E. M. Zielinski

Thermal strain and stress in copper thin films

Abstract Desired improvements in the performance and reliability of integrated circuit interconnects may necessitate a move from aluminum alloys to copper. Before copper is adopted, however, characterization of the thermal stress behavior of copper thin films is necessary in order to identify mechanical reliability concerns and to determine differences from aluminum behavior. In this study, the behavior of copper films is evaluated to determine effects of film texture, thickness, and the presence of a passivation layer. Mechanistic models based on bulk deformation maps and interface-controlled dislocation glide are compared with the measured behavior. A preferred [111] grain orientation is found to slightly increase the stress throughout a thermal cycle as compared with a film with random grain orientation. An inverse relationship between film thickness and strength, similar to that seen in aluminum, is quantified. The presence of a passivation layer significantly reduces stress relaxation at high temperatures, resulting in behavior that closely resembles that of unpassivated aluminum films. Neither model adequately predicts the thickness and passivation effects over the entire temperature and stress range, emphasizing the need for more characterization of the flow processes active in metallic thin films.

Effects of barrier layer and annealing on abnormal grain growth in copper thin films

Abnormal (100) grain growth has been characterized in predominantly (111)‐textured Cu thin films as a function of deposition temperature, annealing temperature and the presence of a Ta or W underlayer. For films deposited at room temperature, bimodal grain size distributions are observed at annealing temperatures at or above 150 °C for Cu on Ta and 100 °C for Cu on W. Suppression of (100) abnormal grain growth was achieved by depositing Cu on either barrier layer at 150 °C. A bimodal grain size distribution was still observed for the film deposited on W at 150 °C but the large grains forming this distribution were found to be (111) oriented. These results are explained as the result of competition between strain energy minimization and surface and interface energy minimization. The (100) growth is shown to be driven by a reduction of the orientation‐dependent strain energy that builds up due to the elastic anisotropy of Cu. Films deposited at higher temperatures have a lower yield stress which limits the ...

The influence of strain energy on abnormal grain growth in copper thin films

Biaxial stress and strain in (100) and (111) oriented grains have been measured as a function of annealing temperature for a Cu film on an oxidized Si substrate which exhibits abnormal (100) grain growth. The observed behavior indicates isostrain averaging, which is consistent with grain growth that is controlled by strain energy density minimization. In contrast, two films which do not exhibit (100) abnormal grain growth appear to follow isostress averaging. Strain energy density minimization in this situation favors (111) grain growth.

Thickness dependence of the twin density in YBa2Cu3O7−δ thin films sputtered onto MgO substrates

The lengths and spacings of twins in YBa2Cu3O7−δ thin films deposited onto MgO substrates have been measured by transmission electron microscopy as a function of film thickness t, for t ranging from 50 to 1400 nm. The twin length is linear in t, while the twin spacing follows a t1/2 dependence. This form for the twin spacing is consistent with the prediction of a simple free energy expression for the twinning transformation.

Effects of barrier layer and processing conditions on thin film Cu microstructure

The crystallographic texture and grain size of sputtered Cu films were characterized as a function of deposition temperature, barrier layer material, and vacuum conditions. For Cu deposited in a HV chamber, (111) Cu texture was found to weaken with increasing deposition temperatures on W, amorphous C and Ta barrier layers, each deposited at 30°C. Conversely, under identical Cu deposition conditions, texture was found to strengthen with increasing deposition temperature on Ta deposited at 100°C. Median Cu grain size varied parabolically with deposition temperature on all barrier layers and was slightly higher on the 100°C Ta at a given Cu deposition temperature, relative to the other underlayers. For depositions in an UHV chamber, Cu texture was found to strengthen with increasing Cu deposition temperature, independent of Ta deposition temperature. Median Cu grain size, however, was still higher on 100°C Ta than on 30°C Ta. The observed differences between the two different chambers suggest that the trend of weak texture at elevated deposition temperatures may be related to contamination. Characterization of the Ta underlayers revealed that the strengthened texture of Cu films deposited on 100°C Ta is likely related to textural inheritance.

The Influence of Strain Energy Minimization on Abnormal Grain Growth in Copper Thin Films

Sputtered Cu films on Si, Al and Cu substrates were thermally cycled to 300 °C at a rate of 6 °/min, which induced an applied thermal strain that was compressive, tensile and zero, respectively. Microstructural characterization of the annealed films revealed abnormal (100) grain growth in the films on Al and Si, but not Cu substrates. In addition, symmetric x-ray diffraction scans demonstrated that the films in which abnormal grain growth was observed were primarily (100) in orientation. In contrast, the Cu film on a Cu substrate was largely randomly oriented after cycling, with a small degree of (111) preferred orientation. These results are consistent with a strain energy driving force for abnormal grain growth, which predicts that the growth should occur in compression, as on the Si substrate, or in tension, as on the Al substrate, but not when there is no applied thermal strain, as on the Cu substrate.

Effect of Copper Film Thickness on Stress and Strain in Grains of Different Orientation

Models describing the effect of grain orientation on dislocation glide in a thin film are reviewed; it is predicted that differences in stress-temperature behavior should exist between grains of different orientation in a polycrystalline copper film. Direct x-ray evaluation of strains in {111} and {100| grains within primarily {111} textured copper films shows that these stresses are film thickness or grain size dependent. The {111} oriented grains behave as expected, with room temperature flow stress increasing linearly with 1/film thickness. The flow stress of grains of {100} orientation, however, is approximately constant with film thickness and is not well represented by the models. It is proposed that simplifying assumptions about orientation dependent yield may only be appropriate for the majority grain orientation population in a textured film.