AFM Fundamental System Components

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AFM Fundamental System Components

• Outline
• Sample preparation
• Instrument setting
• Data acquisition
• Imaging software

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Elements of a Basic Atomic force Microscope
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Potential Diagram
Repulsion
Distance
Potential
Attraction
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Piezoelectric Material
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Sample Preparation

• AFM Does require minimum of sample preparation
• No clean room handling
• No thin film metal coating
• Works in liquids, gases, and vacuum
• Works at elevated or sub ambient temperatures
• Dimensions are not critical

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Instrument Setting

• Sample Center it in the middle of the sample
  plate and immobilize it using dual side sticky
  pads
• Laser Laser beam has to bounce on the tip of the
  cantilever.
• Photodiode Reflected beam signal has to be
  shared equally between the 4 cells
• System adjustment Servo Gain (PI values), Force,
  Raster speed

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Two progressively greater magnifications
(Lowest magnification, over a 10µm grating)
(Highest magnification, over a 10µm grating)
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Camera and Lens Assembly
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Video System Overview

• The NAVITAR zoom lens system provides an optical
  magnification range of 2.1x-13.5x to the camera.
• The degree of magnification at the monitor
  depends on the ratio of the monitor size to the
  CCD chip size. The camera uses a 1/3" CCD (6mm
  diagonal). Using a 12" monitor (305mm diagonal)
  with the 1/3" CCD chip, the total magnification
  of the system would then be (13.5) x (1.8) x
  (305/6) 1230 (1 micron would be seen as 1.2 mm
  on the screen)

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How the sample is scanned

• Positioning the probe
• The main challenge is to move the probe with
  increments as small as 0.05 nm and keep it at the
  right position
• Resolution in the X-Y range is limited by the
  radius of the probe tens nm
• Resolution in the Z-range is limited by the noise
  of the system 0.05 nm
• Introduction to Piezoelectric materials
• Ceramic tube
• Pendulum design

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How to move and to maintain the probe at the
right position?

• For a full scale of 1 micron assuming an image
  area of 1000 x 1000 pixel the x-y resolution is 1
  nm
• No mechanical positioning can meet this
  specification
• Piezoelectric ceramic actuators can meet these
  requirements

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Introduction to Piezo-Electric Properties

• Electric dipoles in domains
• unpoled ferroelectric ceramic
• During and (3) after poling (piezoelectic ceramic)

PZT (Lead zirconium titanate) ceramics must be
poled at an elevated temperature. The ceramic
now exhibits piezoelectric properties and will
change dimensions when an electric potential is
applied.
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Piezoelectric materials and scanners

• The extension or contraction of a piezoelectric
  element is small
• For example, for a 5 cm long piezoelectric
  element, a voltage of100 V will result in an
  extension of 1 micron
• Since voltages can be controlled on the level of
  at least 10 mV, this gives a resolution of 0.1 nm
  or 1 Angstrom

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Ceramic Tubular Actuator
The tube is deformed in a controlled way by
applying a voltage on the X electrodes
There are four electrically isolated parts on the
outside of the tube X, -X, Y, -Y and one
electrical electrode inside of the tube Z
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Errors Introduced by the PZT Scanner
Hysteresis
Voltage
Creep
Voltage
Top PZT materials have hysteresis. When a
voltage ramp is placed on the ceramic, the motion
is nonlinear. Bottom Creep occurs when a voltage
pulse on a PZT causes initial motion followed by
drift.
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Linearity Error
A test pattern with squares, A, will appear
severely distorted if the piezoelectric scanner
in the AFM is not linear as in B. A common
method for correcting the problems of X-Y
non-linearity and calibration is to add
calibration sensors to the X-Y piezoelectric
scanners (Close loop scanner).
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Error due to the Bow
The motion of the probe is nonlinear in the
Z axis as it is scanned across a surface. The
motion can be spherical
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Bow and Tilt
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Balanced Pendulum How Does It Work

• Laser tracking spot remains fixed
• relative to Z-piezo AFM cantilever
• Z-piezo does not bend

Pendulum Design
Tube Design
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Why Balance the Pendulum Moving weight
distribution for scanning accuracy and speed
Simple pendulum scans slower, less accurate
during turn around, more noise

• Balanced pendulum
• Scans faster, less noisy
• More accurate control in XYZ
• Low inertia
• Maintains rigidity
• Minimizes X-Y coupling

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Nose Assemblies
The nose assembly retains the cantilever and
enables its motion. A spring clip on the nose
assembly secures the probe in place. One-piece
nose assemblies are available for different modes
and may include additional electronics and/or
components.
Clockwise from upper left Top MAC, CSAFM,
Contact Mode, AC Mode, STM
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Mounting the Nose Assembly on the Scanner
Push evenly and straight down when inserting the
nose assembly. Small off-axis forces will create
LARGE torques about the anchor point for the
piezoes, where most breakage occurs. Do NOT push
as this will damage the spring clip and/or glass
down on the top of the nose assembly window.
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Controlling and Imaging Software

• PicoView provides control and the first line of
  visual interpretation and has to be understood
  before getting any further. It gives limited
  information about results and requires the use of
  a more sophisticated software to interpret the
  experiments.
• Gwyddion and Imaging Metrology provide sample
  measurements and statistical data. They have to
  be used to prepare professional reports.

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PicoView Powerful SPM Control Software

• Benefits
• Simultaneous real-time display of up to eight
  channels (in all resolutions)
• Simultaneously display real-time image and
  post-processed data
• Unlimited data points in spectroscopy
• 16x16 to 4096x4096 pixels in images
• Parametric data structure in Spectroscopy
• Allow flexible data presentation
• Temporal display of all channels
• Select any channel as x-axis and plot all the
  rest against it.

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PicoView Powerful SPM Control Software

• PicoScript scripting interface for PicoView
• SPM I/O and control function library
• DLL (dynamically linked library) for VB,
  LabView, and more.
• Labview VI
• Allow interface with external acquisition cards
• Benefits
• Empower user to customize their own application
  needs
• No need to understand the source code structures
• Allows the popular LabView program to interface
  with PicoView

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Data Acquisition PicoView Software
Data Types Topography Z height (quantitative
information) Amplitude (AC AFM) rms value of the
cantilevers oscillation at the set frequency
(qualitative info only) Phase (AC- AFM) Phase
difference between driving signal and the
waveform of the tips interaction with the
sample Other types Deflection, Current, Friction
etc
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Gwyddion Imaging Software
Free powerful Imaging Software!
http//gwyddion.net/ recommended by Agilent
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Image Metrology SPIP Imaging Software
Expensive but very versatile!. Free trial.
http//www.imagemet.com\ We have a license for
one station at the time