Force detection for microscopy based on direct tip trajectory observation

Patent metadata

Field	Value
Publication number	US-9562927-B2
Application number	US-201514872332-A
Country	US
Kind code	B2
Filing date	Oct 1, 2015
Priority date	Oct 6, 2014
Publication date	Feb 7, 2017
Grant date	Feb 7, 2017

Abstract

With example embodiments described herein, a probe tip of a scanning probe microscope (such as an atomic force microscope (AFM)) is directly detected as it moves in a tapping mode to determine the tip positions over time, and a force for the tip is computed from these determined tip positions.

First claim

What is claimed is:

1. An apparatus comprising:
a scanning probe microscope, the scanning probe microscope having a moveable tip, and wherein the scanning probe microscope is configured to move the tip in a tapping mode to cause tapping movement for the tip;
a position detector, the position detector configured to (1) directly detect the tapping tip, and (2) generate an output signal indicative of a plurality of positions for the detected tip over time; and
a processor configured for operation in conjunction with the position detector, the processor configured to (1) process data representative of the output signal from the position detector, (2) determine a plurality of positions for the tapping tip over time in at least one dimension based on the processed data, (3) process the determined tip positions, and (4) compute a force for the tapping tip in at least one dimension based on the processed tip positions over time.

2. The apparatus of claim 1 wherein the scanning probe microscope is configured for observation of an object by the tapping tip, and wherein the computed force comprises a force representative of an interaction force between the tapping tip and the object under observation.

3. The apparatus of claim 2 wherein the processor is further configured to (1) compute a first acceleration for the tapping tip based on a plurality of the detected tip positions over time while the tapping tip is not in contact with the object, (2) compute a second acceleration for the tapping tip based on a plurality of the detected tip positions over time while the tapping tip is in contact with the object, and (3) isolate the interaction force between the tapping tip and the object under observation based on a difference between the computed first acceleration and the computed second acceleration.

4. The apparatus of claim 1 wherein the position detector comprises:
a light source positioned and configured to direct light onto the tip as the tip moves in the tapping mode; and
a light-sensitive position sensor positioned and configured to receive and sense back-scattered light from the tapping tip in response to the directed light.

5. The apparatus of claim 4 wherein the scanning probe microscope comprises an atomic force microscope (AFM), wherein the light source comprises a laser source.

6. The apparatus of claim 5 further comprising:
a surface onto which the tip comes into intermittent contact as a result of the tapping mode;
an optical isolator positioned and configured to provide optical communication among the laser source, the tapping tip, and the light-sensitive position sensor; and
a lens positioned optically between the optical isolator and the tapping tip, the lens configured to direct laser light onto the tapping tip and collect the back-scattered light from the tapping tip.

7. The apparatus of claim 6 wherein the optical isolator is configured to separate laser light from the laser source that is propagating toward the tapping tip from the back-scattered light propagating away from the tapping tip, the optical isolator including a polarized beam splitter.

8. The apparatus of claim 6 wherein the surface is configured to pass light, and wherein the surface is positioned optically between the tapping tip and the optical isolator.

9. The apparatus of claim 1 wherein the scanning probe microscope further comprises:
a cantilever arm to which the moveable tip is attached; and
a member of the group consisting of (1) a piezoelectric transducer configured to drive the cantilever arm, and (2) a photothermal driver, the member configured to cause the tapping movement for the tip.

10. The apparatus of claim 1 wherein the processor is further configured to (1) determine the tip positions over time in a plurality of dimensions, and (2) compute the force as a force vector for the moving tip in the plurality of dimensions based on the processed tip positions over time.

11. The apparatus of claim 10 wherein the plurality of dimensions are three dimensions (3D).

12. A method comprising:
directly detecting a tip of a scanning probe microscope as the tip moves in a tapping mode;
based on the direct observations, determining a plurality of positions for the tapping tip over time in at least one dimension;
processing the determined tip positions over time; and
computing a force for the tapping tip in at least one dimension based on the processed tip positions over time.

13. The method of claim 12 further comprising:
observing an object via the tapping tip; and
wherein the computing step comprises computing a force representative of an interaction force between the tapping tip and the object under observation based on the processed tip positions over time.

14. The method of claim 13 wherein the processing step comprises (1) computing a first acceleration for the tapping tip based on a plurality of the detected tip positions over time while the tapping tip is not in contact with the object, and (2) computing a second acceleration for the tapping tip based on a plurality of the detected tip positions over time while the tapping tip is in contact with the object; and
wherein the force computing step comprises isolating the interaction force between the tapping tip and the object under observation based on a difference between the computed first acceleration and the computed second acceleration.

15. The method of claim 12 wherein the directly observing step comprises:
directing light onto the tapping tip; and
sensing back-scattered light from the tapping tip in response to the directed light; and
wherein the determining step comprises determining the positions for the tapping tip over time in at least one dimension based on the sensed back-scattered light.

16. The method of claim 15 wherein the scanning probe microscope is an atomic force microscope (AFM), and wherein the light is a laser from a laser source.

17. The method of claim 16 wherein the directing step further comprises:
directing the laser from the laser source onto the tapping tip via a surface that passes light, the surface being positioned optically between the laser source and the tapping tip.

18. The method of claim 17 further comprising:
performing the method steps while an object is located on the surface.

19. The method of claim 18 wherein the object comprises a lipid bilayer material, the method further comprising:
the tapping tip intermittently contacting the lipid bilayer material.

20. The method of claim 15 further comprising:
optically isolating the back-scattered light from the directed light via an optical isolator that includes a polarizing beam splitter.

21. The method of claim 15 wherein the sensing step comprises sensing the back-scattered light via a light-sensitive position sensor;
wherein the determining step comprises:
generating a first voltage signal based on the sensed back-scattered light, wherein the first voltage signal is proportional to a normalized lateral difference in optical power on a first portion of the sensor;
generating a second voltage signal based on the sensed back-scattered light, wherein the second voltage signal is proportional to a normalized lateral difference in optical power on a second portion of the sensor;
generating a third voltage signal based on the sensed back-scattered light, wherein the third voltage signal is proportional to a total power incident on the sensor as a whole;
computing a plurality of positions for the tapping tip over time with res

Assignees

• Univ Missouri

Inventors

• King Gavin Mclean
• Sigdel Krishna Prasad

Classifications

• G01Q20/00

  Monitoring the movement or position of the probe · CPC title

• G01Q60/24

  AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes · CPC title

• G01Q20/02 Primary

  by optical means · CPC title

• G01Q60/34

  Tapping mode · CPC title

• G01Q30/04

  Display or data processing devices · CPC title