The mechanism of copper chemical vapor deposition (CVD) was studied. CVD of copper film is being intensively studied as an alternative fabrication technique for interconnect metallization in future generations of deep sub-micron ultra large scale integrated (ULSI) circuits. Copper offers many intrinsic advantages over aluminum, including lower resistivity and better electromigration resistance. The CVD process offers conformal coverage over high aspect ratio features, unlike physical vapor deposition (PVD) method. Copper(I) hexafluoroacetylacetonate trimethylvinylsilane, Cu(hfac)(tmvs), is one of the most promising copper precursor, because it exists as liquid even at room temperature and shows high vapor pressure and relatively good thermal stability.
The pure copper films were prepared on titanium nitride (TiN) substrates in a low-pressure warm-wall reactor using Cu(hfac)(tmvs) as the precursor. The microstructure and resistivity of the copper film deposited at various temperature and pressure were investigated. The effects of the thermal annealing on the microstructure and the electrical resistivity of the copper films were also investigated. Copper films were deposited with and without d. c. substrate bias, and the effect of the substrate bias on the deposition was discussed.
The effects of the deposition temperature on the microstructure and the electrical resistivity of the CVD copper films were studied at the deposition temperatures between 160℃ and 330℃. The films deposited at below 200℃, where the deposition is limited by surface reaction, were dense and had low resistivity of approximately 2μ Ωㆍcm. Moreover, they exhibited excellent step coverage. However, the films deposited at above 200℃, where the mass transport processes become important, were composed of poorly connected globular grains, resulting in considerably high resistivities and rough surfaces. The grain size of the copper film increased linearly with deposition temperature in both kinetic regions. However, at the junction between two regions, a transitional behavior was observed. The copper deposited on (111) TiN exhibited a preferential growth in the direction of (111).
The effect of the total reactor pressure and the partial pressure of Cu(hfac)(tmvs) on deposition were studied. The activation energies for the copper deposition was found to be approximately 23 kcal/mol at the total pressure ranged from 0.25 to 1.0 Torr. The deposition rates decreased with increasing total pressure at low temperatures, however, at high temperatures the rate did not change with pressure. The transition temperature of the kinetic region increased with pressure. The decrease of deposition rate was explain by the inhibition of the reaction byproducts. The deposition rate at constant total pressure increased with partial pressure of the precursor from 0.04 to 0.05 Torr after which it became constant over the range of 0.05 to 0.09 Torr. The surface roughness and the resistivity of the film decreased with increasing total pressure. The optimum deposition condition was concluded to be 180℃, 0.5 Torr from the property of the film and the deposition rate. At this condition, the deposition rate was approximately 45 nm/min on TiN substrate, and resistivity was 2.2μ Ωㆍcm at the film thickness of 400 nm.
The changes of the microstructure and the electrical resistivity of the copper films after annealing were studied. The copper films with a thickness of 400 nm were prepared on TiN. The deposited films were subjected to be annealed at 450℃ for 30 min. in He, Ar and $H_2(10%)/Ar$ ambients. After annealing, the surfaces of the copper films had tendency to become smooth, and the grain size enlarged in all deposition temperature regions. After annealing, the resistivity of the copper film deposited in the surface reaction limited region decreased slightly by the grain growth, but ones deposited in the mass transport limited region decreased drastically by coalescence of the grains, causing the improvements of the electrical connections among copper grains. $H_2(10%)/Ar$ was more effective than Ar as an ambient gas for the post annealing of the copper films, because smaller resistivity and larger grain size of the annealed film was given.
The d. c. substrate bias was applied during the CVD process to change the adsorption behaviors of the reactant. When the -30 V was applied to the substrate, the rate of deposition reaction of copper increased and the surface morphology improved both on TiN and on $SiO_2$. When +30 V was applied to substrate, there is negligible effect on the deposition rate compared with the case without bias. No change was observed in the chemical composition of the copper film deposited with substrate bias. These phenomena were explained due to the alignment of Cu(hfac) on the substrate surface. The calculated dipole moment of Cu(hfac) was 1.41 D, and its direction was from copper atom to hfac in Cu(hfac). The local electric fields due to surface roughness may affect the adsorption behavior of the precursor, especially the adsorption direction of the molecular dipole. Resulting from the overlapping population (OP) value analysis, the improvement of deposition rate under negative substrate bias was explained due to the adsorption of copper atom in Cu(hfac) species directly onto the substrate by the local fields generated by substrate bias.
