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Design of RF Class A Tuned Ampli

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					         FACULTY OF ENGINEERING



                                 LAB SHEET


                  ENT 3026 DIAGNOSTIC
                     TECHNOLOGY
                  TRIMESTER 2 2008-2009

                 DT1 – ATOMIC FORCE MICROSCOPY




*Note: On-the-spot evaluation may be carried out during or at the end of the experiment. Students
are advised to read through this lab sheet before doing experiment. Your performance, teamwork
                    effort, and learning attitude will count towards the marks.




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             Atomic Force Microscopy: Magnification Exercise
1.0. Objective
        Atomic Force Microscopy has been one of the most effective techniques for
surface characterization of thin film and low dimension structures. This experiment is
designed to familiarize the student to the practical aspects in the operation principles of
atomic force microscope and its applications in surface characterization of surface
structures.

2.0. Equipment Required
1. Atomic Force Microscope.

3.0. Components Required
1. Standard Calibration Sample (Square grid patterns of 3m, 5m and 10m)

4.0. Introduction

4.1 What is Atomic Force Microscope (AFM)?
An AFM is a mechanical imaging instrument that measures the three dimensional
topography as well as physical properties of a surface with a sharpened probe. The
primary uses for the AFM imaging are:

    Visualization
   The AFM measures three dimensional images of surfaces and is very helpful for
   visualizing surface topography.

    Spatial Metrology
   Nanometer sized dimensions of surface features are measurable with the AFM.

    Physical Property Maps
   With different imaging modes it is possible to measure surface physical property
   maps.

4.2. Basic Operation of Atomic Force Microscope

Typically, when we think of microscopes, we think of optical or electron microscopes.
Such microscopes create a magnified image of an object by focusing electromagnetic
radiation, such as photons or electrons, on its surface. Optical and electron microscopes
can easily generate two dimensional magnified images of an object’s surface, with a
magnification as great as 1000X for an optical microscope, and as large as 100,000X for
an electron microscope. Although these are powerful tools, the images obtained are
typically in the plane horizontal to the surface of the object. Such microscopes do not
readily supply the vertical dimensions of an object’s surface, the height and depth of the
surface features.




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Unlike traditional microscopes, the AFM does not rely on electromagnetic radiation, such
as photon or electron beams, to create an image. An AFM is a mechanical imaging
instrument that measures the three dimensional topography as well as physical properties
of a surface with a sharpened probe, The sharpened probe is positioned close enough to
the surface such that it can interact with the force fields associated with the surface. Then
the probe is scanned across the surface such that the forces between the probe remain
constant. An image of the surface is then reconstructed by monitoring the precise motion
of the probe as it is scanned over the surface. Typically the probe is scanned in a raster-
like pattern.

The force sensor in an atomic force microscope is typically constructed from a light
lever. In the light lever, the output from a laser is focused on the backside of a cantilever
and reflected into a photo detector with two sections. The output of each of the photo-
detector sections is compared in a differential amplifier. When the probe at the end of the
cantilever interacts with the surface, the cantilever bends, and the light path changes
causing the amount of light in the two photo-detector sections to change. Thus the
electronic output of the light lever force sensor is proportional to the force between the
probe and sample.




                  Figure 1: Basic operation of Atomic Force Microscope


Although the AFM is capable of extreme magnification, it is not a large instrument. An
AFM that is capable of resolving features as small as a few nanometers can be easily
installed in a laboratory on a desk top. The greatest deterrent to high magnification with
the AFM is often environmental vibrations that cause the probe to have unwanted
vibrations.




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4.3. Basic Components of Atomic Force Microscope

      Scan Head




                           Figure 2: AFM Scan Head


      Controller




                           Figure 3: AFM Controller



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   Stage
    An AFM stage is where the sample is placed when measured.




                                       (a)




                                       (b)

        Figure 4: (a) Standard stage (b) Motorized high precision xy stage

   Computer and Software
    Software in the computer is used for acquiring and displaying AFM images. Also,
    software for processing and analyzing AFM images typically resides in the
    computer.




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                         Figure 5: Nanosurf Easyscan2 Software

5. Standard Operating Procedures for easyScan 2 AFM system

The basic steps involved in operating an Atomic Force Microscope are as follow:

   1.   Prepare Sample
   2.   Place Sample on Stage
   3.   Probe Approach
   4.   Scan Sample
   5.   Zoom on Feature
   6.   Tip Retract

(1) Preparing the Sample
The easyScan 2 AFM can be used to examine any material with a surface roughness that
does not exceed the height range of the scanning tip (14 μm). Nevertheless the choice and
preparation of the surface can influence the surface tip interaction. Examples of
influencing factors are excess moisture, dust, grease etc. Because of this some of the
samples need special preparation to clean their surface. Generally, clean as little as
possible. If the surface is dusty, try to measure on a clean area between the dust. It is
possible to blow coarse particles away with dry, oil free air, but small particles generally
stick so well to the surface that they cannot be removed. Note that bottled pressurized air


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is generally dry, but pressurized air from an in-house supply is generally not. In this case
an oil filter should be installed. Blowing dust away by breath is not advisable, because it
is not dry. When the surface is contaminated with grease, oil or something else, the
surface should be cleaned with a solvent. Suitable solvents are distilled or deionized
water, alcohol or acetone, depending on the contaminant. The solvent should be very
pure, in order to prevent the collection of impurities on the surface when the solvent
evaporates. The sample should be cleaned several times to remove dissolved and
redeposited contaminants if it is very contaminated. Delicate samples can be cleaned in
an ultrasound bath.

(2) Place Sample on Stage
The sample is placed on the stage by using a suitable tweezer. If vacuum suction is used,
the sample must cover the opening completely to prevent air leakage. The vacuum pump
is then turn on to provide suction to hold the sample at its place.

(3) Probe Approach
Turn on the easyScan 2 Controller and start the easyScan 2 Software on the control
computer. The main program window appears, and all status lights are turned off. Now a
Message ‘Controller Startup in progress’ is displayed on the computer screen, and the
module lights are turned on one after the other. When initialization is completed, a
Message ‘Starting System’ is shortly displayed on the computer screen and the Probe
Status light, Scan Head status light of the detected scan head and modules will light up.
To start measuring, the tip must come within a fraction of a nanometer of the sample
without touching it with too much force. To achieve this, a very careful and sensitive
approach of the cantilever is required. This delicate operation is carried out in two steps:
Manual coarse approach and the automatic final approach.
     Manual Coarse Approach
        Lower the scan head manually until the cantilever is between 1 to 2 mm from the
        sample. This is achieved by referring to the right magnifying lens, where the
        cantilever must be lowered to obtain a reflective image of itself on the sample.
     Automatic Final Approach
        Click the positioning icon on the software, and select ‘approach’. The software
        will perform automatic approach to bring the cantilever down to the set point
        value. A message ‘Approach done’ appears when the approach has been
        completed.




                          Figure 6: Software Automatic Approach Panel




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           Figure 7: View of cantilever after approach: left: side view, right: top view

(3) Scan Sample
The instrument was set to automatically start measuring after the automatic approach.
Two representations of the ongoing measurement are drawn in the Imaging panel. One
representation is a color coded height image, called a Color map, the other is a plot of
height as a function of X* position called a Line graph. Watch the displays for a while
until about a quarter of the measurement has been measured. With the current setting, the
software automatically adjusts the contrast of the Color map, and height range of the Line
graph to the data that have been measured.




                               Figure 8: Imaging Window




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(4) Zoom on Feature
After the measurement has been completed, activate the color map graph by clicking on
it. Click in the imaging toolbar. The mouse pointer becomes pen shaped when moving
over the color map and the Tool Results panel is zooming in on an overview
measurement displayed. Select an interesting region by drawing a square with the mouse
pointer. Click on one corner of the region using the left mouse button, and keep the
button pressed. Drag the mouse to the other corner of the region, and then release it. The
size and the position of the square are shown in the Tool result panel. Release the mouse
button when the square’s size covers approximately one period of the grid. Confirm the
selection by double clicking the color map graph using the left mouse button. Now the
selection is enlarged to the whole display size. You can abort the zoom function by
clicking again. The microscope will now start measuring a single grid period.




                   Figure 9: Zooming in on an overview measurement



 (5) Tip Retract
After completing the scanning, the tip is retracted from the sample by clicking on the
‘Retract’ button on the positioning window. A soft hissing sound is heard as the motor
retracts the tip away from the sample. Perform manual retracting and remove the sample
from the stage.




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6. AFM Experiment Procedures

1. Place the sample with square grid pattern with actual size of 5m on the AFM stage.
   Follow the AFM standard operating procedure. Select the appropriate data filter to get
   the clear square grid pattern on the imaging window.

2. Adjust rotation angle to ensure the image is horizontally and vertically aligned as
   shown in Figure 10 below. Take note that there will be more than four square grid
   patterns on your first measured/scanned overview AFM image. Note down the
   scanning area (dimension of x- and y- axis). Save your AFM image as bitmap file into
   your thumb drive.

                                 Vertically aligned



                                                                        Four square grid
                                                                        patterns
Horizontally aligned




                 Figure 10: AFM image with four square grid patterns.


3. Zoom in the feature to fit for one, four and nine square grid patterns on the image
   monitor. An example of AFM image with four square grid patterns is shown above in
   Figure 10. Save your AFM images with one, four and nine square grid patterns as
   bitmap files into your thumb drive.

4. From the print out of 5m square grid pattern (overview AFM image), draw lines on
   selected edges of square grid pattern as shown in Figure 11. Using a ruler, measure
   the horizontal (dx) and vertical (dy) distances. Repeat the same procedure for images
   with four and nine square grid patterns.




                                                                                      10
                                                     dx




         y


                                                                         dy
                      x




                  Figure 11: AFM image with four square grid patterns.


5. Calculate the x- and y-axis magnification of Mx and My for the square grid pattern of
   5m, respectively using the following formula:

               x-axis magnification           y-axis magnificaiton

                      Dx                             Dy
               Mx =                           My =
                      dx                             dy
               where
               D = measured distance (provide the distance in m)
               d = actual distance (5m +5m)

6. Repeat step (5) for the image which fits four, and nice square grid patterns.
   Calculate Mx and My for these images.

7. What are the advantages and disadvantages of AFM in comparison to SEM (scanning
   electron microscopy) in terms of surface morphology/topography inspection.

8. Comment on the potential applications of AFM in the area of nano-science.


IMPORTANT NOTE:
Report submission: Submit your report within 10 days of performing the
experiment to the same laboratory. Your report should include background
information on AFM, with neat diagram/graph/image of results and recorded data.
Include the discussion on the results obtained and the answers to the questions.




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