A small introduction into Scanning Acoustic Microscopy

A small introduction into

The near-field scanning probe based thermal microscope (SThM) gives information about the temperature distributions and allows to determine the local thermal conductivity quantitatively. We use a resistive probe based method, which provides both the topography with a spatial resolution in the nanometer scale and the temperature/thermal conductivity profile, with a resolution of a few millikelvin and an accuracy of 2% simultaneously.
The probe is made of a wire with a thin platinum core surrounded by a thick silver cladding and is integrated in a Wheatstone bridge which is connected to a signal generator. A small current is applied for the determination of the probe resistance in order to perform measurements of temperature distribution of a device. The application of a significantly higher current at the frequency w is used for heating the probe periodically and the thermal conductivity of the sample can be determined by measuring the 3 w component of the output voltage of the Wheatstone bridge at two different frequencies.

	a)b)
Fig. 1: Set-up of scanning thermal microscope to perform quantitative temperature and conductivity measurements.	Fig. 2: a) Temperature distribution on a short channel n-MOSFET b)Quantitative thermal conductivity analysis on a MW-CVD diamond film

Scanning Acoustic Microcscopy

Mankind is used to see with their eyes by the use of light. In nature, however, there exist also other techniques for exploring the environment. There are some animals which can see by the use of sound, such as bats or whales. Learning from nature, man has adapted this technique for the benefits of human beings and uses it now in many different fields like for example sonar systems and non-destructive testing in industry.
With the development of modern industry, especially with the rapid development of ultra-large-scale integrated circuit technology, there are more and more requirements to research the material properties at the resolution of micro- or nanometer range.
The so-called scanning acoustic microscope (SAM) is developed to analyse the acoustic (mechanical) properties of materials. This kind of microscope has to use focused beams to detect acoustic properties, it has naturally a limit of resolution because of Rayleich diffraction limits of focused beams.
To overcome this resolution limit, the so-called near-field acoustic microscope has been developed. The principle of this kind of microscopes is almost the same: an acoustic wave is produced in a tiny area just in the vicinity of surface (near-field area) through different interaction mechanisms and by detecting this acoustic wave, we can gain the acoustic properties of materials at a high resolution which is not dependent on wavelength.
Scanning Electron Acoustic Microscope (SEAM) and Scanning Probe Acoustic Microscope (SPAM), which are developed from the commercial Scanning Electron Microscope (SEM) and Scanning Probe Microscope (SPM), are two typical microscopes of this kind with highest resolution.
The following figure describes the main structure of SEAM and SPAM:


Fig. 1: Principle of operation of a Scanning electron acoustic and Scanning probe acoustic microscope

The typical results of SEAM and SPAM with ferroelectric BaTiO₃ ceramic sample are as follows:

Topography of SEM Acoustic image of SEAM Topography of SPM Acoustic image of SPAM

Fig. 2: Typical Results of acoustic near-field microscopes

So with these techniques it is possible to obtain the acoustic properties, which are different from topography, at the resolution in the micro- and nanometer range.