Raster Scan Parameters

Scanning Probe Microscopes generate an image of the sample surface by scanning back and forth, line by line, probing the surface and collecting data.

Several key parameters define the size, shape, and rate of the scanner's motion:

Scan Dimensions

Scan Size

The most important parameter defining the dimensions of the scan is the Scan Size. This parameter quite literally defines how far the tip will travel as it scans back and forth across the sample. The Scan Size is typically limited by the range of the scanner, which is roughly 90 μm full range.

Scan Angle

The direction that the scanner travels does not necessarily need to coincide with the XY coordinates of the scanner’s Axis Orientation. The Scan Angle parameter defines the degree of rotation between the axis upon which the scanner travels and the scanner’s x-axis. The axis that the probe scans along is referred to as the fast scan axis, whereas the axis that the probe slowly creeps along is referred to as the slow scan axis. It is possible to set the Scan Angle anywhere between 0 and 360 degrees, resulting in tip scanning motion in any desired direction.

Example: If the scanner is set to scan with a scan angle of 0 degrees, then the fast axis will be parallel to the X-axis of the scanner. However, if the scan angle is set to 90 degrees, then the fast axis will be parallel to the Y-axis of the scanner:

HINT: Typically the scan angle is set to 0 degrees, although certain applications, such as Lateral Force Mode, require scan angles other than 0 degrees.

Aspect Ratio

Whereas the Scan Size parameter defines the length of the scan along the fast scan axis, the Aspect Ratio defines the length of the slow scan axis scan, relative to the fast axis:

Aspect Ratio = Length of fast scan axis / Length of slow scan axis

The minimum possible value for the Aspect Ratio is 1, meaning the fast axis is always equal to or longer than the slow axis.

Example: If the Aspect Ratio is set to “1”, then the shape of the image would be square, as the length of scan along the slow axis would be equal to the length of scan along the fast axis, as defined by the Scan Size. If the Aspect Ratio is set to “2”, then the slow axis scan length will be half as long as the fast axis, and the image will be a rectangle. Changing the Scan Angle from 0 to 90 degrees would rotate the orientation of the fast and slow axes accordingly.

Imaging Speed

The quality of the images captured with any SPM technique will be significantly affected by the rate at which the tip scans along the surface. The slower the tip scans, the longer the feedback loop and sensor electronics have to collect data and react to changes in the sample's surface properties.

Scan Rate

Perhaps the most important scan parameter of all is the Scan Rate, which defines how fast the scanner travels. The scan rate defines how many round trips (trace & retrace) the scanner makes along the fast scan axis in one second, and thus the parameter is shown in units of hertz (Hz).

Example: With the scan rate set to 1 Hz, the scanner will travel back and forth once in one second. With the scan rate set to 2 Hz, the scanner will travel back and forth twice in one second.

HINT: Always begin scans with conservative, slow Scan Rate and small Scan Size. These parameters can always be increased incrementally. Excessive scan speed may damage the tip.

Tip Velocity

Whereas the Scan Rate defines the number of scan lines in one second, this Scan Size is also required to completely define the Tip Velocity, the value that describes how fast the tip is actually moving across the surface. The Tip Velocity is a rate of travel and is thus defined in units of µm/s.

Tip Velocity = Scan Rate / Scan Size

Example: A user has just engaged on the sample surface with the Scan Size set to 500 nm, and the Scan Rate set to 1 Hz. The resulting Tip Velocity is 1 um/s. Now the user expands the Scan Size to 10 um. The resulting Tip Velocity has now increased to 20 um/s. Thus the rate at which the tip scans across the surface has increased by a factor of 20 without changing the Scan Rate parameter!

HINT: A best practice to maintain care for a sharp probe is to decrease the Scan Rate before increasing the Scan Size, such that the Tip Velocity does not inadvertently increase to an undesirably fast rate of travel.

Image Resolution

Three important parameters define the resolution of the digital image that the system collects. We already learned that the Aspect Ratio defines the ratio of the fast scan axis to the slow scan axis. There are two other parameters that define the image resolution: Lines and Samples per Line.

The NanoScope V Controller is capable of capturing up to 8 channels of data simultaneously with a resolution of 5120 Lines and 5120 Samples per Line, resulting in a 8 channels of 25 Mpixel data.

Lines

The Lines parameter defines how many lines along the slow axis the scanner travels in the course of a single image scan. Increasing the number of lines will result in a higher resolution image for a given scan size, but also increase the time required to capture a single scan.

At typical scan rates, a very high resolution and high quality image can take hours to collect. The time remaining to collect the current scan is displayed in the status bar under the Time Remaining tab.

HINT: If you intend to collect a high resolution image, inspect the Time Remaining value to know how long the image capture will take with the current settings.

Samples per Line

The Samples per Line parameter defines how many data points the controller will capture along a single line. A higher number of data points in a single line of an image improves the lateral resolution of the image, but reduces the amount of time that the system has to collect data at each data point.

Example: The system is scanning at 1 Hz over 500 nm, collecting 512 data points per line. The system thus has roughly 1 ms per pixel to collect data. Now the number of Samples per Line is increased to 5120 without touching any of the other parameters. The system now captures many more pixels in a line, but only has 0.1ms per pixel to collect data. In order to maintain the same amount of time at each pixel, the scan rate would need to be reduced by a proportional amount to the increase in Samples per Line.

Typically, the Samples per Line parameter does not need to be changed. Changing the Lines parameter will automatically change the Samples per Line parameter by an equivalent amount, maintaining the aspect ratio of the pixels in the digital image. However, if the Samples per Line parameter is changed, the Lines parameter is not changed by an equivalent amount. Thus changing the Samples per Line parameter will thus result in non-square pixels.

Square vs Non-Square Pixels

Typically, the digital images collected have square pixels, indicating that the Scan Angle, Lines, and Samples per Line parameters all agree such that the pixels in the digital data represent equal dimensions in both axes. However, it is possible to collect images with non-square pixels.

Example: With the Aspect Ratio set to 1 and the Lines and Samples per Line parameters both set to 512 and 512 respecitvely, the image captured will have square pixels with a image resolution of 512 x 512 pixels.

Beginning with the previous example, the Aspect Ratio is changed to 2. The Samples per Line parameter will automatically change to 256 to maintain the pixel geometry. The image captured will still have square pixels, with an image resolution of 512 x 256.
Set the Aspect Ratio back to 1, set the Lines parameter to 16, and the Samples per Line parameter to 16,000. Now the system captures only a few lines in the scan, but with very high resolution on each scan line. The resulting "image" has individual non-square pixels, each with an aspect ratio of 100. In this scenario, there is significantly more resolution along one axis as compared to the other.

The only way to change the pixel geometry is to manually change the Samples per Line parameter. Changing either the Aspect Ratio or Lines parameters will automatically adjust the Samples per Line parameter to keep the pixel geometry at its current value. Unless non-square pixels are desired, do not adjust the Samples per Line parameter!

XY Feedback Control

Closed Loop

The vast majority of applications will benefit from scanning in closed loop to optimize performance and scanner linearity. To enable the XY feedback, the parameter X-Y Feedback Control should be set to On. This is the default setting for most experiments.

Open Loop

Alternatively, the XY feedback parameter may be disabled to run the scanner open loop by switching the X-Y Feedback Control parameter to Off. The Dimension Icon SPM Scanner is equipped with sensors in the X, Y, and Z direction, allowing the scanner to know precisely where it is. This is important because piezoelectric materials do not expand and retract in a linear fashion. A drive signal for the X and Y axes that creates approximately linear motion is obtained via the Calibration process, enabling accurate control while running in an open loop. However, turning on XY Feedback significantly improves linearity of the XY scan signal.

Traditionally, open loop scanning has provided lower noise operation than closed loop scanning due to noise introduced by the sensors. However, with the new Dimension Icon scanner on the Dimension Icon, the sensor noise is extremely low, allowing closed loop operation for practically all use cases. While the open loop scanning option is still retained for legacy purposes, all of the default configurations are set up with X-Y Feedback Control enabled, and this is the recommended operating condition.

www.bruker.com	Bruker Corporation
www.brukerafmprobes.com	112 Robin Hill Rd.
nanoscaleworld.bruker-axs.com/nanoscaleworld/	Santa Barbara, CA 93117

	Customer Support: (800) 873-9750
	Copyright 2010, 2011. All Rights Reserved.