John C. Bravman

Mechanical deflection of cantilever microbeams: A new technique for testing the mechanical properties of thin films

The mechanical deflection of cantilever microbeams is presented as a new technique for testing the mechanical properties of thin films. Single-layer microbeams of Au and SiO 2 have been fabricated using conventional silicon micromachining techniques. Typical thickness, width, and length dimensions of the beams are 1.0,20, and 30 μm, respectively. The beams are mechanically deflected by a Nanoindenter, a submicron indentation instrument that continuously monitors load and deflection. Using simple beam theory and the load-deflection data, the Youngs moduli and the yield strengths of thin-film materials that comprise the beams are determined. The measured mechanical properties are compared to those obtained by indenting similar thin films supported by their substrate.

Separation of film thickness and grain boundary strengthening effects in Al thin films on Si

We have measured stress variations with temperature as a function of film thickness and a given grain size in pure Al and Al–0.5% Cu films on Si substrates. The variation in thickness for a given grain size is brought about by using the same film and the repeated controlled growth and dissolution of a barrier anodic oxide which can be grown uniformly on the film. Stress measurements were made as a function of temperature by measuring wafer curvature after successively removing each 0.1 μm of Al film. The components of strengthening due to the film thickness and the presence of grain boundaries were separated by assuming that the flow stress of the film is simply the sum of these two components. It is found that strengthening due to film thickness varies inversely with the thickness, which is consistent with results obtained by us using laser-reflowed films in an earlier work. The Hall–Petch coefficients calculated from the strengthening due to the grain boundaries are slightly higher than those reported for bulk Al. However, it is also recognized that the variation of the flow stress as g−1, where g is the grain size, is more plausible than that predicted by the Hall–Petch relation (i.e., as g−1/2). The variations of these two components with temperature, and under tension and compression, are discussed.

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.

Scanning X-ray microdiffraction with submicrometer white beam for strain/stress and orientation mapping in thin films

Scanning X-ray microdiffraction (microSXRD) combines the use of high-brilliance synchrotron sources with the latest achromatic X-ray focusing optics and fast large-area two-dimensional-detector technology. Using white beams or a combination of white and monochromatic beams, this technique allows for the orientation and strain/stress mapping of polycrystalline thin films with submicrometer spatial resolution. The technique is described in detail as applied to the study of thin aluminium and copper blanket films and lines following electromigration testing and/or thermal cycling experiments. It is shown that there are significant orientation and strain/stress variations between grains and inside individual grains. A polycrystalline film when investigated at the granular (micrometer) level shows a highly mechanically inhomogeneous medium that allows insight into its mesoscopic properties. If the microSXRD data are averaged over a macroscopic range, results show good agreement with direct macroscopic texture and stress measurements.

Synthesis and properties of YBa2Cu3O7 thin films grown in situ by 90° off-axis single magnetron sputtering

Abstract High quality superconducting films of YBa 2 Cu 3 O 7− x were deposited in situ using single target 90° off-axis sputtering. We have investigated their superconducting DC and RF properties, their normal state properties, and their microstructures. These films are distinctly different from bulk crystals and post-deposition annealed films. Sharp superconducting transition temperatures can be reproducibly obtained by control of deposition parameters. The T c can be varied from 75 to 89 K. The optimization of properties other than T c and the control of film texture occur under conditions different from those for which the highest T c is obtained. Normal state conductivities are as high as or higher than those of single crystals. Critical current densities reach 6 × 10 7 A/cm 2 at 4.2 K. All the above properties are relatively insensitive to compositional variations. The T c s have a much weaker dependence on the c -axis lattice parameters than do those of bulk samples. The measured low-temperature penetration depth is 1400 A and surface resistance at 4.2 K and 10 GHz is as low as 16 μΩ. Microstructural studies show sharp interfaces between films and their substrates and a variety of defect structures. Many of the properties of in situ films can be explained by clean grain boundaries and the characteristics of the surface growth occuring during in situ deposition.

Molecular beam epitaxial growth of layered Bi-Sr-Ca-Cu-O compounds

Abstract The in situ epitaxial growth of Bi-Sr-Ca-Cu-O films by molecular beam epitaxy (MBE) is reported. The suitability of various oxidants for the MBE growth of cuprate superconductors is discussed and the use of ozone described. Molecular beams of the constituents were periodically shuttered to grow various Bi 2 Sr 2 Ca n −1 Cu n O x phases, including 2201, 2212, 2223, 2245, and layered 2212/2223 and 2223/2234 mixtures. Some of the films grown in this way were superconducting as grown. The ability of MBE to grow layered, probably metastable Bi 2 Sr 2 Ca n −1 Cu n O x films is demonstrated. This is a major step in the development of growth method capable of producing custom layered combinations of perovskite-related compounds.

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 ...

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Mechanical properties and microstructural characterization of Al-0.5%Cu thin films

Using a wafer curvature technique we have studied the variation of stress with tem-perature in Al-0.5%Cu thin films deposited on oxidized silicon wafers. Concurrently, the microstructural changes in the films induced by the thermal cycling inherent to this technique were studied with in-situ transmission electron microscopy heating experi-ments. On heating an as-sputtered film a stress drop occurs, corresponding to the onset of grain growth. The in-situ TEM experiments indicate that the extent of grain growth is significantly altered by the presence of compressive stresses in the film. During cool-ing, dislocation loops nucleate on {111} planes inclined to the film surface, although the grain size plays an important role in determining the extent to which this mechanism can account for the deformation. A native oxide can influence the stress levels in the film by pinning one end of the dislocation loops. Upon cooling below 200° C a rapid increase in stress occurs. Although this increase has been attributed to hardening due to the precipitation of excess copper, no evidence of precipitate-dislocation interactions were observed.

High spatial resolution grain orientation and strain mapping in thin films using polychromatic submicron x-ray diffraction

The availability of high brilliance synchrotron sources, coupled with recent progress in achromatic focusing optics and large area 2D detector technology, have allowed us to develop an X-ray synchrotron technique capable of mapping orientation and strain/stress in polycrystalline thin films with submicron spatial resolution. To demonstrate the capabilities of this instrument, we have employed it to study the microstructure of aluminum thin film structures at the granular and subgranular level. Owing to the relatively low absorption of X-rays in materials, this technique can be used to study passivated samples, an important advantage over most electron probes given the very different mechanical behavior of buried and unpassivated materials.