Mechanical Stiffness and Dissipation in Ultrananocrystalline Diamond Films

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Doctor of Philosophy (PhD)
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Materials Science & Engineering
elastic properties
poisson's ratio
mechanical dissipation
quality factor
ultrananocrystalline diamond films
temperature dependence of elastic properties and dissipation
Condensed Matter Physics
Other Materials Science and Engineering
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Tetragonal sp3-bonded diamond has the highest known atomic density. The nature of the bond and its high density enable diamond to have superior physical properties such as the highest Young’s modulus and acoustic velocity of all materials, and excellent tribological properties. Recently, conformal thin diamond films have been grown at CMOS-compatible temperatures in the form known as ultrananocrystalline diamond (UNCD). These make diamond promising for high frequency micro/nanomechanical devices. We have measured the Young’s modulus (E), Poisson’s ratio and the quality factors (Q) for microfabricated overhanging ledges and fixed-free beams composed of UNCD films grown at lower temperatures. The overhanging ledges exhibited periodic undulations due to residual stress. This was used to determine a biaxial modulus of 838 ± 2 GPa. Resonant excitation and ring down measurements of the cantilevers were conducted under ultra high vacuum (UHV) conditions on a customized atomic force microscope to determine E and Q. At room temperature we found E = 790 ± 30 GPa, which is ~20 % lower than the theoretically predicted value of polycrystalline diamond, an effect attributable to the high density of grain boundaries in UNCD. From these measurements, Poisson’s ratio for UNCD is estimated for the first time to be 0.057±0.038. We also measured the temperature dependence of E and Q in these cantilever beams from 60 K to 450 K. Mechanical stiffness of these cantilevers increased linearly with the reduction in temperature until ∼160 K where it then saturates. Reduction in the modulus of the film with temperature is slightly higher than that of single crystal diamond(averaged over all directions). We measured extremely low temperature coefficient of resonant frequency and results are promising for applications in MEMS and NEMS wireless devices and biosensors. The room temperature Q varied from 5000 to 16000 and showed a moderate increase as the cantilevers were cooled below room temperature followed by a characteristic low temperature plateau. Overall, results show that grain boundaries of UNCD films play a key role in determining its thermomechanical stability and mechanical dissipation. These results are extremely useful in understanding and controlling the dissipation in nanocrystalline materials.

Robert W Carpick
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