Secondary ion counts

If the primary ion beam is used to continuously remove material from the surface of a specimen in a given area, the analytical zone is advanced into the sample as a function of the sputtering time. By monitoring the secondary ion count rates of selected... 

A mass scan is acquired in cases when a survey of all impurities present in a volume of material is needed. Rather than measuring the secondary ion count rates of preselected elements as a fimction of sputtering time the count rates of all secondary ions are measured as a fimction of mass. Because a mass scan is continuously acquired over a mass range, no depth profiling or lateral information is available while operating in this mode. Figure 4 shows a mass scan acquired from a zirconia... 

Secondary ion mass spectrometry (SIMS) was used to characterise the coatings for their Ti, Ru and O stoichiometry on the surface and as a function of depth into the coating. A PHI 6650 Quadrupole mass spectrometer, with Cs+ as the ion source was used in these studies. The conversion of the measured secondary ion counts to concentration was performed using relative sensitivity factors, which were first determined with a standard sample containing known amounts of RuC>2 and TiC>2. All of the SIMS profiles were repeated several times, to determine the measurement precision, which was typically +10%. 

The most common species used with SIMS sources are Ar+, 02+, 0 , and N2+. These ions and other permanent gas ions are formed easily with high brightness and stability with the hollow cathode duoplasmatron. Ar+ does not enhance the formation of secondary ions but is popular in static SIMS, in which analysis of the undisturbed surface is the goal and no enhancement is necessary. 02+ and 0 both enhance positive secondary ion count rates by formation of surface oxides that serve to increase and control the work function of the surface. 02+ forms a more intense beam than 0 and thus is used preferentially, except in the case of analyzing insulators (see Chapter 11). In some cases the sample surface is flooded with 02 gas for surface control and secondary ion enhancement. An N2+ beam enhances secondary ion formation, but not as well as 02+. It is very useful for profiling and analysis of oxide films on metals, however. It also is less damaging to duoplasmatron hollow cathodes and extends their life by a factor of 5 or more compared to oxygen. 

Table 3 Secondary Ion Count Rates Under Different Bombardment Conditions...

As an example, the steps followed to make the calibration shown in Figure 52.2 are described. A gas cylinder containing a mixture of methanol, acetaldehyde, acetone, isoprene, benzene, and styrene was used for this calibration. In addition to the ions that correspond to the calibration mixture, other ion masses should also be monitored. The ion at m/z 19 corresponds to the primary ion mass, which amount is used to normalize the secondary ion counts. Because the value of the ion at m/z 19 is a factor 10 higher in intensity than most of the other masses (value 10 cps), it can age the electron multiplier very fast. Therefore, the ion at m/z 21 is used as an indicator for the primary ion signal, taking into account that this is the isotope of with a... 

A major application of the SIMS technique is depth profiling. As illustrated with the ion implant standards, secondary ion counts may be monitored as a primary beam erodes the sample resulting in a depth profile of one or several ions. This section will discuss some of issues associated with depth profiles and the analytical choices available. Three terms of interest that will be discussed are dynamic range, depth resolution, and detection limit. The dynamic range is the ratio of the maximum secondary ion counts to the minimum secondary ion counts and is often measured on an ion implantation profile. The dynamic range can be several orders of magnitude. The depth resolution is the depth across which a secondary ion signal drops from a defined upper limit to a defined lower limit or rises from a defined lower limit to a defined upper limit. The detection limit is the minimum concentration that can be detected for the element of interest and will vary with the element and the matrix. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

The absolute scale of the cross sections was obtained by making measurements with the secondary electron beam produced by (3+ bombardment of the moderator. Comparing the ion count rates measured at detector 2 obtained from both electron and positron impact gave the ratio [[Pg.179]

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