chemistry of additives in damascene copper plating, The

IBM Journal of Research and Development, Jan 2005 by Vereecken, P M, Binstead, R A, Deligianni, H, Andricacos, P C

Redox reaction {6} can be easily demonstrated by a change in solution color from blue to yellow after a Cu solution is mixed with MPS [35, 38, 39]. In [34] the logarithm of the equilibrium constant for reaction {6} was found to be 3.3. Calculation of the free enthalpy for reaction {6} based on this equilibrium constant shows that reaction {6} is spontaneous only when either the SPS and MPS concentrations are large and comparable to the cupric concentration, or the ratio of MPS to SPS is large (1,000× for micromolar concentrations of SPS). Hence, only if SPS were reduced at the surface under diffusion limitation (surface concentration of SPS close to zero and MPS twice the SPS bulk concentration) would a mechanism through reaction {6} be applicable. However, literature studies are ambiguous as to whether direct electrochemical reduction of SPS to MPS at the electrode surface occurs. Zhukauskaite and Malinauskas have claimed the existence of a reversible reduction wave for SPS at a Pt electrode positive from the copper deposition potential [35]. However, we were unable to detect electrochemical reduction of SPS from sulfuric acid solutions at Pt, glassy carbon (GC), or copper before the onset of hydrogen evolution [40]. Also, Healy et al. have reported the absence of a reduction wave for SPS in both the presence and absence of Cu^sup ^[21]. On a gold electrode we were able to measure a reduction wave for 10-50-millimolar solutions of SPS in 10-vol.% sulfuric acid at potentials negative of 0.14 V (SHE) [40], i.e., still in the range of copper deposition (see Figure 5). However, in the case of gold we were most likely examining the electrochemical reaction of SPS and Au(I)thiolate [32, 33].

In order to verify the mechanism of SPS consumption and the formation of adsorbed or soluble cuprous thiolatc complexes during copper deposition, we conducted a rotating ring-disk electrode study in copper sulfate solutions in which the ring was used to detect cuprous ions as well as SPS and MPS or its thiolate anion [^sup -^S(CH^sub 2^)^sub 3^SO^sub 3^H]. A glassy carbon ring and a platinum disk were used. Glassy carbon was chosen as the material for the ring electrode since, in contrast to Pt or Au, it allowed the detection of SPS down to micromolar concentrations without excessive interference from surface adsorption. To minimize the change in collection efficiency due to slow adsorption of the oxidation products, a relatively fast scan rate was used. Distinguishing between the cuprous ion oxidation current and the SPS, MPS, or thiolate oxidation currents at the ring was possible because of the well-separated oxidation waves for Cu^sup ^ (under diffusion limitation at 0.5 V) and SPS and MPS (under diffusion limitation at 0.9 V). When the ring was set at U^sub ring^ = 0.5 V, Cu was detected in collection mode: i.e., an increase in ring current was seen when Cu was formed at the disk. For the ring set at U^sub ring^ = 0.9 V, SPS was detected in the shielding mode (i.e., a decrease in ring current would be measured when SPS is consumed at the disk), and MPS or thiolate species were detected in the collection mode (i.e., an increase in ring current would be measured when thiolate species are formed at the disk). Note that the reduction of one SPS molecule leading to the formation of two MPS molecules at the disk would actually be detected as a net increase in ring current.