Method and apparatus for obtaining quantitative measurements using a probe based instrument
View Patent ↗A method includes determining the point at which a tip of a probe based instrument contacts a sample and/or the area of that contact by dynamically oscillating a cantilever of the instrument in flexural and/or torsional modes. The method additionally includes using oscillation characteristics, such as amplitude, phase, and resonant frequency, to determine the status of the contact and to provide quantitative data. Static and quasi-static measurements, including contact stiffness and elastic modulus, can be obtained from the thus obtained data. Quasistatic measurements, such as creep and viscoelastic modulus, can be obtained by repeating the static measurements for a number of force profiles at different force application rates and correlating the resultant data using known theories.
1. A method comprising:
indenting a sample with a probe of a probe-based instrument, the probe including a cantilever having a base and a free end, and the indenting occurring in an indentation cycle having a loading phase in which the probe is driven into the sample to form the indentation and an unloading phase in which the probe is withdrawn from the sample,
during the indentation cycle, acquiring at least one datum required for obtaining a local mechanical property measurement; and
using the acquired datum, obtaining the local mechanical property measurement.
2. The method of claim 1 , wherein
the probe is driven to oscillate during at least part of the indentation process,
the acquiring step comprises detecting a change in at least one parameter of probe oscillation resulting from probe/sample interaction during the indentation cycle; and wherein
the obtaining step comprises determining at least one of an initial contact point occurring when the probe first contacts the sample during the loading phase and a release point occurring when the probe releases from the sample during the unloading phase.
3. The method in claim 2 , wherein the initial contact point is detected by detecting a change in one or more of 1) probe oscillation amplitude, 2) probe oscillation phase, and 3) probe oscillation resonant frequency occurring as a result initial probe/sample contact.
4. The method in claim 2 , wherein the release point is detected by detecting a change in one or more of 1) probe oscillation amplitude 2) probe oscillation phase, and 3) probe oscillation resonant frequency occurring as a result of initial tip/sample separation.
5. The method in claim 2 , where the datum is obtained by detecting a change in one or more of an oscillation amplitude an oscillation phase of sub-resonant oscillation of the cantilever.
6. The method in claim 2 , where the datum is collected by detecting a change in one or more of an amplitude and a frequency of oscillation of a probe oscillating in torsional resonance.
7. The method of claim 1 , wherein the mechanical property includes one or more of elastic modulus and visco-elasticity and is derived from a determined load/displacement relationship of the cantilever probe at a given contact area.
8. The method of claim 1 , further comprising counteracting lateral forces imposed on the probe during the indentation cycle using a closed loop feedback process to maintain a characteristic parameter of the probe a constant.
9. The method of claim 8 , wherein the probe includes an active cantilever, and wherein the counteracting step comprises energizing the active cantilever under feedback to impose a force impact on the probe that is of opposite sign to a lateral force on the probe induced by motion of the probe upon interacting with the sample.
10. The method of claim 1 , further comprising:
(A) determining an initial contact point occurring when the probe first contacts the sample during the loading phase, and
(B) determining a release point occurring when the probe first separates from a bottom of the indentation during the unloading phase of the indentation cycle, the release point being determined by detecting a change in probe dynamic characteristics resulting from interaction between the probe and the indentation.
11. The method of claim 10 , further comprising driving the probe to oscillate in torsional resonance; and wherein
the initial contact point is determined by noting the onset of a contact peak f c of probe resonant frequency that occurs when the probe first contacts the sample during the loading phase of the indentation cycle.
12. The method of claim 10 , further comprising driving the probe to oscillate in torsional resonance; and wherein
the initial release point is determined by noting the onset of a contact peak f c of probe resonant frequency that occurs when the probe is withdrawn from the sample during the unloading phase of the indentation cycle.
13. The method of claim 12 , further comprising determining a maximum penetration depth P max , of probe penetration into the sample during the indentation cycle by monitoring the position of the probe at the juncture between the loading phase and unloading phase of the indentation cycle.
14. The method of claim 1 , further comprising
driving the probe to oscillate in torsional resonance; and
determining an initial contact point by detecting a change in one or more of one of a torsional oscillation resonant frequency and a torsional oscillation amplitude occurring as a result of initial probe/sample contact during the loading phase.