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Preparing for Imaging. Outline Servo system Light level force sensor Setting the parameters Laser adjustments Photodiode adjustments. Basic Servo System of an AFM. Basic elements of a AFM servo Close loop control Proportional – Integral (PI) controller.

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preparing for imaging
Preparing for Imaging

Outline

  • Servo system
  • Light level force sensor
  • Setting the parameters
  • Laser adjustments
  • Photodiode adjustments

AFM Lab

basic servo system of an afm
Basic Servo System of an AFM
  • Basic elements of a AFM servo
  • Close loop control
  • Proportional – Integral (PI) controller.

AFM Lab

slide4
Introduction to Proportional – Integral controller.http://newton.ex.ac.uk/teaching/CDHW/Feedback/ControlTypes.html

Proportional Control

On-Off Control

Proportional+ Integral+ Derivative control

Y(t)=Kpє(t) + Ki ∫t-∞є(ζ) dζ

AFM Lab

effect of the pi controller setting
Effect of the PI Controller Setting
  • When the gains are set too high the system will overcompensate and will lead to a “ringing” at the leading and trailing edges of the features.
  • When the gains are set too low the system will not adjust the tip fast, blurring the image.

AFM Lab

artifacts showing the effect of servo optimization on the image
Artifacts Showing the Effect of Servo Optimization on the Image

Optimized controller

Poorly-optimized controller

Example of an artifact created by not having the PID parameters

optimized while scanning. In the upper image parameters are optimized, in the

lower image parameter are not optimized and the error signal is large.

AFM Lab

how to adjust the pi parameters
How to adjust the PI Parameters?
  • Typically set P=I ~ 5-10%
  • More to come on the subject!

AFM Lab

x y raster generator
X-Y Raster Generator

Left: Signals output for driving the x and y piezoelectric ceramics

in the AFM scanner. Right: Motion of the probe in the x and y axis when the

piezoelectric ceramics are activated.

AFM Lab

light lever force sensor
Light Lever Force Sensor

Small displacement of the cantilever result in large displacements on the photodetector due to the large “lever arm” of the light path.

AFM Lab

slide10
Scan
  • Size: Size of the square
  • Offset (X and Y): Offset of the square related to the 0,0 origin- Values are limited by the maximum range of the scanner.
  • Typical values for a run:
  • Speed (ln/s): 2
  • Pixel (pix/ln): 256
  • Angle: 0
  • Frames: 1
  • +

AFM Lab

advanced
Advanced

Tip lift: Keep it checked

Overscan % (x and y): 1

Correct for drift (X and Y): 0

Enable Closed loop: Leave it unchecked

AFM Lab

motor
Motor

Stop at: Stepper motors displacement before t he servo is made active (90% for ACAFM mode, X volts for contact mode (to be defined))

Speed (μm/s): Stepper motors speed, typical 3

Withdraw (μm): Vertical displacement of the probe when the withdraw button is clicked, typical 10

Direct: (leave it unchecked)

Position (μm): Relative z position (set it to0 it when the probe is 50 μm above the sample)

Step Close/Open: Down or Up movement of the tip

Travel: Tip traveling when the steps button are actuated

AFM Lab

scanner
Scanner

AFM Lab

detector assembly
Detector Assembly
  • Detector assembly, top and bottom views

AFM Lab

scanner mounting
Scanner Mounting
  • Scanner mounting jig with nose assembly and spring key; scanner in mounting jig
  • Nose assembly removal tool

AFM Lab

nose amplifier assembly
Nose Amplifier Assembly
  • Using the nose assembly removal tool

AFM Lab

inserting the nose assembly
Inserting the Nose Assembly
  • Applying even, vertical pressure at the edges to insert the nose assembly.

AFM Lab

nose assembly removing
Nose Assembly Removing
  • Spring key
  • Do not use tweezers to remove a nose assembly. Doing so can place damaging lateral forces on the scanner.

AFM Lab

probe mounting
Probe Mounting
  • Probe properly situated on AFM nose assembly

AFM Lab

laser adjustment
Laser Adjustment
  • Use the scanner knobs to optimize the laser spot

Goal: to adjust the laser bean on the tip of the cantilever

“F” corresponds to the goal which has to be met. The laser beam is bouncing just on the tip of the cantilever

AFM Lab

photodiode adjustment
Photodiode Adjustment
  • Laser Alignment window in PicoView for AAC Mode
  • Align spot to yellow, dotted line for Contact Mode

AFM Lab

getting an image
Getting an Image
  • Enter scan Speed 1-2 lines/sec. The slower the “speed” the better the resolution
  • Select from X list “256”. The higher the number the better the resolution
  • Set “Size”
  • Set “Angle” to “0”. The “Angle” adjust the direction where the tracking is going to be done. “0” is parallel to “X”

AFM Lab

fine tuning the image
Fine-tuning the Image
  • Set Servo Gain I=P=10%
  • Note the “Deflection” value
  • In the “Setpoint” enter a value slightly above this number
  • In ACAFM the closer the Setpoint is to “0”, the more force the cantilever exerts on the sample
  • Use the “Up” arrow to increase the voltage. At one point, the indicator will change from “green” to “red”. Use the “down” arrow to bring the indicator to the “green” zone. This is the highest value which will keep the tip and sample in contact.
  • During the scan you may decide to lower the value of the setpoint to improve the resolution of the image.

AFM Lab