SIMS dynamic mode

By using dynamic mode SIMS the lateral distribution of phases in three dimensions can be resolved (Fig. 31). Thin films (thickness ca. 500 nm) of binary mixtures of deuterated or partially brominated PS, polyisoprene and poly(vinylpyridine) were investigated with a lateral resolution of approximately 120 nm and composition versus depth profiles with a resolution better than 10 nm [208]. The brominated PS formed continuous phase-domain structures in the interior of the films whereas they were encapsulated by deuterated PS layers at the interfaces. Moreover a very thin layer (ca. 3 nm) of polyisoprene covered the surface of a binary mixtme of poly(isoprene)/deuterated PS [208]. 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

SIMS is, strictly speaking, a destructive teclmique, but not necessarily a damaging one. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one monolayer per several hours, implying that the surface structure does not change during the measurement (between seconds and minutes). In this case one can be sure that the molecular ion fragments are truly indicative of the chemical structure on the surface. 

SIMS is strictly speaking a destructive technique. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one... 

Imaging SIMS has been used to locate the positions of trace amounts of metals such as vanadium and nickel on zeolite-containing cracking catalysts. This technique can be used ex-situ in a dynamic mode to determine the mobility of these poisons. The results... 

Secondary ion mass spectrometry (SIMS) is to measure the secondary ions, ionized clusters, atoms and atomic clusters, which are emitted from the surface of particles, when it is bombarded with a primary beam of ions, such as He", Ne", or Ar", with energies in the range of hundreds of eV to keV scale. The emitted ions and ionized clusters are analyzed directly by using a mass spectrometer. Therefore, chemical composition of the surface can be analyzed with the obtained accordingly. SIMS has two modes of analysis (i) static and (ii) dynamic. Static SIMS uses an ion beam with low current density, so as to confine the analysis to the outermost layers. Dynamic SIMS uses beams of high current density, so that successive atomic layers can be eroded at a relatively high rate. Comparatively, the analytical conditions of dynamic SIMS are less suitable for surface analysis. 

A practical definition could finally also be derived from the capabilities of the instrumentation in use. For instance, SIMS, the most widespread MS technique, applied to surface and thin films can be operated in static mode (giving information from the first atomic layers of a nearly undamaged surface) or dynamic mode (depth profile of the layer). When the material to be analyzed is sputtered, this sputtering could be very slow, providing a practical limit (often in the micrometer range for SIMS) to the thickness range achievable in a reasonable amount of time. 

In SIMS, we distinguish between static and dynamic modes. In static SIMS one uses a primary beam with a very low current density (< 10 nA cm ), which leaves the surface largely unperturbed. In order to obtain sufficient signal intensity, a... 

An immediate distinction needs to be made between dynamic and static SIMS experiments. In the former, a primary ion current density of typically > 10 pA cm is used and this leads to rapid sputtering of material. The surface is eroded at a rate of order nm s and by following the intensity of chosen peaks in the mass spectrum as a function of time, a concentration depth profile can be constructed. In this mode SIMS can be very sensitive, with trace element detection limits in the ppm-ppb range. However, quantification is not straightforward. Secondary ion intensities are strongly matrix-dependent and extensive calibration procedures involving closely related standards of known composition and under identical experimental conditions must be used to extract quantitative concentrations. 

There are static and dynamic modes of operation in SIMS. Static SIMS refers to analysis of only the surface of a sample and is crnisidered nondestructive under certain bombardment conditions. Dynamic SIMS refers to probing below the surface and is considered destructive. 

In SIMS, one distinguishes between the static and the dynamic modes. In static SIMS only a fraction of the first monolayer of the surface layer Is perturbed. This depends on the flux of the primary Ion beam, which Is kept well below 10 par-ticles/cm. This number corresponds to l%o of the number of particles of the first monolayer, which is approximately 10 particles. Past this limit, degradation signatures can be detected. During further bombardment the top-surface will then be completely destroyed, leading to a depth profile type of Information (dynamic mode). 

Conceptually, SIMS can be considered a straightforward and direct technique. In practice, there are many complexities introduced as a result of the various methodologies that can be applied, whether in the static or in the dynamic mode of SIMS. This exists because there are numerous conditions under which SIMS can be carried out. Each condition is optimized to deal with the analysis of a different elemental or molecular species, from different solid matrices. In addition, relating the output to the compositional variations that may occur on or within the sohd being examined can be problematic. This stems, in part, from the complexities surrounding secondary ion generation, or more precisely, the matrix effect. As the term suggests, the matrix effect describes the effect of the matrix on the population of ions emitted. Matrix effects and their associated transient effects are discussed in Section 3.3.3.1.2. 

Sensitivity and Detection Limits Prime attributes associated with SIMS are its detection limits (ability to detect low concentrations) and its concurrent sensitivity (ability to detect concentration differences) to certain elements when analyzed under the appropriate conditions. Indeed, SIMS when operated in the Dynamic mode can routinely provide elemental detection limits extending to parts per billion or even sub parts per billion levels. As an example. Boron in Silicon can be detected to 0.4 ppb or 2x 10 atoms/cm under fully optimized conditions. Note However, that although SIMS can detect all elements and molecules, not all are detectable to the same levels. This is reflected in the detection limits for Hydrogen through Krypton in Silicon, as provided in Appendix A.5. 

Also shown in Fignre 1.4 are the relative detection limit ranges for SIMS when operated in both its Static and Dynamic modes, as well as the detection limits... 

SIMS, whether used in static or dynamic mode, identifies the types of isotopes, elements, or molecules present on or within a substrate by the mass to charge ratio (m/q) of the emitted secondary ions. Identification is possible because ... 

A mass spectrum, derived by scanning the mass filter over some predefined m/q ratio range, constitutes all the secondary ions of the polarity of interest (only one polarity can be collected at a time). Such spectra are of interest when the sample type is unknown (this allows for the identification of the elemental and/or molecirlar constituents) and/or when information on the optimal signals for the acqirisition of images and/or depth profiles is required. This option is available and commonly used in all applications of SIMS, whether in Static or Dynamic modes. 

The poorer detection limits when operated in dynamic mode than Magnetic Sector-based instruments (this loss is not evident in Static SIMS)... 

As surface organic contamination generally covers any targeted trace elements in a sample, the use of a sputter gun (i.e., dynamic mode) is useful for a better detection and counting statistics for trace elements. In contrast, organic compounds are effectively destroyed by dynamic SIMS, such that no diagnostic information is obtained. 

Static SIMS is labeled a trace analytical technique because of the very small volume of material (top monolayer) on which the analysis is performed. Static SIMS can also be used to perform chemical mapping by measuring characteristic molecules and fiagment ions in imaging mode. Unlike dynamic SIMS, static SIMS is not used to depth profile or to measure elemental impurities at trace levels. 

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