Method and apparatus of operating a scanning probe microscope
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 workable across varying environments, including gaseous, fluidic and vacuum.
1. A method of operating a scanning probe microscope (SPM) comprising:
generating relative motion between a probe and a sample,
detecting motion of the probe;
determining, from the detected probe motion, a probe deflection based on a probe-sample interaction, the probe deflection being substantially independent of parasitic probe deflection, wherein the parasitic probe deflection is caused by the background associated with operation of the SPM, and wherein the determining step includes subtracting the background from the detected probe motion by using a digital controller; and
controlling the SPM in real time using the determining step; and
wherein an amplitude of a probe-sample interaction is less than an amplitude of the parasitic probe deflection.
2. The method of claim 1 , further comprising identifying an instantaneous force associated with the interaction.
3. The method of claim 2 , 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.
4. The method of claim 3 , further comprising using the instantaneous force to maintain a setpoint during imaging.
5. The method of claim 4 , wherein the instantaneous force is a repulsive force.
6. The method of claim 4 , wherein a minimum controllable force corresponding to the instantaneous force is less than about 1000 μN.
7. The method of claim 6 , wherein the minimum controllable force is less than about 10 pN.
8. The method of claim 6 , wherein the detected probe motion is synchronously averaged to reduce the minimum controllable force.
9. The method of claim 1 , wherein the recovering step comprises determining a probe deflection magnitude resulting from the probe-sample interaction, and wherein the probe deflection magnitude corresponds to a force between the probe and the sample; and wherein the magnitude of the probe deflection is less than the magnitude of the parasitic probe deflection.
10. The method of claim 1 , wherein the parasitic probe deflection corresponds to any relative periodic motion between the probe and the sample when the probe is not interacting with the sample.
11. The method of claim 10 , wherein the background is caused by the hydrodynamic background associated with operation of the SPM.
12. The method of claim 1 , further comprising acquiring an image with a resolution of less than 100 nm.
13. The method of claim 1 , further comprising acquiring an image with a maximum tracking force of less than 100 pN.
14. The method of claim 1 , wherein the method is operable with any cantilever having a spring constant between about less than 0.1 N/m and 1000 N/m.
15. The method of claim 1 , wherein the generating step is controlled by a feedback loop using a pro-determined synchronous distance in each interaction period.
16. The method of claim 1 , wherein the digital controller performs at least one of lock-in amplification and synchronous averaging to subtract the background.
17. The method of claim 1 , wherein the digital controller is an FPGA.
18. A scanning probe microscope (SPM) comprising:
an actuator to generate relative motion between a probe and a sample,
a detector to detect motion of the probe;
a digital controller to determine, from the detected probe motion, a probe deflection based on a probe-sample interaction, the probe defection being substantially independent of parasitic probe deflection, wherein the parasitic probe deflection is caused by the background associated with operation of the SPM, wherein the parasitic probe deflection corresponds to any relative periodic motion between the probe and the sample when the probe is not interacting with the sample; and
wherein the controller subtracts the background from the detected probe motion, and
controls the SPM in real time using the probe deflection; and
wherein an amplitude of a probe-sample interaction is less than an amplitude of the parasitic probe deflection.
19. The SPM of claim 18 , wherein the digital controller is an FPGA.