Continuous primary ion beams

Why does ToF SIMS use a pulsed, not continuous, primary ion beam ... 

Continuous primary ion beams are applied in Quadrupole-based SIMS instruments, Magnetic Sector-based SIMS instruments, and Time-of-Flight-based SIMS instruments in which secondary ion beam pulsing is induced through bunching of the secondary ion beam alone (see Section 4.2.3.1.5). 

As a lower current density of the analysis beam reaches the sample surface per unit time (relative to instruments utilizing continuous primary ion beams), increased sputter rates can only be realized through the irradiation of the analyzed area of the sample by a second pulsed primary ion beam. These beams are thus referred to as the sputter beam. Note Both beams must be operated in an interleaved manner with respect to each other, i.e. only one can be sticking the sample at a time. 

To ensure effective sputtering, the sputter beam is operated such that the dose is significantly higher (tens of nanoampere) than that of the analysis beam (< 1 picoampere). The sputter rate is then contiolled through the adjustment of the sputter beam pulse width, between tens to hundreds of microseconds, as opposed to the adjustment of the ion optics as used in continuous primary ion beams. The extraction ion optics is also switched off over the interval the sputter beam is directed at the sample (this also aids in providing for additional charge compensation). Depth resolution will then become a function of the sputter beam... 

With the exception of one variant of the last, all nse a single continuous primary ion beam that can be operated over a large range of primary ion dose values. Time-of-Flight instruments utilizing pulsed primaiy ion beams reqnire a low-dose analysis primary ion beam. To contend with this, a second primaiy ion beam is often employed. This is referred to as the sputter primaiy ion beam. The presence of two primary ion beams introduces further variables dependent on how these are operated, which will enhance either the mass resolntion or the spatial image resolution, but at the expense of the other. Additional modes can, however, be implemented to minimize the loss. 

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally, such an ion gun emits a stream of cesium ions (Cs ), which are cheaper to use than xenon but still have large mass (atomic masses Cs, 139 Xe, 131). Although ion guns produce no fragment ions in the primary beam, they can contaminate the mass spectrometer by deposition with continued use. 

If the primary ion beam is used to continuously remove material from the surface of a specimen in a given area, the analytical zone is advanced into the sample as a function of the sputtering time. By monitoring the secondary ion count rates of selected... 

TOF-SIMS can be applied to identify a variety of molecular fragments, originating from various molecular surface contaminations. It also can be used to determine metal trace concentrations at the surface. The use of an additional high current sputter ion source allows the fast erosion of the sample. By continuously probing the surface composition at the actual crater bottom by the analytical primary ion beam, multi element depth profiles in well defined surface areas can be determined. TOF-SIMS has become an indispensable analytical technique in modem microelectronics, in particular for elemental and molecular surface mapping and for multielement shallow depth profiling. 

Bombardment of a sample surface with a primary ion beam followed by mass spectrometry of the emitted secondary ions. Used for the analysis of trace elements in solid materials, continuous erosion by ion beam allows depth analysis... 

In addition, the primary ion beam can be directed at the sample s snrface either in a continuous manner or in a pulsed manner. 

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. 

AES analysis is done in one of four modes of analysis. The simplest, most direct, and most often used mode of operation of an Auger spectrometer is the point analysis mode, in which the primary electron beam is positioned on the area of interest on the sample and an Auger survey spectrum is taken. The next most often used mode of analysis is the depth profiling mode. The additional feature in this mode is that an ion beam is directed onto the same area that is being Auger analyzed. The ion beam sputters material off the surface so that the analysis measures the variation, in depth, of the composition of the new surfaces, which are being continu-... 

Another difference between conventional high pressure Cl and the FTMS Cl experiment is the duration of the electron beam event used to form primary ions and secondary electrons. In the conventional Cl source, the electron beam is on continuously during the experiment. The FTMS, in contrast, uses a pulsed electron beam, and the duration of the electron beam event may be varied from less than a millisecond to over a second. In the NICI studies using the FTMS, the electron beam was typically left on for ten milliseconds. However, it was found that in some cases, which will be discussed later, it was necessary to use a longer beam time, up to 1 sec, in order to observe the product ions normally produced... 

In quantitative mass spectrometry, the signal intensity depends not only on the amount of sample, but also on a number of other variables such as the ionization yield, focusing of the ion beam, and the amplification factor of the detector. As it is very difficult to keep these parameters constant over the whole period of analysis, nearly all quantitative applications of MS are based on a comparison of the ion current obtained from the component of interest, with the ion current obtained from a standard. In quantitative SIM this can be accomplished either by the continuous admission of a reference sample at a constant rate, concurrently with the sample under investigation, or by the use of an internal standard (IS) which is added to the sample prior to MS analysis (Halpern, 1981). The choice of this IS is of primary importance in the design of a new assay and was subject to some controversy in the late 1970s (Claeys et al., 1977 Lee and Millard, 1975 Millard, 1978b Self, 1979). Ideally, an IS should compensate for all possible losses during sample isolation, purification, derivatization, and separation steps and at the same time minimize variances due to the measurement process. In practice, the... 

Although some early research did focus on introducing a pulsed external ion beam from a plasma jet into the acceleration region of the TOF-MS [12], it has not been until recently that continuous ion sources such as plasmas, electrospray devices, and other atmospheric-pressure discharges have been coupled with TOF-MS. Unfortunately, the inherently pulsed nature of the spectrometer can cause this marriage to be quite inefficient. The primary challenge in using the TOF-MS with... 

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