Micro-TA is based on an Atomic Force Microscope (AFM). A schematic diagram of an AFM is shown above. This consists of a probe connected to an array of peizo transducers which can move it in the x, y and z planes. A laser beam is reflected off the back of the cantilever to fall on a photodetector, which senses the degree to which the probe is bent upward or pulled downward. The probe is first lowered onto the sample until the cantilever bends slightly, thus exerting a force onto the surface. The piezo transducers then move the probe over the surface of the sample, rastering line by line in the x and y directions until an area of, for example, 10 µm² has been covered. While the probe is being scanned over the sample surface, a feedback loop from the photodetector ensures that the degree of bending of the cantilever, and thus the force exerted on the surface, is kept constant.

This means that, as the tip encounters a bump, the base of the cantilever must be moved upward over the bump as illustrated in the animation. In this way, by monitoring the z-axis peizo as the probe is moved, a map of the topography of the surface is created. This is illustrated for some grains of gold above. This mode of operation is called contact mode AFM because the probe is at all times touching the surface of the sample.

For Micro-Thermal Analysis a special probe is used that has a resistive heater at the tip. Currently, the most widely used probe is based on a Wollaston wire. This is a wire that has a thick coating of silver over the top of a thin core of platinum. The Wollaston wire probe is illustrated below.

At the tip of the probe the silver is etched away exposing the platinum. This design means that almost all of the electrical resistance of the probe is located at the tip. Consequently, when an electrical current is passed through the probe only the tip heats up. Also, the electrical resistance of the probe is a measure of the temperature at the tip. The outcome is that this probe can serve as a means of carrying out miniature thermal analysis experiments by changing its temperature.

The simplest mode of using the probe is to hold its temperature constant and measure the electric power required to do this. The probe is then rastered over the surface of the sample as described above for contact mode AFM. As the probe encounters an area of the sample with high thermal conductivity more heat is lost from the tip to the sample so the electrical power required to maintain the temperature constant is increased. When the thermal conductivity is low the electrical power is reduced. In this way the thermal conductivity of a sample can be mapped as shown below.

The images of a thick film resisitor (above) show the topography (left) with a low (dark) area in the top right, with the rest being raised with an irregular surface. The thermal conductivity map (middle) shows an area of high thermal conductivity, top left, which is the metal contact, an intermediate thermal conductivity area at the bottom, which is the ruthenium oxide resistance element and low thermal conductivity (dark) area that is the alumina base material. This illustrates how the disposition of materials on a surface can often be mapped simply by looking at differences in thermal properties.

The image on the right is the AC thermal image. In addition to holding the tip at a contant temperature, its temperature can modulated in the same way as Modulated Temperature DSC. The power required to achieve a sinusoidal modulation of a few degrees is measured. The thermal conductivity, or DC image, is obtained by averaging the signal to eliminate the modulation. In this way AC and DC images can be obtained simultaneously. The AC thermal image is dependent of several parameters including thermal diffusivitity. In the example above it simply gives much the same information as the thermal conductivity map, however, it has the advantage that it can look at different depth into the sample by varying the modulation frequency. This capability is explained elsewhere.