Dual-probe scanning probe microscope
An apparatus and method of positioning a probe of an atomic force microscope (AFM) includes using a dual probe configuration in which two probes are fabricated with a single base, yet operate independently. Feedback control is based on interaction between the reference probe and surface, giving an indication of the location of the surface, with this control being modified based on the difference in tip heights of the two probes to allow the sensing probe to be positioned relative to the sample at a range less than 10 nm.
1. A method of positioning a probe of an atomic force microscope (AFM) relative to a sample, the method comprising:
providing a probe assembly including two probes, a reference probe having a tip with a height, h 1 , and an imaging probe having a tip with a different height, h 2 , wherein a mechanical path between the reference probe and the sample and the imaging probe and the sample is the same;
detecting a surface of the sample using the reference probe;
tracking the surface based on the detecting step using a feedback controller having a setpoint; and
changing the setpoint based on a known difference between h 1 and h 2 to position the imaging probe tip relative to the sample;
wherein h 1 >h 2 .
2. The method of claim 1 , wherein the two probes share a common base.
3. The method of claim 2 , wherein a linear offset between the first tip and the second tip is less than 500 nm.
4. The method of claim 2 , wherein the changing step includes positioning at least one of the probes and the sample so that the tip-sample separation is less than 20 nm.
5. The method of claim 4 , wherein the tip-sample separation is sub-nanometer.
6. The method of claim 1 , wherein the first probe has a Tapping Mode AFM resonant frequency, ω 1 , that is different than a Tapping Mode AFM resonant frequency,ω 2 , of the second probe.
7. The method of claim 1 , wherein the first probe is a reference probe operated in a DC AFM Mode, and the second probe is an imaging probe operated in an AC AFM Mode.
8. A method of positioning a probe of an atomic force microscope (AFM) relative to a sample, the method comprising:
providing a probe assembly including two probes, a reference probe having a tip with a height, h 1 , and an imaging probe having a tip with a different height, h 2 , wherein a mechanical path between the reference probe and the sample and the imaging probe and the sample is the same;
engaging a surface of the sample with the reference probe;
scanning the sample with the probe assembly by providing relative scanning motion between the probe assembly and the sample;
using, during the scanning step, the reference probe in a feedback loop of the AFM to control probe-sample separation of the imaging probe; and
measuring a property of the imaging probe concurrently with the using step;
wherein h 1 >h 2 .
9. The method of claim 8 , wherein the property is imaging probe deflection.
10. The method of claim 8 , wherein the two probes share a common base and wherein a linear offset between the first tip and the second tip is less than 500 nm.
11. The method of claim 10 , wherein the first probe is a reference probe operated in a DC AFM Mode, and the second probe is an imaging probe operated in an AC AFM Mode.
12. The method of claim 8 , comprising:
detecting a surface of the sample using the reference probe;
tracking the surface based on the detecting step using a feedback controller having a setpoint; and
changing the setpoint based on a difference between h 1 and h 2 to position the imaging probe tip relative to the sample.
13. The method of claim 12 , wherein the changing step includes positioning at least one of the probes and the sample so that the tip-sample separation is maintained on the sub-nanometer scale.