Closed Loop Controller for Fast Scanning Probe Microscopy
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Solution Overview
Problem
Scanning probe microscopes (SPMs) face limitations in imaging speed due to actuator non-linearities, drift, creep, and sensor noise, which compromise positioning accuracy and image quality, especially at higher scan speeds.
Innovation Solution
An adaptive feed forward algorithm combined with a closed loop feedback controller is used to attenuate high frequency noise and compensate for low frequency position errors, allowing for high-speed scanning while maintaining image integrity by optimizing the feed forward waveform and reducing sensor noise impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If scanning speed is increased to improve productivity, then imaging throughput increases, but positioning accuracy and image quality deteriorate due to actuator non-linearities, drift, and sensor noise
Solution Approach 1:
The system performs preliminary characterization of actuator non-linearities, drift, and sensor noise properties before high-speed scanning. A feedforward control signal is pre-calculated based on this characterization to compensate for these effects in advance, enabling accurate positioning at high scan speeds without real-time feedback delays
Solution Approach 2:
The system implements feedback control using a position sensor to monitor actual actuator position and generate error signals. This feedback is processed through a controller that adjusts the drive signal to maintain positioning accuracy, counteracting the effects of non-linearities, drift, and noise that worsen at higher scanning speeds
2Measurement precision
If closed loop feedback control is used to maintain positioning accuracy, then measurement precision improves, but high frequency sensor noise is amplified and system stability deteriorates
Solution Approach 1:
The system changes the bandwidth parameter of the feedback controller, using a lower bandwidth that filters out high-frequency sensor noise while maintaining effectiveness at the lower frequencies where scanning errors occur. This parameter adjustment allows the feedback loop to improve positioning accuracy without amplifying noise or causing instability
3Speed
If bandwidth is increased to improve response speed, then scanning speed improves, but sensor noise impact increases and measurement precision deteriorates
Solution Approach 1:
The system optimizes the bandwidth parameter to achieve the maximum scanning speed while maintaining image quality. By carefully selecting the bandwidth value, the system allows fast scanning response while filtering out high-frequency noise that would otherwise degrade measurement precision
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
This approach enables scanning speeds significantly greater than current AFM controllers without degrading image quality, achieving peak position errors less than 1% of the scan range and reducing sensor noise to less than 1 Angstrom RMS, allowing for precise and fast imaging over various scan sizes.
Implementation Method 1
scanner 24 often comprises a piezoelectric stack (often referred to herein as a 'piezo stack') or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface
Implementation Method 2
probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15
Implementation Method 3
a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26
Data Source
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Figure 6
AI summary
A method of operating a metrology instrument includes generating relative motion between a probe (14) and a sample (22) at a scan frequency using an actuator (110). The method also includes detecting motion of the actuator using a position sensor (108) that exhibits noise in the detected motion, and controlling the position of the actuator (110) using a feedback loop (104) and a feed forward algorithm (120). In this embodiment, the controlling step attenuates noise in the actuator position compared to noise exhibited by the position sensor over the scan bandwidth. Scan frequencies up to a third of the first scanner resonance frequency or greater than 300 Hz are possible.