Direct Bombardment Mode Research Articles

On the dynamic range in depth profiling with electron-gas SNMS

Despite of the high depth resolution around 1 nm achieved with the direct bombardment mode DBM of electron-gas SNMS, its dynamic range is mostly limited to 3 orders of magnitude. Detailed depth profile studies reveal that crater edge and memory effects contribute with 80% and 20%, respectively, to the limitations of the dynamic range in DBM-SNMS. An experimental remedy has been found by the installation of an additional aperture in the postionization chamber which allows a “geometrical blanking”. By such means the dynamic range is improved by one order of magnitude. Further experiments have shown that the so-called Mesa technique as a specific method for the sample preparation leads to a somewhat smaller improvement.

Advantages of a High-frequency Square Wave in a Sputtered Neutral Mass Spectrometry Study of Insulating Materials

The direct bombardment mode (plasma sputtering) of d.c.-voltage sputtered neutral mass spectrometry (SNMS) achieves better depth profile resolution than ion gun sputtering by minimizing the conic structures and preferential sputtering associated with ion gun sputtering. However, when applied to insulating materials, d.c. SNMS allows a charge to build up on the surface; this charging causes sample decomposition and degrades the depth resolution. This problem can be eliminated by applying a high-frequency square-wave voltage to the target. The negative phase of the square wave produces positive ion bombardment, while the positive phase attracts electrons to the sample surface to neutralize the accumulated positive charge. The analysis of insulators is particularly enhanced by this method.* Author to whom correspondence should be addressed.

Applications of SNMS in archaeometry.

The recently developed High Frequency Mode HFM of electron gas SNMS allows investigations on insulating samples with the well known advantages of the SNMS Direct Bombardment Mode DBM for the analysis of conducting samples. HFM has been applied to analyses of different historic ceramic and glass samples in order to demonstrate the possibilities of SNMS in this field. It is shown that manufacturing places of ceramic samples could be distinguished by SNMS mass spectra ("fingerprints"). Furthermore questions of the constituents of colour remains on a painted ceramic ("Kaisermedaillon") could be answered by our SNMS analyses. SNMS investigations have been also applied to corrosion phenomena on different glass samples.

High resolution depth profiling of non-conducting samples with SNMS

Beat-like signal modulations in sputter depth profiles of multilayer structures are shown to enable an estimation and the optimization of the homogeneity of the sputter erosion process. Using W-Si multilayer structures of 69 doublelayers with a thickness of 40 A, it is shown that the high-frequency mode (HFM) of electron-gas SNMS (e−-gas SNMS) for the analysis of insulators provides the same high depth resolution as the conventional direct-bombardment mode (DBM) of this technique.

Secondary-neutral microprobe with electron-gas post-ionization

The essential features and the performance data of a recently developed secondary-neutral microprobe are described which interfaces the well-established technique of electron gas (plasma) post-ionization with a high-transmission magnetic sector mass spectrometer. Small-area analysis is accomplished by means of a separate high-brightness liquid-metal (Ga+) ion source. For 20 keV Ga+ ions and 1 nA beam current the intensities of post-ionized sputtered neutrals amount to some 103 counts/s for pure elements, and secondary neutral micrographs can be acquired with an image resolution in the μm range. Due to the high transmission of the mass spectrometer secondary-neutral intensities up to 109 counts/s per 1 mA primary current can be obtained in the direct bombardment mode (i.e., by extracting the sputtering beam out of the plasma), thus extending the detection limit down to the 10 ppb regime. Furthermore, the instrument can be operated at high mass resolution (M/ΔM=9500) providing the option of isotope analysis by means of secondary-neutral mass spectrometry.

Theory and improvements for SNMS depth profiling with INA3

Optimization of SNMS depth profiling using the direct bombardment mode (DBM) is possible by improvement of the ion current density distribution, reduction of bombardment energy and independent choice of current density. A model for primary ion extraction is presented and ion current density distributions have been calculated for different sample holder arrangements. Using the simulation procedure optimal extraction conditions can be predicted for conventional arrangements and suggestions for new ones can be made. The best depth resolution has been obtained using a new extraction arrangement with an additional aperture.

Effects of instrumental parameters on quantification and depth resolution in secondary neutral mass spectrometry

The influence of plasma conditions and ion optics of the only commercially available electron gas Secondary (or Sputtered) Neutral(s) Mass Spectrometry (SNMS) instrument Leybold INA3 on relative element sensitivity factors and on the flatness of sputter craters is investigated. For III–V-compounds with mass differences up to 84 amu it is found that realistic plasma and optics variations are, inter alia, the reason for the spread of sensitivity factors by up to a factor of 3 on one and the same instrument. Changes of plasma conditions, for which bombardment voltage has been optimized with respect to crater flatness in the direct bombardment mode, require changes of the bombardment voltage roughly proportional to the ion saturation current on the Langmuir probe.

Secondary neutral mass spectrometric investigations on the formation of sputter-generated molecules by atomic combination

The direct bombardment mode of secondary neutral mass spectrometry (SNMS) has been employed for the investigation of the neutral particle flux from a series of NiW and NiMo alloys under normal bombardment with Ar + ions at energies of up to 1200 eV. Making use of the direct proportionality between the intensities of the SNMS signals and the individual formation probabilities of the detected species, a critical examination of the atomic combination model for the formation of sputter-generated molecules is performed. It is shown that this model applies to the generation of the detected dimers and trimers formed by the atomic constituents of the alloy samples.

Recent applications of secondary neutral mass spectrometry for quantitative analysis of homogeneous and structured samples

The main advantage of secondary neutral mass spectrometry SNMS compared to its sister method SIMS is its high quantificability in consequence of the separation of the emission and the ionization process of the analyzed particles. Electron gas SNMS using electron impact postionization by the electron component of a resonantly excited low pressure high frequency plasma is at present the most developed analytical technique of this type. The characteristic properties and the potentialities of SNMS are demonstrated by comparative SNMS-SIMS measurements, and for the analysis of homogeneous multielement samples. Detection sensitivities down to 100 ppb are obtained with SNMS in such cases. The direct bombardment mode of SNMS is applied to depth profile analysis of a Ta-Si multilayer system. For bombarding energies in the order of 10 2 eV a depth resolution down to the fundamental limits set by the low energy ion-surface interaction itself becomes possible.

Application of glow discharge mass spectrometry and sputtered neutral mass spectrometry to materials characterization

Postsputtering ionization using an inert gas plasma to decouple the sputtering and ionization processes minimizes matrix effects commonly associated with conventional secondary ion mass spectrometry. Glow discharge mass spectrometry (GDMS) utilizes ions generated in a dc inert gas plasma to sputter atoms into the ambient plasma from the surface of a cathode composed of the material to be studied. The sputtered atoms, predominantly neutral, are then ionized in the plasma by Penning and electron impact ionization. GDMS provides excellent sensitivity and signal stability, and has found wide uses for characterization of bulk materials such as metals and GaAs. The sensitivity and large sampling volume also make it ideal for determining nonuniform trace contaminants in materials, e.g., U and Th in metals. Sputtered neutral mass spectrometry uses either low-energy plasma ions (direct bombardment mode or DBM) or an independent focused ion beam (separate bombardment mode or SBM) to sputter atoms into a low-pressure high-frequency plasma for electron impact ionization. DBM offers better depth resolution than conventional surface analytical techniques and is ideal for thin-film studies. The plasma electrons compensate for sample charging in SBM and make this method ideal for the analysis of insulating material such as phosphosilicate glass.

Quantitative chemical surface, in-depth, and bulk analysis by secondary neutrals mass spectrometry (SNMS)

An SNMS/SIMS apparatus is described that allows measurements in three different modes: SNMS direct bombardment mode (DBM), SNMS separate bombardment mode (SBM), and SIMS, with the sample remaining in the same positions. A novel geometrical arrangement that guarantees optimum performance in each operating mode was chosen for this combination. A postdescription of the two SNMS modes is given. From the presented results it could be deducted that direct quantification is possible; the sensitivity is in the ppm range for all elements; in in-depth-profiling high resolution and high speed can be obtained simultaneously; and conductors, semiconductors, and insulators can be analyzed. The results show that the combination of two SNMS modes and SIMS promises to become a valuable tool for surface, interface, and bulk analysis.

High resolution sputter depth profiling of implantation structures in Si by low energy SNMS

The potentialities of the direct bombardment mode of secondary neutral mass spectrometry (SNMS) for depth profile analysis are discussed. In particular the achievement of extremely high depth resolution by using ion bombarding energies on the order of only 102 eV is emphasized. Low energy SNMS is applied for depth profiling of implantation profiles of As and P in Si. The range parameters obtained from the measured profiles are compared with theoretical values. By means of the high depth resolution, detailed structures in the unannealed implantation profiles are quantitatively determined and partially explained by the presence of residual surface oxides.