Measuring anisotropic mechanical properties of thin films
AssigneeMassachusetts Institute of Technology
InventorsJohn A. Rogers, Keith A. Nelson, Lisa Dhar
Life of this patent
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Abstract
Anisotropic mechanical properties of thin films are measured by exciting time-dependent waveguide acoustic modes in the thin film sample with a pair of excitation pulses from an excitation laser. The waveguide acoustic modes are then optically detected by diffracting a probe laser beam off the excited modes. The probe beam is detected to generate an electronic signal. The anisotropic moduli and related properties in the film are determined by analyzing the electronic signal using a mathematical inversion procedure.
Claims18
A method for determining the anisotropy between an out-of-plane and an in-plane material property of a thin sample, wherein an exposed surface of said thin sample extends substantially along said plane, comprising:
- exciting time-dependent waveguide acoustic modes in the sample by directing two time-coincident laser pulses onto the sample so that they overlap in an excitation region and interfere to form an excitation field having a known wavevector,
- detecting said waveguide acoustic modes by directing a probe beam onto the excitation region so that it is diffracted, said probe beam having a temporal width that is comparable to the detectable presence of said waveguide acoustic modes;
- analyzing the diffracted probe beam to measure phase velocities of said waveguide acoustic modes, and
- determining the anisotropy between the out-of-plane and the in-plane mechanical property of said sample by:
- proposing stiffness tensors in directions of interest,
- calculating phase velocities based on said proposed stiffness tensors,
- comparing said calculated phase velocities to said measured phase velocities,
- repeating said proposing, calculating, and comparing until said calculated phase velocities match said measured phase velocities to a desired degree, and
- determining the anisotropy between the out-of-plane and the in-plane mechanical property in the direction of interest from the stiffness tensors for which the calculated phase velocities match the measured phase velocities to said desired degree.
A method for determining the anisotropy between an out-of-plane and an in-plane mechanical property of a thin sample, wherein an exposed surface of said thin sample extends substantially along said plane, comprising:
- exciting time-dependent waveguide acoustic modes in the sample by directing two time-coincident laser pulses onto the sample so that they overlap in an excitation region and interfere to form an excitation field having a known wavevector,
- detecting said waveguide acoustic modes by directing a probe beam onto the excitation region so that it is diffracted, said probe beam having a temporal width that is comparable to the detectable presence of said waveguide acoustic mode;
- measuring a phase velocity or frequency of said waveguide acoustic modes from said diffracted probe beam;
- and
- analyzing the phase velocity or frequency of said acoustic waveguide modes to determine the anisotropy between the out-of-plane and the in-plane mechanical property of said sample.
A method for determining an out-of-plane mechanical property of a thin sample, comprising:
- exciting time-dependent waveguide acoustic modes in the sample by directing two time-coincident laser pulses onto the sample so that they overlap in an excitation region and interfere to form an excitation field having a known wavevector;
- detecting the waveguide acoustic modes by directing a probe beam onto the excitation region so that it is diffracted, with the probe beam having a temporal width that is comparable to the detectable presence of the waveguide acoustic modes;
- measuring a phase velocity or frequency of the waveguide acoustic modes from the diffracted probe beam;
- and
- analyzing the phase velocity or frequency of the waveguide acoustic modes to determine the out-of-plane mechanical property of the sample.
Dependencies are inferred from the claim text.