted in the adhesion enhancement. In this section, based on our observation results and the phase diagrams, the reaction route at the interface and the formation mechanism of the intermixing layer are discussed.

3.4.1. ZnO–SiO₂ reaction at 300–400 °C

Figure 6 shows a Zn–Si–O ternary diagram at 300–400 °C on which the compositions of the observed phases are plotted. The tie lines show possible routes of the interfacial reactions drawn based on our observations and on a ZnO–SiO₂ binary phase diagram,¹⁷⁾ as discussed below. When ZnO layers were deposited on SiO₂ at 350 °C or when a ZnO/SiO₂ stack was annealed at 400 °C, SiO₂(Zn) [$\text{Zn}:\text{Si}:\text{O} = 2:1:2$ (Table I)] and Zn₂SiO₄ phases, which are plotted in Fig. 6 with solid symbols, were formed at the interface. This layer remained as the surface layer after ZnO removal. The SiO₂(Zn) lie on the Zn–SiO₂ tie line and the Zn₂SiO₄ lie on the ZnO–SiO₂ tie line. This fairly proves that the SiO₂(Zn) phase was formed by the reaction between Zn and SiO₂, and the Zn₂SiO₄ phase was produced by a direct reaction between ZnO and SiO₂. The ZnO–SiO₂ phase diagram¹⁷⁾ shows that ZnO and SiO₂ react over 1300 °C and forms Zn₂SiO₄. Our process temperature was lower than this temperature, therefore, the Zn₂SiO₄ was formed by diffusion reaction.

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The reaction between ZnO and SiO₂ does not produce other compounds (2ZnO + SiO₂ → Zn₂SiO₄). Therefore, based on our observations, we formulated a possible overall reaction as follows:

$$\begin{equation} \text{4ZnO} + \text{2SiO$_{2}$}\rightarrow\text{Zn$_{2}$SiO$_{4}$} + \text{SiO$_{2}$(2Zn)} + \text{2O}, \end{equation} \tag{ 1 }$$

where the coefficient of Zn is introduced as 2Zn to satisfy the stoichiometry of Zn. The bracket of Zn shows that Zn is free metal and dissolves in or coexists with the SiO₂. The formation of the Zn₂SiO₄ phase cannot be excluded but is not necessarily assumed here because 1) SiO₂ is very stable and inert, at least, compared to ZnO, 2) the presence of such a compound or Zn–SiO₂ phase diagrams is not known to the extent of the author's survey, and 3) the formation of Zn₂SiO₄ required more phase equilibriums some of those may not satisfy the phase rule.

Reaction (1) indicates that the molar ratio of the formed Zn₂SiO₄ and the SiO₂(Zn) is $1:1$ and that more ZnO is consumed in the reaction than SiO₂. Indeed, from the TEM–EDX analyses, the volumes of these phases are almost identical apart from the molar density of these phases. The stoichiometry also shows that free Zn forms and dissolves in SiO₂. As a result, as observed (Table I), the Zn₂SiO₄ and the SiO₂(Zn) coexist at the interface.

This reaction requires reductive chemistry or release of oxygen. It is well-known that vapor deposited ZnO has oxygen vacancies especially at elevated temperatures. Either when oxygen-deficient ZnO reacts or when the formed oxygen diffuses through or stuff oxygen vacancies, reaction (1) is satisfied in terms of the presence of O on the right-hand side.

Elementary reactions of reaction (1) can be expressed as follows:

$$\begin{align} & \text{ZnO} + \text{SiO$_{2}$}\rightarrow\text{Zn$_{2}$SiO$_{4}$}, \end{align} \tag{ 2 }$$

$$\begin{align} & \text{ZnO}\rightarrow\text{2Zn} + \text{2O}, \end{align} \tag{ 3 }$$

$$\begin{align} & \text{SiO$_{2}$} + \text{2Zn}\rightarrow\text{SiO$_{2}$(2Zn)}. \end{align} \tag{ 4 }$$

In fact, the above formulae simply indicate that ZnO is decomposed to Zn and O. Presumably, reaction (2) ignites these reactions to proceed. Once Zn diffuses into SiO₂, oxygen in ZnO becomes less and promotes oxygen diffusion. This atomic agitation will accelerate reaction (2). Finally, schematic diagrams of the above-mentioned reactions are shown in Fig. 7.

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3.4.2. Pt catalyzed reactions during room-temperature Cu deposition

It should be noted that the Cu₄₂Pt₁₈Zn_0.8Si₁₅O₂₂ phase was formed at the Cu/SiO₂ interface at room temperature (Fig. 5). The Zn–Si–O composition of this phase is Zn_2.1Si_39.7O_58.2 (open symbol in Fig. 8). This composition is very close to the Si–O tie line and has a Si–O ratio close to $2:3$. Therefore, we can safely say that this phase is an oxygen-deficient silicon oxide, presumably Si₂O₃ (SiO₂SiO) that contains impurity Zn. The reactions can be expressed as the following formulae:

$$\begin{align} & \text{Zn$_{2}$SiO$_{4}$} + \text{SiO$_{2}$(2Zn)}\overset{\text{Pt}}{\longrightarrow}\text{SiO$_{2}$}\cdot \text{SiO} + \text{3O} + \text{4Zn}, \end{align} \tag{ 5 }$$

$$\begin{align} & \text{Cu} + \text{Zn}\rightarrow\text{Cu(Zn)}. \end{align} \tag{ 6 }$$

Reaction (5) is a Pt-catalyzed reaction that generates (free) oxygen and (free) Zn. Zn is well miscible in Cu,³⁰⁾ so that the repelled Zn dissolves into the depositing Cu (Fig. 9). The dissolution of Zn is clearly seen in the EDX map of Zn, where Zn exists in the Cu film region [Fig. 5(b)]. Reaction (6) simply expresses the dissolution of Zn in Cu, and thus the stoichiometric balance of Zn is not taken into account here.

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Free oxygen may also dissolve in Cu but the presence of oxygen is not clearly confirmed in our EDX analyses. O atoms in the above reactions are introduced nominally to satisfy the stoichiometry in terms of our EDX observations. Another subject left is a fact that Pt is dissolved in the intermixing layer. We used Pt particles, whereas the Pt was completely mixed with the other elements after the Cu deposition. This was observed in our previous study using Pd.¹⁶⁾ There is still a large possibility that noble metals form other compounds such as oxides and co-dissolve to the existing oxides besides their catalytic role.