August 2010 Lecture IV Contact Mode Getting started with imaging SPM Users Basic Training August 2010 Contact Mode Advantages Easy set up Fast imaging Friction signal Force vs distance measurements ID: 258938
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Slide1Slide2
SPM Users Basic Training
August 2010
Lecture IV – Contact Mode
Getting started with imagingSlide3
SPM Users Basic Training
August 2010
Contact Mode
Advantages
Easy set up
Fast imaging
Friction signal
Force vs. distance measurements
Disadvantages
Lower resolution due to lateral forces
Possible tip and/or sample damage from vertical forces.
Mode choice usually dictated by sample.Slide4
SPM Users Basic Training
August 2010
Contact Mode Feedback LoopSlide5
SPM Users Basic Training
August 2010
Cantilever
Force Applied
A
B
C
D
photodetector
Force exerted on sample by cantilever
is proportional
to deflection of cantilever
(F = -
kx
).
Deflection, or force,
determined by difference between
initial Deflection setting
and
Force Setpoint
.
Feedback loop works to keep deflection (i.e., position of laser spot on photodetector) constant at Setpoint value while scanning.Slide6
SPM Users Basic Training
August 2010
Deflection and Setpoint
Same Setpoints (green circles), different initial deflections (red spots).
What is difference in initial scanning force?
Before Approach
After Approach
Setpoint = 0V
Deflection = -1V
Setpoint = 0V
Deflection = -2VSlide7
SPM Users Basic Training
August 2010
Deflection and Setpoint
Different Setpoints (green circles), same initial deflections (red spots).
What is difference in initial scanning force between them?
How about compared to previous setup?
Before Approach
After Approach
Setpoint = 0V
Deflection = -1V
Setpoint = 1V
Deflection = -1VSlide8
SPM Users Basic Training
August 2010
Deflection and Setpoint
?
Setpoint = -2V
Deflection =
0V
Approach
?
Setpoint =
7V
Deflection =
5V
ApproachSlide9
SPM Users Basic Training
August 2010
Contact Mode Image Channels (Typical)
Topography: Calibrated voltage sent to Z piezo by feedback loop.
Deflection: Same as Error Signal, equals Deflection minus Setpoint. Shows edges more clearly.
Friction: (A+C) – (B+D) / Sum, measures twisting of cantilever due to frictional effects of sample surface.Slide10
SPM Users Basic Training
August 2010
Contact Mode Image Channels: Why Monitor Friction?
Topography image of polymer film
Friction - Trace
Friction - Retrace
Friction reveals hidden information about the sample.
All three images acquired simultaneously from same sample area.Slide11
SPM Users Basic Training
August 2010
AFM Contact Mode Imaging Pre-Engage Check-list
Set the Mode to AFM (Mode
Contact AFM).
Choose correct scanner calibration file (Scanner select correct scanner calibration file).
Verify servo settings (Servo Dialog).
Double check photodetector alignment (Laser Alignment Dialog).
Check for appropriate Setpoint. Make sure Setpoint is more positive than Deflection setting.
Check approach speed and Stop At value (Scan and Motor Window, Motor Tab).
Select Scan Size and Resolution.
Click Approach button.Slide12
SPM Users Basic Training
August 2010
AFM Contact Mode Imaging Pre-Engage Check-list
Servo Gain Settings – Typical values shown. Experience will show what values work best for a particular scanner and sample.
Setpoint (V) – Zero or small positive value is typical starting point.
Range (um) – Should be the full Z range of the scanner to start.
Sample Bias – This is typically not used for standard Contact AFM imaging. Set to 0.000.Slide13
SPM Users Basic Training
August 2010
AFM Contact Mode Imaging Pre-Engage Check-list
Stop At (V) – Deflection value at which approach process is stopped and servo (feedback) engaged. Typically set to 0.00V but can be adjusted based on the initial deflection value and if a softer or harder approach is desired.
Speed (um/s) – Rate at which tip approaches sample. Low speed means more gentle approach but longer wait time. Slide14
SPM Users Basic Training
August 2010
Contact Mode Initial Settings Summary
Sum greater than 1.0V
Deflection on HEB ~ -0.5 to -1.0V
Friction (LFM) on HEB ~0.0V
Setpoint in software ~ 0.0 to +1.0V
Scan speed 2-5 lines/sec
I and P Gains ~2
256 lines
Vary Deflection, Setpoint and/or cantilever force constant to match sample.Slide15
SPM Users Basic Training
August 2010
Approaching the Tip to the Sample
Approach button – Click to begin automated approach. Approach In Progress window opens. High voltage to scanner and feedback loop enabled when sample is detected.
Withdraw button – Withdraws tip from the sample the distance defined in the Motor tab of the Scan and Motor Dialog. Disables HV.
Scanning button – Opens the scan control windows (Servo, Scan and Motor, and
Realtime
Images).
1
2
3Slide16
SPM Users Basic Training
August 2010
Proper Engage Indicators
The servo channel input signal should remain at or near zero. Signal shown in oscilloscope is Set Point minus Deflection, i.e. Error.
The Z piezo position indicator (yellow bar) will be somewhere near the middle of the Range indicator in the Servo window.
1
2Slide17
SPM Users Basic Training
August 2010
False Engage Indicators
The servo channel input signal (the error) will not be at or near zero.
The Z piezo position indicator will be drift to the one or the other end of the range in the Servo dialog.
1
2Slide18
SPM Users Basic Training
August 2010
Preparing Software for Contact ModeSlide19
SPM Users Basic Training
August 2010
Servo Offset Correction
Stepping motors can be manually controlled to move sample away from or towards tip in small steps.
Controls step size
Piezo almost fully extended
Piezo in middle of rangeSlide20
SPM Users Basic Training
August 2010
Scan Controls
Available Scan controls:
Start Scan from bottom
Start Scan from top
Continue: Restart scan from where scan was stopped
Frames: How many frames to complete (with single or continuous frame presets)
Withdraws tip after preset number of frames when selected.
Size: Scan area
Speed: Lines per second
X & Y Offset: Move scan center
Resolution: Pixels per line
Angle: Rotate scan areaSlide21
SPM Users Basic Training
August 2010
Servo Controls
Available Servo controls:
Servo Off: Disable feedback loop
I Gain
P Gain
Setpoint: Target value for feedback loop
Sample Bias
Range: Total Z movement available
Center: Centers Z piezo range around current position. Has effect only if Range is reduced from maximum. Slide22
SPM Users Basic Training
August 2010
Optimizing Contact Mode Images
The user has three parameters to adjust to optimize an AFM image.
1. Setpoint (V) controls deflection of cantilever i.e., strength of interaction between tip and sample.
2. Scan Speed influences how quickly feedback must respond to changes in cantilever deflection.
3. Servo Gains determine how quickly feedback loop can respond to those changes.
Of course, quality of tip is very important in determining ultimate image quality.Slide23
SPM Users Basic Training
August 2010
Using Topography and Deflection to
Adjust I and P Gains
Use Tools -> Realtime Cross Section in Real Time Images window to observe individual scan lines.
Top Plot: Not optimized, I and P Gains too low. Topography doesn’t show sharp corners, Deflection (Error signal) very large.
Bottom Plot: Optimized, I and P Gains correctly set. Topography shows sharp corners, Deflection (Error signal) minimized.
Can also directly compare trace and retrace line scans in PV 1.6.xSlide24
SPM Users Basic Training
August 2010
Setting The Servo Gains Too High
Setting Gains too high (especially I Gain) leads to oscillation in servo signal, as seen in cross sections seen above.Slide25
SPM Users Basic Training
August 2010
What does that green bar mean?
Z Piezo in near the middle of its range.
Good.
Z Piezo fully retracted.
Bad.
Why is it retracted?
How to fix?
Z Piezo fully extended.
Bad.
Why is it extended?
How to fix?Slide26
SPM Users Basic Training
August 2010
Contact Mode Key Points
Feedback loop works to keep cantilever deflection constant by applying voltage to Z piezo.
Topography equals voltage applied to Z piezo times calibration factor of the piezo.
Initial deflection after approach (i.e., force) is determined by difference between Deflection setting before approach and Setpoint defined by user in software.