Multimode扫描探针显微镜仪器及原理

In scanning probe microscopy, “setpoint” refers to tipsample force. In the case of a probe tip, the operator assigns a setpoint value corresponding to a certain amount of tipsample force, then adjusts gains for the feedback to track the surface as closely as possible while maintaining the setpoint. The Z-axis piezo crystal uses voltage to raise and lower the probe. The Scan rate parameter must be also be considered. when Scan rate is set too high,the microscopist could have difficulty imaging. If the AFM setpoint is improperly set a poor image or damage to the probe or sample might occur.
2. Control Mechanisms and Feedback
“Feedback” refers to using the actual result of an action to determine what adjustment or new action is required to produce the desired result. The control of the SPM is accomplished by use of electronic feedback. The “result” is the probe spacing from the sample or contact force and the “action” is the adjustment required to make a correction to the desired value.
4. As the tip encounters a decline in the sample topology, the tip drops. This directs more of the beam onto the “B” portion of the photodiode array. With the “B” photodiode receiving an increased portion of the laser light, its voltage increases while portion “A’s” decreases (A < B). 5. Again, the Vertical Deflection (A-B) voltage differential is sensed by the feedback electronics, increasing voltage to the Z piezo crystal-the piezo extends. As the Z piezo extends, the tip is pushed down until the laser beam recenters on the photodiode array (A =B).
3. The Vertical Deflection (A-B) voltage differential is sensed by the feedback electronics,causing a dropped voltage to the Z piezo crystal—the piezo retracts. As the Z piezo retracts, the cantilever recenters the laser beam onto the photodiode array (A = B).
4. Control Parameters and Feedback Review of General Operating Concepts
The AFM system maintains the tip at the end of the cantilever in contact with the sample surface .The sample is scanned under the tip in X and Y. Features on the sample surface deflect the cantilever, which in turn change the position of the laser spot on the photodiodes. This position change is read by the feedback loop. The feedback loop moves the sample in Z to restore the spot to its original position.
SPM Fundamentals for the MultiMode
1 2 3 4 5
Hardware Control Mechanisms and Feedback:
Feedback and Gains
Control Parameters and Feedback
Review of tapping mode AFM
1. Hardware
The MultiMode SPM consists of seven components: SPM, controller, computer, keyboard, mouse, display monitor and control monitor.
The central component of the system is the SPM.
1. A flat portion of the sample surface is scanned beneath the tip left-to-right, maintaining the laser beam at the center of the photodiode array. 2. As the tip encounters a raised feature, the cantilever is pushed up and the laser beam is deflected onto the “A” portion of the array. With the “A” photodiode receiving an increased portion of the laser light, its voltage increases while portion “B’s” decreases (A > B).
2.1 Overview of SPM control mechanisms The MMSPM uses an “optical lever” to determine probe to sample spacing.
During setup for imaging, the target location or oscillation characteristics of the beam on the photodetector are set by the user established “setpoint”. As tracing is performed, the variations in the sample lead to the beam move off the setpoint which is an “error signal”. The error signal is used in the feedback system to make changes which will reduce the error.
This picture shows the arrangement of the photodiode elements in the MultiMode head.
The top two elements (A) and the two bottom elements (B) provides a measure of the vertical deflection of the cantilever. The two left photodiodes (C) and the two right photodiodes (D) provides a measure of the torsional deflection of the cantilever.
Note:1.All scanners are interchangeable. 2. When a scanner is changed, the parameter file for the new scanner must also be changed.
Figure 2.1f depicts the electrode configuration used on a typical scanner piezo tube.
3. Feedback and Gains
The feedback system used to control tip-sample interactions and render images must be optimized for each new sample. This is accomplished by adjusting gains in the SPM’s feedback circuit.