Adiga, Vivekananda P
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Publication Mechanical Stiffness and Dissipation in Ultrananocrystalline Diamond Films(2010-05-17) Adiga, Vivek PTetragonal 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.Publication Angle-Resolved Environmental X-Ray Photoelectron Spectroscopy: A New Laboratory Setup for Photoemission Studies at Pressures up to 0.4 Torr(2012-09-27) Wabiszewski, Graham E; Mangolini, F.; Adiga, Vivek P; Åhlund, J.; Egberts, P.; Streller, F.; Backlund, K.; Carpick, Robert W; Karlsson, P. G; Wannberg, B.The paper presents the development and demonstrates the capabilities of a new laboratory-based environmental X-ray photoelectron spectroscopy system incorporating an electrostatic lens and able to acquire spectra up to 0.4 Torr. The incorporation of a two-dimensional detector provides imaging capabilities and allows the acquisition of angle-resolved data in parallel mode over an angular range of 14° without tilting the sample. The sensitivity and energy resolution of the spectrometer have been investigated by analyzing a standard Ag foil both under high vacuum (10−8 Torr) conditions and at elevated pressures of N2 (0.4 Torr). The possibility of acquiring angle-resolved data at different pressures has been demonstrated by analyzing a silicon/silicon dioxide (Si/SiO2) sample. The collected angle-resolved spectra could be effectively used for the determination of the thickness of the native silicon oxide layer.Publication Temperature dependence of mechanical stiffness and dissipation in ultrananocrystalline diamond(2009-05-28) Adiga, Vivekananda P; Sumant, A V; Suresh, S; Gudeman, C; Auciello, O; Carpick, R W; Carlisle, J AUltrananocrystalline diamond (UNCD) films are promising for radio frequency micro electro mechanical systems (RF-MEMS) resonators due to the extraordinary physical properties of diamond, such as high Young’s modulus, quality factor, and stable surface chemistry. UNCD films used for this study are grown on 150 mm silicon wafers using hot filament chemical vapor deposition (HFCVD) at 680°C. UNCD fixed free (cantilever) resonator structures designed for the resonant frequencies in the kHz range have been fabricated using conventional microfabrication techniques and are wet released. Resonant excitation and ring down measurements in the temperature range of 138 K to 300 K were conducted under ultra high vacuum (UHV) conditions in a custom built UHV AFM stage to determine the temperature dependence of Young’s Modulus and dissipation (quality factor) in these UNCD cantilever structures. We measured a temperature coefficient of frequency (TCF) of 121 and 133 ppm/K for the cantilevers of 350 ìm and 400 ìm length respectively. Young’s modulus of the cantilevers increased by about 3.1% as the temperature was reduced from 300 K to 138 K. This is the first such measurement for UNCD and suggests that the nanostructure plays a significant role in modifying the thermo-mechanical response of the material. The quality factor of these resonators showed a moderate increase as the cantilevers were cooled from 300 K to 138 K. The results suggest that surface and bulk defects significantly contribute to the observed dissipation in UNCD resonators.Publication Mechanical stiffness and dissipation in ultrananocrystalline diamond microresonators(2009-06-02) Adiga, Vivekananda P; Sumant, A V; Suresh, S; Gudeman, C; Auciello, O; Carpick, Robert W; Carlisle, J AWe have characterized mechanical properties of ultrananocrystalline diamond UNCD thin films grown using the hot filament chemical vapor deposition HFCVD technique at 680 °C, significantly lower than the conventional growth temperature of 800 °C. The films have 4.3% sp2 content in the near-surface region as revealed by near edge x-ray absorption fine structure spectroscopy. The films, 1 m thick, exhibit a net residual compressive stress of 3701 MPa averaged over the entire 150 mm wafer. UNCD microcantilever resonator structures and overhanging ledges were fabricated using lithography, dry etching, and wet release techniques. Overhanging ledges of the films released from the substrate exhibited periodic undulations due to stress relaxation. This was used to determine a biaxial modulus of 8382 GPa. Resonant excitation and ring-down measurements in the kHz frequency range of the microcantilevers were conducted under ultrahigh vacuum UHV conditions in a customized UHV atomic force microscope system to determine Young’s modulus as well as mechanical dissipation of cantilever structures at room temperature. Young’s modulus is found to be 79030 GPa. Based on these measurements, Poisson’s ratio is estimated to be 0.0570.038. The quality factors Q of these resonators ranged from 5000 to 16000. These Q values are lower than theoretically expected from the intrinsic properties of diamond. The results indicate that surface and bulk defects are the main contributors to the observed dissipation in UNCD resonators.Publication Thermomechanical Stability of Ultrananocrystalline Diamond(2012-03-13) Adiga, Vivekananda P; Suresh, Sampath; Datta, Arindom; Carpick, Robert W; Carlisle, John AWe have measured mechanical stiffness and dissipation in ultrananocrystalline diamond (UNCD) from 63 K to 450 K using microcantilever resonators in a custom ultrahigh vacuum (UHV) atomic force microscope. UNCD exhibits a temperature coefficient of modulus that is found to be extremely low: -26 ppm/K, which is close to the previously measured value of -24 ppm/K for single crystal diamond. The magnitude and the temperature dependence of dissipation are consistent with the behavior of disordered systems. The results indicate that defects, most likely at the grain boundaries, create the dominant contribution to mechanical dissipation. These measurements of modulus and dissipation versus temperature in this temperature range in UNCD establish the nanostructure’s effect on the thermomechanical stability and suggest routes for tailoring these properties.