AFM Fundamental System Components Outline •Sample preparation •Instrument setting •Data acquisition •Imaging software Spring 2009 AFM Lab 1 Elements of a Basic Atomic force Microscope Spring 2009 AFM Lab 2 Spring 2009 AFM Lab 3 Potential Diagram Repulsion Distance Attraction Spring 2009 AFM Lab 4 Piezoelectric Material Spring 2009 AFM Lab 5 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 Spring 2009 AFM Lab 6 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 Spring 2009 AFM Lab 7 Two progressively greater magnifications (Lowest magnification, over a 10μm grating) Spring 2009 (Highest magnification, over a 10μm grating) AFM Lab 8 Camera and Lens Assembly Spring 2009 AFM Lab 9 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) Spring 2009 AFM Lab 10 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 Spring 2009 AFM Lab 11 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 Spring 2009 AFM Lab 12 Introduction to Piezo-Electric Properties Electric dipoles in domains; (1) unpoled ferroelectric ceramic (2) 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. Spring 2009 AFM Lab 13 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 Spring 2009 AFM Lab 14 Ceramic Tubular Actuator 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 Spring 2009 The tube is deformed in a controlled way by applying a voltage on the “X” electrodes AFM Lab 15 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. Spring 2009 AFM Lab 16 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). Spring 2009 AFM Lab 17 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 Spring 2009 AFM Lab 18 Bow and Tilt Spring 2009 AFM Lab 19 Balanced Pendulum: How Does It Work • Laser tracking spot remains fixed relative to Z-piezo & AFM cantilever • Z-piezo does not bend Y scan Tube Design Spring 2009 Pendulum Design AFM Lab 20 Why Balance the Pendulum Moving weight distribution for scanning accuracy and speed Traditional tube scanner Pendulum scanner sample Simple pendulum: scans slower, less accurate during turn around, more noise Spring 2009 Balanced pendulum • Scans faster, less noisy • More accurate control in XYZ • Low inertia • Maintains rigidity • Minimizes X-Y coupling AFM Lab 21 Nose Assemblies The nose assembly retains the cantilever and enables its motion. A spring clip on the nose assembly secures the probe in place. Onepiece nose assemblies are available for different modes and may include additional electronics and/or components. Spring 2009 AFM Lab Clockwise from upper left: Top MAC, CSAFM, Contact Mode, AC Mode, 22 STM 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. Spring 2009 AFM Lab 23 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. Spring 2009 AFM Lab 24 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. Spring 2009 AFM Lab 25 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 Spring 2009 AFM Lab 26 Data Acquisition: PicoView Software Data Types: Topography: “Z” height (quantitative information) Amplitude (AC AFM): rms value of the cantilever’s oscillation at the set frequency (qualitative info only) Phase (AC- AFM): Phase difference between driving signal and the waveform of the tip’s interaction with the sample Other types: Deflection, Current, Friction etc Spring 2009 AFM Lab 27 Gwyddion Imaging Software Free powerful Imaging Software! http://gwyddion.net/ recommended by Spring 2009 AFM Lab Agilent 28 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 Spring 2009 AFM Lab 29