Sub-resonant AFM Probe Feedback Control for Tapping Mode
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Solution Overview
Problem
In atomic force microscopy, maintaining a constant peak force while scanning uneven surfaces is challenging due to the lack of a surface map, as the probe may not contact the surface in valleys or contact too early over hills, leading to variable force measurements.
Innovation Solution
A method and apparatus using a compound actuator system with both fine and coarse actuators to control the vertical offset of the probe, relying on forces other than peak force, such as the shape of the force curve or its derivatives, to maintain a consistent tapping force during sub-resonant tapping, allowing for faster scanning without increasing artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the probe scans over uneven surfaces using conventional tapping mode, then the probe can contact the surface, but the peak force varies significantly due to topography (zero force in valleys, high force on hills)
Solution Approach 1:
The system uses feedback control by monitoring a feature of the force curve (other than peak force) and adjusting the vertical offset to maintain constant peak force. The controller continuously compares the measured force feature with a reference value and modifies the probe position accordingly to compensate for topographic variations.
Solution Approach 2:
The invention changes the vertical offset parameter dynamically during scanning based on the measured force curve features. By adjusting this parameter in response to detected force variations, the system adapts to different surface heights and maintains consistent imaging force despite topographic changes.
2Productivity
If the scanning speed is increased to improve productivity, then data acquisition is faster, but artifacts increase and measurement precision deteriorates
Solution Approach 1:
The feedback mechanism allows the system to maintain precision at higher speeds by continuously correcting the vertical offset based on real-time force measurements. This enables faster scanning without sacrificing accuracy because the system can react to and compensate for topographic variations even during rapid scanning.
Solution Approach 2:
The system performs preliminary detection of the force curve feature before the probe reaches problematic areas, allowing it to pre-adjust the vertical offset to anticipate and compensate for upcoming topographic changes, thereby maintaining precision during fast scanning.
3Measurement precision
If the vertical offset is adjusted frequently to maintain constant peak force on uneven surfaces, then measurement precision improves, but the complexity of the control system increases
Solution Approach 1:
The feedback control system automatically adjusts the vertical offset based on measured force curve features, eliminating the need for complex manual control mechanisms. The system uses a straightforward feedback loop that monitors force and makes automatic corrections, achieving precision without excessive complexity.
Solution Approach 2:
The system performs self-adjustment by using its own force measurements to control its vertical position. The probe itself provides the feedback signal needed for adjustment, making the system self-regulating and reducing the need for external complex control mechanisms.
4Device complexity
If the probe uses only peak force as the feedback signal, then the control is simple, but the reliability of maintaining constant force deteriorates due to missed contacts in valleys or early contact on hills
Solution Approach 1:
The system uses feedback from a more reliable feature of the force curve that occurs consistently during proper contact. This feedback mechanism provides more dependable information about actual probe-surface interaction compared to peak force alone, enabling more reliable detection of contact events regardless of topographic variations.
Solution Approach 2:
The invention uses an intermediary force curve feature as a mediator between the probe contact event and the control system. This intermediary signal provides more reliable information about actual contact than peak force, serving as a better basis for determining when the probe should be adjusting its position.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables faster and more accurate scanning by maintaining a constant peak force across varying surface topography, reducing the need for frequent adjustments and minimizing artifacts in the image, while allowing for quicker data acquisition without compromising precision.
Implementation Method 1
a probe attached to a cantilever oscillates above and below some equilibrium position
Implementation Method 2
sub-resonant tapping mode
Implementation Method 3
the sample surface exerts a force on the probe
Data Source
AI summary
A method of carrying out sub-resonant tapping in an atomic force microscope includes causing a probe that is disposed above a sample to be translated in a direction parallel to a horizontal plane defined by the sample and to oscillate in a vertical direction that is perpendicular to the horizontal plane about an equilibrium line that is separated from the horizontal plane by a vertical offset. As a result, the probe repeatedly taps a surface of the sample. Each tap begins with a first contact of the probe on the surface followed by a progressive increase in force exerted by the sample on the probe until a peak force is attained. The vertical offset is controlled by relying at least in part on a feature other than the peak force as a basis for controlling the vertical offset.