Secondary Ion Columns

The secondary ion column is the section of a SIMS instrument in which the ions emanating from the sample surface are collected, filtered, and focused onto the respective detector. Secondary ion columns typically contain ... 

Various electrostatic lenses and deflectors. These are used to efficiently collect and transport the secondary ions through the secondary ion column... 

Figure 4.9 Highly simplified illustrations of the layouts of secondary ion columns based around (a) Quadrupole mass filters, (b) Magnetic Sector mass filters, and (c) Time of Flight mass filters (a linear flight path is depicted for reasons of clarity). Additional lenses and deflector may also be present within these columns to further optimize the capabilities of...

Once the extracted secondary ion beam passes through the respective secondary ion column, it is detected by one of several different detector types. Detectors along with their capabilities and limitations are covered in Section 4.2.3.3. 

When these mass filters are well matched to the remainder of the ion column (in the form of double focusing secondary ion columns), these mass analyzers provide high mass resolution at high transmission, albeit at the cost of instrument size (these are the largest of the commercially available SIMS instruments). This matching also allows for imaging to be carried out in both the microprobe and the microscope modes (see Section 5.3.2.2). Examples of Magnetic Sector SIMS instruments can be found in Appendix A.7.1 -4. 

In the case of ion Magnetic Sector-based instruments operated in the microscope mode, the information on the point of secondary ion ejection is retained throughout the secondary ion column. As a result, both pulse counting and spatial resolving detectors are commonly found in such instruments. DD-EMs are used when maximum sensitivity is required (these also allow for imaging in the microprobe mode), PCs are used when increased dynamic range is needed, and MCPs combined with a phosphor screen or reactive anode encoder when imaging in the microscope mode. 

Instrument designs are defined by the type of mass filter used within the secondary ion column. Indeed, there are three types that have been heavily commercialized in the field of SIMS, with more available in the parent field of Mass Spectrometry. Those commercialized are based around ... 

Electrostatic deflectors, lenses and secondary ion extraction fields make up the remainder of the ion optical elements within the secondary ion column. These are utilized to ensure that the highest possible fraction of the secondary ions reach the detector. Detectors used in SIMS comprise of Faraday Cups for the measurement of intense signals and Electron Multipliers for pulse counting of less intense signals. Electron Multipliers come in several different designs referred to as Discrete... 

S.3.2.3 The O2 Leak Methodology As covered in Section 4.2.1.1, SIMS requires a vacuum of better than 1 X 10 " Torr (that at which the mean free path of an ion equates to 1 m, which equates to the typical path length of a primary or secondary ion column). More typically, a vacuum of better than 1 x 10 Torr is used to control the adsorption of gas phase molecules. As an example, a monolayer of Oxygen will form on Silicon in Is when in the 10 Torr range. 

There are, however, a large variety of modes under which a SIMS instrument can be operated, with each highly specific to a particular type of analysis. The analysis may require the collection of electropositive elements, electronegative elements, and/or molecules of one or the other polarity. Which analysis is required will dictate the type of primary ion beam used, the primary ion incident energy, angle of incidence and dose, the secondary ion column conditions applied, and the secondary ions collected. 

SIMS relies on the formation, collection, and transport of charged particle beams (most specifically ions, although electrons are also used in dealing with insulating samples), whether within the primary or secondary ion columns of the instrument. 

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