4 ppm DVF-B, 20 ppm DVF-C and 1.5 ppm DVF-D were added in the base electrolyte for Cu electrodeposition in the blind holes (Ø 40 µ m × 140 µ m) at room temperature. The applied direct current (DC) density was 3 mA cm^-2 for 10 min plus 10 mA cm^-2 for 270 min. Fig. 2(a) shows the SEM image of the cross-sectional Cu micro-cylinder including the overburden Cu film. The Cu overburden film and Cu micro-cylinder were separated with a green solid line.

Fig. 2.

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	Fig. 2. EBSD grain map of the cross-sectional Cu micro-cylinder with CSL-GBs. (a) SEM image of the cross-sectional Cu micro-cylinder, and the corresponding EBSD grain maps (a1) in the top region, (a2) in the middle region, and (a3) in the bottom region. (b) Color key. (c) Coordinate system.

Fig. 2(a₁-a₃) show the EBSD grain maps with coincidence site lattice-grain boundaries (CSL-GBs) in the top, middle and bottom regions of the cross-sectional Cu micro-cylinder, respectively. As shown in Fig. 2(a₁), the grain of electrodeposited Cu in the overburden film is bigger than that in the top region of the Cu micro-cylinder.

Fig. 2(a₃) shows the EBSD grain map in the bottom region of the Cu micro-cylinder. It can be seen that grains in this region have an average size of about 1.67 µ m. For the middle region of the Cu micro-cylinder (Fig. 2(a₂)), the grains in the lower half have an average size of 1.92 µ m, while some grains in the upper half are so small that their Kikuchi patterns cannot be obtained^[23]. These features are very reproducible. The reason for this phenomenon is probably due to the factor that the concentrated suppressors (DVF-C) inhibit Cu grain growth in the upper part of the blind holes^[24].

Compared to the Cu damascene, the grain size distribution of Cu micro-cylinder is not uniform from the bottom to the top due to the different concentration distributions of the additives in the high-aspect-ratio holes. Considering the high-aspect-ratio of the blind holes and the long electroplating time, it requires a very large consumption of cupric ions and additives, and produces a large amount of by-products at the deposit surface. The by-products at the bottom of the hole cannot diffuse out timely, so they are incorporated into the deposits. The interaction of additives and their by-products at the deposit surface during the hole filling process leads to defects, dislocations, and incorporation of impurities, which might drive or retard the recrystallization process^{[4, 11, 12, 22 , 25]}.

Fig. 3(a) shows the grain map with CSL-GBs of the local region in the middle of the Cu micro-cylinder as indicated by the red solid rectangle in Fig. 2(a). It can be seen that the grain size near the sidewall of the Cu micro-cylinder is smaller than that far from the sidewall. The grain size increases along the growth direction. Besides, many large grains with {111} preferred orientation distribute far from the sidewall. Fig. 3(b) shows the grain size distribution of the Cu deposits. It can be seen that the grain size concentrates in the range of 0-2 µ m.

Fig. 3.

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	Fig. 3. (a) EBSD grain map with CSL-GBs of the local region of Cu micro-cylinder, as indicated by a red solid rectangle in Fig. 2(a); (b) grain size distribution; (c) CSL boundary distribution; (d) GB misorientation distribution.

As shown in Fig. 3(c), it contains the highest fraction of Σ 3 CSL boundaries. The Σ 3 CSL boundary has the lower grain boundary energy than the random high-angle boundary, which is more likely to weaken the electron scattering^[23]. Besides, there exists a fraction of Σ 9, Σ 27b and Σ 25b boundaries. The presence of twin boundaries may enhance the tensile strength of the Cu micro-cylinder without losing its high electrical conductivity^[26].

The grain boundary distribution of Cu deposits in terms of misorientation angles is shown in Fig. 3(d). It can be seen that grain boundaries with misorientation of 55° -60° occupy the largest fraction, indicating that a large amount of Σ 3 boundaries separate two neighboring grains which is rotated by 60° around the < 111 > axis. Besides, there are a large number of grain boundaries with misorientation of 35° -40° indicating the existence of Σ 9 boundaries, where the grains are rotated by 38.94° around the < 110 > axis.

In this section, 11 ppm A, 14 ppm S, and 10 ppm L were added in the base plating electrolyte (80 g L^-1 Cu(CH₃SO₃)₂, 20 g L^-1 CH₃SO₃H and 50 mg L^-1 HCl). The silicon wafer contains an array of blind holes. The thickness of each Cu overburden film is about 38 µ m. Fig. 4 shows XRD spectra of the Cu overburden film at different current densities (from 2 to 80 mA cm^-2). As shown in Fig. 4(a), when the current density of 2 mA cm^-2 is applied, the Cu overburden film shows a strong {111} preferred orientation. An increase in the current density increases the intensities of the {200}, {220} and {311} peaks. There exists a sharp change in the diffraction spectra when the applied current density further increases up to 25 mA cm^-2. As shown in Fig. 4(b), no other peak appears except the strong {220} preferred orientation when the applied current density is above 25 mA cm^-2.

Fig. 4.

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	Fig. 4. X-ray diffraction spectra of electrodeposited Cu overburden films at different current densities: (a) from 2 to 20 mA cm-2; (b) from 25 to 80 mA cm-2.

The results mentioned above can be explained as follows. Pangarov^{[27 , 28]} and Kremmer et al.^[29] linked the preferred orientation of deposits with the energy needed for the formation of two-dimensional nuclei W_hkl, which was regarded as a function of the over-voltage η .

(1)

where A_hkl and B_hkl are materials parameters that depend on the crystallographic direction < hkl > . z is the number of electrons, N_a is the Avogadro constant and F is the Faraday constant. For fcc metals, W₁₁₁ < W₁₀₀ < W₁₁₀ are yielded at a low over-voltage according to Eq. (1), while W₁₁₀ < W₁₀₀ < W₁₁₁ at a high over-voltage η ^{[27], [28] and [29]}. The crystallites of the electrodeposited metal have a preferred orientation when the formation energy of a particular nucleus is lower than that of all the other types of nuclei. At the small current density (< 8 mA cm^-2), the energy W₁₁₁ for the formation of two-dimensional {111} orientation nuclei is the smallest one (W₁₁₁ < W₁₀₀ < W₁₁₀) and the Cu overburden film shows {111} preferred orientation. With the increase in the current density (20-40 mA cm^-2), {110} oriented nuclei have the least formation energy so that the preferred orientation of Cu overburden films changes from {111} to {220} at high current density.

The effect of current density on the morphology of Cu overburden films is also investigated. It can be seen from Fig. 5 that a wide variation in morphological features is obtained. An increase in the current density decreases the crystallite size of the Cu overburden film. The spherical crystallites are observed when the current density is in the range of 8-20 mA cm^-2, while the flocci or needle-like crystallites appear when the current density is in the range of 25-80 mA cm^-2. Besides, some nodules with a diameter of 100-300 µ m appear on the smooth surface when the current density is 80 mA cm^-2, as shown in the insert image of Fig. 5(l). It is found that the nodules always appear when only additive S is used in the electrolyte at the high current density, although the formation mechanism is not clear yet. When the nodule exists near the hole opening, it might seal the hole opening before the blind hole is completely filled. Fortunately, according to our experiments, nodules disappear when another two kinds of additives are added in the electrolyte.

Fig. 5.

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	Fig. 5. SEM images of electrodeposited Cu overburden films at different current densities: (a) 2, (b) 3, (c) 5, (d) 6, (e) 8, (f) 10, (g) 15, (h) 20, (i) 25, (j) 30, (k) 40, and (l) 80 mA cm-2 in Fig. 3.

Changes in the grain size of the Cu overburden film are related to the electrodeposition mechanism, which can be explained by the model of nucleation and crystal growth. According to the principles of electrocrystallization, the formation rate of new nuclei on the electrode surface increases exponentially as the cathodic polarization η increases ^[30]:

(2)

where T and B are temperature and the constant, respectively, ω is the rate of nuclei formation, and η is the cathodic polarization potential. At the low current density (low cathodic polarization potential), limited nuclei are formed and the grains tend to grow from the existing nuclei, so the large size grains are obtained. An increase in the current density enhances the polarization effect, which promotes the copper nucleation. Massive nuclei cause the formation of fine grains (the crystallite size decreases with increasing cathodic polarization).

Two kinds of seed layers are used for electrodepositing the Cu overburden film. XRD patterns of #1 and #2 seed layers are shown in Fig. 6(a, b). The #1 seed layer shows very strong {111} preferred orientation. The #2 seed layer also shows {111} preferred orientation, but the intensity of {200} diffraction peak enhances compared to that of the #1 seed layer.

In this section, the thicknesses of each Cu overburden film shown in Fig. 6(c-f) are around 45, 43, 39 and 42 µ m, respectively. All of the current densities used for copper electroplating are 10 mA cm^-2. Fig. 6(c, d) shows XRD patterns of Cu overburden films electroplated on the #2 seed layer in the electrolyte containing 0-40 ppm A and 2-20 ppm S, respectively. For quantifying the preferred orientation, the texture coefficient is introduced. The texture coefficient M_hkl of an (hkl) plane is defined by Barrett and Massalski ^[31] as follows:

(3)

where I_hklIhkl is the measured intensity of XRD, n is the number of calculated peaks, and Ihkl0 is the intensity of the powder pattern in the JCPDS cards. Three peaks of {111}, {200}, and {220} are used to calculate the texture coefficient. The index value of M_hkl indicates the volume fraction of grains of an (hkl) plane ^[32].

From Fig. 6(c), it can be calculated that by increasing the accelerator from 2 to 40 ppm, the {111} texture coefficient increases from 1.39 to 2.39, and the {200} texture coefficient decreases from 1.50 to 0.54, which demonstrates that the accelerators absorb on the deposit surface and induce certain preferred orientations during the electroplating process. As shown in Fig. 6(d), both the {111} preferred orientation and the {200} preferred orientation almost have no change with an increase in the suppressor concentration in the electrolyte. It indicates that the suppressor has little effect on the crystal orientation of the Cu overburden film when the #2 seed layer is used in the electrodeposition process.

Fig. 6.

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Fig. 6. X-ray diffraction spectra of electrodeposited Cu overburden films on the different seed layer in the different electrolytes: (a) Cu seed layer #1 with a strong {111} preferred orientation; (b) Cu seed layer #2 with both {111} and {200} preferred orientations; (c) electrodeposited Cu overburden film on the #2 seed layer in the electrolyte containing different contents of the accelerator; (d) electrodeposited Cu overburden film on the #2 seed layer in the electrolyte containing different contents of the suppressor; (e) electrodeposited Cu overburden film on the #1 seed layer in the electrolyte containing different contents of the accelerator; (f) electrodeposited Cu overburden film on the #1 seed layer in the electrolyte containing 4 ppm accelerator and different contents of the suppressor.

Fig. 6(e, f) shows the XRD patterns of the Cu overburden films electroplated on the #1 seed layer in the electrolyte containing the 0-10 ppm accelerator and 1-15 ppm suppressor in the presence of the 4 ppm accelerator, respectively. It can be seen from Fig. 6(e, f) that they all show strong {111} preferred orientation. Both the concentrations of the accelerator and the suppressor have little effect on the crystal orientation of the Cu overburden film when the seed layer with a very strong {111} preferred orientation is used. In addition to the seed layer with a strong {111} preferred orientation, the smallest energy needed for the formation of two-dimensional {111} orientation nuclei at the small current density also plays an important role in the formation of strong {111} preferred orientation for the Cu overburden film.

According to the experimental results in Fig. 6, it can be concluded that the crystal orientation of the electrodeposited Cu overburden film is influenced not only by the additives but also by the substrate. When the Cu electrodeposition occurs on a single-crystal or on a polycrystalline substrate with a strong preferred orientation, additives adsorbed on the deposit surface have little influence on the crystal orientation of Cu deposits. The preferred orientation of the grains is epitaxially influenced by the crystal structure of the substrate, which corresponds to the case for the #1 seed layer with a strong {111} preferred orientation. However, when the Cu electrodeposition occurs on an inert substrate, the epitaxial influence of the substrate can be ignored. The additives adsorbed on the Cu deposit surface influence the electrodeposition process dramatically. The crystal structure of the Cu overburden film is determined by the electrocrystallization conditions, which correspond to the case for the #2 seed layer.