What is PhaseMode?

       PhaseMode is to WaveMode as Lateral Force Mode (LFM) is to Contact Mode. That is to say, LFM is another type of scan mode that can be acquired while running the AFM in contact mode (see Applications Note AN-4). The signal for LFM is generated when running in contact mode, whether or not the data is being collected and plotted as an image. Similarly, when running in WaveMode using amplitude feedback with a fixed resonant frequency, a phase-shift signal is generated and this can produce image contrast - and we call this scan mode PhaseMode. The exact nature of the interaction mechanism that creates the phase shift signal is uncertain; however, one mechanism almost certainly involves sticking of the probe to the surface, causing a delay time in the cantilever resonance pattern, relative to before the instant that the tip makes its brief contact.

Why bother with PhaseMode?

       There has been a lot of current interest in PhaseMode. Much of the work has been done with heterogeneous mixtures of polymers, which tend to respond well to PhaseMode because the modulus properties of the polymer are readily resolved. PhaseMode can yield results that are similar in appearance to LFM, however, PhaseMode is generally more sensitive in producing meaningful image contrast. Furthermore, PhaseMode seems to produce image contrast on a greater variety of sample types than does LFM.

Topography & Material Properties that Control Image Contrast

       Subtle topographic features are well resolved with PhaseMode, even better than WaveMode, because these features can produce a strong phase-shift signal. The phase image contrast that results from topographic features has the appearance of a 2-D derivative image, similar to that produced in mapping the tip deflection signal (i.e., error signal), and can be termed a pure phase image, because it is not dominated by the effects of material properties. Figure 1 shows a pure phase image dominated by topographic features.
In addition to topographic features, there are two material properties that should affect the magnitude of the phase shift signal when operating PhaseMode at a high resonance amplitude and a high degree of damping (i.e., hard tapping). First, the degree of sticking between the tip and sample will be related to the chemical affinity for bonding between these two objects. Second, the amount of sample deformation at the surface, as a function of elastic modulus, will affect the contact area between the tip and the sample. These two effects are synergistic, because the more contact area there is between the tip and the sample, the greater the degree of sticking. Figure 2 is a PhaseMode image that was produced using hard-tapping, and consequently shows a map of force-modulus .

Control Variables:

       After careful systematic study, it is clear that a variety of experimental parameters can be used to control image quality in PhaseMode. It is fair to say that there is not necessarily a bad or good image produced with PhaseMode, but rather, some images have contrast characteristics that seem more useful than others. The challenge, therefore, is to control experimental parameters so as to create images that give the best contrast. The following are experimental parameters that used for getting "good" phase mode images:

1. Amplitude damping (%) - for pure phase, damping is set < 50%; for force modulus, damping is set < 65%
2. AC drive voltage (V) - for pure phase, max peak intensity < 1/2 scale; for force modulus, max peak intensity > 1/2 scale
3. Tip radius - a partially dulled probe tip is better for more contrast in force modulus imaging

1. Amplitude damping (%) - If the % damping is too high, that is, if the amount of amplitude damping is very high (i.e., a small negative set-point), then the image contrast can be either too great, or sometimes there is no contrast at all (Figures 3a-3d). Conversely, if the % damping value is too small (i.e., a large negative set-point), then the tip will not properly track the surface because there is not enough energy transferred from the probe to the sample for a good feedback response. There is a range of damping values that produce reasonably good results, varying from 30% to 85%. After setting the %-damping value as prescribed, one can "fiddle" around with this parameter in order to optimize contrast features

2. AC drive voltage - The effect of voltage is controlled in a way similar to resonance damping %. If the resonance voltage is too high, poor contrast results. If the voltage is too low, then artifacts creep into the image as a result of the tip not properly tracking the surface (this is true for WaveMode in general). The effects of varying voltage can be seen in a series of images taken from a piece of rubber tire (Figures 4a-1f). At a lower voltage, the image is that of a purely derivative nature (Figure 4a). With gradually increasing voltages, the images become more dominated by the effects of sample modulus (i.e., Figure 4f shows no remaining pure phase signal).

3. Tip radius - Recall, the amount of contact area between the tip and sample will control the amount of sticking between these objects, which in turn controls the amount of phase lag. Consider that not only will the elastic modulus of a material control the contact area, but the radius of the probe tip is also an important factor. A sharp probe tip is often less than ideal for getting good force modulus contrast with PhaseMode - instead, a dull probe tip often seems to yield a better response. However, a better pure PhaseMode image is obtained with a sharper probe tip simply because topographic features are better resolved with the smaller interaction area of a smaller tip radius. One downfall to this reality is that there exists a point where a probe tip is damaged beyond any usefulness. That is when the probe tip has been dulled so much that image artifacts are produced. At this point, the probe should be discarded.