Pulsed primary ion beams

For static SIMS, the time-of-flight SIMS (TOF-SIMS) technique is most suitable. For TOF-SIMS, a pulsed primary ion beam is used. The secondary ions emitted from the surface after one pulse, are accelerated by an electrical potential U. These ions gain the velocity v that depends on the mass m and charge Q of the ion according to ... 

On a selected number of Cu metallized samples a second system consisting of a Time-Of- Flight -SIMS (Charles Evans spectrometer was employed. In this case, a pulsed primary Ion beam of 12 keV -Ga ions was used. A 600 pA DC current was pulsed at 5 kHz repetition rate and a pulse width of 4 lun. A 1 beam spot was... 

Laser SNMS requires the operation with properly selected duty cycles that control the delay times between the primary ion pulse, a pulsed extraction voltage for separating the secondary ions from post-ionized neutrals, and the firing of the postionizing laser pulse. Such duty cycles have, in addition, to be synchronized with the stepwise motion of the pulsed primary ion beam across the sample surface in the microprobe mode of laser SNMS. The selection of appropriate duration and decay times of the ion and laser pulses, of the laser intensity, and beam shape is important to make the photoion yields independent on the sputtered particle velocities. The detection volume must be matched to the entrance ion optics of the TOP such that it becomes independent of the individual ionization process. Usually, laser intensities in the range from 10 to lO Wcm are applied. While the particle density in the detection volume is monitored at small laser intensities, the particle flux is measured at high photon densities. 

Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface analysis technique used to analyze mass and image constituents that are present on the surface of materials. The equipment (Figure 12.47) uses a pulsed primary ion beam to desorb and ionize species from the sample surface. The resulting secondary ions are accelerated into a mass spectrometer and analyzed by measuring the ToF from the sample surface to the deteetor. The location and distribution of the species on the surface can be identified and an image shown at the detector. The composition is determined from the mass spectrum. Many different primary sources can be used for ionization ... 

The surface conditions of intelligent devices such as electronic and biodevices have been evaluated with TOF-SIMS. For example, a process of patterning and layer-by-layer assembly of synthetic [80] and orientation of polymers [81] and proteins [28,29,82] can be analyzed with TOF-SIMS. TOF-SIMS produces important fragment ions having information of chemical structures at the most upper surfaces of samples. The surface is bombarded with a pulsed primary ion beam... 

The advantage of increased transmission and simultaneous collection in Time-of-Flight SIMS utilizing pulsed primary ion beam instruments does not, however, translate to Dynamic SIMS studies. This is realized as a sizable portion of the analyzed volume is not recorded during the analysis. This stems from the fact that two ion beams are used, one for sputtering and the other for analysis, with the population arising from the sputter beam not recorded. This is further... 

The degradation in detection limits/sensitivity along with increased analysis times applies only to instruments utilizing pulsed primary ion beams when operated in the Dynamic SIMS mode. Reasons for this are discussed in a latter part of this section. 

Highly simplified SIMS instruments based around this concept are commercially available in the form of the miniSIMS instrument developed by Millbrook Instruments Ltd. (now available from Scientific Analysis Instruments Ltd.). Although effective, these suffer greater constraints in the analytical conditions that can be applied (this is discussed further in Section 5.3.2.1.2). On the plus side, such instruments are available at a significandy lower cost than pulsed primary ion beam instruments. Indeed, these have been introduced in the hope of introducing SIMS to a greater audience. 

Smooth surfaces are required as this reduces topographically induced secondary ion yield variations and is important when HMR in Time-of-FUght instruments using pulsed primary ion beams is required. 

Pulsed primary ion beams are used in Time-of-Flight-based SIMS instruments in which secondary ion beam pulsing is derived from primary ion beam pulsing (see Section 4.2.3.1.5). Pulsing can also be carried out using different sequences. 

Pulsed primary ion beams used for generating the secondary ion signal (this beam is henceforth referred to as the analysis beam) are pulsed at frequencies of between 10 and 50 kHz. Each pulse then lasts from a few nanoseconds to several hundred nanoseconds with the time between pulses ranging from tens to hundreds of microseconds. These times are adjusted to control ... 

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. 

In secondary ion mass spectrometry (SIMS), the sample is deposited as a thin film on a metal foil and the sample is then bombarded with a primary ion beam (perhaps a Ar" " pulsed primary ion beam) and secondary ions are sputtered from the surface and analyzed. The SIMS process begins by pulsing a primary ion beam onto the sample surface. This pulse is on the order of one to a few nanoseconds in length and contains from one to several thousand primary ions. The impact of this primary ion beam leads to sputtering of atomic and fragment species and the desorption of molecular ions. 

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