Electron-gas SNMS

In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufFiciendy dense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in Ar, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electron-gas SNMS is still in the developmental stages, while the former has been incorporated into commercial instmmentation. 

The SNMS instrumentation that has been most extensively applied and evaluated has been of the electron-gas type, combining ion bombardment by a separate ion beam and by direct plasma-ion bombardment, coupled with postionization by a low-pressure RF plasma. The direct bombardment electron-gas SNMS (or SNMSd) adds a distinctly different capability to the arsenal of thin-film analytical techniques, providing not only matrbe-independent quantitation, but also the excellent depth resolution available from low-energy sputterii. It is from the application of SNMSd that most of the illustrations below are selected. Little is lost in this restriction, since applications of SNMS using the separate bombardment option have been very limited to date. 

Similar detailed studies of RSFs have been carried out for GDMS, but not for electron-gun electron impact ionization or for SALI. The spread in elemental RSFs for electron-gas SNMS is comparable to that observed for Ar glow-discharge ionization of sputtered neutrals. ... 

Figures Quantitative high depth resoiution profile of a complex Ai Ga. j As laser diode test structure obtained using electron-gas SNMS in the direct bombardment mode, with 600-V sputtering energy. The data have been corrected for relative ion yield variations and summed to Al + Ga = 50%. The 100-A thick GaAs layer is very well resolved.

Figure 8 Quantitaftive high depth resolution profile of O and N in a Ti metal film on Si, using electron-gas SNMS in the direct bombardment mode. Both O and N are measured with reasonably good sensitivity and with good accuracy both at the heavily oxidized surface and at the Ti/Si interface.

Sputtered Neutrals Mass Spectrometry Secondary Neutrals Mass Spectrometry Direct Bombardment Electron Gas SNMS... 

The layout of a commercial apparatus for electron gas SNMS is depicted in Figure 2. The sample head is introduced directly into the postionizing electron gas. Particle emission from the sample surface is excited by extracting ions from the ECWR plasma by a simple ion optical arrangement in front of the sample (direct bombardment mode, DBM). Since the angular distribution of sputtered particles changes from a torus-like configuration to an over-cosine behavior when the... 

Figure 2 Schematic of a commercial instrument for electron gas SNMS. (By IFOS GmbH, Kaiserslautern.)...

Figure 7 Schematic diagram of the external bombardment mode of electron gas SNMS, including an Auger electron spectrometer for complemental analysis.

In Figure 12, the SNMS intensities taken directly from the mass spectra obtained with electron gas SNMS for different standard samples with specified compositions are plotted versus the concentrations Cx of the sample constituents. It can be seen that the relative detection factors for electron gas SNMS vary only little for a large variety of elements. Figure 12 demonstrates that the SNMS signals obtained with the electron gas method describe the sample composition within a factor of 2 without any further evaluation procedure being employed. Such a simple and very convenient behavior can be well understood from eqn [1] since the integral for is not much sensitive to the variations of the individual functions referring to an analyzed species X. This will be... 

The small variations of the relative detection factors in electron gas SNMS for the different species X can be reduced significantly by putting... 

The very favorable features of electron gas SNMS for high-resolution depth profiling being achieved with the DBM technique (see above) become exemplified by Figure 16. This figure shows the depth profile through a W-Si multilayer stack on a ceramic... 

Table 9. Typical Operating Conditions for Electron Gas SNMS, Using Argon...

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

A considerably more effective way of electron impact postionization for mass spectrometric identification of sputtered neutral species was introduced by employing a spatially expanded hot Maxwellian electron gas (see below). Much higher postionization efficiencies of the order of 10 were established by this method for which the acronym SNMS was first introduced. 

Post-ionisation schemes for the detection of sputtered neutral species in SNMS utilise either electron impact ionisation (e-beam SNMS), elecfion gas or plasma ionisation (plasma SNMS) or laser ionisation (L-SNMS). For e-beam SNMS, which is based on the use of a directed flux of essentially mono-energetic electrons towards the sputtered neutrals, high sensitivities have been obtained. Plasma or e-gas SNMS uses a low-pressure plasma (usually inert gas, e.g. Ar) containing ions to sputter the surface and at the same time the e-gas for ionisation of the neutrals. Although e-beam and plasma SNMS suffer from a low ionisation probability, they provide a well-established quantification scheme. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

The basic principle of e-beam SNMS as introduced by Lipinsky et al. in 1985 [3.60] is simple (Fig. 3.30) - as in SIMS, the sample is sputtered with a focused keV ion beam. SN post-ionization is accomplished by use of an e-beam accelerated between a filament and an anode. The applied electron energy Fe a 50 20 eV is higher than the range of first ionization potentials (IP) of the elements (4—24 eV, see Fig. 3.31). Typical probabilities of ionization are in the 0.01% range. SD and residual gas suppression is achieved with electrostatic lenses before SN post-ionization and energy filtering, respectively. 

Fig. 3.32. Basic principles of HF-plasma SNMS. Vhf, PHrarethe HF generator frequency and power, respectively, PArthe plasma gas (Ar) pressure 7e and the electron and plasma gas temperatures, respectively. Hpi = n is the plasma e, Ar" ) density. Bo the...

Element mapping with non-resonant laser- SNM S can be used to investigate the structure of electronic devices and to locate defects and microcontaminants [3.114]. Typical SNMS maps for a GaAs test pattern are shown in Fig. 3.43. In the subscript of each map the maximum number of counts obtained in one pixel is given. The images were acquired by use of a 25-keV Ga" liquid metal ion source with a spot size of approximately 150-200 nm. For the given images only 1.5 % of a monolayer was consumed -"static SNMS". 

In SIMSLAB from VG Scientific, both surface analytical techniques - SIMS and SNMS - have been applied (see Figure 5.34). In this mass spectrometer different types of primary ion sources are available. Ar+, Cs+, Ga+ or O) primary ions are accelerated in the secondary ion source on the solid sample surface. Similar to the CAMECA IMS-7f, with this experimental arrangement, besides depth profiling, a microlocal analysis can also be performed. The sputtered secondary ions (for SIMS) or the post-ionized sputtered neutrals (for SNMS) - the post-ionization is carried out by an electron beam in an ionizer box (right-hand schematic in Figure 5.34) - are separated... 

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