anasys: intoduction to AC thermal imaging

Introduction to AC Thermal Imaging

The advantage of thermal imaging over other modes of imaging is that all materials conduct heat. Futhermore, the technique is able to see beneath the surface of specimens since the heat from the tip penetrates into the sample. An example of this is shown below for a metal grid buried in an epoxy resin.

Topography (left) and DC Thermal images of metal grid buried in epoxy resin (mean probe temperature 50°C, scan speed 50 µm/sec)

The topography (left)is largely featureless apart from an indentation in the top left of the image deliberately created by placing a hot thermal probe down on the sample. This has created a hole through to the metal layer about 1 µm down. The DC thermal image on the right shows a faint "X" of the sub-surface metal mesh as well as a bright region where the metal is exposed in the indentation.

One way of controlling the depth of view of the thermal image is to apply an AC current to the probe (superimposed on the conventional DC heating). This propagates an AC thermal wave from the tip into the sample and the voltage needed to sustain this wave can be monitored and used to build up an AC thermal image in the same way as the power required to keep the tip at a constant temperature can be used to generate a DC thermal image. However, the depth of penetration of the thermal wave is strongly dependent on frequency. High frequencies detect thermal diffusivity differences close to the surface whereas low frequencies "see" further into the sample. This is illustrated below in two AC thermal images of a metal film covered by a polymer. In the centre of the image the polymer layer has been etched away to bring the metal closer to the surface. This shows up bright in the low frequency AC thermal image on the left but is hardly evident when the tip is modulated at 30 kHz:

AC thermal images of buried metal layer in polymer - left: 1 kHz, right: 30 kHz (images courtesy of Lancaster University)

As a rough "rule of thumb", an AC thermal wave of 30 kHz will penetrate 1 µm into a typical polymeric sample.

The next sequence of pictures show images of the buried metal grid described above. The AC modulation frequency was increased from 2 to 100 kHz approximately doubling the frequency between images. It is still possible to acquire the topographic and DC thermal images at the same time.

AC thermal images at different frequencies of the buried metal grid (mean probe temperature 50°C, amplitude 5°C)

It can be seen that the image contrast (using the bright exposed metal area as a reference) decreases as the frequency is increased. From these set of images a 3 dimensional image of the buried metal grid was reconstructed using the histograms of each of the images as a means to decide the cut off between metal and polymer regions:

3D Tomographic reconstruction of the buried metal grid (x,y = 100 µm, z = 2 µm)

The "high" area in the top of the image is, in fact, the hole through the polymer to the metal surface. From this we can estimate the depth of the metal layer. This form of image analysis is still in its early stages but promises to be a powerful form of non-invasive depth-profiling of surfaces.

Futher Reading

L. J. Balk, M. Maywald, R. J. Pylkki, "Nanoscopic detection of the thermal conductivity of compound semiconductor materials by enhanced scanning thermal microscopy", 9th Conf. on Microscopy of Semiconducting Materials Oxford 1995.

A. Hammiche, H. M. Pollock, M. Reading, M. Song, "Scanning thermal microscopy: sub-surface imaging, thermal mapping of polymer blends, localised calorimetry", Journal of Vacuum Science and Technology B: Microelectronics and Nanostructures, 14 (1996) 1486-1491.

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