In FMM mode, the AFM tip is scanned in contact with the sample, and the z feedback loop maintains a constant cantilever deflection (as for constant-force mode AFM). In addition, a periodic signal is applied to either the tip or the sample. The amplitude of cantilever modulation that results from this applied signal varies according to the elastic properties of the sample, as shown in Figure 1.11.
The system generates a force modulation image, which is a map of the sample's elastic properties, from the changes in the amplitude of cantilever modulation. The frequency of the applied signal is on the order of hundreds of kilohertz, which is faster than the z feedback loop is set up to track. Thus, topographic information can be separated from local variations in the sample's elastic properties, and the two types of images can be collected simultaneously. Figure 1-12 shows a topographic contact-AFM image (left) and an FMM image (right) of a carbon fiber/polymer composite.
Phase detection microscopy (PDM) - also referred to as phase imaging - is another technique that can be used to map variations in surface properties such as elasticity, adhesion, and friction. Phase detection images can be produced while an instrument is operating in any vibrating cantilever mode, such as non-contact AFM, intermittent-contact AFM (IC-AFM), or MFM mode. Phase detection information can also be collected while a force modulation image is being taken.
Phase detection refers to the monitoring of the phase lag between the signal that drives the cantilever to oscillate and the cantilever oscillation output signal. (See Figure 1-13.) Changes in the phase lag reflect changes in the mechanical properties of the sample surface.
The system's feedback loop operates in the usual manner, using changes in the cantilever's deflection or vibration amplitude to measure sample topography. The phase lag is monitored while the topographic image is being taken so that images of topography and material properties can be collected simultaneously.
One application of phase detection is to obtain material-properties information for samples whose topography is best measured using IC-AFM rather than contact AFM (see Intermittent-contact AFM). For these samples, phase detection is useful as an alternative to force modulation microscopy, which uses contact AFM to measure topography.
Figure 1-14 shows a topographic non-contact AFM image (left) and a PDM image (right) of an adhesive label. The PDM image provides complementary information to the topography image, revealing the variations in the surface properties of the adhesive label.
EFM maps locally charged domains on the sample surface, similar to how MFM plots the magnetic domains of the sample surface. The magnitude of the deflection, proportional to the charge density, can be measured with the standard beam-bounce system. EFM is used to study the spatial variation of surface charge carrier density. For instance, EFM can map the electrostatic fields of a electronic circuit as the device is turned on and off. This technique is known as "voltage probing" and is a valuable tool for testing live microprocessor chips at the sub-micron scale.
Scanning capacitance microscopy (SCM) images spatial variations in capacitance. Like EFM, SCM induces a voltage between the tip and the sample. The cantilever operates in non-contact, constant-height mode. A special circuit monitors the capacitance between the tip and the sample. Since the capacitance depends on the dielectric constant of the medium between the tip and sample, SCM studies can image variations in the thickness of a dielectric material on a semiconductor substrate. SCM can also be used to visualize sub-surface charge-carrier distributions. For example, to map dopant profiles in ion-implanted semiconductors.
For TSM, a cantilever composed of two different metals is used. (Or, a thermal element made up of two metal wires can be used.) The materials of the catilever resopond differently to changes in thermal conductivity, and cause the cantilever to deflect. The system generates a TSM image, which is a map of the thermal conductivity, from the changes in the deflection of the cantilever. A topographical non-contact image can be generated from changes in the cantilever's amplitude of vibration. Thus, topographic information can be seperated from local variations in the sample's thermal properties, and the two types of images can be collected simultaneously.
Near-field scanning optical microscopy (NSOM) uses a special type of scanning probe microscope that operates using visible light. Traditionally, the resolution of optical microscopes has been limited by the wavelength of light to about half a micron. NSOM improves the resolution of an optical microscope by an order of magnitude.
The probe for NSOM is a "light funnel" scanned over the sample. Visible light emanates from the narrow end of the light funnel, a couple of hundred angstroms in diameter, and either reflects off the sample or travels through it into a detector. The intensity of the optical signal collected by the detector at each measurement point constitutes the data set, the NSOM image. NSOM can be used to generate a visible-light image of the surface with a resolution of about 150Å, provided that the distance between the light source and the sample is very short, about 50Å.
Normally an SPM is used to image a surface without damaging it in any way. However, either an AFM or STM can be used to modify the surface deliberately, by applying either excessive force with an AFM, or high-field pulses with an STM. Not only scientific literature, but also newspapers and magazines have shown examples of surfaces that have been modified atom by atom. This technique is known as nanolithography.
Figure 1-16 shows a photoresistive surface that has been modified using this technique
