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AAC Optimization

AAC Optimization. Outline Mode of operation; intermittent contact, Non-Contact Advantages of NC mode? What are the parameters which have to be set?. Acoustic AC AFM.

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AAC Optimization

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  1. AAC Optimization Outline • Mode of operation; intermittent contact, Non-Contact • Advantages of NC mode? • What are the parameters which have to be set?

  2. Acoustic AC AFM The “set point” controls the oscillation amplitude of the cantilever. “Set point” is automatically set by the value of “Peak Amplitude” The oscillation amplitude is maintained constant by the feedback system P and I parameters Amplitude image Peak Amplitude is used to adjust the voltage applied to the Acoustic generator Drive% has the same effect and is automatically set. The Stop At% determines the mode i.e.: “Intermittent Contact” or “Non-contact”

  3. Phase Imaging • The phase shift between probe excitation and oscillation is available • This phase shift is almost independent of the friction • Phase shift gives sharper images than a topographic image

  4. Phase Imaging Sharpness Topography Imaging Phase Imaging 2.5 µm AFM PicoView image 090702-39 showing additional resolution achieved when comparing the (c) topography and (d) phase images.

  5. AAC Mode AFM Optimization ACAFM Control window AC Mode Tune Window with resonance peak at ~331KHz

  6. Modes of Operation of the AFM Intermittent contact Repulsive Cantilever pushed up Contact mode Distance Potential Non-contact AttractiveCantilever pulled down- Non Contact mode Stop at % limits the distance where “Contact” is defined. We can assume that the values of Stop at % above 90% will set the system in Non-contact and the values below 90% will set the system in Intermittent Contact

  7. AAC Mode of Operation Advantages over Contact Mode • Gentle contact with the sample when properly adjusted. • Virtual elimination of lateral force (LFM) • Use of phase measurements with a lock in amplifier gives lower noise and higher resolution • Clear images of soft samples

  8. AAC Mode: Typical Image Types • Topography: Identical to Contact but oscillation amplitude is feedback loop input, not deflection • Amplitude: Error signal, difference between setpoint amplitude and actual amplitude • Phase: Lag between phase of driving signal and response of cantilever. Very sensitive to material differences in sample and can show details not readily seen in Topography image

  9. AAC Mode- Intermittent Contact mode Select a peak amplitude (typ. 2V). That will set the “Select Drive” AAC Nose amplifier • AAC Mode nose assembly Determine the resonance frequency Choose the peak amplitude Set “Stop at value” to typ. 95%. This is the % of the total oscillation which is considered as a safe stop amplitude. Example: Total oscillation 2V, Stop at Value 90%, the system will stop when it reaches an oscillation amplitude of 1.8V

  10. Measurement of the Resonance Frequency of the Probe and Global Settings • Adjust the peak amplitude between 2V and 6 V (this is the maximum of the peak oscillation, typ. 2V) • Adjust the Stop at % to 90% • Sweep the frequency of the Acoustic generator over the range of frequencies given by the probe manufacturer and determine the resulting deflection of the sensor. • Note the maximum and set a negative offset of typ. 200 Hz. • Set the I=P Gain to 30%

  11. Optimization • Input a stop at % to 95% (soft landing) • Force will increase when the voltage decreases (opposite of Contact mode) • Start with a voltage of 75% of the free amplitude of the cantilever. For instance if the free voltage is 4 V enter a set point of 3 V. Increase it until the indicator is “red”. This is the indication that the system is not in contact. • Decrease the voltage until the “trace” and retrace” image look identical. • Adjust the I and P gain as explained in “Contact” mode by starting first at a low value (0.5-1%) and progressively increase it until it reaches oscillation. Then decrease it.

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