Atomic SIMS

Applied to solid materials, especially semiconductors and thin films, SIMS can determine trace levels of all elements in the periodic table. Spatial microanalysis is provided by collimating the primary ion beam to about 1 pm in diameter and control of where the beam strikes the sanple surface. This way, SIMS provides lateral and depth distributions of these elements within the sample. Curraitly, SIMS is being adapted to achieve lateral resolutions well below 100 nnx The driving force comes from the progress in microelectronics aiming at structures that approach 10 nm. Also, the depth resolution needs to come close to the atomic scale [91]. 

Note A single sputtering event, i.e., penetration, lattice perturbation by impact cascades, and final particle ejection, takes less than 10 s. Interestingly, for primary ion beam densities 10 A cm (equal to 6 x 10 ions s cm ) no interference of processes caused by different primary ions will occur, because the maximum cross section of an impact is only in the order of 10 nm. In other words, each impact is independent of prior, subsequent, or nearby synchrone-ous hits [82,83]. In the time domain this means that a given surface area suffers 10-100 hits min in dynamic SIMS, but only 0.01-0.1 hits min in static SIMS. For a monoatomic layer, observation times in the order of 1 h can thus be realized in static SIMS, i.e., the attribute static refers to the extended preservation of the surface. 

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SIMS Secondary-ion mass spectroscopy [106, 166-168] (L-SIMS liquids) [169, 170] Ionized surface atoms are ejected by impact of -1 keV ions and analyzed by mass spectroscopy Surface composition... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

SIMS Secondary Ion mass spectroscopy A beam of low-energy Ions Impinges on a surface, penetrates the sample and loses energy In a series of Inelastic collisions with the target atoms leading to emission of secondary Ions. Surface composition, reaction mechanism, depth profiles... 

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

Molecu lar mechari ical force fields use the equation s of classical mech an ics to describe th e poteri tial energy surfaces and physical properties of m olecii Ies. A molecu le is described as a collection of atom slhal in teracl with each other by sim pic an alytical fiiriclions. I h is description is called a force field. One component of a force field is th e eri ergy arisiri g from com pression and stretch in g a bond. 

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. ... 

Molecular clynainics sim illations calculate future position s and velocities of atoms, based on their current positions and velocities. A sim Illation first determ in es the force on each atom (lY) as a function of time, ct ual to the negative gradient of the polen tial en ergy (ct]uation 2 I ),... 

To generate characteristic velocities and bring a molecular system toequillbrium at th e sim illation temperature, atom s are allowed to in teract W ith each other through the equation s of motion. For isothermal simulations, a temperature bath" scales velocities to drive the system towards the simulation temperature,. Scaling occurs at each step of a simulation, according to equation 2S. 

Temperature also determines step size. An acceptable time step for room temperature simulations is about 0..5-1 fs for All Atom system s or for sim Illation s that do not con strain hydrogen atoms. For United Atom systems or systems containing only heavy atoms, you can use steps of 1-2 fs. 

Quantum mechanical calculation of molecular dynamics trajectories can sim ulate bon d breakin g and frtrm ation.. Although you dt) n ot see th e appearance or disappearan ce ofhonds, you can plot the distan ce between two bonded atom s.. A distan ce excccdi n g a theoretical bond length suggests bond breaking. 

Molccti lar mcchan ics depends on the con cep I of atom types and parameters associated with these atom types. Since the number of atom types is veiy large foi the tin iverse of possible molecules, parameters will probably he missing for a random new molecule tin less a force field has been developed for molecules sim ilar lo the new molecule. Molecu lar m ech an ics predicts how the new molecule will behave based upon the behavior orknown, similar mole-cu les. 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Th ere are sim ilar expression s for sym m etry related in tegrals (sslyy), etc. For direct comparison with CNDO, F is computed as in CNDO. The other INDO parameters, and F, are generally obtained [J. I. Slater, Quantum Theory of Atomic Structure, McGraw-Hill Book Company, Vol. 1, New York, I960.] from fits to experimental atomic energy levels, although other sources for these Slater-Con don parameters are available. The parameter file CINDO.ABP contains the values of G and F (columns 9 and 10) in addition to the CNDO parameters. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and Cl, in a ratio of. 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -i- 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 36.10). This selection of only certain ion masses is called selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM, an unfortunate... 

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). 

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

In Dynamic Secondary Ion Ma s Spectrometry (SIMS), a focused ion beam is used to sputter material from a specific location on a solid surface in the form of neutral and ionized atoms and molecules. The ions are then accelerated into a mass spectrometer and separated according to their mass-to-charge ratios. Several kinds of mass spectrometers and instrument configurations are used, depending upon the type of materials analyzed and the desired results. 

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