ARTICLE pubs.acs.org/crystal
Manipulating the Crystallographic Texture of Nanotwinned Cu Films by Electrodeposition Tsung-Cheng Chan, Yu-Lun Chueh,* and Chien-Neng Liao* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ABSTRACT: Manipulation of the crystallographic texture and twinning structure of Cu films is achieved through adjustment of the chloride concentration in copper sulfate electrolyte using direct-current (DC) and pulsed-current (PC) deposition methods. With increasing chloride concentration in the electrolyte, the DCdeposited Cu film showed a monotonically strengthening (110) crystallographic texture, while the PC-deposited one revealed a (111) to (110) transition at the chloride concentration 104105 M. A physical explanation based on the distinct exchange current density of different Cu crystallographic planes and the duty cycle of the pulsed current is provided to elucidate the change of Cu film texture with varying chloride concentrations. A transmission electron microscopy analysis reveals that a high density of nanotwins exists in the PC-deposited Cu films but not in the DC-deposited ones. The effect of nanotwin spacing and crystallographic texture on the hardness of electrodeposited Cu films is investigated.
1. INTRODUCTION Damascene Cu technology that includes electroplating of Cu metallization into patterned dielectric trenches and vias followed by chemicalmechanical polishing is the mainstream back-endof-line (BEOL) process for advanced integrated circuit (IC) devices. With constant advancement of semiconductor processing technology, the dimension of Cu interconnects in IC devices also decreases steadily. Thus, microstructural manipulation of thin-and-narrow Cu wires becomes an important task because crystallographic orientation and grain structure affect many physical properties of Cu interconnects. For example, Cu interconnects with ultrafine grains usually have high electrical resistivity due to extensive carrier scattering at grain boundaries.1 Moreover, grain boundaries, especially large-angle tilt boundaries, provide fast diffusion channels that are highly undesired, due primarily to electromigration (EM) reliability concerns.2 In the past decade, a tremendous effort has been dedicated to understanding the influence of processing conditions and thermal treatment on microstructural evolution of Cu metallization in order to control the electrical, mechanical, and mass transport properties of Cu interconnects.24 EM-induced voiding usually occurs at atomic flux divergence sites such as triple junctions of the grain boundary and interface. The coarse-grained Cu interconnects with specific crystallographic orientation would have few fast diffusion channels and flux divergence sites, which would help improve the EM reliability of Cu interconnects.3 Cu metallization with nanoscale twinning structure has received growing attention in recent years. Nanotwinned Cu foils with (110) preferred orientation were prepared on low carbon steel sheets with an amorphous NiP surface layer using pulsed electrodeposition, which demonstrated very high mechanical strength, good ductility, and reasonably low electrical resistivity.5 Moreover, the presence of triple junctions where twin boundaries meet grain boundaries or surface was found to retard currentr 2011 American Chemical Society
driven atomic transport, which may improve the EM reliability of Cu interconnects.6 The excellent mechanical and electrical properties make nanotwinned Cu a perfect interconnect material for the BEOL process and through-silicon-via applications in three-dimensional IC devices. To implement nanotwinned Cu in modern interconnect technology, control of the twinning structure and crystallographic texture of nanotwinned Cu would be essential. Cui et al. have demonstrated the control of the crystallographic texture and twin density of electrodeposited Cu films by changing the substrate and the peak current density.7 The texture of the Cu films electrodeposited at low current density appeared to be modulated by the substrate crystallographic orientation. By increasing the peak current density, a transition from (110) to (111) texture occurred due to the anisotropic growth rate of different crystallographic planes in the electrodeposited Cu films. It has also been reported that with increasing negative electrochemical potential the crystallographic texture of Cu films changed from (111) to (110) or (100) orientation, which is attributed to the competition between surface energy and strain energy of Cu crystallites.8 In addition to the above-mentioned substrate and current density effects, ingredients of electrolyte also play an important role in electrodeposition of Cu films. The reduction of Cu from Cu2+ ions in electrolyte usually follows two successive oneelectron transfer reactions shown below Cu2þ þ e ¼ Cuþ Cuþ þ e ¼ Cu
E ¼ 0:087 V SCE E ¼ 0:281 V SCE
ð1Þ ð2Þ
Received: July 11, 2011 Revised: September 6, 2011 Published: September 16, 2011 4970
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where VSCE is the electrical potential with respect to the saturated calomel electrode. Clearly, the reduction of Cu2+ into Cu+ is the rate-determining step.9 Vasiljevic et al. have investigated the effects of Br, F, SO42, and Cl ions in electrolyte on the crystallographic texture of electrodeposited Cu films.10 The results indicate that Cl is the most effective species in modulating the crystallographic orientation of Cu films. It is because the addition of chloride ions (Cl) in electrolyte can provide an alternative route of Cu reduction through the formation of anionbridged complex CuClads, Cu2þ þ Cl ads þ e ¼ CuClads
E ¼ 0:338 V SCE
ð3Þ
and the transformation of CuClads into Cu on the electrode surface.10 CuClads þ e ¼ Cu þ Cl E ¼ 0:063 V SCE
ð4Þ
The addition of chloride in electrolyte would effectively accelerate electrodeposition of Cu because two parallel reactions are activated. In this study the effect of chloride addition in copper sulfate electrolyte on the crystallographic texture and twinning structure of electroplated Cu films is investigated. A mechanism based on the exchange current density of an individual Cu crystallographic plane and the duty cycle of the pulsed current applied is presented to explain the change of Cu film texture as a function of the chloride concentration in the electrolyte. The influence of crystallographic texture and twinning structure on the mechanical properties of electrodeposited Cu films is also discussed.
2. EXPERIMENTAL SECTION Cu films were electrodeposited at room temperature on thermally oxidized Si substrate with 400-nm-thick SiO2 using copper sulfate electrolytes with different chloride concentrations. Prior to electrodeposition, the oxidized Si substrate was sequentially deposited on a 30nm-thick Ti adhesion layer and a 50-nm-thick Cu seed layer by e-beam evaporation. The electrolyte used contained 1 M Cu2+ cations and different concentrations of Cl anions (03 mM), which were prepared by mixing appropriate amounts of CuSO4 3 5H2O (99.5%, SHOWA), NaCl (99.5%, SHOWA), H2SO4 (97%, SHOWA), and deionized water (electrical resistance ∼18 MΩ), with the pH value of the solution adjusted to 1. To perform the electrodeposition, a piece of Cu sheet (99.5%, Nilaco) was put in the counterelectrode, with the Cu/Ti/SiO2/ Si substrate of 5 mm 5 mm in size in the working electrode. An Ag/ AgCl reference electrode was used to monitor the potential change during electrodeposition. Two electrodeposition methods, direct-current (DC) and pulsed-current (PC), were employed to deposit Cu films on the Cu/Ti/SiO2/Si substrate using an electrochemical analysis system (Jiehan 5000, Jiehan Technology). The current density applied through the specimen at DC mode was 450 A/m2, while that at PC mode was 2 104 A/m2 with a duty cycle of 0.02 s (on)/1 s (off). After electrodeposition, the Cu film specimens were rinsed and blown dry by N2 for subsequent crystallographic and microstructural inspections. The morphology and microstructure of the electrodeposited Cu films were examined with a field-emission scanning electron microscope (SEM, JEOL6500F, JEOL) and transmission electron microscope (TEM, JEM-2010, JEOL). The crystallographic texture of the electrodeposited Cu films was identified with a X-ray diffractometer (XRD, XRD-6000, Shimadzu) with Cu Kα radiation at a scanning rate of 2/min. The electrodeposition rate
Figure 1. Evolution of crystallographic texture of electrodeposited Cu films as a function of chloride concentration in electrolyte: (a) DCdeposited Cu films with 20 min deposition time; (b) PC-deposited Cu films with 20 min deposition time. was determined from the thickness of the electrodeposited Cu films measured using a stylus profiler (Dektak-150, Veeco). A nanoindentation system (CSEM Nano Hardness Tester) was used to measure the hardness of the electrodeposited Cu films with a maximum load of 3 mN, a loading rate of 1 mN/s, and a dwelling time of 5 s.
3. RESULTS AND DISCUSSION The crystallographic texture of Cu films with either (111) or (110) orientation can be manipulated by adjusting the chloride concentration in copper sulfate electrolyte along with DC and PC electrodeposition methods. A texture coefficient (TC) is adapted to indicate the preferred crystallographic orientation of an electrodeposited Cu film. The TC value is calculated from the relative intensities of the (hkl) reflections in XRD spectra,11 TCðhklÞ ¼
IðhklÞ =I0ðhklÞ 1=n IðhklÞ =I0ðhklÞ
∑
ð5Þ
where I0(hkl) and I(hkl) represent the relative intensity of the (hkl) reflection of a randomly oriented powder sample (JCPDS Card Number 04-0836, Fm3m) and that of the electrodeposited Cu film specimen, respectively, and n is the number of reflections. Figure 1 shows the texture coefficients of the DC- and PCdeposited Cu films with respect to chloride concentration for the (111), (100), and (110) reflections, respectively. Here, a unity TC(hkl) value stands for no preferential crystallographic orientation, and the large TC(hkl) value reflects the strong (hkl) texture of the electrodeposited Cu film. Clearly, the DC-deposited Cu films revealed no specific crystallographic texture when they were deposited in the chloride-free electrolyte. With increasing [Cl], an improved (110) texture was developed in the DC-deposited Cu film. On the other hand, the PC-deposited Cu films showed a strong (111) texture when [Cl] < 105 M and transformed into (110) texture when [Cl] > 104 M. How the crystallographic texture of electrodeposited Cu films changes with [Cl] is a subject of interest. 4971
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Figure 3. Plot of electrodeposition rate of the DC- and PC-deposited Cu films averaged over a 20 min deposition time with respect to chloride concentration in electrolyte. Figure 2. Plot of texture coefficient ratio, TC(110)/TC(111), versus film thickness for the DC- and PC-deposited Cu films using the electrolyte with [Cl] equal to 0 M, 0.01 mM, and 3 mM, respectively.
step, the exchange current density for a specific (hkl) plane is given by15
For the Cu film deposited on a constrained substrate, the interplay between strain energy and surface energy may cause the change of crystallographic texture as the film thickness increases.12 A thin electrodeposited Cu film usually exhibits a strong (111) texture because (111) planes have the lowest surface energy among the major low-indices planes.8 With the increase of film thickness, an increasing strain energy is developed and the (110) texture is favored because (110) planes can accommodate larger strains than (111) planes. Figure 2 shows the texture coefficient ratio, TC(110)/TC(111), against the film thickness for the DC- and PC-deposited Cu films deposited in the electrolytes with different chloride concentration. Both the DC- and PC-deposited Cu films indeed revealed the strengthening (110) crystallographic texture with increasing film thickness. It is also noted that the dependence of TC(110)/ TC(111) on film thickness becomes more prominent with the increase of [Cl]. Therefore, we may reasonably speculate that the addition of Cl in electrolyte may raise the Cu deposition rate and give rise to a thick (110)-textured Cu film under the same deposition time. Unfortunately, the speculation does not agree with our observation. The averaged deposition rate of the DC- and PCdeposited Cu films with a 20 min deposition time was found to decrease with increasing [Cl] in electrolyte, as shown in Figure 3. The trend is also against the previous report that the addition of Cl would enhance the Cu electrodeposition rate.10 We believe that the discrepancy results from the different electrodeposition methods. The presented work was conducted using the galvanostatic (constant-current) technique, while the referenced one used the potentiostatic (constant overpotential) technique. It has been pointed out that the overpotential would decrease with time during galvanostatic deposition of Cu crystallites.13 Since the decreasing overpotential is not favored for the reduction of CuCl into Cu (eq 4), a CuCl solid layer may form on the electrode surface.14 The low overpotential under high [Cl] leads to a thick CuCl layer and, in turn, a low Cu deposition rate. Thus, the observed texture transition of electrodeposited Cu films with increasing [Cl] must be associated with some factor other than film thickness. We note that Cu grains of different crystallographic planes may have distinct deposition rate.15 By assuming the electron charge transfer process shown in eq 1 to be the rate-determining
where β, η, and F are symmetry factor, overpotential, and Faradic constant, respectively. The exchange current density in acidic copper sulfate electrolyte is known to follow the order i0(110) > i0(100) > i0(111) with i0(110) = 5i0(111).16 In the case of low electrodeposition rate, Cu grains would grow mainly in a twodimensional manner, and the (110)-oriented grains tend to grow faster than the others in both the thickness and lateral directions. Under high electrodeposition rate, Cu nuclei of different crystallographic orientation may form simultaneously and a threedimensional grain growth with random orientation is expected to occur in the Cu film. Because the addition of Cl in electrolyte decreases the cathodic overpotential and, hence, electrodeposition rate, the DC-deposited Cu film using chloride-free or low [Cl] electrolyte (high deposition rate) shows no preferred orientation, while the films deposited in high [Cl] electrolyte (low deposition rate) show a strong (110) crystallographic texture. However, the model cannot explain why the PCdeposited Cu film with low [Cl] electrolyte reveals a strong (111) texture. We believe that it is associated with desorption and migration of Cu adatoms on the surface during the “off” period of PC electrodeposition. According to the exchange current density of the specific crystallographic plane, the desorption rate of Cu adatoms on (111) planes is about 1 order of magnitude smaller than that on (110) planes.17 This means that Cu atoms in (110)oriented grains may dissolve into the electrolyte more easily than those in (111)-oriented grains during the “off” period of PC electrodeposition. Moreover, Cu adatoms are also expected to have a high probability of reaching the kink sites on the (111) surface because of their high surface diffusivity on (111) planes.18,19 Consequently, the growth of (111)-oriented grains becomes dominant for the Cu films deposited at PC mode in the electrolyte of low Cl concentration. With increasing [Cl] in electrolyte, both cathodic overpotential and electrodeposition rate are reduced, and thus, Cu adatoms have sufficient time to transport and reach stable surface sites on both (111) and (110) planes. The (110) texture is again developed in the PC-deposited Cu films with increasing [Cl] in electrolyte. In summary, the addition of Cl in electrolyte decreases the cathodic overpotential and Cu nucleation rate during galvanostatic electrodeposition, which favors the growth of (110)-oriented Cu grains and causes the texture transition from random to (110) orientation for the DC-deposited Cu films. The higher stability of Cu
iðhklÞ ¼ i0ðhklÞ expðβηF=RTÞ
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nanotwins of 2025 nm in lamella width exist in the PC-deposited Cu films, while there are very few nanotwins in the DC-deposited Cu film. Figure 4e shows that the PC-deposited Cu films have 30 50% higher hardness than the DC-deposited ones. It is generally accepted that the mechanical strength of polycrystalline solids increases with decreasing grain size according to the HallPetch equation.21 For nanotwinned Cu, a similar relationship between yield strength (σy) and twin spacing (t) has also been suggested, σ y ¼ σ 0 þ kt 1=2
ð7Þ
where σ0 is the lattice friction stress and k is the barrier strength of the twin boundary to dislocation transmission.21 On the basis of the reported data of the nanotwinned Cu,22 the values of σ0 and k were determined to be 100 and 3320 MPa 3 nm0.5, respectively. The twin spacing of the PC-deposited Cu films was estimated from the measured hardness (∼3 times of yield strength) to be 17 21 nm, which is in reasonable agreement with the TEM observations shown in Figure 4c and d. As mentioned earlier, the PCdeposited Cu film with chloride-free electrolyte has a strong (111) texture, while 3 mM [Cl] reveals a (110) texture. Considering {111}/Æ110æ to be the major slip systems for crystalline Cu, we can calculate the respective Schmid factor to be 0.272 and 0.408 when the loading is in the directions of [111] and [110]. With the same critical resolved shear stress, the (111)-textured nanotwinned Cu is expected to have higher yield strength than the (110)oriented one, which agrees with our hardness measurements. Indeed, the (111)-oriented Cu film has a hardness of 2.7 GPa, while the (110)-oriented Cu film has one of 2.4 GPa, as shown in Figure 4e. The results demonstrate that, with the adjustment of chloride concentration in electrolyte and an appropriate electrodeposition method, we can manipulate the crystallographic orientation and twinning structure of the electrodeposited Cu films and, hence, change the mechanical property of the Cu films accordingly.
Figure 4. (a) Planar TEM image of 50-nm-thick Cu seed layer prepared by e-beam evaporation; (b) planar TEM image of the DC-deposited Cu film using the chloride-contained (3 mM Cl) electrolyte; (c) crosssectional TEM image of the PC-deposited Cu film using the chloridefree electrolyte; (d) planar TEM image of the PC-deposited Cu film using the chloride-contained (3 mM Cl) electrolyte; (e) respective hardness of the Cu films deposited at DC and PC modes in the chloridefree and chloride-contained (3 mM Cl) electrolytes.
adatoms on (111) planes prevents Cu desorption from the film surface during the off-time of PC electrodeposition, which favors the development of (111) texture in the PC-deposited Cu films. It has been shown that the nanotwinned Cu foil prepared by pulsed electrodeposition has a (110) preferred orientation, and its hardness increases with decreasing twin lamella width due to the blocking of dislocation activity by dense twin boundaries.5,20 In the present work, we showed that the hardness of nanotwinned Cu films can be further modulated by film texture. Figure 4 shows the TEM images and the measured hardness of the Cu films deposited at DC and PC modes using the chloridefree and chloride-contained (3 mM [Cl]) electrolytes, respectively. For comparison, the planar TEM image of the Cu seed layer prepared by e-beam evaporation is shown in Figure 4a. The evaporated Cu film appears to have small grain size (
