Dynamic SIMS

As sputtering removes atoms/molecules present at the outer surface of a solid and damage is of minor concern (opposite to Static SIMS), measurement of the secondary ion signal as a function of sputtering time provides the depth distribution of the signal measured. In other words, the removal of atoms from the outer surface that occurs during SIMS analysis exposes deeper layers, which then become part of subsequent analyzed populations. Profiles over depths ranging from several nm up to 10 pm can then be collected. Under ideal conditions, the depth resolution can surpass 1 run. 

Images in all three dimensions can be constructed by stacking the spatial images collected at every depth. Imaging can be carried out via either the microprobe or the microscope modes (both are discussed in Section 5.3.2.2). When carried out using the microprobe mode, an ultimate spatial resolution approaching 10 to 20 nm is possible (McPhail et al. 2010), although 50 nm and above is more common. This spatial resolution is, however, heavily dependent on primary ion spot size, and hence the primary ion current. When carried ont in the microscope mode, the spatial resolution is fixed at 1 pm irrespective of the primaiy ion spot size/cnrrent. Note Improved detection limits also allow for improved spatial resolntion. 

Sensitivity along with the best possible Detection limits and Dynamic range 

Optimizing one of the above generally comes at the cost of one or more of the others. These aspects are discussed further in Chapter 5. 

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Prepare a molecii le for a molecii lar dynamics sim illation. If the forces on atoms are too large, th e in legralion algorithm may-fail during a molecular dynamics calculation. ... 

Note A molecular dynamics sim u lation cannot overcome con -strain is imposed by covalent bonds, such as disulfide bonds and rings. Check that such constraints are acceptable. Search other possible structures in separate simulations. 

In general, Laiigeviii dynamics sim illation s run much the same as nioleciilar dynamics simulations. There are differences due Lo the presence of additional forces. Most of the earlier discussions (see pages 69-yO an d p. 3 10-327 of this man ual) on simulation parameters and strategies for molecular dyn amics also apply to Lan gevin dynamics exceptions and additional con sideraiion s are noted below. 

Dynamic Secondary Ion Mass Spectrometry (Dynamic SIMS)... 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

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. 

Dynamic SIMS is used to measure elemental impurities in a wide variety of materials, but is almost new used to provide chemical bonding and molecular information because of the destructive nature of the technique. Molecular identihcation or measurement of the chemical bonds present in the sample is better performed using analytical techniques, such as X-Ray Photoelectron Spectrometry (XPS), Infrared (IR) Spectroscopy, or Static SIMS. 

Dynamic SIMS Static SIMS Q-SIMS Magnetic SIMS Sector SIMS TOF-SIMS PISIMS... 

Today dynamic SIMS is a standard technique for measurement of trace elements in semiconductors, high performance materials, coatings, and minerals. The main advantages of the method are excellent sensitivity (detection limit below 1 pmol mol ) for all elements, the isotopic sensitivity, the inherent possibility of measuring depth profiles, and the capability of fast direct imaging and 3D species distribution. 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

The basic instrumental set-up for dynamic SIMS is the same as for SSIMS (Sect. 3.1.2). Depending on the intensity, beam diameter, and ion species needed, dif ferent ion sources are used. Several mass analyzers with different characteristics enable a broad field of applications. 

The Duoplasmatron (Eig. 3.18). In the Duoplasmatron, gas-discharge ion sources are used for bombardment with oxygen or argon. In dynamic SIMS, especially, the use of O2 ions is common because of the chemical enhancement effect. With a duoplasmatron ion beam currents of several microamps can be generated. The diameter of the beam can be focused down to 0.5 pm. 

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. 

The element sensitivity is determined by the ionization probability of the sputtered atoms. This probability is influenced by the chemical state of the surface. As mentioned above, Cs" or OJ ions are used for sample bombardment in dynamic SIMS, because they the increase ionization probability. This is the so-called chemical enhancement effect. 

Production of homogeneous solid-state standards is costly. Dynamic SIMS has the advantage that non-homogeneous ion implantation standards can also be used. Knowing the implantation dose of element (el), its RSF can be calculated by use of the integrated (summed) intensities of a depth profile according to Eq. (3.15) ... 

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