Sputtered atoms ionization

SNMS Spullered Neutral Mass Spectroscopy Surface, bulk Plasma discharge noble gases 0.5-20 keV Sputtered atoms ionized by atoms or electrons then mass analyzed 0.1-0.6 nm (or deeper Ion milling) 1 cm Elemental analysis Z > 3 depth prolile deleclion limit ppm 4,6... 

GDMS Glow Discharge Mass Spectroscopy Sample forms the cathode for a D.C. glow discharge Sputtered atoms ionized in plasma Ions - analyzed in mass spectrometer 0.1-100 pm 3-4 mm (Bulk) trace element analysis deleclion limit sub-ppb 9,10... 

SALI Surface Analysis by Laser Ionization Surface e-beam, ion-beam, or laser for sputtering Sputtered atoms ionized by laser then mass analyzed 0.1-0.6 nm up to 3 pm in milling mode 60 nm Suriace analysis depth proliling 7... 

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

The atom flux sputtered from a solid surface under energetic ion bombardment provides a representative sampling of the solid. Sputtered neutral mass spectrometry has been developed as method to quantitatively measure the composition of this atom flux and thus the composition of the sputtered material. The measurement of ionized sputtered neutrals has been a significant improvement over the use of sputtered ions as a measure of flux composition (the process called SIMS), since sputtered ion yields are seriously affected by matrix composition. Neutral panicles are ionized by a separate process after sputter atomization, and SNMS quantitation is thus independent of the matrix. Also, since the sputtering and ionization processes are separate, an ionization process can be selected that provides relatively uniform yields for essentially all elements. 

If the secondary ion component is indeed negligible, the measured SNMS ion currents will depend only on the ionizing mode, on the atomic properties of the sputtered atoms, and on the composition of the sputtered sample. Matrix characteristics will have no effect on the relative ion currents. SNMS analysis also provides essentially complete coverage, with almost all elements measured with equal facility. All elements in a chemically complex sample or thin-film structure will be measured, with no incompleteness due to insensitivity to an important constituent element. Properly implemented SNMS promises to be a near-universal analytical method for solids analysis. 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Local Thermodynamic Equilibrium (LTE). This LTE model is of historical importance only. The idea was that under ion bombardment a near-surface plasma is generated, in which the sputtered atoms are ionized [3.48]. The plasma should be under local equilibrium, so that the Saha-Eggert equation for determination of the ionization probability can be used. The important condition was the plasma temperature, and this could be determined from a knowledge of the concentration of one of the elements present. The theoretical background of the model is not applicable. The reason why it gives semi-quantitative results is that the exponential term of the Saha-Eggert equation also fits quantum-mechanical expressions. 

The element sensitivity is determined by the ionization probability of the sputtered atoms. This probability is influenced by the chemical state of the surface. As mentioned above, Cs" or OJ ions are used for sample bombardment in dynamic SIMS, because they the increase ionization probability. This is the so-called chemical enhancement effect. 

A plasma is a hot, partially-ionized gas that effectively excites and ionizes atoms [366, 534, 535]. A glow discharge is low-pressure plasma maintained between two electrodes. It is particularly effective at sputtering and ionizing material from solid surfaces. 

Secondary ions return to the sample surface through the influence of the electric field at the cathode. Individual atoms and clusters of atoms undergo collisions that may dissociate the clusters and redeposit material at the surface. A percentage of these sputtered atoms, however, diffuse into the negative glow for subsequent excitation and ionization. 

Only a low net ionization in the discharge is produced. The ionization efficiency is estimated to be 1 % or less. However, glow discharge produces an atomic vapour representative of the cathode constituents and the discharge ionization processes are also relatively non-selective. Also, because most elements are sputtered and ionized with almost the same efficiency in the source, quantitative analysis without a standard is possible. 

The principle of CHARISMA is as follows the sample is ablated with a pulsed Nd YAG laser, focused into a spot of micrometer size. Neutral species drift up while ions are suppressed. Two lasers (Ti Sapphire), tuned to resonantly ionize the element of interest, are bred into the ablated, neutral material. The ions are extracted and accelerated and analyzed with a TOF mass spectrometer where mass separation occurs due to different flight times of species with different mass. The useful yield (detected ions/sputtered atoms) is around 1% in the current set-up. A planned modification is aiming at a useful yield of around 30% which would open up new possibilities for the isotope study of trace elements (e.g. the rare earth elements) in presolar grains. With the new set-up it will also be possible to extend the analyses to sub-micrometer-sized grains, which are much more representative of presolar grains than the currently studied micrometer-sized grains. 

One major strength of GD is the separation or sampling step (sputter atomization) from the subsequent analytical steps of excitation or ionization. The GD process can be... 

Fig. 8.10. Sputter atomization and ionization processes occurring in a glow-discharge. M cathode metal atom, f potential gradient of cathode fall. (Reproduced with permission of the American Chemical Society.)...

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