4 Copper CMP
4.2 High Selectivity for Copper CMP
Cu CMP slurries commonly use submicron-sized colloidal silica abrasive particles dispersed in aqueous solutions that contain an oxidizer, as well as an complexing agent and corrosion inhibiting agents and other chemi-cals. Most of the slurries described in the article by several researchers use H2O2 as the oxidizer and benzotriazole as the inhibitor, with various complexing agents such as organic polymer, alkaline agent, and organic amine in slurry.
The pH value of the polishing slurry is one of the most important parameters influencing the polishing rate, surface roughness, and other performance characteristics of the Cu CMP process. In this section, the slurry’s pH and conductivity were adjusted to the range of pH 10 to 11 and conductivity of 8 to 10 (mS/cm) by adding an alkaline agent, including NH4OH and HCl solution.
Figures 4.4 to 4.7 show the results obtained from an experiment con-ducted by varying the concentrations of complexing agent (alanine) and selectivity control agent (PAM) in aqueous slurry. Figure 4.4 shows the removal rate of Cu and TaN films versus the alanine concentration. The removal rate of Cu film increased with alanine concentrations. In addition, the removal rate of TaN film strongly suppressed and slightly increased with increasing alanine concentrations in aqueous suspension. As with the removal rate of Cu film, the removal rate of TaN film drastically decreased and was essentially saturated with a concentration of alanine beyond 0.5 wt%.
Cu Removal Rate (Å/min)
0
TaN Removal Rate (Å/min)
Cu removal rate TaN removal rate
Figure 4.4 The removal rates of Cu and TaN films versus alanine concentration in slurry.
Copper CMP 83
Alanine could exist in aqueous solution in three different forms, namely, CH3 CH(NH3+)COOH (cation), CH3 CH(NH3+)COO– (zwitterions), and CH3 CH(NH2)COO– (anion). These species are denoted as H2L+, HL, and L–, respectively, for brevity. The equilibrium between these may be depicted, as Babu et al. (2005) previously reported, the dissolution and removal rate probability of the complexing agent, including phthalic acid, citric acid, glycine, oxalic acid, and carboxyl and/or amine functional group, which interact on the Cu film surface should strongly influence the removal rate.
(4.2)
Babu et al. explained that a complexing agent, such as amino group in glycine and hydrogen peroxide system, is protonated at pH <4.0, and thus may not effectively form chelates with positively charged metal ions; thus, the dissolution must be due to the carboxyl group. On the other hand, at pH >4.0 the amino group can chelate Cu2+ ion and cause the dissolution of the metal up to pH 10. However, alanine and H2O2–containing colloidal silica slurry exhibited an enhanced removal rate of Cu film at alkaline pH region. We thought that the alanine could be a very effective complexing agent with an increased removal rate of Cu film through a high dissolu-tion rate of Cu2+ ion in alkaline pH region.
The suppression of the removal rate of TaN film could not be fully explained through the electrochemical phenomena by chemical reaction between complexing agent and the TaN film surface. We thought that the TaN film loss and the Cu-to-TaN removal selectivity are directly related to the electrostatic interaction and electrokinetic behavior due to chemical adsorption and steric hindrance of adsorbed organic chemical.
Figure 4.5 shows the electrokinetic behaviors of Cu film, TaN film, and col-loidal silica slurries with alanine addition as a function of pH. The absolute surface zeta potential of the Cu film was slightly negatively charged above pH 5. The TaN film also exhibited a slightly negative charge at a pH above pH 5.3. Colloidal silica slurry with alanine exhibited a pHiep at pH 4.0.
The surface potentials of the colloidal silica abrasive particles in the aqueous suspension with alanine were strongly negatively charged above
pH 4.0, while the TaN film’s surface potential was weakly negatively charged. The attraction behavior between the abrasive particles and the TaN film results from the different electrostatic potentials exhibited in certain pH regions. Therefore, we suggest that the selective adsorption of alanine added slurry on the abrasive particles and the TaN film surfaces correspond to the differing zeta potential charge. The removal rate of TaN film is drastically supressed, and increased slightly by this difference.
By comparing the removal rate of Cu and TaN films, we can calculate the removal selectivity of Cu-to-TaN films (Figure 4.6). By increasing
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
0 0.5 1 1.5 2
Alanine Concentration (wt%)
Cu-to-TaN Removal Selectivity
Figure 4.6 Removal selectivity of Cu-to-TaN films versus alanine concentration in slurry.
–60 –50 –40 –30 –20 –10 0 10 20
0 2 4 6 8 10 12
pH
Zeta-potential (mV) 0.2 wt%
1.0 wt%
2.0 wt%
Cu film TaN film Alanine
Figure 4.5 Zeta potential of Cu, TaN films, and colloidal silica slurry with alanine as a function of pH.
Copper CMP 85
the alanine concentration, the removal selectivity drastically increased and essentially saturated from 5:1 to 32:1 with increasing alanine concentration.
Figure 4.7 shows the removal rate of Cu and TaN films versus the PAM concentration. The removal rate of Cu film slightly decreased with PAM concentrations. Here, the removal rate of TaN film was strongly sup-pressed and saturated with increasing PAM concentrations in aqueous suspension as shown in Figure 4.7.
To enhance the removal selectivity of Cu-to-TaN films with suppressing the removal rate of TaN film by selective adsorption, we also optionally added organic polymer (PAM) with the concentration of up to 0.7 wt%.
The adsorption of PAM-added slurry on the abrasive particles and the film surfaces corresponds to the differing zeta potential charge. By this zeta potential difference, the removal rate of the Cu and TaN films was more suppressed, and the oxide-to-nitride removal selectivity increased with addition of PAM.
Figure 4.8 shows the electrokinetic behaviors of Cu film, TaN film, and colloidal silica slurries with PAM addition as a function of pH.
Adsorption of PAM on Cu and TaN film surfaces increases and reaches a strong suppressed point of approximately 0.3 wt%. In other words, PAM is more adsorbed on the Cu and TaN film surfaces. This is driven by the difference in zeta potential, which affects the interaction between PAM and each surface.
In addition, above the isotropic point, the slightly negative-charged Cu oxide and TaN films surface can interact with the deprotonated between carboxyl groups of alanine, neutral –NH2 groups, and NH+ functional groups of PAM, which results in the formation of strong complexes with Cu and TaN films. However, with addition of PAM, the removal rate of
0
Cu Removal Rate (Å/min)
0
TaN Removal Rate (Å/min)
Cu removal rate TaN removal rate
Figure 4.7 The removal rates of Cu and TaN films versus PAM concentration in slurry.
Cu and TaN film decreased. By increasing the PAM concentration, the removal selectivity drastically increased and essentially saturated from 30:1 to 130:1 (Figure 4.9).
Potentiodynamic polarization studies were carried out to measure the corrosion current density and potential at various alanine and polyacryl-amide concentrations with H2O2. Polarization plots for Cu film as a function of alanine concentration with H2O2 at pH 10 are presented in Figure 4.10a and b, respectively. The value of corrosion potential reduced gradually
PAM
–60 –50 –40 –30 –20 –10 0 10 20
0 2 4 6 8 10 12
pH
Zeta-potential (mV) 0.3 wt%
0.5 wt%
0.7 wt%
Cu film TaN film
Figure 4.8 Zeta potential of Cu, TaN films, and colloidal silica slurry with PAM as a func-tion of pH.
0 40 80 120 160 200
0 0.3 0.5 0.7
PAM Concentration (wt%)
Cu-to-TaN Removal Selectivity
Figure 4.9 Removal selectivity of Cu-to-TaN films versus PAM concentration in slurry.
Copper CMP 87
with an increasing concentration of alanine (Figure 4.10a). As the alanine solution increases from 0.5 to 2.0, the fraction of carboxyl and amino func-tional group is more pronounced with increased anion fraction at alkaline pH. Since the bidentate (L–) is more reactive than the monodentate (HL), the increased dissolution rate of Cu+ or Cu2+ ions. The removal rate of Cu film increases with an increasing alanine concentration by low corrosion potential. On the other hand, corrosion potential showed no difference with addition of PAM solution (Figure 4.10b).
–10 –8 –6 –4 –2 0
–1.0 –0.5 0.0 0.5
Ec vs. Ag/AgCl
10x (A/cm2) 0.5 wt% Alanine 1.0 wt% Alanine 1.5 wt% Alanine 2.0 wt% Alanine
(a)
–10 –8 –6 –4 –2 0
–1.0 –0.5 0.0 0.5
Ec vs. Ag/AgCl
10x (A/cm2) 1.5 wt% Alanine w/o PAM 1.5 wt% Alanine w/ PAM
(b)
Figure 4.10 (See color insert) Potentiodynamic polarization: (a) various alanine concen-tration, (b) 1.5 wt% alanine with and without PAM.