Transcript Contact Mode Feedback Loop

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
Disadvantages
• Lower resolution due to lateral forces
• Possible tip and/or sample damage from vertical
forces.
Mode choice usually dictated by sample.
SPM Users Basic Training
August 2010
Contact Mode Feedback Loop
SPM Users Basic Training
August 2010
Cantilever  Force Applied
Force exerted on sample by
cantilever is proportional to
deflection of cantilever
(F = -kx).
photodetector
A
B
D
C
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.
SPM Users Basic Training
August 2010
Deflection and Setpoint
Before Approach
After Approach
Setpoint = 0V
Deflection = -1V
Setpoint = 0V
Deflection = -2V
Same Setpoints (green circles), different initial deflections (red spots).
What is difference in initial scanning force?
SPM Users Basic Training
August 2010
Deflection and Setpoint
Before Approach
After Approach
Setpoint = 0V
Deflection = -1V
Setpoint = 1V
Deflection = -1V
Different Setpoints (green circles), same initial deflections (red spots).
What is difference in initial scanning force between them?
How about compared to previous setup?
SPM Users Basic Training
August 2010
Deflection and Setpoint
Setpoint = -2V
Deflection = 0V
Setpoint = 7V
Deflection = 5V
Approach
?
Approach
?
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.
SPM Users Basic Training
August 2010
Contact Mode Image Channels: Why Monitor
Friction?
All three images acquired
simultaneously from same
sample area.
Topography image
of polymer film
Friction - Trace
Friction - Retrace
Friction reveals hidden
information about the
sample.
SPM Users Basic Training
August 2010
AFM Contact Mode Imaging Pre-Engage Check-list
1. Set the Mode to AFM (Mode  Contact AFM).
2. Choose correct scanner calibration file (Scanner  select
correct scanner calibration file).
3. Verify servo settings (Servo Dialog).
4. Double check photodetector alignment (Laser Alignment
Dialog).
5. Check for appropriate Setpoint. Make sure Setpoint is more
positive than Deflection setting.
6. Check approach speed and Stop At value (Scan and Motor
Window, Motor Tab).
7. Select Scan Size and Resolution.
8. Click Approach button.
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.
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.
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.
SPM Users Basic Training
August 2010
Approaching the Tip to the Sample
3
1
2
1. Approach button – Click to begin
automated approach. Approach In
Progress window opens. High voltage to
scanner and feedback loop enabled when
sample is detected.
2. Withdraw button – Withdraws tip from the
sample the distance defined in the Motor
tab of the Scan and Motor Dialog.
Disables HV.
3. Scanning button – Opens the scan control
windows (Servo, Scan and Motor, and
Realtime Images).
SPM Users Basic Training
August 2010
Proper Engage Indicators
1.
The servo channel input signal should
remain at or near zero. Signal shown
in oscilloscope is Set Point minus
Deflection, i.e. Error.
2.
The Z piezo position indicator (yellow
bar) will be somewhere near the
middle of the Range indicator in the
Servo window.
1
2
SPM Users Basic Training
August 2010
False Engage Indicators
1.
The servo channel input signal (the
error) will not be at or near zero.
2.
The Z piezo position indicator will be
drift to the one or the other end of the
range in the Servo dialog.
1
2
SPM Users Basic Training
August 2010
Preparing Software for Contact Mode
SPM Users Basic Training
August 2010
Servo Offset Correction
Piezo almost fully
extended
Stepping motors can be manually
controlled to move sample away
from or towards tip in small steps.
Piezo in middle of range
Controls step size
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 area
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.
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.
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.x
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.
SPM Users Basic Training
August 2010
What does that green bar mean?
Z Piezo in near the middle of
its range.
Z Piezo fully retracted.
Good.
Bad.
Why is it retracted?
How to fix?
Z Piezo fully extended.
Bad.
Why is it extended?
How to fix?
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.
SPM Users Basic Training
August 2010