Static SIMS method

Static SIMS method, 24 107, 108 Static uncertainties, 26 1019-1021 Statins, 2 821-822, 5 137, 138-140t, 142-143... 

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. 

As with any analytical method, the ability to extract semiquantitative or quantitative information is the ultimate challenge. Generally, static SIMS is not used in this mode, but one application where static SIMS has been used successfully to provide quantitative data is in the accurate determination of the coverage of fluropolymer lubricants. These compounds provide the lubrication for Winchester-type hard disks and are direaly related to ultimate performance. If the lubricant is either too thick or too thin, catastrophic head crashes can occur. 

In secondary ion mass spectrometry (SIMS) the sample surface is sputtered by an ion beam and the emitted secondary ions are analyzed by a mass spectrometer (review Ref. [360]). Due to the sputtering process, SIMS is a destructive method. Depending on the sputtering rate we discriminate static and dynamic SIMS. In static SIMS the primary ion dosis is kept below 1012 ions/cm2 to ensure that, on average, every ion hits a fresh surface that has not yet been damaged by the impact of another ion. In dynamic SIMS, multiple layers of molecules are removed at typical sputter rates 0.5 to 5 nm/s. This implies a fast removal of the topmost layers of material but allows quantitative analysis of the elemental composition. 

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. 

There are two varieties of SIMS - static and dynamic. Static SIMS (often referred to as time-of-flight SIMS, TOF-SIMS) is often the method-of-choice, used for... 

The main difference between static and dynamic SIMS lies in the ion dose that the sample receives during a measurement. In static SIMS it typically lies below < 10 ions cm , which means that there is about one incident ion per 1000 surface atoms. In this way one considers the perturbation of the smface by the ion beam negligible. Instead of reducing the primary current density one can also limit the ion dose by reducing the time of the pertmbation. This method is used in time-of-flight SIMS, abbreviated TOF-SIMS. The primary beam is pulsed and the secondary ions are separated according to their time of flight between the sample and the detector. 

The most widely used method of static SIMS is ToF-SIMS. This uses ToF mass spectrometry which provides optimum sensitivity and mass resolution for secondary ions. 

There are two varieties of SIMS - static and dynamic. Static SIMS (often referred to as time-of-flight SIMS, TOF-SIMS) is often the method-of-choice, used for elemental analysis and imaging of the top two to three monolayers of a sample in comparison, dynamic SIMS is used to determine elemental concentrations of the sample, as a function of depth. As such, dynamic SIMS is a destructive technique primarily used for depth profiling, whereas TOF-SIMS does not appreciably deteriorate the surface being analyzed. For instance, due to a slow, controllable sputtering rate, the entire analysis may be performed without removing less than one-tenths of an atomic monolayer. 

Colton has described FAB as a method that provides the molecular results of static SIMS under the high-flux conditions of dynamic SIMS. It is now understood that the important feature of this technique is the liquid matrix, and not the neutral beam. Thus, the technique is more appropriately described as liquid SIMS. The liquid... 

Although static SIMS is applied predominantly for material characterization [92BRU], true surface sensitivity can be achieved when combining the method with powerful and sensitive mass spectrometers (e.g. a time-of-flight mass spectrometer). A particular advantage of SIMS is that aU elements (including H) can be detected. 

With today s technology, the definition of the surface as it effects a material s performance in many cases means the outer one or two monolayers. It is the specific chemistry of these immediate surface molecules that determines many of the chemical and physical properties. Therefore, it is important to have available a tool that is able to characterize the chemistry of these layers. One such method that has met with considerable success is Static Secondary Ion Mass Spectrometry (SIMS). 

---