Basic Operation: Contact Imaging
Table of Contents
- Powering up the System
- Selecting a Probe
- Installing a Probe
- Viewing the Probe in the Camera Window
- Aligning the Laser Optics
- Setting the Scan Parameters (SPM Config Window)
- Engaging the Probe with the Sample Surface
- Scanning the Sample
This page serves two purposes: It provides a fairly detailed overview of the basic steps needed to image a surface and any scanning mode of the instrument, and more specifically, it explains how to image a surface in the scan modes where the probe tip remains in constant contact with the sample surface: Z Height, Broadband, and Lateral.
The basic steps required to image a sample are as follows:
- Power up the system.
- Select and install a probe.
- View the probe in the video camera.
- Align the laser optics.
- Set up the scan parameters for the selected imaging mode.
- Position the sample under the probe.
- Lower the probe onto the sample surface.
- Scan the surface.
These steps will be discussed in more detail below.
Powering up the System
If the SPM system has an AC Power Control Module set the Computer, Monitor, and Aux1 switch positions to ON. If your system does not have an AC Power Control Module then turn on the computer, the computer monitor, and the EIU electronics with the power switches on each device. The computer is usually started by pressing a power button on the front panel of the computer, but note that some computers have an additional AC switch on the back of the case.
The yellow ON light at the front panel of the EIU should be glowing, and the green READY light should be off. This indicates that the EIU is receiving AC power, but has not yet been initialized by the software. If the yellow light does not come on check the position of the power switch located on the back panel of the EIU.
The computer will boot-up automatically with the Windows desktop displayed. To start the SPM control software, double click the ScanAtomic icon on the desktop. (The SPM software folder can also be reached from the desktop. Use the Start button in the lower-left corner of the Windows desktop and navigate to Start > Programs > SPM.) When the software starts the ScanAtomic main window should appear on the screen, and the green READY light on the EIU front panel should glow. The green light indicates that the EIU has been initialized and is ready to control the microscope. If either the READY light does not come on or an error message is generated by the software refer to the Appendix for likely causes and solutions.
Selecting a Probe
Anatomy of a Probe: Commercially manufactured cantilevers are fabricated on wafers of silicon or silicon nitride using semiconductor industry technology, i.e., vacuum chambers, photolithography, sputtering, and ion beams. As manufactured, a single wafer will contain hundreds of 1.5 x 3.6 mm “dies”, each having a tiny cantilever protruding from one end. The dies are very difficult to manipulate by hand because of their size. For ease of handling, Ambios mounts the dies onto magnetic stainless steel mounting preforms, referred to as “crosses.” The crosses fit into a slot in the gold colored “probe holder” at the front of the scanner. The left and right edges of a cross are called “tabs.” It is easiest to manipulate a cross by grasping one of the tabs with tweezers.
Throughout this manual the whole cross-and-die unit is sometimes referred to generically as a “cantilever” or “probe.”
Guidelines: SPM cantilevers fall into two classes: “Contact” type and “Intermittent Contact” type. The characteristics of the surface you wish to scan, and the type of surface information you wish to obtain, dictate which type of cantilever you will choose.
Contact-type cantilevers work best in situations where the material to be imaged is reasonably hard (e.g. metals, ceramics, most polymers) and the surface topography does not have abrupt edges or tall, steep features. They are less expensive than intermittent type cantilevers. And because there are fewer instrument settings involved in using them, imaging a surface with contact cantilevers is easier than with intermittent cantilevers. Anyone new to the field of SPM microscopy should definitely learn to operate the microscope with a contact-type cantilever first. In addition, contact cantilevers have the ability to detect lateral friction forces on the surface, when this type of measurement is desired.
Intermittent-contact cantilevers generally work well on all surfaces. They have the disadvantages of being more expensive and a bit more difficult to use, but they excel in imaging surfaces that are very soft (e.g. organics, polymer coatings) and surfaces with steep features. Intermittent contact cantilevers also have the ability to detect the elastic and adhesive properties of surface materials, when this type of measurement is desired.
The above information provides a rough guideline for selecting a cantilever. Additional information is given in the sections of the manual covering specific modes of operation of the microscope. SPM systems are shipped from Ambios with one box of general purpose contact-type cantilevers and one box of general purpose intermittent-contact cantilevers.
Installing a Probe
The following steps pertains to the standard probe holder, which uses small magnets to hold the magnetic stainless steel cross in place.
- The scanner incorporates a 5 mW class IIIa laser. To avoid eye injury the laser must be turned off whenever the head is removed from the stage. The laser on/off switch is located in the toolbar in the program’s main window.
- The scan head should be several millimeters away from the stage before removing it from the dovetail plate. If it is not, bring up the Probe Position window and hold down the Z Fast Up button until the probe is several millimeters away from the stage.
- Remove the scanner by loosening the thumbscrew on the dovetail mounting plate and carefully lifting the scanner off of the microscope stage.
Note: When removing the scanner from the stage, take care not to bump or strike the probe holder, or the ceramic tube to which it is attached. Doing so may damage the PZT.
Place the scanner on a level surface, resting it on the flat end of the dovetail extension, with the probe holder facing you.
Refer to the photo above. If a cantilever is already seated on the probe holder remove it by sliding the cross in the probe holder so that one of its tabs protrudes beyond the edge of the probe holder, and then lift it away with tweezers.
Open a cantilever box and select a replacement cantilever. The cantilevers are held in place within the box by a magnet pad. Using the points of the tweezers, slide the cantilever to the edge of the magnet pad so that a tab overhangs the edge. Then lift up the cantilever by the tab with the tweezers. Do this carefully because if you drop it, it will most certainly be ruined. Lay the cross into the probe holder. With the points of the tweezers, center the cross laterally in the probe holder and then slide it fully forward.
- Place the scanner back into the mounting plate on the stage. The lower end of the dovetail should rest against a mechanical stop at the bottom of the mounting plate. Tighten the thumbscrew to lock the scanner in place.
The following steps pertains to the optional nonmagnetic probe holder,
- Move the tilt lever to the horizontal position and lightly lift the probe holder spring by pushing the insertion tool under it, as shown in Figure 9-1. Make sure the insertion tool is at either the left or right edge of the slot to allow room for the metal cross.
Note: Be careful. Over-extending the spring will permanently deform it.
Next, holding the metal cross by one of its tabs with tweezers, insert the bottom end of the cross under the spring as shown in Figure 9-2. Gently remove the insertion tool.
Center the probe in the holder by sliding it laterally with the tweezers.
Viewing the Probe in the Camera Window
Opening the Camera Window: The camera view of the cantilever is displayed in the Probe Position window. When the cantilever tilt is set correctly, and the view is properly illuminated and in focus, the camera view will appear similar to the image below. If the camera view is out of focus skip ahead to “Focusing and Rotating the Camera View”.
Compare the cross, die, and cantilever positions in the image above on the left to the image above on the right. Note that the cantilever is the only object in-focus. The front of the cross and the front of the probe holder are above the depth-of-field of the camera optics, and are blurred. The sample surface beneath the cantilever is below the depth-of-field, and is also blurred. The cantilever will appear more distinctly in the camera view when the surface in the background is at least partially reflective. For example, the cantilever tends to disappear against the flat-black surface of the XY translation stage, but it can be viewed much more clearly if a glass slide is placed on the stage.
The illumination of the camera view is adjusted with the light intensity slider control in the Probe Position window.
Note that as the light intensity is increased a point may eventually be reached where the video camera’s automatic brightness control feature will become active, and the camera view will not brighten even though the illuminator light intensity is increased. Avoid unnecessarily bright camera illumination settings. Doing so will reduce extraneous heating effects inside the scan head.
If the view is poor then follow the next three sections to improve the cantilever view.
Adjusting the Cantilever Tilt: The tilt of the cantilever can be varied over a range of about ± 4° with the small tilt lever located at the back of the probe holder. The purpose of the tilt adjustment is to compensate for manufacturing variations in the tilt of the cantilever. Later, when the laser optics in the scanner are aligned the exact setting of the tilt lever will be made. Here, for the purposes of viewing the cantilever, it is only necessary to adjust the tilt lever to approximately the right position. If the tilt lever is set too low, as shown in the image below (A), the cantilever will tilt steeply downward and reflect very little of the illumination light back toward the camera. The cantilever will appear to fade into the background. On the other hand, if the tilt lever is set too high as in (B), the cantilever will be close to horizontal and it will appear mirror-like in the camera window. The correct adjustment falls between these two extremes, as shown below. Adjust the tilt of your cantilever accordingly
Focusing and Rotating the Camera View: Refer to the image below. Loosen the plastic locking screw on the camera body. By sliding the camera body along the extension tube the position of the focal plane of the camera can be varied. Rotating the camera body will rotate the camera image. Set the focal plane so that the cantilever appears sharply in focus in the camera image, and rotate the camera so that the cantilever falls along the y-axis of the video screen. Tighten the locking screw.
Centering the Camera View: The camera view is centered at the factory; normally it will not be necessary to adjust it. If for some reason the cantilever position becomes shifted out of place in the camera view, however, do the following:
- Loosen the locking screw and raise the camera body so that approximately 2 cm of the extension tube is exposed.
- Unscrew the extension tube from the lens assembly by about a quarter of a turn. Tighten the locking screw to hold the camera in place.
- Loosen the locking ring by unscrewing it by no more than a quarter turn.
- The lens assembly will now be free to move laterally by about 1 mm in all directions. As the lens assembly is moved the center of the camera image will also move. Shift the lens assembly as necessary to center the cantilever near the top of the camera window. Then tighten the locking ring.
- Screw the extension tube back down onto the locking ring surface. Refocus the camera image as described earlier. Note that when the locking ring is loose it is also possible to rotate the whole lens assembly into or out of the scan head. Doing so will change the magnification of the camera optics. It is recommended that the magnification is not varies from the factory setting.
Aligning the Laser Optics
The position of the cantilever is measured by bouncing a laser beam off of its mirror-like back surface and detecting the angle of the reflected light. Due to manufacturing variations between different cantilevers, and variations in how the cross is inserted into the probe holder, it is necessary to adjust the laser optics every time the cantilever is changed.
There are three basic steps to aligning the laser optics of the scanner.
- The first step is to direct the laser beam onto the back of the cantilever.
- The second step is to adjust the tilt of the cantilever to direct the light reflecting off of the back of the cantilever toward the optical detector.
- The third step is to adjust the position of the optical detector for the optimum laser signal. These steps will be covered in detail below. The alignment control locations, and the path of the laser beam inside the scan head, are shown below.
First Step: Directing the Laser
To direct the laser beam onto the back of the Cantilever perform the following,
- First, locate the position of the laser beam in the camera window by rotating the Laser Position knobs to direct the laser onto the back of the cross, as shown below in (A).
Next, shift the laser spot on the Cross to a point just to the left of the cantilever, as it appears in (A). The spot is approximately 3 widths of the cantilever to the left of the cantilever.
Now if the laser Y position knob is rotated counterclockwise by about a quarter of a turn the laser spot will slide off of the cross and down onto the end of the cantilever, as shown in (B). It will probably be necessary to also adjust the X position control slightly to center the laser spot on the width of the cantilever.
The camera images in step 1. show a diving board shaped, contact mode cantilever. The performance of the microscope is not sensitive to the exact laser spot position. For this type of cantilever, the laser spot may be positioned anywhere on the lower half of the cantilever’s length. NOTE: Silicon nitride contact-mode cantilevers are V shaped for extra stiffness. They are usually manufactured in pairs at the end of the die. Either of the V’s may be used, but the microscope may perform erratically if both probes contact the surface at the same time. When adjusting the laser beam position it is important to be sure that the laser is being reflected from the apex of the cantilever and not from one of the arms.
Second Step: Adjusting the Tilt
Adjusting the Cantilever Tilt follow the steps below,
Rotate the Mirror Position knob fully counterclockwise. This will flip a small mirror inside the scan head into the path of the laser beam, redirecting the laser beam toward the alignment window on the left side of the scanner. The alignment window is made of frosted glass, and the laser spot will appear as a red dot on its surface.
When the cantilever is positioned properly the laser dot will fall near the middle of the alignment window. Exact centering is unnecessary: anywhere within the circle indicated in the image below will be sufficient.
If adjustments are necessary to bring the laser spot within the acceptance circle it is usually sufficient to just move the laser spot laterally. This is accomplished by adjusting the tilt of the cantilever. Refer to the main diagram. Move the tilt lever at the back of the probe holder up or down slightly as necessary to reposition the laser spot in the alignment window.
It is usually unnecessary to center the laser spot vertically within the alignment window. When required, however, the laser spot can be shifted vertically by sliding the cross laterally within the probe holder. Note that when the cross is moved the laser X and Y position controls will have to be readjusted to direct the laser beam onto the back of the cantilever.
- Be sure to rotate the mirror position knob fully clockwise before continuing with the next alignment step— adjusting the optical detector position. A common mistake is to accidentally leave the mirror in the laser path, and then attempt to adjust the detector position for a laser signal that does not exist
Third Stem: Positioning the Detector
Adjusting the Optical Detector Position follow the steps below,
- Open the Beam Align window as seen below. On the right side of the Beam Align window is a pointer placed against a logarithmic scale which represents the intensity (Beam Intensity) of the laser light striking the optical detector. Within the central area of the window there is a red dot set against a green target pattern (Beam Position) which graphically represents where the laser beam is striking the optical detector.
NOTE: At this step in the alignment process the direction of the laser beam is fixed, and the red dot is moved in the target pattern by shifting the position of the optical detector with the detector’s X and Y position controls. The correct position of the red dot is determined by the type of probe in the probe holder. For contact mode topology imaging (that would be Z Height or Broadband) the correct position of the red dot is at the intersection of the vertical green line and the innermost target circle, as shown in the image above.
If the Beam Intensity pointer falls anywhere in the green zone, as indicated in the image in step 1., then the laser optics alignment process is complete, and the Beam Align window can be closed; YAY!!! You can skip step 3!
If the Beam Intensity pointer falls in the yellow or red zones, however, then the laser signal is too weak for the microscope to work properly. If you are having issues getting the laser optics aligned you are encouraged to ask a Manager for assistance during this step; This does not happen very often, but when it does happen further adjustments will be necessary. There may be a problem with the microscope itself rather than an operator error so ask a Manager just to be safe. NOTE: The problem usually lies in the reflectivity of the cantilever. There are two solutions.
- The first is to use the laser position controls to redirect the laser to a more reflective spot on the back of the cantilever, and then readjust the detector position.
- The second solution is to replace the cantilever altogether and repeat the entire laser optics alignment sequence. (Managers Only!!!!) To better understand the operation of the microscope it helps to understand what is physically being adjusted by the position of the red dot. Ultimately what this adjusts is the contact force between the cantilever point and the surface when the probe is engaged with the surface. The contact force is determined by two factors:
- The spring constant of the cantilever.
- The distance the cantilever is deflected (bent). The distance of the red dot above the horizontal green line in the target pattern determines the distance the cantilever will be deflected when it is in contact with the surface. For example, if the cantilever has a spring constant of 0.2 N/m, and the red dot is positioned in the normal way as in the image in step 1. then the probe point will be pressed into the surface with a force of approximately 10^-7 N during a scan. If the red dot is set higher than this, the cantilever deflection will be larger, and the contact force will be greater than 10^-7 N. This tends to wear down a probe tip more quickly, and generally should be avoided. On the other hand, if the red dot is set lower than the normal position the cantilever deflection will be smaller and the contact force will be lower than 10^-7 N. This is better from the point of view of cantilever tip wear, but it also means that the vertical feedback circuitry will have a smaller force signal to work with during a scan. Scans may have to be performed more slowly, and the resulting image may be noisier. NOTE: The Beam Align window access controls are disabled whenever the probe is engaged with the sample surface. Retracting the probe with the Withdraw command will re-enable the Beam Align access buttons.
Setting the Scan Parameters (SPM Config Window)
The scan parameters in the SPM Configuration window determine where and how an image is obtained. To achieve the best imaging results it is important to choose the most appropriate scan parameters for the sample being studied. This can get a little tricky so you are encouraged to ask a Manager for help during this step!
You will need to set several parameters in this step. See the image below for a basic outline of the different options you can set in the SPM Configuration window,
Below is the SPM Configuration window,
The dial controls work by the click and drag method: position the mouse cursor on the tip of the pointer of a dial, hold down the left mouse button, and rotate the pointer until the desired parameter value appears in the window. To increase the resolution of the rotary adjustment the cursor can be moved further from the center of the dial while the mouse button is held down.
The functions of the various controls in the SPM Configuration window are explained below. NOTE: some controls may be disabled at times, depending on the mode of operation of the microscope.
- Scanner Type, XYZ, Samples Per Point, Point Size, Scan Speed. This is information about the state of the microscope. Scanner indicates the type of scan head attached to the microscope stage. XYZ gives approximate values for the range accessible to the scanner along the three axis of motion. Samples per Point specifies how many signal samples will be averaged to produce the final datum at each point in the image. The value of Samples per Point is determined by the settings for Scan Rate and Resolution. Point Size is the length and width of the square area represented by each image point. Scan Speed is the average speed of the probe’s motion along the fast rastering direction. Its value depends on the Scan Size and Scan Rate settings.
- WAVE CONFIG is enabled whenever the Scan Type control is set to one of the modes requiring an oscillating cantilever. This includes Wavemode, BB Wavemode, Phase mode, and all ME modes. Pressing this button brings up the Wave Configuration window, which has controls for setting the frequency and amplitude of the oscillation. Information about how to use the Wave Configuration window is found in the PDF Manual Sections 4.6 and 12.2.
- OK/Cancel. Changes made to the settings in the SPM Configuration will not come into effect until the OK button is pressed. This action also closes the window. Cancel reverts any control changes to their original settings.
- Scan Size (um) is the length of one side of the square area scanned. Thus, a value of 10 μm means a 10 μm x 10 μm area will be scanned. The smaller the value, the higher the magnification.
- Scan Rate (Hz) sets the number of image lines scanned per second, e.g., a setting of 2 Hz means that two scan lines will appear on the screen per second. Samples with sharp features should be scanned at a slow scan rate (≤ 0.5 Hz) to allow the feedback circuitry more time to react to sharp image contour changes. Slower scan rates imply more time to acquire an image.
- Setpoint (V) functions differently depending on the Scan Type selection:
- In all scanning modes based on rastering the probe with constant surface-tip contact (i.e. Z Height, Broadband, Lateral Force, and BiLateral Force) the Setpoint is normally left fixed at zero.
- Moving Setpoint to nonzero values has the same effect as moving the red dot in the Beam Align window closer or further from the horizontal green line: it changes the contact force during the scan. A positive value decreases the value of the contact force; a negative value increases the contact force.
- In all scanning modes based on intermittent surface-tip contact (i.e. Wavemode, BB Wavemode, Phase, BiPhase, and all ME modes) the Setpoint is adjusted to set the damping of the probe oscillation during scanning. Typical values are in the range of -0.2 to -1.0 volts.
- Scan Direction sets the angle of the scan raster relative to the x-axis, which is defined as a vector running from left to right along the microscope stage. The usual setting is 0.0°; data will be recorded as the probe moves from left-to-right along the x-axis. Increasing the angle will rotate the scan counter-clockwise. The scan direction is normally changed only when it is necessary for the probe to move across certain features of the surface in a specific direction (e.g. perpendicular to a surface crack or step).
- Integral Gain, Proportional Gain, and Derivative Gain. These three controls determine the response of the Z-axis feedback circuitry— the circuitry responsible for maintaining the probe deflection (contact mode), vibration damping (intermittent-contact mode), or tunneling current (STM mode) when the probe is engaged with the surface. These parameters determine how well the probe will follow the contours of the surface as it is rastered back and forth.
- Scan Resolution is the number of image points in a horizontal line of the scan. This is also equivalent to the number of lines in the scan. Higher scan resolutions reveal greater surface detail, but require more time to acquire the image. Exploratory scans with a resolution of 300 are good. Final scans with a resolution of 400 to 600 are better.
- Bias Voltage (V) control sets the voltage of the probe holder. This is used in the STM, EFM, and C-AFM imaging modes. For all other modes the bias voltage should be set to zero.
- Center X (um) and Center Y (um) specify the center coordinates of the scan relative to the center of the total scan area accessible to the scanner. For example, a 40 μm scan head can access a total scan area of 40 μm x 40 μm. The center of this area is defined as position (0 μm, 0 μm), thus Center X and Center Y may be varied over a range of -20 μm to +20 μm.
- NOTE: If the probe is in contact with the surface when either of these coordinates is changed, pressing the OK button will cause the probe to move to the new coordinates at the currently defined scan speed (the speed is shown in the upper-right portion of the window). If the scan speed happens to be very slow, and the distance the probe must move is relatively large, then the time required to move the probe can be many seconds. If the required time is more than 3 seconds a message box will appear asking if it would be preferable to move at a faster rate, or if would be better to cancel the operation so that the Center X,Y coordinates may be redefined.
- XY Disabled disables the X,Y raster during a scan. For normal operation this control box must be unchecked. This is only used in the system diagnostics procedures.
- Scan Type selects between the different types of data which may be measured as the probe is rastered. The choices are listed below. First-time SPM users should select Z-Height, which is the simplest mode of operation.
- Z-Height: This is a contact-mode topology scan based on maintaining a constant force between the probe tip and the surface. The Z feedback loop makes the piezo tube contract and expand as necessary to keep the deflection of the cantilever constant. The image is formed from the Z control voltage of the PID circuitry. The Z control voltage is converted into a vertical distance with the Z calibration factor.
- Broadband: As in Z Height topology scanning, the system attempts to maintain a constant contact force between the tip and the surface during a Broadband scan. The difference is that any deviations from the nominal constant force condition between the probe and the surface are corrected for during image rastering. Deviations from the constant force state imply deviations in the deflection of the cantilever. During a Broadband scan the deviation in the cantilever flexure is measured and this information is combined with the Z control voltage to produce a more accurate measurement of the surface profile.
- Lateral Force: This is a recording of the twist of the end of the cantilever as it is rastered over the surface. During a contact-mode scan there will be a friction-induced shear force applied to the probe tip which will cause the entire cantilever structure to twist. The degree of twisting is, in part, a reflection of the strength of the frictional force between the probe tip and the surface. The frictional force may vary as the probe passes over different materials in the sample surface. In favorable circumstances this can be used to extract information about where different materials are located in the surface topography.
- BiLateral Force: The measurement performed here is the same as in Lateral Force mode, except that data are recorded for both the forward and reverse raster of each scan line.
- Wavemode: This produces a topological view of the sample surface. The cantilever is set into vibration with an amplitude of order 100 nm. Then the probe is lowered to the surface where it makes intermittent contact with the surface, damping the oscillation amplitude. Recall that in the contact imaging modes it is the deflection of the cantilever which is held constant by the Z feedback circuitry. Wavemode differs from the contact modes in that the Z feedback circuitry expands and contracts the piezo tube as necessary to keep the damping of the probe’s oscillation constant as it is rastered.
- BB Wavemode: This is another topological imaging mode. Similar to the Broadband mode, BBWavemode attempts to correct for any deviations from the ideal constant damping condition in a Wavemode-type scan.
- Phase: During a Wavemode surface scan, not only is the amplitude of the cantilever’s motion dampened as it bumps into the surface, but the phase of the motion is also shifted. Contact with different materials on a surface may cause this phase shift to differ. For example, a hard surface will advance the phase, while a soft or sticky surface will impede the phase. In favorable circumstances this can be used to extract information about where different materials are located in the surface topography.
- BiPhase: The measurement performed here is the same as in Phase mode, except that now data are recorded for both the forward and reverse raster of each scan line.
- ME Camp/Cphase/Tamp/Tphase: These four modes are used for both Magnetic Force Microscopy (MFM) and Electrostatic Force Microscopy (EFM). MFM maps the magnetic field gradients above a surface; EFM maps the electric field gradients above a surface. NOTE: If you select one of the ME modes of operation then you will have the Delta Z enabled; this determines how high the probe will be raised above the surface when the field gradient is measured. For more info on MFM or EFM see the PDF Manual chapters 13 and 14.
- C-AFM: This mode produces a map of the contact current between a conducting probe and the surface. The required hardware is optional.
- STM Log, STM Lin: Scanning tunneling microscopy. The hardware required for these modes is not available for this microscope.
- XY Signal Mode selects between the different methods of rastering the probe across the surface. Systems fitted with the metrology option may be operated in Standard or Metrology modes of scanning. The Metrology mode is discussed in the PDF Manual Supplement 11. Microscope systems without the metrology option may only be operated in Standard mode.
- Z Signal Mode selects between the different methods of measuring the Z position of the probe tip. Systems fitted with the metrology option may be operated in Standard or Metrology mode. The Metrology mode is discussed in the PDF Manual Supplement 11. Microscope systems without the metrology option may only be operated in Standard mode.
Scan Parameters for Z Height Imaging
The scan parameters in the table below are a starting point for first-time work in Z Height imaging. Once reasonable images have been obtained with these settings move on to the next step of experimenting with the settings to get a feel for the instrument.
The parameters with the most ‘mystery’ in the table above are the Proportional, Integral, and Derivative (PID) gains. Learning to adjust these parameters for the best performance of the microscope requires practice. It takes a while to develop intuition as to what settings work best in a given situation.
Here are some guidelines to get started:
- The derivative factor does not improve the image in most instances, so set the Derivative gain to 0.
- Useful minimum and maximum settings for both the Integral and Proportional gain controls are typically 100 and 600, respectively.
- Good performance is generally obtained when the Proportional gain is in the range of 1-3 times the Integral setting.
- When in doubt as to where to begin, start with the Integral and Proportional gains at 350 and modify the settings if the image quality is not as good as expected.
- NOTE: all three gain settings can be changed while the surface is being scanned, so the effects can be observed instantly.
- Low Integral and Proportional gains will be sufficient to image flatter surfaces, at slower scan rates, and over smaller areas. Conversely, rough surfaces with abrupt rising or falling edges, fast scanning, and the scanning of large surface areas require higher gains.
- NOTE: If any of the gain factors is too high the scanner pzt tube will oscillate. When a gain control is only slightly too high the scanner oscillation will just produce fuzzy bands in the image near the rising or falling edges of the surface contours. The image quality is compromised in those areas, but otherwise the microscope functions normally. When a gain control is far too high, however, the image quality will be extremely poor, and a high-pitched audio frequency squeal will be heard coming from the scan head. This situation should be avoided. It will certainly damage the probe tip, and may damage the scan tube itself.
Engaging the Probe with the Sample Surface
- Manually lower the cantilever to within about 1.0 mm of the sample and position the area of interest under the probe point.
- Manually lower the probe to within about 0.2 mm of the sample surface and then initiate the automatic routine achieve surface contact.
For more detail on the basic steps continue reading the Positioning the Sample, explanation of the Engage Window, and for additional information about engaging the probe.
Positioning the Sample:
- Making sure there is ample clearance between the cantilever and the sample stage, slide the sample under the scan head.
- Use the Fast Down button in the Probe Position window to bring the probe within 1.0 mm of the sample surface.
- The probe should now be close enough to the surface for the surface features to be roughly in focus in the camera view. Adjust the camera illumination if necessary to get a clearer view.
- Move the sample laterally to position the area of interest under the probe tip.
- NOTE: This can be done by hand by sliding the sample on the sample stage. If the stage has manual X and Y vernier knobs, the sample may be accurately positioned by rotating the vernier knobs. Or if the stage is motorized, the sample may be positioned with the stage software controls.
The Engage Window:
Below you can see the Engage Window,
The major elements of the Engage Window are as follows,
- The Engage button initiates the process of automatically lowering the probe to the sample surface.
- There are three engage methods: Standard, Standard at Z=0, and Auto. The Standard engage method is the simplest method, and is outlined in more detail in the Standard Engage Method section. The Standard at Z=0 and Auto engage methods are advanced features of the software, refer to the PDF Manual Section 5.4 for more information.
- The two optional photodetector signals, Laser Sum and Laser L-R, may be plotted in the top graph by checking the appropriate box in this panel. These signals are rarely used. They are mostly used in diagnostic testing of the instrument.
- The WaveMode control panel is used to adjust the damping factor for intermittent contact AFM scanning.
- The up and down arrows move the scan head up or down one step of the Z-axis motor at a time. Each click of an arrow moves the probe approximately 0.25 μm. This is useful for adjusting the expansion and contraction of the piezoelectric scan tube after the probe is in contact with the surface.
- This graph displays the Error signal in the Z feedback loop as a function of time. Two other signals may also be plotted: the sum of the four photodetector outputs Sum and the difference between the left and right photodetector signals L-R. Normally only the Error signal is of interest.
- This graph shows the voltage applied to the Z electrode of the scan tube as a function of time. The tube expands under positive voltages and contracts under negative voltages.
The Standard Engage Method:
The Standard Engage Method is a two step process,
- Use the Fast Down or Slow Down buttons in the Probe Position window to bring the probe tip to within about 0.2 mm of the sample surface.
- Because the cantilever is so small it is often difficult to judge the distance between the cantilever and the surface by eye. As an alternative, try monitoring the gap between the gold probe holder and the surface as the scan head is lowered. When this gap is about 0.5 mm the cantilever will be within about 0.2 mm of the surface. See below,
Note: If the cantilever becomes very bright in the camera window, or disappears all together, most likely the probe has crashed into the surface and is ruined. When this happens, raise the scanning head clear of the surface and have a Manager replace the cantilever.
- Open the Engage Window, select the Standard engage method, and click the Engage button. The Standard engage method follows this sequence:
- (a) The probe is completely retracted by applying the maximum negative voltage to the z piezo.
- (b) The Z motor moves the scanner downward by a distance of approximately ½ of the Z Range of the scanner. For example, if the scanner has a total vertical range of 4 um, the scanner will be lowered by 2 um.
- (c) The voltage applied to the Z piezo is increased linearly to extend the probe toward the sample surface.
- (d) If the probe does not encounter the surface then the process cycles back to (a) and repeats to search for the surface. If the probe does encounter the surface then two or three small steps are taken by the z motor to position the probe near the center of the voltage range and the engage sequence terminates.
The shape of the traces seen in the Engage window before the probe contacts the surface are indicated in time interval A of the image below.
The relatively straight line in the top graph indicates that the Z feedback loop has a constant positive error voltage, corresponding to the system state with the probe too far above the sample surface. The sawtooth pattern in the bottom graph indicates the expansion and contraction of the z piezo section of the scanner.
After the probe contacts the surface (interval B) the error trace should drop to zero (interval C) indicating that the Z feedback loop is actively keeping the loop error at zero, as it should. The message “Feedback On” will appear above the top window as a second indication that the Z feedback loop is active. The bottom trace should level off, and you should see the message “In Range” flashing above the lower graph. This indicates that not only is the feedback circuit active, but the expansion/contraction of the tube is within the total vertical range available to the scanner for imaging.
Additional Info about Engaging the Probe:
- The level of the Error signal in the top graph before the probe reaches the surface is a direct indication of how stable the feedback loop will be when the probe is in surface contact. If the level is almost zero this usually indicates that the laser optics are not set correctly and need realignment. Whenever this condition is noticed immediately abort the engage process, withdraw the probe from the surface, and realign the optics.
- After the engage process is successfully terminated, as in the Standard Engage sequence image in the previous section within interval C, the position of the trace in the bottom graph indicates the expansion/contraction state of the piezo tube within its total vertical range. For example, a 40 μm scanner has a total vertical range of about 4 μm. When the trace is near the middle of the graph the tube is near its rest state. When the trace is very high in the graph, the tube is expanded toward the bottom end of its range, -2 μm. This means the tube will not be able to expand down to track deep features in the surface. Conversely, when the trace is very low in the graph the tube is contracted toward the top end of its range, +2 μm. This means the tube will not be able to contract upward to track high features in the surface. There is the danger of crashing the probe into the surface when the tube is overly contracted, therefore this situation should be avoided.
- As an advanced feature for experienced SPM operators, the “z landing zone” for the automatic engage routine may be adjusted. See the PDF Manual Section 5.6 for these Advanced Scan Parameters.
- If while using the microscope the “In Range” message ever changes to “Lost Feedback” while the probe is nominally in surface contact it means that the tube is either over-expanded or over-contracted. This may happen after the system has been operated for a long time because of thermal drift, or because the probe happens to move over a very high or very low feature on the surface. When feedback is lost the options are to either Withdraw the probe and repeat the engage process, or use the Z Up and Z Down buttons to move the scanner back to within the Z scan range.
- If the SPM locks up at any point in time while the probe is in contact with the surface, withdraw the probe manually by turning the stepper-motor knob at the top of the microscope stage clockwise by at least a quarter of a turn. This will minimize the probe damage.
Scanning the Sample
YAY!!! Now that you’ve set everything up you’re able to collect your images! First let’s get familiar with the The Realtime Scanning Software. All surface imaging is performed from the Realtime Window. More information not explained above about the various menu controls in the Realtime window is presented in the PDF Manual in Chapters 4, 5, 7, and 8.
The major elements of this window are labeled below,
- The Scan button initiates the scan process. Once a scan is underway this button switches to the Stop button, to allow the operator to stop the scan process at any time.
- When the Continuous Scans check box is unchecked the scan process will stop when the end of the scan is reached.
- When the Continuous Scans box is checked the scan process will be repeated over and over.
- This window graphically displays each line of the image data as it is acquired. In effect, it is showing a cross-section of the surface topology, or phase signal, etc., depending on the image type setting.
- When the AGC button is “up” the vertical scale of the graph corresponds to the maximum vertical scale of the data being measured. For example, this would be the full Z range of the scanner for a topology scan, or 65535 ADC units for a Phase mode scan. The Center and Gain controls can be used to manually shift and expand the section line view.
- When the AGC button is “down” the vertical scale is expanded and shifted so that the highest point in the data line reaches the top of the graph and the lowest point in a data line reaches the bottom of the graph.
- In most scan modes the graph always corresponds to the image being created in the image panel, (5). The exceptions are the BiLateral and BiPhase modes. When scanning in either of these modes the user may display both the forward and reverse trace data with the ↔ button. When the ↔ button is “up” the graph corresponds to the image being created in the image panel, as usual, with the data appearing in green. When the ↔ button is “down”, however, the complementary raster direction data are shown in yellow. The forward-reverse trace pairs are (Z, Z Reverse), (Lateral, Lateral Reverse), and (Phase, Phase Reverse). See (8) below for more details.
The PID gains, Setpoint, and the probe holder Bias voltage controls in this panel are identical to the controls in the SPM Configuration window. They may be adjusted at any time. The PID gain controls allow the Z feedback loop to be optimized while scanning.
- The parameters at the top of this panel, running from XY Mode down to Resolution, indicate where and how the scan will be performed when the Scan button is pressed. All of these parameters are identical to those found in the SPM Configuration window with the exception of RT Tilt, which is an advanced feature discussed in the PDF Manual in Section 5.3.
- NOTE: These parameters are tied to the SPM Configuration window, not the image currently being displayed. Whenever an image is retrieved from the hard disk these parameters are not modified from their SPM Configuration window settings.
- To find information about the scan rate, resolution, scan angle, etc. for an image being viewed right-click on the image to bring up the Scan Summary window.
The scan data appears in this image panel, one line at a time as the surface is rastered, starting at the top of the panel. The scale units for the x and y axis are given at the bottom of the panel. The z-axis units are given at the bottom of the Z color scale. The type of image data being displayed is indicated by the Image Type parameter (See 7 below).
The numerical range of the z-axis of the image is shown in the Z color scale. This scale is updated 21 times during the course of an image scan: once when the very first scan line is acquired, then again as the image is 1/20, 2/20, … , completed. The Contrast and Brightness sliders to the right if the Z color scale adjust how the palette colors are assigned to the height profile of the image.
The Image Type parameter specifies the type of image data being displayed in the image panel. For the various Scan Type settings there are as many as four sets of image information simultaneously measured by the microscope. The table below (8) summarizes which Image Type information (i.e. Signals 1-4) is measured for each Scan Type.
- While scanning the user can flip through the available buffer data using the Realtime window’s Buffer pull-down menu list. The possible selections are given in the table below.
- The raw data may also be viewed after the scan is completed by flipping through the menu list, but note that changing the scan mode at the SPM Configuration window will disable access to the previous scan’s raw data. Further information about the relationship between the software’s raw data buffers and the z-buffer is given in the PDF Manual in Section 7.9.
At this stage of the process the probe should be in contact with the surface, and the system set and ready to scan.
Whenever the Scan Type is changed at the SPM Configuration window the Image Type variable will always reset to Signal 1. So, for a Z Height scan type setting, the system will initially display Z Height image data when the Scan button is pressed.
Until you become familiar with the basic operation of the microscope it is best to turn off any pre-processing of the image data by going into the RT Options menu and making sure all the options are unchecked. This will make the RT Tilt parameter in the panel at the lower-left part of the Realtime window read “none.”
- To initiate a scan simply press the Scan button. When the scan size is about 5 μm or greater the movement of the cantilever can easily be observed in the video camera window. With a scan direction of 0.0º the probe will move to the upper left corner of the scan area and begin rastering from left-to-right, stepping through each scan line from top-to-bottom.
- NOTE: Due to an optical illusion produced by the isotopic focal system, the sample rather than the cantilever will appear to be moving.
- NOTE: The scans may take some time as it is a delicate process. If you absolutely need to increase speeds/scan time (not typically recommended) refer to the PDF Manual Section 3.9 Broadband Imaging. Please ask a Manager before performing any Broadband Imaging.
- To permanently save a completed image select File > Save As…
- NOTE: Partial scan data may be recovered. When a scan is terminated before completion the previous completed scan is always returned to the z-buffer, but the partially scanned image can be recovered by clicking Next.