AM/FM measurements using multiple frequency of atomic force microscopy

• US 9,453,857 B2
• Filed: 04/23/2015
• Issued: 09/27/2016
• 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): BERMAN, JACK I
• Application Number:
  US14/694,980
• Publication Number:
  US 20150309071A1
• Time in Patent Office: 523 Days
• US Class Current: 1/1
• 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. A method of calibrating an atomic force microscope, which is equipped with a cantilever with a tip, a photodetector and a coarse positioning system and a fine positioning system for controlling a position of the cantilever and a sample, comprising the following steps:

turning a drive mechanism off;

positioning a detection laser on a side of the cantilever opposite the tip and in a center of the photodetector;

calibrating a stiffness of the cantilever from a measured thermal spectrum taken far from the surface of the sample and recording information indicative of the stiffness as representing a natural frequency of the cantilever;

approaching the tip of the cantilever to close proximity with the sample, as determined by optical inspection of the tip and the sample using the coarse-positioning system;

after said approaching, turning the drive mechanism on;

setting a drive frequency equal to the natural frequency of the cantilever as determined from said thermal spectrum;

setting a drive amplitude to a desired free oscillation amplitude;

approaching the tip of the cantilever to the surface of the sample until contact is established by setting a feedback set point to a desired interaction amplitude that is slightly less than a free oscillation amplitude;

fully separating the sample from the tip of the cantilever as determined by optical inspection using the fine-positioning system;

acquiring a cantilever tune by either setting the drive frequency to the natural frequency of the cantilever as determined from said thermal spectrum or from setting the drive frequency to a local amplitude maximum of the cantilever near a resonant frequency of the cantilever;

turning off the drive mechanism and acquiring an additional thermal spectrum of the cantilever that is closer to imaging conditions;

using a simple harmonic oscillator model and the data from the additional thermal spectrum to define a new natural frequency of the cantilever and determine a phase-to-drive-frequency relationship of the cantilever;

turning on the drive mechanism and choosing a drive frequency equal either to the new natural frequency of the cantilever or to a local amplitude maximum of the cantilever near the cantilever resonant frequency;

choosing a drive amplitude which achieves the desired cantilever free oscillation amplitude; and

setting phase from a result of the aforesaid simple harmonic oscillator model.