AM/FM measurements using multiple frequency atomic force microscopy

• US 10,444,258 B2
• Filed: 12/11/2017
• Issued: 10/15/2019
• Est. Priority Date: 04/23/2014
• Status: Active Grant

• Current Assignee: Oxford Instruments Asylum Research Inc (Oxford Instruments PLC)
• Original Assignee: Oxford Instruments Asylum Research Inc (Oxford Instruments PLC)
• Inventors: Proksch, Roger, Bemis, Jason, Labuda, Aleksander
• Primary Examiner(s): Stoffa, Wyatt A
• Application Number:
  US15/837,852
• Publication Number:
  US 20180292432A1
• Time in Patent Office: 673 Days
• CPC Class Codes: G01Q 10/04 Fine scanning or positioning
• CPC Class Codes: G01Q 20/00 Monitoring the movement or position of the probe
• CPC Class Codes: G01Q 40/00 Calibration, e.g. of probes
• CPC Class Codes: G01Q 60/24 AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
• CPC Class Codes: G01Q 60/32 AC mode

Abstract

Apparatus and techniques presented combine the features and benefits of amplitude modulated (AM) atomic force microscopy (AFM), sometimes called AC mode AFM, with frequency modulated (FM) AFM. In AM-FM imaging, the topographic feedback from the first resonant drive frequency operates in AM mode while the phase feedback from second resonant drive frequency operates in FM mode. In particular the first or second frequency may be used to measure the loss tangent, a dimensionless parameter which measures the ratio of energy dissipated to energy stored in a cycle of deformation.

Claims

1. An atomic force microscope system, comprising:

a cantilever having a tip at one end and a driving part at its other end;

a cantilever position detector which detects a position of the cantilever;

a positioning system for controlling a position of the cantilever relative to a sample, the positioning system having a controller receiving feedback, and producing outputs that drive movement of the cantilever,

the positioning system driving the cantilever at or near at least two eigenmodes of the cantilever,

where the positioning system operating by;

first operating by exciting the probe tip at a first eigenmode frequency and a first amplitude,

using first and second feedback loops to controls the distance between the tip of the cantilever and the sample surface to maintain said distance,

the first feedback loop being an amplitude modulated feedback loop that controls a separation between the tip and the sample by keeping the amplitude of the cantilever constant and producing a topographic image from a feedback signal used to keep the amplitude constant;

the second feedback loop being a frequency modulated mode feedback loop that controls the separation between the tip and the sample by varying a drive frequency of the cantilever in a frequency modulated mode that maintains a phase comparison between the cantilever and actuator at a specified phase value;

thereby obtaining topographic feedback from the first feedback loop and obtaining frequency feedback from the second feedback loop, where the frequency feedback is a quantitative value of the frequency shift that depends on the sample stiffness;

exciting a second eigenmode frequency of the cantilever at a second amplitude different than the first amplitude, where at the second amplitude the cantilever eigenmode is interacting with predominantly repulsive forces to the surface,

determining amplitude and phase of the first eigenmode and of the second eigenmode, and using said amplitude and phase of the first eigenmode and of the second eigenmode to characterize the surface, wherein said positioning system keeps phase constant at 90°

between a signal from the cantilever oscillation and a signal from the actuator driving the cantilever.

2. The system as in claim 1, wherein said Cantilever position detector includes a photodetector.

3. A method of operating an atomic force microscope system, comprising:

driving a cantilever to different movement positions, the cantilever having a tip at one end and a driving part at its other end;

detecting a position of the cantilever;

controlling said driving for controlling a position of the cantilever relative to a sample,

receiving feedback from a position detector into both first and second feedback loops,

said driving the cantilever comprising driving the cantilever at or near two eigenmodes of the cantilever,

first exciting the probe tip at a first eigenmode frequency and a first amplitude,

the first feedback loop being an amplitude modulated feedback loop that controls a separation between the tip and the sample by keeping the amplitude of the cantilever constant and producing a topographic image from the feedback signal used to keep the amplitude constant;

the second feedback loop being a frequency modulated mode feedback loop that controls the separation between the tip and the sample by varying the drive frequency of the cantilever in a frequency modulated mode that maintains a phase comparison between the cantilever and actuator at a specified value;

thereby obtaining topographic feedback from the first feedback loop and obtaining frequency feedback from the second feedback loop, where the frequency feedback is a quantitative value of the frequency shift that depends on the sample stiffness, to characterize the surface, keeping phase constant at a substantially constant 90°

between a signal from the cantilever oscillation and a signal from the actuator driving the cantilever.