Mechanical stiffness and dissipation in ultrananocrystalline diamond microresonators

Abstract

We 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. We have characterized mechanical properties of ultrananocrystalline diamond ͑UNCD͒ thin films grown using the hot filament chemical vapor deposition ͑HFCVD͒ technique at 680°C, significantly lower than the conventional growth temperature of ϳ800°C. The films have ϳ4.3% sp 2 content in the near-surface region as revealed by near edge x-ray absorption fine structure spectroscopy. The films, ϳ1 m thick, exhibit a net residual compressive stress of 370Ϯ 1 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 838Ϯ 2 GPa. Resonant excitation and ring-down measurements in the kHz frequency range of the microcantilevers were conducted under ultrahigh vacuum ͑UHV͒ 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 790Ϯ 30 GPa. Based on these measurements, Poisson's ratio is estimated to be 0.057Ϯ 0.038. The quality factors ͑Q͒ 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 re...

Temperature dependence of mechanical stiffness and dissipation in ultrananocrystalline diamond

“…A discussion of room temperature modulus and dissipation in HFCVD grown UNCD films is provided elsewhere. [11] The dissipation observed is much higher than single crystal silicon cantilevers and polycrystalline diamond cantilevers. Stress assisted relaxation of the defects in the grain boundaries and at the surface are believed to be responsible observed dissipation.…”

“…The characterization of the films, fabrication of UNCD cantilevers and the experimental set up used for the resonant frequency and dissipation measurement are reported elsewhere. [11] 4 RESULTS AND DISCUSSION:…”

“…[11,12,22] Impurity atoms, dislocations, and other defects at grain boundaries and at surfaces undergo a transition from one state to another similar to two-level systems when under stress. Dissipation in such systems exhibit Debye peaks with a characteristic relaxation time τ which is unique to that transition.…”

“…Stress assisted relaxation of the defects in the grain boundaries and at the surface are believed to be responsible observed dissipation. [11,12] The temperature dependence of the resonant frequency (and hence the Young's modulus) and dissipation in UNCD cantilever beams is useful for determining the nature of such defects. In this article we report the temperature dependence of frequency, hence Young's modulus and quality factor in UNCD cantilever beams grown using the HFCVD process.…”

“…[10] We have recently reported mechanical properties and dissipation in HFCVD-grown UNCD cantilevers (growth temperature 680 °C). [11] We have measured a Young's modulus of 790± 30 GPa by resonant excitation of the fundamental mode of the cantilevers. Ring down measurements yielded a quality factor in the range of 5000 to 16000.…”