1.
Introduction

With the development of integrated circuit technology, the performance has been improved significantly, integration level is higher and higher, and the wiring is more than ten layers ^[1–3]. Thus the multilayer copper interconnection planarization becomes more and more crucial with the feature size of Gita large scale integrated circuit (GLSI) dropping down to 14 nm and even smaller. Chemical mechanical polishing (CMP) is the local and global planarization technology combining mechanical force and chemical effect to reach excellent and optimal polishing performance^[4–6]. Fig. 1 shows the schematic illustration of copper CMP in this experiment. This process can realize the requirements of surface topography imposed by decreasing lithographic depth of focus with increasing resolution^{[7, 8]}.






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Figure1.
(Color online) The schematic illustration of CMP.




The performance of CMP including material removal rate (MRR), thickness uniformity and surface quality is determined by various mechanical and chemical factors^[9–11]. Among the chemical factors, the influence of slurry component concentration on stability can be studied by measuring the variation of the slurry pH value, abrasive particle size, and zeta potential. These factors are known to significantly affect the results of CMP^{[4, 5, 7]}. Since acid slurry is not only detrimental to the low K material and cobalt as barrier material but also will corrode polishing equipment^[12], alkaline slurry becomes the main research direction with the development of IC^[13], so weakly alkaline slurry is the main topic in the paper.



Lin et al. ^[14] studied the colloidal stability of mixed abrasive slurry and their role in CMP by particle size distribution, ζ-potential and settling test of mixed abrasive slurry. Liang Jiang et al. ^[15] investigated the effect of benzotriazole and non-ionic surfactant on copper chemical mechanical polishing. In addition, Liang Jiang et al.^[16] also investigated the synergetic effect of H₂O₂ and glycine on cobalt CMP in weakly alkaline barrier slurry. However, there are few studies on the stability of copper slurry. The stability was systematically studied via particle size, zeta potential, copper removal rate and settling behavior in this paper. Meanwhile in order to achieve a higher polishing rate, the optimizing composition and concentration of copper slurry was provided. Using such slurry the stability of the copper removal rate, root-mean square roughness (Sq) were also studied. All the results showed that the working life of the slurry has been extended to 7 days.

2.
Experiment

2.1
Slurry preparation

The composition of copper slurry includes colloidal silica, FA/O complexing agent, anion surfactant, pH regulator, BTA inhibitor and H₂O₂ oxidizing agent with a constant pH value at 9.0. The initial colloidal silica has a mean particle size of 80 nm. By adjusting the concentration of colloidal silica, glycine, BTA and H₂O₂ respectively, the pH value, particle size and zeta potential were measured in order to observe the stability of slurry.



The pH value was measured by the PHB-4 pH meter produced by INESA at room temperature (the precision is 0.01). The mean particle size and zeta potential were tested by the NiComp380 DLS produced by America PSS. The pH value, mean particle size and zeta potential were the average of three measurements.

2.2
CMP process

2.2.1
Copper wafer polishing

All the CMP experiments were performed on an E460E produced by French Alpsitec company. The polishing pad was IC 1000TM provided by Rohm and Haas Electronic Materials. The copper removal rate was determined by Eq. (1).

$${v_{ m RR}} = frac{{Delta M}}{{ ho pi {R^2}t}}, $$

(1)

where v_RR is the material removal rate of copper, ΔM is the weight loss of copper. R is the radius of copper wafer (purity of 99.99%), ρ is the density of copper (ρ = 8.9 g/cm³) and t is polishing time (t = 3 min). The weight loss was gained by a professional electronic balance (Mettler Toledo AB204-N), which has the resolution of 0.1 mg.

2.2.2
Copper blank wafer polishing

Copper blank wafer was polished by slurry with optimal proportion. After polishing for 1 min, the copper removal rate was obtained by the film thickness lost of 81 test points measured by the four-point probe (4D Model 333A). Atomic force microscopy (Agilent, 5600LS) was used to measure the surface roughness.

Parameter	Value
Down force	2
Back pressure	0 psi
Polishing head speed	87 r/min
Platen rotation speeds	93 r/min
Slurry flow rate	300 mL/min

Table1.
Process parameters of CMP.



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Parameter	Value
Down force	2
Back pressure	0 psi
Polishing head speed	87 r/min
Platen rotation speeds	93 r/min
Slurry flow rate	300 mL/min

3.
Results and discussion

3.1
Effect of colloidal silica concentration on the slurry stability

The concentration of silica sol in the slurry with pH value 9.0 was 1 wt%, 2 wt%, 5 wt%, 10 wt% and 20 wt% respectively, and the other compositions kept constant concentration for anion surfactant 1.2 wt%, FA/O complexing agent 0.4 wt%, glycine 1 wt%, BTA 200 ppm and oxidizing agent (H₂O₂) 0.5 wt% respectively.



Fig. 2 shows the variation removal rate of copper with different colloidal silica concentrations. It can be observed that CuRR progressively increase with the increasing of colloidal silica concentration. According to Fig. 2, 20 wt% is superior according to the higher removal rate of 4569.6 ?/min.






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Figure2.
Effect of colloidal silica concentration on CuRR.




Fig. 3 shows the settling behavior of slurry with 20 wt% colloidal silica after 5 min. A gel phenomenon occurs, which illustrates that slurry with 20 wt% colloidal silica is unstable. Fig. 4 shows the settling behavior of slurries with different colloidal silica concentration. It is shown that the slurries with 5 wt%, 10 wt% colloidal silica delaminated after setting 48 h. After setting for 72 h, the slurry with 2 wt% colloidal silica also delaminated.






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Figure3.
The slurry with 20 wt% colloidal silica after 5 min.






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Figure4.
(Color online) The settling behavior of slurries with different colloidal silica concentrations. (a) After 5 min. (b) After 48 h. (c) After 72 h.




It is shown with the extension of setting time, the stability of slurries decreased with the increasing of silica sol concentration.



Figs. 5(a) and 5(b) show the silica sol particles perform the endless Brownian Motion, and Brownian force overcomes the force of gravity of particles to keep balance. It is easy to imagine that aggregation chance among silica sol particles caused by collision leading becomes greater due to the silica sol particles number increase in constant volume.






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Figure5.
(a) SiO₂ brownian motion. (b) Double concentration of SiO₂.




Lower abrasive concentration is the development trend with the feature size dropping down to 14 nm even smaller, since higher abrasive concentration probably leads to scratch on the copper wafer surface and residual. In addition, a higher copper removal rate is desired in the CMP process, and the removal rate increases with the increasing of silica sol concentration. Hence, 1 wt% silica sol is optimized for keeping a balance between stability of slurry and the copper removal rate.



Fig. 6 shows the SEM photograph of 1 wt% silica sol, which has good stability and dispersity with pH value 9.0. Further work will focus on the complexing agent concentration, which affects the copper removal rate significantly.






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Figure6.
The SEM photograph of 1 wt% silica sol.

3.2
Effect of glycine concentration on the slurry stability

Adding 0, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt% and 2.5 wt% glycine to the slurry with 1 wt% silica sol and pH value 9.0 respectively, the other compositions kept constant concentration as shown before.



Fig. 7 shows the variation trend of the copper removal rate with increasing of glycine from 0 to 2.5 wt%. It can be observed that CuRR shows nearly linear growth from 391.68 to 4993.92 ?/min with the increasing of glycine concentration.






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Figure7.
Effect of glycine concentration on CuRR.




Figs. 8(a), 8(b) and 8(c) show the variations of pH value, mean particle size and the zeta potential of slurries with different glycine concentrations and setting times. It is shown that with the extension of setting time, glycine slightly affects the stability of slurry since with the increasing of concentration, mean particle size and the zeta potential of slurries fluctuate within a small scope. Figs. 9(a) and 9(b) show the settling behavior of slurries with different glycine concentrations and setting times. There is no obvious delamination phenomenon of the slurries due to the low abrasive. According to Figs. 7 and 8, it is inferred that the effect of glycine on stability is weaker than that of silica sol. In order to gain a high copper removal rate which meets industrial requirements and reduces cost, 2.5 wt% was selected as a moderate concentration.






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Figure8.
(Color online) Effect of glycine concentration on slurry. (a) pH. (b) Particle size. (c) Zeta potential.






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Figure9.
(Color online) The settling behavior of slurries with different glycine concentrations. (a) The slurries on 0 day. (b) The slurries after 7 days.




In the pH value range from 2.35 to 9.78 solution, glycine exists as the form of ⁺H₃NCH₂COO^–, when the pH value is greater than 9.78, H₂NCH₂COO^– plays a major role in the condition. For this paper, reactions are shown as following:

$${ m {C{{ u}^{2 + }} + {}^ + {H_3}NC{H_2}CO{O^-} to Cu{({H_2}NC{H_2}COO)^ + } + {H^ + }}}, $$

(2)

$${ m {Cu{({H_2}NC{H_2}COO)^ + } + {H^ + } to Cu{({H_3}NC{H_2}COO)^{2 + }}}}, $$

(3)

$$begin{align} & { m Cu{{({{H}_{3}}NC{{H}_{2}}COO)}^{2+}}{{+}^{+}}{{H}_{3}}NC{{H}_{2}}CO{{O}^{-}}} & qquad qquad to { m Cu{{({{H}_{2}}NC{{H}_{2}}COO)}_{2}}+2{{H}^{+}}}. end{align}$$

(4)

Cu(H₂NCH₂COO)⁺ can accelerate the decomposition of H₂O₂, which facilitates the oxidation process of copper.

$${ m 2Cu + {H_2}{O_2} to C{u_2}O + {H_2}O}, $$

(5)

$${ m C{u_2}O + {H_2}{O_2} to 2CuO + {H_2}O}.$$

(6)

3.3
Effect of BTA concentration on the stability

Adding different concentrations of 0, 100, 200, 300, 400, and 500 ppm BTA to the slurries with 1 wt% silica sol, 2.5 wt% glycine and pH value 9.0 respectively, the other compositions kept constant concentration as shown before.



Fig. 10 shows CuRR markedly decreases with the increasing of BTA concentration. It can be seen that Figs. 11(a), 11(b) and 11(c) show with the setting time extension the variation of the pH value, mean particle size and the zeta potential of slurries with different BTA concentrations. It can be concluded with the extension of setting time, the increasing of BTA concentration is conducive to the stability of slurries. Figs. 12(a) and 12(b) show the settling behavior of slurries with different BTA concentrations with setting time. According to Fig. 12 and Fig. 11(b), although a delaminating phenomenon did not occur, the mean particle size markedly increased in lower BTA concentration within 7 days. With the concentration increasing, the growth rate of mean particle sizes drops slowly. However, BTA not only is very difficult to clean from the copper wafer surface but also contaminates the environment. Considering the copper removal rate and planarization efficiency, 200 ppm was selected as the appropriate concentration.






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Figure10.
Effect of BTA concentration.






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Figure11.
(Color online) Effect of BTA concentration on slurry. (a) pH. (b) Particle size. (c) Zeta potential.






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Figure12.
(Color online) The settling behavior of slurries with different BTA concentration. (a) The slurries on 0 day. (b) The slurries after 7 days.

3.4
Effect of H₂O₂ concentration on the stability

The slurries with pH value 9.0 consisted of 1 wt% silica sol, 2.5 wt% glycine, 200 ppm BTA, 1.2 wt% anion surfactant, 0.4 wt% FA/O complexing agent and different concentrations of 10, 20, 30, 40, 50 mL/L H₂O₂ (30 wt%) respectively.



Fig. 13 shows CuRR markedly increases between 10 and 20 mL/L and gains maximum removal rate value 4047.36 ?/min in 20 mL/L. Fig. 14(a) shows with the extension of setting time, the pH of slurries increases first and then decreases due to the decomposition of hydrogen peroxide. The reactions are shown as the following^[1],






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Figure13.
Effect of H₂O₂ concentration on CuRR.

$${ m {H_2}{O_2} + 2{e^-} to 2O{H^-}},$$

(7)

$${ m {H_2}{O_2} + {e^-} to O{H^*} + O{H^-}}.$$

(8)

The increasing of OH^– concentration will raise the pH value in a short time, then with the consumption of OH^–, the pH value will decrease slightly.



Figs. 14(b) and 14(c) show the changing of mean particle size and the zeta potential of slurries with different H₂O₂ concentration and setting time. It is observed that H₂O₂ can reduce slightly the particle size of silica sol within a small scope. Fig. 14(c) shows the increasing of H₂O₂ concentration is counter to the stability of slurries. H₂O₂ will consume ions in the electrical double layer of SiO₂ particles, which leads to the particle size measured by the light scattering method decreasing. Meanwhile, it can reduce the absolute value of the zeta potential, which reflects the degree of colloidal particles charged. The higher the absolute value of the zeta potential is, the more colloidal particles charge, the thicker the diffusion layer is, and the more stable the slurry is. The relationship between the zeta potential value and colloid stability is shown in Fig. 15.






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Figure15.
Zeta potential and colloid stability.




Figs. 16(a) and 16(b) show the settling behavior of slurries with different H₂O₂ concentrations as the setting time. The slurries are stable due to the appropriate concentration of other components. H₂O₂ easily decomposes and oxidizes other components of the slurry, therefore it is the major cause of the instability. In conclusion, taking the copper removal rate and slurry stability into consideration, 20 mL/L was selected as appropriate and effective for this slurry.






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Figure16.
(Color online) The settling behavior of slurries with different H₂O₂ concentration. (a) The slurries on 0 day. (b) The slurries after 7 days.

3.5
The copper blanket wafer polishing experiment

The final slurry was consisted of 1 wt% silica sol, 2.5 wt% glycine, 200 ppm BTA, 20 mL/L H₂O₂ (30 wt%) and anion surfactant with pH value 9.0.



From Fig. 14 and Fig. 16 it seems the slurries are stable. In order to further study the slurry stability of application, the copper wafer (purity of 99.99%) was polished every day to gain the copper removal rate in 7 days. Fig. 17 shows the variation of copper removal rate in 7 days. The results reveal that the copper removal rate is nearly the same in 7 days, which demonstrates the stability of such weak slurry for 7 days with H₂O₂.






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Figure17.
The variation of copper removal rate in 7 days.






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Figure14.
(Color online) Effect of H₂O₂ concentration on slurry. (a) pH. (b) Particle size. (c) Zeta potential.




At the same time, the copper blanket wafers were polished by fresh slurry and the slurry settling for 7 days. The comparison of the removal rate, which was gained by average value of 81 test points on copper blanket wafer is shown in Fig. 18. The copper removal rate after 7 days is similar to the removal rate in 0 day, which is consistent with the results of the copper wafer (purity of 99.99%). The Sq of copper blanket wafer polished by fresh slurry shown in Fig. 19(a) is 0.595 nm, while the value of copper blanket wafer polished by slurry settling for 7 days shown in Fig. 19(b) is 0.605 nm. The Sq value, which almost remains unchanged can meet the microelectronic industry requirement.






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Figure19.
(Color online) Sq value comparison of copper wafer. (a) Polished by fresh slurry. (b) Polished by slurry settling for 7 days.






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Figure18.
The comparison of copper removal rate.

4.
Conclusion

The effect of slurry main components on the stability of the weakly alkaline copper slurry was studied systematically in this paper. The optimal matching of silica sol, glycine, BTA and H₂O₂ was firstly found for the stability of slurry at least after 7 days. The polishing results of the copper blanket wafer also prove that such slurry can be stable for 7 days in the presence of H₂O₂, which can be applied to industrial production. Meanwhile, it has guiding significance for further research of the stability in weakly alkaline solution, which is a future development tendency in the microelectronics industry.