Method and apparatus of operating a scanning probe microscope
View Patent ↗An improved mode of AFM imaging (Peak Force Tapping (PFT) Mode) uses force as the feedback variable to reduce tip-sample interaction forces while maintaining scan speeds achievable by all existing AFM operating modes. Sample imaging and mechanical property mapping are achieved with improved resolution and high sample throughput, with the mode being workable across varying environments, including gaseous, fluidic and vacuum. Ease of use is facilitated by eliminating the need for an expert user to monitor imaging.
1. A method of operating a scanning probe microscope (SPM) comprising:
generating periodic relative motion between a probe and a sample;
detecting motion of the probe;
recovering, from the detected probe motion, a substantially instantaneous force between the probe and sample;
automatically controlling the generating step in response to the recovering the instantaneous force step to maintain a feedback setpoint; and
wherein said automatically controlling step includes automatically controlling a gain in a corresponding feedback loop.
2. The method of claim 1 , further including automatically controlling Z-limit.
3. The method of claim 1 , further including providing relative scanning motion between the probe and the sample, and automatically controlling a scan rate associated with the providing step.
4. The method of claim 1 , wherein the feedback setpoint is a preset instantaneous force, and further including automatically optimizing the preset instantaneous force.
5. The method of claim 1 , wherein the generating step includes providing relative oscillatory motion between the probe and the sample, and wherein the instantaneous force is identified prior to the completion of one cycle of the oscillatory motion.
6. The method of claim 1 , wherein the instantaneous force is a repulsive force.
7. The method of claim 1 , wherein a minimum controllable force corresponding to the instantaneous force is less than about 1 nN.
8. The method of claim 7 , wherein the minimum controllable force is less than about 10 pN.
9. The method of claim 7 , wherein the detected probe motion is synchronously averaged to reduce the minimum controllable force.
10. A method of operating an AFM, the method comprising:
causing a probe to interact with a sample intermittently; and
maintaining a substantially identical peak interaction force between the probe and the sample in each modulation period of oscillation of the probe.
11. The method of claim 10 , wherein the maintaining step includes detecting an interaction force at a fixed synchronization distance relative to the beginning of a cycle of the causing step.
12. The method of claim 11 , further comprising using a feedback control circuit to control the causing step using the synchronously detected force.
13. The method of claim 12 , wherein the synchronously detected force spans a range of about 5 pN to 1 mN.
14. The method of claim 10 , wherein a controlled distance between the probe and the sample is greater than about 100 nm.
15. A method of operating an AFM, the method comprising:
causing a probe to interact with a sample intermittently;
providing scanning movement between the probe and the sample;
using feedback to maintain a setpoint force between the probe and the sample during the providing step; and
automatically determining the setpoint based on a noise background signal.
16. The method of claim 15 , further comprising automatically determining a feedback gain based on an oscillation magnitude associated with an onset of instability.
17. The method of claim 15 , further comprising detecting motion of the probe so as to detect an interaction force at a fixed synchronization distance relative to the beginning of a cycle of the causing step.
18. The method of claim 15 , further comprising controlling, using feedback, a separation between the probe and the sample using the synchronously detected force.
19. A method of operating a scanning probe microscope (SPM) comprising:
generating periodic relative motion between a probe and a sample;
interacting the probe and the sample;
detecting probe motion based on the interacting step;
recovering, from the detected probe motion, a substantially instantaneous force between the probe and the sample;
controlling, in response to the recovering the instantaneous force step, a mean position between the probe and the sample to maintain a preset instantaneous force between the probe and the sample using a feedback loop; and
automatically optimizing at least one of the feedback gain, the setpoint, and the scan rate of the SPM.
20. The method of claim 19 , wherein the recovering step includes recovering a probe-sample interaction that is substantially independent of parasitic probe deflection.
21. The method of claim 20 , wherein the parasitic probe deflection is caused by the hydrodynamic background associated with operation of the SPM.