AFM operation notes sept 2006

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AFM operation notes sept 2006 Powered By Docstoc
					   Operation Notes for
            the
   Digital Instruments
   DIMENSION 3000
           AFM




NOTE: This operation manual does not replace the Digital Instruments
       handbooks (Instruction Manual and Command Reference
       Manual); they are meant as an introduction only. Please take the
       time to examine these manuals (which are available in the lab)
       and the Scanning Probe Microscopy Training Notebook (which
       you will get at your introductory session).
                                Contents
                                                              Page

    1. Introduction                                            3
           a. The atomic force microscope
           b. Some useful AFM related web sites

    2. Read me now: Laser safety precautions                   4

    3. Preparing the microscope                                5
          a. Switch on the microscope
          b. Mount a probe
          c. Align the beam to the end of the cantilever
          d. Align the photodiode
          e. Initialize the stage

    4. Obtaining an image                                     10
         a. Align the TV camera
         b. Mount the specimen
         c. Focus on the surface of the specimen
         d. Tune the cantilever (tapping mode only)
         e. Set scan parameters
         f. Start scanning
         g. Adjust scan parameters
         h. Stop scanning

    5. Optimising scan parameters                             17
         a. Scope mode
         b. The setpoint
         c. The gain settings
         d. The scan controls
         e. Some hints and suggestions

    6. Saving images                                          20

    7. Exchanging specimens                                   21

    8. Ending the session                                     22

    9. Appendices                                             23
         a. A brief description of the operating parameters
         b. AFM probes
         c. Contact AFM in fluids


AFM Operation Notes
                                                                     2
Dec. 2006
                                        1. Introduction
  a. The Atomic Force Microscope
Atomic Force Microscopy (AFM) is a Scanning Probe Microscopy (SPM) or
proximal probe technique. The atomic force microscope uses a sharp tip to
probe the surface of a specimen. The tip is a couple of microns long and has a
point which is 10-60 nm in diameter. It is located at the free end of a cantilever
that is 100 to 200 µm long. Forces between the tip and the surface of the
specimen cause the cantilever to bend or deflect. A laser aimed at the back of
the cantilever is reflected onto a photodiode detector which measures the
deflections. This information is used by the computer to generate a map of
surface topography. The interactions between the tip and the specimen may
also be used to collect other sorts of information such as specimen elasticity or
electrical potential.

                                   Means of sensing the
                                  vertical position of the tip




                                                                                                     A coarse positioning
                                                                                                      system to bring the
      A feedback system to                                                                            tip into the general
                                                                     A probe tip
                                                                                                     vicinity of the sample
        control the vertical
         position of the tip




                                                                   A piezoelectric scanner that moves the tip
                                                                 over the sample (or the sample under the tip) in
                                                                                a raster pattern




                                                                    A computer system that drives the
                                                                 scanner, measures data, and converts the
                                                                           data into an image


Figure 1: Schematic of a generalized SPM (from Veeco.com).

In contrast to electron microscopes, the AFM does not have to operate under
vacuum and the specimens do not need to be electrically conductive.
Specimens for the AFM need to be stable and relatively flat but otherwise need
minimal preparation. The AFM can be used to examine a wide range of
specimens from hard and dry (such as silicon wafers) to soft and wet (such as
living cells).

The AFM is relatively easy to use, but

                               I F I N D O U B T, A S K F O R H E LP.




AFM Operation Notes
                                                                                                                              3
Dec. 2006
  b. Some useful AFM related web sites


http://www.veeco.com                   Digital Instruments was bought by Veeco;
                                       this site contains information about AFM,
                                       probes and application notes*.

http://www.veeco.com/library/community.php

                                       Select SPM user forum to join a SPM user
                                       discussion group.

http://spmtips.com                     An alternative source of AFM probes and
                                       information.



http://ntmdt.ru                        Techniques, publications and application
                                       notes.




* Note that EMU at UNSW does not have the hardware to support some of the (advanced)
imaging modes.




AFM Operation Notes
                                                                                       4
Dec. 2006
  2. Read me NOW: Laser safety precautions




This microscope features a diode laser with an output of less than 1.0 mW.
Users are cautioned not to attempt adjustments to the laser, its electronic
control circuits, or optical components. If laser malfunction is suspected,
immediately return the entire Dimension head to Digital Instruments for repair
or replacement; the dimension head is NOT a user serviceable part.

                 WARNING: Use of controls or adjustments
                  or performance of procedures other than
                  those specified in these notes and the DI
                 manuals may result in hazardous laser light
                  exposure. The use of optical instruments
                   with this microscope will increase eye
                                   hazard.
        Users are cautioned against staring at or into stray beams reflected from
        shiny surfaces and/or stage components.

        Note that the head features a tilt switch to automatically power down the
        laser whenever the head is tilted out of a vertical position.

        Unplugging tip holders from the end of the scanner tube removes a
        partial barrier to the laser beam; probe holders should always be
        removed by pulling straight down, away from the operator.

        The laser is powered up whenever the Dimension head is plugged in to
        the microscope control electronics.

        Care should be taken when highly reflective specimens are on the stage.
        Avoid looking at reflected laser light.




AFM Operation Notes
                                                                               5
Dec. 2006
                      3. Preparing the microscope
 a. Switch on the microscope
  1.    Switch on both computer monitors.
  2.    Remove the cover from the microscope.
  3.    Check that the AFM head is unplugged. If not, unplug it.
  4.    Log in to the system.
  5.    Double-click on Nanoscope v530r3sr3 icon.
  6.    Turn on the controller at the left end of the bench.
  7.    Turn on the monitor next to the controller.
  8.    Turn on the light source.
  9.    Click on the REAL-TIME microscope icon.


        The AFM may be operated either real-time (as above) for
        acquisition of images or off-line for analysis of images. To
        operate off-line only, click on the off-line icon at step 9.




AFM Operation Notes
                                                                       6
Dec. 2006
 b. Initialize the stage
  1.    Select STAGE/INITIALIZE. This verifies that the motorized subsystems (Z-
        stage, optical zoom and optical focus) are operating correctly.

  2.    Click INITIALIZE.

  3.    Click OK as requested by the program. The afm head will travel to the top
        of its range and the stage motors will be driven to the limits of the travel.

              When the dialogue box closes, initialization is complete.




 c. Mount a probe
Choose an operating mode
The AFM may be operated in either contact mode or tapping mode.

In contact mode the probe is in continuous contact with the specimen during
scanning and the image is formed from the deflection of the cantilever as it
passes across the specimen. In general, this is best for hard specimens.

In tapping mode the probe lightly ‘taps’ the surface of the specimen and the
image is formed from the change in amplitude of the cantilever oscillation. In
general, this is best for soft specimens.

The choice of operation mode depends on the characteristics of the specimen
and the information to be gathered.

  1.    In the OTHER CONTROLS box, select the mode you wish to use by
        clicking on the box and scrolling to the appropriate mode.




AFM Operation Notes
                                                                                   7
Dec. 2006
Choose the appropriate probe
There are two probe holders with mounted AFM probes available in this unit.

              DNP probe
              Designed for use in contact mode. Each substrate has four
              cantilevers, two at each end. In most cases the shorter cantilever
              (spring constant 0.58N/m) is used. (For more details see Appendix
              b).
              OTESPA probe
              Designed for use in tapping mode. Each substrate has a single
              cantilever (spring constant ~42N/m). (For more details see Appendix
              b).

These probes are suitable for the majority of imaging done in this unit.
However, for specialized or advanced applications, users may be required to
purchase their own probes and probe holders.

When you begin your session, the probe used by the previous user will usually
still be mounted on the AFM head.




AFM Operation Notes
                                                                             8
Dec. 2006
          PROBE




        PROBE HOLDER




              AFM HEAD AND
                 CAMERA




Figure 2: Important parts of the AFM.




Mount the tip holder on the microscope head
    1. Release the AFM head from the stage by tightening the screw on the
       right-hand side of the mount.
    2. Slide the AFM head up and out of the mount.
    3. Mount the loaded AFM tip holder on the end of the scanner, matching
       the tip holder's sockets to the scanner end's pins (see figure 2).
    4. Check that the tip holder is mounted securely onto the AFM head.




AFM Operation Notes
                                                                            9
Dec. 2006
Figure 3: The AFM head and camera.




 d. Align the beam to the end of the cantilever
CAUTION: This instrument uses a semiconductor diode laser emitting
 a maximum 1.0mW beam at 670nm. The light is emitted down the
   centre of the scanner tube and normally reflects back into the
system’s optics on the back of the cantilever probe. During and prior
  to set up of the laser, it is especially important to avoid looking
          directly at the laser beam or at the laser spot.
  Note that the laser is powered up whenever the Dimension head is
          plugged in to the microscope control electronics.

    1. Connect the cable from the AFM head to the stage. This will turn the
       laser on (if you do not see the beam, see EMU staff). Be careful not to
       stretch the cable.
    2. Place a piece of white paper on the stage.
    3. Allow the laser to shine on the piece of paper.
    4. Using the knobs on top of the AFM head (A in figure 3) align the laser to
       the end of the cantilever. This often takes a bit of practice.
                Use the rear knob to find the edge of the substrate.
                Use the front knob to find the base of the cantilever.


AFM Operation Notes
                                                                                 10
Dec. 2006
                Use the rear knob to find the tip of the cantilever. (See figure 4)
                Note that the SUM signal (figure 5) should be above 0.5 or the
                microscope will not operate (normally it is between 4 and 6 for a
                correctly aligned cantilever).




                                        a                              b

Figure 4: Laser aligned on a) a contact mode tip and b) a tapping mode tip.


    5. Slide the AFM head into the mount. (Ensure the
       probe will not touch the stage by clicking on the
       WITHDRAW TIP icon, if necessary).

    6. Secure the AFM head by loosening the screw on the right-hand side of
       the mount.

 e. Align the photodiode
    1. Using the knobs on the left-hand side of the AFM (C in figure 3) align the
       photodiode so that the laser reflected from the end of the cantilever is
       centered on the photodiode.
                First centre the beam roughly by centering the red spot in the oval
                window on the microscope head (B in figure 3).
                Then perform a fine adjustment by centering the red dot on the
                secondary computer monitor.
                For operation in contact mode the vertical deflection should be
                set to –2V. (See figure 5)
                For operation in tapping mode the vertical deflection should be
                set to 0V. (See figure 5)
                In both modes the horizontal deflection should be 0V.




       Figure 5: Photodiode alignment for contact mode (left) and tapping mode (right).


AFM Operation Notes
                                                                                          11
Dec. 2006
                         4. Obtaining an image
 a. Locate the tip
    1. Click on the LOCATE tip icon (or select STAGE/LOCATE TIP).
    2. Click on ZOOM OUT. Adjust light intensity as required.
    3. Centre the tip of the cantilever on the monitor, using the knobs at D in
       figure 3. Align the tip to the centre of the cross-hair.
    4. Focus on the tip end of the cantilever, using the trackball control. (Hold
       down the left-hand button and roll the trackball. See figure 6)
    5. Check that the tip of the cantilever is correctly aligned and adjust, if
       necessary, using the knobs at D in figure 3.
    6. Zoom in using the trackball and refocus (if required).
    7. Click on OK.

                              Lock                         Lock




                              Focus                       Zoom

Figure 6: Diagram of the trackball control. To focus – hold down the focus button and roll the
trackball. To zoom – hold down the zoom button and roll the trackball. To move the stage – roll
the trackball. The trackball is only active when either the LOCATE TIP or the FOCUS SURFACE
functions are open.




AFM Operation Notes
                                                                                           12
Dec. 2006
 b. Mount the specimen
The specimen should not be more than 12mm in height and 15cm in diameter.

The specimen should be mounted so that it has as little freedom as possible to
move around. Most specimens are heavy enough that they can just rest on the
stage, because the forces during scanning are very low.
For specimens that need to be stuck down, double-sided tape on a microscope
slide is usually sufficient (be aware that the tape has a tendency to drift initially
and the specimen may need to be prepared some time before it is scanned).
Keep the bottom side of the slide clean. Other adhesives, such as ‘superglue'
may also be used.
    1. Check that the AFM head is high enough for the specimen to fit
       underneath it with at least 0.5cm to spare.
                If not, click several times on the WITHDRAW TIP
                icon until the gap is sufficient.
    2. Place the specimen on the stage.
    3. Rotate the stage (manually) so that the specimen is underneath the
       probe.



 c. Focus on the surface of the specimen
    1. Click on the FOCUS SURFACE icon (or select STAGE/FOCUS
       SURFACE). The optics of the camera will move to a focus
       position which is approximately 1mm below the tip.
       Therefore, the surface of the specimen will be in focus when the tip is
       1mm from the surface.

    2. Click on ZOOM OUT. Adjust light intensity as required.

    3. Hold down the FOCUS button on the trackball and roll the ball towards
       you to lower the AFM head towards the specimen until the distance
       between the tip and the specimen is about 1mm.

                Take care that the tip does not hit the surface of the specimen.

                The trackball will not operate if the FOCUS SURFACE dialogue box is
                not open.

    4. Adjust the focus of the specimen on the monitor, by holding down the
       focus button and rolling the trackball (carefully).

                Fine focus can be achieved by pressing the ZOOM button on the
                trackball and rolling the ball to magnify the image on the TV
                screen.

AFM Operation Notes
                                                                                   13
Dec. 2006
    5. Roll the trackball to align the area of interest with the centre of the cross-
       hair on the monitor.

    6. Click on OK.

    7. Place the cover on the microscope.



 d. Tune the cantilever (tapping mode only)
    1. Click on the CANTILEVER TUNE icon (or select
       VIEW/SWEEP/CANTILEVER TUNE).

                Check that START FREQUENCY is set at 100 kHz and END FREQUENCY
                is set at 500 kHz. (If specialty probes are used, these values may need to be
                altered – please see staff if you need to use a specialty probe).

                Check that the TARGET AMPLITUDE is 1.5 to 2 V.




    2. Click on AUTO TUNE.

                The cantilever is excited through a range of frequencies beginning
                at the START FREQUENCY and ending at the END FREQUENCY. A plot of
                the cantilever’s response curve is shown on the display monitor.

                In most instances the peak will have a sharp Gaussian
                distribution.

    3. Check the shape of the curve.

                The white curve should be symmetrical. If not, the laser may need
                to be realigned or the probe may need to be replaced. See EMU
                staff if you have any questions.




AFM Operation Notes
                                                                                         14
Dec. 2006
                The tapping frequency is now set to a frequency just below the
                resonance frequency of the cantilever.

    4. Click on BACK TO IMAGE MODE.

For most purposes, the auto tune function is sufficient. In some cases, however,
it may be useful to tune the cantilever manually. Please see EMU staff if you
believe that you need to do this.

 e. Set scan parameters
 Once the preceding setup has been completed, the microscope is operated
 entirely by computer. All of the operating parameters can be adjusted (after
 clicking on the appropriate box) in one of the following three ways:

              Use the left or right arrow keys to change the value.

              Type in a new value directly.

              Click in the parameter box and hold the mouse button down while
              dragging the mouse to the left or right to increase or decrease the
              value.




AFM Operation Notes
                                                                                15
Dec. 2006
Suggested initial settings:




            Contact mode      Tapping mode




                                             Note: Tip velocity
                                             should not exceed
                                                  30µm/s




                                                  Set during
                                                  auto tune




AFM Operation Notes
                                             16
Dec. 2006
 f. Start scanning
    1. Click on the ENGAGE icon (or select MOTOR/ENGAGE).

    2. Watch the monitor as the tip comes into focus and moves towards the
       specimen.

                The tip is lowered under program control. When the surface is
                detected, the computer beeps, starts the real-time imaging process
                and displays ‘engaged' in the status bar on the control monitor.

                The tip will travel up to 200µm as it approaches the sample
                surface. If the sample surface is not detected in this distance, an
                error message will be displayed. If this happens click on the
                WITHDRAW icon and check that the surface of the specimen is in
                focus.

                If the tip seems to ‘crash' into the specimen or the specimen
                moves when the tip makes contact then click on the WITHDRAW
                icon immediately.

                Once the tip is engaged it will continue to scan until the
                WITHDRAW icon is clicked.


 g. Stop scanning
    1. Click on the WITHDRAW icon (or select MOTOR/WITHDRAW).

                The tip is raised under computer control back to 1mm above the
                surface of the specimen.




AFM Operation Notes
                                                                                  17
Dec. 2006
                 5. Optimising scan parameters
Careful selection of the scan parameters is important. The optimal settings for
the scan parameters depend on the specimen and the information required.

a. Aligning the image




      •   Using the ZOOM controls.

          1.    Click on ZOOM IN or ZOOM OUT to open a box on the image.

          2.    Use the left-click control to toggle between adjusting the size of
                the box and moving the box.

          3.    Right click when the size and position of the box are correct. The
                cursor will jump to EXECUTE.

          4.    Click on EXECUTE.

          5.    Click on CLEAR to remove the box from the image.

      •   Using the OFFSET control.

          1.    Click on OFFSET to get a cross hair on the image.

          2.    Click on feature to be centered in the image.

          3.    Click on EXECUTE. The selected feature will now be in the middle
                of the scan (be aware that the maximum scan size will be reduced
                when the scan is offset).

          4.    Click on CLEAR to remove crosshair from the image.

      •   The cursor can be placed on the image to measure the distance and
          angle between two points on the image. The distances and angles are
          displayed directly on the screen, but they are not saved on the final
          image.

          1.    Click on the first point.

          2.    Click on the second point.

          3.    Right click to erase the line or measure a different area.




AFM Operation Notes
                                                                                18
Dec. 2006
b. Scope mode




SCOPE MODE shows a plot of tip-specimen interactions for the most recent line of
pixels scanned. This is a good way to monitor the effect of changes applied to
the scan parameters. The line direction in channel 1 (and 2 and 3 if they are
being used) should be set to retrace. The left to right trace of the tip is shown in
white and the right to left trace is shown in yellow. Ideally, the two lines should
be the same shape and be close together.

To go to SCOPE MODE   click on the SCOPE MODE icon (or go to
VIEW/SCOPE MODE).

To exit SCOPE MODE click on the IMAGE MODE icon (or go to VIEW/IMAGE
MODE).

The TMR value is the average distance between the trace and the retrace values.
This is a useful indicator of whether the tip is tracking the surface properly. The
TMR should be kept as small as possible for best results.


c. Z centre position
The piezo has a range of ±220V, but it operates best when in the middle of the
range (-20 to +20 V).

    1. Select MOTOR/STEP MOTOR.

    2. Minimise the step size.

    3. Click TIP UP if the z-centre position is too negative and TIP
       DOWN if it is too positive. Note: The effect may not be
       immediate. Proceed with caution to avoid overcorrection.
       CAUTION. THIS OPERATION RISKS DAMAGE TO THE TIP AND
       SPECIMEN.

    4. Click QUIT when the z-centre position is between ±20V.



AFM Operation Notes
                                                                                19
Dec. 2006
d. Setpoint
The SETPOINT influences the amount of force applied to the specimen by the
probe. It should be adjusted for each specimen to maximise image quality. In
most cases the less force applied to the specimen, the better the image. To
optimise the SETPOINT value, adjust the value until the probe lifts off the surface
of the specimen and then adjust the SETPOINT in the opposite direction until the
probe is lightly touching the surface of the specimen.

        Contact mode

        The SETPOINT should be set as low as possible (negative values allowed),
        which means that minimal force is applied to the specimen.

        Tapping mode

        The SETPOINT should be set as high as possible, which means that
        minimal force is applied to the specimen.

e. The gain settings
The gain settings have to be set empirically. They influence the feedback loop
and should normally be set as high as possible without introducing unwanted
oscillations. The INTEGRAL GAIN is usually the major contributor to the
performance of the feedback loop and is usually adjusted first. Typically,
settings for PROPORTIONAL GAIN are 35 -100% higher than for INTEGRAL GAIN
settings. However, this depends on the specimen so some experimentation is
required.

To optimize the gains for height data slowly increase the gain until the piezo
begins to oscillate (noise in the image) and then eliminate the oscillations by
reducing the gain with two or three clicks of the left arrow key. It is easiest to
monitor the effect of changes to the gain setting in SCOPE TRACE mode.

f. The scan controls
SCAN SIZE can be adjusted during scanning.

SCAN RATE should be adjusted so that the best image is achieved. The
appropriate scan rate depends on the specimen. Streaking in the image can
suggest that the scan rate is too high.

Reducing the SAMPLES/LINE and LINES allows a quick overview of the scan area to
be acquired. Increase the SAMPLES/LINE and LINES to 256 (or 512) to capture an
image.

In some cases it can be helpful to set the SLOW SCAN AXIS to DISABLED so that the
scan parameters can be optimised. This means that the tip is scanning over the


AFM Operation Notes
                                                                                 20
Dec. 2006
same line of pixels of the specimen repeatedly, rather than moving across the
specimen. Set the SLOW SCAN AXIS to ENABLED before capturing an image.

g. Some hints and suggestions
        Always keep in mind: the sharper feature images the duller one.




        Use minimum contact force in most cases. This is particularly important
        for soft specimens. Using excess force on hard specimens will wear the
        tip and cause the image to degrade.

        There are no standard optimum values for the operating parameters.
        There are many factors influencing the image including the tip shape on
        the atomic scale, the thickness of the water layer on the surface of the
        specimen (and tip), the humidity, the temperature etc. The operating
        parameters have to be optimized each time the microscope is used.

        The main limiting factor for high resolution is acoustic noise. Use the
        hood, close the door, turn off the light source and TV and the overhead
        lights to minimize noise in the room.

        To estimate the sharpness of the tip, zoom in and look at the smallest
        features visible. The diameter of the tip is in the size range of those
        features.

        The lateral (x,y) resolution of an AFM image is determined by two
        factors.

            o The step size of the image (eg. for a 10µm2 scan: a 256x256 pixel image
                has a step size of ~40nm and a 512x512 pixel image has a step size of ~20nm.
                If the tip ROC is >40nm, nothing is gained by using 512x512 pixels).

            o The minimum radius of the probe tip (interaction area is often a fraction
              of the tip radius).


AFM Operation Notes
                                                                                        21
Dec. 2006
6. Saving images
    1. To record and save an image click on the CAPTURE icon (or
       select CAPTURE/CAPTURE).

                The status bar will display CAPTURE: ON. If scan parameters are
                altered while capturing, the image capture is postponed until the
                scan regains the beginning of the next frame (the status bar will
                display CAPTURE: NEXT).

                When the capture is completed the status bar will display
                CAPTURE: DONE.

                The file is named with the date and time of recording and stored
                in the !:\ drive.

    2. Click on the OFF-LINE icon (or select DI/OFF-LINE).

    3. Copy your file from the !:\ drive to the m:\ drive.

                Highlight the file. Go to FILE/COPY and enter the destination
                directory (usually m:\images\afm\your login name).

                We recommend that images are saved into your folder in the
                images directory as the files on the m:\images directory are
                permanently backed up on the EMU server.

    4. Analyse your data.

                The data is saved as a Nanoscope file (extension .001). This
                format includes a full list of the parameter settings used in
                recording the image. This file can be analysed using the off-line
                software (eg. 3D presentation, roughness measurements).

                Nanoscope files cannot be displayed using
                other programs. Images can be exported as           Note: The probe will
                .tif files so that they can be displayed            continue to scan the
                elsewhere.                                          specimen while the off-
                                                                    line window is open. This
    5. Export .tif files. (the .tif file will be a screen capture   means that an image
       of the image on the display monitor).                        may be captured while
                Select UTILITY/TIF EXPORT.                          the previous image is
                                                                    being analysed.
                Enter the correct path (m:\images\afm\….)
                Click on reverse (so that you get a white           To minimise wear on the
                background and black text).                         tip, withdraw the tip if
                                                                    an image is not being
                                                                    captured while analysis
                Name the file.
                Click on SAVE.                                      is being performed.


AFM Operation Notes
                                                                                 22
Dec. 2006
                      7. Exchanging specimens
    1. Click on the WITHDRAW icon (or select
       MOTOR/WITHDRAW). Repeat until the tip is at least 5mm
       above the surface of the specimen.

    2. Rotate the stage so that the specimen moves away from the head.

    3. Remove the specimen.

    4. Place the next specimen on the stage.

    5. Check that there is sufficient space for the specimen to fit under the tip.

                Go to section 4c (focus on the surface of the specimen).


                       8. Ending the session
    1. Click on the WITHDRAW icon (or select
       MOTOR/WITHDRAW). Repeat until the tip is at least 5mm
       above the surface of the specimen.

    2. Rotate the stage so that the specimen moves away from the head.

    3. Remove the specimen.

    4. Unplug the afm head. (The afm head must be unplugged before the
       Nanoscope program is shut down.)

    5. Check that you have completed the analysis of your images and saved all
       of your images to the m:\images\afm\your login name.

    6. Exit the Nanoscope program by selecting DI/EXIT.

    7. Log out of the system.

    8. Switch off the controller, located on the left-hand side of the bench.

    9. Switch off the monitor.

    10. Switch off the light source.

    11. Switch off the two computer screens.

    12. Cover the microscope.




AFM Operation Notes
                                                                                23
Dec. 2006
                                9. Appendices
  a. A brief description of the operating parameters
 •   The OTHER CONTROLS panel

Microscope mode       selects the type of microscopy to be employed. To select
                      the mode you need, click on the box and scroll to CONTACT
                      or TAPPING.

Z limit               the z-range parameter is limited to the value of this
                      parameter. Features that are larger (vertically) than this
                      value cannot be imaged.

Units                 sets whether the units of certain scan parameters are METRIC
                      or in VOLTS. In most cases the units should be METRIC.

Color table           selects the range of colours used to map the z data. Usually
                      set to 12.

Engage set point      (tapping mode only) Allows the user to correct for loss of
                      tracking on engage due to sample differences.

 •   The SCAN CONTROLS panel

Scan size             defines the size of the scan by controlling the voltage
                      applied to the X and Y piezos.

Aspect ratio          controls the width-to-height size ratio of scans. An aspect
                      ratio of 1:1 results in a square scan.

X and Y offset        controls the centre position of the scan in the X and Y
                      directions respectively. Non-zero offsets reduce the
                      maximum scan size.

Scan angle            controls the angle of the X scan relative to the specimen.

Scan rate             sets the number of fast scan (X axis) lines performed per
                      second.

Tip velocity          the actual velocity of the tip as it scans over the specimen.
                      The value is related to scan rate and scan size: it should not
                      exceed 30µm/s in tapping mode.

Samples/line          selects the number of sample data points (pixels) per scan
                      line.

Lines                 selects the number of lines (data points in the Y axis). This
                      can be used to reduce the resolution along the Y axis,


AFM Operation Notes
                                                                                   24
Dec. 2006
                      which will speed up the scan and allow an overview of the
                      area to be examined.

Slow scan axis        allows the slow scan (Y axis) to be disabled, causing the
                      fast scan (X axis) to be repeated continually at the same
                      position. For normal imaging this should be enabled.

•    The FEEDBACK CONTROLS panel

SPM feedback          selects the signal to be used for tip feedback according to
                      the selected MICROSCOPE MODE. For contact mode, the
                      choice defaults to deflection; for tapping mode either
                      deflection or amplitude may be selected.

Integral gain         controls the amount of integrated error signal used in the
                      feedback calculation. Is usually the major contributor to
                      the feedback loop due to its “long term” influence. For this
                      reason, it is usually adjusted first.

Proportional gain     controls the amount of the proportional error signal used in
                      the feedback calculation. Typically, the settings for
                      proportional gain are 35 – 100% more than integral gain
                      settings.

Setpoint              contact mode (deflection setpoint):
                      controls the deflection-signal level used as the constant
                      desired voltage in the feedback loop. The lower the value
                      of the setpoint, the lower the force applied to the
                      specimen.

                      tapping mode (amplitude setpoint):
                      defines the amplitude of the cantilever oscillation signal to
                      be maintained by the feedback loop. The higher the value
                      of the setpoint, the lower the force applied to the
                      specimen.

Drive frequency       (tapping mode only) selects the oscillation frequency
                      applied to the piezoelectric crystal that vibrates the
                      cantilever (automatically adjusted by CANTILEVER TUNE
                      command).

Drive amplitude       (tapping mode only) selects the amplitude of the drive
                      voltage applied to the piezoelectric crystal that vibrates the
                      cantilever (automatically adjusted by CANTILEVER TUNE
                      command).




AFM Operation Notes
                                                                                25
Dec. 2006
 •   The CHANNEL 1, 2 or 3 panel

Up to three data channels may be opened simultaneously using the
REALTIME/PANELS menu. As channels are opened, their images are added to the
display monitor. It is possible to have up to three separate images from each
scan. The parameters on each CHANNEL control panel vary slightly, depending
upon the operating mode.

Data type             Options vary depending on operation mode. Selects the
                      type of data to be collected in that channel.

Data scale            Controls the vertical scale corresponding to the full height
                      of the display and colour bar.

Line direction        Selects the direction of the fast scan (X axis) during data
                      collection. When TRACE is selected data is collected when
                      the relative motion of the tip is left to right as viewed from
                      the front of the microscope. When RETRACE is selected data
                      is collected when the relative tip motion is right to left as
                      viewed from the front of the microscope.

Scan line             Controls whether the MAIN or INTERLEAVE scan line is
                      displayed and captured. This parameter is not selectable
                      when the INTERLEAVE MODE parameter is set to disable.

Realtime planefit     Applies a software ‘levelling plane' to each realtime image,
                      thus removing first order tilt. This function does not apply
                      to captured data, and only improves the realtime image.
                      Normally set to LINE (takes the slope and Z-axis average of
                      each scan line and subtracts it from each data point in that
                      scan line).

Offline planefit      Applies a software leveling plane to each offline image for
                      removing first-order tilt. This function applies to captured
                      data. Normally set to FULL (a best fit plane which is derived
                      from the data file is subtracted from the captured image).




AFM Operation Notes
                                                                                 26
Dec. 2006
  b. AFM probes available in this lab

Contact mode probes
Contact mode probes consist of cantilevers with low spring constants (<1N/m)
to minimize the force between the tip and the sample during imaging. For
greater reduction of tip-sample forces, contact mode imaging may also be
performed in a fluid environment with the same probes (see appendix c).


 •   DNP Probes (Digital Silicon Nitride Probes)

Probe Specifications

These are general purpose silicon nitride probes. There are 4 cantilevers on
each probe. In general, the 0.58 N/m cantilever is used in this lab.


                           DNP probe specifications
                                    (as supplied)


Force (or Spring) constants                 0.58, 0.32, 0.12, 0.06 N/m

Nominal tip radius of curvature             >10 nm (as supplied)

Cantilever lengths                          100 and 200 µm

Tip height                                  2.5 – 3.5 µm

Cantilever configuration                    triangular

Reflective coating                          gold

Tip angles                                  35º on each side, 35º front, 35º back




AFM Operation Notes
                                                                               27
Dec. 2006
Tapping mode probes

Tapping mode probes consist of a short, stiff silicon cantilever with an
integrated single crystal silicon tip. The cantilevers have high resonance
frequencies and high spring constants.



 •   OTESPA probe (Olympus tapping mode etched silicon probe)

The tetrahedral tip of the Olympus etched silicon
probes can be used when very high lateral resolution
imaging is required. Features of these probes include:

       Position of tip can be located in Video View

       Aluminized on the back for better reflection

       Tip angle of 35º or less

       With the cantilever at the standard tilt, the tip
       angles are nearly symmetric in the ±x directions




                           OTESPA probe specifications
                                    (as supplied)


Force (or Spring) constants                 ~42 N/m

Resonant frequency                          200 – 400 kHz

Nominal tip radius of curvature             <10 nm (as supplied)

Cantilever length                           160 µm

Tip height                                  10 µm

Cantilever configuration                    rectangular

Reflective coating                          aluminium

Tip angles                                  35º on each side, 0º front, 35º back




AFM Operation Notes, UNSW EMU                                                      28
Revised April 2007
 c. Contact AFM in fluids
Essentially, the procedure for observing specimens under fluid is the same as
that for contact AFM in air; however, a special tip holder is used and minor
adjustments must be made to correct for refractive effects as the laser beam
transmits air-fluid boundaries. Therefore, anyone intending to observe
specimens under fluid must seek training from EMU staff. These instructions are
designed to be used in conjunction with the main Operation Notes for the AFM.
Please make sure that you are familiar with those Notes before you start.

                                 Warning!!!!
   THE PIEZO TUBE MUST NOT BE ALLOWED TO GET WET. IT IS A VERY
  SENSITIVE PIECE OF EQUIPMENT AND VERY COSTLY TO REPLACE. TAKE
  EXTREME CARE NOT TO SPILL FLUIDS ON OR AROUND THE SPECIMEN
      STAGE OR ON ANY OF THE ELECTRONIC EQUIPMENT. SAMPLE
    PREPARATION MUST TAKE PLACE AWAY FROM THE MICROSCOPE.

Instructions
   1. Mount specimen securely and cover with liquid (not more than 7mm in
      depth).

   2. Mount fluid tip holder (probe previously loaded by staff) onto the AFM
      head. [If there is no probe loaded, see EMU staff.]

   3. Mount the protective rubber skirt onto the AFM head.

   4. Ensure that the microscope is set for contact mode.

   5. Align the laser onto the tip in the same way as for imaging in air. (Section
      3c and 3d). The red dot on the left hand monitor should be positioned so
      that the horizontal deflection is 0V and the vertical deflection is -2V.

   6. Locate the tip of the cantilever with the TV camera (Section 4a) and
      remain In the LOCATE TIP function.

   7. Move the laser beam onto the substrate of the probe, using the rear knob
      on the top of the AFM head. You should be able to see this on the TV
      screen. The laser signal sum (bar on the bottom of the left-hand monitor
      screen) should be ~4 (as the laser signal moves the SUM will fall to ~0
      and then rise as the laser moves onto the substrate).

   8. Place the specimen (appropriately mounted) onto the stage, under the
      probe. Be careful that the probe does not touch the surface. (Raise the
      probe by clicking on the WITHDRAW icon.)



AFM Operation Notes, UNSW EMU                                                   29
Revised April 2007
     9. Click on the FOCUS SURFACE icon.

     10. Lower the AFM head until the surface of the specimen is in focus on the
         TV screen. During this process the probe should enter the liquid.

     11. Click on the LOCATE TIP icon.

     12. Focus on the tip.

     13. Move the laser beam back onto the tip of the cantilever using the rear
         screw on top of the AFM head. Check that the laser signal sum is ~4.

     14. Check that the vertical deflection is ~-2V and that the horizontal
         deflection is at ~0V.

     15. Click on the FOCUS SURFACE icon.

     16. Focus on the specimen surface. Then (very carefully) adjust the focus so
         that the camera is focused a little below the surface of the specimen.

     17. Click on the ENGAGE icon.

     18. Scan surface as normal.

 •    At the end of the session

          Click on the withdraw icon as many times as required to get the tip
          clear of the specimen.

          Remove the specimen (being careful to avoid the tip holder).

          Remove the protective skirt and the fluid tip holder and rinse them in
          distilled water to clean them (do this away from the microscope).

          Place the fluid tip holder and the protective skirt into the storage box.

          Save your images and shut down the microscope as normal.




AFM Operation Notes, UNSW EMU                                                      30
Revised April 2007

				
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Description: AFM operation notes sept 2006