Structures on a submicron scale are becoming more and more important in high technology materials and devices. For better characterization and better understanding of the chemical and physical properties of solids on a nanometer scale a variety of high resolution analytical techniques have been developed. One of those is the 3D atom-probe which is an improved combination of the field ion microscope [1] exhibiting atomic resolution and a time-of-flight mass spectrometer with single ion detection efficiency. In the field ion microscope the specimen is a very sharp needle with an average tip radius of 10 to 100 nm. If an electric field is applied to the tip in the presence of an imaging gas (e.g. He or Ne at 10-2 Pa) ions are created at the surface and project a highly enlarged image of the surface onto a screen. The image resolution depends on tip radius, temperature, image gas and netplane size. Usually for tips with radii below 50 nm and temperatures below 80 K, a resolution of 0.3 nm can be achieved. This is just sufficient to resolve the atomic arrangement on most surfaces. If a certain critical field is exceeded, surface atoms or adatoms become unstable and evaporate as positive ions. Using this process of field desorption in a well controlled manner, a chemical analysis can be performed. During this analysis surface atoms are evaporated one by one and analyzed in a time-of-flight mass spectrometer. The arrangement of the time-of-flight mass spectrometer provides simultaneous detection of all masses and a transmission close to unity. The sensitivity is only limited by the detection efficiency of the particle detector. Operation in a single ion counting mode leads to quantitative analysis simply by adding up the registered particles.

Figure 1: Basic principle of the 3D atom probe (imaging atom probe with optical position recording)

Figure 1 shows the basic principle of the optical 3D atom probe. The essential part of the instrument is the specimen tip. On the imaging detector (chevron channel plate with phosphorus screen) a direct image of the surface can be obtained. Time-of-flight measurement of pulse-desorbed individual ions provides the information about the mass. From the recorded light pulse by means of the CCD-camera the position of the desorbed species can be obtained. From the information of position and mass of the individual species a three dimensional reconstruction of the volume analysed can be obtained with nearly atomic resolution laterally and true atomic resolution into depth.

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REFERENCES:

[1] Miller M K, Cerezo A, Hetherington M G,  Smith G D W, Atom Probe Field ion Microscopy, Clarendon Press, Oxford, 1996