Dynamic and Static SIMS

We can estimate the time scale in which the whole surface layer is affected by the primary ions. The lifetime of a surface may be simply estimated from the primary ion flux (Ip) and damage cross-section (er) generated by each impact. Ip is commonly measured in A cm-2 (1 A = 6.2 x 1018 charged particles per second). Assume that each primary ion generates a = 10-13 cm2. Then, 1013 primary ions cm-2 will affect the whole surface area of 1 cm2. It means that the lifetime of a surface with the flux density Ip= 1 pA cm-2 (= 6.2 x 1012 ions cm-2) is less than 1 second. Apparently, 1 p A cm-2 of flux density for primary ions is too high for static SIMS. Since it is commonly accepted for the static SIMS condition to limit the total amount of primary ions up to 1013 ions cm-2, for a 10-min duration of static SIMS examination a primary flux density of about 2.7 nA cm-2 is required to preserve the chemical structure of the surface top layer where the secondary ions are emitted. This flux is extremely low compared with that of dynamic SIMS, which requires a flux density of greater than 1 pA cm-2 to ensure a reasonable erosion rate of surface for depth profiling. 

---

What are the main differences between dynamic and static SIMS ... 

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. 

Principles and Characteristics Ion beam spectroscopy for polymer surface analysis comprises two general classes of experiments. One class uses a primary ion beam to generate secondary ions, which are then mass analysed. This technique, secondary ion mass spectrometry, has evolved into dynamic and static SIMS. Only the latter technique finds frequent application in polymer/additive analysis cfr. Chp. 4.2.1). The second class of ion beam spectroscopy measures the energy loss of a primary ion scattered from a surface. 

Further discussion on the definitions of Dynamic and Static SIMS whether in their conventional forms or otherwise can be found in Sections 4.1.1.1-4.1.1.3. 

SIMS can be performed in two modes dynamic SIMS, which uses a high primary ion beam intensity and static SIMS, which uses a very low primary ion beam intensity. 

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. 

Secondary-ion mass spectrometry (SIMS) is used largely as a surface characterization technique [18]. It is practiced in two formats, dynamic SIMS and static SIMS. In both formats, a beam of keV-energy ions, usually of Ar ions, bombards the surface of the sample directly. The momentum transfer from the beam to the... 

Damage cross section is an important SIMS parameter and provides a measure of the surface area affected by the impact of a single primary ion. The magnitude of the incident ion dose establishes whether a dynamic or static SIMS experiment is performed. For dynamic SIMS, the number of incident ions exceeds the number of surface atoms on the sample and results in erosion due to sputtering and chemical damage to the surface. Dynamic SIMS is primarily used in quantitative elemental imaging applications. In contrast, static SIMS measurements are performed so that the number of incident ions is... 

Three different experiments for surface and interface analysis are possible by SIMS (secondary ion mass spectrometry) SIMS static, dynamic and imaging SIMS. 

SIMS can be operated in two different modes, denoted as dynamic SIMS and static SIMS. 

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. 

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. 

If it is required that the surface of the sample remains undisturbed during analysis, SIMS must be carried out at very low surface removal rates, typically about 10 monolayer/s. The terms static and dynamic are used to divide the sputtering rate of the sample into regimes where only surface species are observed (static SIMS) or where surface and bulk species are observed (dynamic SIMS). The static limit is usually considered to be <10 ions/cm impinging on the sample surface. Under these conditions, only about 1/1000 atoms on the surface of the sample are struck by a primary ion. 

Dynamic SIMS, used for obtaining compositional information as a function of depth below the surface. With higher etch rates, the surface is eroded much more quickly than in static SIMS, and depth profiling can be achieved since each successive layer of atoms can be analysed as it is peeled away . 

Static SIMS is appropriate for obtaining information on the lateral distribution of surface chemical species. A broad, defocussed ion beam is often used in order to minimise surface damage. In dynamic SIMS sample erosion takes place quite rapidly, and depth profiles are obtained by monitoring peak intensities in the mass spectrum of sputtered ions as bombardment proceeds. 

In secondary ion mass spectrometry (SIMS) the sample surface is sputtered by an ion beam and the emitted secondary ions are analyzed by a mass spectrometer (review Ref. [360]). Due to the sputtering process, SIMS is a destructive method. Depending on the sputtering rate we discriminate static and dynamic SIMS. In static SIMS the primary ion dosis is kept below 1012 ions/cm2 to ensure that, on average, every ion hits a fresh surface that has not yet been damaged by the impact of another ion. In dynamic SIMS, multiple layers of molecules are removed at typical sputter rates 0.5 to 5 nm/s. This implies a fast removal of the topmost layers of material but allows quantitative analysis of the elemental composition. 

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

---