Automatic gain tuning in atomic force microscopy

Patent metadata

Field	Value
Publication number	US-9689891-B2
Application number	US-201113117238-A
Country	US
Kind code	B2
Filing date	May 27, 2011
Priority date	May 27, 2011
Publication date	Jun 27, 2017
Grant date	Jun 27, 2017

Abstract

A method for optimizing loop gain of an atomic force microscope (AFM) apparatus includes determining a change in gain of the physical system and adjusting a controller frequency response of the controller in an AFM loop to compensate for the determined change in gain. The AFM loop has a corresponding loop response that includes the product of the controller frequency response and a physical system response of the physical system.

First claim

The invention claimed is:

1. A method for optimizing loop gain of an atomic force microscope (AFM) apparatus comprising a controller and a physical system, the physical system comprising an actuator, a cantilever and a probe tip, the method comprising:
determining a change in gain of the physical system as the AFM apparatus operates in a contact-mode in which a substantially constant force is maintained between the probe tip and a surface of a sample;
adjusting a controller frequency response of the controller in an AFM loop to compensate for the determined change in gain, the AFM loop having a corresponding loop response comprising the product of the controller frequency response and a physical system response of the physical system; and
optimizing and reoptimizing an overall gain of the controller after the physical system has been rescaled and/or the controller has been tuned.

2. The method of claim 1 , wherein adjusting the controller frequency response comprises:
dividing an initial controller frequency response by the determined change in the gain.

3. The method of claim 2 , wherein determining the change in the gain comprises:
measuring the physical system response at a predetermined frequency at a first time;
measuring the physical system response at the predetermined frequency at a subsequent second time; and
determining the ratio of the physical system response measured at the first time and the physical system response measured at the second time.

4. The method of claim 1 , further comprising:
initially shaping the loop response of the AFM loop, wherein shaping the loop response comprises selecting an initial controller frequency response to cause the loop response to have a desired shape.

5. The method of claim 4 , wherein shaping the loop response compensates for resonances of the physical system response.

6. The method of claim 1 , wherein adjusting the controller frequency response to compensate for a subsequent change is performed iteratively.

7. A method for optimizing loop gain of an atomic force microscope (AFM) loop in an AFM apparatus, the AFM apparatus including a controller and a physical system, the method comprising:
(a) initializing a frequency of a signal input to the controller;
(b) measuring a loop response of the AFM loop at the frequency;
(c) determining whether a phase of the loop response is approximately the same as a target phase;
(d) when the phase of the loop response is not approximately the same as the target phase, adjusting the frequency of the input signal and repeating steps (b) and (c), and when the phase of the loop response is approximately the same as the target phase, determining whether a magnitude of the loop response is approximately the same as a target magnitude;
(e) when the magnitude of the loop response is not approximately the same as the target magnitude, adjusting a gain of the controller and repeating steps (b), (c) and (d), and when the magnitude of the loop response is approximately the same as the target magnitude, setting the gain as the loop gain of the AFM loop; and
(f) optimizing and reoptimizing an overall gain of the controller after the physical system has been resealed and/or the controller has been tuned.

8. The method of claim 7 , wherein measuring the loop response comprises determining the product of a controller frequency response of the controller and a physical system response of the physical system.

9. The method of claim 7 , wherein measuring the loop response comprises determining the ratio of a deflection response of deflection of a cantilever in the physical system at a frequency and an error response of an error signal indicating a difference between the deflection of the cantilever and a predetermined setpoint.

10. The method of claim 7 , wherein measuring the loop response comprises:
determining a closed-loop transfer function as the ratio of a deflection of a cantilever in the physical system and a setpoint; and inverting the closed-loop transfer function.

11. The method of claim 7 , wherein measuring the loop response comprises:
determining a sensitivity function as the ratio of an error signal and a setpoint, the error signal indicating a difference between a deflection of a cantilever of the physical system and the setpoint; and inverting the sensitivity function.

12. The method of claim 7 , wherein the target phase is 180 degrees from a predetermined phase margin.

13. The method of claim 12 , wherein the predetermined phase margin comprises a phase of the loop response where the magnitude of the loop response is unity at a lowest frequency.

14. The method of claim 7 , wherein the target magnitude is unity gain.

15. The method of claim 7 , wherein adjusting the frequency comprises increasing the frequency by a predetermined incremental amount.

16. The method of claim 15 , wherein the predetermined incremental amount decreases as the phase of the loop response approaches the target phase.

17. A non-transitory computer readable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method for optimizing loop gain of an atomic force microscope (AFM) loop in an AFM apparatus, the AFM apparatus including a controller and a physical system, the computer readable medium comprising:
a measuring code segment for measuring a loop response of the AFM loop in response to a signal input to the controller, the input signal having a corresponding frequency;
a phase determining code segment for determining whether a phase of the loop response at the frequency is approximately the same as a target phase, and for adjusting the frequency when the phase of the loop response is not approximately the same as the target phase;
a magnitude determining code segment for determining whether a magnitude of the loop response at the frequency is approximately the same as a target magnitude, and for adjusting a gain of the controller when the magnitude of the loop response is not approximately the same as the target magnitude; and
an optimizing and a reoptimizing code segment for adjusting an overall gain of the controller after the physical system has been resealed and/or the controller has been tuned.

18. The non-transitory computer readable medium of claim 17 , wherein the magnitude determining code segment determines whether the magnitude of the loop response at the frequency is approximately the same as the target magnitude and adjusts the gain of the controller after the phase determining code segment determines that the phase of the loop response at the frequency is approximately the same as the target phase.

19. The non-transitory computer readable medium of claim 17 , wherein the target magnitude is selected such that the gain is unity at a crossover point of a loop response signal at a frequency corresponding to the target phase.

Assignees

• Moon Christopher Ryan
• Keysight Technologies Inc

Inventors

• Moon Christopher Ryan

Classifications

• G01Q60/363

  Contact-mode AFM · CPC title

• G01Q10/065 Primary

  Feedback mechanisms, i.e. wherein the signal for driving the probe is modified by a signal coming from the probe itself · CPC title

• B82Y35/00

  Methods or apparatus for measurement or analysis of nanostructures · CPC title