Development of 111 texture in Al films grown on SiO2/Si(001) by ultrahigh-vacuum primary-ion deposition

Abstract

A high degree of 111 preferred orientation with minimal mosaic spread has been shown by many researchers to be essential for electromigration resistance in Al-based interconnects. We have found that 111 texture can be greatly enhanced through the use of low-energy self-ion irradiation during deposition. In these experiments, 300-nm-thick Al layers were grown on SiO2 at 65 °C from highly ionized beams provided by an ultrahigh-vacuum primary-ion deposition (PID) source. Al+ ion energies EAl+ and ion/neutral ratios JAl+/JAl were independently varied from 10 to 120 eV and from 0% to 68%, respectively. All PID Al films exhibited very strong 111 preferred orientations, which increased with increasing EAl+ and/or JAl+/JAl, and azimuthally symmetric x-ray diffraction pole figures with no measurable tilt. The full width at half-maximum intensity Δω of 111 ω-rocking curves decreased continuously from 9.6° with EAl+=10 eV and JAl+/JAl=68% to 2.2° with JAl+/JAl=120 eV compared to 10.6° for films deposited by thermal evaporation. This was accompanied by a continuous decrease in the average grain size from 370 nm for thermal deposition to 90 nm with EAl+=120 eV. The PID Al films exhibited a columnar microstructure with weak competitive column growth. Changing the beam energy after the formation of a continuous layer had only a minor effect on film texture, indicating that the degree of ion-irradiation-induced preferred orientation is controlled during nucleation and/or coalescence while local pseudomorphic forces dominate thereafter. ω-rocking curves from a bilayer film consisting of a 20-nm-thick Al buffer layer grown by PID followed by a 280-nm-thick thermally evaporated Al overlayer were essentially identical to those obtained from 300-nm-thick single-layer PID Al films.