Sputter beam

Corrective action for roughening induced by sputtering has taken several directions. The simultaneous use of two sputtering beams from different directions has been explored however, rotation of the sample during ion bombardment appears to be the most promising. Attention to the angle of incidence is also important... 

All carbon samples could easily be oxidized to C02, a form of the sample greatly preferred by most users, for many reasons. Neither the on-line or dedicated electrostatic accelerators under construction have succeeded in overcoming the problem of too much memory of C02 gas. Consequently, the sample presented to the cesium sputter beam source will probably have to be a solid. The cesium sputter source is the most likely to be used because it can produce negative ions of carbon in microampere beams. 

FeC (Target)CyHjj in FeC By sputtering or heating of the sample these reaction products may evaporate into the gas phase. During this process decomposition induced by thermal reactions or the sputtering beam may take place, but this is by no means always the case. 

Only three methods proved to yield beams of an intensity high enough to perform excitation experiments beam production by ion neutralization, sputtered beams and seeded supersonic beams. The merging beam technique26 does not seem to be suited for the measurement of optical excitation cross sections because of the very long beam interaction path. 

A serious disadvantage of the sputtered beams is the low intensity caused by the need of a velocity selector. 

Negative ion yield is proportional to the electron affinity of the element. Sputter yield depends on the difference between electron affinity of the desired atom and the effective work function. Work function varies upon the environment of the surface of the sample. Physical conditions of the sample affect the properties of atoms on the surface. The probability of negative ion formation is enhanced by the presence of Cs layer at the surface of the sample and electron cloud near the sample surface. Samples are mixed with metallic powder (e.g., Ag or Nb) to improve the thermal and electrical conductivity. Ion-atom collision kinematics reduces the sputter yield for heavy elements. Production of negative ions is at the maximum for normal incidence of the sputtering beam, but the total sputter rate, which means positive, negative, and neutral emission, increases when the angle of incidence is away from the normal. Atomic ion current is very low or zero for some elements. In that case, selection of one molecular ion out of many possible molecular ions (like oxides, hydrides, or carbides) becomes important (Tuniz et al. 1998). 

Rading, D., Moellers, R., Kolhner, E, Paul,W., Niehuis, E. (2011) Dual beam depth profiling of organic materials variations of analysis and sputter beam conditions. Surf. Interface Anal, 43,198-200. 

Two-dimensional separations such as thin layer chromatography (TLC) or gel electrophoresis in combination with SIMS [53], FAB [54], and laser desorption [55] have recently given impressive results and are promising for the future [56]. In principle the technique is identical to FAB or laser desorption, with the crucial difference being that the target is the device used for the separation of the sample mixture. There are problems associated with this difference the sample may be rather dilute, not in an appropriate matrix and, because the layers are rather thick, much of the sample is protected from the sputtering beam. It is... 

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... 

Although 80 pA appears ideal for Static SIMS applications, this is much too low to satisfy the sputter rates required in Dynamic SIMS. To counter this, a second primary ion beam of much higher current is used in such instruments to etch some volume of the sample s surface between analysis beam cycles. This beam, commonly referred to as the sputter beam, is pulsed at the same frequency but out of phase with the analysis beam. 02 or Cs beams are most commonly used for this purpose, as these can also induce a significant enhancement of respective secondary ion yields (see Section 3.3.2) owing to the small fraction implanted into the substrates surface. The downside of this approach is that the sensitivity and detection limit gains resulting from simultaneous ion detection are lost. This is due to the fact that the sputtered population cannot be recorded (only ions produced by the analysis beam are recorded). 

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... 

There also exist different pulse sequences for the analysis and sputter beams during depth profiling. These are referred to as the interlaced or interleaved and the noninterlaced or Phase modes. Both essentially produce the same result on conductive samples when species present in the gas phase are not of interest, i.e. Hydrogen, Carbon, Oxygen, and Nitrogen. When these are of interest, the interlaced mode is preferred. 

The interlaced or interleaved mode describes a situation in which the sputter beam is switched on during the flight time of the secondary ions produced by the analysis beam. To avoid interferences between the analysis and sputter beams, a delay and lead-off time of several ps is implemented. This represents the fester and hence, more commonly used mode for depth profiling. This mode is also better for interface analysis and allows for reduced adsorption of gas phase species. 

In the case of primary ion pulsed Time-of-Fhght-based SIMS instruments, crater edge effects can be removed by rastering the primary ion analysis beam over a smaller region centered within the middle of the primary ion sputter beam raster pattern. 

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