Sputtering effects, dynamic SIMS

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

The element sensitivity is determined by the ionization probability of the sputtered atoms. This probability is influenced by the chemical state of the surface. As mentioned above, Cs" or OJ ions are used for sample bombardment in dynamic SIMS, because they the increase ionization probability. This is the so-called chemical enhancement effect. 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

Another potential difficulty with dynamic SIMS is that, in multicomponent systems, various elements may sputter at different rates. This will lead to segregation of the more slowly sputtering component at the surface. At steady state this surface segregation must exactly compensate for the lower sputtering rate, so that the ratio of sputtered ions will accurately reflect the composition of the material, but transient effects will occur before this steady state is reached. This is a problem if one is interested in the composition profile immediately beneath the surface. What can be done is to coat the sample with a thin, sacrificial layer of polymer (typically about 50 nm thick) steady state is then reached by the time etching has reached the sample... 

In SIMS, the chemical environment has a dramatic influence on the sputtering and/or the ionization yields of the species of interest. For instance, in dynamic SIMS, elements like oxygen and cesium are known to enhance the yields of positive and negative atomic ions of other elements by several orders of magnitude. Matrix effects are also particularly important in organic and biological SIMS and their beneficial effects were recognized very early on. Cooks and coworkers reported remarkable... 

In dynamic SIMS, a primary ion beam of energy, ranging from 0.5 to 20 keV, is used to sputter-remove successive layers of the sample in a well-defined area ranging in size from, typically, 1x1 mm to 10x 10 pm. This yields elemental information on the surface region from a few nanometres to several hundreds micrometres in depth. The detection limits of the technique are in the ppm-ppb range. Unfortunately, quantification by SIMS is bedevilled by matrix effects. They arise because the particle emission and ionisation processes take place simultaneously . If we were able to decouple sputtering from ionisation (e.g. ionisation occurring after the neutrals had been moved away from the surface), the ion yield would be independent of the matrix and quantification would be easier. 

Conversion of the sputter time scale to either the depth (as applies to Dynamic SIMS) or the surface coverage removed (as applies to Static SIMS) can be a relatively straightforward procedure. This is outlined in Section 5.4.2. Owing to the intricate interplay of intrinsic matrix effects with analysis-induced matrix effects, conversion of the intensity scale can be much more difficult. Indeed, effective quantification of the concentration scale requires the complete removal of all matrix... 

A. 10.3.3 SNMS and RIMS Secondary Neutral Mass Spectrometry (SNMS), also referred to as Sputtered Neutral Mass Spectrometry, is a destructive technique primarily used for examining elemental constituents within solid samples. This technique is closely related to Dynamic SIMS in that an ion beam is used to sputter the solid of interest. The difference lies in the fact that the sputtered neutral population, once ionized, is passed through a mass spectrometer. Ionization is induced via the action of a laser, an electron beam, or plasma (ionization yields vary from 10% for lasers to 1% for plasmas). As the greatest fraction of the sputtered population departs in the neutral state, this methodology provides the advantage of improved detection limits and reduced matrix effects relative to SIMS. Depth resolution can extend to 1 nm. Spatial imaging is generally not carried out. No prior sample preparation is needed, but HV or better conditions are required. 

As surface organic contamination generally covers any targeted trace elements in a sample, the use of a sputter gun (i.e., dynamic mode) is useful for a better detection and counting statistics for trace elements. In contrast, organic compounds are effectively destroyed by dynamic SIMS, such that no diagnostic information is obtained. 

In another review, Magee and Honig [24] discuss three important aspects of depth profiling by SIMS depth resolution, dynamic range and sensitivity. First, the depth resolution is a measure of the profile quality. They point out that the depth resolution is limited by atomic mixing effects and the flatness of the sputtered crater within the analyzed area. Second, the dynamic range of depth profiles is limited by crater edge... 

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