A scanning atom probe (SAP) was constructed by modifying an ultrahigh vacuum scanning tunneling microscope. A unique feature of the SAP is the introduction of a funnel-shaped microextraction electrode to a conventional atom probe. The electrode scans over an unsmoothed specimen surface at a negative bias voltage and stands still right above an apex of a microcusp. Then the high electric field required for field evaporation of the apex atoms is confined in an extremely small space between the small open hole of the funnel-shaped electrode and the apex of the microcusp. The Pt and Si extraction electrodes are fabricated by mechanical and lithographic processes, respectively, and the diameter of the open hole at the sharp end of the electrode is in the range of 2 to 50 µm and its height is 0.1 to 0.3 mm. In order to examine the unique capability of the SAP, diamond grown by chemical vapor deposition (CVD) and fabricated by high-pressure high-temperature (HPHT) processes was mass analyzed atom-by-atom. The study has revealed that the diamond contains an unexpectedly large amount of hydrogen and that the clusters of 5, 8, and 16 carbon atoms in the diamond structure are weakly bound by hydrogen bonds.The atom probe (AP) [1, 2] is a combination of a field-ion microscope (FIM) [3] with atomically high resolution and a mass spectrometer with a sensitivity capable of detecting individual incoming atoms. Thus, the atom-by-atom mass analysis of the atoms observed by the FIM allows us to clarify the distribution of the atoms composing an analyzed area on an atomic level. Accordingly, the introduction of the AP was expected to generate a strong impact in the areas of materials science and technology. However, its unique capability has been utilized only in a limited area of materials study, mostly the microanalysis of metals, alloys, semiconductors, and limited kinds of conductive polymers and ceramics because the conventional AP can analyze only an apex area of an extremely sharp and long tip where a high field sufficient to field-evaporate the apex atoms as positive ions can be generated by a relatively low voltage applied to the tip. Furthermore, the fabrication of such a filamentary-long tip is extraordinarily difficult for most materials. In order to overcome this difficulty, Nishikawa proposed the development of a scanning atom probe (SAP) [4-6] introducing a funnelshaped microextraction electrode to the conventional AP in order to confine the high field into a small space between a tip apex and an open end of the electrode. Thus, the SAP can analyze not only a sharp slender tip but also an apex area of a minute cusp which normally exists abundantly on an unsmoothed specimen surface. In order to examine the unparalleled capability of the SAP, thin layers of chemicalvapor-deposited (CVD) and fine grains of high-pressure hightemperature (HPHT) diamonds were mass-analyzed by detecting individual ions field-evaporated from the specimen surfaces.
The analyzing area of a conventional atom probe is an apex of a long, sharp tip. However, the fabrication of such a filamentary tip is extraordinary difficult for most materials, such as organic materials, ceramics and semiconductors with multi-layer structures. Accordingly, the application of the atom probe are severely restricted In order to overcome this difficulty, Nishikawa proposed to develop a scanning atom probe (SAP) introducing a funnel-shaped microextraction electrode to the conventional AP in order to confine the high field into a small space between a tip apex and an open end of the electrode. Thus, the SAP allows to analyze not only an apex of a sharp slender tip but also an apex of a few micron high minute cusp which normally exists on an unsmoothened specimen surface.The microextraction electrode scans over a rugged specimen surface at a bias voltage and stands still right above an apex of a micro cusp, FIG. 1.
Unique capabilities of the scanning atom probe, atomically high resolution and atom-by-atom mass analysis, were utilized to investigate the compositional distribution in the individual apexes of a silicon microtip array and fine grains of chemical vapor deposition diamonds. The dry-etched silicon tips contain a large amount of carbon and hydrogen and the HF-treated tips contain even oxygen and fluorine. Although the carbon concentration of the uppermost surface layer is as high as 50% for the dry-etched tip, it decreases to less than 10% for the second layer and approaches to the constant concentration of less than 2% at the depth of 20 nm suggesting the carbon intermixture during the fabrication and/or etching process of the microtip array. The carbon concentration in the HF-treated tip decreases from 18% to 8% at the depth of 30 nm. On the other hand, the oxygen concentration stays fairly constant at around 25% throughout the analysis. The high hydrogen concentration in the diamonds is attributed to the large difference between the activation energies for hydrogen desorption, 21 kcal/mol, and for methane adsorption, 7.3 kcal/mol.
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