L2 1 AFM Fundamental Components

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1Spring 2009 AFM Lab

AFM FundamentalSystem Components

OutlineSample preparation

Instrument setting

Data acquisitionImaging software


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Elements of a Basic Atomic forceMicroscope


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Potential Diagram

Distance

Repulsion

Attraction


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Piezoelectric Material


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Sample Preparation

AFM Does require minimum of samplepreparation:

No clean room handling

No thin film metal coating

Works in liquids, gases, and vacuum

Works at elevated or sub ambienttemperatures

Dimensions are not critical


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Instrument Setting

Sample: Center it in the middle of thesample plate and immobilize it using dualside sticky pads

Laser: Laser beam has to bounce on the tipof the cantilever.

Photodiode: Reflected beam signal has to

be shared equally between the 4 cellsSystem adjustment: Servo Gain (PI values),

Force, Raster speed


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Two progressively greatermagnifications

(Lowest magnification, over a10m grating)

(Highest magnification, over a 10mgrating)


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Camera and Lens Assembly


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Video System Overview

The NAVITAR zoom lens system provides anoptical magnification range of 2.1x-13.5x to thecamera.

The degree of magnification at the monitordepends on the ratio of the monitor size to theCCD chip size. The camera uses a 1/3" CCD(6mm diagonal). Using a 12" monitor (305mm

diagonal) with the 1/3" CCD chip, the totalmagnification of the system would then be(13.5) x (1.8) x (305/6) 1230 (1 micron wouldbe 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 atthe right position

Resolution in the X-Y range is limited by theradius of the probe ~ tens nm

Resolution in the Z-range is limited by the noiseof the system ~0.05 nm

Introduction to Piezoelectric materials Ceramic tube Pendulum design


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

For a full scale of 1 micron assuming animage area of 1000 x 1000 pixel the x-yresolution is 1 nm

No mechanical positioning can meet thisspecification

Piezoelectric ceramic actuators can meetthese requirements


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Introduction to Piezo-ElectricProperties

PZT (Lead zirconium titanate) ceramics must be poledat an elevated temperature.The ceramic now exhibits piezoelectric propertiesand will change dimensions when an electric potential is applied.

Electric dipoles in domains;

(1) unpoled ferroelectric ceramic

(2) During and (3) after poling(piezoelectic ceramic)
http://www.physikinstrumente.com/en/primages/pi_dipoles_d4c_O_eps.jpg


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Piezoelectric materials andscanners

The extension or contraction of apiezoelectric element is small

For example, for a 5 cm long piezoelectricelement, a voltage of100 V will result in anextension of 1 micron

Since voltages can be controlled on thelevel of at least 10 mV, this gives aresolution of 0.1 nm or 1 Angstrom


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Ceramic Tubular Actuator

There are four electrically isolatedparts on the outside of the tube; +X,-X, +Y, -Y and one electrical

electrode inside of the tube: Z

The tube is deformed in acontrolled way by applying avoltage on the X electrodes


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Errors Introduced by the PZTScanner

Top: PZT materials have hysteresis. When a voltage ramp isplaced on the ceramic, the motion is nonlinear. Bottom: Creepoccurs when a voltage pulse on a PZT causes initial motionfollowed by drift.

Voltage

Voltage

Hysteresis

Creep


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Linearity Error

A test pattern with squares, A, will appear severely distorted if thepiezoelectric scanner in the AFM is not linear as in B.

A common method for correcting the problems of X-Y non-linearityand calibration is to add calibration sensors to the X-Ypiezoelectric scanners (Close loop scanner).


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Error due to the Bow

The motion of the probe isnonlinear in the Z

axis as it is scannedacross a surface. Themotion can be spherical


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Bow and Tilt


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Balanced Pendulum: How Does It Work

Y scan

Laser tracking spot remains fixedrelative to Z-piezo & AFM cantilever

Z-piezo does not bend

Tube Design Pendulum Design


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Why Balance the PendulumMoving weight distribution for scanning accuracy and speed

Simple pendulum: scans slower, lessaccurate during turn around, more noise

sample

Traditional tube scanner Pendulum scanner

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

Clockwise fromupper left: Top MAC,CSAFM, Contact

Mode, AC Mode,STM

The nose assembly retainsthe cantilever and enablesits motion. A spring clip on

the nose assembly securesthe probe in place. One-piece nose assemblies areavailable for differentmodes and may includeadditional electronicsand/or components.


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Mounting the Nose Assembly onthe Scanner

Push evenly and straight downwhen inserting the nose

assembly. Small off-axis forceswill create LARGE torques aboutthe anchor point for the piezoes,where most breakage occurs. DoNOT push as this will damagethe spring clip and/or glass

down on the top of the noseassembly window.


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Controlling and Imaging Software

PicoView provides control and the first line ofvisual interpretation and has to be understoodbefore getting any further. It gives limited

information about results and requires the use ofa more sophisticated software to interpret theexperiments.

Gwyddion and Imaging Metrology provide

sample measurements and statistical data. Theyhave to be used to prepare professional reports.


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PicoView Powerful SPM ControlSoftware

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 ControlSoftware

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: PicoViewSoftware

Data Types:

Topography: Z height(quantitative information)

Amplitude(AC AFM): rms valueof the cantilevers oscillation at

the set frequency (qualitativeinfo only)

Phase(AC- AFM): Phasedifference between drivingsignal and the waveform of the

tips interaction with the sampleOther types: Deflection, Current,Friction etc


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Gwyddion Imaging Software

Free powerful Imaging Software! http://gwyddion.net/recommended by

Agilent
http://gwyddion.net/http://gwyddion.net/


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Image Metrology SPIP ImagingSoftware

Expensive but very versatile!. Free trial. http://www.imagemet.com\

We have a license for one station at the time
http://www.imagemet.com/http://www.imagemet.com/http://www.imagemet.com/

L2 1 AFM Fundamental Components

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