Layered Imaging(tm) Applications in Scanning Probe Microscopy

Ms. R. Jobe, Dr. M. Green, Dr. S. Marchese-Ragona, Dr. G. Collins and B. Christie, TopoMetrix Corporation

Introduction

In this report we will describe Layered Imaging and show its use in mapping surface compliance and adhesion of a sample. Its use in magnetic force microscopy will also be shown. Layered Imaging is an SPM technique in which several measurements of cantilever deflection can be performed at each image pixel. Each measurement is preceded by a step change in the tip/sample distance. After each step, a measurement is performed and the result is stored. When all measurements for the current pixel are completed the process is repeated at the next pixel and so on through the scan area. The resulting three-dimensional data set can be thought of as a stack of "layers" of images. Each horizontal layer is an image which represents the measurements taken throughout the scan area at a specified Z height. Since several measurements were made at each pixel, the data set can also be processed vertically to yield the force distance curve at each pixel.

Figure 1. Ideal force-distance curve. Layered Imaging sample points are marked in green.

Overview - The Force-Distance Curve

Layered Imaging allows the user to record the cantilever deflection as the cantilever is moved through the entire or a partial force distance curve for each pixel in an image. The cantilever deflection is monitored at each layer position, and the Force (F) and Distance (D) data is stored for each layer.

There are several pieces of information which can be derived from the force-distance curve:

During the segment ab the probe and sample are not in contact but the probe is approaching the sample; when there is no net long-range attractive or repulsive force, there is no information content.

The segment be is known as the "jump to contact" and is due to an attractive pull on the cantilever. When operating in air, primary among these attractive interactions are capillary forces, resulting fro contamination layers (absorbed water, etc.) on the probe and sample as they come into contact. Also contributing to the attraction between the probe and sample are electrostatic patch charges and van der Waals forces. It has also been proposed that this "jump to contact" effect can result from "stick-slip" friction as the probe contacts the surface.

Segment cd represents the upward motion of the cantilever in contact with the sample surface in response to the increase in z piezo voltage. What is being recorded is the response of the sensor which measures the cantilever displacement. The shape of segment cd indicates whether the sample is deforming in response to the force from the cantilever and the slope can be used to derive information about the hardness of the sample. The segment need not be a straight line; changes in slope within the segment indicate a differing sample response at different loading forces. This slope is the key information for hardness studies.

The segment de reverses the situation in segment cd. If segment de lies to the right of cd, this can be the result of piezo hysteresis or friction-induced bowing of the cantilever as it is withdrawn from the surface. If de lies to the left of cd, the difference can give information on the elastic or plastic deformation which has a response time slower than the withdrawal rate of the probe.

Segment ef records the motion of the cantilever from its neutral deflection position as it is deflected downwards (therefore representing the adhesive force) until the spring force of the cantilever equals the adhesion.

The segment fg shows the jump of the cantilever away from the surface which occurs when the cantilever force exceeds the adhesive forces; the force at point f is the total adhesive force between the probe and sample. This is the key information necessary for adhesion studies. The sharp break in behavior at point f is not a universal response. If the adhesive interaction can be characterized as being viscous, the probe may not break away form the surface abruptly, but may produce a much more gradual and rounded response. During the segment gh the probe and sample are not in contact and are moving away from each other.

Using Layered Imaging for Force Imaging Spectroscopy (FIS)

In AFM, the Force Imaging Spectroscopy (FIS) mode can be used to measure adhesion, hardness, or deformability of samples. Many probe/sample interaction mechanisms can be studied.

The hardness of the sample surface and the relative adhesion between two surfaces can be derived from the force-distance curve produced by Layered Imaging. The following compression and adhesion experiments demonstrate surface characterization using this technique:

Experimental

Sample: blend of two natural gums
Cantilever: Si
Tip: conical in shape
Scanning Mode: Layered Imaging, AFM contact mode
Instrument: TMX 2000 Discoverer

Compression Experiment

Relative elasticity of a sample can be recorded by monitoring the cantilever deflection as the applied force is changed. In this experiment, the tip is pushed into the sample surface a selected distance while cantilever deflection is sampled at equal intervals. The resulting force curves from raised and recessed areas of the sample were extracted from the layered image and are shown in Figure 2. This corresponds to sections ab, bc and cd on the force-distance curve shown in Figure 1. Initially, the feedback position was set at a force of 0.14 nN. The tip was then "pulled" from the surface a distance of 265nm and cantilever deflection was recorded in 100 equidistant layers from this position to 135nm past the feedback position.

From the two force-distance curves extracted from the 13 layered images and shown in Figure 2 it can be seen that the slope of the first compression curve (red), measured on the recessed gum, is 1.03 nN/nm and the slope of the second compression curve (blue), measured on the raised gum, is 2.02 nN/nm. This indicates that the recessed gum is softer than the raised gum.

Figure 2. Topography (above) and force distance curves (below) extracted from the layered images of the blended gum sample with extraction points noted. The zero force position is marked with a green line. Scan size: 10 x 10 µm.

Adhesion Experiment

This experiment was performed on the blended gum sample using the same cantilever as in the compression experiment. The outward movement (withdrawal) of the cantilever (sections de, ef, fg and gh on the force-distance curve shown in Figure 1 was monitored and plotted. The feedback position was set at a force of 0.17 nN. Initially, the tip was "pushed" into the surface a distance of 150nm. Cantilever deflection was recorded as the tip was withdrawn from this position to a height of 700nm in 24 equidistant layers at each pixel.

The two withdrawal curves extracted from the layered images in Figure 3 show the points at which the tip breaks free from the surface. The first adhesion curve (red) was measured on the recessed gum while the second adhesion curve (blue) was measured on the raised gum. The curves indicate that the recessed gum is more adhesive to the tip than the raised gum.

Figure 3. Top - Withdrawal sections of the force distance curves extracted from the layered images of the blended gum sample with extraction points noted on the topography image. The curve representing the recessed gum is red, the curve representing the raised gum is blue. The feedback position is marked with a black line. Bottom - From top to bottom, the image sampling heights are Onm, 154nm, 266nm and 296nm from the feedback position. Scan size: 10 x 10 µm.

The layered images and force curves shown in Figure 3 were extracted from a 24-layer data set. The Z color scale is identical for all of the images. The lighter areas on the higher layers indicate where the tip pulls off the surface. Darker areas indicate areas of more adhesion.

Layered Imaging can quickly display the relative adhesion over the entire image area. A 200 resolution (x, y) layered image data set containing 40,000 force-distance curves can be acquired in 5 to 20 minutes (depending on the number of layers). To analyze this large data set, the image layers can be rapidly displayed in succession as a ,'movie". This allows the eye to rapidly evaluate the behavior of the surface at all force levels. The movie can highlight features which would probably be missed if only a few force-distance curves were acquired.

The images in Figure 3 are an example of this ability to see subtleties in the data. Although the recessed areas generally have higher adhesion, small spots in the recessed gum areas have very low adhesion. This could be due to a low adhesion phase blended into the softer gum or the result of small contamination particles. Whatever the cause, it probably would have been missed if individual force-distance curves were extracted on raised and recessed areas only.

Another advantage of layered imaging data acquisition is pixel-to-pixel matching between images. This allows unambiguous correlation of force phenomena with topographic features and is inherently superior to approaches which require multiple passes or separate experiments. Multiple pass approaches can be subject to drift and hysteresis effects.

Figure 4. From top to bottom, topography and magnetic force gradient data at feedback and 20nm, 26nm, 75nm and 100nm above the feedback point At each pixel, pullback levels are referenced to the feedback point, which compensates for height variations in the sample and decouples topographic effects from the MFM image. Scan size: 50 x 50 µm.

Using Layered Imaging for Magnetic Force Microscopy (MFM)

Experimental

Sample: recorded thin film hard disk
Cantilever: Si
Tip: conical in shape, magnetically coated
Scanning Mode: Layered Imaging, Non-contact, amplitude detection
Instrument: TMX 2000 Discoverer

Results

In a single scan, Layered Imaging MFM mapped both topography and magnetic field strength at four different heights, producing a map of the magnetic field gradient. The farther the tip is from the sample surface the weaker the magnetic fields appear. The highest resolution image was obtained at 20nm above the feedback position (Figure 4).

Layered Imaging, in MFM mode, is a multipurpose tool for:

1. Finding the height above the sample surface at which the optimum magnetic force image can be acquired.

2. Allowing optimum decoupling of topography from the magnetic force gradient.

Other Capabilities of Layered Imaging

Layered Imaging has a number of additional advantages for AFM and STM data acquisition:

* Decoupling of lateral forces - the tip can be raised from the surface and reengaged at each pixel as it scans across the sample. This can eliminate the damaging effects of lateral forces. The chance that the probe will distort or sweep away soft and weakly bound samples such as live cells is reduced. Decoupling lateral forces also minimizes image distortions resulting from the flexing of SuperTipsª.

* Non-contact in liquids - Layered Imaging can be used to acquire true non-contact images in liquids.

* Imaging at multiple force levels - multiple forces can be applied to the same surface within a single scan. On biological specimens, this allows subsurface cytoskeletal structures to be detected and imaged. It also allows the optimum imaging force to be quickly determined for new sample types.

* Image arithmetic - pixel-to-pixel matching among images allows images to be added, subtracted, averaged and so on. This allows the analyst to subtract topographic or other background effects from modulus, lateral force, electrical force and thermal images.

* STM - current imaging tunneling spectroscopy (CITS), height imaging tunneling spectroscopy (ZITS) and other sophisticated imaging techniques can be performed with Layered Imaging.

Conclusion

The ability to precisely sample cantilever deflection at different locations along the force-distance curve offers the microscopist a new tool for sample characterization. By operating in a variety of imaging modes and recording acquired data in layers, mapping of properties such as magnetic fields or surface compliance and adhesion has been demonstrated.

Additional Information - Determining Force

For the experiments described, the applied force is measured at the feedback position as a photodetector current reading (SO) that changes with laser reflection off the back of the cantilever. Thus, as cantilever deflection changes, so too does the value of S,. According to Hooke's Law, for any spring
F = -kx
where k is the spring constant and x is the displacement distance.
With the displacement distance expressed as n times the change in sensor current:
n = DS/DX
Solving for the value of X, the Force can then be calculated:
F = (-kDx/Ds)(S-So)
The value So is determined by noting that, for a cantilever above the sample surface there is no deflection; F = 0, therefore S=So
(D=delta)

All images were acquired with the TopoMetrix TMX 2000 Discoverer SPM

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