Force measurement with real-time baseline determination
An atomic force microscope (AFM) and corresponding method to provide low force (sub-20 pN) AFM control and mechanical property measurement is provided. The preferred embodiments employ real-time false deflection correction/discrimination by adaptively modifying the drive ramp to accommodate to deflection artifacts.
1. A method of detecting a force between a sample and a probe of an AFM, the method comprising:
positioning at least one of the probe and the sample at a location of interest of the sample;
moving at least one of the probe and the sample to lessen a separation therebetween and cause the two to interact;
measuring a deflection of the probe based on the moving step;
discriminating a deflection artifact from a deflection due to probe-sample interaction from the measured deflection data by continuously identifying a baseline during the moving step to derive an artifact free deflection and compare it with a predefined trigger force;
retracting the probe from the sample if the artifact free deflection substantially corresponds to the trigger force; and
determining the force between the sample and the probe, wherein the force is less than 20 pN.
2. The method of claim 1 , further comprising using the force as a trigger to change a parameter associated with the moving step.
3. The method of claim 2 , wherein the parameter is at least one of a speed, a direction and a force gradient.
4. The method of claim 1 , wherein the discriminating step includes comparing a drive ramp to a fit line based on data corresponding to the deflection, and further comprising extrapolating the baseline based on the comparison.
5. The method of claim 4 , wherein the fit line is determined by performing a least squares fit.
6. The method of claim 5 , further comprising repeating the comparing and extrapolating steps so as to provide a rolling baseline until a threshold trigger is met.
7. A method of detecting a force between a sample and a probe of an AFM, the method comprising:
positioning at least one of the probe and the sample at a location of interest of the sample;
moving at least one of the probe and the sample to lessen a separation therebetween and cause the two to interact;
measuring a deflection of the probe based on the moving step; and
using the measured deflection, in real time during the moving step, to determine an instantaneous baseline.
8. A method of detecting an observable interaction between a sample and a probe of an AFM, the method comprising:
positioning at least one of the probe and the sample at a location of interest of the sample;
moving at least one of the probe and the sample to lessen a separation therebetween and cause the two to interact;
measuring an observable interaction based on the moving step;
determining an observable interaction artifact from an observable interaction due to actual probe-sample interaction during the measuring step to derive an artifact free observable interaction and compare with a trigger;
retracting the probe from the sample if the artifact free observable interaction substantially corresponds to the trigger.
9. The method of claim 8 , wherein the observable interaction includes at least one of a group including: force, deflection, electric interaction (e.g., force, current, voltage), magnetic interaction, thermal interaction and electromagnetic interaction (e.g., scattering near field optical signals).