Quantitative measurements using multiple frequency atomic force microscopy

• US 20130117895A1
• Filed: 10/29/2012
• Published: 05/09/2013
• Est. Priority Date: 06/16/2008
• Status: Active Grant

• Current Assignee: Oxford Instruments Asylum Research Inc (Oxford Instruments PLC)
• Original Assignee: Asylum Research Corporation (Oxford Instruments PLC)
• Inventors: Proksch, Roger B., Bemis, Jason
• Granted Patent: US 9,297,827 B2
• Time in Patent Office: Days
• US Class Current: 850/1
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01B 17/08 for measuring roughness or irregularity of surfaces
• CPC Class Codes: G01Q 10/00 Scanning or positioning arrangements, i.e. arrangements for actively controlling the movement or position of the probe
• CPC Class Codes: G01Q 60/32 AC mode

Abstract

The imaging mode presented here combines 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 second resonant drive frequency operates in FM mode and is adjusted to keep the phase at 90 degrees, on resonance. With this approach, frequency feedback on the second resonant mode and topographic feedback on the first are decoupled, allowing much more stable, robust operation.

Claims

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

using a feedback loop to control the distance between the base of the cantilever and the sample surface to maintain the probe tip of the cantilever in a pre-established relationship with respect to the surface of the sample in the Z direction while scanning the sample by creating relative movement between the probe tip of the cantilever and the sample;

exciting the chip of the cantilever at the first two resonant frequencies of the cantilever which have been summed together by a circuit element;

providing each resonant frequency as a reference signal to a lock-in amplifier;

controlling the distance between base of the cantilever and the sample so that the amplitude of oscillation at the first frequency of the probe tip is maintained essentially constant at an amplitude setpoint;

detecting changes in phase and/or amplitude in the oscillation at the first frequency while the amplitude of oscillation at the first frequency is maintained essentially constant at the amplitude setpoint; and

detecting changes in phase and/or amplitude in the oscillation at the second frequency while the amplitude of oscillation at the first frequency is maintained essentially constant at the amplitude setpoint; and

measuring the amplitude and/or phase of the cantilever at the different excitation frequencies.

2. The method according to claim 1, comprising:

using the amplitude and/or phase to calculate a signal indicative of the loss tangent of the cantilever as it interacts with the sample surface.