SPM Imaging Modes

�1993, �1995 TopoMetrix Corporation

Table of Contents

1.0 Introduction

2.0 Basic Concepts

2.1 Probe/Sample Forces

2.2 Instrumentation

3.0 AFM Imaging Modes

3.1 Introduction

3.2 D.C. Contact

3.3 Lateral Force Microscopy

3.4 Force Modulation

3.5 High Amplitude Resonances (HAR)

3.6 DC - Non-Contact

3.7 Low Amplitude Resonance (LAR)

3.8 Additional Information

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SPM IMAGING MODES

1.0 Introduction

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The scanning probe microscope (SPM) and more specifically the atomic force microscope (AFM) can be operated in many ways. These different techniques provide a variety of capabilities for imaging different types of samples and generating a wide range of information. Imaging modes (sometimes referred to as "scan modes" or "operating modes") are the methods that are used to move the AFM probe over the sample surface and sense the surface in order to create an image. There is a continuum of possible imaging modes, due to differing interactions between the probe tip and sample, as well as the detection scheme used. The choice of the appropriate mode depends on the specific application. This booklet is intended to help understand AFM imaging modes, to select the mode appropriate to the application, and to interpret the resulting images.

2.0 Basic Concepts

2.1 Probe/Sample Forces

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The AFM operates by measuring the forces between the probe and sample. These forces depend, in part, on the nature of the sample, the distance between the probe and sample, the probe geometry, and any contamination on the sample surface. To understand the various AFM imaging modes, it is helpful to understand these contributing factors.

Force/Distance Relationship.

As the probe is brought close to the sample, it is first attracted to the sample surface. A variety of long range attractive forces, such as van der Waals forces, are at work.

When the probe gets very close to the surface, the electron orbitals of the atoms on the surface of the probe and sample start to repel each other. As the gap decreases, the repulsive forces neutralize the attractive forces, and then become dominant.

***** FIGURE 1. Forces between the probe and sample as the

probe approaches the sample surface.

In the area of the curve below the line of zero force, the net force is attractive. In the area above the line of zero force, the net force is repulsive.*****

Sample Surface Contamination.

The sample surface in ambient conditions will have a thin contamination layer (Figure 2). This layer is composed of water and other ambient contaminants, as well as contaminants that remain from the production of the sample. Its thickness will vary (for instance with humidity), and ranges from 25 and 500 A.

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FIGURE 2. A thin contamination layer covers the sample surface in ambient conditions.

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As a probe tip is moved toward a sample surface that has a contamination layer, at some point it is pulled strongly toward the surface by capillary attraction within the contamination layer. Thus, the attractive forces are much greater when a contamination layer is present than when the surface does not have this layer.

The effect of this is also seen when the probe is retracted from the surface. Capillary attraction will tend to hold the probe tip firmly. Thus, the forces at a given distance may be less when moving the probe toward the sample than when moving the probe away from the sample.

Probe Tip Geometry.

The shape of the probe tip is critical to AFM imaging, since the image is the result of both sample surface and probe shape. The probe tip geometry is also important to AFM imaging modes. A very dull, large radius, low aspect ratio tip will have a large area that interfaces with the contamination layer, resulting in very strong attractive forces.

A sharp, small radius, high aspect ratio probe tip will have a much smaller area that interfaces with the contamination layer. Thus, sharp probes have a lower capillary attraction with the sample, since they have a small area of contact within the contamination layer. They can also be moved in and out of this layer more readily than a dull tip.

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FIGURE 3. A sharp probe tip has a much smaller wetted area than a broad tip, resulting in lower forces from capillary attraction.

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Nature of Sample.

The sample itself affects the probe/ sample forces. Some samples are much more likely to have surface contamination than others. In addition, some samples develop a static electric charge readily. Static electricity on the sample surface can have a significant effect on the probe-sample interaction, making AFM imaging difficult.

The force/distance relationship can also depend on the compliance of the sample surface. A surface that is soft will deform as a result of forces from the probe.

2.2 Instrumentation

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Cantilevers.

Force is determined by measuring the deflection of a spring cantilever. This force is calculated by Hooke's Law: F = - kx, where k is the "spring constant" and x is the displacement of the cantilever.

There are two important properties of the AFM cantilever: spring constant and resonant frequency. The spring constant determines the force between the probe and sample when they are in close proximity, and is determined by the material used to construct the cantilever, as well as the cantilever geometry. In conventional "contact" AFM, a very weak cantilever, with a very low spring constant is used.

When a cantilever is moved from its equilibrium position and released, it will oscillate at a "resonant frequency" that is determined by the mechanical properties of the cantilever. A stiff cantilever (with a high spring constant) will resonate at a higher frequency than a weak cantilever. The resonant frequency depends on the material and physical dimensions of the cantilever and the forces that act on the probe. For cantilevers used in AFM, these frequencies vary from about 15 kHz to more than 500 kHz. Resonant frequency depends critically on the mass at the end of the cantilever.

Force/Distance Curves in Practice.

In actual operation, the force/distance (F/s) curve observed by an AFM instrument differs from that shown in Figure 1. This curve can be measured by moving the probe toward the sample surface and measuring its deflection. A typical curve showing deflection as the probe is moved toward the sample and then retracted is shown in Figure 4.

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FIGURE 4. Typical cantilever deflection curve. At the right, the cantilever deflections in the repulsive and attractive force regions are shown

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Measuring Forces.

The AFM measures forces between the sample and probe in order to generate images of the sample surface. There are two important methods of measuring these forces, depending on whether or not the cantilever is being modulated.

Non-Modulated Methods.

In conventional AFM, the sensor detects deflection of the cantilever and the force applied to the sample by the probe is calculated by Hooke's Law (F = -kx). Calculation of this force is achieved by moving the cantilever a fixed distance and measuring the sensor output (Figure 5).

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FIGURE 5. In non-modulated sensing methods, the laser light is reflected from the back of the AFM cantilever, and detected in a multiple-stage photo detector. Changes in photo detector output are used to adjust the z-piezoelectric ceramic, and as z-data.

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Modulation Techniques.

In these techniques, the cantilever is modulated, generally by mounting the base of the cantilever on a piezoelectric ceramic. This ceramic is driven by an alternating voltage, causing it to oscillate Figure 6 .

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FIGURE 6. In modulation techniques, the AFM cantilever is mounted on a piezoelectric ceramic which is used as an oscillator.

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The relationship between the input oscillation and the oscillation of the probe depends on the frequency of oscillation and the "resonant frequency" of the cantilever. Below the resonant frequency, the motion of the probe is the same as the input oscillation (Figure 7a.).

When the frequency is increased toward the resonance condition, the probe "whips" up an down at a higher amplitude for a given driving voltage to the piezo. (Figure 7b). It also lags the input signal. For instance, when the piezoelectric ceramic reaches the top of one oscillation and begins downward, the probe is still rising. The tag in the probe oscillation compared to the input oscillation is called the "phase shift." At even higher frequencies, the phase shift can reach 180', and the amplitude will decrease to very low levels (Figure 7c). The amplitude and phase shift as functions of frequency are shown in Figure 7d. ***** FIGURE 7-1. Amplitude and phase shift (0) as a function of oscillation frequency. a)below resonance, b) at resonance, c) above resonance.

FIGURE 7-2. Amplitude and phase shift (0) as a function of oscillation frequency, d) changes in amplitude and phase with frequency.

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When a force acts on the probe, it behaves as though a mass were placed on the end of the cantilever. When an oscillating probe is moved in close proximity to a sample, the forces from the sample on the probe will cause the resonance frequency to shift as shown in Figure 8.

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FIGURE 8. The resonance frequency is changed by the probe/sample forces when the probe is moved into close proximity to the sample surface.

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Feedback Methods.

The AFM feedback circuit controls the motion of the z-piezoelectric ceramic and the z-data acquisition. There are two basic AFM feedback methods, those in which the probe is not oscillating, and those which use phase shift and/or amplitude change in oscillating cantilever systems.

In feedback methods that are not based on cantilever oscillation, the sensor output is used to adjust the z-piezoelectric ceramic and generate z-data, as shown in Figure 9.

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FIGURE 9. Feedback loop in methods that do not use an oscillating probe.

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The change in phase or amplitude associated with an oscillating probe near a sample can be used to control the feedback loop in the AFM. When this is done, the cantilever is oscillated, and the oscillating photo detector output is compared with the input oscillation through a phase-lock loop. The output is proportional to either the change in amplitude or the phase shift, which is used to control the feedback to the z-piezoelectric ceramic and to generate the z data point (Figure 10).

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FIGURE 10. Feedback circuitry for oscillating cantilever, using phase shift or amplitude change.

This basic concept can be used in a number of scan modes which use cantilever oscillation.*****

3.0 AFM Imaging Modes

3.1 Introduction

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Contact and Non-Contact AFM. Imaging modes can be classified as "contact" or "non-contact" depending on the net forces between the probe and sample.

When the AFM is operating in the attractive region, it is called "non-contact." In this region, the AFM cantilever is curved toward the sample, since it is being pulled by attractive forces. Operation in the repulsive region is called "contact" imaging, and the cantilever is curved away from the sample due to the repulsive forces.

When operating in "non-contact" modes, the probe is in the attractive force region and the cantilever is pulled toward the sample.

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FIGURE 11. a) In "non-contact" modes, the probe is pulled toward the surface primarily by capillary forces on contaminated samples and van der Waals forces on clean samples. b) In "contact" modes, the probe is in the repulsive force region and the cantilever is pushed away from the sample.

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TABLE 1. Imaging Modes Continuum

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The continuum of AFM imaging modes is summarized in Table 1. In the sections that follow, we will discuss each of these modes in more detail. Images illustrating these modes can be found following these descriptions and also in the appendix.

These imaging modes take place in different parts of the force/distance relationship, as shown in Figure 12.

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FIGURE 12. Operating regimes of different scan modes.

In the pages that follow, we will discuss each of these modes.*****

3.2 D.C. Contact

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� Description. This is the standard method of AFM imaging. It has the following characteristics:

� Force: Repulsive

� Cantilever: Soft. Hard cantilevers can be used, but must be operated at high forces. When this is done, the cantilever pushes down on the sample surface, distorting the surface of soft samples.

� Feedback: Displacement. In standard operation, the displacement of the probe is used by the feedback loop to adjust the z-piezoelectric ceramic so that the force between the probe and sample stays constant. The voltage required to do this is used as the z data for imaging. This is called "constant force," or "slow scan" mode. If the scan is done very fast (or the feedback loop is slowed), the z-piezoelectric ceramic will not be able to keep up with features in the sample surface. In this case, the sensor output (from deflection of the cantilever) is used as the z-data. This mode is called "variable force " or "fast scan."

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FIGURE 13. Operation of DC-Contact imaging in constant force mode. The probe is scanned over the surface and the z-piezoelectric ceramic is adjusted to maintain a constant cantilever deflection, and thus a constant force.

Applications. This mode is useful for a wide range of applications. It can be used for both hard and soft samples, although soft samples require use of a weak cantilever and light forces. it is simple to operate.*****

Constant force is used when there is a significant change in height of features over the scan area. Variable force is most often used for very small areas in which the sample is very flat, such as when imaging atomic corrugations or steps.

This mode is very effective when imaging submerged samples. When samples are submerged, there is no meniscus fanned between the probe and the surface of the contamination layer, and imaging may be done with very light forces. This is particularly useful with soft samples.

3.3 Lateral Force Microscopy

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LFM is a modification of standard DC topographic imaging, in which the sideways forces on the probe are imaged. This shows changes in surface friction as well as enhanced contrast at edges. It is sometimes called "frictional force microscopy."

� Description. The force regime and cantilevers used in LFM are similar to those used in DC topographic imaging:

� Force: Repulsive (lateral)

� Cantilever: Soft

Lateral force microscopy works on a very similar principle to variable force AFM. In variable force AFM, movement of the cantilever in response to changes in surface topography are detected by changes in the current from the photo detector. In variable force mode, this is done by measuring the difference between the output on the upper and lower halves of the photo detector [(I +2)-(3+4) on Figure 14]. As the cantilever is deflected in the z axis, the relative amounts of light striking the upper and lower halves of the photo detector change, providing topographic data.

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FIGURE 14. Light lever sensor circuit, with 4 sector photo detector

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Lateral force data is obtained by monitoring the difference between the signals from the left and right sides of the photo detector [(1+3)-(2+4)]. As the cantilever twists slightly due to changes in the lateral force on the AFM probe, the relative amount of light striking the left and right halves of the photo detector changes, providing LFM data for imaging.

Since output from all four sectors can be detected simultaneously, topographic and lateral force data can be obtained during the same scan. When this is done in constant force mode, the output of the top and bottom halves of the photo detector are used to control the z piezoelectric ceramic.

Applications.

LFM is used in conjunction with topographic imaging. It shows changes in material as well as enhanced contrast on sharp edges. In addition to helping to interpret images, it can be used for tribology studies.

3.4 Force Modulation

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This mode is usually done simultaneously with conventional AFM topographic imaging (as described under "DC-contact"). It provides additional data about the sample due to changes in the force/distance characteristics of the sample during scanning.

� Description. Force modulation imaging is performed by modulating the probe at a relatively low frequency during normal AFM imaging. In doing this, the force between the probe and sample is varied, allowing the slope of the force distance curve to be calculated for each data point.

� Force: Repulsive. The cantilever is modulated to vary the force, but this is all done within the repulsive region.

� Cantilever: Soft. This mode uses the same cantilever as in standard AFM constant or variable force imaging.

� Cantilever Modulation: The cantilever is modulated at about 5 kHz, well below its resonant frequency. Since the probe is always in contact with the sample, it does not vibrate, but rather imposes an oscillating force on the sample surface.

� Feedback: The cantilever displacement is used to determine both the slope of the force/distance curve as well as the surface topography. This is done simultaneously. The circuitry to do this is shown in Figure 15.

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FIGURE 15. Instrument control and feedback for simultaneous topography and dF/ds imaging.

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Applications.

Force modulation is used to help analyze topographic images, and provides enhanced contrast for surfaces in which the sample compliance (hardness) changes, as well as at sharp edges. It is thus useful for samples in which the material varies, such as the grooved glass sample shown in Figure 16.

It should also be noted that this technique can be performed in a mode where the cantilever is lifted off the surface between data acquisition times and during the translation to the next point to be sampled. Such an "intermittent contact" mode reduces smearing effects that would otherwise occur on certain samples while scanning under continuous contact.

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FIGURE 16. Image of grooved glass, showing topography, force modulation, and lateral force data. A region that appears to be of a different material, possibly due to contamination, can be seen in the lower right. The force modulation and LFM data shows a change in surface despite no topographical changes.

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3.5 High Amplitude Resonances (HAR)

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� Description. In this mode, the probe is oscillated in the attractive regime at its resonant frequency and at a very high amplitude. The amplitude of the oscillations are sufficiently high that the probe rapidly moves in and out of the contamination layer. In some cases, the probe may enter the contact (or repulsive) regime during its oscillations. In the extreme, an undesired effect may be that the probe touches the sample, resulting in another form of "intermittent contact" in which there is a very short dwell time during contact.

� Force: Attractive and repulsive

� Cantilever: Stiff. A soft cantilever will flop around uncontrollably.

� Oscillation: Resonance, at 50-500 kHz, and an amplitude of 100- 1,000 �.

� Feedback: Phase shift or amplitude change.

It should also be noted that, although the resonance condition may enhance some aspects of image quality, it is not a necessary condition. High quality imaging can also be performed above or below the cantilever resonance frequency.

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FIGURE 17. In high amplitude resonance, the probe rapidly oscillates at a large amplitude.

Applications. In this mode, the image is not affected by the contamination layer, since the probe rapidly penetrates this layer. Under extreme conditions, the probe may actually touch the surface of the sample when imaging with this mode. If this occurs, the probe may damage the sample. This mode may be used to image very soft samples,or those which are not well adhered to their substrates.

3.6 DC - Non-Contact

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There are two ways that AFM systems can be operated in the attractive region without oscillating the cantilever. In one, the net force is attractive, but the probe tip is actually contacting the surface. The attractive capillary forces are greater than the repulsive forces of tip contact. As long as the tip is in contact, this is identical to DC- Contact imaging.

As soon as the probe tip comes out of contact with the sample surface, important changes occur. The feedback loop must change, since when the probe moves away from the surface (due to reduction in the attractive force), the feedback loop must act to move the probe and sample closer rather than farther apart as in contact imaging. This mode is not widely used, due to problems in maintaining stable feedback.

� Description. This method is similar to the DC - Contact mode, except that the probe operates in the attractive region.

� Force: Attractive. The cantilever bends toward the sample, and is held primarily by meniscus forces between the contamination layer and the probe. Despite the fact that the net force is attractive, the probe tip may actually contact the sample surface.

� Cantilever: Soft. The cantilever must be soft enough to be bent by meniscus forces.

� Feedback: Feedback is the same as DC- Contact, with changes in cantilever deflection being used to control the z-piezoelectric ceramic in order to maintain a constant cantilever deflection. Feedback is different depending on whether the probe tip is contacting the sample or if the tip has lifted off the sample, as described above.

When the probe is not contacting the surface, stability of imaging is difficult because of changes in the contamination layer and the meniscus forces on the probe tip. As these change, due to changes in humidity, sample warming, or surface features, the attractive forces can change. If the probe breaks contact with the contamination layer, the system will come out of feedback.

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FIGURE 18. In DC Non-Contact mode, the cantilever is bent toward the sample by the meniscus forces of the contamination layer.

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Applications.

In all non-contact modes, images may be made of very soft surfaces, or of samples that are not well adhered to their substrates. Examples of non-contact imaging are shown following these descriptions.

3.7 Low Amplitude Resonance

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� Description. The cantilever is oscillated at its resonant frequency with a low amplitude. The probe remains within the contamination layer, and the imaging can be done at any point on the force curve (attractive or repulsive). It is usually performed in the attractive region, providing non contact imaging. It is also possible to operate this mode such that there is intermittent contact with the surface as discussed earlier.

� Force: Attractive

� Cantilever: Stiff

� Oscillation: Resonance (50-500 kHz), at low amplitude (2- 100 �).

� Feedback: Phase shift or amplitude change.

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FIGURE 19. The probe is resonated at low amplitude within the contamination layer.

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Applications.

This mode shows promise for imaging samples which can be damaged by the forces between the probe and sample during contact mode imaging.

Images made in this mode depend on the forces of the contamination layer, and can change when this layer changes (e.g. due to sample warming, changes in humidity, surface features, etc.).

3.8 Additional Information

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Additional discussion of AFM imaging modes may be found in the following publications and articles.

1). Atomic- Force Microscope - Force Mapping and Profiling on a Sub 100 A Scale, Martin et al., J. Appl. Phys. 61 (10),4723 (1987)

2). A Scanning Force Microscope Designed for Applied Surface Studies, Erlandsson et al., Microsc. Microanal Microstruct 1, 471 (1990)

3). Rapid Measurement of Static and Dynamic Surface Forces, Druker et al., Appl. Phys. Lett. 56 (24), 2408 (1990)

4). Force Microscopy, Bumham & Colton, Scanning Tunnelina Microscopy: Theory and Application. VCH Publishers, scheduled for publication Spring 1993

All images were acquired with the TopoMetrix TMX SPM System

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