SIMS

Dynamic SIMS has long been an established technique for the detection of low concentration species in bulk samples (e.g. dopant levels in semiconductors). While the sample is being sputtered (see Sec. 2.3), a quadrupole mass analyzer detects the resulting fragments in real time, resulting in a depth profile of the desired species in the sample or, with calibration, an indication of the concentration of the desired species in the bulk of the sample. The method is fast and simple, but a quadrupole mass analyzer can only tune into one particular mass-to-charge ratio at a time, or 

In its most elementary form, a SIMS system consists of a source of primary ions, a sample holder, secondary ion extraction optics, a mass spectrometer, and an ion detector, all housed in a UHV compartment. Systems are also equipped with data processing and output systems. A schematic diagram of part of a SIMS system with a quadrupole mass analyzer is shown in Fig. 14.38. The design and operation of mass spectrometers is covered in Chapter 9 and will be only briefly reviewed here. 

The three most common types of mass analyzers in SIMS systems are (1) double focusing magnetic sector instmments, (2) time of flight (TOF) mass spectrometers, and (3) quadru-pole mass spectrometers. The choice of mass analyzer depends on whether dynamic or static SIMS is needed, on the requirements of mass range and resolution, and on transmission efficiency, among other factors. The mass analyzers have been discussed in Chapter 9 in detail and this chapter should be reviewed as necessary. 

The detectors used for SIMS are electron multipliers (discussed in Section 14.2.1.1 and in Chapter 9) array detectors are used for imaging. 

In order to independently confirm the actual In content and structural parameters, we performed high-resolution SIMS measurements. SIMS was carried out with a CAMECA IMS 4f-E6 system, employing Gs+ primary ions with an impact energy of 5.5 keV and InCs+ ions as species for detection. The depth resolution imder these conditions is about 1 nm/decade and 2 nm/decade at the leading and trailing edge, respectively. 

The lower In content found for the former even suggests that it is stronger in the present case. In other words, the In incorporation efficiency on the M plane, phenomenologically lumping together processes such as segregation and desorption, seems to be lower when compared to the C plane. 

This technique uses a non-pulsed or DC primary ion beam of Cs or Ga ions. Alternating between a DC and a pulsed beam permits the discrete depth profile data to be obtained. In this mode, the technique would be considered destructive in order to obtain the depth concentration profiles. 

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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. 

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]. 

Other methods of sample introduction that are commonly coupled to TOP mass spectrometers are MALDI, SIMS/PAB and molecular beams (see section (Bl.7.2)). In many ways, the ablation of sample from a surface simplifies the TOP mass spectrometer since all ions originate in a narrow space above the sample surface. 

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... 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface teclmique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. For a more extensive treatment of SIMS the reader is referred to [3] and [14. 15 and 16]. The principle of SIMS is conceptually simple When a surface is exposed to a beam of ions... 

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. 

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. 

The advantages of SIMS are its high sensitivity (ppm detection limit for certain elements), its ability to detect hydrogen and the emission of molecular fragments which often bear tractable relationships with the parent... 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Figure Bl.25.9. Positive SIMS spectra of a Zr02/Si02 catalyst, (a) after preparation from Zr(OC2Ftj), (b) after drying at 40 °C and (c) after calcination in air at 400 °C (from [17]).

Figure Bl.25.10. SIMS spectra of the Rli(l 11) surface after adsorption of 0.12 ML NO at 120 K (bottom), after heating to 400 K (middle) and after reaction with H2 at 400 K (top) (from [19]).

As in Auger spectroscopy, SIMS can be used to make concentration depth profiles and, by rastering the ion beam over the surface, to make chemical maps of certain elements. More recently, SIMS has become very popular in the characterization of polymer surfaces [14,15 and 16]. 

Helmut Grubmuller, Helmut Heller, Andreas Windemuth, and Klaus Schulten. Generalized Verlet algorithm for efficient molecular dynamics simulations with long-range interactions. Mol. Sim., 6 121-142, 1991. 

C. Niedermeier and P. Tavan. Fast version of the structure adapted multipole method — efficient calculation of electrostatic forces in protein dynamics. Mol. Sim., 17 57-66, 1996. 

An important though deman ding book. Topics include statistical mechanics, Monte Carlo sim illation s. et uilibrium and non -ec iiilibrium molecular dynamics, an aly sis of calculation al results, and applications of methods to problems in liquid dynamics. The authors also discuss and compare many algorithms used in force field simulations. Includes a microfiche containing dozens of Fortran-77 subroutines relevant to molecular dynamics and liquid simulations. 

Monte Carlo sim u lat ion s pro vide an altern ate approach to the generation of stable con form ation s. As with HyperCh ern s o th er simulation methods, the effects of temperature changes and solvation arc easily incorporated into th c ealcii lation s. 

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 ),... 

If the coiiplin g parameter (the Bath relaxation constan t in IlyperChem), t, is loo Tight" (<0.1 ps), an isokinetic energy ensemble results rather than an isothermal (microcan on leal) ensemble. The trajectory is then neither canonical or microcan on-ical. You cannot calculate true time-dependent properties or ensemble averages for this trajectory. You can use small values of T for Ih CSC sim ii lalion s ... 

IlyperChem can either use initial velocilies gen eraled in a previous simulation or assign a Gaussian distribution of initial velocities derived from a random n iim her generator. Random numbers avoid introducing correlated motion at the beginn ing of a sim illation. ... 

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