Static SIMS instrument

Cation emitters The alkali metal zeolites, and other alkali metal aluminosilicates, are efficient emitters of alkali metal cations. The cation emitters have been known for a much longer time than the anion emitters, but the anion emitters are better understood from a chemical perspective hence they are discussed here. Both types of emitters, however, can be scaled up in intensity readily to be used for the primary ion guns in static SIMS instruments. Ion beams of 50 pA to 1 nA focused to a 1-mm spot size are routinely produced by using these emitters. These emitters are primarily used in SIMS guns, as opposed to being used for isotope ratio analyses. 

Static SIMS. The static SIMS instrument was briefly described in the instrumental section. Static SIMS has been applied to the study of metal surfaces (fig.) oxide formation (11), and catalysts (ii fi6) but static SIMS is not widely used in microelectronics materials characterization. A major limitation of static SIMS for microelectronics applications is the inability to obtain a detectable signal from a very small area on the sample. The low energy low current density primary ion source used in static SIMS produces a lower count rate per unit area of sample than does the higher energy higher current density used in dynamic SIMS. 

How does static SIMS instrumentally differ from dynamic SIMS How does the information obtained from static SIMS differ from that obtained from dynamic SIMS What is imaging SIMS What type of information is obtained with imaging SIMS ... 

In modern static SIMS instruments, the most efficient spectrometer is the time of flight spectrometer. It has typical current densities of the order of 1 nA/cm, which corresponds to approximately lO particles/cm s [23, 24]. The emitted secondary ions are separated according to their mass In this spectrometer. Figure 6 shows schematically the principle of the time of flight measurement. 

A major advantage of static SIMS over many other analytical methods is that usually no sample preparation is required. A solid sample is loaded directly into the instrument with the condition that it be compatible with an ultrahigh vacuum (10" —10 torr) environment. Other than this, the only constraint is one of sample size, which naturally varies from system to system. Most SIMS instruments can handle samples up to 1-2 inches in diameter. 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

Selected topics in Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry instrumentation are discussed in depth, and numerous analytical application examples are given. In particular, optimization ofthe single-cell FTMS design and some of its analytical applications, like pulsed-valve Cl and CID, static SIMS, and ion clustering reactions are described. Magnet requirements and the software used in advanced FTICR mass spectrometers are considered. Implementation and advantages of an external differentially-pumped ion source for LD, GC/MS, liquid SIMS, FAB and LC/MS are discussed in detail, and an attempt is made to anticipate future developments in FTMS instrumentation. 

Figure 7 Static secondary ion mass spectrometry (SIMS) instrument of Benninghoven and Loebach. (From Ref. 34.)...

The results presented here demonstrate that static SIMS has unique capabilities for the characterization of the surfaces of polymers that have been modified by metal deposition or by plasma or corona techniques. Especially, the introduction of unsaturation and crosslinking are aspects that in some polymers can be observed directly. The formation of low-molecular oxidized material that can be inferred from XPS studies, can also be observed directly. A limitation of the quadrupole-type instrument, which is still the most widely used, is its limited mass range and mass resolution. It can be expected that a considerably more detailed description of modified polymer surfaces can be obtained by application of the more powerful reflectron-type Time-of-Flight SIMS spectrometers, but such studies have, to date, not yet been published. 

Morrison has shown that even these are irreproducible (6). Relative quantitation of atomic ions has been quite successful, with sensitivity factor approaches the most widely used (7). However, no such treatment has been available for molecular ions in static or low damaqe SIMS owing to the difficulty of detection and low signal levels. In addition, the inability to reproduce molecular ion emission is related to instrumental factors, irreproducibility of samples, and the role of surface chemistry. An advance in the use of static SIMS could be made with a suitable method for quantitation. The present paper will discuss such an approach as it applies to surface chemistry of metal-organic systems. 

Such features make SIMS a powerful technique for surface analysis. However, SIMS as a surface analysis technique has not yet reached a mature stage because it is still under development in both theoretical and experimental aspects. This lack of maturity is attributed to the complicated nature of secondary ion yield from a solid surface. Complexity of ion yield means that SIMS is less likely to be used for quantitative analysis because the intensity of secondary ions is not a simple function of chemical concentrations at the solid surface. SIMS can be either destructive or non-destructive to the surface being analyzed. The destructive type is called dynamic SIMS it is particularly useful for depth profiling of chemistry. The nondestructive type is called static SIMS. Both types of SIMS instruments are widely used for surface chemical examination. 

Analysis of surface chemical structure requires use of the static SIMS technique to ensure that the major portion of the surface should not be affected by secondary ion emission. Time-of-flight SIMS (ToF SIMS) is the most widely used static SIMS technique. As its name indicates, ToF SIMS uses the ToF mass analyzer to measure mz l of secondary ions. ToF SIMS is a stand-alone instrument, not incorporated into or attached to other SIMS instruments as for dynamic SIMS. A typical structure is illustrated in Figure 8.13. 

The proliferation of adequate lasers and commercial TOF-SIMS instruments with advanced features for focused primary ion bombardment under static conditions, highly efficient mass analysis, and detection... 

The three most common types of mass analyzers in SIMS systems are (1) double-focusing magnetic sector instruments, (2) TOF mass spectrometers, and (3) quadrupole mass spectrometers. The choice of mass analyzer depends on whether dynamic or static SIMS is needed, on the requirements of mass range and resolution, and on transmission efficiency, among other factors. The mass analyzers have been discussed in Chapter 9 in detail and this chapter should be reviewed as necessary. 

However, when applied to chemical analysis conditions other than those approximating static SIMS, such a primitive instrument gives very poor resolving power in practice as a result of a distribution of values of Vx- In most cases the neutral analyte molecules and/or ions occupy... 

In contrast, static SIMS measurements are performed with a number of incident ions (<10 ions/cm ) about one order of magnitude less than the number of atoms at the surface of the sample (one monolayer of Si contains 10 atoms/cm ). In this case, the damage to the sample surface is minimized and ions are mainly emitted from the first atomic layers, also promoting desorption of large fragments. However, count rates are low and information is restricted to relatively abundant species from the very superficial layers of the target. In both cases, instrumental parameters need to be selected according to the most critical factors in the analysis (e.g., lateral resolution, depth resolution, or sensitivity), and the optimal conditions represent a compromise between those different factors. 

TOF-SMS not only allows one to perform static SIMS experiments, but many instruments also permit chemical imaging of surfaces as well as depth profile analysis. 

Instrumental Aspects. The early developments of static SIMS (largely driven by polymer surface analysis requirements in terms of practical application) were carried out with noble gas ion sources and quadrupole mass analyzers (QMS). The latter had the benefits of compactness, ready availability, and relatively straightforward adaptability to SIMS use in UHV systems. Although all the essential features of polymer surface analysis were introduced using this technology, the QMS has major limitations for static SIMS. Firstly, it is a serial device, so that only one mass at a time can be detected. Secondly, it has a limited mass range (<1000 Da) and the transmission (ie, sensitivity) decreases with mass (by at least m ). Thirdly, it is only possible to achieve a uniform mass resolution... 

Kratos Prism ToF-SIMS spectrometer (Manchester, UK) or similarly designed static SIMS imaging instrument. 

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