SIMS instrumentation

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

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. 

Perkin-Elmer Physical Electronics Division, Eden Prairie, MN, model 7000 SALI/TOF-SIMS instrument. 

The first SIMS instrument was built by Herzog and Viehboeck [3.42] in Vienna in the nineteen-forties. In the early nineteen-sixties, Herzog and Liebl [3.43] built the first sophisticated SIMS instrument at about the same time as Castaing and Slodzian in Paris [3.44]. In 1970, Benninghoven was the first to use the acronym SIMS [3.45]. 

Apart from the quadrupole and TOP analyzers described in Sect. 3.2.2, the most important types of mass analyzer used in common dynamic SIMS instruments employ a magnetic-sector field. 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

TOF analyzer it is critical for the mass resolution that the secondary ions are ejected at a precisely defined time. This means that the primary ion pulse should be as narrow in time as possible, preferably < 1 ns. At the same time maximum lateral resolution is desired. Unfortunately, there is a trade-off between these two parameters if the primary ion intensity is not to be sacrificed [122], Therefore, TOF-SIMS instruments have two modes of operation, high mass resolution and high lateral resolution. An advantage with the pulsed source is that an electron flood gun can be allowed to operate when the primary ion gun is inoperative. Thus, charge-compensation is effectively applied when analyzing insulating materials. 

SIMS instrumentation, 24 108-109. See also Secondary ion mass spectroscopy (SIMS)... 

There are two basic types of detectors used to measure ion signals, current detectors and ion counters. Each type has different implementations. A third type of detector is an imaging detector. In some SIMS instruments, the mass spectrometer is also an ion microscope, which transmits a stigmatic image of the sample to a detector plane. 

Time of flight ion probes (TOF SIMS) have unique capabilities not found in other mass spectrometers. A pulsed ion beam, typically either cesium or gallium, ejects atoms and molecules from the sample. Ionized species are accelerated down the flight tube and the arrival time in the detector is recorded, giving the mass of the species (see discussion of time-of-flight mass analyzers above). TOF SIMS instruments used in cosmochemistry have spatial resolutions of <1 pm. They are used to determine elemental abundances in IDPs and Stardust samples. The spatial distribution of elements within a small sample can also be determined. TOF SIMS instruments can obtain good data with very little consumption of sample. 

A few SIMS and SNMS instruments for surface analysis187-189 are commercially available on the analytical market. These are SIMS instruments using a double-focusing sector field mass spectrometer (e.g., CAMECA IMS-7f), time-of-flight secondary ion mass spectrometers (ToF-SIMS IV from CAMECA, Cedex, France, or ToF-SIMS 5, the ToF-SIMS 300 from ION-TOF, Munster, Germany and the PHI TRIFT IV from Physical Electronics, USA) and quadrupole based SIMS (SIMS 4550 and 4600 CAMECA, Cedex, France) or the quadrupole based SNMS instruments with SIMS option (INA-X, SPECS GmbH, Berlin, Germany). 

The silane films were prepared by dipping the substrates in 1 vol% aqueous solutions for 30-60 s. Some were then blown dry with nitrogen and introduced into the SIMS instrument within 2 h. In general, relatively thick films were prepared by dipping for 60 s, followed by dripping for 60 s. The films were then dried in air and not blown dry. Thin films were prepared by dipping for 30 s after which they were blown dry immediately. Some films were heated for 30 min at 150°C in air prior to analysis. 

For the TOF SIMS analysis, only slides treated with a natural pH HAPS solution were used. These were subsequently extracted with warm and hot water. They were mounted into a grid sample holder for transportation into a VG IX23S time-of-flight (TOF) SIMS instrument operating at a vacuum of < 10 Torr with a microfocused liquid Ga metal ion primary beam source (30 keVx 1.0 nA). For charge compensation, an electron flood gun was used. The working resolution of the spectrometer was determined from a lead phthalocyanine spectrum for Pb+ at mlz = 208 and the molecular ion at mlz = 720, it was 500 and 1000, respectively. 

Not surprisingly, however, all of these capabilities are generally not achieved within a single SIMS Instrument. It Is necessary to examine critically the requirements at hand and choose those design features which lead to an Instrument best suited to the defined need. 

The features from which one has to choose In building a SIMS Instrument are as numerous and varied as the problems which the technique can address. Shown conceptlonally In Figure 1 are the various choices for primary bombarding beam formation and emitted particle detection, all encompassed within a vacuum system. 

When designing a SIMS Instrument to give the best possible results for a given type of analysis one must know which components are necessary for the desired performance, and Indeed, what kind of performance Is required. Table I lists the three basic areas of SIMS analysis to be discussed here. Under each type of analysis are listed the most appropriate components and design features. 

The vacuum requirements for this type of SIMS Instrument are extremely stringent because the uppermost monolayer of the sample must not become contaminated by the residual gas In the Instrument, otherwise the spectrum observed will bear little resemblance to the actual sample surface. 

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