NanoWiki

version 1.3, 21 March 2007

1. Switch on the three RHK boxes ( SPM 1000, AIM-DI and VScan)

2. Mount Probe

Inspect the cantilevers in the optical microscope. Broken cantilevers should be discarded. Mount a probe into the cantilever holder. Be sure that it is in firm contact with the end of the groove and sitting flat in the base. For contact mode use a silicon nitride probe.

3. Scanner

This AFM uses a EV type scanner (EV 9 x 9 μm).

4. Mount Sample

Mount a sample on a metal disk with a “sticky-tab”, or carbon tape. Other types of glueing might cause a long term drift in the Z-direction. The sample must not hang over the edges of the disk’s 15mm diameter. You can only access the central 3-5mm of the disk for imaging. Mount the disk and sample on top of the scanner. The scanner top is magnetic to hold the disk.

5. Put the cantilever holder in the optical head.

First make sure the tip will not touch the sample surface when you place the cantilever holder in the optical head. Hold the up/down motor to UP (it is the optical head that moves up, not the scanner that moves down) until the tops of the ball bearing points in the optical head are higher than the sample surface. Lift the holder over your sample being careful not to scratch it. There are three ball bearing points inside the optical head that fit into a hole, a groove and a flat on the underside of the cantilever holder. Lower the clamp (it has six pins on it that connect to the cantilever holder) to secure the holder by tightening the screw in the back of the optical head. The screw should be firm, but not over tightened. Bring the tip closer to the sample by holding the up/down motor to DOWN, but do NOT allow the tip to touch the surface. You should be able to see a gap with the naked eye.

6. View sample though optical microscope and position cantilever.

Turn off the laser. Obtain an image of the sample and cantilever through the optical microscope. Turn on the light source. Move the AFM under the optical microscope until the circle of light from the optical microscope is centered over the cantilever. The sample surface will probably be out of focus since they are at different heights. If you need to image a particular area of your sample, focus the sample. You should still be able to see a shadow of the cantilever. Roughly position the cantilever over the area of interest by using the sample translation screws at the base of the optical head, or carefully push the magnetic disk across the scanner. Be sure not to lift the disk and smash into the tip. Refocus the microscope on the cantilever.

7. Align Laser

Turn on the laser. You should see a red laser inside the optical head. It is very important not to insert any reflective objects into the cavity while the laser is on, or you could reflect the laser into your eye. This includes samples, tweezers and the AFM parts. Be sure to turn off the laser when changing the sample, removing the tip holder, or dismantling the microscope. Use the two laser alignment screws at the top of the optical head to move the red laser spot onto the end of the cantilever. Next use the “paper method” to fine position the laser spot on the cantilever. Insert a piece of paper into the left side of the optical head cavity to intercept the laser before it enters the photodiode. You may need to adjust the mirror angle to image the reflected laser on the paper. If the laser is positioned correctly on the cantilever, you should see a very sharp bright laser spot. Adjust the laser alignment screws slightly to optimize the image of the laser. Hold the up/down motor to DOWN to bring the tip close to the surface. Watch the space between the laser spots. Do not allow the laser spots to meet. You do not want the tip to touch the surface. Refocus the image in the optical microscope. You should find that the sample surface and the cantilever are both close to focus.

8. Adjust Photodiode Signal

The photodiode is split into 4 sections: ABCD. Different combinations of signal from the four sections are displayed on the front of the AFM base. With the mode switch in AFM & LFM, the top number is the vertical difference, which is the signal from the top half of the photodiode minus the bottom half (A+B – C+D). The bottom number is the horizontal difference, which is the signal from the left half of the photodiode minus the right half (A+C – B+D). Around the bottom number is a circular meter that shows the sum signal (A+B+C+D). With the mode switch in AFM & LFM, first maximize the SUM signal by adjusting the mirror angle (lever on back of optical head). The actual maximum will depend on what cantilever you are using. Wide cantilevers reflect a lot of the laser signal and will give a large maximum. Thinner cantilevers have less area to reflect the laser and will give a smaller maximum. Adjust the horizontal difference value (bottom LCD number) to zero using the photodetector adjustment screw on the back left side of the optical head. CONTACT mode : Leave the mode switch at AFM & LFM. Adjust the Vertical Difference value (top LCD) to -2.0 volts using the screw on the top left side of the optical head.

9. Open Software

Open the AFM software by double clicking on the Xpmpro icon on the desktop. Click on Opt on the menu bar. Select the appropriate parameter file (Xpmpro_contact.prm for contact mode). Open the acquisition menu and the navigation menu.

10. Set Initial Scan Parameters.

On the frontpanel of the RHK box set the time constant to a minimal value (without the piezo's starting to oscillate). The gain should be set to 5.0 (middle of potmeter range).

11. Tip approach

Be sure that the potmeter for the adjustment of the Z-direction is at about 5.0 (middle position).
On the front panel of the RHK box the red light of out of range should be on.
The command for approach is in the “Navigation window”.
Set tip approach.
The stepping motor starts rotating and will stop when the deflection signal= 0. The green light of “in range” will be on, and the needle indicator will be out of the corner. Adjust the Z-potmeter until the needle indicates 0.

12. Plane leveling

Go to Aquisition menu, choose the “image line” option. Perform a scan in X-direction. Adjust the slope to get a horizontal, flat line by changing the potmeter position of the X-slope on the frontpanel of the RHK-box. Swap XY direction and do the same for the Y position.

13. Start Scanning

F6 initiates a scan. F7 toggles the fast scan direction between the X and Y axes.

14. Optimize the image by adjusting the scan parameters.

Open windows for viewing the topography and force error for forward (trace) and backwards scanning direction (retrace). Check to see if the Trace and Retrace lines are tracking each other well (i.e. look similar). If they are tracking, the lines should look the same, but they will not necessarily overlap each ot horizontally or vertically. If they are tracking well, then your tip is scanning on the sample surface.

The Deflection or Force error image is a map of the voltage indicated on the upper digital indicator, i.e. the normalized voltage difference (A-B)/(A+B) (times 12 volts); see previous page. The Deflection signal drives a feedback circuit that displaces the piezoelectric sample scanner vertically (Z) to maintain the Deflection setpoint (in volts). The Height image is the vertical position, Z(X,Y), needed to maintain constant Deflection. The Deflection sensitivity depends on the cantilever used; for the short, wide-legged cantilever, the Deflection sensitivity is about 40 nm/V. Note this implies constant applied load. The Deflection signal may be used to optimize topographic imaging as follows. Adjusting the scan rate and gains, for a given scan size, will change the contrast seen in the Deflection image; a minimal contrast indicates that the tip is tracking the surface topography optimally. (Often Deflection images are published as “error signal” images.) Increasing the gains (but no so high as to cause feedback oscillations) or decreasing the scan rate usually improves tracking.

If there is a slow drift in Z-direction that causes the feedback to go out of range, this can be avoided using the “auxilary feedback option”. This option should be switched off after scanning.

Streaks appear on the trailing edge of surface features

Streaks are an indication of the tip not tracking the surface due to:

1. the scan rate is too fast
2. the gain settings are too low
3. there is not enough imaging force

It can also be any combination of these things. Try one of the following to try and eliminate the condition.

1. Reduce the scan rate. Typically it should be around 1 sec per line.
2. Increase the gain. This will speed up the response time of the Piezo transducer.
3. Increase the amount of force on the surface (by adjusting the offset voltage on the phot detector eg. from -2.0 V to -3.0 V) . This will probably be the most effective. However be careful when doing this on soft samples, the sample surface can be disturbed even though the force is very small.

Scanners that move the probe in an atomic force microscope in the X, Y and Z directions are typically made from piezoelectric ceramics. As electromechanical transducers, piezoelectric ceramics are capable of moving a probe very small distances. However, when a linear voltage ramp is applied to piezoelectric ceramics, the ceramics move in a nonlinear motion. Further, the piezoelectric ceramics exhibit hysteresis effects caused by self-heating. Artifacts can also be introduced into images because of the geometry of the scanner. The positioning of the scanner relative to the sample can also create artifacts.

Probe/Sample Angle

If the features that are being imaged by the AFM are much larger in profile than the probe, and the image does not seem “correct”, the artifact may be caused by a non-perpendicular probe surface angle. Ideally, the probe of the microscope should be perpendicular to the surface.

Background Bow/Tilt

The piezoelectric scanners that move the probe in an atomic force microscope typically move the probe in a curved motion over the surface. The curved motion results in a “Bow” in the AFM image. Also, a large planar background or “Tilt” can be observed if the probe/sample angle is not perpendicular. Often the images measured by the AFM include a background “Bow” and a background “Tilt” that are larger than the features of interest. In such cases the background must be subtracted from the image. This is often called “leveling” or “flattening” the image. After “leveling” the desired.

Scanner Drift

Drift in AFM images can occur because of “creep” in the piezoelectric scanner, creep of the glue with which the sample is mounted on the metal disk and because an AFM can be susceptible to external temperature changes. The most common type of drift occurs at the beginning of a scan of a zoomed-in region of an image. This artifact causes the initial part of a scan range to appear distorted. Drift artifacts are most easily observed when imaging test patterns. Drift will cause lines that should appear straight to have curvature.

Multiple or repeating patterns

The tip is probably chipped. This is usually caused by too much imaging force on the surface.

1. Change the tip.
2. Operate with a minimum force between the tip and the sample.

The image has weird vertical/diagonal bands lines all over it, or oscillations in the force curve.

This problem is a common artifact in AFM. It is due to the laser light spilling over the edge of the cantilever, being reflected off the sample, and travelling back up towards the photodetector. The reflected light interferes with light reflected from the cantilever, causing typical wave-like oscillations in the image (oscillations in the fast scan axis, or if you look at the images, bands running near-vertically in the slow scan axis) or in the zero force line of force curves. Typically, the wavelength of these oscillations is two wavelengths of the laser i.e. it is about 1.3 micrometers for a red laser. It is more common with reflective samples, with high coherence lasers, with narrow cantilevers, and when the laser alignment is not perfect. The typical way to fix it is to re-align the laser, trying to make sure the spot is right in the middle of the cantilever. Newer instruments often have low coherence lasers to reduce this problem. However, with some instrument/cantilever/sample combinations it is very hard to avoid.

* AFM contact mode
xpmpro_contact_prm.txt