The interaction of Cu with Si separated by thin (50 nm) layers of tantalum, Ta2N, and a nitrogen alloy of Ta has been investigated to determine the factors that affect the success of these materials as diffusion barriers to copper. Intermixing in these films was followed as a function of annealing temperature by in situ resistance measurements, Rutherford backscattering spectra, scanning electron microscopy, and cross-section transmission electron microscopy. Ta prevents Cu-silicon interaction up to 550 °C for 30 min in flowing purified He. At higher temperatures, copper penetration results in the formation of η″-Cu3Si precipitates at the Ta-Si interface. Local defect sites appear on the surface of the sample in the early stages of this reaction. The Ta subsequently reacts with the substrate at 650 °C to form a planar hexagonal-TaSi2 layer. Ta silicide formation, which does not occur until 700 °C in a Ta-Si binary reaction couple, is accelerated by the presence of Cu. Nitrogen-alloyed Ta is a very similar diffusion barrier to Ta. It was found that Ta2N is a more effective barrier to copper penetration, preventing Cu reaction with the substrate for temperatures up to at least 650 °C for 30 min. In this case, local Cu-Si reaction occurs along with the formation of a uniform Ta5Si3 layer at the Ta2N-Si interface.

We have investigated the effectiveness and failure mechanism of thin tantalum layers as diffusion barriers to copper. 50 nm tantalum films were sputtered onto unpatterned single-crystal 〈100〉 Si wafers and overlaid with 100 nm Cu. Material reactions in these films were followed as a function of annealing temperature by in situ resistance measurements, and characterized by Rutherford backscattering spectroscopy and cross-section transmission electron microscopy. While pure Cu on Si reacts at 200 °C, the Ta film prevents Cu silicon interaction up to 600 °C. At higher temperatures, reaction of the Si substrate with Ta forms a planar layer of hexagonal TaSi2. Cu rapidly penetrates to the Si substrate, forming η″-Cu3Si precipitates at the Ta-Si2-Si interface.

The structure and interfacial reaction in sputtered Mo-Si multilayers have been studied using cross-section transmission electron microscopy, electron diffraction, Rutherford backscattering, and low-angle x-ray diffraction. Low-temperature (T<550 °C) annealing was performed in a rapid-thermal-annealing furnace and in situ in the microscope. No solid-state amorphization was observed, in spite of the presence of amorphous alloy interfacial layers in the as-deposited structure. Instead, the amorphous interlayers crystallize, and growth of the crystalline product, hexagonal-MoSi2, proceeds. The bilayer period contracts during the reaction, as the disilicide is more dense than its constituents.

Reactions upon rapid thermal annealing of sputtered Ti-Si multilayers have been studied by cross-section and through-foil transmission electron microscopy, glancing-angle Rutherford backscattering, and x-ray diffraction. The compositions of the samples are 40 at. % Ti, 60 at. % Si and 60 at. % Ti, 40 at. % Si, and the bilayer periodicity is about 10 nrn. The silicon layers in the as-deposited films are amorphous; the titanium layers are polycrystalline hcp. After a 30-8 anneal at 455 ·C, significant interdiffusion occurs and we observed the formation of an amorphous Ti-Si alloy by interfacial reaction. The metastable disilicide, C49 TiSi z , nucleated along with a small amount of TiSi in the sample with higher silicon content (60%) upon annealing at 550 ·C for 10 s, but the amorphous alloy remained as the only product of reaction in the 40-at. % Si sample.

We demonstrate that depositing Ta diffusion barriers under ultra-high vacuum conditions without in situ oxygen dosing allows for variations both in microstructure and in the concentration of chemical impurities that severely degrade barrier performance. The effects of deposition pressure, in situ oxygen dosing at interfaces, hydrogen and oxygen contamination, and microstructure on diffusion barrier performance to Cu diffusion for electron-beam deposited Ta are presented. 20 nm of Ta diffusion barrier followed by a 150 nm Cu conductor were deposited under ultra-high vacuum (UHV, deposition pressure of 1×10−9 to 5 ×10−8 Torr) and high vacuum (HV, deposition pressure of 1×10−7 to 5×10−6 Torr) conditions onto 〈100〉 Si. In situ resistance furnace measurements, Auger compositional depth profiling, secondary ion mass spectrometry, and forward recoil detection along with scanning and transmission electron microscopy were used to determine the electrical, chemical, and structural changes that occurred in thin-film Ta diffusion barriers upon annealing. Undosed HV deposited Ta barriers failed from 560 to 630 °C, while undosed UHV barriers failed from 310 to 630 °C. For UHV Ta barriers, in situ oxygen dosing during deposition at the Cu/Ta interface increased the failure temperatures by 30–250 °C and decreased the range of failure temperatures to 570–630 °C. Undosed UHV Ta barriers have no systematic relationship between failure temperature and deposition pressure, although correlations between breakdown temperature, oxygen and hydrogen concentrations, and microstructural variations were measured.